Reconstructing past deep-water circulations of Indian Ocean

Subject: Geography

Section: Oceanography

Why in news:

Studies have indicated that tectonically driven changes in the ocean gateways such as the closure of the Central American Seaway, a body of water that once separated North America from South America, since the late Miocene period, had a dramatic impact on the global overturning circulation.

What is global overturning circulation:

Global overturning circulation (GOC) — 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.

Panama closure hypothesis:

The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene.

What were previous assumptions?

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.
Consequently, it is also hypothesised that there would have been large-scale changes in the Deep Water Circulation (DWC) in the oceans across the world, thus impacting global climate through ocean-atmosphere carbon dioxide and heat exchanges.

Lack of data and evidence in support of the hypothesis:

Data for deep water circulations were only available for the Atlantic and Pacific oceans. Those data might not reflect the global impact and change in deep water circulations.

Role of the Indian ocean in deep water circulation:

The Indian Ocean does not have any major deep-water formations of its own.
It acts only as a host for NCW and AABW.
Further, the northern parts of the Indian Ocean are located at one of the terminal ends of the GOC, far away from the deep-water formation regions and oceanic seaways.
These specific features could make the northern Indian Ocean an ideal basin to do this.

Deep water circulation studies in the Indian Ocean:

Few studies have been carried out in the Indian Ocean to reconstruct past deep water circulations based on iron-manganese crust records and the authigenic neodymium isotope composition of sediment cores.
There were some hurdles. For example:
- Data are available only for the Bay of Bengal region
- Himalayan rivers bring a substantial amount of neodymium particulates.

New Study:

A new study 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 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).
The 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.

Significance of the study:

The study is highly significant since it provides unequivocal evidence in support of the theory that the closing of the gap between North and South America had led to the evolution of the modern form of GOC.