Ocean Circulation 55 Million Years Ago

In our publication that came out this month in Climate of the Past we focus on the Paleocene-Eocene Thermal Maximum, or PETM (check out our full paper here -open access journal so there is no pay wall!).  This was a time in Earth’s history characterized by high atmospheric CO2 and warm global temperature.  It is important to note, even the release of CO2 during the PETM pales in comparison to the modern CO2 release by humans (for a recent summary, see science daily’s summary of work out of the University of Hawai’i here).  However it is still important to understand how ocean currents responded to the warming climate of Earth million years ago and whether changing currents caused, exaggerated, or mediated the increasing CO2.

We propose that ocean circulation was fundamentally different during the PETM than it is today.  We use sediment cores collected by international efforts from seven sites distributed throughout the global ocean.  Recorded in these sediments are many important climate indicators, including carbon and neodymium isotopes.  The PETM is an time period easily identified in sediment records by carbon isotopes. Specifically, there was a significant change to more negative carbon isotope values at this time (called a “Carbon Isotope Excursion” or “CIE”).  The neodymium isotopes of ancient seawater can tell us about where that seawater has traveled from, and this record is believed to be recorded in the sediments. Based on these isotopes, we observe distinct changes in deep ocean circulation in each of the Atlantic, Southern, and Pacific Ocean basins. A significant change in neodymium prior to CIE in the Pacific leads us to believe that a change in circulation was, at least in part, the trigger of carbon release.

Unlike in the modern ocean, where there is no deep water formation occurring in the North Pacific we suggest that immediately prior to and during the beginning of the PETM there was sudden (but short lived) intensification of deep water formation in the North Pacific, supporting many existing modelling studies.  Also dissimilar from today, our records support an Atlantic circulation regime that was largely isolated and an Atlantic Ocean that remained stratified through much of the PETM.

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