Abstract
Abstract. A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane), a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle.
Highlights
The Mid-Cretaceous is characterized by an overall warm climate (Huber et al, 2002), punctuated by several colder periods (e.g., Bornemann et al, 2008; Forster et al, 2007)
One of the most pronounced and widespread ocean anoxic events (OAEs) is Oceanic Anoxic Event 2 (OAE-2), which occurred at the Cenomanian/Turonian boundary (C/T; 93.5 Ma, Gradstein et al, 2004) and which is known as the Cenomanian Turonian Boundary Event (CTBE)
At the onset of the OAE-2 interval, δ13Cphytane values rapidly rise to −26 ‰, maintaining this value until 426.2 mcd (Fig. 2b)
Summary
The Mid-Cretaceous is characterized by an overall warm climate (Huber et al, 2002), punctuated by several colder periods (e.g., Bornemann et al, 2008; Forster et al, 2007). Turgeon and Creaser, 2008; Kerr, 1998; Blattler et al, 2011; Jenkyns, 2010) Superimposed on this period of high atmospheric pCO2 levels, several short lived episodes of increased organic matter (OM) deposition, so-called ocean anoxic events (OAEs) (Schlanger and Jenkyns, 1976; Arthur et al, 1988; Jenkyns, 1980), occurred. A positive carbon isotopic excursion accompanying OAE-2 has been observed both in marine carbonates and in marine and terrestrial OM (Hasegawa, 1997) This excursion has been attributed to enhanced OM burial (Arthur et al, 1988; Scholle and Arthur, 1980), because organisms preferentially take up light carbon 12C leaving the remaining carbon in the ocean–atmosphere reservoir enriched in 13C
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