Average shell size vs. in-life dissolution
The sediment cores retrieved within Aly were subjected to study the in-life pteropod dissolution resulting from the past variations in deep ocean chemistry (spade cores SPC 05, 06, 09, 11, 12, 13, and 14) (Figs. 2, 3 and 4). Here, the LDX values indicate increased in-life dissolution and the FR values show increased shell breakage of thinner and weaker-formed shells. The periods coinciding with the larger shells corresponded to lower atmospheric CO2 concentration and vice versa in a few cores (SPC 05, SPC06, SPC 09, and SPC 12). However, the offsets in the variability in dissolution could be attributed to the age. Now consider the shorter cores of younger age (maximum of 2.8 kyr) when the atmospheric CO2 variability lies within more or less 6–8 ppmv (Figs. 2 and 3). Here, the variability in shell size is attributed to the carbonate saturation, hence in-life dissolution that leads to breakage of the shells. However, compared to MIS 3 and 2 of longer records, the atmospheric CO2 concentration shows an exponential increase during the Late Holocene. Since the variability within the late Holocene is not very pronounced, we could assume a multi-stressor environment influencing the carbonate saturation and dissolution of aragonite shells in this study region. The enhanced in-life dissolution and smaller shell size of H. inflatus in the shallow cores (SPC 5–14) (Figs. 2 and 3) are indicative of variability in carbonate ion concentrations that restrict the shell calcification and thinning of shells. This relationship highlights the effort required to repair the shells that have undergone dissolution rather than their growth (Comeau et al. 2012; Wall-Palmer et al. 2013). The increased in-life dissolution in the Andaman basin and the possible interference in the calcification process may be caused by the low saline water conditions and the OA in the study site. A laboratory culture experiment has shown salinity-induced pH /alkalinity fluctuations in benthic foraminifera and suggested that the salinity variations could control the carbonate inventory in the coastal regions affected by seasonal freshwater input (Saraswat et al. 2015). Unlike salinity, seasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB) ranges from ~ 27 to 29°C throughout the basin, and slightly higher surface water temperatures from 28 to 30°C are seen in the Andaman Sea with near homogeneous temperatures up to a depth of 50 m (Sijinkumar et al. 2021). The combined thermal and freshwater forcing leads to strong stratification, tropical cyclone intensity, and primary productivity (Prasanna Kumar et al. 2002; Gauns et al. 2005; Neetu et al. 2012; Dave and Lozier 2013). So, in this study, the larger shells with lower dissolution values in terms of LDX and FR are considered to be due to the absence of preferential dissolution of smaller shells. However, an increased value of FR along with larger shell size can be attributed to the preferential dissolution of smaller shells. Smaller shells coinciding with increased dissolution i.e., LDX and FR can be attributed to in-life dissolution of pteropod shells. The hydrographic conditions related to the climate variability viz., monsoon, productivity, salinity, and temperature variation could be the reason for the visible changes in aragonite dissolution in this particular region. Moreover, the volcanic ash inputs can dramatically reduce the oceanic pH in a particular area during and after an eruption (Wall-Palmer et al. 2011). A recent study under laboratory conditions has shown that volcanic materials entering seawater significantly reduce pH (Jones and Gislason 2008), reducing the carbonate ion concentration. Its impact on the pteropod fauna has been inspected and described by a few researchers (Jones et al. 2007; Wall-Palmer et al. 2011, 2012). Our observations suggest that, in this case, the increased in-life dissolution in the northernmost cores from the Andaman Sea could be attributed to the salinity-induced aragonite undersaturation in the north (SPC 05) and the volcanogenic inputs near Barren volcano in the south (SPC 06). The possibility of submarine volcanic activity has also been predicted for the Andaman Sea. A cratered seamount, fresh volcanic, underwater volcanic chain (Raju et al. 2012), and a volcanogenic component and ash layers in sediments (Kurian et al. 2008; Awasthi et al. 2010) all suggest volcanogenic input in the Andaman Sea. The laboratory work on the living pteropods and pteropods from sediment traps in the Southern Ocean also agrees with the assumption that the LDX profile is the result of changing carbonate availability (Roberts et al. 2008; Fabry et al. 2008; Comeau et al. 2009, 2010a, b; Wall-Palmer et al. 2012). The aragonite undersaturation of the surface water could also affect the increased in-life dissolution and shell calcification due to freshwater mixing and volcanogenic inputs. These factors affecting aragonite undersaturation could also vary on the basin of deposition.
The increased FR, decrease in absolute abundance, and progressive modification of pteropod assemblages with depth are accompanied by reduced aragonite content in the core U1467 on a longer scale (See Sreevidya et al. 2019). Within the preserved record, the dissolution proxies, such as pteropods FR, LDX, and the average shell size of H. inflatus, all point toward better preservation of aragonite during the glacial stages (MIS 16, 14, 8, 6, 4, 3 and 2) with the exception to MIS 12 and 10 during Mid-Brunhes Dissolution Interval (MBDI) (Sreevidya et al. 2019) (Fig. 4). The shell size curves from Site U1467 show an inverse relation to the atmospheric CO2 concentration over glacial/interglacial time scales, where the increased shell size of all the cores coincides with the reduced atmospheric CO2 concentration. The published laboratory studies on pteropods also show a reduction in calcification rates and greater in-life shell dissolution under falling carbonate saturation conditions (Lischka et al. 2011; Comeau et al. 2012; Bednaršek et al. 2012b). Examination of multiple dissolution proxies in this study has shown that in-life dissolution is the dominant process modifying the characteristics of the size factor of pteropod shells, as indicated by the downcore variation in the size of H. inflatus (Sreevidya et al. 2019). Only very few studies have presented the downcore abundance/preservation record of pteropod fossils from the Indian Ocean for a longer time scale (Cullen and Droxler 1990; Droxler et al. 1990; Wall-Palmer et al. 2013). Wall-Palmer et al. (2013) utilized fossil pteropod proxies in a core ODP 716B and Vostok atmospheric CO2 record to understand calcification rate and surface ocean carbonate saturation (Fig. 4). The assumption behind this finding was the surface water carbonate saturation was directly influenced by dissolved CO2 content. Another view on the carbonate saturation is predicted by analyzing the trend in the shell weights of planktonic foraminifera, Globigerinoides trilobus by decreasing atmospheric CO2 (Mungekar et al. 2020). In their study, the shell weights show an increasing trend by decreasing atmospheric CO2 except for the last 1 Ma where shell weights decrease drastically. As they have suggested, the oxygen minimum zone (OMZ) is known to reach its modern strength at about 1.0 Ma closely following the enhanced surface water productivity (Tripathi et al. 2017) which points to intensified South Asian Monsoon (Derry and France-Lanord 1996; Clift et al. 2008). The increased OMZ intensity from 1.0 Ma again created conditions favorable for an increase in bottom water CO2 leading to the dissolution of shells which was reflected in lower shell weights for the past 1 Ma. These results reveal that the Maldives Sea behaved similarly to other tropical oceanic regions in terms of its surface water carbonate chemistry (Mungekar et al. 2020). Since the equatorial Indian Ocean has moderate surface ocean productivity, minimal terrigenous input, and a low sedimentation rate the dissolution would have been the primary control of carbonate content in sediments in the past (Yadav et al. 2022).
Average shell size vs. post-depositional dissolution
Sediments recovered below the present-day Aly are studied for post-depositional dissolution of the pteropod shells. Here, the cores NGHP-17, SK343, SK168, AAS11, and the RVS cores are located well beneath the Aly (~ 300 m) (Sarma and Narvekar 2001; Böning and Bard 2009; Reid et al. 2009). The post-depositional dissolution can be understood by the presence of abundant larger, thicker, and corroded shells with increased dissolution features as the smaller shells undergo preferential dissolution in the sediments (Sreevidya et al. 2019). Here, the higher aragonite dissolution values (LDX and FR) during the interstadials correspond with the shell size maxima in most cores, such as in SK343, SK168, AAS11, and RVS2 (Fig. 5). In contrast, the NGHP-17 record exhibits results similar to the cores collected above the Aly (Fig. 4), with the shell size maxima in NGHP-17 coinciding with the dissolution minima during the glacial stages. Hence, the presence of more giant shells during the interstadials in the other four cores can be attributed to the post-depositional dissolution of smaller shells (Fig. 5). The comparison between U1467 and the NGHP-17 exhibits similar variations of shell calcification which is higher during the glacial stages compared to interglacials (Fig. 4). However, the average shell sizes documented in core NGHP-17 is lower than the core U1467. The reason could be the water column's physicochemical characteristics or the regional hydrography. Furthermore, the longer records show a similar trend in the pteropod dissolution. Hence, there exists a regional pattern in carbonate preservation or dissolution during the glacial/interglacial scale, though the local water column characteristics are different. However, we suppose that the initial dissolution of pteropod shells in the deeper cores below ACD occurred mainly at the water column or sediment-seawater interface due to the extended time the sediments are exposed to dissolving conditions. After the burial, further dissolution might have taken place in cores SK168, AAS11, and the RVS2 (Fig. 5) depending on the aragonite corrosiveness of pore water environments and microbial degradation of organic matter within the sediment, given the high productivity during the southwest monsoon (SWM) (Gerhardt et al. 2000). Under the interstadial conditions, increased SWM-induced upwelling and productivity lead to increased organic carbon deposition. When deposited in an oxic environment, organic matter tends to degrade at the sediment-water interface through microbial degradation, producing carbon dioxide within the water column and sediments (Emerson and Bender 1981), promoting carbonate shells’ dissolution. These conditions would have prevailed during the interglacial/interstadial periods, characterized by relatively low CaCO3 preservation in the northern Indian Ocean (NIO). During periods of intense OMZ, shell dissolution due to corrosive bottom waters appears to obscure the understanding of surface water carbonate ion concentrations. Moreover, the high input and demineralization of organic matter leading to a high concentration of dissolved inorganic carbon (DIC), lowers the pH (Millero et al. 1998; Sijinkumar et al. 2010). So, the global upwelling regions preserve pteropods poorly (Berger 1978; Ganssen and Lutze 1982; Gerhardt and Henrich 2001). For example, the dissolution features and higher shell size values of the core SK343 during the interstadial periods (Fig. 5) can be attributed to the increased monsoon-induced productivity, organic matter production, and degradation, resulting in the dissolution of carbonates in the sediments. The observed higher shell masses of foraminifera shells during glacial stages are documented by several authors (Broecker and Clark 2004; Barker et al. 2006; Moy et al. 2009). However, from a study using foraminiferal faunal assemblage data, Anderson and Archer (2002) found slightly lower CO32− concentrations in the glacial Atlantic Ocean but with no change in the Pacific and Indian Oceans (de Villiers 2005). This increase in shell masses during deglaciation is assumed to increase in deepwater CO32− concentration by about 10 mmol/kg in the deep Atlantic, Indian, and Pacific Oceans (Yu et al. 2010). The study based on boron isotope in foraminifera shells suggests that the ocean was significantly more basic during the last glacial maximum (Sanyal et al. 1995). The faunal proxy and Corg content reveal that productivity was high during MIS 3 and 2, especially during the LGM (Yadav et al. 2021) which corroborates with the lower shell sizes and increased shell dissolution in the Andaman and the Laccadive Sea. The calcite dissolution during MIS 3 and the LGM is probably related to increased productivity, leading to increased organic matter export to the sediments. Degradation of this organic matter in sediments can lead to increased CO2 and undersaturation with respect to carbonate ion that finally reduces the preservation of CaCO3 in the study region (Emerson and Bender 1981; Archer et al. 1989; Hales et al. 1994; Hales and Emerson 1997).
In the case of BoB and the Andaman Sea freshwater flux through riverine input is another compound factor affecting the pH by modifying the OMZ (Panchang and Ambokar 2021). The freshwater input from the rivers or precipitation in these basins causes surface stratification during the SWM (Sijinkumar et al. 2021 and references therein). During such times, a high freshwater influx in the northern BoB and the Andaman Sea transports heavy suspended load, restricting light penetration which affects biological productivity (Panchang and Ambokar 2021). The reduced vertical mixing causes the strengthening of OMZ, thereby increasing the pH in the intermediate waters. Bhattacharjee (2005), in his comparison of the ACDs at the Andaman Sea and Carlsberg Ridge, suggested that the shoaling of ACD in the Andaman Sea is because of the excessive influx of freshwater from the Irrawaddy to the Andaman Sea though both these basins are at comparable latitudes. The variability in aragonite saturation in relation to the pH determines the depth of ACD in the Andaman Sea. As the core depths are well beyond the present-day ACD for SK168, AAS11, and RVS2 (~ 1200 m) (Sijinkumar et al. 2010), the variability in shell size could be due to the post-depositional dissolution and the preferential corrosion of the smaller shells in the sediments, leaving the more giant, thick shells during the interstadials. However, the shell size values during the stadials were low compared to the interstadials but within the average values calculated for cores collected above the Aly (275–300 µm) (Fig. 4). All these cores exhibit lower shell sizes during the stadials despite the minimum dissolution rates indicated by FR. The reason behind this would be the variable ACD during glacial and interglacial periods. The ACD was more profound during the glacial periods than the present day due to amplified thermohaline ventilation and decreased biological productivity (Panchang and Ambokar 2021). A weaker OMZ and deeper ACD were marked glacial/stadial period. During colder periods, the monsoon/biological productivity strength was reduced, leading to weaker OMZ and deep ACD and vice versa (Panchang and Ambokar 2021). The intermediate waters were rich in O2 due to low oxygen demand (Böning and Bard 2009). In addition, the influence of increased ventilation of oxygen-rich intermediate waters to the NIO resulted in good preservation of shelled microfossils during the glacial/stadial periods (Panchang and Ambokar 2021). The reason for the direct inflow of oxygen-rich intermediate waters (Böning and Bard 2009; Klöcker and Henrich 2006) is considered to be the shutting-down of the inflow of Persian Gulf Waters (PGW) and the Red Sea Water (RSW) to the NIO, during lowered sea level (Panchang and Ambokar 2021). Therefore, the post-depositional dissolution was significantly higher during the interstadial periods than in the stadials when the ACD was shallow. However, the post-depositional dissolution of pteropod shells was comparatively weakened during the stadials as the ACD was deeper, i.e., > 1340 m in the Laccadive Sea and > 2900 in the Andaman Sea.