Overturning circulation in the Pacific during the last glacial period

Situated between Australia and New Zealand, the Tasman Sea is a crucial but previously underappreciated part of the global ocean conveyor belt. A recent study led by Dr. Torben Struve from the University of Oldenburg reveals that during the last ice age, this marginal sea in the South Pacific played a significant role in the exchange of water masses between major ocean basins. These insights contribute to refining climate models and enhancing our understanding of ocean circulation and carbon storage.

The researchers analyzed 62 fossil specimens of the stony coral Desmophyllum dianthus, collected at depths of 1,400 to 1,700 meters south of Tasmania by the remotely operated vehicle JASON. These corals, dating from 10,000 to 70,000 years ago—a span covering the peak and end of the last glacial period—grow in areas with strong currents that prevent sediment buildup.

Because the corals’ skeletons preserve the chemical fingerprint of surrounding seawater, their analysis revealed details about past ocean chemical composition at those depths. Specifically, radiogenic neodymium isotope ratios indicated that “young,” or relatively recently surface-contacted, Pacific Ocean water flowed through the Tasman Sea’s depths during the ice age.

This suggests that during the last glacial period, the upper Pacific Ocean was more mixed than today, with deep layers more isolated from the atmosphere. Such circulation patterns helped sequester carbon dioxide long-term, contributing to cooler global climates. Water from the North Pacific sank to about 2,000 meters, traveled south past Tasmania, and likely entered the Indian Ocean, reinforcing the global conveyor belt responsible for distributing heat among the world’s oceans.

These findings highlight the Tasman Sea’s dynamic role in past ocean circulation and offer vital context for understanding deep ocean currents’ influence on climate across glacial-interglacial cycles.

They demonstrate how fossil coral archives can illuminate the interactions within the global ocean system during key climatic periods, helping improve climate and carbon cycle projections.