| Summary: | The temperature and chemistry of early seawater have both been inferred from the isotopic composition of Precambrian chert (SiO2), a precipitated mineral formed on or within marine sediments. The O of chert shows a robust quasi-linear increase through time - a signal that has been interpreted in a number of conflicting ways. For example, changing O has been hypothesized to reflect the product of cooling surface ocean temperatures, a signature of evolving seawater O composition, or the product of later stage diagenesis (where measured O reflects the composition of diagenetic fluids). We suggest this uncertainty can be resolved through the additional measurement and interpretation of the minor oxygen isotope 17O (noted as Δ'17O) in conjunction with O. In this study, we present a suite of triple oxygen isotope data on stratigraphically constrained Precambrian chert (both peritidal chert nodules in carbonates and iron formation silica). These mineralogically well-defined data allow for the first stratigraphic tests of the fidelity of 17O in SiO2. We then apply a Monte Carlo resampling technique to test the features of the competing hypotheses noted above, here now including critical constraints from 17O. The most parsimonious interpretation of these data suggests that secondary alteration with higher-temperature, meteoric-derived groundwater has skewed an original geochemical signature. This treatment can allow for some change in the oxygen isotope composition of seawater itself, however this does not appear to be the most statistically defensible single solution - clearly, some combination of multiple mechanisms is always possible and even likely. What is definitively the case is an equitable, modern-like Archean surface ocean temperature.
|
|---|