Jessica Whiteside , Brown University
Long-duration elevated global temperatures and increased atmospheric pCO2 levels (~1000-2000 ppm) characterized the earliest portion of the Eocene (Ypressian; ~55 to 49 Ma). This extended period of global warmth was also punctuated by a series of short (millennial-long) hyperthermal events in which atmospheric CO2 (>2000 ppm) and global temperatures rose with unprecedented and (as of yet) unexplained rapidity. This interval is arguably one of the best temporal analogs for assessing contemporary response of the biosphere and global carbon cycle to increased CO2 emissions.
Furthermore, highly oil-productive formations were deposited during this time interval. Analysis of fossil leaves suggests North America maintained much more humid conditions than present (Wilf et al., 1998, Sheldon and Retallack, 2004; Lyle et al., 2008) and large lakes grouped to anomalously large (>60,000 km2), sizes (Roehler, 1991; 1993 Smith et al., 2008). In particular, the Eocene Green River Formation, a highly oil-productive formation, consists of open- and marginal-lacustrine rocks deposited in and around Lake Uinta. In these locales, we have shown that hyperthermals provided environmental conditions that favored burial and preservation of extensive amounts of organic material. A relict of one of these rapid warming events is observed here in the oil industry marker bed, the Mahogany interval, an extremely rich zone.
Although hyperthermals appear paced by 100 Ka and 1 Ma scale orbital (eccentricity) cycles in the marine realm, high frequency forcing processes have not yet been examined, and long continental records have yet to be explored for their expression. To identify sub-eccentricity (<100,000 year) scale variability in Early Eocene carbon cycling, we examined lacustrine records of organic carbon isotopes and carbon content from a ~5 Ma record in the Green River Formation in the Uinta Basin of Utah, U.S.A. We demonstrate that in addition to the expected 100 Ka eccentricity cycle, the 40 Ka cycle of obliquity is also an important component of climate variability as reflected in the lacustrine carbon cycle and hence a potential driver of global carbon cycling.
We further investigated carbon cycle dynamics by examining biomarker evidence for changes in the terrestrial methane cycle during this time interval. Due to their increased volumes, highly stratified and cyclically anoxic lakes of the Eocene could have provided enough methane to alter global radiative forcing. This is consistent with our data, which demonstrate that the Green River Formation exhibits strongly reducing conditions as well as abundant methanogen and methanotroph biomarkers. Further, the lacustrine environment was highly stratified with, at times, euxinic waters extending into the photic zone, as inferred from the presence of isorenieratene derivatives. Thus, the Green River Formation was likely a lacustrine environment with elevated methanogenic activity during this time. Increasing input of terrestrial matter into the Green River Formation correlates with shifts in the pristane/phytane ratio and isorenieratane abundances, suggesting that increased runoff intensified the stratification of the lake with a transition to more anoxic conditions. Following this transition, it is likely that methane production in the Green River Formation lake increased, which released more into the atmosphere. Our new results clearly demonstrate that the global carbon cycle of the early Eocene greenhouse world was strongly mediated by both astronomical forcing (including obliquity) and increased methane production in large stratified lakes.
Additionally, the Milankovitch properties of the oil-shale yield records promise predictability in exploration, suggesting intervals that while at the basin margin might be relatively low-grade, could improve dramatically towards the center.
This grant has greatly impacted my career by (1) extending my work on rapid continental climate change and source rock characterization to a new geological interval; (2) facilitating collaboration with organic geochemist Rich Pancost, Bristol University, to research the types of environmental conditions that favored extensive organic matter burial and preservation (hyperthermals) as well as the chemical transformation of biological organic matter into kerogen; (3) facilitating collaboration with Matthew Huber, Purdue University, to understand the sensitivity of the climate system to insolation variation by more extensive modeling of the changes in orbital parameters amplified by carbon cycle feedbacks for the Eocene.
An undergraduate research assistant, Maria Dunlavey, was supported in part by this award. Maria was able to experience the end-to-end process of hypothesis-development, proper field protocol and sampling techniques, and laboratory analysis and hypothesis-testing. I was extremely satisfied to witness the development of her technical and analytical skills as a function of this project, which matched her interests and benefited from her enthusiasm. The skills that she gained in this project are currently being applied to broader studies that will compose the majority of her senior thesis, which is currently in progress.