The application of silica Δ'17O and δ18O towards paleo-environmental reconstructions

Abstract

The oxygen isotope composition of silica provides a valuable record of the temperature at which it precipitated and the isotopic composition of the fluid with which it equilibrated. This temperature-δ18O_H2O pair, however, is non-unique. The chert δ18O record shows a robust quasi-linear increase through time, but this signal has been interpreted as reflecting either cooling oceans, shifting marine δ18O composition, or diagenesis. We suggest that Δ′17O measurements may resolve this uncertainty. In chapter 2 we collect Precambrian chert from many well preserved Precambrian deposits and measure their δ18O and Δ′17O. Given equilibrium thermodynamic predictions for chert O-isotope evolution as a function of precipitation temperature, ocean O-isotope evolution, and diagenesis, we applied a Monte Carlo re-sampling model to test these alternate hypotheses. The results indicate the chert O-isotope record is best described as a product of alteration with higher-temperature, meteoric-derived groundwater. Neither changes in seawater temperature nor changes in seawater O-isotope composition are required. While this result is applicable to the Precambrian, it may not be useful in interpreting Phanerozoic data. We must first determine whether biological fractionation imposes a significant vital effect in Δ′17O. In chapter 3 we measure the δ18O and Δ′17O of diatom frustules from semi-continuous cultures. To ascertain the degree of natural variability we measured the O-isotope composition from two different diatom species across a range of growth rates and temperatures. Variations in the cultured growth rate and species produced no significant signal in the frustule δ18O or Δ′17O. Temperature variations in the culture, however, do produce a measurable O-isotope fractionation. The absolute value of the measured Δ′17O compositions were systematically negative with respect to the thermodynamic prediction. This offset we interpret to represent a vital effect which may be produced by the silica concentrating mechanisms diatoms use to precipitate their frustules. These results confirm the utility of diatom δ18O in reconstructing temperature, but suggest that Δ′17O may not be a suitable temperature proxy for diatomaceous sediment. Instead, Δ′17O may be used to distinguish biologically from abiologically precipitated silica. In chapter 4 we analyze carbon isotope fractionations from the stratigraphy immediately following the Neoproterozoic Marinoan snowball Earth event. We observe a temporary excursion where εp is anomalously small due to an interval of heavy δ13Corg deposition. This signal may reflect the unique biogeochemical conditions that persisted in the aftermath of snowball Earth. To explain this record, we developed a model that tracks the fluxes and isotopic values of carbon between the surface ocean, deep ocean, and atmosphere. From this, we conclude the post-Marinoan conditions will not intrinsically generate the observed isotopic signal. Reproducing the heavy δ13Corg requires the progressively diminishing contribution of an additional anomalous source of organic matter.