SR/CA VARIATIONS IN CRETACEOUS CARBONATES: RELATION TO PRODUCTIVITY AND SEA LEVEL CHANGES

Abstract

We present high resolution Sr/Ca measurements on coccolith-dominated pelagic carbonates of Berriasian-Valanginian, Albian-Santonian, and Campanian-Maastrichtian age from DSDP sites and Tethyan land sections. Carbonate Sr/Ca varies by up to 80% over both long and short timescales and may reflect either temporal variations in seawater Sr/Ca or variable Sr partitioning in biogenic carbonates. Numerical models of seawater Sr and Ca budgets indicate that while sea level changes can cause comparably large variations in the seawater Sr/Ca ratio, the timing and pattern of predicted Sr/Ca variations agrees poorly with the carbonate record. Reef crises unrelated to sea level changes may also elevate seawater Sr/Ca, and in some cases, the record of reef crises corresponds to increasing carbonate Sr/Ca. Changes in Sr partitioning, likely due to changes in calcareous nannoplankton productivity, also appear to control carbonate Sr/Ca in these records. Cretaceous carbonate Sr/Ca broadly covaries with indicators of productivity derived from microfossil ecology. Recent studies in sediment core tops and cultures have shown that Sr partitioning in coccolith carbonate is strongly influenced by changes in coccolithophorid growth and calcification rate. Where the effects of changing seawater Sr/Ca can be constrained by independent geological data, carbonate Sr/Ca measurements could provide a valuable perspective on changes in the productivity of calcareous nannoplankton and variations in the carbonate and carbon cycle in the Cretaceous.