We apply a new approach for the δ13C analysis of single organic‐walled microfossils (OWM) to three sites in the Appalachian Basin of New York (AB) that span the Late Devonian Biotic Crisis (LDBC). Our data provide new insights into the nature of the Frasnian–Famennian carbon cycle in the AB and also provide possible constraints on the paleoecology of enigmatic OWM ubiquitous in Paleozoic shale successions. The carbon isotope compositions of OWM are consistent with normal marine organic matter of autochthonous origins and range from −32 to −17‰, but average −25‰ across all samples and are consistently13C‐enriched compared to bulk sediments (δ13Cbulk) by ~0–10‰. We observe no difference between the δ13COWMof leiospheres (smooth‐walled) and acanthomorphic (spinose) acritarch OWM, indicating that our data are driven by ecological rather than taxonomic signals. We hypothesize that the offset between δ13COWMand δ13Cbulkis in part due to a large δ13C gradient in the AB water column where OWM utilized relatively13C‐enriched dissolved inorganic carbon near the surface. Thus, the organisms producing the balance of the total organic carbon were assimilating13C‐depleted C sources, including but not limited to respired organic carbon or byproducts of fermentation. We also observe a systematic decrease in both δ13COWMand δ13Cbulkof 3‰ from shoreward to open‐ocean facies that may reflect the effect of13C‐enriched dissolved inorganic carbon (DIC) derived from riverine sources in the relatively enclosed AB. The hypothesized steep carbon isotope gradient in the AB could be due to a strong biological pump; this in turn may have contributed to low oxygen bottom water conditions during the LDBC. This is the first time single‐microfossil δ13Corganalyses of eukaryotes have been directly compared to bulk δ13Corgin the deep‐time fossil record.
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.
more » « less- NSF-PAR ID:
- 10075920
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Geobiology
- Volume:
- 17
- Issue:
- 1
- ISSN:
- 1472-4677
- Page Range / eLocation ID:
- p. 27-42
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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