Paleoenvironment and post-depositional changes recorded in the chemical composition of marine sediments from the Gulf of Alaska

Abstract

The composition of marine sediments is controlled by an interplay of primary deposition and post-depositional changes, for example organic matter decomposition and authigenic mineral formation. These changes are called early diagenesis and are influenced by external factors, such as sedimentation rates and the supply of organic matter and electron acceptors. IODP Expedition 341 drilled the sites U1417 and U1419 in the Gulf of Alaska. Both sites record a unique feature of diagenetic and primary depositional interplay. At Site U1417 varying sedimentation rates and organic matter input caused cementation and a self-sealing of discrete sediment layers which trapped a methane reservoir in the sediments. Also, a deep pool of sulphate rich waters exists close to the basement. This pool is either invoked by tectonic or volcanic processes or caused by the cut-off of a seawater pool due to varying sedimentation rates. The diagenetic overprint of this site is investigated by inorganic, porewater and isotope geochemistry and the application of a reaction transport model. At Site U1419 the variability of the oxygen deficient zone in the Gulf of Alaska in response to paleoclimate is recorded by the novel BHT isomer biomarker for marine anammox. Usually oxygen limited conditions also cause trace element accumulations in marine sediments. However, at Site U1419 trace element concentrations do not follow the trends recorded by BHT isomer. Their distribution is affected by other external factors. Copper and nickel are affected by bioproductivity, chromium and vanadium by sedimentary provenance and the accumulation of molybdenum and uranium is prevented by overall high sedimentation rates. This PhD project brings these two features of the interplay between primary deposition and diagenetic overprint in the context of global biogeochemical cycles of organic matter and elements.