HR: 1340h
AN: ED13A-0715    [Abstracts]
TI: Geochemical Cycles of the Major Elements in the Oceans: What do we Know and how do we Know it?
AU: * Drever, J I
EM: drever@uwyo.edu
AF: University of Wyoming, Dept. Geology & Geophysics, 1000 E. University Ave., Laramie, WY 82071 United States
AB: In the context of the cycles of the major elements in the oceans, the emphasis forty years ago was on the relative constancy of the composition of the ocean and atmosphere over geologic time. More recently, the focus has been on variations in input and output fluxes, with corresponding variations in the composition of seawater and the atmosphere. The present-day riverine input (corrected for human activities) is fairly well constrained. Fluxes associated with reaction between seawater and the ocean floor (hydrothermal processes at spreading centers, low-temperature alteration of oceanic crust and detrital sediments) are not well constrained at the global scale: some of the data (particularly for potassium) are contradictory. We need to maintain a clear distinction between fluxes that are measured directly and fluxes that are computed on the basis of mass balance and the assumption of steady-state on some time scale. Modeling of past variations in seawater composition are based on assumptions regarding past weathering rates (which are not well constrained by direct observation, except to a limited extent by Sr isotopes and Ge/Si ratios), and past rates of hydrothermal circulation. Seafloor spreading rates are used as a proxy for hydrothermal circulation in the Cenozoic and Mesozoic: global sea level has been used as a proxy for hydrothermal circulation in the Paleozoic. The data that constrain past seawater compositions can be classified as direct (e.g. composition of ancient marine evaporite minerals and enclosed fluid inclusions; the composition of marine carbonate minerals) and indirect (e.g. S, C, Sr isotopic records). Although there are good geological records of these isotopic proxies, their interpretation in terms of past fluxes and the composition of the ocean and atmosphere is dependent on models that contain various assumptions. Despite a great deal of uncertainty, models based on relatively simple assumptions give results that seem reasonable and consistent with limited data.
DE: 1030 Geochemical cycles (0330)
DE: 1010 Chemical evolution
SC: Education and Human Resourcese [ED]
MN: 2004 AGU Fall Meeting