Marine reservoir ages for coastal West Africa

Guillaume Soulet, Philippe Maestrati, Serge Gofas, Germain Bayon, Fabien Dewilde, Maylis Labonne, Bernard Dennielou, Franck Ferraton, Giuseppe Siani

► To cite this version:

Guillaume Soulet, Philippe Maestrati, Serge Gofas, Germain Bayon, Fabien Dewilde, et al. Marine reservoir ages for coastal West Africa. Geochronology, 2023, 10.5194/gchron-2023-5. hal-04420390

HAL Id: hal-04420390

https://hal.science/hal-04420390

Submitted on 4 Apr 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_1.png" /></figure>

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# Marine reservoir ages for coastal West Africa

Guillaume Soulet¹, Philippe Maestrati², Serge Gofas²,³, Germain Bayon¹, Fabien Dewilde¹, Maylis Labonne⁴, Bernard Dennielou¹, Franck Ferraton⁴, Giuseppe Siani⁵

1 Ifremer, Univ Brest, CNRS, Geo-Ocean UMR6538, F-29280, Plouzané, France

2 Muséum National d'Histoire Naturelle, Paris, DGD-Collections, France

3 Departamento de Biología Animal, Facultad de Ciencias, Universidad de Málaga, Málaga, Spain

4 MARBEC, Univ Montpellier, CNRS, Ifremer, IRD, Montpellier, France

5 GEOPS, UMR 8148 Université Paris-Saclay, Orsay, France

*Correspondence to:* Guillaume Soulet (gsoulet@ifremer.fr)

**Abstract.** We measured the ¹⁴C age of pre-bomb suspension-feeding bivalves of known-age from coastal West Africa (n=30) across a latitudinal transect extending from 33°N to 15°S. The specimens are from the collections of the Muséum National d'Histoire Naturelle (Paris, France). They were carefully chosen to ensure that the specimens were alive when collected or died not long before collection. From the ¹⁴C-dating of these known-age bivalves, we calculated the marine reservoir age (as ΔR and R values) for each specimen. ΔR values were calculated relative to the Marine20 calibration curve and the R values relative to Intcal20 or SHcal20 calibration curves. Except five outliers, the ΔR and R values were quite homogeneous to a mean value of -77 ± 47 ¹⁴C yrs (1sd, n = 25), and of 400 ± 59 ¹⁴C yrs (1sd, n = 25), respectively. These values are typical of low latitude marine reservoir age values. Five suspension-feeding species living in five different ecological habitats were studied. For localities were different species were available, the results yielded similar results whatever the species considered suggesting that the habitat has only a limited impact on the marine reservoir age reconstruction. We show that our measured marine reservoir ages follow the declining trend of the global marine reservoir age starting ca. 1900 AD, suggesting that marine reservoir age of coastal West Africa is driven, at least at first order, by the global carbon cycle and climate rather than by local effects. Each outlier was discussed. Sub-fossil specimens likely explain the older ¹⁴C age and thus larger marine reservoir age measured for these samples. *Bucardium ringens* might not a best choice for marine reservoir age reconstructions.

## 1 Introduction

The marine reservoir age (R) at a given calendar time/year (t) is the difference between the radiocarbon age (¹⁴C) of the dissolved inorganic carbon (DIC) of the ocean (¹⁴C_m) and that of atmospheric CO₂ (¹⁴C_atm) (Stuiver and Polach, 1977; Ascough et al., 2005; Soulet et al., 2016; Skinner and Bard, 2022; Soulet, 2015):

$$R(t) = {}^{14}\text{C}_{\text{m}}(t) - {}^{14}\text{C}_{\text{atm}}(t) \qquad (1)$$

At global scale, the marine reservoir age of the surface mixed layer of the ocean is set by the exchange of “young” CO₂ at the atmosphere-ocean interface, plus the exchange of DIC between oceanic surface waters and deep waters that contain large
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

# Marine reservoir ages for coastal West Africa

Guillaume Soulet¹, Philippe Maestrati², Serge Gofas²,³, Germain Bayon¹, Fabien Dewilde¹, Maylis Labonne⁴, Bernard Dennielou¹, Franck Ferraton⁴, Giuseppe Siani⁵

1 Ifremer, Univ Brest, CNRS, Geo-Ocean UMR6538, F-29280, Plouzané, France

2 Muséum National d'Histoire Naturelle, Paris, DGD-Collections, France

3 Departamento de Biología Animal, Facultad de Ciencias, Universidad de Málaga, Málaga, Spain

4 MARBEC, Univ Montpellier, CNRS, Ifremer, IRD, Montpellier, France

5 GEOPS, UMR 8148 Université Paris-Saclay, Orsay, France

*Correspondence to:* Guillaume Soulet (gsoulet@ifremer.fr)

**Abstract.** We measured the ¹⁴C age of pre-bomb suspension-feeding bivalves of known-age from coastal West Africa (n=30) across a latitudinal transect extending from 33°N to 15°S. The specimens are from the collections of the Muséum National d'Histoire Naturelle (Paris, France). They were carefully chosen to ensure that the specimens were alive when collected or died not long before collection. From the ¹⁴C-dating of these known-age bivalves, we calculated the marine reservoir age (as ΔR and R values) for each specimen. ΔR values were calculated relative to the Marine20 calibration curve and the R values relative to Intcal20 or SHcal20 calibration curves. Except five outliers, the ΔR and R values were quite homogeneous to a mean value of -77 ± 47 ¹⁴C yrs (1sd, n = 25), and of 400 ± 59 ¹⁴C yrs (1sd, n = 25), respectively. These values are typical of low latitude marine reservoir age values. Five suspension-feeding species living in five different ecological habitats were studied. For localities were different species were available, the results yielded similar results whatever the species considered suggesting that the habitat has only a limited impact on the marine reservoir age reconstruction. We show that our measured marine reservoir ages follow the declining trend of the global marine reservoir age starting ca. 1900 AD, suggesting that marine reservoir age of coastal West Africa is driven, at least at first order, by the global carbon cycle and climate rather than by local effects. Each outlier was discussed. Sub-fossil specimens likely explain the older ¹⁴C age and thus larger marine reservoir age measured for these samples. *Bucardium ringens* might not a best choice for marine reservoir age reconstructions.

## 1 Introduction

The marine reservoir age (R) at a given calendar time/year (t) is the difference between the radiocarbon age (¹⁴C) of the dissolved inorganic carbon (DIC) of the ocean (¹⁴C_m) and that of atmospheric CO₂ (¹⁴C_atm) (Stuiver and Polach, 1977; Ascough et al., 2005; Soulet et al., 2016; Skinner and Bard, 2022; Soulet, 2015):

$$R(t) = {}^{14}\text{C}_{\text{m}}(t) - {}^{14}\text{C}_{\text{atm}}(t) \qquad (1)$$

At global scale, the marine reservoir age of the surface mixed layer of the ocean is set by the exchange of “young” CO₂ at the atmosphere-ocean interface, plus the exchange of DIC between oceanic surface waters and deep waters that contain large
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

amounts of “old” DIC (Bard, 1988; Skinner and Bard, 2022). The ¹⁴C age of the global ocean over time, i.e., the Marine20 calibration curve (Heaton et al., 2020), has been modelled using the global carbon cycle box model BICYCLE (Köhler et al., 2006; Köhler and Fischer, 2006, 2004; Köhler et al., 2005) and the Northern Hemisphere atmospheric ¹⁴C calibration curve (IntCal20; Reimer et al., 2020). While the global marine calibration curve (Marine20) is widely used to derive calibrated ages from ¹⁴C dating of marine samples, it does not account for local marine ¹⁴C offsets due to, for instance, continental carbon inputs to the coastal ocean, regional winds, and changes in the oceanic circulation and climate (Bard, 1988; Alves et al., 2018; Skinner and Bard, 2022; Heaton et al., 2023). Hence, the usefulness of the ΔR metric (Stuiver et al., 1986; Stuiver and Braziunas, 1993; Reimer and Reimer, 2017) that is the difference between the ¹⁴C age of any marine sample (¹⁴Cₘ) and that of the marine calibration curve (¹⁴CₚMarine20) at the same time (t):

$$ \Delta R(t) = {}^{14}\text{C}_{\text{m}}(t) - {}^{14}\text{C}_{\text{Marine20}}(t) \qquad (2) $$

The local marine reservoir age offset (ΔR) is known to vary largely as demonstrated by pre-bomb values ranging between – 500 to + 2000 ¹⁴C years (Reimer and Reimer, 2001) depending on the location. Larger ΔR values are located at high-latitudes while values close to ΔR = 0 ¹⁴C years are located at low latitudes (Bard, 1988; Bard et al., 1994).

From a geochronological perspective (i.e., calibration of marine ¹⁴C dates and building age-depth model from marine ¹⁴C dates), knowing ΔR(t) is of crucial interest to correct marine ¹⁴C dates for a local ¹⁴C offset compared to the global marine calibration curve and hence a pre-requirement to derive accurate calendar ages. Reconstructing ΔR(t) values from unstudied areas is also valuable as it could contribute to deriving regional/local marine calibration curves from the global one using 3-D large-scale ocean circulation model (Butzin et al., 2017; Alves et al., 2019).

From a carbon cycle perspective, the R(t) and ΔR(t) are also very important as they reflect ¹⁴C disequilibria between the ocean and the atmosphere and hence they are key proxies to understand local variations of the global carbon cycle, and its evolution over time with changing climate and environment (Skinner et al., 2015, 2010; Lindsay et al., 2016; Soulet et al., 2011; Siani et al., 2001; Schefuß et al., 2016; Heaton et al., 2021).

On a whole, efforts to estimate R(t) and ΔR(t) values wherever possible are valuable to the understanding of both modern and past carbon cycle, and the reconstruction of climate and environmental changes based on sedimentary archives.

Pre-bomb R(t) and ΔR(t) values for coastal West Africa are very sparse. According to the Marine Reservoir Correction Database (Reimer and Reimer, 2001; http://calib.org/marine/; last seen 15/11/2022), from Oran on the Mediterranean coast of Algeria (Siani et al., 2000) to Hondelkip Bay on the Atlantic coast of South Africa (Dewar et al., 2012), only few marine reservoir ages from Mauritania and Senegal were reported (Ndeye, 2008) (Fig. 1). For Mauritania, collection sites were Nouadhibou (formerly Port-Étienne, 2 samples), the area of the Cape Timiris – El Mamghar (3 samples). Two samples were collected from an unknown location from coastal Mauritania. For Senegal, collection sites were restricted to the Dakar area (Almadies, Dakar harbour, Gorée Island and Rufisque; 5 samples). Thirteen additional samples were from unknown locations from coastal Senegal.

In this study we report new marine reservoir age values (n=30) based on the ¹⁴C dating of bivalves with a known pre-bomb collection date and collected across a latitudinal transect extending from Mohammed (Morocco, 33°N) to Moçâmedes
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

amounts of “old” DIC (Bard, 1988; Skinner and Bard, 2022). The ¹⁴C age of the global ocean over time, i.e., the Marine20 calibration curve (Heaton et al., 2020), has been modelled using the global carbon cycle box model BICYCLE (Köhler et al., 2006; Köhler and Fischer, 2006, 2004; Köhler et al., 2005) and the Northern Hemisphere atmospheric ¹⁴C calibration curve (IntCal20; Reimer et al., 2020). While the global marine calibration curve (Marine20) is widely used to derive calibrated ages from ¹⁴C dating of marine samples, it does not account for local marine ¹⁴C offsets due to, for instance, continental carbon inputs to the coastal ocean, regional winds, and changes in the oceanic circulation and climate (Bard, 1988; Alves et al., 2018; Skinner and Bard, 2022; Heaton et al., 2023). Hence, the usefulness of the ΔR metric (Stuiver et al., 1986; Stuiver and Braziunas, 1993; Reimer and Reimer, 2017) that is the difference between the ¹⁴C age of any marine sample (¹⁴Cₘ) and that of the marine calibration curve (¹⁴CₚMarine20) at the same time (t):

$$ \Delta R(t) = {}^{14}\text{C}_{\text{m}}(t) - {}^{14}\text{C}_{\text{Marine20}}(t) \qquad (2) $$

The local marine reservoir age offset (ΔR) is known to vary largely as demonstrated by pre-bomb values ranging between – 500 to + 2000 ¹⁴C years (Reimer and Reimer, 2001) depending on the location. Larger ΔR values are located at high-latitudes while values close to ΔR = 0 ¹⁴C years are located at low latitudes (Bard, 1988; Bard et al., 1994).

From a geochronological perspective (i.e., calibration of marine ¹⁴C dates and building age-depth model from marine ¹⁴C dates), knowing ΔR(t) is of crucial interest to correct marine ¹⁴C dates for a local ¹⁴C offset compared to the global marine calibration curve and hence a pre-requirement to derive accurate calendar ages. Reconstructing ΔR(t) values from unstudied areas is also valuable as it could contribute to deriving regional/local marine calibration curves from the global one using 3-D large-scale ocean circulation model (Butzin et al., 2017; Alves et al., 2019).

From a carbon cycle perspective, the R(t) and ΔR(t) are also very important as they reflect ¹⁴C disequilibria between the ocean and the atmosphere and hence they are key proxies to understand local variations of the global carbon cycle, and its evolution over time with changing climate and environment (Skinner et al., 2015, 2010; Lindsay et al., 2016; Soulet et al., 2011; Siani et al., 2001; Schefuß et al., 2016; Heaton et al., 2021).

On a whole, efforts to estimate R(t) and ΔR(t) values wherever possible are valuable to the understanding of both modern and past carbon cycle, and the reconstruction of climate and environmental changes based on sedimentary archives.

Pre-bomb R(t) and ΔR(t) values for coastal West Africa are very sparse. According to the Marine Reservoir Correction Database (Reimer and Reimer, 2001; http://calib.org/marine/; last seen 15/11/2022), from Oran on the Mediterranean coast of Algeria (Siani et al., 2000) to Hondelkip Bay on the Atlantic coast of South Africa (Dewar et al., 2012), only few marine reservoir ages from Mauritania and Senegal were reported (Ndeye, 2008) (Fig. 1). For Mauritania, collection sites were Nouadhibou (formerly Port-Étienne, 2 samples), the area of the Cape Timiris – El Mamghar (3 samples). Two samples were collected from an unknown location from coastal Mauritania. For Senegal, collection sites were restricted to the Dakar area (Almadies, Dakar harbour, Gorée Island and Rufisque; 5 samples). Thirteen additional samples were from unknown locations from coastal Senegal.

In this study we report new marine reservoir age values (n=30) based on the ¹⁴C dating of bivalves with a known pre-bomb collection date and collected across a latitudinal transect extending from Mohammed (Morocco, 33°N) to Moçâmedes
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

(Angola, 15°S). Our suite of sample includes specimens from Mauritania, Senegal, Republic of Guinea, Sierra Leone, Ivory Coast, Benin, Gabon and Republic of Congo (Fig. 1, Table S1 in the Supplement). We used specimens of five different species: *Senilia senilis*, *Bucardium ringens*, *Donax rugosus*, *Ostrea stentina* and *Pseudochama gryphina*. We briefly discuss our results in the context of local environmental setting of the studied bivalves and regional oceanography of the Eastern Atlantic Ocean.

# 2 Material and methods

## 2.1 Material

Bivalve shells were selected from the collections of the Muséum National d'Histoire Naturelle (MNHN) (Paris, France). We carefully chose pre-bomb specimens of known-age and ensured that they were collected alive or very soon after death. For example, specimens with articulated valves and exhibiting flesh remains inside the shell were clearly collected alive. For *Senilia senilis*, the presence of the fragile periostracum provides evidence that the specimen was collected fresh. For *Bucardium ringens*, remains of the hinge ligament indicate that the bivalve death occurred not long before collection. The collection date was also carefully checked. Below, we provide background information for the five different bivalve species investigated in this study. Additional information for each sample is given in section 3.1.

*Senilia senilis* (Linnaeus, 1758) can be found from Mauritania to northern Angola. It lives in fine sand in estuaries, creeks or lagoon with regular tidal influence from the lower intertidal zone to about 2 meters water depth. The specie is tolerant to seasonal salinity changes (von Cosel and Gofas, 2019). *S. senilis* is a suspension feeder that lives in the top 5-10-cm layer of the sediment (Okera, 1976; Catry et al., 2017).

*Bucardium ringens* (Bruguière, 1789) occurs from Mauritania to southern Angola. It lives in clean fine sand and mixed sand on open coast from shallow (5-10 meters depth) to about 50 meters depth. Shells and valves are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019). *B. ringens* is likely a suspension feeder as typically are cardiids (Herrera et al., 2015).

*Donax rugosus* (Linnaeus, 1758) occurs from Mauritania to Ghana and from northern Angola to southern Angola. It lives in mixed and coarse sand in the surf zone of open beaches (von Cosel and Gofas, 2019). *D. rugosus* is a suspension feeder (Smith, 1971).

*Ostrea stentina* (Payraudeau, 1826) can be found southern Portugal to Ghana, then from Gabon to northern Angola. It is common and occurs on various types of hard substrate such as rocks, stones, pebbles and other oysters from 1 to 30 meters depths. It can also be found in lagoons, inlets and creeks under marine condition (von Cosel and Gofas, 2019). *O. stentina* is a suspension feeder (Türkmen et al., 2005).

*Pseudochama gryphina* (Lamarck, 1819) occurs from Southern Portugal to Mauritania and from Gabon to southern Angola (von Cosel and Gofas, 2019) and lives on hard substrate such as rocks and stones in clear water offshore about 10 to 60 meters water depth. *P. gryphina* is a suspension feeder (Sessa et al., 2013).
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

(Angola, 15°S). Our suite of sample includes specimens from Mauritania, Senegal, Republic of Guinea, Sierra Leone, Ivory Coast, Benin, Gabon and Republic of Congo (Fig. 1, Table S1 in the Supplement). We used specimens of five different species: *Senilia senilis*, *Bucardium ringens*, *Donax rugosus*, *Ostrea stentina* and *Pseudochama gryphina*. We briefly discuss our results in the context of local environmental setting of the studied bivalves and regional oceanography of the Eastern Atlantic Ocean.

# 2 Material and methods

## 2.1 Material

Bivalve shells were selected from the collections of the Muséum National d'Histoire Naturelle (MNHN) (Paris, France). We carefully chose pre-bomb specimens of known-age and ensured that they were collected alive or very soon after death. For example, specimens with articulated valves and exhibiting flesh remains inside the shell were clearly collected alive. For *Senilia senilis*, the presence of the fragile periostracum provides evidence that the specimen was collected fresh. For *Bucardium ringens*, remains of the hinge ligament indicate that the bivalve death occurred not long before collection. The collection date was also carefully checked. Below, we provide background information for the five different bivalve species investigated in this study. Additional information for each sample is given in section 3.1.

*Senilia senilis* (Linnaeus, 1758) can be found from Mauritania to northern Angola. It lives in fine sand in estuaries, creeks or lagoon with regular tidal influence from the lower intertidal zone to about 2 meters water depth. The specie is tolerant to seasonal salinity changes (von Cosel and Gofas, 2019). *S. senilis* is a suspension feeder that lives in the top 5-10-cm layer of the sediment (Okera, 1976; Catry et al., 2017).

*Bucardium ringens* (Bruguière, 1789) occurs from Mauritania to southern Angola. It lives in clean fine sand and mixed sand on open coast from shallow (5-10 meters depth) to about 50 meters depth. Shells and valves are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019). *B. ringens* is likely a suspension feeder as typically are cardiids (Herrera et al., 2015).

*Donax rugosus* (Linnaeus, 1758) occurs from Mauritania to Ghana and from northern Angola to southern Angola. It lives in mixed and coarse sand in the surf zone of open beaches (von Cosel and Gofas, 2019). *D. rugosus* is a suspension feeder (Smith, 1971).

*Ostrea stentina* (Payraudeau, 1826) can be found southern Portugal to Ghana, then from Gabon to northern Angola. It is common and occurs on various types of hard substrate such as rocks, stones, pebbles and other oysters from 1 to 30 meters depths. It can also be found in lagoons, inlets and creeks under marine condition (von Cosel and Gofas, 2019). *O. stentina* is a suspension feeder (Türkmen et al., 2005).

*Pseudochama gryphina* (Lamarck, 1819) occurs from Southern Portugal to Mauritania and from Gabon to southern Angola (von Cosel and Gofas, 2019) and lives on hard substrate such as rocks and stones in clear water offshore about 10 to 60 meters water depth. *P. gryphina* is a suspension feeder (Sessa et al., 2013).
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

A small piece (30-100 mg) of the outermost layers of each shell was cut using a Dremel™ rotary tool fitted with a cut-off wheel. We focused on the external part of the shell to ensure that we sampled and dated the most recent part (likely the last few months) of the specimen. The shell carbonate samples were then sonicated and rinsed in deionized water at least 5 times. Samples were coarsely crushed and split into a subsample for stable isotopic analysis and a subsample for ¹⁴C analysis.

## 2.2 Radiocarbon measurements

Samples were washed with dilute HNO₃ (0.01M) for 15 min then rinsed to neutral pH. Then the shell carbonate was converted into CO₂ following LMC14 laboratory (Laboratoire de Mesure du Radiocarbone, Saclay, France) standard phosphoric acid hydrolysis procedure (Tisnérat-Laborde et al., 2001; Dumoulin et al., 2017). The CO₂ was then converted to graphite (Cottereau et al., 2007; Dumoulin et al., 2017) and analyzed for its ¹⁴C composition by Accelerator Mass Spectrometry (AMS) using the Artémis ¹⁴C AMS facility (Moreau et al., 2013). Results are corrected for the ¹³C/¹²C ratio as measured on the AMS (Santos et al., 2007) and are reported in the F¹⁴C notation (Reimer et al., 2004). F¹⁴C is identical to the A<sub>SN</sub>/A<sub>ON</sub> metric (Stuiver and Polach, 1977), and the ¹⁴a<sub>N</sub> notation (Mook and van der Plicht, 1999). Corresponding conventional ¹⁴C ages reported in ¹⁴C years Before Present (AD 1950) were calculated according to:

$$^{14}\text{C} = -8033\ln(F^{14}\text{C}) \qquad (3)$$

## 2.3 Stable carbon isotopes

Stable carbon and oxygen isotopic analyses of the dated samples were performed at the Pôle Spectrométrie Océan (PSO, Plouzané, France) using a MAT-253 (Thermo Scientific) stable isotope ratio mass spectrometer (IRMS) coupled with a Kiel IV Carbonate Device (Thermo Scientific). The measurements are reported versus Vienna Pee Dee Belemnite standard (VPDB) defined with respect to two international carbonate standards: NBS-19 (δ¹⁸O = -2.20 ‰ and δ¹³C = +1.95 ‰) and NBS-18 (δ¹⁸O = -23.20 ‰ and δ¹³C = -5.01 ‰). The mean external reproducibilities (1σ), based on repeated measurements of an in-house standard, was ±0.04‰ and ±0.02‰ for δ¹⁸O and δ¹³C values, respectively.

## 2.4 Marine Reservoir Age calculation

The marine reservoir age R of the selected shells is calculated according to equation (1) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), ¹⁴C<sub>m</sub> is the measured shell ¹⁴C age, and ¹⁴C<sub>atm</sub> is the ¹⁴C age of the atmosphere. For shells picked from the northern hemisphere, ¹⁴C<sub>atm</sub> is obtained from the IntCal20 calibration curve (Reimer et al., 2020). For shells from the southern hemisphere, we used instead the southern hemisphere calibration curve SHCal20 (Hogg et al., 2020). The uncertainty is calculated (Soulet, 2015) according to:

$$\sigma_{R(t)} = \sqrt{\sigma_{14C_m(t)}^2 + \sigma_{14C_{atm}(t)}^2} \qquad (4)$$

Note that mean SHCal20 offset compared to IntCal20 is estimated to be 36 ± 27 ¹⁴C yrs. Thus, the R values calculated with IntCal20 or SHcal20 are essentially the same if one take uncertainties into account.
<figure><img src="image_10.png" /></figure>

A small piece (30-100 mg) of the outermost layers of each shell was cut using a Dremel™ rotary tool fitted with a cut-off wheel. We focused on the external part of the shell to ensure that we sampled and dated the most recent part (likely the last few months) of the specimen. The shell carbonate samples were then sonicated and rinsed in deionized water at least 5 times. Samples were coarsely crushed and split into a subsample for stable isotopic analysis and a subsample for ¹⁴C analysis.

## 2.2 Radiocarbon measurements

Samples were washed with dilute HNO₃ (0.01M) for 15 min then rinsed to neutral pH. Then the shell carbonate was converted into CO₂ following LMC14 laboratory (Laboratoire de Mesure du Radiocarbone, Saclay, France) standard phosphoric acid hydrolysis procedure (Tisnérat-Laborde et al., 2001; Dumoulin et al., 2017). The CO₂ was then converted to graphite (Cottereau et al., 2007; Dumoulin et al., 2017) and analyzed for its ¹⁴C composition by Accelerator Mass Spectrometry (AMS) using the Artémis ¹⁴C AMS facility (Moreau et al., 2013). Results are corrected for the ¹³C/¹²C ratio as measured on the AMS (Santos et al., 2007) and are reported in the F¹⁴C notation (Reimer et al., 2004). F¹⁴C is identical to the A<sub>SN</sub>/A<sub>ON</sub> metric (Stuiver and Polach, 1977), and the ¹⁴a<sub>N</sub> notation (Mook and van der Plicht, 1999). Corresponding conventional ¹⁴C ages reported in ¹⁴C years Before Present (AD 1950) were calculated according to:

$$^{14}\text{C} = -8033\ln(\text{F}^{14}\text{C}) \qquad (3)$$

## 2.3 Stable carbon isotopes

Stable carbon and oxygen isotopic analyses of the dated samples were performed at the Pôle Spectrométrie Océan (PSO, Plouzané, France) using a MAT-253 (Thermo Scientific) stable isotope ratio mass spectrometer (IRMS) coupled with a Kiel IV Carbonate Device (Thermo Scientific). The measurements are reported versus Vienna Pee Dee Belemnite standard (VPDB) defined with respect to two international carbonate standards: NBS-19 (δ¹⁸O = -2.20 ‰ and δ¹³C = +1.95 ‰) and NBS-18 (δ¹⁸O = -23.20 ‰ and δ¹³C = -5.01 ‰). The mean external reproducibilities (1σ), based on repeated measurements of an in-house standard, was ±0.04‰ and ±0.02‰ for δ¹⁸O and δ¹³C values, respectively.

## 2.4 Marine Reservoir Age calculation

The marine reservoir age R of the selected shells is calculated according to equation (1) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), ¹⁴C<sub>m</sub> is the measured shell ¹⁴C age, and ¹⁴C<sub>atm</sub> is the ¹⁴C age of the atmosphere. For shells picked from the northern hemisphere, ¹⁴C<sub>atm</sub> is obtained from the IntCal20 calibration curve (Reimer et al., 2020). For shells from the southern hemisphere, we used instead the southern hemisphere calibration curve SHCal20 (Hogg et al., 2020). The uncertainty is calculated (Soulet, 2015) according to:

$$\sigma_{R(t)} = \sqrt{\sigma_{14C_m(t)}^2 + \sigma_{14C_{atm}(t)}^2} \qquad (4)$$

Note that mean SHCal20 offset compared to IntCal20 is estimated to be 36 ± 27 ¹⁴C yrs. Thus, the R values calculated with IntCal20 or SHcal20 are essentially the same if one take uncertainties into account.
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

The local marine reservoir offset ΔR of the selected shells is calculated according to equation (2) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), $^{14}$C$_m$ is the measured shell $^{14}$C age, and $^{14}$C$_{Marine20}$ is the $^{14}$C age of the global marine calibration curve. The uncertainty is calculated as follows:

$$ \sigma_{\Delta R(t)} = \sqrt{\sigma_{14C_m(t)}^2 + \sigma_{14C_{Marine20}(t)}^2} \qquad (5) $$

Note that Reimer and Reimer (2017) do not propagate the uncertainty of Marine20 calibration curve.

# 3 Results and Discussion

## 3.1 Radiocarbon measurements results

The results are shown in Table S1 in the Supplement. Below we provide the detailed list of the samples used in this study. We classified the samples by location with corresponding geographic coordinates, then by species. Code numbers “MNHN-IM-2022-xxxx” allows one to find the samples in the collections of the MNHN of Paris (France). Code numbers “SacA-xxxxx” are the radiocarbon laboratory number for the sample.

### 3.1.1 Samples from Morocco

MNHN-IM-2022-4615

*Ostrea stentina*

Mohammedia (33.71°N, 7.37°W)

Articulated specimen with remains of flesh.

SacA-68834: 482 ± 18 BP

$F^{14}C = 0.9418 \pm 0.0021$

$\delta^{13}C = 1.19 \pm 0.00$ VPDB

Collection date: 1921

Collector: Jacques de Lépiney

Museum label: Ostrea stentina Payr, Fedhala 1921, donat J de Lépiney 1939

MNHN-IM-2022-4609

*Ostrea stentina*

El Jadida, beach (33.25°N, 8.49°W)

An isolated valve looking quite fresh.

SacA-68828: 817 ± 18 BP

$F^{14}C = 0.9033 \pm 0.0020$

$\delta^{13}C = 0.71 \pm 0.00$ VPDB
<figure><img src="image_11.png" /></figure>

The local marine reservoir offset ΔR of the selected shells is calculated according to equation (2) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), $^{14}$C$_{m}$ is the measured shell $^{14}$C age, and $^{14}$C$_{Marine20}$ is the $^{14}$C age of the global marine calibration curve. The uncertainty is calculated as follows:

$$ \sigma_{\Delta R(t)} = \sqrt{\sigma_{14C_m(t)}^2 + \sigma_{14C_{Marine20}(t)}^2} \qquad (5) $$

Note that Reimer and Reimer (2017) do not propagate the uncertainty of Marine20 calibration curve.

# 3 Results and Discussion

## 3.1 Radiocarbon measurements results

The results are shown in Table S1 in the Supplement. Below we provide the detailed list of the samples used in this study. We classified the samples by location with corresponding geographic coordinates, then by species. Code numbers “MNHN-IM-2022-xxxx” allows one to find the samples in the collections of the MNHN of Paris (France). Code numbers “SacA-xxxxx” are the radiocarbon laboratory number for the sample.

### 3.1.1 Samples from Morocco

MNHN-IM-2022-4615

*Ostrea stentina*

Mohammedia (33.71°N, 7.37°W)

Articulated specimen with remains of flesh.

SacA-68834: 482 ± 18 BP

$F^{14}C = 0.9418 \pm 0.0021$

$\delta^{13}C = 1.19 \pm 0.00$ VPDB

Collection date: 1921

Collector: Jacques de Lépiney

Museum label: Ostrea stentina Payr, Fedhala 1921, donat J de Lépiney 1939

MNHN-IM-2022-4609

*Ostrea stentina*

El Jadida, beach (33.25°N, 8.49°W)

An isolated valve looking quite fresh.

SacA-68828: 817 ± 18 BP

$F^{14}C = 0.9033 \pm 0.0020$

$\delta^{13}C = 0.71 \pm 0.00$ VPDB
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

Collection date: October 26th 1909

Collector: Louis Gentil

Museum label: Plage de Mazagan 26 octobre 1909, Maroc, Louis Gentil

160 MNHN-IM-2022-4608

*Ostrea stentina*

Lagoon of Sidi Moussa (32.98°N, 8.75°W)

Articulated specimen with remains of flesh.

SacA-68827: 1058 ± 19 BP

165 F¹⁴C = 0.8766 ± 0.0021

δ¹³C = -0.48 ‰ VPDB

Collection date: 1924

Collector: Jacques de Lépiney

Museum label: Ostrea stentina Payr., lagune de Sidi Moussa (région de Mazagan), 1924, donat. J de Lépiney 1939

170 **3.1.2 Samples from Mauritania**

MNHN-IM-2022-4612

*Ostrea stentina*

Nouadhibou, Pointe Chacal (20.91°N, 17.04°W)

Articulated specimen.

175 SacA-68831: 514 ± 17 BP

F¹⁴C = 0.9381 ± 0.0020

δ¹³C = 2.28 ‰ VPDB

Collection date: 1948

Collector: Roger Sourie

180 Museum label: Port Etienne (Pointe des Chacals) M. Sourie, 1948

MNHN-IM-2022-4610

*Ostrea stentina*

Cansado Bay (20.88°N, 17.04°W)

185 Articulated specimen with remains of flesh.

SacA-68829: 516 ± 17 BP

F¹⁴C = 0.9378 ± 0.0020
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Collection date: October 26th 1909

Collector: Louis Gentil

Museum label: Plage de Mazagan 26 octobre 1909, Maroc, Louis Gentil

160 MNHN-IM-2022-4608

*Ostrea stentina*

Lagoon of Sidi Moussa (32.98°N, 8.75°W)

Articulated specimen with remains of flesh.

SacA-68827: 1058 ± 19 BP

165 F¹⁴C = 0.8766 ± 0.0021

δ¹³C = -0.48 ‰ VPDB

Collection date: 1924

Collector: Jacques de Lépiney

Museum label: Ostrea stentina Payr., lagune de Sidi Moussa (région de Mazagan), 1924, donat. J de Lépiney 1939

170 **3.1.2 Samples from Mauritania**

MNHN-IM-2022-4612

*Ostrea stentina*

Nouadhibou, Pointe Chacal (20.91°N, 17.04°W)

Articulated specimen.

175 SacA-68831: 514 ± 17 BP

F¹⁴C = 0.9381 ± 0.0020

δ¹³C = 2.28 ‰ VPDB

Collection date: 1948

Collector: Roger Sourie

180 Museum label: Port Etienne (Pointe des Chacals) M. Sourie, 1948

MNHN-IM-2022-4610

*Ostrea stentina*

Cansado Bay (20.88°N, 17.04°W)

185 Articulated specimen with remains of flesh.

SacA-68829: 516 ± 17 BP

F¹⁴C = 0.9378 ± 0.0020
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

δ¹³C = 2.09 ‰ VPDB

Collection date: 1911-1912

190 Collector: Mission Gruvel

Museum label: Ostrea stentina Payr. = lacerans Hanl., Baie de Cansado, Mission Gruvel, 1911-1912

MNHN-IM-2022-4599

*Bucardium ringens*

195 Nouadhibou (20.88°N, 17.04°W)

An isolated valve of a juvenile with remains of the hinge ligament.

SacA-68811: 863 ± 18 BP

F¹⁴C = 0.8981 ± 0.0020

δ¹³C = 0.38 ‰ VPDB

200 Collection date: 1908

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin; Port Etienne; 1908; Mission Gruvel

MNHN-IM-2022-4603

205 *Donax rugosus*

Ndiago, beach (16.17°N, 16.51°W)

Articulated specimen with remains of flesh and hinge ligament.

SacA-68815: 518 ± 19 BP

F¹⁴C = 0.9376 ± 0.0022

210 δ¹³C = 0.68 ‰ VPDB

Collection date: January 21st 1908

Collector: Mission Gruvel

Museum label: Donax rugosus Linné, N'Diago plage, 21.I.08, Mission Gruvel

### 3.1.3 Samples from Senegal

215 MNHN-IM-2022-4607

*Donax rugosus*

Saint Louis (16.02°N, 16.51°W)

Articulated specimen with remains of flesh and hinge ligament.

SacA-68819: 574 ± 18 BP

220 F¹⁴C = 0.9310 ± 0.0021
<figure><img src="image_15.png" /></figure>

δ¹³C = 2.09 ‰ VPDB

Collection date: 1911-1912

190 Collector: Mission Gruvel

Museum label: Ostrea stentina Payr. = lacerans Hanl., Baie de Cansado, Mission Gruvel, 1911-1912

MNHN-IM-2022-4599

*Bucardium ringens*

195 Nouadhibou (20.88°N, 17.04°W)

An isolated valve of a juvenile with remains of the hinge ligament.

SacA-68811: 863 ± 18 BP

F¹⁴C = 0.8981 ± 0.0020

δ¹³C = 0.38 ‰ VPDB

200 Collection date: 1908

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin; Port Etienne; 1908; Mission Gruvel

MNHN-IM-2022-4603

205 *Donax rugosus*

Ndiago, beach (16.17°N, 16.51°W)

Articulated specimen with remains of flesh and hinge ligament.

SacA-68815: 518 ± 19 BP

F¹⁴C = 0.9376 ± 0.0022

210 δ¹³C = 0.68 ‰ VPDB

Collection date: January 21st 1908

Collector: Mission Gruvel

Museum label: Donax rugosus Linné, N'Diago plage, 21.I.08, Mission Gruvel

### 3.1.3 Samples from Senegal

215 MNHN-IM-2022-4607

*Donax rugosus*

Saint Louis (16.02°N, 16.51°W)

Articulated specimen with remains of flesh and hinge ligament.

SacA-68819: 574 ± 18 BP

220 F¹⁴C = 0.9310 ± 0.0021
<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

δ¹³C = 1.29 ‰ VPDB

Collection date: December 1901

Collector: Buchet's mission

Museum label: Sénégal, Saint Louis, Coquilles; Donax, Món Buchet, Xbre 1901

MNHN-IM-2022-4592

*Senilia senilis*

Dakar, backwaters of the "Marigot de Hann" (14.74°N, 17.39°W)

Articulated specimen with remains of flesh and well-preserved periostracum.

SacA-68824: 560 ± 17 BP

F¹⁴C = 0.9327 ± 0.0020

δ¹³C = -0.29 ‰ VPDB

Collection date: May 1908

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, Marigot de Hann V.1908, se vend sur le marché de Dakar env. 2 sous la douzaine, Mission Gruvel

Note: The Marigot of Hann seems to have been a creek more or less connected to the ocean. It was drawn on a map of Dakar in 1905 but does not exist any longer. The map can be accessed from the Gallica website managed by the Bibliothèque Nationale de France: https://gallica.bnf.fr/ark:/12148/btv1b53197802m

MNHN-IM-2022-4593

*Senilia senilis*

Dakar, Bay of Hann, Pointe Bel Air, beach at low tide (14.71°N, 17.42°W)

Articulated specimen with remains of flesh and well-preserved periostracum.

SacA-68825: 544 ± 18 BP

F¹⁴C = 0.9346 ± 0.0020

δ¹³C = 0.07 ‰ VPDB

Collection date: December 1st 1909

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, Baie de Hann, Pointe de Bel Air, plage à basse mer, M Gruvel, 1.XII.1909

MNHN-IM-2022-4606

*Donax rugosus*

Dakar, Bay of Hann (14.71°N, 17.42°W)
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

δ¹³C = 1.29 ‰ VPDB

Collection date: December 1901

Collector: Buchet's mission

Museum label: Sénégal, Saint Louis, Coquilles; Donax, Món Buchet, Xbre 1901

MNHN-IM-2022-4592

*Senilia senilis*

Dakar, backwaters of the "Marigot de Hann" (14.74°N, 17.39°W)

Articulated specimen with remains of flesh and well-preserved periostracum.

SacA-68824: 560 ± 17 BP

F¹⁴C = 0.9327 ± 0.0020

δ¹³C = -0.29 ‰ VPDB

Collection date: May 1908

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, Marigot de Hann V.1908, se vend sur le marché de Dakar env. 2 sous la douzaine, Mission Gruvel

Note: The Marigot of Hann seems to have been a creek more or less connected to the ocean. It was drawn on a map of Dakar in 1905 but does not exist any longer. The map can be accessed from the Gallica website managed by the Bibliothèque Nationale de France: https://gallica.bnf.fr/ark:/12148/btv1b53197802m

MNHN-IM-2022-4593

*Senilia senilis*

Dakar, Bay of Hann, Pointe Bel Air, beach at low tide (14.71°N, 17.42°W)

Articulated specimen with remains of flesh and well-preserved periostracum.

SacA-68825: 544 ± 18 BP

F¹⁴C = 0.9346 ± 0.0020

δ¹³C = 0.07 ‰ VPDB

Collection date: December 1st 1909

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, Baie de Hann, Pointe de Bel Air, plage à basse mer, M Gruvel, 1.XII.1909

MNHN-IM-2022-4606

*Donax rugosus*

Dakar, Bay of Hann (14.71°N, 17.42°W)
255 Articulated specimen with remains of flesh and hinge ligament.

SacA-68818: 526 ± 18 BP

$$F^{14}C = 0.9366 \pm 0.0022$$

$$\delta^{13}C = 0.33 \, \text{‰} \text{ VPDB}$$

Collection date: April 1908

260 Collector: Mission Gruvel

Museum label: Donax rugosus Linné, Baie de Hann à basse mer, IV 08, Mission Gruvel

MNHN-IM-2022-4598

*Bucardium ringens*

265 Dakar, Bay of Hann at low tide (14.71°N, 17.42°W)

An isolated valve with remains of the hinge ligament.

SacA-68810: 628 ± 17 BP

$$F^{14}C = 0.9247 \pm 0.0019$$

$$\delta^{13}C = 0.68 \, \text{‰} \text{ VPDB}$$

270 Collection date: April 1908

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin, Baie de Hann à basse mer, IV.08, Mission Gruvel

MNHN-IM-2022-4616

275 *Ostrea stentina*

Dakar, beach of Hann, posts of the pontoon (14.71°N, 17.42°W)

An isolated valve with remains of flesh

SacA-68835: 539 ± 19 BP

$$F^{14}C = 0.9351 \pm 0.0022$$

280 $\delta^{13}C = 1.56 \, \text{‰} \text{ VPDB}$

Collection date: 1947

Collector: Roger Sourie

Museum label: Ostrea stentina Payr Dakar (plage de Hann, piles du ponton) M Sourie 1947

### 3.1.4 Samples from Republic of Guinea

285 MNHN-IM-2022-4601

*Bucardium ringens*

Los Islands, Roume Island at low tide (9.46°N, 13.79°W)
<figure><img src="image_19.png" /></figure>

255 Articulated specimen with remains of flesh and hinge ligament.

SacA-68818: 526 ± 18 BP

$$F^{14}C = 0.9366 \pm 0.0022$$

$$\delta^{13}C = 0.33 \, \text{‰} \text{ VPDB}$$

Collection date: April 1908

260 Collector: Mission Gruvel

Museum label: Donax rugosus Linné, Baie de Hann à basse mer, IV 08, Mission Gruvel

MNHN-IM-2022-4598

*Bucardium ringens*

265 Dakar, Bay of Hann at low tide (14.71°N, 17.42°W)

An isolated valve with remains of the hinge ligament.

SacA-68810: 628 ± 17 BP

$$F^{14}C = 0.9247 \pm 0.0019$$

$$\delta^{13}C = 0.68 \, \text{‰} \text{ VPDB}$$

270 Collection date: April 1908

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin, Baie de Hann à basse mer, IV.08, Mission Gruvel

MNHN-IM-2022-4616

275 *Ostrea stentina*

Dakar, beach of Hann, posts of the pontoon (14.71°N, 17.42°W)

An isolated valve with remains of flesh

SacA-68835: 539 ± 19 BP

$$F^{14}C = 0.9351 \pm 0.0022$$

280 $\delta^{13}C = 1.56 \, \text{‰} \text{ VPDB}$

Collection date: 1947

Collector: Roger Sourie

Museum label: Ostrea stentina Payr Dakar (plage de Hann, piles du ponton) M Sourie 1947

### 3.1.4 Samples from Republic of Guinea

285 MNHN-IM-2022-4601

*Bucardium ringens*

Los Islands, Roume Island at low tide (9.46°N, 13.79°W)
<figure><img src="image_22.png" /></figure>

A fresh-looking isolated valve.

SacA-68813: 857 ± 39 BP

290 F¹⁴C = 0.8988 ± 0.0043

δ¹³C = -0.40 ‰ VPDB

Collection date: December 20th 1909

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin; Ile Roumé, archipel de Los, à basse mer, 20.XII.09, Mission Gruvel

295

MNHN-IM-2022-4618

*Pseudochama gryphina*

Los Islands, Tamara Island (9.46°N, 13.83°W)

An articulated specimen.

300 SacA-68820: 502 ± 19 BP

F¹⁴C = 0.9395 ± 0.0022

δ¹³C = 1.52 ‰ VPDB

Collection date: 1909-1910

Collector: Mission Gruvel

305 Museum label: Chama gryphina Lm, Tamara, Guinée, mission Gruvel, 1909-1910

Note: It is possible that this sample was also collected in December 1909 as sample MNHN-IM-2022-4601

### 3.1.5 Sample from Sierra Leone

MNHN-IM-2022-4611

*Ostrea stentina*

310 Near Cape Saint Ann (7.56°N, 12.94°W)

Articulated specimen with remains of flesh.

SacA-68830: 464 ± 18 BP

F¹⁴C = 0.9439 ± 0.0021

δ¹³C = 1.04 ‰ VPDB

315 Collection date: 1912

Collector: Mission Gruvel

Museum label: Sierra Léone près Cap Ste Anne, m. Gruvel, 1912

### 3.1.6 Samples from Benin

MNHN-IM-2022-4591
<figure><img src="image_20.png" /></figure>

A fresh-looking isolated valve.

SacA-68813: 857 ± 39 BP

290 F¹⁴C = 0.8988 ± 0.0043

δ¹³C = -0.40 ‰ VPDB

Collection date: December 20th 1909

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin; Ile Roumé, archipel de Los, à basse mer, 20.XII.09, Mission Gruvel

295

MNHN-IM-2022-4618

*Pseudochama gryphina*

Los Islands, Tamara Island (9.46°N, 13.83°W)

An articulated specimen.

300 SacA-68820: 502 ± 19 BP

F¹⁴C = 0.9395 ± 0.0022

δ¹³C = 1.52 ‰ VPDB

Collection date: 1909-1910

Collector: Mission Gruvel

305 Museum label: Chama gryphina Lm, Tamara, Guinée, mission Gruvel, 1909-1910

Note: It is possible that this sample was also collected in December 1909 as sample MNHN-IM-2022-4601

### 3.1.5 Sample from Sierra Leone

MNHN-IM-2022-4611

*Ostrea stentina*

310 Near Cape Saint Ann (7.56°N, 12.94°W)

Articulated specimen with remains of flesh.

SacA-68830: 464 ± 18 BP

F¹⁴C = 0.9439 ± 0.0021

δ¹³C = 1.04 ‰ VPDB

315 Collection date: 1912

Collector: Mission Gruvel

Museum label: Sierra Léone près Cap Ste Anne, m. Gruvel, 1912

### 3.1.6 Samples from Benin

MNHN-IM-2022-4591
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

320 **Senilia senilis**
Ahémé Lake (6.42°N, 1.96°E)
Articulated specimen with well-preserved periostracum

SacA-68823: 414 ± 18 BP

$$F^{14}C = 0.9498 \pm 0.0021$$

325 $\delta^{13}C = -4.76 \, \text{‰}$ VPDB

Collection date: February 1910

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, Lac Ahémé Dahomey, Mission Gruvel, II.1910

330 MNHN-IM-2022-4600
*Bucardium ringens*
Cotonou, dredging at a water depth of 20-25 meters (6.33°N, 2.39°E)

A fresh isolated valve of a juvenile.

SacA-68812: 606 ± 18 BP

335 $F^{14}C = 0.9273 \pm 0.0020$

$$\delta^{13}C = 0.26 \, \text{‰}$ VPDB

Collection date: February 1910

Collector: Mission Gruvel

Museum label: Cardium ringens, Cotonou, mer, II.1910, sac 372, Mission Gruvel

340 Note: The information that the sample came from a dredging at 20-25 meters water depth in front of Cotonou can be found in Dautzenberg (1912).

MNHN-IM-2022-4602

*Bucardium ringens*

345 La Bouche du Roi, Grand Popo, beach (6.29°N, 1.92°E)

An isolated valve with remains of the hinge ligament.

SacA-68814: 527 ± 17 BP

$$F^{14}C = 0.9365 \pm 0.0020$$

$$\delta^{13}C = 0.25 \, \text{‰}$ VPDB

350 Collection date: February 1910

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin, Bouche du Roi, Gd Popo, plage, II.1910, Mission Gruvel.
<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

320 **Senilia senilis**
Ahémé Lake (6.42°N, 1.96°E)
Articulated specimen with well-preserved periostracum
SacA-68823: 414 ± 18 BP
$F^{14}C = 0.9498 \pm 0.0021$
325 $\delta^{13}C = -4.76 \, \text{‰} \text{ VPDB}$
Collection date: February 1910
Collector: Mission Gruvel
Museum label: Arca (Senilia) senilis Linné, Lac Ahémé Dahomey, Mission Gruvel, II.1910

330 MNHN-IM-2022-4600
*Bucardium ringens*
Cotonou, dredging at a water depth of 20-25 meters (6.33°N, 2.39°E)
A fresh isolated valve of a juvenile.
SacA-68812: 606 ± 18 BP
335 $F^{14}C = 0.9273 \pm 0.0020$
$\delta^{13}C = 0.26 \, \text{‰} \text{ VPDB}$
Collection date: February 1910
Collector: Mission Gruvel
Museum label: Cardium ringens, Cotonou, mer, II.1910, sac 372, Mission Gruvel

340 Note: The information that the sample came from a dredging at 20-25 meters water depth in front of Cotonou can be found in Dautzenberg (1912).

MNHN-IM-2022-4602

*Bucardium ringens*

345 La Bouche du Roi, Grand Popo, beach (6.29°N, 1.92°E)
An isolated valve with remains of the hinge ligament.
SacA-68814: 527 ± 17 BP
$F^{14}C = 0.9365 \pm 0.0020$
$\delta^{13}C = 0.25 \, \text{‰} \text{ VPDB}$
Collection date: February 1910
Collector: Mission Gruvel
Museum label: Cardium ringens Gmelin, Bouche du Roi, Gd Popo, plage, II.1910, Mission Gruvel.
<figure><img src="image_26.png" /></figure>

Note: Three labels mention the same location, but one label mentions the Catumbella estuary (Angola, June 17th 1910). We believe the sample is from Grand Popo.

## 3.1.7 Samples from Ivory Coast

MNHN-IM-2022-4595

*Bucardium ringens*

Grand Bassam, beach (5.19°N, 3.73°W)

An isolated valve with remains of the hinge ligament.

360 SacA-68807: 507 ± 18 BP

$$F^{14}C = 0.9388 \pm 0.0021$$

$$\delta^{13}C = 0.29 \, \text{‰} \text{ VPDB}$$

Collection date: 1909-1910

Collector: Mission Gruvel

365 Museum label: Cardium ringens Gmelin, plage de Gd Bassam 1909-10, mission Gruvel.

MNHN-IM-2022-4597

*Bucardium ringens*

Jacqueville, beach (5.19°N, 4.42°W)

370 An isolated valve of a juvenile with remains of the hinge ligament.

SacA-68809: 995 ± 18 BP

$$F^{14}C = 0.8835 \pm 0.0020$$

$$\delta^{13}C = 0.03 \, \text{‰} \text{ VPDB}$$

Collection date: January 10th 1910

375 Collector: Mission Gruvel

Museum label: Cardium ringens Gmel., Jacqueville Côte d'Ivoire, plage, 19.I.10, Mission Gruvel.

## 3.1.8 Sample from Gabon

MNHN-IM-2022-4613

*Ostrea stentina*

380 Port-Gentil (0.71°S, 8.79°E)

An isolated valve with remains of flesh.

SacA-68832: 497 ± 19 BP

$$F^{14}C = 0.9401 \pm 0.0022$$

$$\delta^{13}C = 2.24 \, \text{‰} \text{ VPDB}$$
<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

Note: Three labels mention the same location, but one label mentions the Catumbella estuary (Angola, June 17th 1910). We believe the sample is from Grand Popo.

### 3.1.7 Samples from Ivory Coast

MNHN-IM-2022-4595

*Bucardium ringens*

Grand Bassam, beach (5.19°N, 3.73°W)

An isolated valve with remains of the hinge ligament.

360 SacA-68807: 507 ± 18 BP

$$F^{14}C = 0.9388 \pm 0.0021$$

$$\delta^{13}C = 0.29 \, \text{‰} \text{ VPDB}$$

Collection date: 1909-1910

Collector: Mission Gruvel

365 Museum label: Cardium ringens Gmelin, plage de Gd Bassam 1909-10, mission Gruvel.

MNHN-IM-2022-4597

*Bucardium ringens*

Jacqueville, beach (5.19°N, 4.42°W)

370 An isolated valve of a juvenile with remains of the hinge ligament.

SacA-68809: 995 ± 18 BP

$$F^{14}C = 0.8835 \pm 0.0020$$

$$\delta^{13}C = 0.03 \, \text{‰} \text{ VPDB}$$

Collection date: January 10th 1910

375 Collector: Mission Gruvel

Museum label: Cardium ringens Gmel., Jacqueville Côte d'Ivoire, plage, 19.I.10, Mission Gruvel.

### 3.1.8 Sample from Gabon

MNHN-IM-2022-4613

*Ostrea stentina*

380 Port-Gentil (0.71°S, 8.79°E)

An isolated valve with remains of flesh.

SacA-68832: 497 ± 19 BP

$$F^{14}C = 0.9401 \pm 0.0022$$

$$\delta^{13}C = 2.24 \, \text{‰} \text{ VPDB}$$
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

385 Collection date: 1948

Collector: Charles Roux' mission

Museum label: Port Gentil, M Roux, 1949.

Note: Charles Roux writes in 1949 (Roux, 1949) that he was in the Port-Gentil area during the year 1948. We can understand that he was already back to France in 1949. Hence, the collection date must be 1948.

390 **3.1.9 Samples from Republic of Congo**

MNHN-IM-2022-4614

*Ostrea stentina*

Loango (4.66°S, 11.80°E)

An isolated valve with remains of flesh.

395 SacA-68833: 571 ± 19 BP

$$F^{14}C = 0.9314 \pm 0.0022$$

$$\delta^{13}C = 1.38 \, \text{‰} \text{ VPDB}$$

Collection date: 1890

Collector: Augusto Nobre

400 Museum label: Ostrea stentina Payr Loango M. Nobre 1890

MNHN-IM-2022-4604

*Donax rugosus*

Pointe-Noire (4.76°S, 11.84°E)

405 A fresh isolated valve from a juvenile specimen.

SacA-68816: 447 ± 18 BP

$$F^{14}C = 0.9459 \pm 0.0021$$

$$\delta^{13}C = 0.84 \, \text{‰} \text{ VPDB}$$

Collection date: December 1936 – April 1937

410 Collector: Edgard Aubert de la Rüe

Museum label: Pte Noire, Aubert de la Rüe, 1937

Note: Edgard Aubert de la Rüe was in Congo from 18/12/1936 to 16/04/1937 as evidences by his field books kept in the archives of the Musée du Quai Branly (files 2AP/62 to 2AP/64)

3.1.10 Samples from Angola

415 MNHN-IM-2022-4590

*Senilia senilis*
<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

385 Collection date: 1948

Collector: Charles Roux' mission

Museum label: Port Gentil, M Roux, 1949.

Note: Charles Roux writes in 1949 (Roux, 1949) that he was in the Port-Gentil area during the year 1948. We can understand that he was already back to France in 1949. Hence, the collection date must be 1948.

390 **3.1.9 Samples from Republic of Congo**

MNHN-IM-2022-4614

*Ostrea stentina*

Loango (4.66°S, 11.80°E)

An isolated valve with remains of flesh.

395 SacA-68833: 571 ± 19 BP

$$F^{14}C = 0.9314 \pm 0.0022$$

$$\delta^{13}C = 1.38 \, \text{‰} \text{ VPDB}$$

Collection date: 1890

Collector: Augusto Nobre

400 Museum label: Ostrea stentina Payr Loango M. Nobre 1890

MNHN-IM-2022-4604

*Donax rugosus*

Pointe-Noire (4.76°S, 11.84°E)

405 A fresh isolated valve from a juvenile specimen.

SacA-68816: 447 ± 18 BP

$$F^{14}C = 0.9459 \pm 0.0021$$

$$\delta^{13}C = 0.84 \, \text{‰} \text{ VPDB}$$

Collection date: December 1936 – April 1937

410 Collector: Edgard Aubert de la Rüe

Museum label: Pte Noire, Aubert de la Rüe, 1937

Note: Edgard Aubert de la Rüe was in Congo from 18/12/1936 to 16/04/1937 as evidences by his field books kept in the archives of the Musée du Quai Branly (files 2AP/62 to 2AP/64)

3.1.10 Samples from Angola

415 MNHN-IM-2022-4590

*Senilia senilis*
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

Cabinda (5.55°S, 12.20°E)

Articulated specimen with well-preserved periostracum.

SacA-68822: 584 ± 17 BP

F¹⁴C = 0.9298 ± 0.0020

δ¹³C = -1.15 ‰ VPDB

Collection date: 1885-1887

Collector: Paul Hesse

Museum label: Cabinda, Cabinda, Angola; C.R. Boettger coll. 1909

Note: The shell was donated by Caesar R. Boettger to the MNHN in 1909 (Oliver and von Cosel, 1992) but collected earlier
by Paul Hesse when Hesse was leaving in Banana (Democratic Republic of Congo) south of Cabinda (Boettger, 1912).

Boettger (1912, p. 110) writes that Hesse's collection includes a number of *Senilia senilis* specimens from Cabinda. The
collection date is unfortunately not provided. However, Hesse was employed by a trading company in Banana by the
end/beginning 1884/1885 since at least after March 1886 (Westhoff, 1886). Mollusc specimens reported in Boettger (1912)
were collected by Hesse between 1885 and 1886. Also, Hesse collected reptile specimens in Cabinda in 1885 and 1887
(Boettger, 1898). Thus, we believe that the MNHN specimen must have been collected between 1885 and 1887.

430

MNHN-IM-2022-4594

*Senilia senilis*

Cabinda (5.55°S, 12.20°E)

An isolated valve with well-preserved periostracum.

SacA-68826: 536 ± 19 BP

F¹⁴C = 0.9355 ± 0.0022

δ¹³C = -3.11 ‰ VPDB

Collection date: June 6th 1921

Collector: Unknown

Museum label [Staadt collection]: Arca senilis Lin, Cabenda, Africa, Guinea, bought just over 1d on June 6th 1921 in Grays
Jun rd, (some of the specimens at B. M. are more than double the size of mine).

445 MNHN-IM-2022-4589

*Senilia senilis*

Luanda, beach (8.78°S, 13.27°E)

Articulated specimen with well-preserved periostracum.

SacA-68821: 538 ± 19 BP

F¹⁴C = 0.9353 ± 0.0022
<figure><img src="image_29.png" /></figure>

Cabinda (5.55°S, 12.20°E)

Articulated specimen with well-preserved periostracum.

SacA-68822: 584 ± 17 BP

F¹⁴C = 0.9298 ± 0.0020

δ¹³C = -1.15 ‰ VPDB

Collection date: 1885-1887

Collector: Paul Hesse

Museum label: Cabinda, Cabinda, Angola; C.R. Boettger coll. 1909

Note: The shell was donated by Caesar R. Boettger to the MNHN in 1909 (Oliver and von Cosel, 1992) but collected earlier
by Paul Hesse when Hesse was leaving in Banana (Democratic Republic of Congo) south of Cabinda (Boettger, 1912).

Boettger (1912, p. 110) writes that Hesse's collection includes a number of *Senilia senilis* specimens from Cabinda. The
collection date is unfortunately not provided. However, Hesse was employed by a trading company in Banana by the
end/beginning 1884/1885 since at least after March 1886 (Westhoff, 1886). Mollusc specimens reported in Boettger (1912)
were collected by Hesse between 1885 and 1886. Also, Hesse collected reptile specimens in Cabinda in 1885 and 1887
(Boettger, 1898). Thus, we believe that the MNHN specimen must have been collected between 1885 and 1887.

430

MNHN-IM-2022-4594

*Senilia senilis*

Cabinda (5.55°S, 12.20°E)

An isolated valve with well-preserved periostracum.

SacA-68826: 536 ± 19 BP

F¹⁴C = 0.9355 ± 0.0022

δ¹³C = -3.11 ‰ VPDB

Collection date: June 6th 1921

Collector: Unknown

Museum label [Staadt collection]: Arca senilis Lin, Cabenda, Africa, Guinea, bought just over 1d on June 6th 1921 in Grays
Jun rd, (some of the specimens at B. M. are more than double the size of mine).

445 MNHN-IM-2022-4589

*Senilia senilis*

Luanda, beach (8.78°S, 13.27°E)

Articulated specimen with well-preserved periostracum.

SacA-68821: 538 ± 19 BP

F¹⁴C = 0.9353 ± 0.0022
<figure><img src="image_32.png" /></figure>

δ¹³C = -0.16 ‰ VPDB

Collection date: May 18th 1910

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, St Paul de Loanda, plage, Mission Gruvel, 18.V.1910.

455

MNHN-IM-2022-4605

*Donax rugosus*

Luanda, beach (8.82°S, 13.21°E)

Articulated specimen with remains of flesh and hinge ligament.

460

SacA-68817: 528 ± 18 BP

F¹⁴C = 0.9364 ± 0.0021

δ¹³C = 0.82 ‰ VPDB

Collection date: May 18th 1910

Collector: Mission Gruvel

465

Museum label: Donax rugosus Linné, St Paul de Loanda plage, 18.V.10, Mission Gruvel.

MNHN-IM-2022-4596

*Bucardium ringens*

Bay of Lobito, near the peninsula (12.33°S, 13.56°E)

470

An isolated valve.

SacA-68808: 595 ± 17 BP

F¹⁴C = 0.9286 ± 0.0020

δ¹³C = 1.51 ‰ VPDB

Collection date: June 1910

475

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin, Baie de Lobito côté presqu'île, VI.1910, mission Gruvel.

MNHN-IM-2022-4617

*Ostrea stentina*

480

Moçâmedes (15.18°S, 12.14°E)

A fresh isolated valve.

SacA-68836: 568 ± 18 BP

F¹⁴C = 0.9317 ± 0.0021

δ¹³C = 1.75 ‰ VPDB
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

δ¹³C = -0.16 ‰ VPDB

Collection date: May 18th 1910

Collector: Mission Gruvel

Museum label: Arca (Senilia) senilis Linné, St Paul de Loanda, plage, Mission Gruvel, 18.V.1910.

455

MNHN-IM-2022-4605

*Donax rugosus*

Luanda, beach (8.82°S, 13.21°E)

Articulated specimen with remains of flesh and hinge ligament.

460

SacA-68817: 528 ± 18 BP

F¹⁴C = 0.9364 ± 0.0021

δ¹³C = 0.82 ‰ VPDB

Collection date: May 18th 1910

Collector: Mission Gruvel

465

Museum label: Donax rugosus Linné, St Paul de Loanda plage, 18.V.10, Mission Gruvel.

MNHN-IM-2022-4596

*Bucardium ringens*

Bay of Lobito, near the peninsula (12.33°S, 13.56°E)

470

An isolated valve.

SacA-68808: 595 ± 17 BP

F¹⁴C = 0.9286 ± 0.0020

δ¹³C = 1.51 ‰ VPDB

Collection date: June 1910

475

Collector: Mission Gruvel

Museum label: Cardium ringens Gmelin, Baie de Lobito côté presqu'île, VI.1910, mission Gruvel.

MNHN-IM-2022-4617

*Ostrea stentina*

480

Moçâmedes (15.18°S, 12.14°E)

A fresh isolated valve.

SacA-68836: 568 ± 18 BP

F¹⁴C = 0.9317 ± 0.0021

δ¹³C = 1.75 ‰ VPDB
<figure><img src="image_33.png" /></figure>

Collection date: 1910
Collector: Mission Gruvel
Museum label: Mossamédès, m Gruvel, 1910

## 3.2 West African marine reservoir ages

The vast majority of the calculated ΔR values, with an average of -77 ± 47 ¹⁴C yrs (1sd, n = 25), which corresponds to an averaged R value of 400 ± 59 ¹⁴C yrs (1sd, n = 25), are typical of low latitudes marine reservoir age values (Bard, 1988; Bard et al., 1994) (Table S1 in the Supplement, Fig. 1). Our results agree perfectly with those already obtained (Ndeye, 2008) from the Nouadhibou-Cansado Bay area (Mauritania; Nh in Fig. 1) and the Dakar area (Senegal; Dk in Fig. 1); the only two areas that we can compare our results to.

No significant interspecific differences were observed. This is best illustrated for the localities where reservoir age values were obtained from at least two different species for the same calendar time. In the Dakar area (Senegal; Dk in Fig. 1) for years 1908-1909, we present data for 5 species (*Bucardium ringens*, *Donax rugosus*, *Mactra glabrata*, *Ostrea stentina*, *Senilia senilis*) (Ndeye, 2008; this study) all clustering within a range of [413;546] ([min; max]) with an average ΔR value of -16 ± 56 ¹⁴C yrs (1sd, n = 6) (corresponding to an average R value of 478 ± 55 ¹⁴C yrs). This was also the case for Luanda (Angola; Lu in Fig. 1) in the year 1910, with two species (*Donax rugosus* and *Senilia senilis*) yielding the same reservoir age values. This was further supported for the area of Nouadhibou-Cansado Bay (Mauritania) showing the same pattern (Ndeye, 2008; this study), although one sample over four was likely an outlier (*Bucardium ringens* with # MNHN-IM-2022-4599). The fact that species living in very different ecological habitats (e.g., *Senilia senilis* in lagoons/semi-enclosed bays and *Donax rugosus* on beaches exposed to heavy surf; see also section material) show similar reservoir age values (R or ΔR) suggests that the habitat only exerts a minor influence on measured reservoir age. The fact that all investigated species in this study correspond to suspension feeders further implies that suspension feeders are suitable material for reservoir age reconstruction.

Unlike semi-isolated basins such as the Black Sea (Soulet et al., 2019), where the radiocarbon system is closely linked to the local carbon stable isotopic system, the open-ocean coastal region of West Africa is characterized by the lack of any relationship between reservoir age values (R or ΔR) and stable oxygen and carbon isotope compositions (r² of 0.02 and 0.001, respectively), as inferred from our results.

## 3.3 Marine reservoir evolution over time

The local marine reservoir age were averaged over 5-yrs windows ([1886-1890]-[1891-1895] and so on), excluding the five outliers discussed in section 3.5. Sample with radiocarbon lab # AA-70015 (see Table S1 in the Supplement) is a single value from 1916 and was averaged with samples from years 1912. We also calculated global marine reservoir age as the difference between the Marine20 and IntCal20 calibration curves. The evolution of the marine reservoir age of coastal West Africa (pink symbols in Fig. 2) shows a similar trend as that of the global marine reservoir age (black line in Fig. 2) with values declining steadily with time since ca. 1900 AD. The ¹⁴C age evolution of the global ocean (Marine20 calibration curve; Heaton et al.,
<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

Collection date: 1910

Collector: Mission Gruvel

Museum label: Mossamédès, m Gruvel, 1910

## 3.2 West African marine reservoir ages

The vast majority of the calculated ΔR values, with an average of -77 ± 47 ¹⁴C yrs (1sd, n = 25), which corresponds to an averaged R value of 400 ± 59 ¹⁴C yrs (1sd, n = 25), are typical of low latitudes marine reservoir age values (Bard, 1988; Bard et al., 1994) (Table S1 in the Supplement, Fig. 1). Our results agree perfectly with those already obtained (Ndeye, 2008) from the Nouadhibou-Cansado Bay area (Mauritania; Nh in Fig. 1) and the Dakar area (Senegal; Dk in Fig. 1); the only two areas that we can compare our results to.

No significant interspecific differences were observed. This is best illustrated for the localities where reservoir age values were obtained from at least two different species for the same calendar time. In the Dakar area (Senegal; Dk in Fig. 1) for years 1908-1909, we present data for 5 species (*Bucardium ringens*, *Donax rugosus*, *Mactra glabrata*, *Ostrea stentina*, *Senilia senilis*) (Ndeye, 2008; this study) all clustering within a range of [413;546] ([min; max]) with an average ΔR value of -16 ± 56 ¹⁴C yrs (1sd, n = 6) (corresponding to an average R value of 478 ± 55 ¹⁴C yrs). This was also the case for Luanda (Angola; Lu in Fig. 1) in the year 1910, with two species (*Donax rugosus* and *Senilia senilis*) yielding the same reservoir age values. This was further supported for the area of Nouadhibou-Cansado Bay (Mauritania) showing the same pattern (Ndeye, 2008; this study), although one sample over four was likely an outlier (*Bucardium ringens* with # MNHN-IM-2022-4599). The fact that species living in very different ecological habitats (e.g., *Senilia senilis* in lagoons/semi-enclosed bays and *Donax rugosus* on beaches exposed to heavy surf; see also section material) show similar reservoir age values (R or ΔR) suggests that the habitat only exerts a minor influence on measured reservoir age. The fact that all investigated species in this study correspond to suspension feeders further implies that suspension feeders are suitable material for reservoir age reconstruction.

Unlike semi-isolated basins such as the Black Sea (Soulet et al., 2019), where the radiocarbon system is closely linked to the local carbon stable isotopic system, the open-ocean coastal region of West Africa is characterized by the lack of any relationship between reservoir age values (R or ΔR) and stable oxygen and carbon isotope compositions (r² of 0.02 and 0.001, respectively), as inferred from our results.

## 3.3 Marine reservoir evolution over time

The local marine reservoir age were averaged over 5-yrs windows ([1886-1890]-[1891-1895] and so on), excluding the five outliers discussed in section 3.5. Sample with radiocarbon lab # AA-70015 (see Table S1 in the Supplement) is a single value from 1916 and was averaged with samples from years 1912. We also calculated global marine reservoir age as the difference between the Marine20 and IntCal20 calibration curves. The evolution of the marine reservoir age of coastal West Africa (pink symbols in Fig. 2) shows a similar trend as that of the global marine reservoir age (black line in Fig. 2) with values declining steadily with time since ca. 1900 AD. The ¹⁴C age evolution of the global ocean (Marine20 calibration curve; Heaton et al.,
<figure><img src="image_34.png" /></figure>

2020) is constructed using the global carbon cycle model BICYCLE (Köhler et al., 2006; Köhler and Fischer, 2006, 2004; Köhler et al., 2005). This box model incorporates a globally averaged atmospheric box and modules of the terrestrial (7 boxes) and oceanic (10 boxes) components of the carbon cycle. It is driven by temporal changes in the boundary conditions mimicking changing climate and simulates changes in the carbon cycle including ¹⁴C. To construct the Marine20 calibration curve, the BICYCLE model was revised to allow the atmospheric CO₂ and F¹⁴C to be specified externally (Heaton et al., 2020). The global marine reservoir age evolution calculated as the difference between Marine20 and Intcal20 calibration curves (Fig. 1) is thus also an output of the revised BICYCLE model. The fact that the trend in our measured marine reservoir age is similar to that of the globally modelled ones strongly suggest that global carbon cycle and climate are the first order drivers of the coastal West African marine reservoir age rather than local effects.

### 3.4 Marine reservoir age off equatorial Ogooué and Congo rivers

Large rivers draining equatorial Africa as the Ogooué and Congo rivers inject massive amounts of freshwater into the Atlantic Ocean (Lambert et al., 2015; Milliman and Farnsworth, 2011) leading to extensive sea surface salinity negative anomalies (Martins and Stammer, 2022). The sea surface salinity negative anomalies are associated with net primary productivity positive anomalies that are likely caused by the nutrient-rich river plumes from the Ogooué and Congo Rivers (Martins and Stammer, 2022). From a radiocarbon perspective, such net primary productivity positive anomalies should imply an increased uptake of atmospheric CO₂ through intensified biological pump. As a result, the reservoir age should be lower than average. The Congo River represents the second largest supplier of dissolved organic carbon (DOC) to global ocean with ~5% of the land to ocean DOC flux (Spencer et al., 2016; Coynel et al., 2005; Richey et al., 2022). The DOC exported by the Congo river is ¹⁴C-modern (Marwick et al., 2015; Spencer et al., 2012) and experiments showed that 45 % of the Congo River DOC can be photo-mineralized by sunlight (Spencer et al., 2009; Richey et al., 2022). Dissolved inorganic carbon (DIC) released from the photo-mineralisation of the Congo River DOC should also be ¹⁴C-modern. Thus, this modern DOC-derived DIC should impact the marine reservoir age towards values lower compared to average. There is a lack of available data to estimate the age and flux of dissolved CO₂ discharged by the Congo river into the ocean (Richey et al., 2022). Nevertheless, the marine reservoir age value measured at Port-Gentil (Gabon) close to the Ogooué river outlet is lower than average value (ΔR = -106 ± 63 ¹⁴C years, corresponding to R = 329 ± 21 ¹⁴C yrs) (PG in Fig. 1). The marine reservoir age measured in Pointe-Noire (Republic of Congo) ~150 km north of the Congo river outlet is also lower than average (ΔR = -156 ± 64 ¹⁴C yrs; R = 289 ± 20 ¹⁴C yrs) (PN in Fig. 1). These values could be interpreted as having been influenced by the Ogooué and Congo Rivers discharges. However, all other localities close to the Congo River outlet had marine reservoir age close to the average (Lo, Ca and Lu, in Fig. 1). Instead the lower values observed in Port-Gentil (Gabon) and Pointe-Noire (Republic of Congo) are from years 1948 and 1937 suggesting that these lower values are in line with the declining global marine reservoir evolution linked to global climate and carbon cycle changes (see section 3.3). The impact of the African equatorial rivers on the local/regional coastal marine reservoir age, if any, cannot be inferred from our results.
<figure><img src="image_35.png" /></figure>

2020) is constructed using the global carbon cycle model BICYCLE (Köhler et al., 2006; Köhler and Fischer, 2006, 2004; Köhler et al., 2005). This box model incorporates a globally averaged atmospheric box and modules of the terrestrial (7 boxes) and oceanic (10 boxes) components of the carbon cycle. It is driven by temporal changes in the boundary conditions mimicking changing climate and simulates changes in the carbon cycle including ¹⁴C. To construct the Marine20 calibration curve, the BICYCLE model was revised to allow the atmospheric CO₂ and F¹⁴C to be specified externally (Heaton et al., 2020). The global marine reservoir age evolution calculated as the difference between Marine20 and Intcal20 calibration curves (Fig. 1) is thus also an output of the revised BICYCLE model. The fact that the trend in our measured marine reservoir age is similar to that of the globally modelled ones strongly suggest that global carbon cycle and climate are the first order drivers of the coastal West African marine reservoir age rather than local effects.

### 3.4 Marine reservoir age off equatorial Ogooué and Congo rivers

Large rivers draining equatorial Africa as the Ogooué and Congo rivers inject massive amounts of freshwater into the Atlantic Ocean (Lambert et al., 2015; Milliman and Farnsworth, 2011) leading to extensive sea surface salinity negative anomalies (Martins and Stammer, 2022). The sea surface salinity negative anomalies are associated with net primary productivity positive anomalies that are likely caused by the nutrient-rich river plumes from the Ogooué and Congo Rivers (Martins and Stammer, 2022). From a radiocarbon perspective, such net primary productivity positive anomalies should imply an increased uptake of atmospheric CO₂ through intensified biological pump. As a result, the reservoir age should be lower than average. The Congo River represents the second largest supplier of dissolved organic carbon (DOC) to global ocean with ~5% of the land to ocean DOC flux (Spencer et al., 2016; Coynel et al., 2005; Richey et al., 2022). The DOC exported by the Congo river is ¹⁴C-modern (Marwick et al., 2015; Spencer et al., 2012) and experiments showed that 45 % of the Congo River DOC can be photo-mineralized by sunlight (Spencer et al., 2009; Richey et al., 2022). Dissolved inorganic carbon (DIC) released from the photo-mineralisation of the Congo River DOC should also be ¹⁴C-modern. Thus, this modern DOC-derived DIC should impact the marine reservoir age towards values lower compared to average. There is a lack of available data to estimate the age and flux of dissolved CO₂ discharged by the Congo river into the ocean (Richey et al., 2022). Nevertheless, the marine reservoir age value measured at Port-Gentil (Gabon) close to the Ogooué river outlet is lower than average value (ΔR = -106 ± 63 ¹⁴C years, corresponding to R = 329 ± 21 ¹⁴C yrs) (PG in Fig. 1). The marine reservoir age measured in Pointe-Noire (Republic of Congo) ~150 km north of the Congo river outlet is also lower than average (ΔR = -156 ± 64 ¹⁴C yrs; R = 289 ± 20 ¹⁴C yrs) (PN in Fig. 1). These values could be interpreted as having been influenced by the Ogooué and Congo Rivers discharges. However, all other localities close to the Congo River outlet had marine reservoir age close to the average (Lo, Ca and Lu, in Fig. 1). Instead the lower values observed in Port-Gentil (Gabon) and Pointe-Noire (Republic of Congo) are from years 1948 and 1937 suggesting that these lower values are in line with the declining global marine reservoir evolution linked to global climate and carbon cycle changes (see section 3.3). The impact of the African equatorial rivers on the local/regional coastal marine reservoir age, if any, cannot be inferred from our results.
<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

## 3.5 Outlier specimens

Mean marine reservoir age values (R and ΔR) are provided for West Africa based on our data, excluding 5 samples. These particular samples display much larger values with ΔR values ranging from 209 to 454 ¹⁴C yrs or R values ranging from 701 to 912 ¹⁴C yrs. Three specimens out of the five outlier samples correspond to *Bucardium ringens* specimens. We analysed 8 *Bucardium ringens* specimens. These 3 outlier specimens display reservoir age (R and ΔR) values that clearly disagree with neighbouring data (Nouadhibou-Cansado Bay, Loos Islands and Ivory Coast areas; Nh, LI and IC in Fig. 1). The Museum number of these specimens are MNHN-IM-2022-4597, MNHN-IM-2022-4599 and MNHN-IM-2022-4601. We do not expect that these larger values compared to those for neighbouring individuals come from the species feeding practice as they are all suspension feeders like all other investigated specimens. Similarly, we showed that the difference in the habitat does not impact the species reservoir ages. Instead *Bucardium ringens* lives in the open coast from 5-10 meters to about 50 meters depth. Shells are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019). Two of the samples that are outlier were collected at low tide (Roume Island in the Loos Islands; Republic of Guinea) or on a beach (Jacqueville; Ivory Coast). It is thus possible that these outlier samples were transported subfossil samples that died a century or more before collection date. Nevertheless, we cannot fully rule out that these higher values represent some variability in the local marine reservoir age. Although, five *Bucardium ringens* samples out of eight displayed reservoir age values in agreement with the neighbouring reservoir age values, this species might not be the best suited for reservoir age reconstruction or for sediment/archaeologic dating.

The two remaining outliers are *Ostrea stentina* specimens from the El Jadida area (Morocco; eJ in Fig. 1). The sample from El Jadida beach was a single valve looking fresh and collected from the beach (museum # MNHN-IM-2022-4609). Based on the older ¹⁴C age of this specimen, we cannot rule out that this sample could actually be a subfossil specimen. The specimen with museum # MNHN-IM-2022-4608 collected in the Sidi Moussa lagoon (south of El Jadida) was a specimen with the articulated valves and remains of flesh was still inside the shell, meaning the specimen was still alive when collected. Variations in the reservoir age could be explained by coastal upwelling that impacts some regions of the Atlantic coast of Morocco and Western Sahara (Freudenthal et al., 2001; Barton et al., 1998). Upwelled waters can be depleted in ¹⁴C relative to the sea surface potentially causing larger reservoir age values (R or ΔR) like off Portugal (Monge Soares, 1993; Monge Soares and Alveirinho Dias, 2006), California (Kennett et al., 1997), or Southern Arabian coast (Southon et al., 2002). Conversely, upwelled waters can also be nutrient-rich causing intensified ocean CO₂ uptake through enhanced primary production and biological pump (Williams and Follows, 2011), in that case, one could expect low-latitude average or decreased reservoir age values (R or ΔR). Off Morocco and Western Sahara, the second hypothesis appears most likely as coastal upwelling in this area is known to bring nutrient-rich waters to the surface ocean (Barton et al., 1998; Freudenthal et al., 2001), although to our knowledge no direct measurement of the ¹⁴C content of coastal waters in this region has been published yet. However, according to recent studies the El Jadida area is only weakly impacted by upwelling (Lourenço et al., 2020; Cropper et al., 2014), suggesting average reservoir age values instead of larger ones. Another explanation could be linked to the local
<figure><img src="image_36.png" /></figure>

## 3.5 Outlier specimens

Mean marine reservoir age values (R and ΔR) are provided for West Africa based on our data, excluding 5 samples. These particular samples display much larger values with ΔR values ranging from 209 to 454 ¹⁴C yrs or R values ranging from 701 to 912 ¹⁴C yrs. Three specimens out of the five outlier samples correspond to *Bucardium ringens* specimens. We analysed 8 *Bucardium ringens* specimens. These 3 outlier specimens display reservoir age (R and ΔR) values that clearly disagree with neighbouring data (Nouadhibou-Cansado Bay, Loos Islands and Ivory Coast areas; Nh, LI and IC in Fig. 1). The Museum number of these specimens are MNHN-IM-2022-4597, MNHN-IM-2022-4599 and MNHN-IM-2022-4601. We do not expect that these larger values compared to those for neighbouring individuals come from the species feeding practice as they are all suspension feeders like all other investigated specimens. Similarly, we showed that the difference in the habitat does not impact the species reservoir ages. Instead *Bucardium ringens* lives in the open coast from 5-10 meters to about 50 meters depth. Shells are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019). Two of the samples that are outlier were collected at low tide (Roume Island in the Loos Islands; Republic of Guinea) or on a beach (Jacqueville; Ivory Coast). It is thus possible that these outlier samples were transported subfossil samples that died a century or more before collection date. Nevertheless, we cannot fully rule out that these higher values represent some variability in the local marine reservoir age. Although, five *Bucardium ringens* samples out of eight displayed reservoir age values in agreement with the neighbouring reservoir age values, this species might not be the best suited for reservoir age reconstruction or for sediment/archaeologic dating.

The two remaining outliers are *Ostrea stentina* specimens from the El Jadida area (Morocco; eJ in Fig. 1). The sample from El Jadida beach was a single valve looking fresh and collected from the beach (museum # MNHN-IM-2022-4609). Based on the older ¹⁴C age of this specimen, we cannot rule out that this sample could actually be a subfossil specimen. The specimen with museum # MNHN-IM-2022-4608 collected in the Sidi Moussa lagoon (south of El Jadida) was a specimen with the articulated valves and remains of flesh was still inside the shell, meaning the specimen was still alive when collected. Variations in the reservoir age could be explained by coastal upwelling that impacts some regions of the Atlantic coast of Morocco and Western Sahara (Freudenthal et al., 2001; Barton et al., 1998). Upwelled waters can be depleted in ¹⁴C relative to the sea surface potentially causing larger reservoir age values (R or ΔR) like off Portugal (Monge Soares, 1993; Monge Soares and Alveirinho Dias, 2006), California (Kennett et al., 1997), or Southern Arabian coast (Southon et al., 2002). Conversely, upwelled waters can also be nutrient-rich causing intensified ocean CO₂ uptake through enhanced primary production and biological pump (Williams and Follows, 2011), in that case, one could expect low-latitude average or decreased reservoir age values (R or ΔR). Off Morocco and Western Sahara, the second hypothesis appears most likely as coastal upwelling in this area is known to bring nutrient-rich waters to the surface ocean (Barton et al., 1998; Freudenthal et al., 2001), although to our knowledge no direct measurement of the ¹⁴C content of coastal waters in this region has been published yet. However, according to recent studies the El Jadida area is only weakly impacted by upwelling (Lourenço et al., 2020; Cropper et al., 2014), suggesting average reservoir age values instead of larger ones. Another explanation could be linked to the local
<figure><img src="image_37.png" /></figure>

hydrology of the Sidi Moussa lagoon. Despite the lagoon is permanently connected to the ocean, it receives waters from rainfall and resurgences that can have an impact on the salinity in the upstream section of the lagoon (Cheggour et al., 2001). As the surrounding rocks are calcareous sandstones (Manaan, 2003), one could hypothesise that freshwaters feeding the lagoon might be depleted in ¹⁴C due to carbonate dissolution in the lagoon watershed causing a hardwater effect and thus a larger reservoir age. A last explanation could be due to an imperfect cleaning of the shell. For *Ostrea stentina*, sediment can be trapped between the growing layers of the shell. If this sediment contains old detrital carbonates and was not perfectly removed before ¹⁴C measurement, the ¹⁴C age of the shell will appear older, and the reservoir age larger. Additional reservoir age reconstructions from this region on different species would be require to validate the larger reservoir age values reconstructed from the El Jadida area.

## 4 Conclusion

The analysis of pre-bomb suspension-feeding bivalves collected along coastal West Africa from 33°N to 15°S provides marine reservoir ages that are quite homogenous, with a mean ΔR value of -77 ± 47 ¹⁴C yrs (1sd, n = 25) and a mean R value of 400 ± 59 ¹⁴C yrs (1sd, n = 25). When including the robust dataset from Ndeye (2008), the resulting mean ΔR and R values for coastal West Africa are -62 ± 51 ¹⁴C years (1sd, n = 33) and 416 ± 61 ¹⁴C years (1sd, n = 33), respectively. We show that the marine reservoir age of coastal West Africa is mainly driven by the global carbon cycle and climate rather than by local effects. Our results for different species yield similar marine reservoir age values, indicating that the ecological habitat only has a second-order impact on the reservoir age reconstruction, if any. Nevertheless, we suspect that *Bucardium ringens* might not be best suited for marine reservoir age reconstruction as corresponding shells are typically not found alive on sample collecting sites. Additionally, ages obtained on *Ostrea stentina* could be possibly influenced by the presence of sediment within the growing shell layers if not fully removed after the cleaning process.

Despite these new data, large portions of the West African coast still remain to be investigated for reservoir age reconstructions, in particular off Western Sahara and Canarias Islands, Sierra Leone-Liberia, Nigeria and Namibia.

## Author contribution statement

GSoulet designed the study and raised the funding. SG, PM, GSoulet and GSiani selected the specimens in the MNHN collections. GSoulet carried out the sample preparation with assistance of ML and FF. FD performed stable isotopes measurements. GSoulet performed reservoir calculations and analysed and discussed the data with GB, GSiani and SG. GSoulet wrote the manuscript with inputs from all co-authors.
<figure><img src="image_37.png" /></figure>

hydrology of the Sidi Moussa lagoon. Despite the lagoon is permanently connected to the ocean, it receives waters from rainfall and resurgences that can have an impact on the salinity in the upstream section of the lagoon (Cheggour et al., 2001). As the surrounding rocks are calcareous sandstones (Manaan, 2003), one could hypothesise that freshwaters feeding the lagoon might be depleted in ¹⁴C due to carbonate dissolution in the lagoon watershed causing a hardwater effect and thus a larger reservoir age. A last explanation could be due to an imperfect cleaning of the shell. For *Ostrea stentina*, sediment can be trapped between the growing layers of the shell. If this sediment contains old detrital carbonates and was not perfectly removed before ¹⁴C measurement, the ¹⁴C age of the shell will appear older, and the reservoir age larger. Additional reservoir age reconstructions from this region on different species would be require to validate the larger reservoir age values reconstructed from the El Jadida area.

## 4 Conclusion

The analysis of pre-bomb suspension-feeding bivalves collected along coastal West Africa from 33°N to 15°S provides marine reservoir ages that are quite homogenous, with a mean ΔR value of -77 ± 47 ¹⁴C yrs (1sd, n = 25) and a mean R value of 400 ± 59 ¹⁴C yrs (1sd, n = 25). When including the robust dataset from Ndeye (2008), the resulting mean ΔR and R values for coastal West Africa are -62 ± 51 ¹⁴C years (1sd, n = 33) and 416 ± 61 ¹⁴C years (1sd, n = 33), respectively. We show that the marine reservoir age of coastal West Africa is mainly driven by the global carbon cycle and climate rather that by local effects. Our results for different species yield similar marine reservoir age values, indicating that the ecological habitat only has a second-order impact on the reservoir age reconstruction, if any. Nevertheless, we suspect that *Bucardium ringens* might not be best suited for marine reservoir age reconstruction as corresponding shells are typically not found alive on sample collecting sites. Additionally, ages obtained on *Ostrea stentina* could be possibly influenced by the presence of sediment within the growing shell layers if not fully removed after the cleaning process.

Despite these new data, large portions of the West African coast still remain to be investigated for reservoir age reconstructions, in particular off Western Sahara and Canarias Islands, Sierra Leone-Liberia, Nigeria and Namibia.

## Author contribution statement

GSoulet designed the study and raised the funding. SG, PM, GSoulet and GSiani selected the specimens in the MNHN collections. GSoulet carried out the sample preparation with assistance of ML and FF. FD performed stable isotopes measurements. GSoulet performed reservoir calculations and analysed and discussed the data with GB, GSiani and SG. GSoulet wrote the manuscript with inputs from all co-authors.
<figure><img src="image_38.png" /></figure>

Acknowledgements

We are grateful to the Muséum National d'Histoire Naturelle (Paris, France) for providing the samples. We thank the LMC14 staff (Laboratoire de Mesure du Carbone-14) and the CNRS-INSU ARTEMIS national radiocarbon AMS facility for providing radiocarbon measurements published in this study. The study was funded by the French national program INSU-LEFE (ResWA project – Pre-bomb Reservoir ages for the Western coast of Africa). I (GS) thank Ifremer in Brest and Sète for supporting my research. Finally, this study was carried out and the paper written during my part-time parental leave. I dedicate this article to Marian, with whom I have had a wonderful time during this year, growing up together, him as a toddler and me as a father.

Competing interests

The authors declare that they have no conflict of interest.

Data availability

All data present in the paper are available in the text section 3.1 and in Table S1 in the Supplement.

References

Alves, E. Q., Macario, K., Ascough, P., and Bronk Ramsey, C.: The worldwide marine radiocarbon reservoir effect: Definitions, mechanisms, and prospects, Rev. Geophys., 56, 278–305, https://doi.org/10.1002/2017RG000588, 2018.

Alves, E. Q., Macario, K. D., Urrutia, F. P., Cardoso, R. P., and Bronk Ramsey, C.: Accounting for the marine reservoir effect in radiocarbon calibration, Quat. Sci. Rev., 209, 129–138, https://doi.org/10.1016/j.quascirev.2019.02.013, 2019.

Ascough, P., Cook, G., and Dugmore, A.: Methodological approaches to determining the marine radiocarbon reservoir effect, Prog. Phys. Geogr. Earth Environ., 29, 532–547, https://doi.org/10.1191/0309133305pp461ra, 2005.

Bard, E.: Correction of accelerator mass spectrometry ¹⁴C ages measured in planktonic foraminifera: Paleoceanographic implications, Paleceanography, 3, 635–645, https://doi.org/10.1029/PA003i006p00635, 1988.

Bard, E., Arnold, M., Mangerud, J., Paterne, M., Labeyrie, L., Duprat, J., Mélières, M.-A., Sønstegaard, E., and Duplessy, J.-C.: The North Atlantic atmosphere-sea surface ¹⁴C gradient during the Younger Dryas climatic event, Earth Planet. Sci. Lett., 126, 275–287, https://doi.org/10.1016/0012-821X(94)90112-0, 1994.

Barton, E. D., Arístegui, J., Tett, P., Cantón, M., García-Braun, J., Hernández-León, S., Nykjaer, L., Almeida, C., Almunia, J., Ballesteros, S., Basterretxea, G., Escánez, J., García-Weill, L., Hernández-Guerra, A., López-Laatzen, F., Molina, R., Montero, M. F., Navarro-Pérez, E., Rodríguez, J. M., van Lenning, K., Vélez, H., and Wild, K.: The transition zone of the Canary Current upwelling region, Prog. Oceanogr., 41, 455–504, https://doi.org/10.1016/S0079-6611(98)00023-8, 1998.
<figure><img src="image_38.png" /></figure>

Acknowledgements

We are grateful to the Muséum National d'Histoire Naturelle (Paris, France) for providing the samples. We thank the LMC14 staff (Laboratoire de Mesure du Carbone-14) and the CNRS-INSU ARTEMIS national radiocarbon AMS facility for providing radiocarbon measurements published in this study. The study was funded by the French national program INSU-LEFE (ResWA project – Pre-bomb Reservoir ages for the Western coast of Africa). I (GS) thank Ifremer in Brest and Sète for supporting my research. Finally, this study was carried out and the paper written during my part-time parental leave. I dedicate this article to Marian, with whom I have had a wonderful time during this year, growing up together, him as a toddler and me as a father.

Competing interests

The authors declare that they have no conflict of interest.

Data availability

All data present in the paper are available in the text section 3.1 and in Table S1 in the Supplement.

References

Alves, E. Q., Macario, K., Ascough, P., and Bronk Ramsey, C.: The worldwide marine radiocarbon reservoir effect: Definitions, mechanisms, and prospects, Rev. Geophys., 56, 278–305, https://doi.org/10.1002/2017RG000588, 2018.

Alves, E. Q., Macario, K. D., Urrutia, F. P., Cardoso, R. P., and Bronk Ramsey, C.: Accounting for the marine reservoir effect in radiocarbon calibration, Quat. Sci. Rev., 209, 129–138, https://doi.org/10.1016/j.quascirev.2019.02.013, 2019.

Ascough, P., Cook, G., and Dugmore, A.: Methodological approaches to determining the marine radiocarbon reservoir effect, Prog. Phys. Geogr. Earth Environ., 29, 532–547, https://doi.org/10.1191/0309133305pp461ra, 2005.

Bard, E.: Correction of accelerator mass spectrometry ¹⁴C ages measured in planktonic foraminifera: Paleoceanographic implications, Paleoceanography, 3, 635–645, https://doi.org/10.1029/PA003i006p00635, 1988.

Bard, E., Arnold, M., Mangerud, J., Paterne, M., Labeyrie, L., Duprat, J., Mélières, M.-A., Sønstegaard, E., and Duplessy, J.-C.: The North Atlantic atmosphere-sea surface ¹⁴C gradient during the Younger Dryas climatic event, Earth Planet. Sci. Lett., 126, 275–287, https://doi.org/10.1016/0012-821X(94)90112-0, 1994.

Barton, E. D., Arístegui, J., Tett, P., Cantón, M., García-Braun, J., Hernández-León, S., Nykjaer, L., Almeida, C., Almunia, J., Ballesteros, S., Basterretxea, G., Escánez, J., García-Weill, L., Hernández-Guerra, A., López-Laatzen, F., Molina, R., Montero, M. F., Navarro-Pérez, E., Rodríguez, J. M., van Lenning, K., Vélez, H., and Wild, K.: The transition zone of the Canary Current upwelling region, Prog. Oceanogr., 41, 455–504, https://doi.org/10.1016/S0079-6611(98)00023-8, 1998.
<figure><img src="image_39.png" /></figure>

Boettger, C. R.: Zur Molluskenfauna des Kongogebiets, Ann. la Société R. Zool. Malacol. Belgique, 47, 89–117, 1912.

Boettger, O.: Katalog der Reptilien-Sammlung im Museum der Senckenbergischen Naturforschenden Gesellschaft in Frankfurt am Main, II Teil (Schlangen), Gebrüder Knauer, Frankfurt am Main, 1898.

Butzin, M., Köhler, P., and Lohmann, G.: Marine radiocarbon reservoir age simulations for the past 50,000 years, Geophys. Res. Lett., 44, 8473–8480, https://doi.org/10.1002/2017GL074688, 2017.

Catry, T., Figueira, P., Carvalho, L., Monteiro, R., Coelho, P., Lourenço, P. M., Catry, P., Tchantchalam, Q., Catry, I., Botelho, M. J., Pereira, E., Granadeiro, J. P., and Vale, C.: Evidence for contrasting accumulation pattern of cadmium in relation to other elements in Senilia senilis and Tagelus adansoni from the Bijagós archipelago, Guinea-Bissau, Environ. Sci. Pollut. Res., 24, 24896–24906, https://doi.org/10.1007/s11356-017-9902-8, 2017.

Cheggour, M., Chafik, A., Langston, W., Burt, G., Benbrahim, S., and Texier, H.: Metals in sediments and the edible cockle Cerastoderma edule from two Moroccan Atlantic lagoons: Moulay Bou Selham and Sidi Moussa, Environ. Pollut., 115, 149–160, https://doi.org/10.1016/S0269-7491(01)00117-8, 2001.

von Cosel, R. and Gofas, S.: Marine Bivalves of Tropical West Africa: From Rio de Oro to Southern Angola, edited by: MNHN and IRD, Faune et Flore tropicales, 48, Paris, Marseille, 1104 pp., 2019.

Cottereau, E., Arnold, M., Moreau, C., Baqué, D., Bavay, D., Caffy, I., Comby, C., Dumoulin, J.-P., Hain, S., Perron, M., Salomon, J., and Setti, V.: Artemis, the new ¹⁴C AMS at LMC14 in Saclay, France, Radiocarbon, 49, 291–299, https://doi.org/10.1017/S0033822200042211, 2007.

Coynel, A., Seyler, P., Etcheber, H., Meybeck, M., and Orange, D.: Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River, Global Biogeochem. Cycles, 19, n/a-n/a, https://doi.org/10.1029/2004GB002335, 2005.

Cropper, T. E., Hanna, E., and Bigg, G. R.: Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981–2012, Deep Sea Res. Part I Oceanogr. Res. Pap., 86, 94–111, https://doi.org/10.1016/j.dsr.2014.01.007, 2014.

Dautzenberg, P.: Mission Gruvel sur la côte occidentale d'Afrique (1909-1910): Mollusques marins. Annales de l'Institut Océanographique, Paris (Nouvelle Série). 5(3): 1-111, pl. 1-3, 1912.

Dewar, G., Reimer, P. J., Sealy, J., and Woodborne, S.: Late-Holocene marine radiocarbon reservoir correction (ΔR) for the west coast of South Africa, The Holocene, 22, 1481–1489, https://doi.org/10.1177/0959683612449755, 2012.

Dumoulin, J.-P., Comby-Zerbino, C., Delqué-Količ, E., Moreau, C., Caffy, I., Hain, S., Perron, M., Thellier, B., Setti, V., Berthier, B., and Beck, L.: Status report on sample preparation protocols developed at the LMC14 laboratory, Saclay, France:

From Sample Collection to ¹⁴C AMS Measurement, Radiocarbon, 59, 713–726, https://doi.org/10.1017/RDC.2016.116, 2017.
Freudenthal, T., Neuer, S., Meggers, H., Davenport, R., and Wefer, G.: Influence of lateral particle advection and organic matter degradation on sediment accumulation and stable nitrogen isotope ratios along a productivity gradient in the Canary Islands region, Mar. Geol., 177, 93–109, https://doi.org/10.1016/S0025-3227(01)00126-8, 2001.

Heaton, T. J., Köhler, P., Butzin, M., Bard, E., Reimer, R. W., Austin, W. E. N., Bronk Ramsey, C., Grootes, P. M., Hughen, K. A., Kromer, B., Reimer, P. J., Adkins, J., Burke, A., Cook, M. S., Olsen, J., and Skinner, L. C.: Marine20—The Marine
<figure><img src="image_39.png" /></figure>

Boettger, C. R.: Zur Molluskenfauna des Kongogebiets, Ann. la Société R. Zool. Malacol. Belgique, 47, 89–117, 1912.

Boettger, O.: Katalog der Reptilien-Sammlung im Museum der Senckenbergischen Naturforschenden Gesellschaft in Frankfurt am Main, II Teil (Schlangen), Gebrüder Knauer, Frankfurt am Main, 1898.

Butzin, M., Köhler, P., and Lohmann, G.: Marine radiocarbon reservoir age simulations for the past 50,000 years, Geophys. Res. Lett., 44, 8473–8480, https://doi.org/10.1002/2017GL074688, 2017.

Catry, T., Figueira, P., Carvalho, L., Monteiro, R., Coelho, P., Lourenço, P. M., Catry, P., Tchantchalam, Q., Catry, I., Botelho, M. J., Pereira, E., Granadeiro, J. P., and Vale, C.: Evidence for contrasting accumulation pattern of cadmium in relation to other elements in Senilia senilis and Tagelus adansoni from the Bijagós archipelago, Guinea-Bissau, Environ. Sci. Pollut. Res., 24, 24896–24906, https://doi.org/10.1007/s11356-017-9902-8, 2017.

Cheggour, M., Chafik, A., Langston, W., Burt, G., Benbrahim, S., and Texier, H.: Metals in sediments and the edible cockle Cerastoderma edule from two Moroccan Atlantic lagoons: Moulay Bou Selham and Sidi Moussa, Environ. Pollut., 115, 149–160, https://doi.org/10.1016/S0269-7491(01)00117-8, 2001.

von Cosel, R. and Gofas, S.: Marine Bivalves of Tropical West Africa: From Rio de Oro to Southern Angola, edited by: MNHN and IRD, Faune et Flore tropicales, 48, Paris, Marseille, 1104 pp., 2019.

Cottereau, E., Arnold, M., Moreau, C., Baqué, D., Bavay, D., Caffy, I., Comby, C., Dumoulin, J.-P., Hain, S., Perron, M., Salomon, J., and Setti, V.: Artemis, the new ¹⁴C AMS at LMC14 in Saclay, France, Radiocarbon, 49, 291–299, https://doi.org/10.1017/S0033822200042211, 2007.

Coynel, A., Seyler, P., Etcheber, H., Meybeck, M., and Orange, D.: Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River, Global Biogeochem. Cycles, 19, n/a-n/a, https://doi.org/10.1029/2004GB002335, 2005.

Cropper, T. E., Hanna, E., and Bigg, G. R.: Spatial and temporal seasonal trends in coastal upwelling off Northwest Africa, 1981–2012, Deep Sea Res. Part I Oceanogr. Res. Pap., 86, 94–111, https://doi.org/10.1016/j.dsr.2014.01.007, 2014.

Dautzenberg, P.: Mission Gruvel sur la côte occidentale d'Afrique (1909-1910): Mollusques marins. Annales de l'Institut Océanographique, Paris (Nouvelle Série). 5(3): 1-111, pl. 1-3, 1912.

Dewar, G., Reimer, P. J., Sealy, J., and Woodborne, S.: Late-Holocene marine radiocarbon reservoir correction (ΔR) for the west coast of South Africa, The Holocene, 22, 1481–1489, https://doi.org/10.1177/0959683612449755, 2012.

Dumoulin, J.-P., Comby-Zerbino, C., Delqué-Količ, E., Moreau, C., Caffy, I., Hain, S., Perron, M., Thellier, B., Setti, V., Berthier, B., and Beck, L.: Status report on sample preparation protocols developed at the LMC14 laboratory, Saclay, France:

From Sample Collection to ¹⁴C AMS Measurement, Radiocarbon, 59, 713–726, https://doi.org/10.1017/RDC.2016.116, 2017.
Freudenthal, T., Neuer, S., Meggers, H., Davenport, R., and Wefer, G.: Influence of lateral particle advection and organic matter degradation on sediment accumulation and stable nitrogen isotope ratios along a productivity gradient in the Canary Islands region, Mar. Geol., 177, 93–109, https://doi.org/10.1016/S0025-3227(01)00126-8, 2001.

Heaton, T. J., Köhler, P., Butzin, M., Bard, E., Reimer, R. W., Austin, W. E. N., Bronk Ramsey, C., Grootes, P. M., Hughen, K. A., Kromer, B., Reimer, P. J., Adkins, J., Burke, A., Cook, M. S., Olsen, J., and Skinner, L. C.: Marine20—The Marine
<figure><img src="image_40.png" /></figure>

Radiocarbon Age Calibration Curve (0–55,000 cal BP), Radiocarbon, 62, 779–820, https://doi.org/10.1017/RDC.2020.68, 2020.

Heaton, T. J., Bard, E., Bronk Ramsey, C., Butzin, M., Köhler, P., Muscheler, R., Reimer, P. J., and Wacker, L.: Radiocarbon: A key tracer for studying Earth’s dynamo, climate system, carbon cycle, and Sun, Science (80-. ), 374, https://doi.org/10.1126/science.abd7096, 2021.

Heaton, T. J., Bard, E., Bronk Ramsey, C., Butzin, M., Hatté, C., Hughen, K. A., Köhler, P., and Reimer, P. J.: A response to community questions on the Marine20 radiocarbon age calibration curve: Marine reservoir ages and the calibration of ¹⁴C samples from the oceans, Radiocarbon, 65, 247–273, https://doi.org/10.1017/RDC.2022.66, 2023.

Herrera, N. D., ter Poorten, J. J., Bieler, R., Mikkelsen, P. M., Strong, E. E., Jablonski, D., and Steppan, S. J.: Molecular phylogenetics and historical biogeography amid shifting continents in the cockles and giant clams (Bivalvia: Cardiidae), Mol. Phylogenet. Evol., 93, 94–106, https://doi.org/10.1016/j.ympev.2015.07.013, 2015.

Hogg, A. G., Heaton, T. J., Hua, Q., Palmer, J. G., Turney, C. S., Southon, J., Bayliss, A., Blackwell, P. G., Boswijk, G., Bronk Ramsey, C., Pearson, C., Petchey, F., Reimer, P., Reimer, R., and Wacker, L.: SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP, Radiocarbon, 62, 759–778, https://doi.org/10.1017/RDC.2020.59, 2020.

Kennett, D. J., Ingram, B. L., Erlandson, J. M., and Walker, P.: Evidence for Temporal Fluctuations in Marine Radiocarbon Reservoir Ages in the Santa Barbara Channel, Southern California, J. Archaeol. Sci., 24, 1051–1059, https://doi.org/10.1006/jasc.1996.0184, 1997.

Köhler, P. and Fischer, H.: Simulating changes in the terrestrial biosphere during the last glacial/interglacial transition, Glob. Planet. Change, 43, 33–55, https://doi.org/10.1016/j.gloplacha.2004.02.005, 2004.

Köhler, P. and Fischer, H.: Simulating low frequency changes in atmospheric CO₂ during the last 740 000 years, Clim. Past, 2, 57–78, https://doi.org/10.5194/cp-2-57-2006, 2006.

Köhler, P., Fischer, H., Munhoven, G., and Zeebe, R. E.: Quantitative interpretation of atmospheric carbon records over the last glacial termination, Global Biogeochem. Cycles, 19, n/a-n/a, https://doi.org/10.1029/2004GB002345, 2005.

Köhler, P., Muscheler, R., and Fischer, H.: A model-based interpretation of low-frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric Δ¹⁴C, Geochemistry, Geophys. Geosystems, 7, n/a-n/a, https://doi.org/10.1029/2005GC001228, 2006.

Lambert, T., Darchambeau, F., Bouillon, S., Alhou, B., Mbega, J.-D., Teodoru, C. R., Nyoni, F. C., Massicotte, P., and Borges, A. V.: Landscape control on the spatial and temporal variability of chromophoric dissolved organic matter and dissolved organic carbon in large African Rivers, Ecosystems, 18, 1224–1239, https://doi.org/10.1007/s10021-015-9894-5, 2015.

Lindsay, C. M., Lehman, S. J., Marchitto, T. M., Carriquiry, J. D., and Ortiz, J. D.: New constraints on deglacial marine radiocarbon anomalies from a depth transect near Baja California, Paleoceanography, 31, 1103–1116, https://doi.org/10.1002/2015PA002878, 2016.

Lourenço, C. R., Nicastro, K. R., McQuaid, C. D., Krug, L. A., and Zardi, G. I.: Strong upwelling conditions drive differences in species abundance and community composition along the Atlantic coasts of Morocco and Western Sahara, Mar. Biodivers.,
<figure><img src="image_40.png" /></figure>

Radiocarbon Age Calibration Curve (0–55,000 cal BP), Radiocarbon, 62, 779–820, https://doi.org/10.1017/RDC.2020.68, 2020.

Heaton, T. J., Bard, E., Bronk Ramsey, C., Butzin, M., Köhler, P., Muscheler, R., Reimer, P. J., and Wacker, L.: Radiocarbon: A key tracer for studying Earth's dynamo, climate system, carbon cycle, and Sun, Science (80-. ), 374, https://doi.org/10.1126/science.abd7096, 2021.

Heaton, T. J., Bard, E., Bronk Ramsey, C., Butzin, M., Hatté, C., Hughen, K. A., Köhler, P., and Reimer, P. J.: A response to community questions on the Marine20 radiocarbon age calibration curve: Marine reservoir ages and the calibration of ¹⁴C samples from the oceans, Radiocarbon, 65, 247–273, https://doi.org/10.1017/RDC.2022.66, 2023.

Herrera, N. D., ter Poorten, J. J., Bieler, R., Mikkelsen, P. M., Strong, E. E., Jablonski, D., and Steppan, S. J.: Molecular phylogenetics and historical biogeography amid shifting continents in the cockles and giant clams (Bivalvia: Cardiidae), Mol. Phylogenet. Evol., 93, 94–106, https://doi.org/10.1016/j.ympev.2015.07.013, 2015.

Hogg, A. G., Heaton, T. J., Hua, Q., Palmer, J. G., Turney, C. S., Southon, J., Bayliss, A., Blackwell, P. G., Boswijk, G., Bronk Ramsey, C., Pearson, C., Petchey, F., Reimer, P., Reimer, R., and Wacker, L.: SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP, Radiocarbon, 62, 759–778, https://doi.org/10.1017/RDC.2020.59, 2020.

Kennett, D. J., Ingram, B. L., Erlandson, J. M., and Walker, P.: Evidence for Temporal Fluctuations in Marine Radiocarbon Reservoir Ages in the Santa Barbara Channel, Southern California, J. Archaeol. Sci., 24, 1051–1059, https://doi.org/10.1006/jasc.1996.0184, 1997.

Köhler, P. and Fischer, H.: Simulating changes in the terrestrial biosphere during the last glacial/interglacial transition, Glob. Planet. Change, 43, 33–55, https://doi.org/10.1016/j.gloplacha.2004.02.005, 2004.

Köhler, P. and Fischer, H.: Simulating low frequency changes in atmospheric CO₂ during the last 740 000 years, Clim. Past, 2, 57–78, https://doi.org/10.5194/cp-2-57-2006, 2006.

Köhler, P., Fischer, H., Munhoven, G., and Zeebe, R. E.: Quantitative interpretation of atmospheric carbon records over the last glacial termination, Global Biogeochem. Cycles, 19, n/a-n/a, https://doi.org/10.1029/2004GB002345, 2005.

Köhler, P., Muscheler, R., and Fischer, H.: A model-based interpretation of low-frequency changes in the carbon cycle during the last 120,000 years and its implications for the reconstruction of atmospheric Δ¹⁴C, Geochemistry, Geophys. Geosystems, 7, n/a-n/a, https://doi.org/10.1029/2005GC001228, 2006.

Lambert, T., Darchambeau, F., Bouillon, S., Alhou, B., Mbega, J.-D., Teodoru, C. R., Nyoni, F. C., Massicotte, P., and Borges, A. V.: Landscape control on the spatial and temporal variability of chromophoric dissolved organic matter and dissolved organic carbon in large African Rivers, Ecosystems, 18, 1224–1239, https://doi.org/10.1007/s10021-015-9894-5, 2015.

Lindsay, C. M., Lehman, S. J., Marchitto, T. M., Carriquiry, J. D., and Ortiz, J. D.: New constraints on deglacial marine radiocarbon anomalies from a depth transect near Baja California, Paleoceanography, 31, 1103–1116, https://doi.org/10.1002/2015PA002878, 2016.

Lourenço, C. R., Nicastro, K. R., McQuaid, C. D., Krug, L. A., and Zardi, G. I.: Strong upwelling conditions drive differences in species abundance and community composition along the Atlantic coasts of Morocco and Western Sahara, Mar. Biodivers.,
<figure><img src="image_41.png" /></figure>

705 50, 15, https://doi.org/10.1007/s12526-019-01032-z, 2020.

Manaan, M.: Étude sédimentologique du remplissage de la lagune de Sidi Moussa, Maroc, PhD thesis, Université Chouaib Doukkali, El Jadida, https://theses.hal.science/tel-00124571, 2003.

Martins, M. S. and Stammer, D.: Interannual variability of the Congo River plume-induced sea surface salinity, Remote Sens., 14, 1013, https://doi.org/10.3390/rs14041013, 2022.

710 Marwick, T. R., Tamooh, F., Teodoru, C. R., Borges, A. V., Darchambeau, F., and Bouillon, S.: The age of river-transported carbon: A global perspective, Global Biogeochem. Cycles, 29, 122–137, https://doi.org/10.1002/2014GB004911, 2015.

Milliman, J. D. and Farnsworth, K. L.: River discharge to the coastal ocean, Cambridge University Press, https://doi.org/10.1017/CBO9780511781247, 2011.

715 Monge Soares, A.: The ¹⁴C content of marine shells: Evidence for variability in coastal upwelling off Portugal during the Holocene, in: Isotope techniques in the study of past and current environmental changes in the hydrosphere and the atmosphere proceedings, edited by: IAEA, Vienna, 471–485, 1993.

Monge Soares, A. M. and Alveirinho Dias, J. M.: Coastal upwelling and radiocarbon-evidence for temporal fluctuations in ocean reservoir effect off portugal during the holocene, Radiocarbon, 48, 45–60, https://doi.org/10.1017/S0033822200035384, 2006.

720 Mook, W. G. and van der Plicht, J.: Reporting ¹⁴C activities and concentrations, Radiocarbon, 41, 227–239, https://doi.org/10.1017/S0033822200057106, 1999.

Moreau, C., Caffy, I., Comby, C., Delqué-Količ, E., Dumoulin, J.-P., Hain, S., Quiles, A., Setti, V., Souprayen, C., Thellier, B., and Vincent, J.: Research and development of the Artemis ¹⁴C AMS facility: Status report, Radiocarbon, 55, 331–337, https://doi.org/10.1017/S0033822200057441, 2013.

725 Ndeye, M.: Marine reservoir ages in Northern Senegal and Mauritania coastal waters, Radiocarbon, 50, 281–288, https://doi.org/10.1017/S0033822200033580, 2008.

Okera, W.: Observations on some population parameters of exploited stocks of Senilia senilis (=Arca senilis) in Sierra Leone, Mar. Biol., 38, 217–229, https://doi.org/10.1007/BF00388935, 1976.

730 Oliver, P. G. and von Cosel, R.: Taxonomy of tropical west African Bivalves. IV. Arcidae., Bull. Muséum Natl. d’Histoire Nat., 14, 293–381, 1992.

Reimer, P. J. and McCormac, F. G.: Marine radiocarbon reservoir corrections for the Mediterranean and Aegean Seas, Radiocarbon, 44, 159–166, https://doi.org/10.1017/S0033822200064766, 2002.

Reimer, P. J. and Reimer, R. W.: A marine reservoir correction database and on-line interface, Radiocarbon, 43, 461–463, https://doi.org/10.1017/S0033822200038339, 2001.

735 Reimer, P. J., Brown, T. A., and Reimer, R. W.: Discussion: Reporting and calibration of post-bomb ¹⁴C data, Radiocarbon, 46, 1299–1304, https://doi.org/10.1017/S0033822200033154, 2004.

Reimer, P. J., Austin, W. E. N., Bard, E., Bayliss, A., Blackwell, P. G., Bronk Ramsey, C., Butzin, M., Cheng, H., Edwards, R. L., Friedrich, M., Grootes, P. M., Guilderson, T. P., Hajdas, I., Heaton, T. J., Hogg, A. G., Hughen, K. A., Kromer, B.,
<figure><img src="image_41.png" /></figure>

705 50, 15, https://doi.org/10.1007/s12526-019-01032-z, 2020.

Manaan, M.: Étude sédimentologique du remplissage de la lagune de Sidi Moussa, Maroc, PhD thesis, Université Chouaib Doukkali, El Jadida, https://theses.hal.science/tel-00124571, 2003.

Martins, M. S. and Stammer, D.: Interannual variability of the Congo River plume-induced sea surface salinity, Remote Sens., 14, 1013, https://doi.org/10.3390/rs14041013, 2022.

710 Marwick, T. R., Tamooh, F., Teodoru, C. R., Borges, A. V., Darchambeau, F., and Bouillon, S.: The age of river-transported carbon: A global perspective, Global Biogeochem. Cycles, 29, 122–137, https://doi.org/10.1002/2014GB004911, 2015.

Milliman, J. D. and Farnsworth, K. L.: River discharge to the coastal ocean, Cambridge University Press, https://doi.org/10.1017/CBO9780511781247, 2011.

715 Monge Soares, A.: The ¹⁴C content of marine shells: Evidence for variability in coastal upwelling off Portugal during the Holocene, in: Isotope techniques in the study of past and current environmental changes in the hydrosphere and the atmosphere proceedings, edited by: IAEA, Vienna, 471–485, 1993.

Monge Soares, A. M. and Alveirinho Dias, J. M.: Coastal upwelling and radiocarbon-evidence for temporal fluctuations in ocean reservoir effect off portugal during the holocene, Radiocarbon, 48, 45–60, https://doi.org/10.1017/S0033822200035384, 2006.

720 Mook, W. G. and van der Plicht, J.: Reporting ¹⁴C activities and concentrations, Radiocarbon, 41, 227–239, https://doi.org/10.1017/S0033822200057106, 1999.

Moreau, C., Caffy, I., Comby, C., Delqué-Količ, E., Dumoulin, J.-P., Hain, S., Quiles, A., Setti, V., Souprayen, C., Thellier, B., and Vincent, J.: Research and development of the Artemis ¹⁴C AMS facility: Status report, Radiocarbon, 55, 331–337, https://doi.org/10.1017/S0033822200057441, 2013.

725 Ndeye, M.: Marine reservoir ages in Northern Senegal and Mauritania coastal waters, Radiocarbon, 50, 281–288, https://doi.org/10.1017/S0033822200033580, 2008.

Okera, W.: Observations on some population parameters of exploited stocks of Senilia senilis (=Arca senilis) in Sierra Leone, Mar. Biol., 38, 217–229, https://doi.org/10.1007/BF00388935, 1976.

730 Oliver, P. G. and von Cosel, R.: Taxonomy of tropical west African Bivalves. IV. Arcidae., Bull. Muséum Natl. d’Histoire Nat., 14, 293–381, 1992.

Reimer, P. J. and McCormac, F. G.: Marine radiocarbon reservoir corrections for the Mediterranean and Aegean Seas, Radiocarbon, 44, 159–166, https://doi.org/10.1017/S0033822200064766, 2002.

Reimer, P. J. and Reimer, R. W.: A marine reservoir correction database and on-line interface, Radiocarbon, 43, 461–463, https://doi.org/10.1017/S0033822200038339, 2001.

735 Reimer, P. J., Brown, T. A., and Reimer, R. W.: Discussion: Reporting and calibration of post-bomb ¹⁴C data, Radiocarbon, 46, 1299–1304, https://doi.org/10.1017/S0033822200033154, 2004.

Reimer, P. J., Austin, W. E. N., Bard, E., Bayliss, A., Blackwell, P. G., Bronk Ramsey, C., Butzin, M., Cheng, H., Edwards, R. L., Friedrich, M., Grootes, P. M., Guilderson, T. P., Hajdas, I., Heaton, T. J., Hogg, A. G., Hughen, K. A., Kromer, B.,
<figure><img src="image_42.png" /></figure>

<figure><img src="image_43.png" /></figure>

<figure><img src="image_44.png" /></figure>

Manning, S. W., Muscheler, R., Palmer, J. G., Pearson, C., van der Plicht, J., Reimer, R. W., Richards, D. A., Scott, E. M., Southon, J. R., Turney, C. S. M., Wacker, L., Adolphi, F., Büntgen, U., Capano, M., Fahrni, S. M., Fogtmann-Schulz, A., Friedrich, R., Köhler, P., Kudsk, S., Miyake, F., Olsen, J., Reinig, F., Sakamoto, M., Sookdeo, A., and Talamo, S.: The IntCal20 northern hemisphere radiocarbon age calibration curve (0–55 cal kBP), Radiocarbon, 62, 725–757, https://doi.org/10.1017/RDC.2020.41, 2020.

740

Reimer, R. W. and Reimer, P. J.: An online application for ΔR calculation, Radiocarbon, 59, 1623–1627, https://doi.org/10.1017/RDC.2016.117, 2017.

745

Richey, J. E., Spencer, R. G. M., Drake, T. W., and Ward, N. D.: Fluvial carbon dynamics across the land to ocean continuum of great tropical rivers, 391–412, https://doi.org/10.1002/9781119657002.ch20, 2022.

Roux, C.: Compte rendu sommaire d'une mission en Afrique Équatoriale Française, Bull. du Muséum Natl. d'Histoire Nat., 21, 500–503, 1949.

Santos, G. M., Southon, J. R., Griffin, S., Beaupre, S. R., and Druffel, E. R. M.: Ultra small-mass AMS ¹⁴C sample preparation and analyses at KCCAMS/UCI Facility, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, 259, 293–302, https://doi.org/10.1016/j.nimb.2007.01.172, 2007.

750

Schefuß, E., Eglinton, T. I., Spencer-Jones, C. L., Rullkötter, J., De Pol-Holz, R., Talbot, H. M., Grootes, P. M., and Schneider, R. R.: Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin, Nat. Geosci., 9, 687–690, https://doi.org/10.1038/ngeo2778, 2016.

755

Sessa, J. A., Callapez, P. M., Dinis, P. A., and Hendy, A. J. W.: Paleoenvironmental and paleobiogeographical implications of a middle Pleistocene mollusc assemblage from the marine terraces of Baía Das Pipas, southwest Angola, J. Paleontol., 87, 1016–1040, https://doi.org/10.1666/12-119, 2013.

Siani, G., Paterne, M., Arnold, M., Bard, E., Métivier, B., Tisnerat, N., and Bassinot, F.: Radiocarbon reservoir ages in the Mediterranean Sea and Black Sea, Radiocarbon, 42, 271–280, https://doi.org/10.1017/S0033822200059075, 2000.

760

Siani, G., Paterne, M., Michel, E., Sulpizio, R., Sbrana, A., Arnold, M., and Haddad, G.: Mediterranean Sea surface radiocarbon reservoir age changes since the Last Glacial Maximum, Science (80-. ), 294, 1917–1920, https://doi.org/10.1126/science.1063649, 2001.

Skinner, L., McCave, I. N., Carter, L., Fallon, S., Scrivner, A. E., and Primeau, F.: Reduced ventilation and enhanced magnitude of the deep Pacific carbon pool during the last glacial period, Earth Planet. Sci. Lett., 411, 45–52, https://doi.org/10.1016/j.epsl.2014.11.024, 2015.

765

Skinner, L. C. and Bard, E.: Radiocarbon as a dating tool and tracer in paleoceanography, Rev. Geophys., 60, 1–64, https://doi.org/10.1029/2020RG000720, 2022.

Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E., and Barker, S.: Ventilation of the deep Southern Ocean and deglacial CO₂ rise, Science (80-. ), 328, 1147–1151, https://doi.org/10.1126/science.1183627, 2010.

770

Smith, D. A. S.: Polymorphism and population density in Donax rugosus (Lamellibranchiata: Donacidae), J. Zool., 164, 429–441, https://doi.org/10.1111/j.1469-7998.1971.tb01327.x, 1971.
<figure><img src="image_42.png" /></figure>

Manning, S. W., Muscheler, R., Palmer, J. G., Pearson, C., van der Plicht, J., Reimer, R. W., Richards, D. A., Scott, E. M., Southon, J. R., Turney, C. S. M., Wacker, L., Adolphi, F., Büntgen, U., Capano, M., Fahrni, S. M., Fogtmann-Schulz, A., Friedrich, R., Köhler, P., Kudsk, S., Miyake, F., Olsen, J., Reinig, F., Sakamoto, M., Sookdeo, A., and Talamo, S.: The IntCal20 northern hemisphere radiocarbon age calibration curve (0–55 cal kBP), Radiocarbon, 62, 725–757, https://doi.org/10.1017/RDC.2020.41, 2020.

Reimer, R. W. and Reimer, P. J.: An online application for ΔR calculation, Radiocarbon, 59, 1623–1627, https://doi.org/10.1017/RDC.2016.117, 2017.

Richey, J. E., Spencer, R. G. M., Drake, T. W., and Ward, N. D.: Fluvial carbon dynamics across the land to ocean continuum of great tropical rivers, 391–412, https://doi.org/10.1002/9781119657002.ch20, 2022.

Roux, C.: Compte rendu sommaire d'une mission en Afrique Équatoriale Française, Bull. du Muséum Natl. d'Histoire Nat., 21, 500–503, 1949.

Santos, G. M., Southon, J. R., Griffin, S., Beaupre, S. R., and Druffel, E. R. M.: Ultra small-mass AMS $^{14}$C sample preparation and analyses at KCCAMS/UCI Facility, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms, 259, 293–302, https://doi.org/10.1016/j.nimb.2007.01.172, 2007.

Schefuß, E., Eglinton, T. I., Spencer-Jones, C. L., Rullkötter, J., De Pol-Holz, R., Talbot, H. M., Grootes, P. M., and Schneider, R. R.: Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin, Nat. Geosci., 9, 687–690, https://doi.org/10.1038/ngeo2778, 2016.

Sessa, J. A., Callapez, P. M., Dinis, P. A., and Hendy, A. J. W.: Paleoenvironmental and paleobiogeographical implications of a middle Pleistocene mollusc assemblage from the marine terraces of Baía Das Pipas, southwest Angola, J. Paleontol., 87, 1016–1040, https://doi.org/10.1666/12-119, 2013.

Siani, G., Paterne, M., Arnold, M., Bard, E., Métivier, B., Tisnerat, N., and Bassinot, F.: Radiocarbon reservoir ages in the Mediterranean Sea and Black Sea, Radiocarbon, 42, 271–280, https://doi.org/10.1017/S0033822200059075, 2000.

Siani, G., Paterne, M., Michel, E., Sulpizio, R., Sbrana, A., Arnold, M., and Haddad, G.: Mediterranean Sea surface radiocarbon reservoir age changes since the Last Glacial Maximum, Science (80-. ), 294, 1917–1920, https://doi.org/10.1126/science.1063649, 2001.

Skinner, L., McCave, I. N., Carter, L., Fallon, S., Scrivner, A. E., and Primeau, F.: Reduced ventilation and enhanced magnitude of the deep Pacific carbon pool during the last glacial period, Earth Planet. Sci. Lett., 411, 45–52, https://doi.org/10.1016/j.epsl.2014.11.024, 2015.

Skinner, L. C. and Bard, E.: Radiocarbon as a dating tool and tracer in paleoceanography, Rev. Geophys., 60, 1–64, https://doi.org/10.1029/2020RG000720, 2022.

Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E., and Barker, S.: Ventilation of the deep Southern Ocean and deglacial $CO_2$ rise, Science (80-. ), 328, 1147–1151, https://doi.org/10.1126/science.1183627, 2010.

Smith, D. A. S.: Polymorphism and population density in Donax rugosus (Lamellibranchiata: Donacidae), J. Zool., 164, 429–441, https://doi.org/10.1111/j.1469-7998.1971.tb01327.x, 1971.
<figure><img src="image_45.png" /></figure>

Soulet, G.: Methods and codes for reservoir-atmosphere ¹⁴C age offset calculations, Quat. Geochronol., 29, 97–103, https://doi.org/10.1016/j.quageo.2015.05.023, 2015.

Soulet, G., Ménot, G., Garreta, V., Rostek, F., Zaragosi, S., Lericolais, G., and Bard, E.: Black Sea “Lake” reservoir age evolution since the Last Glacial — Hydrologic and climatic implications, Earth Planet. Sci. Lett., 308, 245–258, https://doi.org/10.1016/j.epsl.2011.06.002, 2011.

Soulet, G., Skinner, L. C., Beaupré, S. R., and Galy, V.: A note on reporting of reservoir ¹⁴C disequilibria and age offsets, Radiocarbon, 58, 205–211, https://doi.org/10.1017/RDC.2015.22, 2016.

Soulet, G., Giosan, L., Flaux, C., and Galy, V.: Using stable carbon isotopes to quantify radiocarbon reservoir age offsets in the coastal black sea, Radiocarbon, 61, https://doi.org/10.1017/RDC.2018.61, 2019.

Southon, J., Kashgarian, M., Fontugne, M., Metivier, B., and Yim, W. W. S.: Marine reservoir corrections for the Indian Ocean and Southeast Asia, Radiocarbon, 44, 167–180, https://doi.org/10.1017/S0033822200064778, 2002.

Spencer, R. G. M., Stubbins, A., Hernes, P. J., Baker, A., Mopper, K., Aufdenkampe, A. K., Dyda, R. Y., Mwamba, V. L., Mangangu, A. M., Wabakanghanzi, J. N., and Six, J.: Photochemical degradation of dissolved organic matter and dissolved lignin phenols from the Congo River, J. Geophys. Res., 114, G03010, https://doi.org/10.1029/2009JG000968, 2009.

Spencer, R. G. M., Hernes, P. J., Aufdenkampe, A. K., Baker, A., Gulliver, P., Stubbins, A., Aiken, G. R., Dyda, R. Y., Butler, K. D., Mwamba, V. L., Mangangu, A. M., Wabakanghanzi, J. N., and Six, J.: An initial investigation into the organic matter biogeochemistry of the Congo River, Geochim. Cosmochim. Acta, 84, 614–627, https://doi.org/10.1016/j.gca.2012.01.013, 2012.

Spencer, R. G. M., Hernes, P. J., Dinga, B., Wabakanghanzi, J. N., Drake, T. W., and Six, J.: Origins, seasonality, and fluxes of organic matter in the Congo River, Global Biogeochem. Cycles, 30, 1105–1121, https://doi.org/10.1002/2016GB005427, 2016.

Stuiver, M. and Braziunas, T. F.: Modeling atmospheric ¹⁴C influences and ¹⁴C ages of marine samples to 10,000 BC, Radiocarbon, 35, 137–189, https://doi.org/10.1017/S0033822200013874, 1993.

Stuiver, M. and Polach, H. A.: Discussion: Reporting of ¹⁴C Data, Radiocarbon, 19, 355–363, https://doi.org/10.1017/S0033822200003672, 1977.

Stuiver, M., Pearson, G. W., and Braziunas, T.: Radiocarbon age calibration of marine samples back to 9000 cal yr BP, Radiocarbon, 28, 980–1021, https://doi.org/10.1017/S0033822200060264, 1986.

Tisnérat-Laborde, N., Poupeau, J. J., Tannau, J. F., and Paterne, M.: Development of a semi-automated system for routine preparation of carbonate samples, Radiocarbon, 43, 299–304, https://doi.org/10.1017/S0033822200038145, 2001.

Türkmen, A., Türkmen, M., and Tepe, Y.: Biomonitoring of heavy metals from Iskenderun Bay using two bivalve species Chama pacifica Broderip, 1834 and Ostrea stentina Payraveau, 1826, Turkish J. Fish. Aquat. Sci., 5, 107–111, 2005.

Westhoff, F.: Die Fauna der Kongo-Mündung, Jahresbericht der Zool. Sekt. Westfälischen Prov. Vereins für Wiss. und Kunft für das Etatsjahr 1885-86, 38–40, 1886.

Williams, R. G. and Follows, M. J.: Ocean dynamics and the carbon cycle: Principles and mechanisms, Cambridge, 2015.
<figure><img src="image_43.png" /></figure>

Soulet, G.: Methods and codes for reservoir-atmosphere ¹⁴C age offset calculations, Quat. Geochronol., 29, 97–103, https://doi.org/10.1016/j.quageo.2015.05.023, 2015.

Soulet, G., Ménot, G., Garreta, V., Rostek, F., Zaragosi, S., Lericolais, G., and Bard, E.: Black Sea “Lake” reservoir age evolution since the Last Glacial — Hydrologic and climatic implications, Earth Planet. Sci. Lett., 308, 245–258, https://doi.org/10.1016/j.epsl.2011.06.002, 2011.

Soulet, G., Skinner, L. C., Beaupré, S. R., and Galy, V.: A note on reporting of reservoir ¹⁴C disequilibria and age offsets, Radiocarbon, 58, 205–211, https://doi.org/10.1017/RDC.2015.22, 2016.

Soulet, G., Giosan, L., Flaux, C., and Galy, V.: Using stable carbon isotopes to quantify radiocarbon reservoir age offsets in the coastal black sea, Radiocarbon, 61, https://doi.org/10.1017/RDC.2018.61, 2019.

Southon, J., Kashgarian, M., Fontugne, M., Metivier, B., and Yim, W. W. S.: Marine reservoir corrections for the Indian Ocean and Southeast Asia, Radiocarbon, 44, 167–180, https://doi.org/10.1017/S0033822200064778, 2002.

Spencer, R. G. M., Stubbins, A., Hernes, P. J., Baker, A., Mopper, K., Aufdenkampe, A. K., Dyda, R. Y., Mwamba, V. L., Mangangu, A. M., Wabakanghanzi, J. N., and Six, J.: Photochemical degradation of dissolved organic matter and dissolved lignin phenols from the Congo River, J. Geophys. Res., 114, G03010, https://doi.org/10.1029/2009JG000968, 2009.

Spencer, R. G. M., Hernes, P. J., Aufdenkampe, A. K., Baker, A., Gulliver, P., Stubbins, A., Aiken, G. R., Dyda, R. Y., Butler, K. D., Mwamba, V. L., Mangangu, A. M., Wabakanghanzi, J. N., and Six, J.: An initial investigation into the organic matter biogeochemistry of the Congo River, Geochim. Cosmochim. Acta, 84, 614–627, https://doi.org/10.1016/j.gca.2012.01.013, 2012.

Spencer, R. G. M., Hernes, P. J., Dinga, B., Wabakanghanzi, J. N., Drake, T. W., and Six, J.: Origins, seasonality, and fluxes of organic matter in the Congo River, Global Biogeochem. Cycles, 30, 1105–1121, https://doi.org/10.1002/2016GB005427, 2016.

Stuiver, M. and Braziunas, T. F.: Modeling atmospheric ¹⁴C influences and ¹⁴C ages of marine samples to 10,000 BC, Radiocarbon, 35, 137–189, https://doi.org/10.1017/S0033822200013874, 1993.

Stuiver, M. and Polach, H. A.: Discussion: Reporting of ¹⁴C Data, Radiocarbon, 19, 355–363, https://doi.org/10.1017/S0033822200003672, 1977.

Stuiver, M., Pearson, G. W., and Braziunas, T.: Radiocarbon age calibration of marine samples back to 9000 cal yr BP, Radiocarbon, 28, 980–1021, https://doi.org/10.1017/S0033822200060264, 1986.

Tisnérat-Laborde, N., Poupeau, J. J., Tannau, J. F., and Paterne, M.: Development of a semi-automated system for routine preparation of carbonate samples, Radiocarbon, 43, 299–304, https://doi.org/10.1017/S0033822200038145, 2001.

Türkmen, A., Türkmen, M., and Tepe, Y.: Biomonitoring of heavy metals from Iskenderun Bay using two bivalve species Chama pacifica Broderip, 1834 and Ostrea stentina Payraveau, 1826, Turkish J. Fish. Aquat. Sci., 5, 107–111, 2005.

Westhoff, F.: Die Fauna der Kongo-Mündung, Jahresbericht der Zool. Sekt. Westfälischen Prov. Vereins für Wiss. und Kunft für das Etatsjahr 1885-86, 38–40, 1886.

Williams, R. G. and Follows, M. J.: Ocean dynamics and the carbon cycle: Principles and mechanisms, Cambridge, 2015.
<figure><img src="image_46.png" /></figure>

Cambridge, 434 pp., 2011.

<figure><img src="image_47.png" /><figcaption>Figure 1: The geographic distribution of marine reservoir age values along the West African coast. A. ΔR values. B. R values. Data shown in black are from this study. Other are selected results from previous studies discussed in the text, converted from their original format (conventional 14C ages and collection dates) to ΔR and R values using the latest calibration curves Marine20 (Heaton et al., 2020) and Intcal20 or SHcal20 (Reimer et al., 2020; Hogg et al., 2020), respectively. Data in blue are from Ndeye (2008), data in green are from (Reimer and McCormac, 2002), data in purple are from (Siani et al., 2000) and data in pink are from (Dewar et al., 2012). eJ, Nh, Dk, LI, IC and Lu stand for el Jadida (Morocco), Nouadhibou (Mauritania), Dakar (Senegal), Loos Islands (Republic of Guinea), Ivory Coast and Luanda (Angola). The map was drawn using Ocean Data View (Schlitzer, Reiner, Ocean Data View, https://odv.awi.de, 2022).</figcaption></figure>
<figure><img src="image_44.png" /></figure>

Cambridge, 434 pp., 2011.

<figure><img src="image_45.png" /><figcaption>Figure 1: The geographic distribution of marine reservoir age values along the West African coast. A. ΔR values. B. R values. Data shown in black are from this study. Other are selected results from previous studies discussed in the text, converted from their original format (conventional 14C ages and collection dates) to ΔR and R values using the latest calibration curves Marine20 (Heaton et al., 2020) and Intcal20 or SHcal20 (Reimer et al., 2020; Hogg et al., 2020), respectively. Data in blue are from Ndeye (2008), data in green are from (Reimer and McCormac, 2002), data in purple are from (Siani et al., 2000) and data in pink are from (Dewar et al., 2012). eJ, Nh, Dk, LI, IC and Lu stand for el Jadida (Morocco), Nouadhibou (Mauritania), Dakar (Senegal), Loos Islands (Republic of Guinea), Ivory Coast and Luanda (Angola). The map was drawn using Ocean Data View (Schlitzer, Reiner, Ocean Data View, https://odv.awi.de, 2022).</figcaption></figure>
<figure><img src="image_48.png" /></figure>

<figure><img src="image_49.png" /></figure>

<figure><img src="image_50.png" /></figure>

<figure><img src="image_51.png" /><figcaption>Figure 2: Left axis: The radiocarbon age evolution of the atmosphere (IntCal20; green curve with its light green 1-σ envelope) and of the global ocean (Marine20; blue curve with its light blue 1-σ envelope) between 1850 and 1950 AD. Right axis: The global marine reservoir age (black curve with its grey 1-σ envelope) calculated as the difference between Marine20 and Intcal20 curves. Pink symbols are the coastal West African marine reservoir age calculated averaging data over 5-yrs windows. The reported error bars are the maximum of the standard deviation of the averaged data and the individual uncertainty of the averaged data.</figcaption></figure>

820
<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /><figcaption>Figure 2: Left axis: The radiocarbon age evolution of the atmosphere (IntCal20; green curve with its light green 1-σ envelope) and of the global ocean (Marine20; blue curve with its light blue 1-σ envelope) between 1850 and 1950 AD. Right axis: The global marine reservoir age (black curve with its grey 1-σ envelope) calculated as the difference between Marine20 and Intcal20 curves. Pink symbols are the coastal West African marine reservoir age calculated averaging data over 5-yrs windows. The reported error bars are the maximum of the standard deviation of the averaged data and the individual uncertainty of the averaged data.</figcaption></figure>