GLACIOEUSTATIC CONTROL OF δ13C EXCURSIONS IN LATE ORDOVICIAN AND EARLY SILURIAN EPEIRIC SEAS: INFLUENCE OF CARBONATE PLATFORM WEATHERING

Detailed records of positive δ¹³C excursions occur in Late Ordovician and Early Silurian carbonate successions of the Cape Phillips and Baltic basins. They show a striking degree of coincidence with local lithostratigraphic and fossil indicators of sea level change, and eustatic sea level curves attributed to ice volume changes in Gondwana. Sediments hosting the δ¹³C excursions are low in organic carbon content, which does not fit well with the organic C-burial hypothesis that is often invoked to explain positive δ¹³C excursions. Peak δ¹³C excursions are larger in basin proximal sections and smaller in basin distal sections. This finding is consistent with local C-cycles in both the Cape Phillips and Baltic basins, which caused gradients in seawater δ¹³C values. The gradients were oriented such that δ¹³C values were higher in basin proximal settings and lower in basin distal settings.

A simple and predictable consequence of glacioeustatic control of sea level in tropical epeiric seas is a fluctuation in δ¹³C of the local C-weathering flux with time. Carbonate sediments were deposited during sea level high stands and eroded during sea level low stands. Weathering of exposed carbonates during the low stands increased the δ¹³C value of the local C-weathering flux, which caused the δ¹³C value of the epeiric sea to shift towards more positive values. Because proximal regions of epeiric seas were more influenced by coastal processes, and distal regions were more influenced by mixing with the open ocean, the epeiric sea developed a strong gradient in δ¹³C values during the sea level drop. The model predicts the largest δ¹³C excursions in deposits of the shallowest facies in tropical epeiric seas. Smaller excursions would be recorded in ocean sediments because large changes in the isotope value of the global C-weathering flux were damped by silicate weathering contributions from non-tropical settings.

If the record of positive δ¹³C excursions in Ordovician and Silurian successions in the Cape Phillips and Baltic basins is dominantly related to variations in rates of carbonate weathering from sea level changes, then C-isotope curves may prove to be a useful proxy for determining the timing and relative magnitude of eustatic fluctuations through this time interval.