Isotopic approaches for quantifying the rates of marine burial diagenesis

Diagenetic reactions in marine sediments have the potential to alter geochemical proxy records and affect the global carbon cycle over tens of thousands to millions of years. This article describes advances in the use of Ca, Sr, and U series isotopes in constraining carbonate recrystallization and silicate dissolution rates in marine systems. We specifically focus on recent efforts that interpret isotope variability in marine pore fluids using reactive transport models of varying complexity. Such studies suggest that calcite recrystallization rates are significant over time scales <1 Myr, approaching exchange rates of 0.4-1 Myr^-1. Over longer time scales, isotopic data point to continued exchange between calcite and coexisting pore fluid, though at lower rates than in young sediments. Extrapolating these recrystallization rates over tens of millions of years, we quantify the extent to which geochemical climate proxies such as Mg/Ca in calcite can be altered diagenetically. In some cases, such diagenetic effects significantly affect the interpretation of long-term climatic trends, as well as determinations of absolute paleotemperatures. Silicate dissolution rates in siliclastic marine sediments appear to be similar in magnitude to dissolution rates in terrestrial environments (10^-7-10^-6 yr^-1). Such estimates suggest that silicate weathering in marine sediments may play an important role in the carbon cycle, at least over time scales approaching 400-500 kyr.