Although oxygen isotopes have been widely used in paleotemperature estimation, knowledge of Archean seawater temperatures remains limited. It is widely documented that δ18O values in carbonates, phosphorites, and cherts are more depleted with increasing age, a pattern due either to primary changes in seawater δ18O or temperature, or to secondary alteration. Samples of the Carawine Dolomite, of late Archean age (2.5 Ga) of western Australia were analyzed with the goal of constraining Archean seawater temperature estimates. These samples exhibited substantial inter- and intrasample variation in δ18O values, reflecting a complex geochemical history. The most uniformly enriched δ18O values were consistently associated with coated grains, which in different samples yielded values ranging from -3.5 ± 1.5% to -7.5 ± 2.2%. The corresponding cements are much more depleted in 18O and exhibit a greater range of δ 18O values (-8.2 ± 4.1% to -8.9 ± 1.3% PDB); these components also exhibit enrichment in Fe, implying a relatively strong diagenetic signature. Dolomitization of the ooids was probably synsedimentary or early diagenetic, because some samples show early chert cementation that postdates the dolomitization event. If one assumes no change in seawater δ18O through time, the δ18O values of the coated grains suggest seawater temperatures of 53 to 88°C. These estimates may be minima because the Carawine Formation was probably deposited in a hypersaline lagoonal environment, and, hence, there is a possible salinity effect (enrichment of seawater δ 18O, corresponding to increased salinity). While it is not possible to factor in temporal changes in seawater δ18O with any degree of certainty, modeling of isotopic exchange between seawater and mid-ocean ridge basalts suggests that δ18O values of Archean seawater were between 0.0 to +1.0% SMOW (Holland, 1984). Such an enrichment of Archean seawater δ18O would further increase paleotemperature estimates over those given above. Such high paleotemperatures may have been a limiting factor on the pace of evolution of life in the Precambrian, accounting for the deferred appearance of eukaryotic and metazoan organisms.
|School:||University of Cincinnati|
|School Location:||United States -- Ohio|
|Source:||MAI 57/06M(E), Masters Abstracts International|
|Keywords:||Archean paleotemerature analysis, Carawine dolomite, Coated grains, Hamersley basin, Stable isotopes|
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