Evolution of deep-ocean carbon storage in the Indian Ocean since the Last Glacial Maximum

The Last Glacial Maximum (LGM) and the subsequent deglacial period likely featured climate-ocean dynamics and deep-ocean carbon storage states that contrasted with those of today. In this study, we focus on the Indian Ocean and place regional reconstructions in a global context by integrating published results from other ocean basins. Differences in deep-ocean carbon storage between basins across the LGM and deglaciation reflect changes in (1) deep-water mass sources and circulation structure and (2) regional regulation processes within the ocean basin. We reconstruct deep-water oxygen concentrations ([O₂]) between 26 and 10 ka BP using sediments from IODP Site 353-U1445 (Bay of Bengal) and IODP Site 361-U1479 (Cape Basin). Deep-water [O₂] is inferred from the carbon isotope gradient between epifaunal and infaunal benthic foraminifera (Δδ¹³C_epi-in). Changes in biological pump efficiency are assessed from the carbon isotope gradient between planktonic and benthic foraminifera (Δδ¹³C_p-b). Reconstructed [O₂] records are combined with outputs from the TraCE-21ka simulations and CMIP6 EC-Earth3-CC model to estimate respired carbon storage (Pg C).

During the LGM, deep-water [O₂] variations in the Cape Basin and the Bay of Bengal showed broadly synchronous trends, with major inflection points occurring at similar times. However, changes in the Cape Basin systematically preceded those in the Bay of Bengal. This temporal offset indicates a more rapid response in the Cape Basin relative to the Bay of Bengal. From the LGM to the deglaciation, increasing deep-water [O₂] and declining carbon storage in the Cape Basin are closely associated with reduced biological pump efficiency. In contrast, the Bay of Bengal exhibited stronger variability during the deglaciation, with a pronounced response during the Bølling–Allerød (B/A) interval, when deep-water [O₂] sharply decreased. During the B/A stage, the Antarctic Cold Reversal in the Southern Ocean was characterized by weakened AABW formation and reduced deep-water [O₂]. These changes slowed deep-water renewal and enhanced deep-water organic carbon remineralization, which probably resulted in increased deep-water respired carbon storage in the Indian Ocean. The larger LGM–deglacial amplitude in the Cape Basin reflects its location at the confluence of Atlantic, Southern Ocean, and Indian Ocean water masses, resulting in a more rapid and pronounced response to circulation reorganization, whereas the Bay of Bengal exhibits weaker and delayed responses as a distal deep-water reservoir. Estimated respired carbon storage efficiency in the Cape Basin is higher during the LGM by~0.03 mol m⁻³ and ~0.05 mol m⁻³ relative to Heinrich Stadial 1 (H1) and the B/A, respectively. Consistent with this difference, mean respired carbon storage decreased from ~5.51 Pg C (~2.58 ppm CO₂ equivalent) during the LGM to ~2.84 Pg C (~1.33 ppm) and 1.10 Pg C (~0.52 ppm CO₂) during H1 and the B/A, respectively. In contrast, the Bay of Bengal exhibits higher respired carbon storage during the B/A (1.24 Pg C; ~0.58 ppm CO₂ equivalent) than during the LGM (0.51 Pg C; ~0.24 ppm CO₂ equivalent). This study highlights the heterogeneous response of the Indian Ocean deep carbon reservoir during glacial-interglacial transitions.