MICROFOSSIL RESPONSE TO MID-CRETACEOUS OCEANIC ANOXIC EVENT 2 AT NORTH ATLANTIC IODP SITE U1407

During Oceanic Anoxic Event 2 (OAE 2; ~94 Ma) major paleoceanographic perturbations led to dysoxic to anoxic conditions that resulted in deposition of organic-carbon-rich sediments worldwide. Burial of isotopically light carbon during OAE 2 is recorded as a positive 2-4‰ carbon isotope excursion (CIE). This global change in the carbon cycle provides an important tool for identifying the stratigraphic extent of the event. Changes associated with this extreme event are investigated to determine the cause of enhanced burial of organic matter (OM) at this site and provide insight into the long-term response of the carbon cycle. This study uses OAE 2 sediments cored in the western North Atlantic at Site U1407 by IODP Expedition 342. To determine the relative importance of mechanisms behind the enhanced OM accumulation that occurred during this time, 44 samples were collected from cores U1407A and U1407B. Here, we present a composite curve of stable carbon isotopes of organic carbon, values of total organic and inorganic carbon, and counts of characteristic microfossil groups (benthic foraminifera and radiolarians) in order to reconstruct oceanic conditions.

Benthic foraminifera, useful in reconstructing deep-water conditions and higher inputs of OM, are usually scarce or absent in OAE sediments. However, at Site U1407, they are abundant and well preserved throughout, with numbers as high as 936/g of dry sediment. At the start of the CIE, benthic counts average ~550 specimens/g but decrease to 6 specimens/g at the peak of the CIE. The low benthic counts coincide with increased OM content (as high as 16.04 wt%), decreased carbonate content (as low as 0.24%), and a higher density of (>63μm) radiolarians (1706 specimens/g). High OM content and radiolarian abundances together with increased fish debris, indicate deposition of sediments under high surface productivity that resulted in low to no oxygen at the bottom and impoverished benthic foraminifera. Microfossil abundances in OAE 2 sediments at Site U1407 are different from other North Atlantic sites. At Site U1407, radiolarians dominate OM-rich intervals and are rare in underlying calcareous OM-poor sediments, a pattern similar to Tethyan deep-water sections (Rehkogelgraben, Austria; Gubbio, Italy; and Roter Sattel, Switzerland) that share a common lithologic pattern.