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

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 (C_i) and organic (C_o) 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 C_o 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 C_i concentrations effectively buffered oceanic d¹³C values against short-term isotopic change. These time-dependent models, which examine the combined effects of reservoir size, C_o burial, and evolving carbonate precipitation regime strongly suggest that secular changes in the size of the marine C_i reservoir may have provided a primary control on both baseline d¹³C values and the magnitude of isotopic excursions observed during the Proterozoic.