Ocean Temperatures Through Early Cenozoic Climate Maxima Across a Latitudinal Transect from the North to the South Pacific - A Multi-Proxy In Situ Approach

Lead PI: Reinhard Kozdon

Unit Affiliation: Biology and Paleo Environment, Lamont-Doherty Earth Observatory (LDEO)

June 2020 - May 2023
Pacific Ocean
Project Type: Research

DESCRIPTION: Records of Earth's past climate hold important clues to our future climate. For example, the early Eocene (56 to 49 million years ago) was characterized by higher-than-modern atmospheric greenhouse gas levels and much warmer mean global temperatures. This period may be a good analog for the 21st or 22nd century. By that time, climate models predict that greenhouse gas emissions will produce warmer conditions than the Earth has experienced for at least 35 million years. One common feature of all past periods of warm climate is the exceptional warmth of high latitude regions. However, current climate models fail to reproduce these warm polar temperatures. This disagreement between paleoclimate records and model simulations poses a major challenge in climate research. Are the paleoclimate data biased, are the climate models inaccurate, or both? Testing of climate models is currently limited by the scarcity of robust paleoclimate data. For instance, many records of past sea surface temperature are based on the isotopic and chemical composition of fossil shells of planktic foraminifera. These microorganisms live throughout the surface ocean and their shells are preserved in seafloor sediments. However, these fossil chemical records can be degraded by sedimentary processes acting over millions of years. High-resolution microscopic imaging now allows for the identification of better-preserved areas within fossil foraminifera shells. And recent analytical developments make it possible to measure the chemistry of these tiny areas. The proposed study will use these new methods to re-assess equator-to-pole sea surface temperature gradients in the Pacific Ocean during the early Eocene warm period. These data will shed new light on the data-model mismatches and help improve the climate models' accuracy. The study will support a graduate student. The project results will be highlighted in a science outreach display to introduce paleoclimate studies to non-scientists.

Our ability to accurately simulate warm climates in Earth history provides one of the most important tests of our understanding of the Earth's atmospheric system. Of particular interest is the early to middle Eocene 56 to 49 million years ago) which is the last time that greenhouse gas levels exceeded ~600 ppm CO2 (modern 412 ppm, increasing more than 2 ppm/year). A puzzling feature of this time is the exceptional warmth of high-latitude regions associated with relatively cool tropical temperatures and a weak latitudinal temperature gradient. However, even current climate models fail to simulate such a climate regime. This data-model mismatch may be partially caused by diagenetic alteration of paleoclimate archives. After millions of years in the sediment, diageneses may alter the original isotopic and chemical composition of foraminifer shells, sand-grain-sized marine microfossils that are by far the most important recorders of the Earth's past climate. Within the past years, it was found that minute (just tens of microns) domains within these foraminifer shells are better preserved than the remaining material. Recent developments and improvements of in situ (in place) analytical approaches in combination with high-resolution imaging now allows for the identification and analysis of these better-preserved domains within foraminifer shells. By using these new and emerging in situ technologies, we aim to re-asses meridional temperature gradients through early Eocene climate maxima across a South Pacific longitudinal transect. Thereby, we will focus on three time intervals that are of highest interest for climate modeling community: (1) The Early Eocene Climate Optimum (~53, 51 Ma); (2) The Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), (3); The period just before the PETM to assess to pre-warming background conditions. State-of-the-art imaging approaches to identify better preserved domains within foraminifer shells will be used in combination with Secondary Ion Mass Spectrometry (SIMS) for oxygen isotope analysis, Electron Probe Microanalysis (EPMA) for the determination of Magnesium/Calcium (Mg/Ca) ratios, and Laser-Ablation ICP-MS for multielement depth profiling through foraminifer chamber walls (chemical fingerprinting of diagenesis). The in situ data will be paired with conventional oxygen isotope measurements to assess the potential bias of previously published paleorecords.