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Geochemical and Biomarker Evidence of Microbial Community Changes During Lower Cretaceous OAE 1b: Comanche Shelf, Glen Rose Formation, Central Texas

Abstract

Oceanic Anoxic Events (OAE) were major carbon cycle perturbations that occurred mainly in Mesozoic times. These intervals are associated with major ecological turnover and often include carbon isotope excursions and associated intervals of organic-rich black shale deposition. However, in contrast with major OAE (e.g., OAE1a and OAE 2), OAE 1b is not associated with substantial mudrock deposition and does not display well-developed carbon isotope excursions. Positive shifts in δ13C values of marine carbonate for OAE 1b intervals are significantly smaller than other mid-Cretaceous OAEs. It is important to investigate if organic matter (OM) origin in OAE 1b mudstones has a similar origin as other major OAE. We studied Apto-Albian OAE 1b calcareous mudstones in Central Texas and compared the results with earlier findings for the Eagle Ford (EF) OAE 2. Samples selected in this study were collected from Bexar Shale and Glen Rose Formation from Central Texas. The total organic carbon content is low for all samples (between 0.31 to 0.75 wt.%.). Biomarker analysis results show the OM source of OAE 1b in Central Texas mainly originated from marine archaea which is different from the marine algae dominated EF OAE 2 OM source. The results also indicate a marine archaea expansion during OAE 1b in Central Texas. The profile of total isoprenoids versus total hopane and steranes serves as a proxy for the input of marine archaea versus bacteria and marine algae. It closely resembles the organic carbon isotope curve, with the increasing abundance of archaeal biomarkers mimicking the increase in δ 13Corg. This observation agrees with a study for OAE 1b from North Atlantic Ocean that linked increasing archaea source input with increasing δ 13Corg values during OAE 1b. Therefore, our study provides evidence that the archeal expansion was likely a global event during OAE 1b, which occurred not only in the North Atlantic Ocean, but also in the Western interior seaway. Molecular fossils (e.g., isorenieratane) of a pigment indicative of anoxygenic photosynthetic bacteria were recovered from Glen Rose Formation indicating that anoxic conditions extended into the photic zone. We also observed that H2S-rich photic zone euxinic development coincides elevation of cyanobacteria in the water column. This implies cyanobacteria were likely anoxygenic photosynthesizers during this time. Given the close association of the marine achaea, anoxygenic phototrophs, and green sulfur bacteria, we conclude that these principal components of the community were adapted to the same anoxic or oxygen-deficient ecologic setting. More detailed biomarker data will be collected to focus on the marine archaea source indicator and microbial community changes in the OAE 1b event in Central Texas.