Hansen, K. and Wallmann, Klaus (2003) Cretaceous and Cenozoic evolution of seawater composition, atmospheric O2 and CO2: a model perspective. American Journal of Science, 303 . pp. 94-148. DOI 10.2475/ajs.303.2.94.

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Abstract

A new box model for the global cycles of sulfur, oxygen, and magnesium has been developed and coupled with a carbon-calcium-strontium model (Wallmann, 2001). The model accounts for sulfur masses in the ocean, in evaporite rocks, in sedimentary pyrite, in oceanic crust, and in the mantle. Sulfur fluxes considered are burial and weathering of evaporites and sedimentary pyrite, volcanic emission of sulfur gases originating from the mantle and subducted reservoirs, and alteration of oceanic crust and serpentinization of oceanic peridotites. Oxygen fluxes are derived from sulfur turnover, carbon cycling, and iron redox processes. Model outputs are seawater concentrations of SO4, Ca, Mg, Sr, CO2, and HCO3, atmospheric partial pressures of O2 (pO2) and CO2 (pCO2), as well as the sulfur, carbon, and strontium isotopic composition of seawater. The secular trends of δ34S, δ13C, and 87Sr/86Sr recorded in marine evaporites and carbonates are used for model optimization and control. According to sensitivity analysis, tectonic/volcanic processes affect oceanic δ34S values considerably, whereas δ13C values are determined mainly by turnover of particulate organic carbon (POC). The different control mechanisms induce a significant deviation between the secular trends of sulfur and carbon isotope data. Dependence of POC burial on atmospheric pO2 via redox-dependent phosphate recycling provides an effective negative feedback on pO2. Moreover, phosphate supply to the ocean and thus POC burial depends on weathering, which in turn is related to the prevailing pCO2 level. Therefore, emission of volcanic CO2 induces O2 production via POC burial while O2 is consumed by the coeval release and oxidation of reduced sulfur compounds from the mantle. This coupling between tectonic/volcanic processes and POC burial provides additional stabilization of pO2. Consequently, predicted pO2 values fall into a narrow range of 0.17 to 0.25 atm during the entire model period. Changes in pO2 are mainly driven by changes in tectonic activity, erosion rates, and size of exposed carbonate areas. The model indicates that carbonate weathering, unlike silicate weathering, establishes a positive feedback on atmospheric pCO2 via enhanced burial and recycling of pelagic carbonates at subduction zones.

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