THALLIUM ISOTOPES TRACK CHANGING OCEANIC OXYGEN ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM

The geologically instantaneous release of massive quantities of isotopically light carbon during the most recent abrupt climate perturbation, the Paleocene-Eocene Thermal Maximum (PETM; ~55.9 Ma), makes it the best analog to better understand future climate scenarios. A ~3‰ negative carbon isotope excursion defines the PETM but the exact cause(s) and associated magnitudes are debated. However, this excursion undoubtedly records a major perturbation to the global carbon cycle through the release of isotopically light carbon to the ocean-atmosphere system. In addition to warming, other environmental perturbations during this event include ocean acidification, permafrost loss, and a small increase in global euxinia (anoxic and sulfidic water column). However, there is limited evidence for widespread deoxygenation or organic carbon burial, which contrasts the PETM with similar climatic perturbations in the Mesozoic termed oceanic anoxic events. This research aims to better constrain the global spatiotemporal redox structure across the PETM global oceans using both novel and traditional geochemical tools.

We analyzed samples from the Arctic (IODP Expedition 302, Site M0004-A) and the North American Atlantic Coastal Plain (Cambridge-Dorchester Airport) using traditional and novel geochemical proxies including trace metal concentrations, iron speciation, and thallium isotopes to constrain the local and global redox conditions pre-, syn-, and post-PETM. Thallium isotopes – a new global proxy that responds to the global burial of manganese oxides for short-term events and thus tracks the earliest marine oxygen perturbation – record conflicting trends in the two sections. During the Paleocene, however, the Arctic was a restricted basin and the observed shift from reducing to oxic conditions over the peak of the excursion suggests that the basin became progressively more well-connected to the open ocean during the event. The Atlantic Coastal Plain likely records a global signal that documents a steep decline in marine oxygen at the onset of the PETM followed by a return to pre-event values at the recovery stage. Further research will aim to understand the Arctic’s changing marine and lacustrine system and additional sections are required to corroborate the Atlantic Coastal Plain’s global signature.