The pervasiveness of black shale preservation in association with Late Devonian biological crises suggests marine anoxia played a major role in driving ecological perturbations. However, Devonian black shale deposition is still mechanistically poorly understood. We have compiled detailed biomarker lipid chemostratigraphic records for 83 different rock samples using molecular constituents of bitumens of Upper Kellwasser equivalent black shales from two foreland basins: from the low paleolatitude Appalachian Basin (New York State) and from the high paleolatitude Madre de Dios Basin (Bolivia), in order to better understand local environmental conditions and organic source inputs during this depositional event. Despite strong indications from stable nitrogen isotopic signatures for fixed nitrogen nutrient limitation, the biomarker assemblages with consistently low-moderate hopane/sterane ratios (< 0.8) indicate that algae were major marine primary producers in both basins throughout the Frasnian/Famennian (F/F) stratigraphic coverage. Consistently higher C 28 /C 29 sterane ratios at higher paleolatitude in the more nutrient-replete Madre de Dios Basin suggest prasinophyte microalgae flourished in this setting in accordance with palynological evidence for high contributions of Tasmanites cysts in these strata. All samples contain only very low absolute amounts of aryl isoprenoids (with 2,3,6-trimethyl substitution) and other aromatic carotenoids, up to several orders of magnitude lower than concentrations reported from other Phanerozoic euxinic basins. These data are consistent with local marine paleo-redox models for both basins lacking a persistently shallow sulfidic aquatic zone and demonstrate that temporally persistent or spatially pervasive photic zone euxinia is not necessarily associated with all black shale sequences in the Late Devonian.
The end‐Devonian Hangenberg Crisis constituted one of the greatest ecological and environmental perturbations of the Paleozoic Era. To date, however, it has been difficult to precisely constrain the occurrence of the Hangenberg Crisis in the Appalachian Basin of the United States and thus to directly assess the effects of this crisis on marine microbial communities and paleoenvironmental conditions. Here, we integrate organic and inorganic chemostratigraphic records compiled from two discrete outcrop locations to characterize the onset and paleoenvironmental transitions associated with the Hangenberg Crisis within the Cleveland Shale member of the Ohio Shale. The upper Cleveland Shale records both positive carbon (δ13Corg) and nitrogen (δ15Ntotal) isotopic excursions, and replenished trace metal inventories with links to eustatic rise. These dual but apparently temporally offset isotope excursions may be useful for stratigraphic correlation with other productive end‐Devonian epeiric marine locations. Deposition of the black shale succession occurred locally beneath a redox‐stratified water column with euxinic zones, with signs of strengthening denitrification during the Hangenberg Crisis interval, but with an otherwise stable and algal‐rich marine microbial community structure sustained in the surface mixed layer as ascertained by lipid biomarker assemblages. Discernible trace fossil signals in some horizons suggest, however, that bioturbation and seafloor oxygenation occurred episodically throughout this succession and highlight that geochemical proxies often fail to capture these rapid and sporadic redox fluctuations in ancient black shales. The paleoenvironmental conditions, source biota, and accumulations of black shale are consistent with expressions of the Hangenberg Crisis globally, suggesting this event is likely captured within the uppermost strata of the Cleveland Shale in North America.
It is widely agreed that the Earth's atmosphere and oceans have undergone major redox changes over the last 2.5 billion years. However, the magnitude of these shifts remains a point of debate because it is difficult to reconstruct concentrations of dissolved O 2 from indirect proxies in sedimentary archives. In this study, we show that an additional complicating factor that is rarely considered may be the pH of the water column. We analyzed rock samples from the early Jurassic Towaco Formation in the Newark basin (eastern USA), comprising deposits of a rift lake that became temporarily redox stratified. New biomarker evidence points to increasingly saline aquatic conditions during the second half of the lake's history, with a salinity stratification that induced redox stratification, including evidence for water column anoxia, and that state may also explain the disappearance of macrofauna at this time. Distinctive lipid biomarker assemblages and stable nitrogen isotope data support previous mineralogical indications that the lake was alkaline (pH ≥ 9) during its saline episode. Despite the biomarker and macrofaunal evidence for anoxia, ratios of Fe/Al and Fe HR /Fe T show only small to no enrichments in the anoxic horizon compared to oxic facies in the same sectioncounter to what is commonly observed in anoxic marine settings. Molybdenum, As, V, U and to some degree Cd show enrichments in the anoxic interval, whereas Co, Ni, Cu, Zn and Cr do not. These patterns are most parsimoniously explained by differential pH effects on the solubility of these elements. Extrapolating from these observations in lacustrine strata, we speculate that a secular increase in seawater pH over Earth's history as recently proposed may have helped modulate the magnitude of trace metal enrichments in marine shales, although other factors such as atmospheric and oceanic redox likely dominated the observed enrichment patterns. Further, a decrease in the solubility of ferrous iron, a major O 2 sink, with increasing pH may have contributed to ocean oxygenation. In summary, our results highlight the potential importance of pH in influencing global biogeochemical cycles for multiple elements and for the interpretation of ancient nitrogen isotope signatures.
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