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45060-G2
A New Feedback on the Carbon Cycle Involving Clays and the Hydrologic Cycle and the Stabilization of Climate During the Paleocene-Eocene Thermal Maximum
Methods for the physical fractionation of sediment samples have been adapted from the soil science literature for application to marine sediments. A suite of 28 samples spanning the Paleocene-Eocene thermal maximum (PETM) were obtained from the marginal marine Wilson Lake core curated by the USGS and have been processed by a project-supported undergraduate researcher to isolate particulate organic matter. Free organic fractions, which appear to be dominated by charcoal fragments, were analyzed for their carbon isotopic composition and compared with measurements of carbonate carbon. Free fraction values are consistent with those expected for organic matter of terrestrial origin and preserve a high-fidelity record of the carbon isotopic excursion through the PETM without measurable leads or lags relative to the authigenic carbonate fraction. Recovery of free organics increases dramatically within the PETM, implying the enhanced delivery of young, terrestrial organic particulates to the marginal marine site during the PETM warm interval.
Concentrations of mineral-bound organic carbon were measured. These values increase by a factor of 3 at the base of the PETM and then drop to values 1.5 - 2 x pre-event levels through the remainder of the event. Grain size analysis of these samples is ongoing to determine if and to what degree these changes are controlled by changes in particle size distributions. The carbon isotopic composition of carbon in this pools was also analyzed using traditional off-line combustion and purification techniques. Interestingly, carbon isotope ratios of the mineral-bound organic fraction does not record the PETM carbon isotope excursion, but are correlated with wt% organic carbon in this fraction (R2 = 0.41). This suggests three non-exclusive possibilities: 1) the mineral-bound carbon is of diagenetic origin, 2) the age of the mineral-bound carbon is older than that of the free fraction (e.g., due to residence and transport times), or 3) the isotopic composition of this carbon fraction is dominantly controlled by its decay dynamics.
The work conducted to date provides an improved characterization of land-ocean carbon cycle coupling, and changes therein, across a prominent geological global warming episode. Based on results from this site, the dominant response appears to be in terms of particulate organic export and burial, although heterogeneous changes in mineral-bound carbon burial also occurred through the event. A combination of factors, including a transient drop in sea level, increase riverine discharge, and perhaps elevated rates of biomass burning on the continents, may have contributed to the observed responses. Research during project year 2 will focus on improved understanding of these changes through: 1) more detailed characterization of the make-up of particulate and mineral fractions from the Wilson Lake site, 2) complimentary analysis of samples from marginal marine sites in New Zealand, and 3) modeling studies of land-ocean carbon fluxes and their change through the PETM.
