HR: 15:00h
AN: PP53D-06    [Abstracts]
TI: Isotopic Evolution of Soil Organic Matter Affects Paleo-vegetation and Paleo-pCO$_{2}$ Reconstructions
AU: * Bowen, G J
EM: gbowen@biology.utah.edu
AF: Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112 United States
AU: Beerling, D J
EM: d.j.beerling@sheffield.ac.uk
AF: Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN United Kingdom
AB: The stable carbon isotope ratio (\delta$^{13}$C) of fossil terrestrial organic matter is used to study several aspects of biosphere/atmosphere coupling in the geologic past. These range from vegetation response to climatic and pCO$_{2}$ shifts to reconstruction of paleo-pCO$_{2}$ levels. Although screening for diagenesis is typical in these studies, few have taken into account the ubiquitous but poorly understood phenomenon of progressive $^{13}$C-enrichment of soil organic matter during its decay, which is observed in modern soils worldwide. We present a simple model that describes this phenomenon and the interaction of soil organic carbon and CO$_{2}$ concentrations, fluxes and \delta$^{13}$C values. At its most basic level, the model suggests that bulk organic matter from sub-surface soil horizons will be variably enriched in $^{13}$C relative to the vegetation living on the soil surface. This complicates interpretation of paleo-isotopic records used in C3/C4 vegetation reconstructions, and may account for anomalously heavy fossil organic carbon isotope values measured in some paleosols pre-dating the end-Miocene expansion of C4 floras. The model also demonstrates that the \delta$^{13}$C evolution of soil organic carbon during its decay generates 2 types of biases that may affect soil mineral paleo-pCO$_{2}$ proxies. The first type of bias results from a steady-state inequality between the \delta$^{13}$C of organic carbon at a single depth within the soil and that of respired CO$_{2}$ in the soil. This bias is present when fossil organic matter is used to reconstruct the \delta$^{13}$C of soil-respired carbon, and can be minimized with appropriate sampling methods. The second type of bias results from a dynamic, seasonal imbalance in respiration, which may cause the soil \delta$^{13}$CO$_{2}$ flux during times of soil mineral formation to deviate from that of the annually integrated flux. At present, this bias can not be fully described or corrected for due to inadequacies in our knowledge of soil \delta$^{13}$C dynamics and the timing of soil mineral formation. Given the strong dependence of paleo-pCO$_{2}$ reconstructions on data from soil mineral isotopes, further work on these topics is warranted.
DE: 4806 Carbon cycling
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting