Abrupt upwelling and CO₂ outgassing episodes in the north-eastern Arabian Sea since mid-Holocene

Abstract

Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO₂ due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ¹¹B) based seawater pH and pCO₂ (or pCO₂^sw) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO₂ exchange. Here we present the first foraminifera-based δ¹¹B record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8–1 ka). Our record suggests that the region was overall a moderate to strong CO₂ sink during the last 7.7 kyr. The region behaved as a significant CO₂ source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO₂ outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO₂ outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.

Introduction

The recent rapid increase in anthropogenic CO₂ has led to ocean acidification negatively impacting marine carbonate ecosystems¹. The level of carbon dioxide (CO₂) in the atmosphere has a direct role in regulating the chemistry of the ocean. The dissolution of anthropogenic CO₂ in seawater has lowered ocean pH and carbonate ion concentrations and potentially reduced the ability of marine organisms to calcify². Pre-industrial variations in atmospheric CO₂ are believed to be strongly influenced by changes in circulation and carbon cycling in the ocean^3,4,5 where the interaction between large deep ocean carbon reserves and dynamic ocean circulation plays an important role in regulating the ocean carbonate system and air-sea gas exchange of CO₂^3,4. The mechanism involved in this exchange of CO₂ consists of several complex processes which alter the magnitude of oceanic sinks (mainly cold, high latitude regions) and sources (mainly warm, low latitude and tropical regions)^3,6,7. The ocean carbonate system consists of six co-varying parameters (pH, [CO₂], [HCO₃⁻], [CO₃²⁻], total alkalinity (TA) and dissolved inorganic carbon (DIC)), with the full system resolvable provided any two of these six components are known. Moreover, surface water pH and pCO₂ are very tightly coupled in natural systems and thus knowledge of one is enough to infer the other with high confidence⁸. On a glacial-interglacial timescale, the processes influencing the concentration of CO₂ in the atmosphere are thought to leave a fingerprint on the surface water pH⁹. Also, the pH of the surface water is the primary control of the boron isotope composition in planktonic foraminifera^{10,11,12,13,14}. Therefore, the boron isotope signatures (δ¹¹B) of planktonic foraminifera provides a good opportunity to better understand the changes in surface ocean chemistry (pH) and the mechanisms involved in regulating the ocean carbonate system before human influence and industrialization, as well as providing a way to produce reconstructions of past pH and atmospheric CO₂¹⁵.

The Arabian Sea is one of the most productive basins of the world, having the capability to release around 90 TgC yr⁻¹ to the atmosphere^16,17. The monsoon-influenced lateral and vertical circulation of nutrients in the water column plays an important role in regulating the productivity of the basin¹⁸. The majority of the southern, western and central parts of Arabian Sea are characterised by high productivity during the summer season (during south-west monsoon or SWM), due to high riverine flux and SWM influenced upwelling^19,20. However, the northern and north-eastern parts of Arabian Sea experience comparatively high productivity during the winter season^21,22, this is due to the combined effect of northeast monsoon (NEM) influenced winter mixing^23,24,25 as well as the nutrient-rich water advected from Bay of Bengal via the south-eastern Arabian Sea (West Indian Coastal Current or WICC).

Surface water pCO₂ (or pCO₂^sw) in the Arabian Sea is strongly influenced by physical, chemical, and biological processes which ultimately depend on the Indian Summer Monsoon and related sea surface circulation^17,26,27. The available palaeo-pCO₂^sw records from Arabian Sea suggest that the north central sector (Site NIOP464 around Murray Ridge) was a consistent source of the CO₂ between 29 and 5 ka, whereas the south-eastern Arabian Sea (Site AAS9/21) is thought to be a significant CO₂ sink from 23 to 5 ka^17,28 (Fig. 1). Interestingly, both studies point towards the strength of Southwest Monsoon (SWM) as the main driver of pCO₂^sw variation in the Arabian Sea in the past. Modern studies from Arabian Sea show a significant role for regional oceanography and climate on pCO₂^sw variation during pre-, post-, and inter-monsoon seasons²⁹. Studies from coastal regions suggest that the near shore waters tend to show much more short-term temporal and spatial pCO₂^sw variations than the open ocean due to large input of terrestrial carbon and nutrients from rivers and ground waters^30,31. More specifically, the continental shelves are CO₂ sinks with increased pH, whereas nearshore ecosystems behave as a strong CO₂ sources with decreased pH³⁰. Studies from upwelling areas suggest that the coastal Arabian Sea, in general, has potential to behave as both a source and sink of carbon dioxide³². This is due to the supply of terrestrial fresh water input mixed with terrestrial detritus and nutrients (including C). On a short-term, such coastal areas show lower pH estimates than further offshore and behave as CO₂ source. However, on the longer term, the biologically fixed carbon is deposited with the sediments in the deeper parts, which make the region a CO₂ sink with relatively high pH³³. Studies focused on the interaction between surface layer pCO₂ and air-sea exchange suggest that overall the modern Arabian Sea is a weak CO₂ source having a potential to degas ~ 460 mmol C/m²/yr³⁴. However, studies related to the quantification of monsoon-controlled export flux to the interior parts of Arabian Sea suggests that the carbon-captured by sediments during upwelling is a potential sink of ~ 820 mmol C/m²/yr³⁵. By virtue of such air-sea interaction, several modelling studies suggest that the inter-annual SST variability in the Indian Ocean is linked to internal oceanic cycling as well as the external atmospheric forcing^36,37,38,39. However, these existing teleconnections are yet to be explored in past climate records.

Figure 1

Map of modern sea surface ΔpCO₂ distribution in the world ocean (60°N–60°S)²⁶ (https://www.pmel.noaa.gov/co2/story/Surface+CO2+Flux+maps). Red triangle shows the current study site SK-240/485 nearby offshore Saurashtra, north-eastern Arabian Sea. Black circles show the other boron isotope records referred to in this study. The inset figure shows ΔpCO₂ distribution in the northern Indian Ocean. The Map was prepared using Ocean Data View (http://www.odv.awi.de)⁴⁰.

Therefore, studying a sediment core from coastal Arabian Sea may provide new insights into the mechanism(s) of past air-sea interaction, related variations in pCO₂^sw and surface water acidification, and their possible global teleconnection. Here we used the coupled analysis of δ¹¹B and Mg/Ca ratio in planktonic foraminifera (Globigerinoides ruber sensu stricto (white), hereafter G. ruber) in a marine sediment core SK-240/485 collected from continental shelf offshore Saurashtra, NE Arabian Sea (Fig. 1). This study provides insights into the past pH and pCO₂^sw, sea surface temperature (SST) and salinity (SSS) variations to understand the role of the monsoon and associated regional oceanographic settings in regulating the past air-sea interaction in the area during mid-late Holocene.

Results and discussion

The surface water above core SK-240/485, is strongly influenced by the SWM with very little contribution from the NEM^41,42, despite being located in the NE Arabian Sea. The SWM is considered as the major regulator of SST and SSS, particularly in coastal regions. This can also be observed in the monthly salinity record of nearby location (21.5°N; 68.5°E) which shows an extreme SSS decrease around October (post-SWM) (Supplementary Fig. 1). The surface-dwelling foraminifera G. ruber inhabits the upper water column (down to ~ 25 m) when the surface water is well stratified due to increased fresh water discharge in the basin during the SWM season. Therefore, the reconstructed signals from the G. ruber proxy may be biased towards the SWM season (JJAS)^43,44,45. The Mg/Ca ratio of G. ruber at SK-240/285 shows an average value of 4.70 mmol/mol (n = 20) which corresponds to an average SST of 27.9°C in the area during mid-late Holocene (7.72–1.44 ka) (Fig. 2a). At present, the average annual SST in the region is 27.2°C with maximum (29.3°C) and minimum (24.3°C) recorded in June and February respectively (Supplementary Fig. 1). The SST recorded by core SK-240/485 ranges between a minimum of 26.7°C (around 3.65 ka) to a maximum of 29.7°C (around 1.56 ka), which shows an overall 3°C variation of SST during the last 7.7 kyr (Fig. 2a). Mg/Ca based SST records from other parts of Arabian Sea suggest 2–3 °C variation in SST during the Holocene^{46,47,48,49,50} (site specific details in Supplementary Table 1), which is similar to the alkenone-based SST records from northern Arabian Sea (offshore Pakistan) which show ~ 3°C variation in SST during the Holocene attributed to the effect of winter monsoon in the area^51,52.

Figure 2

Down core variation of (a) Sea surface temperature (SST) (°C) and Mg/Ca ratio in G. ruber (mmol/mol) (b) Salinity (psu) and δ¹⁸O_sw (‰ VSMOW) (c) δ¹¹B (‰) (d) pH (total scale) (e) pCO₂^sw (µatm) and (f) ΔpCO₂ (pCO₂^sw–pCO₂^atm) (µatm) in the core SK-240/485. Point at y-axis (left side) represents the modern (2000 AD) value around the core site. Shaded region around the curve represents 2SD error of the data. In panel (a) dark shade = 2SD error of Mg/Ca and light shade = 2SD error of SST. Vertical grey bands mark the abrupt CO₂ outgassing episodes.

The δ¹⁸O_sw derived SSS in the core SK-240/485 varies between 36.0 and 34.4 psu and shows a freshening trend during mid-late Holocene (Fig. 2b). The δ¹⁸O_sw has been extensively used as a proxy for SWM-influenced evaporation-precipitation budget in the northern Indian Ocean^50,53. The δ¹⁸O_sw at our site varies between +0.93 and −0.70 ‰ (VSMOW), with most negative value observed during the late Holocene (after ~ 4 ka; Fig. 2b and Supplementary Fig. 2b). Such a lowering of δ¹⁸O_sw suggests a significant decrease in the evaporation-precipitation budget of the area, perhaps due to an overall intensification of the SMW during 8–1 ka. A Mann–Kendall trend test reveals a significant increasing trend in δ¹⁸O_sw over our study interval [Ʈ = 0.686; p value (two tailed) < 0.0001] (Supplementary Fig. 2b). These observations thus point towards the increased intensification of SWM between 8 and 1 ka, as the major contribution of fresh water to the core site comes from the Indus River and small seasonal tributaries during the SWM. Previously, a δ¹⁸O_G.ruber based statistical and spectral study from the same site (SK-240/485) also suggested an intensifying SWM trend during the Holocene^41,42. In addition, it is in good agreement with other studies from eastern Arabian Sea during this interval^54,55.

The boron isotopic composition of the foraminifera (δ¹¹B) shows an average value of 21.0‰ and ranges between 21.7‰ (at around 1.44 ka) and 19.8‰ (at around 4.44 ka; Fig. 2c). The reconstructed pH in the core SK-240/485 has an average value of 8.19 pH units for last ~ 7.7 kyr (Fig. 2d), which shows a good agreement with the existing pH records of the Holocene from the Pacific (~ 8.15 pH units), Atlantic (~ 8.16 pH units), and Indian (~ 8.22 pH units) Oceans^{9,27,28,56,57,58}. More specifically, the variation of pH in the core SK-240/485 ranges between 8.02 (around 4.44 ka) and 8.28 (around 1.44 ka; Fig. 2d). The Mann Kendall trend test suggests a significant increasing trend [Ʈ = 0.386; p-value (two tailed) < 0.028] in surface water pH since 7.7 ka (Supplementary Fig. 2a). The calculated average pCO₂^sw at the study site for last 7.7 kyr is 247 µatm which is ~ 22 µatm (or ppm) lower than the average atmospheric CO₂ value (around 269 ppm) obtained from EPICA Dome-C ice core record (pCO₂^atm) over the same period⁵⁹. The pCO₂^sw attains a maximum of 415 µatm at around 4.4 ka and minimum of 170 µatm at around 1.4 ka (Fig. 2e). While the average pCO₂^sw at the site SK-240/485 is considerably lower than the average Holocene pCO₂^sw values recorded in Atlantic and Pacific Ocean^9,58,60, it is significantly higher (~ 30 µatm) than that recorded at site AAS9/21 in the south-eastern Arabian Sea²⁸ since ~ 8 ka (247 µatm at SK-240/485 vs ~ 214 µatm at AAS9/21). A modelling-based study of the evolution of pCO₂^sw from the Arabian Sea suggests the existence of a strong north–south gradient in the Arabian Sea due to the seasonal sea surface circulation dynamics¹⁷. The south-eastern Arabian Sea shows comparatively lower pCO₂^sw due to the fresh water input from the Bay of Bengal through the West Indian Coastal Current (WICC) during the NEM⁴¹. On the contrary, the NE Arabian Sea (including the present site) does not receive fresh water input from the WICC and therefore shows comparatively higher pCO₂^sw during modern times^41,55.

For a better understanding of the source/sink nature and quantification of air-sea CO₂ exchange in the studied region, we calculated the ΔpCO₂ (the difference of pCO₂^sw and pCO₂^atm) at each time interval (Fig. 2f). The cross-plot between ΔpCO₂ and salinity at the site SK-240/485 shows a significant positive correlation (r = 0.65; p < 0.01; R squared = 0.41 with exclusion of two points of high ΔpCO₂ (which are probably related to intense upwelling, see below, Supplementary Fig. 3). This suggests that the SWM-influenced freshwater input plays a significant role in the ΔpCO₂ variation of the area in the absence of strong upwelling. We found that the area was an overall CO₂ sink during the last 7.7 kyrs with average ΔpCO₂^sw around − 25 µatm (ranging between 170 and 415 µatm) which is a value characteristic of shelf regions³⁰. Between 7.7 and 6.1 ka, the region was in quasi-equilibrium with the atmosphere with respect to CO₂ (ΔpCO₂ < 10 µatm).

The record shows two abrupt periods since 7.7 ka when the region behaved as a significant CO₂ source, ~ 5.4–4.1 ka and ~ 2.8–2.6 ka. During these periods, pCO₂^sw and ΔpCO₂ were as high as 415 µatm and 145 µatm, respectively and surface water shows a notable 0.25 units decline in pH. During the same intervals, the δ¹³C_G.ruber (δ¹³C_GR) from the same site (SK-240/485) shows a decrease of 0.5‰ centred around 4.5 kyr⁴¹ (Supplementary Fig. 4). Hence, the excess CO₂ in the surface water we see at site SK-240/485 around ~ 4.4 ka may be a result of weaker export production and/or enhanced nutrient-rich water masses reaching to the sea surface (e.g. upwelling). A negative excursion in δ¹³C_G.ruber associated with the increased pCO₂^sw due to increased upwelling of nutrient-rich water has also been observed in the south-eastern Arabian Sea during the Bølling–Ållerød (warm period)²⁸. A similar pace of pH decrease along with high amounts of CO₂ outgassing was also observed around site MD01-2416 in sub-polar North Pacific during the Bølling–Ållerød which shows the significant response of local upwelling on the pCO₂ outgassing⁶¹. Based on these previous findings, we suggest that the outgassing at ~ 4.4 ka at SK-240/485 may be due to the supply of respired photosynthetic products to the surface along with the upwelled water, which increased the DIC/alkalinity ratio and thereby lowered pH and increased surface water CO₂⁶¹. However, the increase in Mg/Ca SST at the same time may either be related to the near-surface habitat of G. ruber and upper water column stratification, rather than upwelling to the surface; the replacement of surface water currents from a lower pH and warmer region; or the influence of non-thermal effects on Mg/Ca⁶². This latter possibility is discussed in more detail in the next section.

Non-thermal influences on foraminiferal Mg/Ca

Several studies suggest that the foraminifera bound Mg/Ca may be impacted by other environmental variables than temperature known as non-thermal effects⁶², including salinity⁶³ or carbonate system parameters^61,64,65. These studies highlight that the local changes in seawater pH may exert a significant bias on the planktonic foraminifera Mg/Ca which lessens its correspondence to sea surface temperature (SST)^61,65. Based on such observations, a correction protocol has been suggested for the non-thermal influence on planktonic foraminifera Mg/Ca using MgCarb package⁶². We compared the original data with MgCarb-adjusted results to check the effect of such influences in our Arabian Sea record (Fig. 3). As expected, for much of the record, the MgCarb-adjustment causes little variation. However, a difference in calculated SST of ~ 2°C is observed at 4.4 and 2.7 ka (Fig. 3) where the lowest pH is observed. Because SST has a more minor control on the calculated pH, the adjusted pH record is largely within error of the original record but with a difference of ~ 0.06 units during 4.4 and 2.7 ka, corresponding to a maximum difference of ~ 50 µatm in ΔpCO₂ (with the adjusted pCO2 values always being lower). Nevertheless, this adjustment represents only a minor change in the overall pH and ΔpCO₂ trends and hence the main inferences of our results remain unchanged (Fig. 3). Additionally, the adjusted SST during the periods of abrupt drop in pH and high CO₂ outgassing (marked by dark grey bands in Fig. 3) further supports the increased upwelling in the area at 2.7 ka.

Figure 3

Comparison of original (solid lines) SST, pH and ΔpCO₂ and nonthermal influence adjusted data (dashed lines) of site SK-240/485 with the frequency of ENSO events per 100 year⁶⁶. Shaded region around the curve represents 2SD error of the data. Vertical grey bands mark the abrupt variation in the proxy record.

Teleconnection of Arabian Sea upwelling with the El-Nino Southern Oscillation

The upwelling of sub-surface waters plays an important role in controlling the CO₂ saturation of the ocean surface. During intense upwelling, the surface waters are higher in CO₂ saturation relative to the atmospheric CO₂ level, decreasing surface water pH and causing CO₂ to outgas. The modern Arabian Sea experiences intense upwelling due to the action of southwest monsoon winds over the sea surface through Ekman pumping⁶⁷. Moreover, recent studies suggest a link between SWM and related upwelling in the Arabian Sea with the El Niño Southern Oscillation (ENSO)⁶⁸. In this context, we compared our record of pH and ΔpCO₂ from site SK-240/485 with the proxy record of long-term ENSO activity during the Holocene (Figs. 3, 4): the variation of sedimentation around Laguna Palcacocha, South Ecuador that is thought to reflect the variation of ENSO activity in the area⁶⁶. The comparison reveals that the abrupt spikes in SST, ΔpCO₂ and pH at 4.4 and 2.7 ka coincide with periods with high ENSO activity (Figs. 3, 4).

Figure 4

Comparison of Arabian Sea pH record around site SK-240/485 with the global pH composite⁶⁹ and ENSO events frequency/100 yr⁶⁶ since 8 ka. Vertical green band marks the industrial period. The modern (2010 AD) and pre-industrial (1770 AD) pH values (global and around site SK-240/485) are obtained from global ocean pH dataset⁷⁰. Shaded regions around the curve marks the uncertainty of the dataset. Vertical grey bands mark the abrupt drop in Arabian Sea pH during high ENSO activity.

The ENSO is an important phenomenon of the tropical climate system. During positive ENSO phases basin-wide warming occurs throughout the tropical Indian Ocean⁶⁸, with several studies suggesting that SST variations in the Indian Ocean have a strong correlation with the ENSO variability^71,72,73,74. The Indian Ocean Basin Mode (IOBM) is the primary mode of variation of SST which has a strong impact on ENSO-induced heat-flux anomalies around Equatorial and northern Indian Ocean (including Arabian Sea)⁶⁸. During frequent ENSO events, the IOBM shows warming causing an increase in precipitation over tropical Indian Ocean and further strengthening the South Asian High⁶⁸. These climatic features influence the rising southwest monsoon winds over the Arabian Sea and result in the intensification of the Findlater Jet (FLJ)⁷⁵. In turn, the FLJ transports the moisture from the southern Indian Ocean to the Indian landmass through the Arabian Sea and inter-annual variation of FLJ has been linked to the strength of El Niño and La Niña episodes⁷⁶. The northward advancement of FLJ is reported to induce upwelling in the Arabian Sea^76,77,78 and during an El Niño event the intensified FLJ advances towards the northern Arabian Sea where it induces strong coastal upwelling in the Arabian Sea via coastal Kelvin waves^{68,77,79,80,81}. These existing teleconnections are yet to be explored in records of past climate and are difficult to test, however, our observations suggest a strong influence of ENSO events on the upwelling around the NE Arabian Sea during mid-late Holocene, possibly similar to the modern-day relationship via coastal Kelvin waves. We therefore propose such a mechanism may have induced strong upwelling around the study area during the periods of high ENSO activity, increasing ΔpCO₂ and surface water pH.

Comparison of global pre-industrial records with the modern analogue

For a better insight of our results in a global perspective, we have compared our pCO₂^sw record with the existing records from Arabian Sea and other parts of the world ocean over the last 8–1 ka (Fig. 5 and Supplementary Table 2). We have also compared our results with global pCO₂/pH composite curves prepared by considering several δ¹¹B based pH and pCO₂^sw records to understand the behaviour of north-eastern Arabian Sea in context of global source/sink patterns⁶⁹ (See Supplementary Table 2 for more details). The pCO₂ records from equatorial and southwest Pacific Ocean (Sites ODP1238, EDRC-92 and PC 83-1) suggest that these areas were significant sources of CO₂ outgassing to the atmosphere during 8–1 ka^58,60,69. In contrast, a recent study from western equatorial Pacific region (Site KR05-15) indicates that the area is a modest CO₂ sink since the last glacial period⁸², whereas the eastern equatorial Pacific has continued to be a significant CO₂ source, the equatorial-western Pacific is currently in near equilibrium (pCO₂^sw≈ pCO₂^atm) and the south-western Pacific has changed to a modest sink of atmospheric CO₂ (Fig. 1)⁸³. The pCO₂^sw record from the ODP Site 999 in Caribbean Sea (Atlantic) indicates that the region was in near equilibrium (pCO₂^sw ≈ pCO₂^atm) over the last 8 kyr and continues to remain this way to the present⁹. On the other hand, the site PS2498 in the south of Atlantic (~ Southern Ocean) appears as a larger CO₂ source ~ 8 ka and is currently a modest CO₂ sink during modern times⁵⁸. A recent study from site TAN1106/28 around offshore New Zealand in the sub-Antarctic Pacific shows high pCO₂^sw leading to the CO₂ outgassing from the region around 4 ka⁸⁴ and a similar increase in CO₂ outgassing potentially caused by enhanced upwelling of CO₂-rich deep waters around 4.5 ka, this has also been reported from site MD972106 in Atlantic sector of Sub Antarctic Zone⁸⁵. The change in the Southern Ocean from a significant CO₂ source (during the deglacial and into the Holocene) to a significant sink in modern times has been attributed to the resumption of Antarctic upwelling and northward advection of CO₂-rich waters via Ekman pumping^58,86.

Figure 5

Comparison of published pCO₂^sw results from Atlantic, Pacific and Indian Oceans and Dome C Ice Core with the global pCO₂^sw composite⁶⁹ during 8–1 ka. Gray shaded in Atlantic, Pacific and Indian Ocean panels mark the uncertainty of global pCO₂ composite. Gray shaded region in the Ice Core panel indicates the sink. Refer text for more details.

The currently available δ¹¹B based records from Indian Ocean are restricted to the Arabian Sea^27,28. The ΔpCO₂ record around site AAS9/21, albeit at low resolution, indicates that the south-eastern Arabian Sea was a significant sink during last ~ 8 kyr²⁸. The present study provides new information about the changes in carbonate system around the coastal NE Arabian Sea during 7.7–1.4 ka. The data shows an overall sink in the area along with the high CO₂ outgassing episodes in the area (as discussed in the previous sections). At the present time, the average ΔpCO₂ and pH nearby site SK-240/485 is around 37 µatm and 8.04 units respectively^26,70. It is worth mentioning here that the recent (2010 AD) pH nearby site SK-240/485 (8.04) is 0.03 units lower than the average pH of global ocean in 2010 AD⁷⁰ (Fig. 4 and Supplementary Fig. 5). We have compared the pre-industrial (1770 AD) and modern (2010 AD) pH at our core site based on a recent study of global pre-industrial, modern and future pH estimates⁷⁰. The pre-industrial pH value (during 1770 AD or ~ 0.18 BP) nearby the study area is estimated to be 8.15 which was also 0.03 units lower than the average pH of the global ocean (~ 8.18) at that time⁷⁰. During industrialisation (between 1770 AD—present) the surface water pH around the study area varied at a similar pace to the global surface water pH (i.e. dropped ~ 0.15 pH units) due to the significant increase in global atmospheric CO₂ levels⁷⁰. Furthermore, with the current rate of pH change, it is estimated that the Arabian Sea, similar to the global ocean, may experience around 0.2–0.3 pH units drop in pH by 2100 if no attempt is made to mitigate emissions⁷⁰.

Conclusion

The present δ¹¹B based pH and pCO₂^sw results from the site SK-240/485 in NE Arabian Sea suggest that the region overall behaved as a net CO₂ sink during Mid-Late Holocene, which is a typical characteristic of a shelf region. In addition, the region was nearly in equilibrium with the atmosphere during 7.7–6.1 ka. Thereafter, the area behaved as an abrupt CO₂ source during two short periods of strong ENSO activity during 5.5–4 ka (very strong CO₂ source) and 2.8–2.5 ka (moderate CO₂ source). The periods of abrupt increase in ΔpCO₂ are likely associated with the enhanced upwelling of nutrient-rich waters to the surface. It is estimated that the two intense upwelling episodes (centring around 4.4 and 2.7 ka) triggered CO₂ outgassing and preindustrial ocean acidification similar to the modern levels. Subsequently, during 2.5–1.4 ka, the region behaved as a strong CO₂ sink probably due to the absence of upwelling. We suggest therefore that there is evidence of intense natural ocean acidification episodes in the region over the past at times of sudden increase in carbon and nutrient supply to the surface due to increased coastal upwelling. We found that the reconstructed pH from site SK-240/2485, as well as the pre-industrial and modern value, appears to be in the range of global pH variation during the Holocene (within uncertainty) except during these two abrupt periods of intense upwelling. A comparison of our data with available records of past ENSO activity suggests a possible teleconnection of NE Arabian Sea upwelling with the timing and intensity of ENSO events during the Holocene. However, this aspect in particular needs to be confirmed in future studies from other regions of the northern Indian Ocean.

Methods

Study area

The analysed marine sediment core (SK-240/485) was recovered from offshore Saurashtra (Lat. 21° 16′; Long. 68° 55.99′ E; 88 m water depth), NE Arabian Sea (Fig. 1). The core covers period between the Pleistocene-Holocene boundary and the Holocene⁴² (last 12.4 kyr). However, the present study is restricted to cover the period between ~ 7.7 and ~ 1.4 ka due to insufficient foraminiferal abundance towards the bottom of the core.

The Arabian Sea is the north-western extension of Indian Ocean. It spreads around the coasts of India, Pakistan, Yemen, Oman, Somalia and the Addu Atoll (Maldives). The Indus River is the primary source of freshwater and sediment deposition in the NE Arabian Sea which receives fluxes of meltwater discharge from the Himalaya as well as runoff from monsoon precipitation^87,88,89. The Indus Fan is the most extensive physiographic feature of the Arabian Sea and considered to be the second-largest submarine fan in the world⁸⁹. Apart from the primary source, minor freshwater input also comes from the Bhadar River (a seasonal river) during the SWM season. The average annual salinity around the study area today is 36.2 psu, and the average annual sea surface temperature (SST) and ΔpCO₂ (the difference between pCO₂^sw and atmosphere) are 27.1 °C and + 36.7 µatm respectively^26,90 (Supplementary Fig. 1) with the latter having an annual cycle of ~ 18 µatm.

Geochemical proxies for studying ocean carbonate system

We analysed Mg/Ca ratios and δ¹¹B in planktonic foraminifera (G. ruber). The ratio of Mg to Ca (or Mg/Ca ratio) in calcite (particularly foraminifera) serves as a proxy of past seawater temperature and has been extensively used in the world oceans^{62,91,92,93,94,95,96}.The boron isotopic composition, expressed as δ¹¹B (Eq. 1), of foraminifera is strongly influenced by the pH of the surface water^9,10,11,14. Hence, the δ¹¹B of planktonic foraminiferal shells is used as a proxy to reconstruct the pH of surface seawater and further to decipher other parameters of the ocean carbonate system, most notably pCO₂^sw9.

$$\updelta^{11} {\text{B}}\left( \textperthousand \right) = \left( {\frac{{\frac{{^{11} {\text{B}}}}{{^{10} {\text{B}}}}\left( {{\text{sample}}} \right)}}{{\frac{{^{11} {\text{B}}}}{{^{10} {\text{B}}}}\left( {{\text{standard}}} \right)}} - 1} \right) \times 1000$$

(1)

The geochemical analyses of foraminifera (Mg/Ca ratio and δ¹¹B) were carried out at the University of Southampton, UK. For determination of Mg/Ca as well as δ¹¹B for each sample 170 individuals of Globigerinoides ruber (white variety; sensu-stricto type) with a size range of 300 to 350 µm were hand separated. The hand-separated specimens were then cleaned following Barker et al.⁹⁷, without applying the reductive cleaning step. Briefly, a multi-step cleaning protocol was applied consisting of crushing the foraminifera shells, followed by the removal of clay, silicate and organic matter, and a weak acid leaching with final dissolution of the carbonates using 0.5 M HNO₃ as detailed in Foster et al.⁹⁸. A small aliquot (~ 7%) of the cleaned and dissolved sample was analyzed for Mg and Ca (and other elemental ratios) by inductively coupled plasma mass spectrometry⁹ (ICP-MS) following Henehan et al.⁹⁹ while the remaining sample was analysed for δ¹¹B using Multi collector inductive coupled plasma mass spectrometry (MC-ICP-MS) following Foster⁹. External reproducibility of the MC-ICPMS δ¹¹B method (at 95% confidence, 2SD) is based upon repeat measurements of an in house carbonate standard and is a function of sample concentration this ranges from 0.4‰ to 0.2‰ for the samples presented here^57,98,100. Analytical precision for elemental ratios is determined at 95% confidence by the reproducibility of several in house standards and is < 5% for all elements. Strict criteria are used to rule out clay contaminated results (samples with Al/Ca > 100 μmol/mol removed). All samples are matrix matched to a gravimetrically determined standard, to eliminate Ca concentration effects on the ratios produced⁹⁹.

The measured Mg/Ca (mmol/mol) ratio were converted to SST (°C) by using the calibration equation¹⁰¹

$$SST = \left( {\frac{1}{0.09}} \right)*ln\left( {\frac{Mg/Ca}{{0.38}}} \right)$$

(2)

Further, δ¹⁸O of seawater (δ¹⁸O_SW) was calculated by using the SST derived from Mg/Ca to deconvolve ice volume effect values⁹⁵ from δ¹⁸O of G. ruber and following equation of Bemis et al.¹⁰²

$$\updelta^{18} {\text{O}}_{{{\text{sw}}}} = 0.27 + \left( {\frac{{{\text{T}}{-}16.5 + 4.8*\updelta^{18} {\text{O}}_{{{\text{calcite}}}} }}{4.8}} \right)$$

(3)

The Sea Surface Salinity (SSS) was then calculated by using δ¹⁸O _SW-salinity relation for Arabian Sea suggested in equation¹⁰³.

$$Salinity = \left( {\frac{{\updelta^{18} {\text{O}}_{{{\text{sw}}}} + 20}}{0.57}} \right)$$

(4)

The pH was calculated from the analysed δ¹¹B values using the species-specific calibration for G. ruber (300–350 µm size range) of Henehan et al.⁵⁷ (See Eqs. 1, 5).

$${\text{pH}} = {\text{pK}}_{{\text{B}}}^{*} - \log \left( {\frac{{ - \updelta^{11} {\text{B}}_{{{\text{sw}}}} - \left( {\updelta^{11} {\text{B}}_{{{\text{borate}}}} } \right)}}{{\updelta^{11} {\text{B}}_{{{\text{sw}}}} -^{11 - 10} {\text{K}}_{{\text{B}}} \left( {\updelta^{11} {\text{B}}_{{{\text{borate}}}} } \right) - 1000 \left( {^{11 - 10} {\text{K}}_{{\text{B}}} - 1} \right)}}} \right)$$

(5)

where pK^*_B is the dissociation constant for boric acid dissociation in the in-situ (temperature, salinity and pressure) conditions and calculated as per Dickson¹⁰⁴, δ¹¹B_sw is the isotopic composition of seawater having value 39.61‰¹⁰⁵. ^11-10KB is the equilibrium constant for the two existing forms of boron in seawater i.e. borate and boric acid¹⁰⁶ having a value of 1.0272 ± 0.0006¹⁰⁷. Since the borate ion is predominantly incorporated into the foraminifera calcite⁹, the δ¹¹${\text{B}}_{{{\text{CaCO}}_{3} }}$ can be used to obtain the pH with the species and size specific (G. ruber) coefficients as estimated by Henehan et al.⁵⁷ (See Eq. 5)

$$\updelta^{11} {\text{B}}_{{{\text{borate}}}} = \left( {\frac{{\updelta^{11} {\text{B}}_{{{\text{CaCO}}_{3} }} - 8.87 \pm 1.52}}{0.6 \pm 0.09}} \right).$$

(6)

Further, the pCO₂^sw was calculated from pH (derived from δ¹¹B), SST and SSS (derived from Mg/Ca ratio) and TA (calculated using modern day relationship between SSS and TA around the area using the Global Data Analysis Project data set)⁷⁰.

All the original data calculations are done with the R computer programming platform¹⁰⁸ (R Core Team 2014) using “seacarb” package, version 6.8¹⁰⁹. Foraminifera bound Mg/Ca may be impacted by non-thermal effects⁶² (e.g. Gray and Evans 2019) including salinity⁶³ or carbonate parameters⁶⁴. We explored the influence of carbonate parameters using the parametrisation using MgCarb package as mentioned in Ref⁶². The uncertainties were calculated using Monte Carlo approach (n = 10,000) with consideration of the uncertainty in all relevant input parameters (with 95% confidence level) δ¹¹B ± 0.25‰; Mg/Ca derived SST ± 1 °C; SSS ± 1psu; TA ± 50 µmol/kg.