# At the edge of the desert: The evolution of the Holocene environment in Lake Rkiz, Senegal River valley

Sandrine Grouard, Anne-Marie Lézine

► To cite this version:

Sandrine Grouard, Anne-Marie Lézine. At the edge of the desert: The evolution of the Holocene environment in Lake Rkiz, Senegal River valley. Quaternary International, 2023, 667, pp.41-50. 10.1016/j.quaint.2023.05.023. hal-04201971

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1 At the edge of the desert: The evolution of the Holocene environment in Lake Rkiz, Senegal

2 River Valley

3

4 Sandrine Grouard<sup>a,*</sup>, Anne-Marie Lézine<sup>b</sup>

5

6 <sup>a</sup> Archéozoologie et Archéobotanique : Sociétés, Pratiques et Environnements (AASPE),

7 CNRS UMR 7209, Muséum national d'Histoire naturelle, Paris, France

8 <sup>b</sup> Laboratoire d'Océanographie et du Climat - Expérimentations et Approches Numériques

9 (LOCEAN/IPSL), CNRS UMR 7159, Sorbonne Université, Paris, France

10 * Corresponding author.

11 E-mail address: sandrine.grouard@mnhn.fr (S. Grouard).

12

13

14 Abstract

15 The analysis of faunal remains, pollen grains and other palynomorphs from a 4 m long core
16 recovered at Lake Rkiz, Mauritania (16.911420 N, -15.172668 W, 0 m asl) documents the
17 major phases of marine intrusion in the Senegal River system during the Holocene. Lake Rkiz
18 was exposed to marine waters from 10 to 4 ka: a brackish lagoon developed at 10 ka,
19 followed by a mangrove between 8.7 and 7 ka both in a calm environment. *Rhizophora*
20 populations were never developed, however, either because they did not reach the core site at
21 the northern extremity of the lake, or because they were restricted to the Senegal River bed.

22 After 7 ka, the environment became increasingly unstable and the mangrove was replaced by
23 a more clearly marine environment. During the last two millennia, a new, discrete phase of
24 marine intrusion is recorded at Lake Rkiz. Contemporary to that already noted in the Ferlo
25 River valley which feeds the Senegal River through Lake Guiers to the south, it also testifies
26 to a positive oscillation of the sea level. The evolution of the regional vegetation of Lake Rkiz
27 follows that of North tropical Africa with the expansion of a gallery forest of tropical humid
28 affinity dominated by *Celtis* during the AHP. As elsewhere in the Sahara and Sahel, tropical
29 plant types co-occurred with Saharan ones attesting to a mosaic-like landscape.

30

31 **Keywords:** Senegal River; Holocene mangrove; Marine intrusion upstream; Sea level;

32 Pollen; Microfauna & mollusks

33

34

35 **1. Introduction**

36
37 The recent paleoenvironmental evolution of the Senegal River delta has been impacted by a
38 combination of two distinct factors: The massive increase in humidity during the “African
39 Humid Period” (AHP) that led to the wide dispersion of tropical plants across the Sahel and
40 the Sahara (Watrin et al., 2009; Hély et al., 2014) and the post-glacial sea level rise from
41 about -130 m during the Last Glacial Maximum (LGM) to the present level about 7000 years
42 ago (Fleming et al., 1998) responsible for the penetration of the sea in low-lying coastal areas
43 of western Africa (Faure et al., 1980). Tropical trees (e.g., *Alchornea*, *Macaranga*, *Elaeis*
44 *guineensis*, *Tetracera*, *Celtis*, *Ficus*, *Syzygium*) formed dense gallery forests as soon as 10.7
45 ka in the interdunal fens of the Niayes area of Senegal located behind the Atlantic coastal
46 strand between 15° and 16°N. Humid tropical trees were also present in the catchment area of
47 Lake Guiers (Lézine, 1988) and in the Senegal River bed (Michel and Assémien, 1969;
48 Médus, 1984). At the end of the AHP between 3000 and 2500 cal yr BP (labelled as 3 and 2.5
49 ka in the text), the degradation of environmental conditions has led to the destruction of these
50 tropical gallery forests (Lézine, 2022). They still persist locally as relict vegetation formations
51 in the Niayes owing to the presence of a water table near the surface (Trochain, 1940), but
52 they disappeared from the banks of the Senegal River and its tributaries during the last few
53 centuries (Fofana et al., 2020; Lézine et al., 2023).

54 The increase in humidity during the AHP was also responsible for the massive development
55 of tropical humid *Rhizophora* mangroves on the West African coast during phases of
56 stabilization of the post-glacial sea level rise (Lézine, 1997). The morphology of the coastal
57 areas played a major role in the wide expansion of these mangroves, particularly in the
58 Senegal River area (Faure et al., 1980). In its lower valley, from the coast to approximately
59 225 km to the east, the bottom of the Senegal River bed is about 5 m below the present-day
60 sea level (Michel, 1973). This geomorphological feature facilitated the penetration of marine
61 waters eastward several hundred kilometers from the present coast and the development of
62 mangroves during periods of sea level rise in the Holocene (Trochain, 1940; Michel and
63 Assémien, 1969; Médus, 1975; Médus and Monteillet, 1981; Lézine et al., 1988). However,
64 our understanding of the Holocene evolution of the West African mangrove is severely
65 hampered by the lack of continuous records. Mangrove levels (most of the time in the form of
66 a peat layer a few centimeters thick) (in Lézine, 1997 and references therein) and shell
67 accumulations forming marine terraces in the interior were used to reconstruct the coastline
68 and its variations over the last millennia (e.g., Faure et al., 1980). The irregular and often
69 truncated nature of the sedimentary archives does not permit a continuous reconstruction of
70 the littoral evolution throughout the Holocene. Possible shore level tilt along the Senegal
71 River delta during the post glacial sea level rise may have introduced an additional difficulty
72 in our understanding of paleoenvironmental changes in this area (Faure et al., 1980).

73 In this paper we present a micro-faunal and palynological record from a continuous
74 sedimentary sequence from lake Rkiz (16.911420 N, -15.172668 W, 0 m asl) in order to
75 identify the timing and amplitude of the successive phases of marine intrusion upstream in the
76 Senegal River system from 10 ka to the present.

77
78
79
## 2. Environmental setting

80 Lake Rkiz occupies a large interdunal depression (Aftout ech Cherguil) north of the Senegal
81 River, about 110 km east of the present coastline (Fig. 1). The NE-SW orientation of the
82 depression is governed by that of the sand dune alignments established during the LGM over
83 much of southern Mauritania and northern Senegal (Michel, 1973). ). Lake Rkiz is about 5 km
84 wide and 30 km long. It is a shallow lake (maximum depth 2.5 m), hydrologically connected
85 to the Senegal River through a network of multiple channels. The hydrological regime of the
86 lake reflects that of the river in its lower valley. During the rainy season (from June to
87 October) Lake Rkiz is supplied by the floodwaters of the Senegal River and its level can rise
88 by 3.5 m. During the dry season, when the inflow from the Senegal River ceases, the lake may
89 dry up completely.

90 The regional climate is characterized by ca. 185 mm annual rainfall during the rainy season
91 from June to October (maximum 79 mm in August) and maximum temperatures between 33
92 and 40°C. As a consequence, the vegetation is of Sahelo-Saharan steppic type with a
93 discontinuous herbaceous cover mainly composed of *Cenchrus biflorus*. Trees (*Acacia
94 raddiana*, *A. scorpioides*, *Salvadora persica*, *Balanites aegyptiaca*)... are scarce (Adam,
95 1960; 1965).

96
97
98
99
## 3. Material and Methods

100 Core Rkiz-1 was collected in 1985 at the mouth of Wadi Mrea at the NE end of the lake (16.911420 N, -15.172668 W, 0 m asl) using a Russian manual corer (Fig. 1). The
sedimentary sequence is 4m long (Fig. 2). Three main units can be distinguished: from the base to 349 cm (Unit I), the sediment consists of a homogeneous light-yellow sand likely derived from the dune sands deposited during the LGM. It is overlaid by a greenish clay layer containing abundant malacological, crustacean and fish bones fragments, witnesses to the Holocene marine transgression (Unit II: 349-190 cm). The upper unit (Unit III: 190-0 cm) is composed of sandy clay with fine sand and gravel intercalations. This testifies for a less homogeneous environment with runoff deposits from the lake edges (such as the ferruginous gravels, Michel, 1973), lake and mangrove organic material and eolian sands. Thirty-six samples were taken at 10 cm intervals for faunal and vegetation analysis.

10

## 3.1 Dating control

11

The core chronology was constructed using ¹⁴C accelerator mass spectrometry (AMS) dates on seven samples of marine shells (*Pachymelania fusca*) and bulk sediment (Fig. 2). Radiocarbon dates on shells and shell fragments were calibrated using the Marine 20 calibration curve (Heaton et al., 2000) from Calib 8.2 software (Stuiver and Reimer, 1993) taking into consideration the reservoir-age of 511 ± 176 years, calculated for the sector of the West African coast closest to the site, facing the Senegal River delta (Ndeye, 2008). The bulk organic sample at 95 cm was calibrated using the IntCal 20 calibration curve (Reimer et al., 2020). Dates are given in calibrated 1000 years (ka) BP. The age-depth model was then derived from a linear interpolation between two calibrated ¹⁴C ages using CLAM (Blaauw, 2010). We assume a constant reservoir-age throughout the Holocene time.

12

## 3.2 Faunal remains

15

The faunal samples were sieved under water on a 0.5 mm mesh and then sorted under a binocular microscope. Only 18 samples between 370 and 190 cm depth yielded identifiable remains and the uppermost samples were found to be sterile (Fig. 3).

Anatomical and taxonomic identifications were conducted using a comparative reference collection from AASPE (library of skeletons and tests), literature (Dautzenberg 1910, 1912, Gregory 1933, Bouchet 1977, Pointier & Lamy 1998, Humann & DeLoach 1993, Grouard 2001, Serrand 2002, Ardovini & Cossignani 2004, Ahmedou Salem et al. 2014, Carpenter & De Angelis 2016, Diedhiou et al. 2018, von Cosel & Gofas 2019, Taylor & Glover 2021), and internet resources (MolluscaBase 2022, World Register of Marine Species 2022). The nomenclature was updated from the WoRMS site.

Quantification of faunal remains used multiple measures often used in zooarchaeological analyses (NRd: Number of Determined Remains and MNI: Minimal Number of Individuals). Bivalves were classified into three groups (intact valve, broken valve but with umbo, and fragments (Claassen, 1998; Giovas, 2009; Manca, 2017)). The Bivalve MNI was estimated from the lateralized complete valves, lateralized hinges and hooks, edge and complementary non-repetitive elements which allow, according to the cases, the maximum estimate. The Gastropod fragmentation degrees were classified in several groups (complete, sub-complete, complete without apex, complete without opening, margin and lip, lip, apex, apex and spire, first spire, fragment of spire, parietal margin, spine, fragment of columella, fragment of opening or base, umbilicus, fragment of base with umbilicus, columella and opening, operculum. The Gastropod MNI was calculated based on complete shells, number of apexes and complementary non-repetitive elements which allows, on a case-by-case basis, the maximum estimate of complete labials (Serrand, 2002; Giovas et al., 2013; Tomadini, 2018).

In total twenty-eight taxa of Bivalvia, Gastropoda, Cnidaria, Crustacea and Teleostei were identified out of 2610 remains. They were grouped according to their ecological affinity in 4
groups (marine, estuarine, mangrove, freshwater) according to World Register of Marine Species 2022 (S-table 1; S-fig. 1).

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## 3.3 Pollen and NPP

155

Pollen samples were processed using the standard HF method described by Faegri and Iversen (1977). After sieving in a 5 µm mesh, the residue was stained with safranin and mounted in glycerine. Pollen identification was based on the reference slide and photo collections of the African Pollen Database (APD) (https://africanpollendatabase.ipsl.fr). Pollen nomenclature, which follows the standards defined by the APD (Vincens et al., 2007), was based on Lebrun and Stork (1991-2015). The pollen microflora was composed of 136 taxa (S-table 2). Pollen taxa were grouped according to the phytogeographical and ecological affinity of the parent species according to Trochain (1940) and Lebrun (1973), among others, with distinction of Saharan, Sahelian, Sudanian and Guineo-Congolian pollen types, mangrove, wet indicators and undifferentiated or common to all the phytogeographical entities mentioned above (including Poaceae). Pollen percentages were calculated against a sum excluding mangrove pollen types, wet pollen types (including Cyperaceae) and ferns, the percentages of which were calculated separately against a sum including all the pollen grains and fern spores counted. Non-Pollen Palynomorphs (NPP) include algae (mainly *Pediastrum* and *Botryococcus*), undifferentiated Foraminifera, Dinocysts and Fungi. Their percentages were calculated against the total NPP.

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The Shannon Diversity index (Pielou, 1966) which accounts for both abundance and evenness of the taxa present was used to evaluate the diversity of pollen and faunal remains using Past 4.03 software (https://www.nhm.uio.no). Both pollen and faunal data are presented under the form of percentage diagrams using Tilia software (Grimm, 1991).

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# 4. Results

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## 4.1 Faunal remains

181

### 4.1.1 Diversity

The MNI calculated is 1312. Gastropoda largely dominates this collection with an MNI of 684 calculated from 886 identified remains (NRd), immediately followed by Crustacea (MNI= 426; NRd=848), and Bivalvia (MNI=185, NRd762). Cnidaria and Teleostei are poorly represented with MNI= 13 (NRd=122) and MNI=4 (NRd=4), respectively (Fig. 3).

187

Maximum of Gastropoda and Crustacea is present at the 8 ka level. These two groups are then replaced by Bivalvia which reach peaks at two distinct levels respectively dated at 7.3 then 6.3 ka. Bivalvia are the most diverse with 12 species and 9 families present, largely accounting for the maximum Shannon diversity index reached at 7 ka. They are followed by Gastropoda with 7 species and 7 families present. On the contrary, only one species of Teleostei, 1 Cnidaria undiff., and 2 Crustacea (Cirripedia and Ostracoda) were present.

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### 4.1.2 The mollusk assemblages

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Figure 4 shows the diversity and MNI of the main mollusk taxa, the preservation of which allows distinguishing two main zones. From 10 to 7 ka (zone 1), the preservation rate is high (mean=76%), though fluctuating from 100% to 32%. It abruptly falls to values as low as 3% in the overlying zone 2 (mean=13%).
201
202 Three sub-zones can be distinguished within zone 1:
203 - Zone 1a: From 10 to 8.7 ka, the presence of *Peringia ulvae* and the highest values of the preservation rate indicate a sheltered lagoon, out of the direct influence of waves;
204
205
206 - Zone 1b: From 8.7 to 7.5 ka: *Peringia ulvae* was associated with *Pachymelania fusca*, both largely dominating the mollusk assemblages, suggesting the progressive development of a mangrove at the coring site. Freshwater inputs are indicated by the presence of Ampullariidae (*Lanistes ovum*). The lower values of the preservation rate compared to zone 1a (75-50% versus 100%) suggest more dynamic environment;
207
208
209
210
211 - Zone 1c: From 7.5 to 7 ka: *Peringia ulvae* and *Pachymelania fusca* strongly decreased and the mollusk assemblage greatly diversified with the appearance of marine and estuarine species such as various Veneridae and *Ungulina rubra*. The high amplitude of the variations in the preservation rate suggests the alternation of phases of calm seas and strong swells.
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213
214
215
216 Zone 2, from 7 to 5.6 ka, is characterized by the disappearance of the mangrove species and their replacement by marine ones, as shown by the high MNI of Veneridae and Lucinidae species. The lowest values of the preservation rate for this zone indicate significant tidal and backwash movements.
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218
219
220

221 4.2 Pollen and NPP

222
223 The pollen diagram (Fig. 5) is characterized by the dominance of the herbaceous taxa Poaceae undiff. and various types of Amaranthaceae whose cumulative percentages reach up to 90% of the pollen sum, indicating the presence of an open environment throughout the Holocene.
224
225
226 As usual in tropical Africa, the pollen percentages of trees are rarely higher than 1% with the exception of Combretaceae, *Celtis* and *Rhizophora* which reach up to 9%, 23% and 7% respectively.
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228
229

230 Three main zones can be distinguished from the pollen grain, spores and NPP diagram:
231
232 Zone 1 (10-8.7 ka) is typified by exceptionally high percentages of *Celtis* (maximum: 23%).
233 Among the herbaceous plants, taxa from the lake margin such as *Typha* and Cyperaceae, are present. *Botryococcus* dominates the NPPs suggesting shallow and nutrient poor fresh to brackish water. The presence of *Ephedra* pollen grains, even in percentages lower than 1% suggest a Saharan (desert) influence. (This pollen zone corresponds to Zone 1a of the mollusk assemblages).
234
235
236
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238

239 Zone 2 (8.7-4.5 ka) records the decline of *Celtis*. The other tropical tree taxa, notably
240 *Alchornea* and various types of Combretaceae remain present though in small percentages.
241 Among the herbaceous plants, the abundance of Amaranthaceae and in particular
242 *Amaranthus/Chenopodium*-type and *Suaeda* allows identification of sub-zone 2a (8.7-7.1 ka)
243 (= Zones 1b and 1c of the Mollusk assemblages). The presence of *Suaeda* suggests saline
244 conditions while the presence of Fungi confirms the, partial or seasonal, desiccation of Lake
245 Rkiz at the core site. The replacement of *Botryococcus* by dinoflagellates within the NPP
246 indicates that a marine influence was present, which is confirmed by the occurrence of
247 *Rhizophora* mangrove pollen grains. Pollen types present in subzone 2a continue into the
248 overlying subzone 2b (7.1-4.5 ka) (= Zone 2 of the Mollusk assemblages) though in smaller
249 percentages, suggesting drier conditions. The NPPs suggest an unstable environment with
alternating saline (Dinoflagellate cysts in particular at 6.2 ka) and freshwater to brackish
(Botryococcus) conditions.

252

Zone 3 (4.5 ka-modern) records a change in the environment with the disappearance of mangrove taxa. In sub-zone 3a (4.5-2.4 ka) *Celtis* and *Alchornea* disappear but the presence of e.g., *Lannea* suggests that the tropical influence was still present, although significantly reduced. After 2.4 ka (sub-zone 3b (2.4-1.6ka)) the Sahelian taxa *Acacia* and *Commiphora* which are associated with the Fabaceae family increase, indicating the transition to the modern environment. Rare tropical plants of Sudanian or more humid types (e.g., *Lannea*, *Macaranga*, *Bridelia...*) are occasionally present but in percentages lower than 1%, and the reappearance of the Saharan taxa (*Ephedra*) indicates the return to dry conditions. The lacustrine environment is dominated by *Botryococcus*. The populations of the lake edge develop progressively with *Typha* in sub-area 3b then Cyperaceae in sub-zone 3c. The predominance of fresh-water during sub-zone 3c (1.6-0.83 ka) and 3d (0.83-modern) indicated by the dominance of *Pediatrum* in the NPP assemblages is interrupted around 1.3 ka by the discrete reappearance of mangrove taxa and dinoflagellates, testifying for the brief return of marine influence.

267

5. Discussion

269

5.1 The Holocene sea level variations and their impacts in the Lake Rkiz area

271

5.1.1 The *Rhizophora* mangrove

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The Senegal River delta is the northernmost limit of the major West African mangrove
species, *Rhizophora racemosa* (Trochain, 1940; Adam, 1965; Spalding et al., 1997). Today
this tropical humid species is present only as small populations or isolated individuals along
the creeks closest to the coast in the Senegal River delta near Saint Louis (Senegal) or on the
Thiong island (Mauritania) at roughly 16°N. As the sea level rose after the LGM, and thanks
to the intensification of the monsoon rains, it spread widely along the West African coasts
during the AHP (Lézine, 1997) as far north as 21.25°N (core GIK16017-2, Hooghiemstra
1988). In the Senegal River area, *Rhizophora* was present on the continental shelf opposite the
present-day delta at 13.8 ka (core Rosilda N 110-Z; Lézine, 1987). It penetrated eastward in
the rear delta after 11.4 ka (Médus and Monteillet, 1981) and in the Potou interdunal
depression at the southern limit of the delta (Lézine, 1987). In this context, and given the
topography of the Senegal River bed, the intrusion of the coastal and marine influence up to
Lake Rkiz from 10 to 4.7 ka is not unlikely. Between 10 and 8.7 ka, the low faunal diversity,
the only occasional presence of dinoflagellates and the very low percentages of *Rhizophora*
pollen, show that the mangrove was only slightly developed and that *Rhizophora* populations
did not reach the core site, i.e., the most distant sector from the channels connecting Lake
Rkiz to the Senegal River. Excellent faunal preservation eliminates the potential long-distance
transport and suggests the presence of a lagoon protected from the direct influence of swells
and strong water currents. The time interval between 8.7 and 4.7 ka during which the
mangrove developed at Rkiz encompasses the transgressive episode of the "Nouakchottien"
(Elouard, 1968). During this episode centered around 6-5 ka, the sea level was higher by 1 to
2 m compared to its current position (Faure et al., 1980). Sea water intrusion inland, in the
rear delta, and along the Senegal River and its tributaries, is attested by numerous
accumulations of marine shells forming terraces (e.g., Faure et al., 1980 and references
therein) (Fig. 6 and 7A; S-table 3), palaeobotanical (*Rhizophora* pneumatophores) and
botanical (relict species of mangrove forests) observations (Trochain, 1940), and pollen
analyses (Médus, 1975; Médus and Monteillet, 1981; Lézine, 1988). According to Michel and Assémien (1969), *Rhizophora* populations could have spread up to Bogué, 250 km east of the present coast in the bed of the Senegal River during this episode (Fig. 6). At Rkiz, the mangrove was progressively exposed to increased marine influence after 7 ka and a more dynamic environment subject to waves, marine and freshwater inputs was established as shown by the important presence of Bivalvia, Veneridae (notably *Pelecyora isocardia*) and Lucinidae in the fauna, the strong fragmentation of faunal remains and the alternation of dinoflagellates and freshwater algae within the NPP assemblage. Increased currents and wave turbulence led to the complete destruction of the faunal remains from the sediment at 5.6 ka. However, *Rhizophora* pollen grains show that the mangrove influence persisted up to 4.2 ka. These results agree with previous studies showing that the decline of *Rhizophora* mangroves along the northern coast of Senegal occurred, according to the sites, between 5.6 ka at 14.9°N (Thiaye, Lézine et al., 1985) and 3.6 ka off the Senegal River delta at 16.0°N (Core GeoB 9503-5, Bouhimetarhan et al., 2009). Evidence of marine influence disappeared from the pollen and NPP assemblages between 4.2 and 2.1 ka that closely corresponds to the “Tafolian” sea level regression (Hébrard, 1972). Then, the renewed presence of a few *Rhizophora* pollen grains between 2.1 and 0.6 ka shows that the last, discrete episode of saline intrusion into the Senegal and Ferlo Rivers during the last millennium (Faure et al., 1980; Monteillet et al., 1981; Fofana et al., 2020) also reached Lake Rkiz. The absence of faunal remains in the sediment and the very low *Rhizophora* pollen percentages confirm that this event was minor.

321

### 5.1.2 Vertical movements in the Rkiz Lake area during the Holocene?

322

The elevation of mangrove levels and shell accumulations have been used to reconstruct the long-term evolution of sea level rise off Senegal: The peat layer in the Rosilda N 110 Z core located at -60 m below sea level is dated 13.8 ka (Lézine, 1987). In the rear delta of the Senegal River Médus and Monteillet (1981) identified a *Rhizophora* mangrove level at -33.6 m dated at 11.4 ka in line with the dating of the Eudoxie and Lea long cores by Faure et al. (1980) (9.9 ka and 9.4 ka at -23 and -20.5 m, respectively). All these dates are consistent with the general trend of the post-glacial sea level rise (Fleming et al., 1998). The most surprising result of our study is the depth at which the early Holocene mangrove development has been identified at Rkiz. There is a difference of about 20 m with the depth of the contemporary levels identified in the rear delta of the Senegal River (Fig. 7B). Early Holocene vertical movements are therefore suggested by our data in the sector of Lake Rkiz located in a region known to have also been affected by significant deformations during the Quaternary, such as the subsidence of the Senegal River delta, and the faulting of the “Dôme de Guiers” (Michel, 1973; Barusseau et al., 2009).

338

## 5.2 The regional vegetation

340

During the AHP, lakes and wetlands expanded in the present day Sahara and Sahel (Lézine et al., 2011) while large paleodrainages were formed or re-activated (Drake et al., 2011), in response to the increase in monsoon rainfall. As a result, tropical plants widely spread into the Sahel and the Sahara (Watrin et al., 2009). Due to the scarcity of data in these (semi-) arid environments where lacustrine archives are poorly preserved, the major difficulty in understanding this profound environmental change was to identify how the tropical humid plants could have migrated north and where they developed. Instead of a general displacement of the vegetation zones towards the north and therefore the development of a more or less wooded savannah in the Sahara and the Sahel (Hoelzmann et al., 1998), pollen data show the
350

presence of gallery forests of tropical humid type (Sudanian to Guineo-Congolian) in a drier
regional environment. It has been suggested that tropical plants have used the banks of the
rivers where they benefited from the moisture of the soil as migration routes to enter the Sahel
and Sahara (Watrin et al., 2009; Hély et al., 2014). At Rkiz the regional landscape has always
been extremely open as shown by the percentages of herbaceous taxa, which may be equal to
or higher than 90% in total. Tropical plants (Guinean-Congolian, Sudanian) have co-existed
with typically Sahelian species (e.g., Acacia, Commiphora) and even Saharan species
(Ephedra, Artemisia). This mixture of plants, which today can only be found in distinct
vegetation zones, explains the peak in diversity observed between 8.8 and 8.3 ka, which is
consistent with the Holocene forest optimum in tropical North Africa (Lézine, 1989) (Fig. 8).

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Tropical influence at Lake Rkiz persisted until 2.4 ka which is consistent with what we know from the end of the AHP in Western Africa (Lézine, 2021). In the nearby Niaye region along the Atlantic coast between Dakar and St Louis for instance, the dense gallery forests, which widely developed during the AHP dramatically collapsed around 2.5 ka. At Rkiz, the decrease of tropical taxa was gradual and an arid phase emerged as early as 4 ka. The discrete reappearance of tropical taxa after 2 ka suggests the forest galleries that extended along the Senegal River during the Holocene had not completely disappeared over the last millennia.

368

They were still present in the Saint Louis area as late as 0.35 ka (Fofana et al., 2020).

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Thirty-six pollen taxa of humid tropical type were found at Rkiz. This number is surprisingly similar to that observed on the opposite bank of the Senegal River, about 95 km to the south at Guiers 3 (16.28°N) (Lézine, 1988). However, the percentages calculated over the entire period covered by the pollen series are on average two and a half times lower at Rkiz (6%) than at Guiers (15.5%). Comparing all pollen sites in Senegal and Mauritania, humid tropical taxa are 68 at Diogo (15.2°N) where they account for an average of 25% of the pollen sum. These values drop to 37 taxa and 15.5% at Guiers 3, 36 taxa and 6% at Rkiz and finally 15 taxa and 1.5% at Chemchane (20.93°N). The contrast between the percentages of tropical taxa between Guiers and Rkiz only less than a hundred kilometers apart suggests that the Senegal River may have constituted a barrier to the northward migration of tropical plants during the AHP in north west Africa. One exception is *Celtis*, which has reached exceptionally high percentages at Rkiz (23%) and so far unreported in tropical North Africa during the Holocene.

386

The modern distribution of *Celtis toka* does not exceed 16°N (Fig. 9). During the Holocene, it expanded widely into Mauritania (Monod, 1983) and more widely across the whole Sahara to southern Libya at 24.8°N (Mercuri, 2008). However, *Celtis* percentages never exceeded 4% at any of the Saharan and Sahelian pollen sites, suggesting that, with the exception of Rkiz, this species never expanded much within the gallery forests.

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## 6. Conclusion

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398

399

Our study shows that, despite a substantial distance of more than 180 km from the mouth of the Senegal River, Lake Rkiz was subject to marine water influx during two distinct episodes of sea level rise. The first one, from 10 to 4.5 ka, is a long period during which the environment of Lake Rkiz changed from a calm coastal environment evolving from a lagoon to a mangrove to a more pronounced, unstable, marine environment. Two major features emerge from our study: (1) the elevation of the marine influence reported in core Rkiz-1, is significantly higher than what is known from the global sea level curve, which suggests important vertical movements at Lake Rkiz (2) the “Nouakchottian” transgression, that led to the development of mangroves in Lake Guiers and in the Senegal River bed up to Bogué during the Middle Holocene, resulted in a highly variable environment at Rkiz as evidenced by the increased fragmentation in the faunal remains. A second discrete phase of marine
intrusion is detected during the last 2 millennia, after the regressive phase of the “Tafolian”. 400
Regional vegetation at Rkiz was open, dominated by herbaceous plants throughout the 401
Holocene. Tropical trees however formed gallery forests along the river and lake beds. These 402
results are consistent with the general pattern observed in the landscape evolution of West 403
Africa during the Holocene. 404

405

## Author contributions 406

AML and SG designed research; AML performed fieldwork; AML and SG performed 407
research: AML palynology and SG zooarchaeology; AML and SG analyzed data; AML and 408
SG wrote the paper with contributions from both authors. 409

410

## Data availability 411

Pollen data are deposited in the African Pollen Database (APD) 412
(https://africanpollendatabase.ipsl.fr: DOI:https://doi.org/10.14768/apd.0bba-rk05). Data for 413
fossil *Celtis* distribution are from the APD (https://africanpollendatabase.ipsl.fr), and for the 414
modern one from GBIF (https://www.gbif.org) and MNHN (https://www.mnhn.fr) databases 415
and the literature. Microfauna data are presented in the Supplementary Material. 416

417

## Declaration of competing interest 418

The authors declare that they have no known competing financial interests or personal 419
relationships that could have appeared to influence the work reported in this paper 420

421

## Acknowledgements 422

The field extraction of the Rkiz core dates from AML's thesis period in the 1980s. We thank 423
the Mauritanian authorities for research authorizations, J.-L. Saos (IRD) and B. Duboscq 424
(IFAN) for their assistance in the field and K. Lemonnier (CNRS) in the laboratory. Thanks to 425
N. Tomadini (MNHN) for assisting with the mollusk's identification, to T. Cucchi (CNRS) 426
and M. Lemoine (MNHN) for providing access to the MNHN bioarcheological image 427
facilities, to K. Debue (CNRS) and S. Brehard (MNHN) for providing access to the osteo- 428
library of the AASPE Laboratory, to M. Mandeng-Yogo (IRD) and M. Leydet (CNRS) for 429
their help in improving the chronological framework, and to A. Bain (U. Laval) and S. 430
Perdikaris (U. Nebraska) for language revision on the manuscript. 431

432

433

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627
628
629 List of Figures
630

630
631 Figure 1: Lake Rkiz. A: The general map is from en-us.topographic-map.com. The dashed rectangle indicates the Lake Rkiz area detailed in Figure 1B. B: The detailed map of Lake Rkiz is redrawn after Vache-Grandet (1959). The shaded area within the lake indicates the area of the lowest level identified by the author in 1954. The Lake was entirely dry at the time of coring. The star indicates the location of core Rkiz1 (this study).

636
637 Figure 2: Core Rkiz1: Lithostratigraphy and ¹⁴C ages.

638
639 Figure 3: Core Rkiz 1 Minimum Number of Individuals (MNI) versus age. From left to right, this figure shows the MNI of all the faunal remains encountered. On the right, the preservation index and the Shannon diversity index are shown.

642
643 Figure 4: Core Rkiz 1 synthetic diagram of MNI of the mollusk taxa versus age. On the right, the preservation and diversity indexes are shown. Pollen zones are shown to the right of the figure for easy comparison.

646
647 Figure 5: Core Rkiz 1 synthetic diagram showing, versus age from left to right, the main pollen and NPP taxa. Percentages are shown in black, gray shading shows the same results,
multiplied by 10 for highlighting. Mollusk zones are shown to the right of the figure for easy comparison.

Figure 6: Location map of sites attesting to the marine influence in the area of the Senegal River and its delta during the Holocene. Black circles indicate pollen sites, gray circles indicate marine shells, and white squares represent peat samples. At Lake Rkiz, the black square show our coring site, while the diamonds indicate shell samples from Faure et al.(1980). Satellite image from Google Earth.

Figure 7: Dated evidence of marine intrusion along the Senegal River and Lakes Guiers and Rkiz (S-Table 3). The colored circles correspond to dates extracted from marine shells collected in 3 major locations: from the West and East: the delta, Lake Guiers and the Ferlo river, and Lake Rkiz. Red circles correspond to the dates of this study at Lake Rkiz, pink circles to those of Faure et al. (1980). Squares correspond to the dates of the long cores from the bed of the Senegal River (white squares are dates on bulk sediment (peat); blue squares are dates from shells. A: the dates are presented as a function of time and longitude in order to evaluate the distance of the marine intrusion from the coast, B: as a function of (non corrected) altitude/depth when indicated by the authors. The dark gray arrows indicate the average position of the two Holocene marine transgressions. The light gray double arrow highlights the difference in depth between the sea level records in the Senegal River bed (Eudoxie, Leo, Keur Marsal) and in Lake Rkiz.

Figure 8: Synthetic diagrams showing the main features of the local (on the right) and regional (on the left) environment of Lake Rkiz from 10 ka to the present. On the right, mangrove pollen percentages (dark purple) are calculated against the sum of all pollen and fern spores. Light shading shows the same results, multiplied by 10 for highlighting. Percentages of mangrove (purple triangles) and marine (black dots) fauna are calculated against the sum of all the determined mollusk shells (in black: power trend line). Only samples with more than 20 determined remains are represented. On the left, pollen data are presented under the form of phytogeographical group of plants from the most humid (Guinean/Sudanian tropical taxa) to the most arid (Saharan desert) (aquatics and ferns excluded from the pollen sum).

Figure 9: Modern (red dots) and Holocene (green dots) distribution of *Celtis toka* in tropical West Africa. Pollen data are from the APD (https://africanpollendatabase.ipsl.fr), modern data are from the GBIF (https://www.gbif.org) and MNHN (https://www.mnhn.fr) databases, and the literature (Trochain, 1940; Hutchinson and Dalziel, 1954-1972). Simplified vegetation map from White (1986).
<figure><img src="image_2.png" /></figure>

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

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /></figure>

Radiocarbon dating

<table><thead><tr><td>Mean depth (cm)</td><td><sup>14</sup>C age BP</td><td>error</td><td>Laboratory number</td><td>Material dated</td></tr></thead><tbody><tr><td>5</td><td>101,63 +/-0,38</td><td></td><td>Beta-564997</td><td>Bulk organic</td></tr><tr><td>95</td><td>1790</td><td>30</td><td>Beta-564998</td><td>Bulk organic</td></tr><tr><td>205</td><td>6340</td><td>30</td><td>Beta-564999</td><td>Shells</td></tr><tr><td>245</td><td>6850</td><td>30</td><td>Beta-616716</td><td>Shells</td></tr><tr><td>285</td><td>7710</td><td>30</td><td>Beta-616717</td><td>Shells</td></tr><tr><td>305</td><td>8640</td><td>30</td><td>Beta-565000</td><td>Shells</td></tr><tr><td>325</td><td>9160</td><td>30</td><td>Beta-616718</td><td>Shells</td></tr><tr><td>345</td><td>9680</td><td>30</td><td>Beta-565001</td><td>Shells</td></tr></tbody></table>
<figure><img src="image_6.png" /></figure>

Sample age (cal yr BP)
<figure><img src="image_7.png" /></figure>

Molluks main taxa

Preservation rate

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

Pollen Zones

3d

3c

3b

3a

2b

Sterile

2

1c

1b

2a

1

1a
<figure><img src="image_9.png" /></figure>

Atlantic Ocean

N

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

St Louis

<figure><img src="image_11.png" /></figure>

Potou

50 km

Lake Rkiz

Senegal River

Bogué

Lake Guiers

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

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

Regional environment

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

Celtis toka