INTERPRETING THE CARBON ISOTOPE RECORD OF ANCIENT EPEIRIC SEAS: LESSONS FROM THE RECENT

Carbonate platforms along with marginal and coastal seas serve as the best modern analog for ancient epicontinental seas and are significant components of the global carbon budget, manifested in part by their high primary productivity. Although these seas cover less than 10% of the global sea surface, primary productivity in coastal seas is estimated to represent up to 30% of total marine productivity. Thus, coastal marine primary productivity was likely far more significant during the non-glaciated geological past when shallow epicontinental seas covered 20-50% of the total cratonic area. The interaction between the global carbon cycle, seawater chemistry, and climate is probably best exemplified by the great carbonate platforms of the Phanerozoic that formed in vast epicontinental seas. Carbonate rocks thereby serve as the primary archive of variability in paleoclimate and paleoceanographic parameters.

Reconstruction of palaeo-seawater characteristics such as temperature, chemistry and pH rely predominantly upon proxy data derived from carbonate precipitated by organisms that inhabited these ancient epeiric seas. When applying isotope proxy data to palaeoceanography it is generally assumed that epeiric water masses are appropriate representatives of open oceanic conditions. A growing number of studies have demonstrated, however, that chemical and physical characteristics of shallow epeiric seas are often decoupled from those of the open ocean. This has implications for the interpretation of global secular trends in seawater chemistry that rely heavily on data from epicontinental settings.

For this purpose, data from modern carbonate platforms is compared to three studies of ancient platforms that show marked isotope gradients. Numerous observational patterns in data sets from Lower Cretaceous, Pennsylvanian, and Ordovician carbonate platforms are best explained using modern parameters, while other patterns may be explicitly non-actualistic.

The goals of this presentation are to (1) assess and quantify the primary variability of epeiric sea aquafacies in modern and fossil case examples, (2) to analyze and discuss the significance of the main factors that control epeiric seawater properties, and (3) to evaluate the implications for global secular isotope curves.