Using the clumped isotope signature in to constrain diagenetic temperature in oceanic and periplatform sediments
Abstract:Carbonate platform margins provide a rare insight into the chemistry of the ancient oceans. As subduction destroys most pelagic sediments, periplatform sediments, mixtures of platform and oceanic derived carbonates, often are the only extant records of ancient ocean conditions.
During times when the Mg/Ca ratio of the oceans is elevated, such as during the present day, metastable aragonite is the favored form of calcium carbonate produced on the platform top. Consequently periplatform sediments formed during these times initially possess high amounts of aragonite and as such have a significantly higher diagenetic potential than periplatform sediments formed during calcite seas.
The rate and extent of diagenesis can be ascertained by petrographic, chemical and isotopic examination of the sediments. Numerous models can be employed to interpret the porewater and sediment chemistry. One such model involves the modeled loss of strontium over time compared to the amount available from sediment recrystallization. The degree of equilibration between sediments and their corresponding porewaters can also provide an estimate for recrystallization. Reactive transport models examine changes in the concentration of elements such as Sr and its isotopes (87Sr/86Sr) or calcium (44Ca/40Ca) in the pore fluids and the sediments.
The d18O of the sediments and pore water provides an additional mechanism with which to understand diagenesis as during burial the temperatures increases while the pore waters become evolved in response to the higher temperatures and mineral-water reactions. For example, in the case of the Bahamas, the pore waters from sites drilled during ODP 166 show a dramatic increase in the d18O with depth below the sea floor. This has been interpreted as being a result of carbonate dissolution and precipitation and increasingly higher temperatures. The d18O of the carbonates decrease over the same interval, presumably as a result of higher formation temperatures at depth. In this presentation we present D47 data from the ODP 166 sediment cores to test his interpretation. We have combined these results with estimates of sediment recrystallization based on Sr flux and d18O.