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-Permian mass extinction was the largest Phanerozoic biotic crisis that resulted in significant and permanent ecological change. In order to examine ecological aspects of the recovery, shell beds deposited in environments ranging from nearshore to storm wave base were examined from three stratigraphic intervals in the Lower Triassic of the western U.S. Shell beds of the first interval, the Griesbachian Dinwoody Formation, are low-diversity, monospecific beds of Claraia and Promyalina, commonly with the inarticulate brachiopod Lingula. Data from the Nammalian Sinbad Limestone (Moenkopi Formation) provide a small window into the second time interval, in which common low-diversity (bivalves and microgastropods) shell beds occur. Within the third interval, represented by the Spathian Virgin Limestone (Moenkopi Formation) and Thaynes Formation, the bivalves Promyalina and Permophorus are found in both monospecific and polytaxic beds. Crinoids are also commonly found as encrinites and as significant contributors to the matrix of these beds. Shell beds range in thickness from pavements to 10s of centimeters and show variable internal complexity. The persistence of monospecific shell beds throughout these three intervals is significant as support for long-term stress during the recovery interval that is not apparent from sedimentological data alone. Although these Early Triassic beds primarily are comprised of members of the Modern Evolutionary Fauna, they are more similar to beds from the Paleozoic in thickness and taphonomic characteristics.

No abstract

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.

No abstract