Controls on the δ13C of dissolved inorganic carbon in marine pore waters: An integrated case study of isotope exchange during syndepositional recrystallization of biogenic carbonate sediments (South Florida Platform, USA)

Abstract

Abstract The carbon isotope systematics of marine carbonates, organic matter and dissolved inorganic carbon (DIC) play a critical role in quantifying carbonate dissolution fluxes from modern deep-ocean sediments to paleoocean–atmospheric modeling. However, there is a growing body of evidence that C mass and isotope balances in marine pore waters appear incompatible, suggesting that some processes other than mass transport, carbonate dissolution, and organic matter decomposition have significantly increased the value of δ13C(DIC). We present a comprehensive data set of pore water and sediment geochemistries in biogenic carbonates from well-characterized depositional environments of the South Florida platform. Pore water elemental and δ13C(DIC) values are integrated with δ13C values of carbon sources (seawater, organic and inorganic carbon), sediment mixing rates (210Pb profiles), microbial sulfate reduction rates (SRR) (radiotracer 35SO42−), and incubation experiments spiked with low δ13C(DIC) to estimate the rate and extent of C isotope exchange. Together, these data indicate that biogenic carbonates undergo extensive syndepositional recrystallization at rates comparable to net dissolution rates, permitting significant exchange between isotopically depleted organic C and isotopically enriched inorganic C pools. Significant amounts of net carbonate dissolution are common in the pore waters of these low-Fe sediments, as manifested by Ca2+/Cl− ratios increased by up to 25% relative to overlying seawater. Despite rapid microbial SRR, degrees of pore water SO42− reduction usually are maintained below 5% by H2S oxidation, the main acid source for dissolution. These processes increase pore water DIC concentrations by more than 6 mM, over a 5-fold increase relative to overlying seawater values. Pore water δ13C(DIC) values are usually greater than −5‰, and sometimes as high as +2‰, despite decomposition of organic matter with low δ13C values (−9‰ to −15‰ VPDB). Pore water δ13C(DIC) values thus require inputs from inorganic C that exceed those from organic C, even though less than 25% of the DIC can be accounted for by the amount of net carbonate dissolution indicated by Ca2+/Cl− ratios. Incubation experiments attain DIC concentrations in excess of 20 mM during closed system SO42− reduction, but δ13C(DIC) values remain buffered between –4‰ and –6‰. Exchange of 12C for 13C during carbonate recrystallization in pore waters is required to reconcile the significant imbalance between mass and isotope budgets. Past work and data presented here for Sr2+/Ca2+ in pore waters show that recrystallization of biogenic aragonites and high-Mg calcites from South Florida platform sediments can occur without significant mineralogic change. Although these phases are metastable relative to lower Mg-calcite, the drive for recrystallization does not appear to be mineralogic stabilization. Instead, the high surface areas of many biogenic carbonates from algal and foraminiferal sources (about 2 m2 g−1) contribute excess surface free energy that is released by recrystallization to larger crystallites of the same mineralogy. Studies of carbonate dissolution and precipitation kinetics have firmly established that high ratios of solid surface area: solution volume causes reaction of CO2 with grain surfaces to become the rate-limiting step. Thus, in sediment-pore water systems containing carbonates with high specific surface areas, pore water δ13C(DIC) may be buffered either by the value of the bulk CaCO3, producing δ13C(DIC) values approaching +2‰, or by the relatively large fractionation factor between CO2 and solid CaCO3 (Δ∼−8‰), producing δ13C(DIC) values that approach −6‰ VPBD. The observed magnitude of isotopic exchange between organic and inorganic carbon pools during the earliest stages of sedimentation has ramifications for the interpretation of the C isotopic record of marine carbonate rocks and contributes significant uncertainty to reconstructions of paleoocean and atmospheric chemistries.