UCLA

Though the Precambrian geologic record has to date elucidated numerous features of the ancient geobiological environment, several fundamental aspects of the Precambrian Earth remain enigmatic. This dissertation describes studies using tools from mineralogy, paleobiology, and molecular evolution to explore two of these aspects: (1) the history of atmospheric oxygenation through the Neoproterozoic-Cambrian transition, associated with the diversification of oxygen-requiring metazoans and (2) the long-term evolution of Earth surface temperature since the Archean (~3000 Ma).

It has long been postulated that the Neoproterozoic-Cambrian transition was correlated with an increase in environmental oxygen on the basis of minimum physiological requirements of metazoans through the Cambrian Explosion, as well as isotopic and redox-sensitive metal analyses of sediments. However, the history of oxygenation during this geobiologically significant interval is not sufficiently or quantitatively constrained. Experimental calibrations reported here of fluorescence signatures associated with an oxygen-dependent mechanism for Sm3+-substitution in apatite are used to devise a semi-quantitative apatite oxygen paleobarometer indicative of ambient O2 concentrations during Sm3+ emplacement. These calibrations are used for the interpretation of comparable fluorescence signatures of microfossil-associated apatite specimens of the late Neoproterozoic Doushantuo and Early Cambrian Chulaktau Formation phosphorites, as well as biomineral apatite scale microfossils of the mid-Neoproterozoic Fifteenmile Group. The characterized fluorescence spectral features evidence generally low, but locally variable O2 concentrations during apatite precipitation, and are consistent with moderate increased oxygenation of the shallow sediment environment of the Chulaktau relative to the Doushantuo phosphorite. The expanded use of this apatite oxygen paleobarometer approach can be expected to provide a more refined and quantitative understanding of Neoproterozoic-Cambrian oxygenation during the Cambrian Explosion.

The long-term evolution of Earth’s surface temperature through the Precambrian is similarly unconstrained. Paleotemperatures inferred from isotopic compositions of marine cherts suggest Earth’s oceans cooled from 70 � 15 �C in the Archean. However, this interpretation has been met with skepticism due to uncertainties regarding post-depositional isotopic alterations of ancient samples, the isotopic composition of the Archean ocean, and the possibility of a local geothermal depositional environment. Thermostability analyses of ancestral enzymes, reconstructed by molecular evolutionary models applied to phylogenies of extant descendants, provide an independent method by which to assess this temperature history. Reported experimental thermostability measurements of ancestral kinases derived from modern photosynthetic taxa limit interpretations of ancient temperatures to the photic zone. Our results suggest that Earth’s surface temperature has cooled from ~65-80 �C in the Archean, a finding consistent with previous isotope- and protein-reconstruction-based interpretations. Interdisciplinary studies such as this hold promise for providing new insight into the coevolution of life and the environment through Earth history.