Quantification of diagenetic transformation of continental margin sediments at the Holocene time scale

Abstract

Early diagenesis processes in the Bay of Biscay have been studied previously on short interface cores. In order to complete the data base and to characterize anoxic biogeochemical processes that affect Quaternary deposits, 1–3 m long gravity cores were collected at 150, 550, 1000 and 2000 m water depth on the continental slope of the south-eastern part of the Bay of Biscay. The aim of this work was to determine how a recorded signal could be changed in the several thousand-year situation of the Holocene period. For that, we determined radiocarbon and 210Pb dating, sedimentological characteristics, and the vertical distribution of particulate and pore water biogeochemical compounds. The sediment consists of a muddy facies deposited continuously during the Holocene. Results show that the distribution of redox sensitive compounds follows the well-established depth sequence of diagenetic reactions governed by the preferential use of the electron acceptor for the bacterially mediated oxidation of organic matter. Mineralization of organic matter occurs down to the bottom of the cores studied, indicating that Holocene sediment of the Bay of Biscay are in a transient state and they keep on being diagenetically transformed today. Effects of anoxic processes on buried sediments were estimated from fluxes induced by chemical gradients in pore waters. Anoxic processes consume up to 3.6 mg particulate organic carbon (POC) per g of dry sediment. They yield also precipitation of secondary calcium carbonate, which represent up to 5% of total buried carbonates. Early diagenetic reactions, which affect proxies commonly used in paleo-oceanography, such as POC, carbonates, phosphorus, or manganese, should be quantified in order to distinguish the part of a sedimentary record being the result of past environmental conditions from that due to post-depositional diagenetic processes. Diagenesis of Holocene deposits also contributes to a low, but significant fraction of N and P flux to the Bay of Biscay sea water.