70 kyr record of denitrification and oxygenation changes in the eastern Arabian Sea

Abstract

A 70 kyr record of δ¹⁵N from the eastern Arabian Sea, Core AAS9/21 was investigated. High δ¹⁵N values during interglacials and low δ¹⁵N values during glacials are documented, which mimics the global δ¹⁵N records in the marginal ocean basins. However, during the Holocene, when the world’s Oxygen Minimum Zone (OMZ) gradually retreated as seen from a decreasing trend in δ¹⁵N, we see a δ¹⁵N increase through the entire Holocene which is also seen in other northeastern Arabian Sea records. Higher δ¹⁵N values recorded in the core (AAS9/21) from beyond the OMZ depth than the core (AAS9/19) from within the OMZ, during the Holocene, suggests that intense denitrification occurred in the eastern Arabian Sea within the OMZ. Furthermore, U/Th ratios also suggest that the shallower core was bathed by anoxic waters during the entire Holocene. Conversely, the deeper core was bathed by oxic waters during Holocene as well as during MIS4. These observations suggest that the vertical extent of the OMZ was nearly the same throughout the Holocene.