Eocene Paradox: Intensified Bottom Water in Greenhouse Climate

During the most recent episode of greenhouse warmth, 52 million years ago, deep ocean circulation unexpectedly became more intense in the southwest Pacific. A new bottom water current flowed towards Zealandia, the currently submerged continent whose highest mountains make up New Zealand. The chemical composition of fossil fish shows that this new water mass was able to form near Antarctica, which is difficult to explain under past ice-free conditions.

The discovery was made by an international team of researchers, including PhD student Irene Peñalver and Professor Laia Alegret from the University of Zaragoza, Spain, and Dr. Sietske Batenburg from Utrecht University, the Netherlands. They investigated the chemical signature of fish remains to track past ocean currents at the sea floor. The relative abundances of different isotopes of the element neodymium are indicative of where a water mass was at the surface, before it sank to the deeper parts of the ocean.

Sediments and organisms from the sea floor were analysed from Site U1510, which was drilled and recovered by the International Ocean Discovery Program, a long-standing scientific collaboration. Laia Alegret, who participated in the ocean drilling expedition to Zealandia in 2017, stresses that they “managed to recover sediments that record the evolution of Zealandia and the climate and ocean currents over the last 70 million years”. “We extracted pieces of fossil fish from the ocean sediments” says PhD student Irene Peñalver, “and their neodymium composition indicates that the water flowing over the sea floor at drilling Site U1510 came from near the Antarctic continent”. The formation of dense water on or near the Antarctic Shelf drove the formation of a new bottom current that flowed into the southwest Pacific Ocean.

At 52 million years ago, in the Eocene, global temperatures were ~13 degrees higher than today, and there was no ice cap on Antarctica. “The formation of a new bottom water current is surprising in a greenhouse climate” explains researcher Sietske Batenburg, “as waters need to cool at the surface to become dense enough to sink to depth”. The complex behaviour of ocean circulation under warm conditions is important to understand future global change, as the ocean distributes heat over the planet.

“We do not have a definite explanation for the formation of this new bottom water mass”, adds Batenburg. “It may be caused by either bathymetric changes in the Tasman Sea, or climatic mechanisms, including reduced deep-water formation in the northern hemisphere or local changes in monsoon intensity. In terms of effects, there are coincident changes in the global carbon cycle in which ocean circulation changes may have played a role.”

Article

Peñalver-Clavel, I., Batenburg, S.J., Sutherland, R., Dallanave, E., Dickens, G.R., Westerhold, T., Agnini, C., and Alegret, L., 2025. ' Intensified bottom water formation in the southwest Pacific during the early Eocene greenhouse—Insights from neodymium isotopes', Geology 2025; https://doi.org/10.1130/G52974.1