Isotopic approaches for quantifying the rates of marine burial diagenesis

Abstract

[1] 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.Citation: Fantle, M. S., K. M. Maher, and D. J. DePaolo (2010), Isotopic approaches for quantifying the rates of marine burial diagenesis, Rev. Geophys., 48, RG3002,

Variability of carbonate diagenesis in equatorial Pacific sediments deduced from radiogenic and stable Sr isotopes

“…The recrystallisation coefficients of the model, determined by fitting the pore water properties, also indicate a higher background recrystallisation rate (k) at Site U1336, as well as a smaller depth increment at which the rate of recrystallisation changes (c) compared to Sites U1338 and 807 (Tables S4 and S5). Being located near the fracture zone, forced advection (Fantle et al, 2010) may have played a role in the high reactivity at Site U1336. However, without samples from the entire sediment column it is not possible to be more specific about what drives the enhanced reactivity observed at this site.…”

Modeling the effects of diagenesis on carbonate clumped-isotope values in deep- and shallow-water settings

“…The effects of diagenesis on the chemical and isotopic composition of deep-sea carbonates has been explored extensively using both numerical models and measurements of the isotopic and chemical composition of carbonates and pore fluids for a variety of elements including oxygen, magnesium, and strontium (Killingley, 1983;DePaolo, 1987, 1988;Schrag et al, 1992;Richter and Liang, 1993;Schrag et al, 1995;Fantle and DePaolo, 2006;Fantle et al, 2010;Higgins and Schrag, 2012). Here, we first review the conceptual and quantitative framework of these models (section 3.1.1).…”

“…We first take established mathematical frameworks used to quantify how dissolution-reprecipitation reactions in carbonate sediments affect chemical and bulk isotopic compositions (i.e.,  18 O and  13 C values) of carbonate minerals DePaolo, 1987, 1988;Schrag et al, 1992Schrag et al, , 1995Fantle and DePaolo, 2006;Fantle et al, 2010;Higgins and Schrag, 2012) and extend them to include predictions of how  47 values change during diagenesis. We then ground truth the model.…”

Diagenetic alteration in low-Mg calcite from macrofossils: a review

“…Some in fer ences about ex pected diagenetic trends can be made on the ba sis of ob ser va tions in ODP sites (Fantle et al, 2010), pre dict ing in creas ing d…”