Influence of Large Lakes on Methane Greenhouse Forcing in the Early Eocene

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 (sub-precessional) hyperthermal events in which atmospheric CO₂ (>2000 ppm) and global temperatures rose with unprecedented and (as of yet) unexplained rapidity. This interval is perhaps the best temporal analog for assessing contemporary response of the biosphere and global carbon cycle to increased CO₂ emissions. Although these 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 (GRF) in the Uinta Basin of Utah, U.S.A. and a ~1 Ma record from the Messel Shale, (Darmstadt, Germany.) 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 (>60,000 km²), 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 GRF and Messel Shale both exhibit strongly reducing conditions as well as abundant methanogen and methanotroph biomarkers. Further, the GRF 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 GRF was likely an area of elevated methanogenic activity during this time. Increasing input of terrestrial matter into the GRF 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 GRF lake increased, which released more into the atmosphere. Our new results suggest 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.