Diagenesis of Lower Cretaceous pelagic carbonates, North Atlantic: Paleoceanographic signals obscured

The stable isotope and minor element geochemistry of Neocomian (Lower Creta ceous) pelagic carbonates of the North Atlantic Basin (Deep Sea Drilling Pr oject Sites 105, 367, 387, 391, and 603) was examined to develop a diagenet ic model for pelagic limestones, In particular, we hoped to test the fideli ty of whole-rock geochemical records as paleoceanographic indicators for pe lagic deposits of pre-Aptian age, in which individual microfossils are not available for analysis. Data indicate that in addition. to depth of burial, rhythmic variations in primary carbonate content have strongly controlled diagenetic patterns and associated geochemical signatures in these Neocomia n sequences. Samples become. increasingly depleted in Sr and O-18 with incr easing CaCO3 content. Within individual sedimentary sections, substantial d ecreases in Sr/Ca ratios and delta(18)O values are evident over a range of 4 to 98% CaCO3. However, even over a relatively narrow range of 50 to 98% C aCO3 a 2.5 parts per thousand variation in delta(18)O values and a change o f a factor of 1.7 in Sr/Ca ratios are observed. Carbon isotope compositions do not vary as extensively with CaCO3 content, but carbonate-rich interval s tend to be relatively depleted in C-13. Petrographic analysis reveals tha t these geochemical patterns are related to the transfer of CaCO3 from carb onate-poor intervals (calcareous shales and marlstones) to adjacent carbona te-rich intervals (limestones) during burial compaction and pressure soluti on. This process results in the addition of diagenetic cement to carbonate- rich intervals to produce a bulk composition that is relatively depleted in Sr and O-18 and, at the same time, enables the retention of more-or-less p rimary carbonate that is relatively enriched in Sr and O-18 in adjacent car bonate-poor intervals. Thus, although cyclic variations in CaCO3 content ar e primary in the Neocomian sequences examined, measured variations in Sr/Ca ratios and delta(18)O values are not and, as such, do not provide reliable proxies for past variations in climate, oceanographic conditions, or globa l ice volume.