Modelling the distribution of stable carbon isotopes in porewaters of deep-sea sediments

Abstract

We present a numerical transport-reaction model that describes the interaction between oxic organic matter decay and CaCO 3 dissolution in deep-sea sediments. The model simulates the distribution and isotopic composition of carbonate species (H 2CO ∗ 3, HCO 3 −, CO 3 2−) in sediment porewaters. Forcing is provided by organic matter degradation by O 2, whereas calcium carbonate dissolution is included as a first order kinetic reaction with respect to porewater undersaturation. Modelling of the isotopic data strongly constrains rate estimates of TCO 2 production by organic carbon mineralization and CaCO 3 dissolution. Compared to previous models, this approach is independent of assumptions on reaction stoichiometry. The validity of the model is tested by simulating TCO 2, δ 13C and Ca 2+ profiles for two sites, taken from Sayles and Curry (1988) located in the Eastern Equatorial Pacific (Ω c = 0.64) and Western North Atlantic (Ω c = 1.30). The model successfully reproduces observed porewater distributions of TCO 2, δ 13C, and Ca 2+. Compared to estimates of TCO 2 produced by organic matter respiration derived by Sayles and Curry (1988), model integrated rates of TCO 2 production are larger by a factor of 1.6 at the Eastern Equatorial Pacific station and a factor of 4.1 at the Western North Atlantic station. The model allows for a quantification of the alkalinity flux induced by metabolic CO 2 released to porewaters during organic matter decay. CaCO 3 dissolution driven by metabolic decay gives rise to a substantial alkalinity flux at the Western North Atlantic station and contributes up to 53% to total alkalinity exchange at the station from the Eastern Equatorial Pacific. Model-derived calcite dissolution rate constants are within a factor of 4 at the two sites. This low variability contrasts with previous studies, which reported dissolution rate constants differing by several orders of magnitude.