Significance of euxinic condition in the middle Eocene paleo-Arctic basin: A geochemical study on the IODP Arctic Coring Expedition 302 sediments

Integrated Ocean Drilling Program (IODP) Expedition 302 Arctic Coring Expedition (ACEX) obtained the first relatively continuous long sediment cores from the Lomonosov Ridge in the central Arctic Ocean in 2004. Preceding microfossil studies indicated the dominance of low salinity surface waters in the early to middle Eocene Arctic basin. The main purpose of this study is to reconstruct paleoceanographic conditions including the extent of saline (seawater) mass presence. To attain this goal we performed geochemical analyses of total sulfur (%TS), total organic carbon (%TOC) and stable sulfur isotopic composition (delta S-34) on the early to middle Eocene section of the ACEX cores. The %TS were high in all the examined intervals and the sedimentary sulfur occurred mainly as framboidal pyrite, indicating that sufficient sulfate, indicative of seawater, was present in the deep layer of the paleo-Arctic basin and that the pyrite was formed in the sediments under sufficient iron input. The high %TS values with low delta S-34 values also indicate the continuous existence and supply of seawater. The high accumulation of sulfide in Unit 1/6 was due to a significant increase of TOC supply which increased sulfate reduction rates by bacteria. The %TOC-%TS diagram shows excess sulfur content relative to the TOC, suggesting euxinic condition of the bottom water during the studied period. Such an oxygen depleted environment was brought about by salinity stratification and restricted water circulation. The patterns observed in the ACEX data can be comparable with the Mediterranean sapropels. The global delta S-34 of seawater sulfate abruptly increased from +17 to +22 parts per thousand in the early to middle Eocene. Previous studies suggested that enhanced pyrite burial caused the isotopic shift during this period. The large pyrite burial in the anoxic Arctic basin could have contributed to the remarkable isotopic event accounting for about 3 parts per thousand of the global increase during this period. (C) 2009 Published by Elsevier B.V.