Reconstructing past deep-water circulations of Indian Ocean
Global overturning circulation—the equatorward transport of cold, deep waters and the poleward transport of warm, near-surface waters— controls ocean heat distribution and atmospheric carbon dioxide levels, thus playing a critical role in global climate.
It is thought that tectonic changes might have led to the formation of two separate water bodies — northern component water in the North Atlantic and Antarctic Bottom Water (AABW) in the Southern Ocean.
But these formulations have remained untested due to lack of adequate data. Some records that are available are from near the deep-water formation regions mostly from the Pacific and the Atlantic Oceans. Hence, they might not necessarily reflect the impact and change in deep water circulation.
Few studies have been carried out in the Indian Ocean to reconstruct past deep water circulations based on iron-manganese crust records and authigenic neodymium isotope composition of sediment cores. But iron-manganese crusts are situated at deeper depthsand are bathed only by AABW, making it suitable only for the reconstruction of the history of AABW, and authigenic neodymium isotope records are available only from the Bay of Bengal region.
A new study (Nature Communications) by a team of researchers from the Goa-based National Centre for Polar and Ocean Research and the School of Earth, Ocean and Atmospheric Sciences in Goa University has now sorted out the issue.
The scientists have generated an authigenic neodymium isotope record from the Arabian Sea and reconstructed the DWC record of the Indian Ocean for the period from 11.3 million years ago (Miocene era) to 1.98 million years ago (Pleistocene era).
“The record shows a clear shift from the Pacific water dominated deep circulation system before about nine million years ago, to the onset of a modern-like deep water circulation system in the Indian Ocean comprising of Antarctic bottom water and northern component water during the Miocene-Pliocene transition (about six million years ago). Our finding suggests a widespread impact of the late Miocene Central American Seaway closure on the evolution of ocean deep water circulation and validates the so-called Panama Closure Hypothesis,” says Dr. Waliur Rahaman from the National Centre for Polar and Ocean Research (NCPOR) and one of the corresponding authors.
(SunderarajanP. is a freelance science journalist based in New Delhi)