INVITED: INTERPRETING THE GLOBAL CARBON ISOTOPE SECULAR CURVE IN THE CONTEXT OF EPEIRIC SEA DYNAMICS: A RECORD OF EXCEPTIONAL ENVIRONMENTS?

The Paleozoic carbon isotope (d¹³C) record is derived from carbonate rocks that were deposited in epeiric seas.  This record is used to reconstruct the history of global C-cycling by assuming that epeiric sea dissolved inorganic carbon (DIC) mixed rapidly with the ocean/atmosphere C reservoir, so that the epeiric sea d¹³C signal reflects that of the global ocean.  It is an oversimplification, however, to assume that epeiric seas were homogeneous watermasses indistinguishable from the oceans in d¹³C.  For example, whole rock d¹³C values from across the well-constrained 454 Ma time slice of the Mohawkian Sea of Eastern Laurentia range by 4.5‰ due to a combination of local C-cycling processes and restricted circulation.  Epeiric sea d¹³C excursions are interpreted in terms of shifts in carbon storage between organic and inorganic carbon reservoirs, yet in a box model where the epeiric sea is treated as a reservoir distinct from the surface ocean, C transfer between organic and inorganic reservoirs appears to be only part of the story.  Changes in the circulation patterns that maintained the d¹³C gradient across the epeiric sea may also have provided a mechanism for driving d¹³C excursions.  Sea level rise may have increased the rate of DIC exchange between the epeiric sea and the surface ocean, causing the d¹³C of both reservoirs to become more alike.  With sea level fall, circulation may have been restricted, making continental influences more important for the epeiric sea, and driving the epeiric sea and ocean d¹³C apart.  This affects not only the epeiric sea d¹³C, but the d¹³C of the ocean, and the effects of sea level rise and fall may have been transmitted globally.  From this point of view, an increase or decrease in the scatter within the carbon isotope secular curve could be interpreted as change in the rate of exchange between the ocean/atmosphere system and the continental seas.  This is important for understanding the long term history of global C-cycling because it implies that the mean of the d¹³C secular curve, which is used as a descriptor of the d¹³C of the global ocean at a given time, could be biased toward d¹³C  values reflecting localized conditions within epeiric seas.