What led to the end of last ice age? New study offers an explanation

For this study, the team went down the history road by reconstructing the ocean temperature and salinity of the ice age cycle of the last 1.5 million years. The findings revealed that during the peak time of each ice age, the Indian Ocean's waters became extremely salty. The study discovered that until the end of the last ice age, salty water was restricted to the Indian Ocean, but changes in winds and currents allowed it to flood into the Atlantic.

"Under the hot sunshine of the subtropics, seawater evaporates and gets saltier. Typically in the Indian Ocean, this salt is diluted by fresher waters flowing in from the Pacific, but during the ice ages this current was cut off by a drop in sea level, so the salt could build up undiluted,” explained Sophie Nuber, the lead author of this study, in an official statement.

This study was made possible thanks to the chemical fingerprints locked in microscopic fossil shells. These fossils were recovered from the 40-meter-long core of deep-sea mud. The measurement enabled the reconstruction of the temperature and salinity of the seawater in which the shells grew. 

Dr. James Rae, from the University of St Andrews, said, "Our work shows how different parts of the climate system are surprisingly inter-connected. Changes in circulation and salinity in one part of the ocean can have huge impacts on the other side of the planet, so we need to stop global warming to prevent further disruption to these critical circulation systems."

The results have been published in the journal Nature.

Study abstract:

The Indian Ocean provides a source of salt for North Atlantic deep-water convection sites, via the Agulhas Leakage, and may thus drive changes in the ocean’s overturning circulation. However, little is known about the salt content variability of Indian Ocean and Agulhas Leakage waters during past glacial cycles and how this may influence circulation. Here we show that the glacial Indian Ocean surface salt budget was notably different from the modern, responding dynamically to changes in sea level. Indian Ocean surface salinity increased during glacial intensification, peaking in glacial maxima. We find that this is due to rapid land exposure in the Indonesian archipelago induced by glacial sea-level lowering, and we suggest a mechanistic link via reduced input of relatively fresh Indonesian Throughflow waters into the Indian Ocean. Using climate model results, we show that the release of this glacial Indian Ocean salinity via the Agulhas Leakage during deglaciation can directly impact the Atlantic Meridional Overturning Circulation and global climate.