Abstract:
The stable isotope and minor element geochemistry of Neocomian (Lower Cretaceous) carbonates of the North Atlantic Basin (DSDP Sites 105, 367, 387, 391, and 603) was studied to develop a diagenetic model for pelagic limestones. In particular, we hoped to test the fidelity of whole-rock geochemical records as paleoceanographic indicators for pelagic deposits in which individual microfossils are not available for analysis. In agreement with previous diagenetic studies, our data indicate that the two major controls on the geochemical composition of pelagic limestones are primary carbonate content and depth of burial. With increasing CaCO (sub 3) content and burial depth, limestones become increasingly depleted in Sr and 18O. Within individual sedimentary sections, substantial variations in Sr/Ca ratios and in delta-18O values are evident over a range of 10 to 98% CaCO (sub 3) . However, even over a relatively narrow range of 50 to 98% CaCO (sub 3) , we observe, on average, a 2.5 per mill variation in delta (super 18) O values and changes of a factor of 2 in carbonate Sr/Ca ratios. Carbon isotope compositions do not vary as extensively with carbonate content, but CaCO (sub 3) -rich intervals tend to be relatively depleted in 13C. Without consideration of the strong diagenetic overprint on geochemical signatures as a function of more-or-less primary variation in carbonate content, one could be led to interpret significant geochemical variations in terms of paleoceanographic processes. For example, because the terrigenous flux to deep-sea basins increases during sea level lowstands and decreases during highstands, basin-wide fluctuations in the CaCO (sub 3) content of pelagic limestones would be expected to correlate with sea level. Even without any significant primary variation, therefore, substantial variations in Sr/Ca and delta (super 18) O values would be expected to result from burial diagenesis. As such, although an intrabasinal correlation of geochemical patterns might correlate with sea level variations, neither the relative nor the absolute magnitudes of Sr/Ca ratios and delta (super 18) O values would reflect primary ocean-climate variations. Specific examples of how geochemical variations in pelagic carbonates may have already led to misinterpretation of Cretaceous oceanography and climate will be presented.