Descent from the Hyperthermals: Persistent Organic-Matter Rich Lakes in the Eocene

Abstract

In this doctoral thesis, I reconstruct continental paleoclimate from the early Cenozoic, when the Earth’s surface experienced a long-term warming trend punctuated by a series of shortlived global warming (hyperthermal) events that culminated in an extended interval of elevated CO2 and extreme warmth, the Early Eocene Climatic Optimum (EECO, 53.26 to 49.14 Ma). The hydrological cycle’s response to climate perturbations is thought to be highly variable, and can promote feedbacks that induce further warming or cooling, thus serving as critical lessons for future warm worlds. However, the response of the hydroclimate regime operating during the EECO is poorly constrained, especially for the mid-to-low latitudes and particularly from continental interior sites.

Here, I adopt a multi-proxy approach, integrating geochemical, organic petrographic, and sedimentological perspectives from organic-rich sediments from the Green River Formation of the Uinta Basin, Utah and the Piceance Basin, Colorado, U.S.A., which were deposited in long-lived, large lakes at ~40ºN. I observed a wide range of organic matter types and distribution, with accumulation of organic matter at the lamination scale controlled by longer term sub-orbital cycles suggesting decadal periodicities in large organic-matter fluxes and associated carbon drawdown. Spatial and temporal variations in salinity conditions were the
result of long-term tectonic controls driving the water balance between fresher and brinerich inputs into the Uinta Basin, increasing ecological stress on biota living in the water column and leading to the cessation of conditions favouring prodigious organic
accumulation in the Mahogany Zone, a regionally extensive marker unit. Lastly, hydrogen isotopes from organic molecules indicate that the hydrological cycle operates differently during gradual vs. transient warming events, and that a stable hydrological regime may have supported deep lake development and promoted organic matter preservation. These organic-rich lake systems acted as an important negative feedback during the termination of the EECO, sequestering at least ~76 Gt of organic carbon over the ~400 kyr history of the Mahogany Zone.

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