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# Wind-driven sediment exchange between the Indian marginal seas over the last 18 000 years

To cite this article: Xiaoying Kang et al 2024 *Environ. Res. Lett.* **19** 084004

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LETTER

Wind-driven sediment exchange between the Indian marginal seas over the last 18 000 years

Xiaoying Kang<fnref n="1" />‚<fnref n="5" /> ID, Zhaojie Yu<fnref n="1" />‚<fnref n="2" />‚<fnref n="3" />‚<fnref n="4" />,* ID, Lina Song<fnref n="1" />‚<fnref n="3" />,* ID, Christophe Colin<fnref n="6" />, David J Wilson<fnref n="7" />, Zehua Song<fnref n="1" />, Bai Su<fnref n="1" />, Xiaojie Tang<fnref n="1" />‚<fnref n="6" />, Fengming Chang<fnref n="1" />, Franck Bassinot<fnref n="8" /> and Shiming Wan<fnref n="1" />

E-mail: yuzhaojie@qdio.ac.cn and linasong@qdio.ac.cn

Keywords: Northern Indian Ocean, Indian Coastal Current, clay minerals, TraCE-21 model, iTraCE model

Supplementary material for this article is available online

Abstract

The Indian Coastal Current is the only channel for material exchange between the two largest marginal seas in the northern Indian Ocean: the Bay of Bengal and the Arabian Sea. However, its past history is poorly known, limiting accurate predictions of its future changes. Here, we present a new clay mineral record from south of India supported by interpretations of model simulations to trace its variability over the last 18 000 years. Decreased smectite/(illite + chlorite) ratios during the cold intervals suggest that a stronger northeasterly wind led to a mean southward flow of the Indian Coastal Current in the Bay of Bengal. In contrast, increased smectite/(illite + chlorite) ratios during the warm intervals suggest the opposite scenario. Combining the proxy record with model simulations, we infer that atmospheric circulation changes were the main driver of the changes. Moreover, a possible link is observed between a positive Indian Ocean Dipole (IOD) and weakened southward flow of the Indian Coastal Current in the Bay of Bengal during the Holocene. These findings imply that future warming scenarios, if associated with more intense positive IOD events as proposed, may lead to a reduction in fresh water transport from the Bay of Bengal to the Arabian Sea.

**1. Introduction**

The Bay of Bengal and the Arabian Sea can only con-
nect through the ocean channel at the southern tip of
the South Asian Continent, where material exchange
can occur. Such exchange depends mainly on the
boundary currents: the East Indian Coastal Current
(EICC) in the Bay of Bengal and the West Indian
Coastal Current (WICC) in the Arabian Sea, both
of which reverse seasonally (Schott and McCreary
Jr 2001) (figure 1(a)), linked to the strong bian-
nual reversal of monsoon winds. The Arabian Sea
is more saline due to strong evaporation, while the

Bay of Bengal is less saline due to strong precipita-
tion and freshwater input from its surrounding rivers
(Subramanian 1993, Prasad 1997), so changes in the
EICC and WICC play an important role in the salin-
ity budgets of the two basins. Notably, long-distance
transport of clay minerals (<2 µm) by ocean cur-
rents over thousands of kilometers has been observed
in a range of settings, including the western Pacific
Ocean (Wu et al 2012, Dang et al 2020) and the
South China Sea (Liu et al 2010), as well as the
Bay of Bengal (Liu et al 2019, Yu et al 2020) and
the Arabian Sea (Phillips et al 2014). In the west-
ern Bay of Bengal, the transport of both sediments

<footnote n="1">¹ Key laboratory of Ocean Observation and Forecasting, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, People's Republic of China</footnote>

<footnote n="2">² Laboratory for Marine Geology, Qingdao Marine Science and Technology Center, Qingdao 266237, People's Republic of China</footnote>

<footnote n="3">³ Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, People's Republic of China</footnote>

<footnote n="4">⁴ College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, People's Republic of China</footnote>

<footnote n="5">⁵ University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China</footnote>

<footnote n="6">⁶ Université Paris-Saclay, CNRS, GEOPS, 91405 Orsay, France</footnote>

<footnote n="7">⁷ Institute of Earth and Planetary Sciences, University College London and Birkbeck, University of London, Gower Street, London WC1E 6BT, United Kingdom</footnote>

<footnote n="8">⁸ LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France</footnote>

<footnote>* Authors to whom any correspondence should be addressed.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Bathymetric map showing the location of core MD77-191 (black star) and other cores: SO130-289KL (Deplazes *et al* 2013), SS-3101G and SS-3104G (Goswami *et al* 2012), VM29-18 to 21 (Colin *et al* 1999), SK187/PC33 (Tripathy *et al* 2011), and SK148/2 (Kessarkar *et al* 2005) (black circles). Arrows show the schematic directions of the winter and summer monsoons, and the EICC and WICC (white solid line, winter; purple dashed line, summer). Pie charts show the clay mineral content of the Indus River (Kessarkar *et al* 2003, Alizai *et al* 2012), Ganga-Brahmaputra River (Sarin *et al* 1989, Heroy *et al* 2003, Khan *et al* 2019), western Indian Peninsula rivers (Kessarkar *et al* 2003), and the Godavari and Krishna rivers on the eastern Indian Peninsula (Bejugam and Nayak 2016), as well as the mean data from core MD77-191 (this study). The discharges of the main rivers are also labelled (Milliman *et al* 1984, Milliman and Syvitski 1992, Alagarsamy and Zhang 2005, Milliman and Farnsworth 2013). (b) and (c) Mean distributions of the European Centre for Medium-Range Weather Forecasts reanalysis of 10-m wind (arrows) and sea level pressure (colour shading) over the northern Indian Ocean during 1979–2018 (Hersbach *et al* 2020) for (b) summer (June to August), and (c) winter (December to February).</figcaption></figure>

and seawater signals from the Ganga-Brahmaputra (G-B) River system to the southern tip of India and the Arabian Sea has been demonstrated by both modern studies (Prasanna Kumar *et al* 2004, Goswami *et al* 2012) and palaeo-reconstructions (Chauhan and Gujar 1996, Liu *et al* 2019), indicating that the mineralogy and geochemistry of clays provide tools for tracing such currents in the past.

The seasonal changes of the Indian monsoon winds reverse the EICC (WICC) (Dandapat *et al* 2018), with the southward (northward) flow occurring during the winter monsoon (figure 1(c) cf figure 1(b)). In terms of the seasonal dynamics, the modern seasonal timing of the strongest monsoon does not completely coincide with the timing of the strongest currents (Dandapat *et al* 2018). These slight discrepancies between the EICC/WICC and local winds are due to Ekman pumping, coastal Kelvin waves, and remote forcing from the equator during specific periods (McCreary *et al* 1996, Shankar *et al* 1996, Mukherjee *et al* 2014, Mukhopadhyay *et al* 2017). Nevertheless, wind forcing is the primary source of the seasonal variability in the large-scale ocean circulation and the seasonal reversals of boundary currents in the north Indian Ocean (Suryanarayana *et al* 1993, Shankar *et al* 2002, Rao *et al* 2010). Research efforts to date have mostly focused on modern features of the EICC/WICC, such as their roles in tropical air-sea interaction (Patnaik *et al* 2014), their seasonal variability (Das *et al* 2020,

Sen *et al* 2022), and their contributions to material exchange between the Bay of Bengal and the Arabian Sea (Varna *et al* 2021, Zhu *et al* 2022). However, their long-term history is less well known, limiting predictions of their future evolution.

The Indian Ocean Walker circulation has a long-term average westerly wind band over the equatorial Indian Ocean (Mohtadi *et al* 2017), but both the intensity and location of the westerly wind band are influenced by the Indian Ocean Dipole (IOD) (Mohtadi *et al* 2017). A positive IOD is characterised by anomalous cooling of the equatorial eastern Indian Ocean, associated with an enhanced equatorial easterly wind anomaly, while a negative IOD indicates the opposite scenario (Saji *et al* 1999). Previous studies showed that a positive IOD can enhance the modern Indian summer monsoon (ISM) (Ashok and Saji 2007, Anil *et al* 2016) and reduce the southward migration of the Intertropical Convergence Zone (ITCZ) (Weller *et al* 2014, Kurniadi *et al* 2021), whereas a negative IOD weakens the ISM and enhances the southward ITCZ migration. The observed interannual link between the ISM and the IOD might also persist on millennial and centennial timescales (Abram *et al* 2009), but this link requires verification. In addition, reconstructions of the IOD over the last 1 ka from coral records reveal significant variability, as well as a trend towards a more frequent positive IOD in the last few decades (Abram *et al* 2020), while climate simulations suggest that a
global warming of 1.5 °C will result in a positive IOD occurring twice as often as during the pre-industrial period (Cai *et al* 2018). Therefore, further studies are needed to explore the detailed influence of the IOD on regional and global climate systems.

In this study, we present new clay mineral data from core MD77-191 offshore of the southern tip of the South Asian Continent (figure 1(a)) to assess changes in the sediment provenance and hence transport by the EICC and WICC to this site over the last 18 ka. We further combine these data with TraCE-21 and iTraCE simulations to explore the dynamics driving past variability of the EICC and the WICC.

## 2. Materials and methods

### 2.1. Sediment core and age model

Core MD77-191 (7°30′ N, 76°43′ E, figure 1(a)) was collected at a water depth of 1254 m and approximately 100 km offshore of the southern tip of the South Asian Continent, during cruise OSIRIS III of the R. V. *Marion Dufresne* in 1977. Its age model was established previously using linear interpolation between 13 accelerator mass spectrometry ¹⁴C dates (Bassinot *et al* 2011, Ma *et al* 2020) (figure S1). Based on this chronology, core MD77-191 spans the last ~18 ka, with linear sedimentation rates ranging from 14 to 89 cm kyr, with a mean of 48 cm/kyr (figure S1).

### 2.2. Clay mineralogy measurements

Clay mineral analyses were conducted on 392 samples from core MD77-191 spanning the last 18 ka, with an average sample resolution of ~45 years. First, samples were treated with 15% hydrogen peroxide solution to remove organic matter, and with 20% acetic acid solution to remove inorganic carbonates. Then, the sediment was washed 4–5 times with deionised water, and the clay fraction (grain size < 2 µm) was separated from the detrital sediments according to Stokes’ law. The clay mineral compositions were determined by x-ray diffraction, using a D8 ADVANCE diffractometer with CuKα radiation at IOCAS. Oriented mounts of the non-calcareous clay-sized (<2 µm) particles were analyzed (Wan *et al* 2012). The Jade 6.5 software was used to semi-quantitatively obtain the relative content of the clay minerals, with an uncertainty better than 5% (2SD).

Combined Sr-Nd isotopes, and to a lesser extent clay mineralogy, are extensively used as robust tracers of sediment sources and transport processes (Kessarkar *et al* 2003, Li *et al* 2018). For core MD77-191, Sr-Nd isotopes were previously analysed on the clay-sized detrital fraction (<2 µm), which minimises grain-size effects, such that provenance is the main driver of variations (Yu *et al* 2022).

### 2.3. Transient climate evolution modeling

The Transient Climate Evolution (TraCE-21) model is a fully-coupled, non-accelerated atmosphere-ocean-sea ice-land surface simulation of the last 21 ka completed using the CCSM3 (Collins *et al* 2006, Liu *et al* 2009). This model allows the investigation of coupled atmosphere-ocean-sea ice-land surface interactions in the climate system. The iTraCE simulations are performed in the iCESM1.3, with realistic forcings applied in the time range from 21 ka to 11 ka before present (He 2021, He *et al* 2021). The iTraCE model contains 4 simulations: (1) ice sheets, greenhouse gases, orbital insolation, and meltwater fluxes, all forcing runs; (2) factorised-forcing runs with ice sheets, greenhouse gases, and orbital forcing; (3) factorised-forcing runs with ice sheets and orbital forcing; and (4) factorised runs with only ice sheet forcing. We note that the resolution of the TraCE-21 model (3.75° latitude-longitude resolution) (Collins *et al* 2006, Liu *et al* 2009) is lower than the iTraCE model (atmosphere and ocean model resolution are nominally 2° and 1°, respectively) (He 2021, He *et al* 2021). However, the time span of the TraCE-21 model is longer than the iTraCE model, so the two models are complementary.

In this study, we used the output from these two models to simulate upper-ocean currents, surface winds, and sea level pressure in the South Asian continental and marine areas during the intervals of Heinrich Stadial 1 (HS1: 18–14.7 ka), the Bølling-Allerød (B/A: 14.7–12.9 ka), the Younger Dryas (YD: 12.5–11.5 ka), the early Holocene (EH: 10–8 ka), and the late Holocene (LH: 2–0 ka), for summer, winter, and the annual mean (figures S3–S10). The datasets used are the monthly and annual outputs from the full-forcing, which are available at www.cgd.ucar.edu/ccr/TraCE/ and www.earthsystemgrid.org/dataset/ucar.cgd.ccsm4.iTRACE.html.

## 3. Results and discussion

### 3.1. Clay mineral sources in core MD77-191

It is difficult to distinguish sediment provenance in core MD77-191 using Sr-Nd isotopes alone, because some of the potential end members are very similar and cannot be effectively distinguished (figure 2(a)). The Sr-Nd isotopic compositions of core MD77-191 overlap with the compositions of the G-B River system and the eastern Indian Peninsula rivers, but could theoretically also be explained by a mixture of sediments from the Indus River and the western/southern Indian Peninsula rivers (figure 2(a)). Several VM and SK cores located in the western Bay of Bengal were also suggested to derive their sediment mainly from the G-B River system and the eastern Indian Peninsula rivers (Colin *et al* 1999, Kessarkar *et al* 2005, Tripathy *et al* 2011, Goswami *et al* 2012) (figures 1(a)
<figure><img src="image_4.png" /></figure>

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

**Figure 2.** (a) εNd versus ⁸⁷Sr/⁸⁶Sr cross plot for Holocene and deglacial sediments from core MD77-191 (Yu et al 2022). These data are compared to potential sources (shaded fields): Sri Lanka (Perera and Kagami 2011), Deccan Traps (Lightfoot and Hawkesworth 1988, Dessert et al 2001), eastern Indian Peninsula rivers (mainly Godavari and Krishna rivers) (Ahmad et al 2009), Ganga-Brahmaputra (G-B) River (Singh and France-Lanord 2002, Lupker et al 2013), Indus River (Kessarkar et al 2003, Clift et al 2008, 2010, Yu et al 2019), western Indian Peninsula rivers (Goswami et al 2012), and southern Indian Peninsula rivers (no data available, so based on its source region: Peninsular Gneisses) (Goswami et al 2012). They are also compared to data from other sediment cores (symbols): SS-3101G (Last Glacial Maximum, LGM) and SS-3104G (Goswami et al 2012), VM29-18 to 21 (Colin et al 1999), SK187/PC33 (Tripathy et al 2011), and SK148/2 (Kessarkar et al 2005). (b) Smectite-(illite + chlorite)-kaolinite ternary diagram, showing clay mineral assemblages in core MD77-191 (green dots and triangles; this study) compared to the G-B River system (blue dots) (Sarin et al 1989, Heroy et al 2003, Khan et al 2019), eastern Indian Peninsula rivers (Godavari and Krishna; blue triangles) (Bejugam and Nayak 2016), southern Indian Peninsula rivers (orange triangles) (Mascarenhas-Pereira et al 2023), western Indian Peninsula rivers (pink triangles) (Kessarkar et al 2003), and the Indus River (pink dots) (Kessarkar et al 2003, Alizai et al 2012).

and 2(a)). In addition, sediment transported from the Bay of Bengal was proposed to have led to an excursion in the Sr-Nd isotopic compositions in core SS-3101G located in the southeastern Arabian Sea during the Last Glacial Maximum (Goswami et al 2012).

The Sr-Nd isotopic compositions in core MD77-191 are close to the values in core SS-3101 G from the Last Glacial Maximum and in some of the cores from the western Bay of Bengal (figure 2(a)), attesting to significant sediment sources from the Bay of Bengal.
The clay mineral assemblage of core MD77-191 consists mainly of illite (14%-70%, average 40%) and smectite (0%-71%, average 34%), with lower kaolinite (5%-40%, average 16%) and chlorite proportions (1%-23%, average 10%) (figure S2). In general, the illite content is inversely correlated to the smectite content (R = -0.98, P < 0.01), while the illite and chlorite contents show similar patterns through time (R = 0.56, P < 0.01), although with some differences in detail (figure S2). Given the high sedimentation rate in core MD77-191 (~48 cm kyr⁻¹), it is clear that the clays are mainly riverine-derived, while local authigenic clay formation and wind-blown dust deposition would have made negligible contributions. Combining the Sr-Nd isotopic compositions and the clay mineral data (figure 2), we suggest that the sediments in core MD77-191 are likely to result from the mixing of sediments from the Bay of Bengal (G-B River system and/or eastern Indian Peninsula rivers) and the Arabian Sea (Indus River system and/or western Indian Peninsula rivers).

The smectite content in core MD77-191 was generally higher during the warm Holocene and the B/A periods (figures 2(b) and S2), which corresponds to sediment sources from the Arabian Sea. Hence, we consider that smectite was derived mainly from the Arabian Sea side, such as from the western Indian Peninsula rivers and/or the Indus River (figure 2(b)). In contrast, the higher illite and chlorite content in core MD77-191 during the cold YD and HS1 intervals (figures 2(b) and S2) indicates a trend towards the composition of clays in the G-B River system, suggesting their derivation from the Bay of Bengal side (figure 2(b)). Although the eastern Indian Peninsula rivers also supply smectite (figure 2(b)), those sediments can only be transported to core MD77-191 by southward flow of the EICC driven by the winter monsoon (figure 1(a)). In contrast, core MD77-191 had a very low smectite content during the intervals with a strong winter monsoon, such as HS1 and the YD (figures S2 and 2(b)), which does not support a major role for inputs from the eastern Indian Peninsula rivers. Additionally, although Sri Lanka and the southern Indian Peninsula (Peninsular Gneisses) are geographically closer to the core site, these regions have lower river runoff, and their Sr-Nd isotopes and clay mineral compositions are distinct from the MD77-191 sediments (figure 2). Specifically, the average clay mineral composition for the southern Indian Peninsula is 73% smectite, 8% illite, 1% chlorite, and 18% kaolinite (Mascarenhas-Pereira et al 2023), whereas the MD77-191 sediments average 34% smectite, 40% illite, 10% chlorite, and 16% kaolinite. Therefore, we consider their contributions may also be an almost continuous, but minor, background input.

Overall, the smectite/(illite + chlorite) ratios in core MD77-191 are an effective indicator of changes

through time in the sediment sources to the core site (figures 2(b) and 3(a)) and could be used to represent the material exchange history between the Bay of Bengal and the Arabian Sea during the last deglacial and Holocene intervals. Specifically, during the warm Holocene (11.7-0 ka) and B/A (14.7-12.9 ka) periods, a strong summer monsoon could drive a southward flow of the WICC that transported more sediments from the Arabian Sea, but restricted sediment transport from the Bay of Bengal to core MD77-191 (figure 1). Conversely, during the cold YD (12.9-11.7 ka) and HS1 (18-14.7 ka) periods, the opposite scenario could have occurred, with strong southward flow of the EICC driving sediment export from the Bay of Bengal to core MD77-191.

## 3.2. Sediment transport to core MD77-191 by the Indian Coastal Current

There are two possible major controls on the variability in clay mineral assemblages in core MD77-191: (1) changes in riverine inputs over South Asia controlled by summer monsoon precipitation, and (2) changes in sediment transport by ocean currents from the river mouths to our study site. Previous studies suggested that during the Holocene and B/A periods, the increased ISM precipitation and the melting of Himalayan glaciers meant that the G-B River system transported more sediments from the Himalayas into the ocean (Tripathy et al 2011, Joussain et al 2016, Li et al 2018). Therefore, at first glance, those findings seem inconsistent with the findings in this study, which instead show a reduced G-B River contribution at this time (figure 3(a)).

We argue that this apparent discrepancy arises because core MD77-191 is located offshore of the southern tip of the South Asian Continent, with its sediments being mainly transported from the Bay of Bengal and Arabian Sea by the EICC and WICC, respectively (figure 1(a)). Specifically, during the Holocene and B/A periods, the mean position of the ITCZ was further north (figure 3(d)), and the enhanced ISM could have led to higher precipitation (figure 3(c)). The enhanced precipitation would have increased erosion, and thereby increased riverine inputs to the ocean. Meanwhile, the strong ISM wind (figure 1(b)) could have led to a strong southward-flowing WICC (and northward-flowing EICC), thereby transporting sediments containing smectite from the Indus River and western Indian Peninsula rivers to core MD77-191, and restricting the supply of sediments containing illite and chlorite from the G-B River system (figure 3(a)). Conversely, during the YD and HS1, the southward movement of the ITCZ and the weakened ISM decreased the precipitation intensity (figures 3(c) and (d)), which would have weakened erosion and reduced riverine sediment fluxes. However, crucially, the enhanced winter monsoon during cold periods (figure 1(c))
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Comparison of clay mineralogy in core MD77-191 to regional climate proxies. (a) Smectite/ (illite + chlorite) ratio in core MD77-191 (this study). (b) Sea surface temperature (SST) gradient between the Eastern and Western Equatorial Indian Ocean (Kuhnert *et al* 2014, Mohtadi *et al* 2014, Romahn *et al* 2014, Weldeab *et al* 2022), as an indicator of the Indian Ocean Dipole (IOD). Within the Holocene, we show the average (mean) values of the SST gradient (black dashed line in panel (b)) and the smectite/(illite + chlorite) ratio (red dashed line in panel (a)). When the SST gradient is lower than the average value, it implies a positive IOD-like mode; when it is higher, it implies a negative IOD-like mode. We also show the average values of the smectite/(illite + chlorite) ratio (AVG<sub>G</sub>) and corresponding average temperature gradient (AVG<sub>TG</sub>) in the different intervals (marked by vertical blue dotted lines) to enable a quantitative comparison. (c) Integrated Indian Summer Monsoon (ISM) proxy based on stalagmite oxygen isotope records (bars show 2σ uncertainty) (Yu *et al* 2022). The bold curves in (a)–(c) are 5-point running means. (d) Total reflectance (lightness, L*) from the Arabian Sea (Deplazes *et al* 2013), which reflects the latitudinal position of the intertropical convergence zone (ITCZ). (e) Upper-ocean (100 m) zonal current anomalies in the iTraCE model near site MD77-191. (f) Comparison between TraCE-21 and iTraCE modeled mean annual zonal wind speed anomalies near site MD77-191. Only the zonal wind speeds are shown here, since these can transport material between the basins.</figcaption></figure>

could have driven a strong southward-flowing EICC (and northward-flowing WICC), thereby transporting more sediments containing illite and chlorite from the G-B River system and eastern Indian Peninsula rivers to core MD77-191, and preventing the supply of smectite-rich sediments from the Indus River and western Indian Peninsula rivers (figure 3(a)).

Therefore, we suggest that the Indian Coastal Current transportation, rather than the river sediment fluxes from the South Asian Continent, was the major factor controlling smectite/(illite + chlorite) ratios in core MD77-191. Hence, the ratio of smectite/(illite + chlorite) can indicate the variability of the EICC/WICC. Higher ratios indicate stronger southward-flowing WICC/northward-flowing EICC, while lower ratios indicate stronger southward-flowing EICC/northward-flowing WICC. We also note that the smectite content reached zero during some periods (e.g. HS1) (figures S2 and 3(a)), which was probably related to a very weak southward WICC at these times (Yang *et al* 2023). However, the proxy is not expected to provide a quantitative measure of the EICC and/or WICC strength, due to possible non-linearities over its range, such as the above feature, as well as the potential for variable monsoon-driven sediment inputs to exert a secondary control through time.

### 3.3. Indian coastal current changes linked to Indian Ocean atmospheric changes

While the positive IOD mode was prevalent throughout the last deglacial period, during HS1 and the YD, lower smectite/(illite + chlorite) ratios are consistent with a southward ITCZ and a weaker ISM (figures 3(a)–(d)). Comparatively, during the B/A period, higher smectite/(illite + chlorite) ratios coincided with a northward ITCZ and a strong ISM (figures 3(a)–(d)). These observations suggest that major millennial-scale fluctuations in the ISM and ITCZ jointly drove the Indian Coastal Current changes. Therefore, the Holocene may be the key period in which the individual influences of these three factors can be better distinguished, because each of them followed a different temporal evolution (figures 3(b)–(d)). In addition, the potential effect of global sea level on sediment transport can be excluded as a main driver during the Holocene given that sea-level changes were modest during this interval, with sea level being relatively stable since ~8 ka (Waelbroeck *et al* 2002).

During the Holocene, the long-term trends in smectite/(illite + chlorite) ratios in core MD77-191 are consistent with the variations of the ISM proxy. In contrast, the sub-millennial scale fluctuations in the smectite/(illite + chlorite) record do not correspond to changes in the ISM proxy, but are similar to the IOD proxy record. From 9.5 to 7.8 ka and from 6.0 to
4.9 ka, the negative IOD-like conditions were gener-
ally associated with lower smectite/(illite + chlorite)
ratios, whereas from 11.7 to 9.5 ka and from 7.8 to
6.0 ka, the positive IOD-like conditions were gener-
ally accompanied by higher smectite/(illite + chlor-
ite) ratios (figures 3(a) and (b)). There are some
anomalies in the above relationship from 4.9 ka to
the present, possibly because the IOD modes shifted
frequently during this time. Nevertheless, the above
observation implies that negative IOD conditions are
associated with strengthened southward flow of the
EICC and increased contributions of illite and chlor-
ite to core MD77-191, consistent with modern studies
(Dandapat et al 2018, Sherin et al 2018). In compar-
ison, neither the ISM nor the ITCZ proxies show com-
parable millennial- to centennial-scale fluctuations
during the Holocene.

We use model simulations to further explore the
changes in the Indian Coastal Current and its driv-
ing mechanisms. In the modern day, wind forcing
plays an important role in driving the Indian Coastal
Current (McCreary et al 1996, Shankar et al 2002,
Dandapat et al 2018, Mukherjee and Kalita 2019, Sen
et al 2022). Our proxy reconstruction can be com-
pared with the simulated annual upper-ocean current
velocity and surface wind speed near core MD77-191
derived from both the TraCE-21 and iTraCE mod-
els (figures 3(e) and (f)). Compared to the long-
term time series of TraCE-21, iTraCE only covers the
deglacial interval, including the HS1, the BA, and
the YD. During these three time periods, the trends
of wind speed and current velocity are consistent
between TraCE-21 and iTraCE, although their amp-
litudes are different (figures 3(e), (f) and S3–S7).
These results suggest that both the high-resolution
iTraCE and the low-resolution TraCE-21 are capable
of simulating the past atmospheric circulation in the
Indian Ocean. In addition, the TraCE-21 model, with
its longer time series, shows relatively stable decreas-
ing trends in wind speed through the Holocene near
core MD77-191 that are consistent with the long-term
trend of smectite/(illite + chlorite) ratios (figures 3(a)
and (f)). Owing to the high spatio-temporal resol-
ution of the iTraCE model, the large-scale surface
ocean currents it simulates in the Indian Ocean are
consistent with modern summer and winter surface
currents (figures 8 and 9 in (Schott and McCreary
2001)). Based on the above evidence, we consider that
the wind derived from the low-resolution but longer
TraCE-21 model could generally be expected to rep-
resent the large-scale circulation changes during the
last deglaciation.

Considering the velocity and direction of the
Indian Coastal Current in the TraCE-21 model is
mainly driven by the winds (figures 3(e) and (f)),
we further used the surface winds and sea level

pressure outputs (figure S8–S10) to calculate anom-
alies between time periods. Specifically, we used
B/A minus HS1, and EH minus YD, to show the
changes during the last deglacial transition period
(figures 4(a)–(d)). We also used EH minus LH to
reveal the changes during the more stable Holocene
period (figures 4(e) and (f)).

The calculated anomalies comparing deglacial
cold and warm states, and comparing the EH and
LH, indicate two key features. Firstly, for the B/A-
HS1 wind and sea level pressure anomalies, and to a
lesser extent the EH-YD anomalies, during both sum-
mer and winter, there is an anticyclone anomaly in
the Southern Hemisphere (blue box in figures 4(a)–
(d)) and a cyclone anomaly on the South Asian
Continent (red box in figures 4(a)–(d)). The anti-
cyclone anomaly in the Southern Hemisphere could
induce a southern-sourced equatorial easterly wind
anomaly and a positive IOD mode, while the cycl-
lone on the South Asian Continent could cause a
prevailing southeasterly wind (stronger ISM) along
the eastern coast of the Indian Peninsula. This wind
could induce a mean flow into the Bay of Bengal,
thereby blocking the illite and chlorite derived from
the Bay of Bengal, which is consistent with higher
smectite/(illite + chlorite) ratios (figure 3(a)). In con-
trast, for the EH-LH anomaly during summer, while
the anticyclone anomaly in the Southern Hemisphere
and the positive IOD mode persist (blue box in
figure 4(e)), the cyclone anomaly on the South
Asian Continent disappears and a new anticyclone
anomaly appears in the Bay of Bengal (red box in
figure 4(e)). This new anticyclone anomaly would
strongly enhance the flow into the Bay of Bengal, also
leading to higher smectite/(illite + chlorite) ratios
as observed (figure 3(a)). For the EH-LH anom-
aly simulated during winter, both the wind and
sea level pressure anomalies are different, with no
clear cyclone/anticyclone observed in those locations
(figure 4(f)).

Secondly, the wind direction over the equatorial
Indian Ocean is generally the same between the sim-
ulations of the B/A-HS1, EH-YD, and EH-LH anom-
alies in the summer (figures 4(a), (c) and (e)), but
is reversed in the winter simulation of the EH-LH
anomaly (blue ellipse in figure 4(f)). The B/A-HS1
and EH-YD anomalies during winter in the equatorial
Indian Ocean exhibit easterly intensification, similar
to the summer simulations, implying a positive IOD-
like anomaly (figures 4(e) and (d)). However, this
state changes to an enhanced westerly wind, particu-
lar for the western part, in the EH-LH anomaly simu-
lated during winter, indicating a more negative IOD-
like anomaly (blue ellipse in figure 4(f)). Despite the
appearance of a negative IOD anomaly in winter, the
enhanced smectite/(illite + chlorite) ratios for the EH
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Wind (arrows) and sea level pressure anomalies (colour shading) simulated in the TraCE-21 model. (a), (b) Bølling-Allerød (B/A) minus Heinrich Stadial 1 (HS1) anomaly. (c), (d) Early Holocene (EH) minus Younger Dryas (YD) anomaly. (e), (f) Early Holocene (EH) minus late Holocene (LH) anomaly. Panels (a)–(c) with JJA (June, July, August) show the summer mean, and panels (d)–(f) with DJF (December, January, February) show the winter mean.</figcaption></figure>

compared to the LH (figure 3(a)) suggest that the positive IOD mode during summer (figures 3(b), 4(e) and S9), and/or the effect of the strong ISM during the EH, dominated the sediment transport in the Indian Coastal Current.

In recent decades, more frequent and more intense positive IOD events have been observed, potentially linked to global warming and enhanced zonal sea-surface temperature gradients across the equatorial Indian Ocean (Abram et al 2008, Cai et al 2014). These intense positive IOD events under global warming also coincide with extreme climate variability in the tropical Indo-Pacific (Abram et al 2020). Based on the iTrace modeling of seawater exchange between the Bay of Bengal and Arabian Sea (figure S11), and the past relationship between the Indian Coastal Current and the IOD observed in our study for the Holocene (figure 3), a more positive IOD

could be expected to reduce the net fresh water inputs from the Bay of Bengal into the Arabian Sea, but it could possibly increase the intensity or variability of water exchange.

## 4. Conclusions

Based on clay mineral data from core MD77-191 in combination with modeling results, we reconstructed changes in sediment transport by the Indian Coastal Current and evaluated its controlling factors over the last 18 ka. During the B/A and the Holocene, a generally stronger mean southward-flowing WICC transported more smectite from the Arabian Sea, while a northward-flowing EICC restricted illite and chlorite from the Bay of Bengal, leading to higher smectite/(illite + chlorite) ratios. During HS1 and the YD, the opposite scenario occurred, with a
strengthened southward-flowing EICC. The modeling results show that changes in atmospheric circulation patterns could have exerted an important control on the Indian Coastal Current flow strength and/or direction. In addition, during the Holocene, we observe a possible link between a positive IOD and a strengthened northward-flowing EICC in the Bay of Bengal. Hence, if future climate warming leads to a more frequent or stronger positive IOD state, as has been proposed, less fresh water could be transported from the Bay of Bengal to the Arabian Sea, thereby enhancing the existing salinity gradients.

## Data availability statement

The data are available at Zenodo (https://zenodo.org/records/11257419).

All data that support the findings of this study are included within the article (and any supplementary files).

## Acknowledgments

This study was supported by the National Natural Science Foundation of China (42376055, 42176034 and 91958107), Strategic Priority Research Program of Chinese Academy of Sciences (XDB42010402), Natural Science Foundation of Shandong (ZR2022YQ33), Youth Innovation Promotion Association, CAS (2020210) and the National Key Research and Development Program of China (2022YFF0800503). D J W was supported by a Natural Environment Research Council independent research fellowship (NE/T011440/1). The CESM project and NCAR CISL supercomputing resources (doi:10.5065/D6RX99HX) are acknowledged for running iTraCE modelling. For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising.

## Conflict of interest

The authors declare no competing financial or non-financial interests.

## ORCID iDs

Xiaoying Kang ◦ https://orcid.org/0009-0001-2243-8543

Zhaojie Yu ◦ https://orcid.org/0000-0003-4135-9418

Lina Song ◦ https://orcid.org/0000-0002-5191-6719

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