Controls on sedimentary accumulation of organic matter during Cretaceous Oceanic Anoxic Event 2, IODP site U1407, Southeast Newfoundland Ridge

Abstract

The pelagic sediments from Integrated Ocean Drilling Program (IODP) Site U1407 in the western North Atlantic are characterized by a sequence of organic-lean and organic-rich sediments representing a major global episode of organic matter (OM) burial during Oceanic Anoxic Event 2 (OAE2). The OM-rich deposits (up to 16% total organic carbon) consist primarily of radiolarians which are rare/absent in the organic-lean sediments. New nannofossil biostratigraphy and the correlation with the global ẟ13C curve were used to constrain the timing of paleoceanographic changes below, within, and above OAE2 sediments. Data were combined from elemental concentrations, biomarkers, and abundances of benthic foraminifera, biserial (heterohelicids) planktic foraminifera, and radiolarians to reconstruct the evolution of these changes. The ẟ13C curve records a global negative excursion within organic-lean sediments not typically recorded in other sections preceding the initial rise in isotopic values that signals OM burial worldwide. Maximum OM accumulation occurred during the second positive isotopic excursion, which is associated with higher terrigenous input (indicated by long-chain n-alkanes and terrestrially derived elements) and water column stratification (suggested by the paleoindicator gammacerane). In addition, sesquiterpene biomarkers derived from cypress and other conifers suggest flooding of gymnosperm forests from increased rainfall coupled with rising sea levels and resulting higher water tables. Within mid-OAE2 sediments, a sharp increase in radiolarians and fish teeth/debris and a decrease in benthic foraminifera implies deposition of OM from increased productivity/upwelling under low oxygen in the lower part of the water column. Upwelling and water column stratification occurring asynchronously at the surface resulted from changes in precipitation, with stratification developing during more humid/wet conditions. Greenhouse warming during OAE2 likely increased precipitation, weathering and nutrient inputs. However, prolonged anoxia caused buried phosphorus to be recycled as indicated by higher redox-sensitive trace element and biolimiting element concentrations, sustaining primary production and further enriching the sediments in OM. We suggest that increased marine paleoproductivity was the primary control on the OM-rich sediment accumulation during OAE2, while low-oxygen conditions resulting from stratification of surface waters were essential to its preservation.