Rapid shoaling of aragonite saturation horizon in the northern Indian Ocean

Anthropogenic carbon dioxide (CO2) penetrates up to 1000 m of water column in the Indian Ocean resulting in acidification and corrosion of aragonite skeletal material. The shallowest aragonite saturation horizon (ASH) was observed in the Bay of Bengal (BoB; 219 ± 10 m) within the tropical Indian Ocean. The ASH shoaled at the rate of 6.3 ± 5 and 4.4 ± 3 m yr−1 in the past four decades in the BoB and Arabian Sea respectively. As a result, an increase in total alkalinity (TA) was observed at the rate of 0.5 ± 0.3 and 0.25 ± 0.2 μmol kg−1 yr−1 at the depth of ASH in the BoB, and Arabian Sea respectively. While the shoaling rate of ASH remained the same in the Arabian Sea over the past four decades, in contrast, rapid shoaling was observed in the BoB in the recent decades due to higher accumulation of anthropogenic CO2 in the upper ocean associated with an increase in river discharge and decomposition of riverine organic matter. These two processes decreased the pH resulting in corrosion of aragonite skeletal material and increased TA at the depth of ASH in the BoB. Under a business-as-usual scenario, aragonite-secreting organisms will be seriously affected by the middle of this century in the BoB.


Introduction
Marine calcifiers synthesize calcium carbonate (CaCO 3 ) from calcium and carbonate/bicarbonate in the seawater forming shells and other skeletal structures that dissolve into the water column when sinking to deeper depths (Milliman 1993).The CaCO 3 produced by the calcifiers mainly exist in the forms of aragonite and calcite.Aragonite is more susceptible to ocean acidification than calcite (Caldeira and Wickett 2005) and their dissolution is being increased in the water column due to decrease in pH (Feely et al 2002).Ocean acidification reduces the aragonite saturation (Ω), making it harder for marine calcifying organisms to form and maintain their shells (Kroeker et al 2013, Le Quéré et al 2018).Ocean is acidified due to uptake of one-third of the CO 2 released into the atmosphere through fossil fuel burning, agriculture and industrial activities (Feely et al 2004, Ciais et al 2014, Le Quéré et al 2018).This results in decrease in carbonate ion concentration and Ω of CaCO 3 , which is the ratio of the concentration of dissolved carbonate ions in the water and their concentration at the saturation for aragonite ([Ca 2+ ][CO 3 2-]/K sp ).The depth, where this ratio falls below 1, is referred to as the aragonite saturation horizon (ASH).Below Ω, where aragonite skeletal material starts to dissolve.Sabine et al (2002) found that the depth of ASH varies between 400 and 600 in the northern Indian Ocean and 600 and 1200 m in the south Indian Ocean.The accumulation of anthropogenic CO 2 in the upper 1000 to 1500 m of water column (Sabine et al 1999), which is deeper than the depth of ASH, leading its impact on corrosion of aragonite shells and shoaling of ASH (Sarma et al 2002).
Accumulation of anthropogenic CO2 in the upper ocean leads to shoaling of ASH by 50-70 m was reported between 1970s and 1990s in the Pacific (Feely and Chen 1982, Feely et al 1984, Feely et al 2002), Indian (Sarma et al 2002), and Southern Ocean (Negrete-García et al 2019) and it was attributed to an increase in anthropogenic CO 2 .However, these estimated rates may carry large errors for the data collected in the aegis of GEOSECS due to a lack of certified reference materials whereas numerical models do not have appropriate data to validate.In addition to increase in CO 2 levels, atmospheric pollutants increased significantly after 1990s in the South and Southeast Asia due to industrialization resulting in a significant warming of the upper 750 m of the water column in the Indian Ocean (Lee et al 2015, Cheng et al 2017, Desbruyères et al 2017, Wenegrat et al 2022).Goes et al (2020) reported decrease in Himalayan ice cover in the past two decades resulting in increase in discharge and decrease in surface salinity of the BoB (Sridevi and Sarma 2021).Increase in river discharge is expected to bring more allochthonous organic matter to the BoB and subsequent decomposition may decrease pH that may influence depth of ASH.Sarma et al (2021) reported rapid acidification of northeastern Indian Ocean due to deposition of aerosols.Therefore, rapid warming, increased river discharge and aerosol deposition may have significant impact on the depth of ASH in the northern Indian Ocean.
Under Representative Concentration Pathway (RCP) 8.5, which is the highest base line emission scenario corresponding to warming of 8.5 W m −2 , Zheng and Cao (2014) predicted that the global mean ASH may rise to 308 m by the end of this century from 1139 m in the preindustrial period.The dissolution of CaCO 3 neutralizes anthropogenic CO 2 and increases total alkalinity (TA) via dissolution.Sarma et al (2002) showed that ASH shoaled by 16 to 124 m in the Indian Ocean between 1978 and 1995, at the rate of 0.8 to 5.9 m yr −1 , associated with an increase in TA at the depth of ASH due to the dissolution of aragonite.However, these estimations were made based on a few data points measured in the aegis of GEOSECS and WOCE programs that covers only 6 data points in the northern Indian Ocean.Based on the large data collected between 1995 and 2022, the recent trends in ASH were estimated in the northern Indian Ocean using high-quality data collected under different national and international programs.The objective of this study is to 1) compile all the available ASH data in the northern Indian Ocean to obtain annual mean spatial variability, 2) estimate the shoaling rate of depth of ASH in the past 4 decades, and 3) evaluate the potential reasons responsible for shoaling of ASH in the northern Indian Ocean.

Data and methods
The hydrography (temperature, salinity, dissolved oxygen and nutrients), dissolved inorganic carbon (DIC) and total alkalinity (TA) data collected under multiple expeditions carried out between 1991 and 2022 were used (table 1; figure 1).The accuracy of the DIC and TA measurements were ± 3 and ± 5 μmol respectively (Lamb et al 2001).Though the I09 transect covered the entire tropical eastern Indian Ocean during 1995, 2007 and 2016, the transect was changed in the BoB during 2007 and 2016 from that of 1995.Hence, we compared the data collected during 2007 and 2016 to obtain ASH shoaling rates between two measurements.Though both I07 and I09 transects collected data from South Indian Ocean to north Indian Ocean, we have considered here only data north of equator where more data available from other programs.In the case of the Arabian Sea, the data collected during 1995 and 2018 along I07N cruise track was used to derive shoaling rates of ASH.While comparing the data collected during two different years, we considered salinity and temperature difference of 0.02-0.04 and <0.4 °C respectively to ensure the same water mass.The available data between 1977 and 2022 within 1°latitude and longitude from that of GEOSECS stations 416 and 446 in the Arabian Sea and BoB respectively was used to derive time-series variations in the depth of ASH.In the case of the GEOSECS data, there was no certified reference material (CRM) available, hence we followed the procedure of Sarma et al (2002).The concentration of CO 3 2-was derived from TA and DIC using the dissociation constants of carbon (Millero et al 2006), bisulfate (Dickson et al 1990), hydrogen fluoride (Dickson and Riley 1979) and total boron (Lee et al 2010) and the solubility product is calculated by Mucci (1983) for aragonite.The aragonite saturation (Ω) is derived using the CO2SYS program version 3.0 (Pierrot et al 2021).The ASH is derived as the depth where Ω is 1.Since the data were collected at standard depth, the exact depth of ASH, and associated DIC, TA, and AOU were computed based on interpolation between two depths.

Spatial variations in ASH in the Indian ocean
The meridional variability in the Ω over depth in the eastern Indian Ocean (WOCE transect I09) during 2016 is depicted in figure 2(a).The ASH was deep (>1000 m) at the 30°S and shoaled to ∼600 m between 12°S and 2°N and shoaled further to ∼200 m between 5°N and 20°N and it is consistent with Sabine et al (2002).The meridional variability in Ω followed the anthropogenic CO 2 accumulation in the water column which is deeper in the south and shallowed towards the northern Indian Ocean (Sabine et al 1999).The depth of ASH varied between 400 and 1200 m in the tropical Indian Ocean with deeper in the south than north (Sabine et al 2002, Jiang et al 2015).The shallowest ASH was reported in the north and equatorial Pacific (<200 m) followed by the northeastern Indian Ocean (BoB; mean ± standard deviation; 251 ± 73 m) and it is attributed to thermohaline circulation (Feely et al 2009, Jiang et al 2015).Since the North Pacific Ocean contains the oldest water mass (Broecker 1991), it experiences a high amount of organic matter degradation resulting in the release of CO 2 and decrease in pH that shoals the depth of ASH (Sabine et al 2002, Jiang et al 2015).Due to the closing of the northern boundary with landmasses, the existence of intense oxygen minimum zones (OMZ; Sarma et al 2020, Udaya Bhaskar et al 2021), where a high amount of organic matter decomposition occurs, compared to other basins (Sarma et al 2006), leading to a decrease in pH and shoaling of ASH in the northern Indian Ocean (figure 2(a)).
The compilation of ASH data collected between 1991 and 2022, based on the high-quality dissolved inorganic carbon data collected under different international and national programs (table 1; figure 1

Rate of shoaling of ASH in the northern Indian ocean
The annual mean shoaling rate of ASH was estimated following three methods, i.e., 1) based on the repeat measurements of vertical profiles of inorganic carbon components along I09N and I07N transects (north of equator) from 1995 to 2016 and 1995 to 2018 in the BoB and Arabian Sea respectively.2) Time-series variations in the depth of ASH between 1978 and 2022 at GEOSECS stations 416 and 446 in the Arabian Sea and BoB respectively.3) The mean ASH observed in the entire BoB and Arabian Sea using all available data between 1978 and 2022.The annual mean shoaling rate of ASH was observed between 0.1 and 15.5 m (mean of 5.0 ± 3 m) and 0.2 to 39.3 m (mean of 8.3 ± 5 m) between 1995-2018 and 2007-2016 in the Arabian Sea and BoB respectively along I09N and I07N transects (north of equator; figure 3(a)).Sarma et al (2002) estimated that mean ASH shoaled at the rate of 4.6 ± 2 m yr −1 in the BoB between 1978 and 1995 and the present estimate (8.3 ± 5 m; table 2) is higher than earlier.The dissolution of CaCO 3 may increase inorganic carbon components such as DIC and TA at the depth of ASH (Sarma et al 2002).The increase in salinity normalized DIC (nDIC) (table 2), at the depth of ASH, varied between −0.48 to 5.2 μmol kg −1 yr −1 with a higher mean rate of increase in the BoB (1.0 ± 0.8 μmol kg −1 yr −1 ) than the Arabian Sea (0.25 ± 0.2 μmol kg −1 yr −1 ).The disparity in the rate of increase in nDIC at the depth of ASH between the BoB and the Arabian Sea can be attributed to differences in the depth of ASH.The depth of ASH is shallower in the BoB than Arabian Sea that facilitates the accumulation of  The increase in nDIC at the depth of ASH is also caused by the release of CO 2 due to the decomposition of organic matter, change in TA and anthropogenic CO 2 accumulation.The AOU at the depth of ASH varied between −0.2 to 0.38 μmol kg −1 yr −1 (mean of 0.15 ± 0.1 μmol kg −1 yr −1 ) and −0.18 to 1.1 μmol kg −1 yr −1 (mean of 0.6 ± 0.2 μmol kg −1 yr −1 ) between 1995-2016 and 2007 and 2018 in the Arabian Sea and BoB respectively.Assuming the C/O ratio of 0.78 (Redfield et al 1963), the mean nDIC increase due to the decomposition of organic matter is estimated as 0.47 ± 0.1 and 0.1 ± 0.05 μmol kg −1 yr −1 respectively in the BoB and Arabian Sea (figure 2(b)).The mean AOU difference reported between 1978 and 1995 in the BoB (0.7 ± 0.3 μmol kg −1 yr −1 ; Sarma et al 2002) is consistent with the present estimate (0.6 ± 0.2 μmol kg −1 yr −1 ) suggesting that microbial respiration rates did not change significantly in the past 4 decades.The decrease in oxygen concentrations within the OMZ in the northern Indian Ocean is consistent with the long-term trends reported in the Arabian Sea (Banse et al 2014, Lachkar et al 2018).Due to the dissolution of carbonate skeletal material, an increase in TA was observed at the depth of ASH in the BoB (−0.8-4.67 μmol kg −1 yr −1 with a mean of 0.5 ± 0.3 μmol kg −1 yr −1 ) and Arabian Sea (0.12 to 0.75 μmol kg −1 yr −1 with a mean of 0.25 ± 0.2 μmol kg −1 yr −1 ) between 2007-2018 and 1995-2016 respectively (figure 3(d)).This estimate is close to that of increase in TA at the depth of ASH in the Indian Ocean between 1978 and 1995 of 0.4 ± 0.2 μmol kg −1 yr −1 (Sarma et al 2002).The contribution of change in AOU and increase in TA at the depth of ASH contributes to an increase in DIC of 0.71 ± 0.4 μmol kg −1 yr −1 in the BoB and 0.23 μmol kg −1 yr −1 in the Arabian Sea.The remaining increase in DIC may be contributed by the accumulation of anthropogenic CO 2 at the depth of ASH.
Since the two transects (I07 and I09) were samples only twice in the past two decades, the rate of shoaling of ASH was estimated assuming a linear rate of change between the two years sampled in the north of equator.This approach may carry some errors due to linear assumptions.In order to confirm the rate of change in the past four decades is linear, we analyzed the depth of ASH data available at the GEOSECS stations 416 and 446 in the Arabian Sea and BoB respectively (figure 4).At both the locations 6 times sampling was done between 1978 and 2018 and displayed linear shoaling trend at the rate of 3.7 ± 1.6 and 5.9 ± 2 m y −1 in the Arabian Sea and BoB respectively (figure 4).The basin-mean variability in depth of ASH in the Arabian Sea and BoB was estimated using all available data between 1978 and 2022 by averaging annual mean (figure 5).The available data in the northern Indian Ocean has bias with reference to space and time, therefore, some caution is required while interpreting the data.The basin mean data between 1978 and 2022 suggests that the rate of shoaling of ASH was 6.3 ± 3 m yr −1 in the BoB but 4.4 ± 2 m yr −1 in the Arabian Sea.The shoaling rate of ASH in the Arabian Sea (4.4 ± 2 m yr −1 ) during the present study is close to that of the same found between 1978 and 1995 (5.0 ± 3 m yr −1 ) whereas it was higher in the recent decades (6.3 ± 3 m yr −1 between 2007 and 2016) than earlier (4.4 ± 2 m yr −1 , between 1978 and 1995; Sarma et al 2002) in the case of BoB.

Potential reasons responsible for rapid shoaling of ASH in the BoB
The increased DIC (0.25 ± 0.1 μmol kg −1 yr −1 ) at the depth of ASH between 1995 and 2018 in the Arabian Sea is mostly contributed by DIC released due to oxidation of organic matter (AOU increase) and dissolution of CaCO 3 (TA increase of 0.23 ± 0.1 μmol kg −1 yr −1 ; table 2).The impact of anthropogenic CO 2 input is less in the Arabian Sea on shoaling of depth of ASH due to the deeper depth of ASH than BoB.The concentration of anthropogenic CO 2 is 35-40 μmol kg −1 at a depth of ASH (∼200 m) in the BoB, whereas it was <20 μmol kg −1 at a depth of 500 m in the Arabian Sea (Sabine et al 1999).Therefore, the impact of anthropogenic CO 2 is more in the BoB than in the Arabian Sea on depth of ASH.In addition to this, higher accumulation of anthropogenic CO 2 is observed in the upper ocean of the BoB than Arabian Sea due to the existence of low-saline waters that absorb atmospheric CO 2 (Sabine et al 1999).Goes et al (2020) showed that snow extent anomaly in the

Typeset image
Himalayan mountains was negative from the mid-1990s to the present due to warming in recent decades.The retreat of the Himalayan glacier increased the freshwater discharge to the BoB (Becker et al 2020) which facilitates the absorption of more atmospheric CO 2 .An increase in rainfall over the BoB was also reported in the recent decades (CDKN and ODI 2014) that decreases salinity.Sridevi and Sarma (2021) reported a decrease in salinity between 1997 and 2018 in the BoB associated with the melting of the Himalayan glacier.Since the Ganges River water is relatively basic compared to the seawater (Kumar et al 1996, Sarma et al 2012), undersaturation of CO 2 with respect to the atmosphere is reported associated with increase in pH by 0.001 to 0.003 yr −1 and decrease in pCO 2 levels by 1-3 μatm yr −1 in the BoB (Sridevi and Sarma 2021).In the case of Arabian Sea, no significant changes in salinity were observed in the past 4 decades (Sridevi et al 2023).This results in the absorption of more CO 2 from the atmosphere in the BoB than Arabian Sea.The increase in CO 2 flux into the BoB is reported between 1980 and 2018 in the global and regional models (Sarma et al 2023a) leading to the accumulation of anthropogenic CO 2 in the BoB and decrease in pH.Such conditions are corrosive for the aragonite form of CaCO 3 leading to shoaling of ASH.In order to further examine the variability in water column properties between 1995 and 2016 in the Arabian Sea whereas 2007 and 2016 in the BoB, the vertical profiles of AOU and pH were examined (figure 6).This figure suggests that higher mean AOU in the upper 200 m of the water column in the BoB (by 28 ± 3 μmol kg −1 ) than in the Arabian Sea is due to the remineralization of riverine organic matter.Higher bacterial respiration rates in the upper ocean in the BoB was reported and it may be supported by DOC brought by the river discharge (Fernandes et al 2008, Sarma et al 2023).Sarma (2009) noticed existence of strong heterotrophy in the BoB during summer monsoon period associated with peak in river discharge.At the mean depth of ASH (251 m) in the BoB, the AOU difference between BoB and the Arabian Sea is minimal (6 μmol kg −1 ).Between 300 and 1000 m, higher AOU was observed in the Arabian Sea (by 43 μmol kg −1 ) than BoB (figure 6(a)).This study suggests that organic matter decomposition (both in situ and riverine) contributed significantly to decrease in pH and shoaling of ASH in the BoB than Arabian Sea.This is further confirmed by the lower pH (by 0.046) between 100 and 300 m depth in the BoB compared to the Arabian Sea [figure 6(b)].At the similar depth range, higher AOU by ∼9 μmol kg −1 was observed that would decrease pH by 0.014 (figure 6).Hence the lower pH in the upper 300 m of water column in the BoB cannot be explained by variations in AOU alone and it may be a combined effect of the decomposition of organic matter, and dissolution of anthropogenic

Summary and conclusion
The time-series data collected along the fixed transects and also basin mean data during different years suggest that ASH is situated at a shallower depth (251 ± 73 m) in the BoB than in the Arabian Sea (472 ± 120 m).The rate of shoaling of ASH is higher in the BoB (6.3 ± 3 m yr −1 ) than in the Arabian Sea (4.4 ± 2 m yr −1 ) and it is consistent with the lower pH and higher AOU in the upper 200-300 m of the water column of the BoB than the Arabian Sea.Increased river discharge due to the melting of the Himalayan glacier and increase in precipitation over the BoB resulted in enhanced anthropogenic CO 2 absorption and decomposition of riverine organic matter.Both these processes decreased pH and corroded aragonite skeletal material.Nevertheless, the low pH conditions in the upper 300 m of the water column are unfavourable for aragonite leading to corrosion in the BoB.If the rate of shoaling of ASH continues in a similar manner, ASH may surface by the middle of this century in the BoB.IPCC Assessment Report 6 (IPCC 2022) suggested an increase in runoff from Brahmaputra, Ganga and Meghna in the past decade and is predicted to increase by 16, 33 and 40% respectively by the end of this century due to a rise in precipitation and accelerated melting of snow.This may aggravate the dissolution rate in the future.The decrease in decadal mean Ω at the sea surface from ∼4.5 to 2 is projected between 1875 and 2095 in the tropical Indian Ocean (Feely et al 2009, Mei-Di andLong 2014) and shoaling of the Ω to the surface is projected by the end of this century (Orr et al 2005, Hauri et al 2016, Fabry et al 2009, McNeil and Matear 2008).This may impact the biogeochemistry of carbon in the water column and ecosystem structure (Hunt et al 2008, Moy et al 2009, Bednaršek et al 2012), particularly the survival of aragonite-secreting organisms (Bednaršek et al 2012).Recently Sarma et al (2023) compared the performance of 16 hindcasts, 9 empirical and 2 atmospheric inversion models in the Indian Ocean with observations and noticed that all models lack real-time river discharge data and atmospheric pollutant inputs.As a result, the prediction given by the models may not be accurate and necessary modifications are required for better prediction in future.
), is depicted in the figure 2(b) in the northern Indian Ocean.The depth of ASH in the BoB varied between 118 and 435 m with a mean of 251 ± 73 m followed by the Arabian Sea (Range: 273-812; 472 ± 120 m).Within the Arabian Sea, shallower ASH was observed in the eastern basin compared to the western, in contrast, the shallower ASH was observed in the western than in the eastern BoB (figure 2(b)).This pattern is consistent with the OMZ which is thick on both the sides of the Indian subcontinent (Sarma et al 2020, Udaya Bhaskar et al 2021).The deeper ASH was observed (>500 m) close to the equator (figure 2(b)) where upwelling does not occur, unlike the Pacific and Atlantic Oceans (Murtugudde and Busalacchi 1999, Xie et al 2002).

Figure 1 .
Figure 1.The map showing the station locations of the data used for ASH long-term trends.The data collected in different periods and years are given in different color/symbol shown in the box.

Figure 2 .
Figure 2. (a) Vertical profile of aragonite saturation along the WOCE transect (cruise no.GOSHIP I09N) collected during 2016 is depicted.The 100% aragonite saturation is shown as thick line and below this line, aragonite dissolves.(b) The available depth of ASH data is plotted based on the data collected between 1991 and 2022 during different expeditions in the northern Indian Ocean.The major rivers discharge to the BoB are shown.

Figure 4 .
Figure 4.The time-series variations in the depth of ASH (m) at (a) GEOSECS station 416 (19.75°N and 64.61°E) in the Arabian Sea and (b) GEOSECS station 446 (12.6°N and 84.6°E) in the BoB.
CO 2 .Due to the low pH in the BoB, the waters are more corrosive for aragonite skeletal material compared to the Arabian Sea.The recent increase in river discharge due to melting of Himalayan glacier (Goes et al 2020) might have enhanced organic matter inputs to the BoB and subsequent remineralization and decrease in pH.Due to lack of time-series data, it is not possible to confirm this speculation.Recently Kurian et al (2023) observed that annual mean carbonate fluxes at the 1000 m depth decreased between 1987 and 1997 (13.1 ± 1.4 g m −2 yr −1 ; Rixen et al 2019) to 2011-2019 (8.2 ± 1 g m −2 yr −1 ) in the northern BoB.The decrease in carbonate flux is attributed to the large input of freshwater, that can lower the ASH and reduces the carbonate precipitation within the organisms (Riebesell et al 2000, Rixen et al 2009).Kurian et al (2023) further attributed that decrease in carbonate flux may also be caused by ocean acidification (Rashid et al 2013, Sarma et al 2015) as evidenced from lowering of pH observed between 2007 (7.54) and 2014-2015 (7.43) at 500 m water depth.Therefore, the decrease in CaCO 3 flux to depth in the recent years attributed to increase of riverine freshwater input and associated organic matter input followed by decomposition, as such conditions are corrosive for calcifying organisms.