T95 From Geochemistry of the Geosphere, Atmosphere, and Cosmos to Forensic Environmental Geochemistry II: A Tribute to Ian Kaplan

Monday, 28 October 2002
1:30 PM - 5:30 PM

THE ROLE OF RESERVOIR SIZE IN CONTROLLING THE RATE AND MAGNITUDE OF SECULAR VARIATION IN CARBON ISOTOPIC COMPOSITION DURING THE MESOPROTEROZOIC ERA

Paper number: T95-16
Program: Technical Sessions
Time: 5:15 PM – 5:30 PM

Authors

Abstract

In contrast to the Phanerozoic C-isotopic record, the Mesoproterozoic record is characterized by long intervals of isotopic stasis, punctuated by a series of short-term isotopic excursions. The Mesoproterozoic record is divided into two main intervals: an older interval (>1.3 Ga), characterized by baseline d13C values near 0‰ and isotopic excursions <2‰, and a younger interval (<1.3 Ga), with baseline d13C values near +3.5‰ and isotopic excursions up to ~4‰. Typically, these isotopic patterns are described using steady-state isotopic models, which can predict relative burial fluxes of inorganic (Ci) and organic (Co) carbon. Such models describe the overall state of carbon cycling during each interval and suggest that the shift to elevated d13C values ~1.3 Ga was related to a significant increase in Co burial driven by long-term evolutionary, environmental, and/or tectonic change. Isotopic excursions, on the other hand, are short-term in nature and represent intervals of non-steady state behavior in the global carbon cycle. These intervals of isotopic change provide equally critical insight into the long-term evolution of the Proterozoic exogenic cycle.

Short-term C-isotopic excursions are best examined using a time-dependent carbon cycle model. In contrast to steady-state models, which suggest that isotopic stasis prior to ~1300 Ma was driven by long-term stability of input and output fluxes, time-dependent models of Mesoproterozoic excursions are quite sensitive to reservoir size and suggest that high marine Ci concentrations effectively buffered oceanic d13C values against short-term isotopic change. These time-dependent models, which examine the combined effects of reservoir size, Co burial, and evolving carbonate precipitation regime strongly suggest that secular changes in the size of the marine Ci reservoir may have provided a primary control on both baseline d13C values and the magnitude of isotopic excursions observed during the Proterozoic.