The Early Triassic records a ~5 My period of marine biological recovery following the end-Permian mass extinction (EPME, ~252 My). Global oceans were characterized by warm sea-surface temperatures (SSTs), widespread anoxia, and major perturbations to the carbon cycle, the collective result of repeated massive Siberian Traps eruptions and associated global climate feedbacks. Globally widespread microbial deposits (stromatolites, thrombolites) developed in shallow marine environments. Several marine isotope records (C, O, S, U) indicate repeated, large-magnitude shifts related to paleoceanographic events during the recovery of marine ecosystems following the EPME.
Chapter 1 describes and interprets a widespread (~400 km along strike) Smithian (~251-250 Ma) shallow marine microbial carbonate mound complex, with facies and morphological variation across the shelf related to local environmental conditions (accommodation space, terrigenous influx). Many authors link Lower Triassic microbial deposits with global SST/anoxic events; results of this study indicate that the primary control on microbial deposition in Utah were local environmental conditions related to transgression (increased accommodation, reduced detrital influx).
Chapter 2 reconstructs the timing of microbial carbonate accumulation in Utah (~600 km along dip) to determine a relationship with any specific Smithian paleoceanic event. The temporal reconstruction is conducted using new carbon isotope (δ13Ccarb) trends and existing ammonite biostratigraphy. Results indicate that microbial limestone deposition was time-transgressive with a protracted (~1 My) transgression, which implies that microbial deposition was not associated with any specific Lower Triassic SST/anoxic event or C-isotope excursion.
Chapter 3 reconstructs a high-resolution Lower Triassic marine δ34S record using carbonate-associated sulfate (CAS) from the western U.S. (eastern Panthalassa), which indicates three large, positive δ34SCAS excursions. The timing of positive and negative δ34SCAS shifts corresponds with existing data from South China (Tethys), indicating a global seawater δ34S record. Positive δ34SCAS shifts are interpreted to result from massive increases in pyrite precipitation by anaerobic sulfur-reducing bacteria beneath globally expanded oxygen minimum zones; coeval positive δ13Ccarb shifts suggests contemporaneous preservation and burial of organic matter. Positive δ34S excursions do not match the timing of eustatic sea level changes, SST trends, redox histories of regional basins, or globally distributed shallow marine microbial deposits.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Second Committee Member
Third Committee Member
Spencer G. Lucas
Fourth Committee Member
Gary S. Weissmann
Fifth Committee Member
John W. Geissman
Early Triassic, microbial limestone, sequence stratigraphy, sulfur isotope, oxygen minimum zone
Jeffrey, Brad M.. "WIDESPREAD, PROLONGED MICROBIAL LIMESTONE DEPOSITION AND LARGE, RAPID SULFUR ISOTOPE EXCURSIONS: A NEW PERSPECTIVE ON THE EARLY TRIASSIC FROM THE WESTERN U.S.." (2018). https://digitalrepository.unm.edu/eps_etds/244