# Variations of Primary Productivity in the Northwestern Arabian Sea During the Last 23,000 Years and Their Paleoclimatological Implications

Xinquan Zhou, Stéphanie Duchamp-Alphonse, Masa Kageyama, Franck

Bassinot, Florian Doressoundiram, Catherine Kissel

► To cite this version:

Xinquan Zhou, Stéphanie Duchamp-Alphonse, Masa Kageyama, Franck Bassinot, Florian Doressoundiram, et al.. Variations of Primary Productivity in the Northwestern Arabian Sea During the Last 23,000 Years and Their Paleoclimatological Implications. Paleoceanography and Paleoclimatology, 2022, 37 (10), pp.1306. 10.1029/2022PA004453. hal-03874269

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Distributed under a Creative Commons Attribution 4.0 International License
# Paleoceanography and Paleoclimatology®

## RESEARCH ARTICLE

10.1029/2022PA004453

**Key Points:**

* Past primary productivity (PP) and upwelling in the NW Arabian Sea are reconstructed by coccolith assemblages from a marine sediment core

* PP was relatively high during the cold periods compared to the warm periods of the last 23 Kyr

* PP changes were mainly controlled by winter monsoon through changes in convective mixing and by aeolian inputs

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

X. Zhou, xinquan_zhou@tongji.edu.cn

**Citation:**

Zhou, X., Duchamp-Alphonse, S., Kageyama, M., Bassinot, F., Doressoundiram, F., & Kissel, C. (2022). Variations of primary productivity in the northwestern Arabian Sea during the last 23,000 years and their paleoclimatological implications. *Paleoceanography and Paleoclimatology*, 37, e2022PA004453. https://doi.org/10.1029/2022PA004453

Received 29 MAR 2022
Accepted 6 OCT 2022

**Author Contributions:**

**Conceptualization:** Xinquan Zhou, Stéphanie Duchamp-Alphonse

**Data curation:** Xinquan Zhou

**Investigation:** Xinquan Zhou, Florian Doressoundiram

**Methodology:** Xinquan Zhou, Masa Kageyama

**Resources:** Franck Bassinot, Catherine Kissel

**Supervision:** Stéphanie Duchamp-Alphonse, Masa Kageyama

**Visualization:** Xinquan Zhou

© 2022. The Authors.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Variations of Primary Productivity in the Northwestern Arabian Sea During the Last 23,000 Years and Their Paleoclimatological Implications

Xinquan Zhou¹ ID, Stéphanie Duchamp-Alphonse¹, Masa Kageyama², Franck Bassinot² ID, Florian Doressoundiram¹,², and Catherine Kissel² ID

¹Université Paris-Saclay, CNRS, GEOPS, Orsay, France, ²Laboratoire des Sciences du Climat et de l'Environnement, CEA/CNRS/UVSQ, Université Paris-Saclay, Centre CEA-Saclay, Gif-sur-Yvette, France

**Abstract** The Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon-related processes that can increase nutrients contents in the euphotic zone. Previous studies of the imprint of oceanic primary productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi-decadennial PP and coastal upwelling dynamic records off northern Oman over the last 23 Kyr, based on the analysis of coccoliths from sediment core MD00-2354. Our results have been compared with previous paleoenvironmental records as well as new modeling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial-scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during the cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early-Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs.

**Plain Language Summary** Ocean primary productivity (PP) is the production rate of organic carbon from inorganic carbon by ocean phytoplankton through photosynthesis, in which nutrients and sunlight are necessary. In tropical oceans, this rate is usually limited by the availability of nutrients in the sunlit ocean, which is controlled by physical processes such as seawater upward motion, ocean mixing, and dust storms. All these processes play a significant role in the northwestern Arabian Sea (AS) today. It is surrounded by arid lands that provide the highest amount of airborne material in the world and it is influenced by the Indian monsoon system that drives seawater upward motion and mixing in summer and winter, respectively. However, not much is known about their impact on PP in the past. This work aims to reconstruct changes of PP in the northwestern AS over the past twenty thousand years and understand by which process(es) they are impacted, based on a model-data comparison approach. We have found that PP was higher during cold periods than during warm ones. The changes were mainly controlled by both, winter monsoon through changes in convective mixing, and dust storms through changes in nutrient-enrich aeolian inputs.

## 1. Introduction

The Arabian Sea (AS) is one of the most productive oceanic regions in the world due to the important supply of nutrients to the euphotic zone either from below, through wind-driven coastal upwelling or surface cooling-driven convective mixing, and/or from surrounding continents, through the transport and deposition of fertilizing aeolian dust (Bali et al., 2019; Jickells et al., 2005; Lee et al., 2000; Prasanna Kumar & Narvekar, 2005). This semi-enclosed basin is under the influence of the Indian monsoon system, characterized by seasonal shifts of prevailing surface winds and precipitations that shape the AS upper circulation and stratification and thus, nutrient and primary productivity (PP) distributions (Lévy et al., 2007; Schott & McCreary, 2001; Wang & Ding, 2008). During the summer monsoon that is, from May to September, faster heating of the Asian continent relative to the Indian Ocean creates a land-sea temperature (pressure) gradient that drives strong and wet
**Writing – original draft:** Stéphanie Duchamp-Alphonse

**Writing – review & editing:** Masa Kageyama, Franck Bassinot, Catherine Kissel

southwesterly winds running diagonally across the AS, which contribute to the development of a clockwise upper ocean circulation. Associated with intense alongshore winds and Ekman pumping, a strong coastal upwelling develops off Somalia and Oman that brings cold and nutrient-rich waters to the euphotic zone and helps sustaining a high PP cell extending up to the central part of the basin (Bartolacci & Luther, 1999; Lee et al., 2000; Prasanna Kumar, Madhupratap, et al., 2001; Figure 1). During the winter monsoon that is, from November to March, as the land-sea temperature (pressure) gradient is opposite due to faster land cooling relative to the Indian Ocean, dry and cold northeasterly winds blow from the Asian continent. They result in an anti-clockwise upper ocean circulation associated with a convective mixing north of 15°N that leads to the development of a high PP cell in the northern part of the AS as cold and nutrient-rich intermediate waters resurface (Schott & McCreary, 2001; Prasanna Kumar & Narvekar, 2005; Prasanna Kumar, Ramaiah, et al., 2001; Figure 1). The AS is also surrounded by vast (supra-) arid lands that provide the highest amount of airborne material in the world during dust storms (Francis et al., 2021; Mashat et al., 2018; Notaro et al., 2015; Ramaswamy et al., 2017; Rea, 1994; Rezazadeh et al., 2013; Yu et al., 2013). This material brings a combination of essential nutrients to the euphotic zone of the AS, such as nitrogen, phosphorus, and iron that contribute to maintain high PP through the year (Bali et al., 2019; Banerjee & Prasanna Kumar, 2014; Guieu et al., 2019; Jickells et al., 2005; Patra et al., 2007; Singh et al., 2008).

The imprint of PP in marine sediments from the AS has already been the focus of studies that aimed at understanding the interaction of the Indian monsoon and global climate. Numerous organic (total organic carbon [TOC], biomarkers) and inorganic (biogenic silica and barium, planktonic foraminifera) proxies have been measured in sediment cores from the western and northwestern AS in order to identify changes in the southwest summer monsoon (indicated by coastal upwelling dynamic) at glacial-interglacial (Anderson & Prell, 1993; Emeis et al., 1995; Reichart et al., 1997; Shimmield et al., 1990; Weedon and Shimmield, 1991; Ziegler et al., 2010) and tectonic (Bialik et al., 2020) time scales. Authors concluded about higher PP during interglacial periods compared to glacial ones due to stronger summer monsoon conditions as minimum ice volume in the Northern Hemisphere reduce snow cover over central Asia and increase the land-sea temperature (pressure) gradient. These results are consistent with the centennial to millennial PP changes recorded over the last deglaciation that show higher PP during the mild interstadial Bølling-Allerød (B-A; 14.7–12.9 ka), and lower PP during the near-glacial stadials Heinrich Stadial 1 (HS1; 17–14.7 ka) and Younger Dryas (YD; 12.9–11.7 ka), in reaction to rapid temperature fluctuations of high northern latitudes (Balaji et al., 2018; Caley et al., 2013; Deplazes et al., 2014; Gupta et al., 2013; Ivanochko et al., 2005; Schulz et al., 1998). Such fluctuations reflect the injection of melt water into the North Atlantic Ocean that regulates the strength of the Atlantic Meridional Overturning Circulation (AMOC), an important process that modulates the interhemispheric heat transport and thus, the intensity of the summer monsoon circulation (Kageyama, Merkel, et al., 2013; Marzin et al., 2013; McManus et al., 2004; Murton et al., 2010). However, these results differ from a few studies that testify for higher PP during glacial periods due to stronger winter monsoon and enhanced fertilization by aeolian input due to increased aridity on the adjacent continents (Pourmand et al., 2007; Tamburini et al., 2003). One could have hypothesis a regional difference between nearshore and offshore studied areas, the latter possibly experiencing further influence of winter monsoon conditions than the former ones. However, the aforementioned records do not follow this hypothesis, as the PP reconstruction from Tamburini et al. (2003) comes from the exact Oman margin. Another could have question the accuracy of the organic and inorganic proxies that have been used to infer PP as they additionally depend upon oxygenation and preservation conditions at the seafloor (especially TOC and biogenic barium) (e.g., Lückge et al., 1999), which make it difficult to disentangle PP variations from changes in bottom water conditions. However, even if these data are not representative of quantitative PP that is a signal restricted to the euphotic zone, they have been obtained based on multi-proxy approaches from several sites, associated with specific preservational conditions that, in no case, may lead to similar PP signals. Answer is more likely to be found in model-proxy modeling studies that include marine biogeochemical components. Bassinot et al. (2011) and Le Mézo et al. (2017) are the only ones to simulate PP patterns in response to changed boundary conditions in the past (e.g., Mid-Holocene [MH] and Last Glacial Maximum [LGM]) considering both summer and winter monsoons, based on the IPSL-CM4 and IPSL-CM5A-LR models. Particularly, Le Mézo et al. (2017) weakens the usual paradigm that a stronger summer monsoon should induce a stronger upwelling and thus increased PP in the AS, further highlighting the urge to compare modeling results to high-resolution PP reconstructions from sites under the influence of both, summer and winter monsoon conditions.

In this study, we provide the first millennial-scale records of paleo-PP and coastal upwelling dynamics in the northwestern AS over the last ~23 Kyr, based on the analysis of coccolith assemblages from the sediment core
<figure><img src="image_3.png" /><figcaption>Figure 1.</figcaption></figure>
MD00-2354 (61.48°E and 21.04°N). Surface waters at that site are under the influence of the coastal upwelling off Oman and the convective mixing cell off Iran/Pakistan that drive high PP during summer and winter, respectively (Figure 1), and receive important amount of airborne nutrients derived from dust storms in the Arabian Peninsula and Iranian Plateau (Kumar et al., 2020). Overall, they are far from the influence of main rivers that flow in the southeastern region of Pakistan (especially the Indus River) (Shimmield et al., 1990). The last ~23 Kyr encompasses a glacial-interglacial transition, an entire precessional cycle, and important fluctuations in the AMOC during the last deglaciation (McManus et al., 2004). It is therefore a perfect case study to explore the impact of key climate forcing mechanisms on PP for both, past and future climate conditions, such as ice sheet volume, sea level, and atmospheric CO₂ concentration. Coccoliths, which are the calcite scales produced by unicellular coccolithophore algae, are abundant in marine sediments, and have been used worldwide to infer physio-chemical conditions of the euphotic zone in the past. The species *Florisphaera profunda* in particular, that is restricted to the lower euphotic zone (water depths of 100–200 m), has been widely used to reconstruct variations in the nutricline depth (e.g., Bassinot et al., 1997; C. Liu et al., 2008; Molfino & McIntyre, 1990; Okada & Honjo, 1973; Okada & Mastsuoka, 1996; Su et al., 2013), and hence in PP (e.g., Beaufort et al., 1997; Beaufort et al., 2010; de Garidel-Thoron et al., 2001; Zhang et al., 2016; Zhou et al., 2020). The species *Calcidiscus leptoporus* that flourishes in mixed and nutrient-rich waters (Gravalosa et al., 2008; Ziveri et al., 1995), has been used for reflecting nutrient contents in coastal upwelling settings (Baumann et al., 2016; Boeckel et al., 2006; Fincham & Winter, 1989; Giraudeau, 1992; Giraudeau & Rogers, 1994; Saavedra-Pelitero et al., 2010). In the AS, highest fluxes of *C. leptoporus* were found in sediment traps off Somalia during summer coastal upwelling events (H. A. Andruleit et al., 2000; Broerse et al., 2000; Renaud et al., 2002), and it is most probable that this pattern remained so in the past. We combine our micropalaeontological data (*F. profunda*-based PP and *C. leptoporus* abundance) with previous records of direct and indirect PP (low-resolution *F. profunda*-based PP and TOC mass accumulation rates), upwelling dynamic (percentage of foraminifera *Globigerina bulloides*), sea surface temperature (SST; alkenones), and aeolian inputs (²³⁰Th-normalized detrital flux and TiₓRF) in the northern and northwestern AS. We also compare our reconstructed PP with outputs of the Earth System model IPSL-CM5A-LR (termed IPSL-CM5A in the following) and the transient simulation TraCE-21 (termed TraCE in the following). This allows us to evaluate the response of PP to changing surface oceanic and atmospheric conditions over the last ~23 Kyr, with a special focus on the LGM (21 ka) and the MH (6 ka).

## 2. Materials and Methods

### 2.1. Core Location and Age Model

Core MD00-2354 was collected in the northwestern AS, about 210 km off the Oman Margin, onboard the *R.V. Marion Dufresne*, in 2000. It is located on the southwestern slope of the Murray Ridge, at 2,740 m water depth that is, above the modern lysocline (~3,300 m; Cullen & Prell, 1984). Fossil coccolith records from this area have great potentials to successfully monitor and reconstruct paleoceanographic conditions (H. Andruleit & Rogalla, 2002; H. Andruleit et al., 2004). The lithology of this sediment core mainly consists of homogenous gray clay and foraminifera- and coccolith-bearing oozes. Its age model has been established by Böll et al. (2015) based on the correlation of the δ¹⁸O signal obtained on foraminifera *Globigerinoides ruber* (δ¹⁸O₉. ruber) to the δ¹⁸O₉. ruber record of 74KL sediment core retrieved in the western AS, in turn, correlated to the δ¹⁸O signal of GISP2 ice core (Stuiver & Grootes, 2000). The upper 3 m of the sediments span the last 23.3 Kyr, with a sedimentation rate ranging between ~10 and 110 cm Kyr⁻¹ (Figure S1 in Supporting Information S1).

**Figure 1.** (a) Geographic setting and bathymetric map created by the Ocean Data View software (©Reiner Schlitzer, Alfred Wegener Institute) with its built-in global high-resolution bathymetric data (GlobHR). The red dot marks the location of the studied core MD00-2354. The black dots and crosses mark the locations of cores with published micropaleontological and geochemical data that have been used in this study. Numbers 1 to 11 refer to site names as follow: 1 = 136KL; 2 = 94KL; 3 = 93KL; 4 = NIOP464; 5 = RC-27-42; 6 = ODP723A; 7 = GeoB 3005; 8 = Moomi Cave; 9 = Konor Sandal peat boot; 10 = Mawmluh Cave; 11 = 126KL. The green and blue arrows schematically depict the summer southwesterlies (Findlater Jet) and winter northeasterlies over the Arabian Sea (AS). The green dashed circle indicates the area of summer monsoon rainfall (Wang & Ding, 2008). The yellow dashed circle shows potential source areas of the aeolian dust transported to the AS (Sirocko et al., 1991, 2000). (b–m) Climatology (period 2003–2019) of PP (color, gC m⁻² yr⁻¹), sea surface temperature (SST; contours, °C), and surface wind (vectors, m s⁻¹). PP was calculated by the standard VGPM model based on the MODIS chlorophyll-a concentration data set (Behrenfeld & Falkowski, 1997). PP data are from the “Ocean Productivity” website (http://sites.science.oregonstate.edu/ocean productiv ity). SST data are from the NOAA OI SST V2 High-Resolution Data set (Reynolds et al., 2007; https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html). Wind speed data are from the ERA5 reanalysis data set (Hersbach et al., 2020; https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5). The pink dots mark core MD00-2354.
## 2.2. Coccolith Analysis

A total of 300 samples at 1 cm interval was analyzed at the Geoscience Paris-Saclay laboratory (Orsay, France) for coccolith data. This depth-spacing translates into resolutions ranging from ~20 to 100 years. The “settling” technique of Duchamp-Alphonse et al. (2018) modified after Beaufort et al. (2014) was used for preparing coccolith smear slides. Briefly, dry sediment was diluted in a high pH and bicarbonate rich water, ultra-sonicated and passed through a 20 µm cellulose membrane filter. The suspension was poured in a flat beaker for decantation on a coverslip, dried and mounted on a smear slide. This technique ensures an homogenous distribution of coccoliths and allow quantifying both, relative (%) and absolute abundances (number per gram of sediment) of specimens. For each slide, a total of at least 500 coccolith specimens was counted using a polarized light microscope (Leica DM6000B) at 1,000× magnification. The relative abundance (%) of coccolith species has been obtained as follows: Species% = 100 × (Species/total coccolith number). Particularly, the 95% confidence interval for Fp% was calculated following the method of Patterson and Fishbein (1984), and the errors are smaller than ±5%. We converted Fp% into PP by using the tropical Indian Ocean empirical equation proposed by Hernández-Almeida et al. (2019) as follow: PP (gC m⁻² yr⁻¹) = [10⁽³.²⁷ - ⁰.⁰¹ × Fp%)] × 365/1,000. *C. leptoporus* absolute abundance has been calculated as follows: A = (Nc × Sf)/(No × So × Ws), where A is the absolute abundance (n/g of sediment), Nc is the number of counted *C. leptoporus* specimens; Sf is the surface of the flat beaker (diameter = 70 mm) in which suspended sediments (and total coccoliths) settled; No is the number of view fields; So is the surface of a view field (diameter = 0.25 mm); and Ws is the weight of sediment that settled in the flat beaker.

## 2.3. Climate Model and Simulations

### 2.3.1. IPSL-CM5A-LR

Paleoclimate simulations were run with the IPSL-CM5A coupled model (Dufresne et al., 2013), which has three model components: the LMDZ5A atmospheric general circulation model (Hourdin et al., 2013), the ORCHIDEE land surface model (Krinner et al., 2005), and the NEMO-PISCES ocean-biogeochemistry model (Aumont & Bopp, 2006; Madec, 2008). Simulations were run for the pre-industrial control (CTRL) in the framework of the Coupled Modeling Intercomparison Project phase 5 (Taylor et al., 2012) and for the LGM and the MH in the framework of Paleoclimate Modeling Intercomparison Project phase 3 (Braconnot et al., 2012). The comparison of LGM and MH mean states with CTRL are expressed in terms of differences (LGM–CTRL and MH–CTRL) to highlight specific atmospheric and oceanic responses under LGM and MH conditions. Compared to CTRL, MH is characterized by a smaller precession parameter, while larger ice sheets, lower greenhouse gases concentrations, and lower sea level are set for the LGM. Nutrient supplies from dust, rivers and sediments to the ocean are set to the CTRL values for all simulations (Kageyama, Braconnot, et al., 2013).

In brief, surface wind speed (m s⁻¹) and wind stress curl (N m⁻³) were extracted to describe atmospheric conditions while surface nutrient content (mmol m⁻³), mixed-layer depth (m), seawater vertical mass transport (upper 200 m mean, kg s⁻¹; Lee et al., 2000), together with temperature (°C), salinity (psu), and potential density (kg m⁻³), were extracted to describe oceanic conditions. Temperature (salinity) vertical gradients were simplified by the difference between 0 (200 m) and 200 m (0 m) water depths, termed ΔT₀–200 m (ΔS₂₀₀–0 m) in the following. The vertically averaged stable stability of the upper 200 m (termed E₀–200 m, m⁻¹) represents the upper seawater stratification (Prasanna Kumar et al., 2002). Single layer stable stability (E) were calculated based on the normalized vertical density (ρ, kg m⁻³) gradient: E = -(1/ρ)·(∂ρ/∂z), where z is the water depth (m). Variables used here are seasonally and annually averaged. Details of the model and the boundary conditions for these three experiments can be found in Dufresne et al. (2013) and Le Mézo et al. (2017).

### 2.3.2. TraCE-21

TraCE-21 is a transient simulation of the global climate evolution over the last 22 Kyr run with Community Climate System Model version 3 (Collins et al., 2006; Z. Liu et al., 2009). The simulation was initialized from a previous LGM equilibrium simulation of CCSM3 (Otto-Bliesner et al., 2006) and was then forced by a set of realistic transient climate forcing including orbital parameters, greenhouse gases concentrations, continental ice sheets, and glacial meltwater discharges. The last one can force the AMOC strength to change. Outputs of the single forcing experiments are also available (Z. Liu et al., 2009). We extracted annual mean temperature and salinity at 0 and 200 m and calculated annual mean vertically averaged stable stability of the upper 200 m (E₀–200 m, m⁻¹) to describe oceanic conditions around site MD00-2354 (21°–23°N and 60°–68°E).
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Relative abundance of the most significant contributors of the coccolith assemblage in core MD00-2354 (black), and quantitative primary productivity (PP; red) estimated by the Indian Ocean empirical equation of Hernández-Almeida et al. (2019). The detailed assemblage (i.e., the entire species %) is shown in Figure S2 in Supporting Information S1.</figcaption></figure>

# 3. Results

## 3.1. Coccolith Assemblage and Reconstructed PP

At site MD00-2354, the Noelaerhabdaceae family (that includes *Emiliania huxleyi*, *Gephyrocapsa oceanica* and small *Gephyrocapsa*), together with *F. profunda*, represent more than 70% of the coccolith assemblage while *C. leptoporus*, *Umbilicosphaera* spp. and *Helicosphaera* spp. account for up to ~25% over the last ~23 Kyr (Figure 2 and Figure S2 in Supporting Information S1). Other species such as *Syracosphaera* spp. and *Rhabdosphaera* spp. are rare and altogether, never exceed 5%. Here, we focus on the Noelaerhabdaceae family and *F. profunda*, the most significant contributors of the assemblage, as well as *C. leptoporus* related to upwelling dynamic (Figure 2). Interestingly, *F. profunda* and the Noelaerhabdaceae depict opposite trends over the last 23 Kyr. While *F. profunda* shows a minimum (~15%) during the LGM, a two-step increase during the deglaciation, and a maximum (~60%) during the Holocene, the Noelaerhabdaceae show high values (~55%) during LGM, a two-step decrease during the deglaciation and a minimum (~25%) during the Holocene. During the deglaciation, millennial-scale variations of the Noelaerhabdaceae and *F. profunda* remain opposite. *F. profunda* (the Noelaerhabdaceae) shows averaged abundances of ~30% (~60%) and ~35% (~50%) during HS1 and YD and of ~39% (~54%) during B-A. *C. leptoporus* shows values as low as 0.5% during HS1 and depicts an increasing trend during the B-A with proportions as high as 2.5%.

Reconstructed PP varies between 150 and 500 gC m⁻² yr⁻¹ (Figure 2). Despite rather scattered values during the LGM, it appears that PP during this period (average of ~382 gC m⁻² yr⁻¹; 23–19 ka) is about ~90% higher than during the Holocene (average of ~203 gC m⁻² yr⁻¹; 11–0 ka). The maximum and minimum PP are found during the LGM and at 10 ka, respectively. During the deglaciation, PP averages are 346 and 305 gC m⁻² yr⁻¹ during HS1 and YD, and do not exceed 300 gC m⁻² yr⁻¹ during the B-A.

## 3.2. Simulated PP

Simulated PP obtained by IPSL-CM5A (CTRL, LGM-CTRL, and MH-CTRL) are shown in Figure 3. CTRL simulations show that the areas off Somalia-Oman and off Northern Oman and Pakistan, with PP as high as 400 gC m⁻² yr⁻¹, are the most productive areas during preindustrial summer and winter, respectively, which is consistent with the VGPM modern features (Behrenfeld & Falkowski, 1997). The difference in annual PP between LGM and CTRL reveals a strong increase (>50 gC m⁻² yr⁻¹) in the northern and northeastern parts of the basin and a strong decrease (<-50 gC m⁻² yr⁻¹) along the coastal areas off Somalia and Oman. Such patterns are mainly related to winter conditions while the PP decrease in the western part may also be associated to the summer ones (Le Mézo et al., 2017). The difference in annual PP between MH and CTRL shows a general PP decrease over the basin. It is only in the northwestern AS off Oman that a PP increase (10–25 gC m⁻² yr⁻¹) is simulated, as a result of a relatively high PP during summer (Le Mézo et al., 2017).

# 4. Discussion: Atmospheric and Oceanographic Implications Behind PP Patterns

## 4.1. Present Day Scenario

The modern seasonal pattern of PP simulated by CTRL in the studied area is shown in Figure 4. During summer, southwesterly winds alongshore Oman and Somalia, are accompanied by positive wind stress curl and upward seawater movements related to the coastal upwelling system. This mechanism increases the nutrient content in surface layers and results in relatively high PP. This high PP pattern is clearly not related to a weaker upper seawater stratification in the northwestern AS as no deeper mixed-layer, no decreased stable stability, and no
<figure><img src="image_5.png" /><figcaption>Figure 3.</figcaption></figure>
smaller temperature and salinity gradients are simulated. During winter, no important positive wind stress curl and upwelling are found. Northeastern winds across AS are associated with a strong sea surface cooling in the northern part of the basin, which results in the collapse of the temperature vertical gradient and hence of the upper seawater stratification. These processes help sustaining a substantial amount of nutrients in the upper layers, which appears to drive high PP.

Despite the model being run at low resolution, the results of CTRL simulations document well the processes encountered in the upper layers of the AS during summer and winter seasons, processes by which, wind-driven coastal upwelling off Somalia and Oman, and surface cooling-driven convective mixing off North Oman and Pakistan drive high PP, respectively. They are also consistent with the findings of previous studies based on in-situ PP/chlorophyll-a and physical parameters data (e.g., Lee et al., 2000; Madhupratap et al., 1996; Prasanna Kumar, Madhupratap, et al., 2001). As a result, modern annual PP near site MD00-2354 is relatively important, as the surface ocean is under the direct influences of the two major seasonal phytoplankton blooms of the basin.

## 4.2. Glacial Condition Forcing During the LGM

Changes in PP, oceanographic and climate conditions in the AS over the last 23 Kyr are provided in Figure 5, based on new and published micropaleontological, geochemical and numerical data. The high PP signal (~380 gC m⁻² yr⁻¹) obtained at site MD00-2354 mimics previous evidence of high PP (~500 gC m⁻² yr⁻¹) at sites 94KL and ODP723A based on Fp% and indirectly at sites GeoB 3005 and NIOP464 based on TOC measurements, which reveals the occurrence of an extended high (exported-) PP cell in the northern and northwestern parts of the AS. Relatively low abundances of *C. leptoporus* (0.8 10⁸ n/g) at site MD00-2354 coincide with relatively low percentage (~32%) of *G. bulloides* at site ODP723A, a planktonic foraminifer indicator for upwelling strength (Conan & Brummer, 2000; Curry et al., 1992). This indicates that higher PP in that area is not associated to the development of a strong upwelling system off Somalia and Oman, as the Indian summer monsoon was weaker than during the Holocene (Braconnot et al., 2007; Jiang et al., 2015; Kathayat et al., 2000). Such an interpretation is testified by the positive δ¹⁸O anomalies obtained on speleothems from Moomi and Mawmluh caves, which indicate relatively low Indian summer monsoon moisture transport and rainfall. It is also supported by the positive seawater δ¹⁸O anomalies from site 126KL (northern Bay of Bengal), which documents relatively high sea surface salinity. In parallel, the relatively cold alkenone-based SST data from core 136KL located in the northeastern AS, away from the coastal upwelling area, show that forcing factors driving this higher PP cell might be found within mechanisms involving cooling-driven convective mixing and a weaker stratification in the euphotic zone. Such an interpretation is testified by TraCE that simulates the exact same SST trends as those obtained at site 136KL and depicts relatively low annual mean seawater stratification in the northern AS, due to a smaller vertical temperature gradient rather than a smaller salinity one (Figures 5b and 6). It is also well expressed by IPSL-CM5A. Even if the model simulates a summer season with anomalous northeasterly winds along the coastal area off Somalia and Oman that drive positive wind stress curl anomalies over the western AS and a slight increase of PP, it clearly appears that the summer monsoon is reduced and that annual mean conditions (and thus PP patterns), reflect a stronger winter one (Figures 3 and 7). High PP in the northern AS are driven by strong northwesterly winds which, together with a smaller vertical SST gradient, generate a weaker stratification and higher nutrient contents in surface layers (Figures 3 and 7). Overall, such scenario is consistent with extended ice sheets and lower greenhouse gas concentration that characterize the LGM (Joos & Spahni, 2008; Peltier, 2004; Tierney et al., 2020). As the global annual cooling occurred during the LGM with a strongest impact over the ice sheet regions than the tropics (Shakun et al., 2012; Tierney et al., 2020), it drove a stronger and weaker temperature contrast between Eurasia and northern Indian Ocean during winter and summer, respectively, and thus, a weaker summer monsoon and a stronger winter monsoon over the AS (Böll et al., 2015; Braconnot et al., 2007; Jiang et al., 2015; Reichart et al., 2004; Tian & Jiang, 2016).

**Figure 3.** (a–c) Annual mean (ANN), summer (June–July–August–September, JJAS), and winter (December–January–February–March, DJFM) integrated primary productivity (PP; gC m⁻² yr⁻¹) calculated by VGPM model based on MODIS chlorophyll concentration data (Behrenfeld & Falkowski, 1997) and surface wind (m s⁻¹) in the ERA5 reanalysis data set. (d–f) Annual mean, summer, and winter integrated PP and surface wind simulated by CTRL experiment of IPSL-CM5A. (g–i) Differences of annual mean, summer, and winter integrated PP and surface wind between Last Glacial Maximum and CTRL experiments of IPSL-CM5A. (j–l) Differences of annual mean, summer, and winter integrated PP and surface wind between Mid-Holocene (MH) and CTRL experiments of IPSL-CM5A. The dots mark the locations of cores MD00-2354 (dots with inside crosses), OPD723A, NIOP464, GeoB 3005, and 94KL (see Figure 1a) with published indirect PP data (shown in Figure 5).
IPSL-CM5A CTRL JJAS

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

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

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

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

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

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

IPSL-CM5A CTRL DJFM

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

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

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

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

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Figure 4.</figcaption></figure>
Such conditions are confirmed by $^{230}$Th-normalized detrital flux records at sites 93KL and RC-27-42 that show, despite low time resolution, that aeolian inputs were higher in the northern and western AS, in conjunction with higher PP during the LGM. Following Tamburini et al. (2003), it is most probable that enhanced fertilization by aeolian input due to increased aridity on the adjacent continents account for a part of the high PP signal obtained at that time (Pourmand et al., 2004, 2007). Glacial conditions do promote the increase of global dust emission and deposition (e.g., Albani et al., 2016, 2018; Lambert et al., 2013; Lamy et al., 2014; Maher et al., 2010; Martínez-García et al., 2009; Winckler et al., 2008) and this is particularly the case during the LGM (Sirocko et al., 1991, 2000). Guieu et al. (2019) demonstrated, based on sensitivity experiments of dust deposition run with a coupled model, that null dust accumulation in the AS could reduce PP by 50%. Following their findings and if the response is linear, the ~40% stronger aeolian flux recorded during the LGM relative to the Holocene (Figures 5q and 5r) could increase PP by ~20%, thus explaining a part of the recorded ~90% higher PP signal.

In summary, the high PP observed in the northwestern part of the basin during the LGM most likely reflects higher nutrient inputs in the euphotic zone due to the combined effects of stronger surface cooling-driven mixing and stronger aeolian input. They resulted from both, stronger winter and weaker summer monsoon conditions.

### 4.3. Glacial Waning and AMOC Forcing During the Last Deglaciation

On the long term (between 19 and 10 ka), the decreasing trend of PP (from ~380 to 200 gC m⁻² yr⁻¹) and the increasing trend of *C. leptoporus* abundance (from 0.8 to 1.2 10⁸ n/g) obtained at site MD00-2354, mimic Fp%-PP, TOC and *G. bulloides* signals previously obtained in the studied area (Figure 5). These results indicate that during the deglaciation, reducing PP in the northwestern part of the AS is associated with intensifying coastal upwelling off Somalia-Oman and strengthening summer monsoon. In parallel, rising SST are recorded at sites 136KL and MD00-2354 and simulated by TraCE in conjunction with rising stratification in the upper layers (Figure 5). This highlights a reduction of the surface cooling-driven convective mixing, compatible with a weakening of the winter monsoon. These findings are in agreement with the $\delta^{18}O_{speleothem}$ signals obtained in Moomi and Mawmluh caves and the SSS record from the northern Bay of Bengal, that point to a transition from cold and dry conditions during the LGM, to warm and wet ones during the early Holocene, as the summer monsoon strengthens. This is expected when increased Northern Hemisphere summer insolation, smaller ice sheets and increased atmospheric greenhouse gas concentration, induce faster warming of Eurasia with respect to the northern Indian ocean (e.g., Blaga et al., 2013; Cheng et al., 2016; Clark et al., 2012; Joos & Spahni, 2008; Peltier, 2004; Renssen & Isarin, 2001; Shakun et al., 2012; Sowers & Bender, 1995; Tierney et al., 2008). These findings also show that PP patterns that document an overall decrease in upper layer nutrient contents in the northern AS are rather sensitive to the weakening of the surface cooling-driven convective mixing than the strengthening of the wind-driven coastal upwelling. The long-term decreasing trends in PP also match the long-term decreasing trends in the $^{230}$Th-normalized detrital flux. Therefore, it is most possible that the concomitant reduction in the aeolian dust activity and hence, in dust-derived nutrients in surface waters, also contributed in driving the overall decrease in PP during the deglaciation relative to the LGM.

On millennial-scale, PP and *C. leptoporus* records obtained at site MD00-2354 between 19 and 10 ka, highlight HS1, B-A and YD climate periods that is, the three stages that correspond to significant changes in the AMOC strength (Figure 5). During the cold HS1 and YD periods relatively high PP signals are accompanied by relatively low *C. leptoporus* abundances while opposite trends are recorded during the warm B-A. Therefore, in a similar way as for the long-term trends, high PP characterizes periods when the coastal upwelling off Oman weakened and when SST in the northern AS decreased. TraCE, which is forced by high latitude melt water fluxes (AMOC strength) during the deglaciation in addition to long-term changes in insolation, greenhouse gases and ice sheets (Figure S3 in Supporting Information S1), simulates a weaker upper seawater stratification due to lower SST and hence, stronger surface cooling-driven convective mixing during HS1 and YD, while opposite conditions

**Figure 4.** Analysis of primary productivity (PP) using CTRL outputs of IPSL-CM5A in (a–f) summer (JJAS) and (g–l) winter (DJFM). (a and g) Integrated PP (color, gC m⁻² yr⁻¹) and surface wind (vectors, m s⁻¹). (b and h) Surface concentration of nitrate (NO₃, mmol m⁻³). (c and i) Vertically averaged mass transport of the upper 200 m seawater (WMO, color, kg s⁻¹) and wind stress curl (VR, contours, 10⁷ N m⁻³). (d and j) Upper seawater stratification quantified by vertically averaged stable stability of the upper 200 m (E₀~200 m, color, 10⁶ m⁻¹) and mixed-layer depth (MLD; contours, m). (e and k) Upper seawater temperature gradient quantified by the difference between the surface and 200 m (ΔT₀~200 m, color, °C) and sea surface temperature (SST, contours, °C). (f and l) Upper seawater salinity gradient quantified by the difference between 200 m and the surface (ΔS₂₀₀~0 m, color, psu) and sea surface salinity (SSS, contours, psu). The dots have the same representations as those in Figure 3.
<figure><img src="image_18.png" /><figcaption>Figure 5.</figcaption></figure>

are simulated during the B-A (Figures 5 and 6). More precisely, results for HS1 versus B-A events show that HS1, as defined by a weakened AMOC, is characterized by annual mean anomalous northerly winds, together with weaker temperature-related stratification over the AS compared to B-A (Figure 6). Therefore, it appears that high PP and weakened coastal upwelling off Oman occur when stronger (weaker) Indian winter (summer) monsoon conditions prevailed, and vice versa. Compared to the temperature vertical gradient in the upper waters,
the salinity gradient has limited variations and a weak control on the upper seawater stratification (Figure 6 and Figure S4 in Supporting Information S1). This is reflected by the much stronger correlation between the temperature gradient and stratification over the deglaciation (Figure S4 in Supporting Information S1). These variations of the upper seawater stratification demonstrate the control millennial-scale changes in the AMOC state (high-latitude) exerts on the AS conditions and PP distribution (low-latitude) (Ivanochko et al., 2005; Kageyama, Merkel, et al., 2013; Marzin et al., 2013; Obase & Abe-Ouchi, 2019; Schulz et al., 1998; Shakun et al., 2007; Z. Liu et al., 2009). According to Marzin et al. (2013), a weakened AMOC can drive a reduction of the temperature gradient between Euro-Asia and the Indian Ocean, a southward shift of the Intertropical Convergence Zone, and a weaker Indian summer monsoon. However not much is known about millennial-scale changes of the Indian winter monsoon during the last deglaciation (Böll et al., 2015). Wen et al. (2016) proposed that the East Asian winter monsoon became stronger during HS1 and the YD and was therefore anti-phased with the East Asian summer monsoon. Here, our results are consistent with such an anti-phased pattern. Weaker AMOC during HS1 and YD reduced the annual northward heat transport from the tropical Atlantic and resulted in Eurasian cooling throughout the year. It therefore reduced and enhanced the land-sea temperature gradients over the Indian monsoon region during summer and winter, respectively, and thus, drove a weaker summer monsoon and a stronger winter one while opposite conditions are recorded during the B-A (Wen et al., 2016; Figure S5 in Supporting Information S1). Different from the East Asian, the winter monsoon over the AS might be more directly influenced by the cooling over the subtropical Mid-East and Tibetan plateau than the Mongolia-Siberian high-pressure cell (Chang et al., 2006; Goes et al., 2020; Y. Liu et al., 2020). However, further data of sensitivity modeling experiments are required to better understand the connexions of the Indian winter monsoon with the AMOC.

At last, since PP changes in concert with $^{230}$Th-normalized detrital flux in the northern AS and with Ti$_{XRF}$ signal in the southeastern Iran (Figure 5), one cannot exclude that dust storms contributed in shaping millennial-scale PP patterns as well. In that scenario, more (less) frequent and/or stronger (weaker) dust storms during HS1 and YD (B-A) that is, when continental aridity increases (reduces), would provide higher (lower) bioavailable nutrients to PP in this area. Indeed, one could have expected an increase of PP during the last deglaciation as SST raised and the coastal upwelling off Somalia and Oman reinforced. However, it clearly appears that fertilization of the euphotic zone due to stronger winter monsoon conditions and higher aeolian inputs, is much more efficient on annual PP than is the summer monsoon-driven upwelling.

### 4.4. Precession Forcing During the Holocene

Values of PP (average of ~200 gC m⁻² yr⁻¹) and TOC obtained at sites MD00-2354, GEOB 3005 and NIOP464, together with values of *C. leptoporus* (1.2 10⁹ n/g) and *G. bulloides* abundances obtained at sites MD00-2354 and ODP723A, respectively, show that the Holocene is characterized by the lowest PP in northern AS, and the strongest coastal upwelling off Somalia-Oman over the past 23 Kyr (Figure 5). Together with relatively high SST at sites 136KL and relatively low SSS at site 126KL, they show that warm and humid conditions installed and document that summer monsoon intensified as winter monsoon weakened compared to the glacial period

**Figure 5.** (a) Alkenone-derived sea surface temperature (SST) anomalies in core 136KL (Schulte & Müller, 2001). (b) Annual mean SST anomalies in the northern Arabian Sea (AS) (21°–23°N and 60°–68°E; AS) simulated by TraCE-21. (c) Alkenone-derived SST anomalies in core MD00-2354 (Böll et al., 2015). (d) Relative abundance of *G. bulloides* in core OPD723A indicating the variations of Oman upwelling intensity (Naidu & Malmgren, 1996). (e) Absolute abundance (n/g) of *Calcidiscus leptoporus* in core MD00-2354 (5-point moving average). (f) Annual mean seawater stratification in the northwestern AS simulated by TraCE-21. Mass accumulation rates (MAR) of total organic carbon (TOC) of cores (g) GeoB 3005 from the western AS (Budziak et al., 2000) and (h) NIOP464 from the northern AS (Reichart et al., 1997). Reconstructed primary productivity (PP) in (i) core 94KL (northern AS) and (j) core ODP723A (western AS) based on *Florisphaera profunda* percentages from Rogalla and Andruleit (2005) using the same PP empirical equation (Hernández-Almeida et al. (2019) as that for core MD00-2354. The lines show the results of three-point moving average. (k) Reconstructed PP in core MD00-2354 (this study). (l) Summer (June-July-August) insolation at 30°N (Laskar et al., 2004). (m) Atlantic Meridional Overturning Circulation strength revealed by the $^{231}$Pa/$^{230}$Th ratio in marine sediments from the western subtropical Atlantic Ocean (McManus et al., 2004). (n) Speleothem $\delta^{18}$O record from the Moomi Cave indicating the variations of summer moisture transport over the western AS (Shakun et al., 2007). (o) Speleothem $\delta^{18}$O record from the Mawmluh Cave indicating the variations of Indian summer monsoon rainfall (Dutt et al., 2015). (p) Seawater $\delta^{18}$O record of core 126KL indicating the variations of sea surface salinity and Indian summer monsoon rainfall (Kudrass et al., 2001). $^{230}$Th-normalized detrital flux of core (q) 93KL from the northern AS (Pourmand et al., 2004) and (r) core RC-27-24 from the western AS (Pourmand et al., 2007), both indicating aeolian dust flux. (s) Downcore Ti abundance measured by XRF of a sediment core from the Konar Sandal peat bog, southeastern Iran (Safaierad et al., 2020). (t) Global mean surface temperature anomalies referenced to the period of 1961–1990 (Marcott et al., 2013; Shakun et al., 2012). The data of Shakun et al. was aligned to that of Marcott et al.'s. The thin curves represent $1\sigma$. (u) Reconstructed PP in core MD00-2354 (this study).
## TraCE LGM-LH

<figure><img src="image_19.png" /></figure>

## TraCE HS1-BA

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

## TraCE MH-LH

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

**Figure 6.** Differences of annual mean $E_{0-200\,m}$ (10$^6\,m^{-1}$), surface wind (vectors, m s$^{-1}$), $\Delta T_{0-200\,m}$ (°C), and $\Delta S_{200-0\,m}$ (psu) between (a–c) Last Glacial Maximum (LGM; 21.5–21 ka) and late-Holocene (LH; 1–0 ka), between (d–f) HS1 (17–15.5 ka) and BA (14.7–13 ka), and between (g–i) Mid-Holocene (MH; 6.5–6 ka) and LH of TraCE-21.
<figure><img src="image_22.png" /><figcaption>**Figure 7.** Differences between Last Glacial Maximum (LGM) and CTRL experiments of IPSL-CM5A-LR in (a–e) summer, (f–j) winter, and (k–o) annual mean. The parameters are the same as those in Figure 4.</figcaption></figure>

(Figures 3 and 7). In parallel, the relatively low $^{230}$Th-normalized detrital flux documents a reduction in aeolian inputs with a probable impact on PP.

In detail, despite rather scattered values, it appears that PP and *C. leptoporus* reach minima and maxima values during the Early-Mid Holocene (EMH; 10–8 ka), before showing slightly increasing and decreasing trends up to the late Holocene (2 ka), respectively (Figure 5). These results are consistent with TOC, *G. bulloides* and MD00-2354 SST records indicating PP minimum and upwelling maximum at EMH (Figure 5), as well as with the aforementioned PP and climate data, that testify for slightly lower annual PP and stronger coastal upwelling dynamics off Oman and Somalia during the EMH (e.g., Gupta et al., 2003; Ivanochko et al., 2005). Such patterns are expressed by TraCE and IPSL-CM5A with MH–LH and MH–CTRL experiments, respectively. Indeed, despite nearly unchanged upper layer stratification due to nearly unchanged temperature and salinity gradients along Somalia and Oman, the imprint of stronger summer monsoon during MH is seen in the annual mean simulations that are characterized by stronger southwesterly winds (Figures 3 and 8). Interestingly, such conditions are not associated with a strong increase of nutrient and hence PP, in surface waters (Figure 8), because the negative wind stress curl anomalies, simulated during summer in the western AS, limits the advection of the coastal upwelling off Somalia and Oman (Le Mézo et al., 2017). In parallel, the winter season is characterized by slightly stronger northeasterly winds and slightly lower PP in the northern AS (Figure 3), with limited changes in upper layer temperature- or salinity-related stratification, and no significant impact on the annual mean setting (Figure 8). Overall, simulations are in good agreement with a smaller precession parameter during the EMH that results in both, stronger summer and winter monsoons due to enhanced summer and lower winter insolation, respectively (Wen et al., 2016), but with a more important imprint of summer monsoon on the annual mean. Such
<figure><img src="image_23.png" /><figcaption>**Figure 8.** Differences between Mid-Holocene (MH) and CTRL experiments of IPSL-CM5A-LR in (a–e) summer, (f–j) winter, and (k–o) annual mean. The parameters are the same as those in Figure 4.</figcaption></figure>

trends are also well expressed by the $^{230}$Th-normalized detrital flux recorded in the northern AS that documents minimum aeolian input during the EMH (Figure 5I) and probably reflect wettest climate and densest vegetation cover around the AS (Davies, 2006; Engel et al., 2012; Hoelzmann et al., 1998; Jiménez-Espejo et al., 2014; Kuper & Kröpelin, 2006; Swann et al., 2014). In such a scenario, it appears that the EMH annual PP minimum recorded in the northwestern AS results from (a) restricted extension of coastal upwelling cell due to the negative wind stress curl anomalies, (b) limited strengthening in the convective mixing associated to the slight increase of winter wind intensity, and (c) weaker aeolian input associated to a wetter period. Further model-data comparisons on the AS are needed to better unravel the impact the summer versus winter monsoons exert on PP during the Holocene and particularly during EMH.

**5. Conclusions**

We provide for the first time, high-resolution PP and coastal upwelling dynamic records of the northwestern AS over the last 23 Kyr, based on coccolith assemblages from the sediment core MD00-2354. Comparisons of these signals with previous micropalaeontological and geochemical data and new model outputs help us to evaluate the impact of summer and winter Indian monsoons, as well as aeolian dust inputs, on PP. We demonstrate that changes in the surface cooling-driven mixing (winter monsoon), together with changes in aeolian inputs, have a greater impact on nutrient budget in the euphotic and hence PP, than changes in the coastal upwelling (summer monsoon). Thus, this study questions the usual paradigm that PP distribution in the (north-) western AS is necessarily shaped by changes in summer monsoon. Higher PP during the LGM relative to the Holocene resulted from the increase of nutrient concentration in surface layers due to the strengthening of both, surface cooling-driven
mixing and aeolian activity as the winter monsoon was stronger and the summer monsoon was weaker. Similar conditions are recorded during the HS1 and YD cold periods, when weaker AMOC conditions reinforced winter monsoon conditions. Lower PP are recorded during the warm B-A and the Holocene despite the reinvigoration of the coastal upwelling off Somalia and Oman under stronger summer monsoon conditions. Such patterns result from weaker winter monsoon conditions that are accompanied by a weaker surface cooling-driven mixing, a reduction of aeolian activity and overall, lower nutrient inputs in ocean surface layers. Over the Holocene, PP was minimum during the EMH, despite stronger summer and winter monsoon conditions. This is due to the combination of restricted advection of summer upwelling cell under negative wind stress curls with limited winter mixing and lower aeolian input.

## Data Availability Statement

Coccolith and estimated PP data of core MD00-2354 are available in the PANGAEA data repository (Zhou et al., 2022). The data of Last Glacial Maximum, Mid-Holocene, and CTRL experiments of IPSL-CM5A-LR model are respectively available through Kageyama et al. (2016), Braconnot et al. (2016), and Caubel et al. (2016), respectively. The data of TraCE-21 simulation and the single forcing experiments are available in the Earth System Grid of the National Center for Atmospheric Research (https://www.earthsystemgrid.org/project/trace.html; Z. Liu et al., 2009).

## Acknowledgments

The simulations presented in this work have been run on the TGCC (Très Grand Centre de Calcul du CEA) supercomputing equipment under the gen2212 project, provided by GENCI (Grand Equipment National de Calcul Intensif). The authors would like to thank the Institut Pierre Simon Laplace (IPSL) and the researchers who developed the IPSLCM5A model, those who ran the model for the Coupled Modeling Intercomparison Project phase 5 and Paleoclimate Modeling Intercomparison Project phase 3, and the TraCE-21 group for sharing the modeling output via the Earth System Grid of the National Center for Atmospheric Research. The authors wish to thank the on-board scientists of R.V. Marion Dufresne in 2000 for retrieving sediment core MD00-2354. Xinquan Zhou thanks the China Scholarship Council for providing him a PhD grant at the Geoscience Paris-Saclay laboratory (Paris Saclay University). The authors also thank the editors and the two anonymous reviewers for their constructive remarks.

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