Abstract
Abstract. A global increase in the strength of the hydrologic cycle drove an increase in the flux of terrigenous sediments into the ocean during the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE2) and was an important mechanism driving nutrient enrichment and thus organic carbon burial. This global change is primarily known from isotopic records, but global average data do not tell us anything about changes at any particular location. Reconstructions of local terrigenous flux can help us understand the role of regional shifts in precipitation in driving these global trends. The proto-North Atlantic basin was one of the epicenters of enhanced organic carbon burial during OAE2, so constraining terrigenous flux is particularly important in this region; however, few local records exist. Here, we present two new OAE2 records from the Atlantic Coastal Plain of North Carolina, USA, recognized with calcareous nannoplankton biostratigraphy and organic carbon isotopes. We use carbon / nitrogen ratios to constrain the relative contribution of marine and terrestrial organic matter; in both cores we find an elevated contribution from vascular plants beginning just before OAE2 and continuing through the event, indicating a locally strengthened hydrologic cycle. Terrigenous flux decreased during the brief change in carbon isotope values known as the Plenus carbon isotope excursion; it then increased and remained elevated through the latter part of OAE2. Total organic carbon (TOC) values reveal relatively low organic carbon burial in the inner shelf, in contrast to black shales known from the open ocean. Organic carbon content on the shelf appears to increase in the offshore direction, highlighting the need for cores from the middle and outer shelf.
Highlights
The Cretaceous was characterized by intermittent periods of enhanced organic carbon burial linked to widespread black shale deposition and anoxia, termed oceanic anoxic events (OAEs; Schlanger and Jenkyns, 1976; Jenkyns, 2010)
OAEs were originally defined by the widespread occurrence of black shales (Schlanger and Jenkyns, 1976), they were soon found to be associated with positive carbon isotope excursions driven by the excess global burial of organic carbon representing a perturbation of the global carbon cycle (Scholle and Arthur, 1980; Arthur et al, 1987; Jenkyns, 2010; Owens et al, 2017)
Benthic foraminifera, which are powerful tools to determine paleoenvironment in marginal marine settings (e.g., Tibert and Leckie, 2004), are absent here due to poor preservation. In both cores we recognize two informal members of the Clubhouse Formation: a marine lower member characterized by bivalves, calcareous nannoplankton, finer grained sediments, thinner beds, and sedimentary features common to inner shelf environments; and a marginal marine upper member characterized by coarser grain size, thicker beds, and woody plant debris instead of calcareous marine fossils, indicating deposition in a delta front or distributary environment
Summary
The Cretaceous was characterized by intermittent periods of enhanced organic carbon burial linked to widespread black shale deposition and anoxia, termed oceanic anoxic events (OAEs; Schlanger and Jenkyns, 1976; Jenkyns, 2010). Trends in the stable carbon isotope ratios (δ13C) of the global carbon pool reveal global changes in organic carbon burial rates (e.g., Jenkyns, 2010) but do not tell us anything about where that organic matter was buried. This is important because local organic matter enrichment can vary significantly in both timing (e.g., Tsikos et al, 2004) and magnitude (e.g., Owens et al, 2018) during an OAE. The calcium isotope proxy used by Blättler et al (2011) and Published by Copernicus Publications on behalf of the European Geosciences Union
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