The role of low temperature alteration of the oceanic crust in global carbon cycling

The role of low-temperature (10's of degrees Celsius) seafloor alteration in the long-term carbon cycle has received far less attention than the parallel process on the continents of 'chemical weathering'. This difference in the amount of study of these two sinks for CO2 degassed from the solid Earth does not appear to be based on their relative importance to global C-cycling. A large amount of CO2 is precipitated as carbonate minerals in the upper oceanic crust and this appears to be largely due to in situ alkalinity generation due to reactions within the oceanic crust [1,2]. Better understanding of the role of this CO2 sink in the global carbon cycle requires better constraints on: (i) the controls on alkalinity production during low-temperature alteration of the upper oceanic crust; and (ii) when in the life cycle of a piece of crust most low-temperature reaction occurs. Alkalinity production due to low-temperature alteration of the oceanic crust appears to have varied over the last 160 Myr [3], but whether this reflects differences in ocean composition or ocean bottom temperature is unclear. New data, largely from the Troodos ophiolite, are being collected to determine the reactions involved in alkalinity generation and how these vary as a function of the environmental conditions. To this end, a new suite of volcanic glass major and trace element data have been collected to determine the protolith composition; this is required in computing alkalinity generation from the altered rock compositions. Overall the data collection (bulk-rock compositions, carbonate O-isotopes) is designed to allow us to determine the roles of protolith composition and fluid temperature in alkalinity generation. Sample processing is underway and these new data will be presented. Understanding the timing of low-T alteration of the oceanic crust is important in understanding the impacts of this process on global geochemical cycling. We are applying novel dating approaches to this problem including directly dating carbonates using in situ MC-ICP-MS U-Pb analyses. Preliminary data suggest that carbonate minerals precipitate early (within 20 Myr of crustal accretion). We have also re-modeled the Sr-isotopic compositions of carbonates, via comparison to the paleo-seawater isotopic, composition as another chronometer. A numerical inversion procedure minimizes the misfit across all data for both the fraction of basaltic Sr and precipitation age. We are currently coupling the modeled fraction of basaltic Sr to the O-isotope derived fluid temperature. Current versions of the model also indicate carbonate precipitation largely occurs within <20 Myr of crustal accretion. This relatively rapid alteration of the crust suggests that changes in extent of alteration of the oceanic crust in response to changing environmental parameters can provide a relatively rapidly feedback on the Earth system. [1] Spivack, A.J. and Staudigel, H., 1994. Chem. Geol., 115: 239-247. [2] Coogan, L.A. and Gillis, K.M., 2013. Geochem. Geophys. Geosys., 14(6): doi:10.1002/ggge.20113. [3] Gillis, K.M. and Coogan, L.A., 2011. Earth and Planetary Science Letters, 302: 385-392.