Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.111..305s&link_type=abstract
Earth and Planetary Science Letters, Volume 111, Issue 2-4, p. 305-317.
Other
12
Scientific paper
A numerical model is presented for the transport and exchange of oxygen isotopes in an idealized two-layer oceanic crust (basalt-carbonate) that is saturated with a non-advecting pore fluid open to the overlying ocean. The model allows for time-varying sedimentation rate, porosity, chemical diffusivity, and temperature-dependent 18O fractionation factors. Reaction is modeled as solution-precipitation of calcite in the carbonate sediment and as modified solution-precipitation (basalt dissolves, smectite precipitates) in the basalt layer. Generic models, using input parameters that are typical of those measured in DSDP-ODP cores, and constant sedimentation rates, illustrate the behavior of the system sufficiently well to allow first-order effects to be discerned that have important implications for paleotemperature and ocean floor weathering studies.
The solution-precipitation reactions allow oxygen to be exchanged between the low-δ18O basalt layer and the high-δ18O carbonate sediment through the medium of the pore fluids. This results in δ18O increasing upwards in the basalt and decreasing downwards in the sediment. Pore fluid δ18O values decrease with depth from zero to values of -1 to -3‰ at the basalt-sediment interface, and to values of -7 to -15‰ within the basalt. Because reaction rates are slow relative to diffusion in the pore fluid, high-frequency δ18O variations in the solid are preserved indefinitely, but the amplitude of the variations decreases as e-Rt where mR is the reaction rate (fraction reacted per unit time). The results suggest that the diagenetic shifts of the δ18O values of benthic and high-latitude planktonic foraminifera samples younger than Oligocene age are typically negligible. For samples older than 50 Ma, diagenetic effects are typically significant, and generally cause estimates of bottom and high-latitude surface ocean temperatures in the Cretaceous and early Tertiary to be too high. On the other hand, the model suggests that diagenesis will shift the δ18O values of low-latitude planktonic foraminifera to higher values, and that paleotemperature estimates of low-latitude surface waters, particularly in the Eocene and Oligocene, may be too low. Using a synthetic δ18O record for DSDP Site 167, we demonstrate how numerical models can be used to correct specific measured records for diagenesis. This approach could provide a means of extracting paleotemperature information from heavily altered Late Cretaceous and older samples, and for quantifying the subtle effects of diagenesis on the δ18O record at all sites.
Depaolo Donald J.
Richter Frank M.
Schrag Daniel P.
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