Nutrient trap for Late Cretaceous organic-rich black shales in the tropical North Atlantic

Abstract

Neodymium isotopes of fi sh debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian-early Santonian). Unusually low Nd isotopic data (ε Nd(t) ~−11 to ~−16) on Demerara Rise during the Cenomanian are confi rmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-fl owing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, effi cient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafl oor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y.

Early Cenomanian “hot greenhouse” revealed by oxygen isotope record of exceptionally well‐preserved foraminifera from Tanzania

“…As for the former, the illustrated early‐middle Cenomanian lowering of deepwater paleotemperatures [ Friedrich et al , ] can be regarded as an artifact of data availability, such that the δ 18 O data used for this interval are solely from a single relatively deep site (ODP Site 1050; middle‐lower bathyal transition). As for the latter, the TEX 86 profile is from ODP Site 1258, Demerara Rise, wherein early‐middle Cenomanian strengthening of the upper ocean current is inferred from a temporarily greater vertical gradient of Nd isotopes [ Jiménez Berrocoso et al , ]. This fact may point to a possibility that the reported cooler TEX 86 temperatures are regional in scale and better explained by the overturning of cooler subsurface waters [e.g., Tierney , ] than by global climatic factors.…”

“…A substantial amount of δ 18 O measurements on exquisitely preserved Cenomanian foraminifera does exist for ODP sites at Demerara Rise, equatorial Atlantic [ Forster et al , ; Moriya et al , ; Friedrich et al , ]. However, these Demerara Cenomanian δ 18 O data sets are interpreted to have been strongly influenced by local paleoceanographic conditions including high net evaporation and the formation of hypersaline, oxygen‐poor Demerara Bottom Water [ Jiménez Berrocoso et al , ], thereby hindering a straightforward paleotemperature reconstruction. In addition, the only planktonic foraminifera available for the Demerara Rise case studies were two opportunistic species, for which depth ecologies are unpredictable [ Ando et al , ].…”

An astronomically calibrated stratigraphy of the Cenomanian, Turonian and earliest Coniacian from the Cretaceous Western Interior Seaway, USA: Implications for global chronostratigraphy

“…Sageman and Lyons, 2003). This indicates that massive carbon sequestration occurred in some other region than the KWIS (e.g., Kuypers et al, 2004;Trabucho Alexandre et al, 2010;Jim enez Berrocoso et al, 2010;Eldrett et al, 2014). Carbonate mass accumulation rates are highest in both marl and limestone lithologies in the underlying Buda Limestone and overlying Austin Chalk suggesting relatively reduced carbonate productivity during Eagle Ford deposition.…”

Response and recovery of the Comanche carbonate platform surrounding multiple Cretaceous oceanic anoxic events, northern Gulf of Mexico

“…Regardless of the mechanism invoked to trigger these events, it is clear that enhanced nutrient delivery to the marine realm dramatically increased surface-water primary productivity Handoh and Lenton, 2003;Weissert and Erba, 2004;Mort et al, 2007;Elrick et al, 2009;Jimenez-Berrocoso et al, 2010). These perturbations are manifest in the stratigraphic record by the anomalous occurrence of organic-rich shale units deposited under dysoxic to anoxic conditions (Schlanger and Jenkyns, 1976;Jenkyns, 1980;Arthur and Sageman, 1994).…”

“…The diverse paleogeographic locations in which Cretaceous OAEs are documented illustrate their global extent, and include sites from the paleo-Tethys Ocean, Atlantic Ocean, Pacific Ocean, the western Gulf of Mexico, and the Western Interior Seaway of North America (Pratt et al, 1984;Weissert, 1989;Vahrenkamp, 1996;Menegatti et al, 1998;Bralower et al, 1999;Stoll and Schrag, 2000;Wilson and Norris, 2001;Jarvis et al, 2002;Price, 2003;Herrle et al, 2004;Locklair et al, 2004;Tsikos et al, 2004;Wendler et al, 2009;Jimenez-Berrocoso et al, 2010).…”