Significance of euxinic condition in the middle Eocene paleo-Arctic basin: A geochemical study on the IODP Arctic Coring Expedition 302 sediments Yusuke Ogawa a , Kozo Takahashi a, ⁎, Toshiro Yamanaka b , Jonaotaro Onodera c a Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Hakozaki6-10-1, Higashi-ku, Fukuoka, 812-8581, Japan b Graduate School of Natural Science and Technology, Okayama University,1-1, Naka 3-chome, Tsushima, Okayama, 700-8530, Japan c Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, 783-8502, Japan abstract article info Article history: Received 27 April 2008 Received in revised form 5 June 2009 Accepted 8 June 2009 Available online 4 July 2009 Editor: P. DeMenocal Keywords: Arctic basin TOC TS sulfur isotope pyrite euxinic bottom water salinity stratification Integrated Ocean Drilling Program Expedition 302 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 (δ 34 S) 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 δ 34 S 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 δ 34 S of seawater sulfate abruptly increased from +17 to +22‰ 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‰ of the global increase during this period. © 2009 Published by Elsevier B.V. 1. Introduction Today significant perennial sea–ice cover is found only in the Arctic Ocean in the northern hemisphere. The region with such a perennial sea–ice cover sometimes plays a major role in the global climatic change since most of the sunlight is reflected due to high albedo. The formation of sea-ice with brine rejects leads to the formation of cold dense water which in turn influences the global abyssal circulation (Holland et al., 2001). Despite its importance, paleoceanographic studies on the Arctic Ocean have been limited to few studies which employed short piston cores (e.g., Kitchell and Clark, 1982; Bukry, 1984; Ling, 1985; and Dell'agnese and Clark, 1994). In 2004 Integrated Ocean Drilling Program (IODP) Expedition 302, Arctic Coring Expedi- tion (ACEX), recovered sediment cores on the Lomonosov Ridge in the central Arctic (Fig. 1) focusing on the reconstruction of the paleoenvironmental evolution of the Arctic Ocean/basin. During the early Eocene paleogeographic conditions of the Arctic were different from those of the modern era. The connection between the Arctic and the world oceans was limited, and thus the Arctic Ocean was nearly isolated from the outside basins (e.g., McNeil, 1990a; Akhmetiev and Beniamovski, 2004). The Lomonosov Ridge broke away from the Eurasian continental margin about 57 Ma (e.g., Vogt et al., 1979) and subsided as the sea-floor spreading proceeded along the Gakkel Ridge. The microfossil studies of the ACEX cores indicated a low salinity surface waters layer in the Arctic basin during the early to middle Eocene (Backman et al., 2006). Brackish water and freshwater planktonic microfossils were abundantly found in middle Eocene ACEX sediments (Onodera et al., 2008; Stickley et al., 2008). The analyses of total sulfur content (%TS) and stable sulfur isotopic composition (δ 34 S) can provide pertinent information with respect to sulfate availability in the deep water masses of the basin (e.g., Nakai et al., 1982). Sulfur in reduced marine sediments is present primarily as pyrite (Berner, 1970, 1984). In anaerobic environments such as anoxic sediments and/or water column, H 2 S is generated by sulfate- reducing bacteria which reduce seawater sulfate ion using organic matter as an electron donor. The H 2 S reacts readily with detrital iron minerals, resulting in pyrite formation (Berner and Raiswell, 1983). Earth and Planetary Science Letters 285 (2009) 190–197 ⁎ Corresponding author. Tel.: +81 92 642 2657; fax: +81 92 642 2686. E-mail address: kozo@geo.kyushu-u.ac.jp (K. Takahashi). 0012-821X/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.epsl.2009.06.011 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl