Scientists from St Andrews and Cardiff Universities, in collaboration with international researchers, have made a groundbreaking discovery regarding the cause of the last ice age’s demise. Their study, published in the prestigious journal Nature, is particularly significant given the current rise in ocean temperatures and the increasing freshness of waters in the Atlantic and Southern Oceans due to melting ice sheets. These factors have the potential to disrupt ocean circulation and global climate patterns.
To unravel the mysteries of the past, the team analyzed the chemical composition of microscopic fossil shells, using them as indicators of the temperature and salinity of ancient seawater. By studying fossil samples from a 40-meter-long core of deep-sea mud, they were able to reconstruct the history of ocean temperature and salinity during each ice age cycle spanning the past 1.5 million years.
Their findings indicate that during the peak of each ice age, the Indian Ocean experienced an influx of highly salty water. The researchers propose that this phenomenon occurred when fresh-water currents, usually flowing into the Indian Ocean from the Pacific, were obstructed. As a result, the Indian Ocean became saltier, resembling an expansive salt pan.
Sophie Nuber, the lead author of the study, explained that under the scorching subtropical sun, seawater evaporates, leading to increased salinity. Ordinarily, this salt is diluted by fresher waters entering from the Pacific. However, during the ice ages, a drop in sea level cut off this current, allowing the salt to accumulate undiluted.
During the ice ages, global sea levels dropped by as much as 120 meters due to the formation of massive ice sheets that retained water evaporated from the oceans. Consequently, the freshwater currents that usually flowed through the Indonesian archipelago into the Indian Ocean were repeatedly severed as the landmass expanded during these periods.
The highly saline waters remained trapped within the Indian Ocean until the conclusion of the last ice age. At that point, shifts in winds and ocean currents allowed these waters to surge into the Atlantic. This influx played a crucial role in re-establishing the powerful Atlantic current system, responsible for warming the UK and Northwest Europe following the weakened glacial state.
Professor Steve Barker, one of the study’s authors, revealed that over a decade ago, they had uncovered evidence of a significant surge in Atlantic currents at the end of the ice age, which facilitated the transition to warmer interglacial conditions. The recent study demonstrates that the source of the salt, which rendered these currents dense and forceful, originated over 10,000 kilometers away in the Indian Ocean.
While warm, salty currents continue to flow through the Atlantic today, they are gradually weakening due to the influx of freshwater from melting ice sheets. This reduction in the currents’ strength could trigger a series of consequential climatic effects.
Dr. James Rae, from the University of St Andrews, emphasized the surprising interconnectedness of different components within the climate system. Changes in circulation and salinity in one part of the ocean can have far-reaching consequences on the opposite side of the planet. Therefore, it is imperative to address global warming and prevent further disruptions to these critical circulation systems.
Source: University of St Andrews