A 16‐kyr Record of Ocean Circulation and Monsoon Intensification From the Central Bay of Bengal

Abstract

The monsoonal signal dominates most climate records from the Indian Ocean. As a consequence, conventional stable isotope tracers have not been effective at tracking ocean circulation in the Indian Ocean and distinguishing between monsoonal and ocean circulation changes in the Bay of Bengal. Here we present a deglacial through Holocene Nd isotope record of seawater in the central Bay of Bengal, spanning the last 16 kyr. Comparison with a published record from the deep equatorial Indian Ocean (Piotrowski et al., 2009(Piotrowski et al., , https://doi.org/10.1016(Piotrowski et al., /j.epsl.2009.06.007 shows that Nd isotope ratios of these cores were similar until~8 ka (Ɛ Nd = −7 to −8 until~13 ka, and −8.5 to −9.5 between~13 and 8 ka), after which the Bay of Bengal record diverges to lower values (Ɛ Nd =~−10.5). We interpret the Nd isotope record until~8 ka in both cores to represent the influence of the global overturning ocean circulation. The divergence of the record in the Bay of Bengal since~8 ka to lower Ɛ Nd values is coincident with a major increase in the strength of the rain-bearing southwest monsoon and a high sediment load from the Ganga-Brahmaputra Rivers to the Bengal Fan that reflects major impacts from the intensified monsoon. Thus, the Nd isotopes are sensitive recorders of the changes in monsoon intensity in the Bay of Bengal.

Changes in Intermediate Circulation in the Bay of Bengal Since the Last Glacial Maximum as Inferred From Benthic Foraminifera Assemblages and Geochemical Proxies

“…(a) Ice core atmospheric CO 2 from Antarctic Dome C (Monnin et al, ), (b) comparison of the intermediate water B‐P 14 C age offset: Arabian Sea records, shown in pink (RC 27‐23; Bryan et al, ), dark red (SO161‐SL22; De Pol‐Holz et al, ) and MD 77‐176 (black, B‐P 14 C age offset calculated by benthic H. elegans age, red diamond, calculated by benthic Cibicidoides spp. ), (c) benthic δ 13 C records for MD77‐176, (d) comparison of seawater ɛ Nd records in BoB obtained from planktonic foraminifera in core MD77‐176 (Yu et al, ), core RC12‐343 (Stoll et al, ), SK157‐20 (Naik et al, ), and ODP site 758 (Burton & Vance, ). The gray‐shaded intervals mark the two‐step increase in atmospheric CO 2 ; the yellow‐shaded interval marks the 14–13 cal kyr BP interval and the early Holocene (10.5–9 cal kyr BP), respectively.…”

“…Higher values of B‐P age offset could indicate intrusions of deep old water masses and a strong stratification of the upper water column. Moreover, the decrease in ɛ Nd at deep water depth of the BoB could be interpreted as the increased influence of NADW (−13), and/or enhanced nonradiogenic Nd inputs from Ganges‐Brahmaputra river system during the Holocene (Naik et al, ; Yu et al, ). Combined with the δ 13 C, δ 18 O and assemblage records, higher values of B‐P 14 C age offset may also suggest the contribution of NADW and SSW, respectively, during the late Holocene and LGM.…”

“…Thus, assemblage 2 could indicate relatively low-oxygen and mesotropic to eutrophic bottom water conditions during the LGM. This environment may be linked to the higher C records for MD77-176, (d) comparison of seawater ɛ Nd records in BoB obtained from planktonic foraminifera in core MD77-176 (Yu et al, 2018), core RC12-343 (Stoll et al, 2007), SK157-20 (Naik et al, 2019), and ODP site 758 (Burton & Vance, 2000). The gray-shaded intervals mark the two-step increase in atmospheric CO 2 ; the yellow-shaded interval marks the 14-13 cal kyr BP interval and the early Holocene (10.5-9 cal kyr BP), respectively.…”

Glacial events during the last glacial termination in the Pagele valley, Qiongmu Gangri peak, southern Tibetan Plateau, and their links to oceanic and atmospheric circulation

“…This explains why the mean annual temperatures in Greenland and northern Europe decreased by 12°C to 17°C relative to today's values in GS-1, with a 22°C to 28°C decrease in winter and a 3°C to 6°C decrease in summer (Denton et al, 2005, 2010). Although the HTC also controlled the Indian Ocean (where the Indian summer monsoon transports water vapor to the TP) for a major portion of the last glacial termination (Naik et al, 2019; Sun et al, 2019), our estimated GS-1 temperature depression of 1.4°C–1.5°C for the Qiongmu Gangri peak is much less than the values from the North Atlantic region. This argument is in agreement with the view of Rehfeld et al (2018) that the temperature decrease from the LGM to Holocene had a clear zonal pattern, with a progressive reduction in change from the high latitudes toward the tropics.…”

Millennial Variability in Intermediate Ocean Circulation and Indian Monsoonal Weathering Inputs During the Last Deglaciation and Holocene

“…In contrast, the studies of Abbott et al (2022) and Blaser et al (2019) are from high-latitude North Atlantic cores that were influenced by either significant inputs of reactive volcanics from Iceland (with highly (Yu et al, 2018). Deep water Nd isotope records are from ODP 758 (Burton & Vance, 2000), SK157-20 (Naik et al, 2019), and SK129-CR2 . Blue bars indicate the consistent millennial-scale variations during HS1, the YD, and the Holocene.…”