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 were adapted from the soil science literature and applied to sample suites spanning the Paleocene-Eocene thermal maximum (PETM) from marginal marine sites at Wilson Lake (New Jersey), Tawanui (New Zealand), and ODP leg 301 (Arctic Ocean). Undergraduate and postdoc personnel on the project isolated and quantified concentrations of free particulate (POC) and mineral-associated (MAC) organic carbon. Carbon/nitrogen ratios and stable isotope ratios were determined by elemental analysis-isotope ratio monitoring mass spectrometry in the Purdue Stable Isotope Laboratory.
At all sites the particulate organic fractions exhibited high C/N ratios characteristic of resilient, land-derived compounds and/or black carbon. At the two site characterized by high sedimentation rates particulate carbon concentrations increased by a factor of 2 – 4 during the PETM and tracked the carbon isotopic excursion recorded in other substrates without measureable leads or lags. The carbon buried was dominated by young material derived from recent photosynthesis, and thus provided a direct, immediate feedback on atmospheric CO2 levels. At the Tawanui site, where sediment accumulation rates were 1 – 2 orders of magnitude lower, POC concentrations were very low and showed little change in concentration or isotopic composition through the PETM. This may indicate the presence of a limiting sedimentation rate threshold below which particulate burial is limited to small amounts of particularly resilient, kerogen-derived organic carbon. Overall, the results suggest that POC preservation was sensitive to sediment accumulation rate, but in high accumulation-rate systems this burial mechanism was a component of a dynamic, negative feedback loop involving increased burial of predominately land-derived organic carbon.
Despite major changes in grain size distributions and mineralogy, changes in mineral-associated carbon concentrations through the PETM were minimal at the Wilson Lake site. This contrasts with results from the other sites, where MAC concentrations increased by a factor of ~2 during the PETM. These results suggest that sediment accumulation rate was not a primary control on MAC preservation. Rather, the efficiency of this burial mechanism may have been linked to sediment source and transport dynamics in riverine and costal systems that controlled carbon fixation to and remobilization from clay mineral surfaces. In particular, prolonged sediment residence in estuarine environments as mineral grains traversed the low-gradient passive margin of New Jersey may have provided enhanced opportunities for re-mobilization of land-derived organics from mineral surfaces relative to the higher-gradient systems present at the other sites. This inference is supported by changes in C/N ratios and carbon isotope ratios of MAC at the sites, which suggest increased preservation of young, land-derived MAC during the PETM at the Arctic and New Zealand sites but a shift to more marine-dominated carbon during the PETM and an older relative age (meaning the time between photosynthesis and burial, here on the order of 10,000 years) for MAC at Wilson Lake.
Our results demonstrate that strikingly different controls govern the dynamic response of marginal marine carbon burial via two major pathways (POC, MAC) during a major global climate change event. Decoupling of responses characterizing these two pathways increases the complexity involved in model projections of negative carbon cycle feedbacks on global warming. Changes in sedimentary systems appear to be involved in governing carbon burial by both mechanisms, but whereas bulk sediment accumulation rate may be a good 1st-order proxy for lower limits on POC preservation, the dynamics of MAC burial response appear to be tied to more complex aspects of catchment hydrology and marginal marine sedimentary transport systems. Across the sites investigated, bulk organic carbon burial increased by a factor of 1.5 – 3 at all sites, however, suggesting that marginal marine organic carbon burial does represent a dynamic feedback on the carbon cycle. In all cases, terrestrial carbon was a major component of the total burial, signifying the importance of land-ocean fluxes in establishing this feedback.
Our continued work is focusing on manuscript preparation and investigations of continental source environments to elucidate potential controls on terrestrial organic carbon sources through the PETM (e.g., hydroclimatology, ecosystem change). Combined with the results from the coastal ocean sites this data will provide a more comprehensive picture of land-ocean carbon cycle coupling across the PETM climate change event.
