Interpreting carbon-isotope excursions: carbonates and organic matter

Variations in the carbon isotopic compositions of marine carbonate and orga nic carbon provide a record of changes in the fraction of organic carbon bu ried through time and may provide clues to changes in rates of weathering a nd sources of organic carbon. Paired carbonate and organic carbon isotope d eterminations provide a possibility of interpreting not only changes in the global carbon cycle through time, but changes in atmospheric pCO(2) as wel l. Interpretations of these types of data are typically rather qualitative; a quantitative basis is required to develop a better understanding of chan ges in the carbon cycle. For this purpose, we employ a simple model of the global carbon cycle which is subjected to a number of different perturbatio ns, each lasting 500 ky, i.e., much longer than the residence times of carb on and phosphorus in the ocean-atmosphere system. In addition to standard c onsiderations of carbon mass and isotopic fluxes to the ocean-atmosphere sy stem from weathering and volcanism and fluxes of organic carbon and carbona te-carbon to sediments, the model ;incorporates sensitivity of the photosyn thetic carbon isotope effect to changes in pCO(2). The inclusion of this pa rameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial. A series of simple to more compl ex simulations illustrates the significant effects of varying differences b etween the carbon isotopic composition of sedimented carbonate and organic carbon (Delta(B)). With constant Delta(B) a 50% increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopi c compositions. However, the same simulation with Delta(B) responsive to pC O(2) changes produces an initial parallel delta(13)C increase, but this is followed by an even greater C-13-enrichment in organic carbon because pCO(2 ) falls in response to increased organic carbon burial. The counterintuitiv e overall result of the enhanced organic carbon burial event is that the ca rbonate carbon isotopic composition actually decreases because of the more substantial increase in delta(13)C(org). In addition, we illustrate the eff ects on carbon isotopic compositions of the oceanic inorganic carbon reserv oir and buried organic matter of a 50% increase in volcanic CO2 outgassing, a 50% increase in weathering rate (with coupled phosphate and riverine car bon flux responses), a 50% decrease in shale-associated organic carbon weat hering, a 50% decrease in silicate weathering rate, and the possible effect s of the rise in abundance of C-4 plants in the late Miocene to Recent. We compare the model simulated carbon isotopic responses for some of these exp eriments to paired carbonate- and organic-carbon records to illustrate how these records might be interpreted in light of the model response. (C) 1999 Elsevier Science B.V. All rights reserved.