Sulfur speciation in the upper Black Sea sediments Mustafa Yücel a, ⁎, Sergey K. Konovalov b , Tommy S. Moore a , Christopher P. Janzen c , George W. Luther III a a University of Delaware, College of Earth, Ocean and Environment, 700 Pilottown Road, Lewes, DE 19958, USA b Marine Hydrophysical Institute, 2 Kapitanskaya Street, Sevastopol 99011, Ukraine c Susquehanna University, Selinsgrove, PA 17870, USA abstract article info Article history: Received 8 May 2009 Received in revised form 9 October 2009 Accepted 17 October 2009 Editor: J.D. Blum Keywords: Black Sea Anoxic sediment Sulfur Iron Elemental sulfur Humic sulfur Pyrite We report solid phase sulfur speciation of six cores from sediments underlying oxic, suboxic and anoxic- sulfidic waters of the Black Sea. Our dataset includes the five sulfur species [pyrite-sulfur, acid volatile sulfides (AVS), zerovalent sulfur (S(0)), organic polysulfides (RS x ), humic sulfur] together with reactive iron and manganese, as quantified by dithionite extraction, and total organic carbon. Pyrite – sulfur was the major phase in all cores [200-400 μmol (g dry wt) -1 ] except for the suboxic core. However, zerovalent sulfur and humic sulfur also reached very significant levels: up to about 109 and 80 μmol (g dry wt) -1 , respectively. Humic sulfur enrichment was observed in the surface fluff layers of the eastern central basin sediments where Unit-1 type depositional conditions prevail. Elemental sulfur accumulated as a result of porewater sulfide oxidation by reactive iron oxides in turbidities from the anoxic basin margin and western central basin sediments. The accumulation of elemental sulfur to a level close to that of pyrite-S in any part of central Black Sea sediments has never been reported before and our finding indicates deep basin turbidites prevent the build-up of dissolved sulfide in the sediment. This process also contributes to diagenetic pyrite formation whereas in the non-turbiditic parts of the deep basin water column formed (syngenetic) pyrite dominates the sulfur inventory. In slope sediments under suboxic waters, organic sulfur (humic sulfur + organic polysulfides) account for 33-42% of total solid phase S, indicating that the suboxic conditions favor organosulfur formation. Our study shows that the interactions between depositional patterns (Unit 1 vs. turbidite), redox state of overlying waters (oxic-suboxic-sulfidic) and organic matter content determine sulfur speciation and enable the accumulation of elemental sulfur and organic sulfur species close to a level of pyrite-S. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The marine sedimentary sulfur cycle is a complex web of biogeochemical interactions driven by the generation of dissolved sulfide by the actions of sulfate reducing prokaryotes (Jørgensen, 1982). The usual fate of sulfide in normal marine sediments (that lie under oxygenated waters) is either reoxidation to a number of sulfur species, such as S 2 O 3 2- ,S x 2- , S(0), SO 4 2- or reacting with metals, mostly minerals of Fe, to give FeS and then ultimately pyrite – FeS 2 , the latter being the stable end product. If the sediment is relatively Fe-poor and rich in organic matter, both dissolved sulfide and sulfur intermediates can react with organic matter to give organic sulfur (Vairavamurthy and Mopper, 1987; Francois, 1987a,b; Kohnen et al., 1989; Luther and Church, 1992). Especially in the sediments that lie under permanently anoxic and sulfidic (euxinic) waters, abiotic reoxidation becomes less important and the sulfur system is governed by the competitive balance between reactions of sulfide with reactive Fe minerals or with reactive organic matter. Upper Black Sea sediments provide possibil- ities to study the entire marine sedimentary sulfur cycle because oxic, suboxic, and anoxic/sulfidic conditions permanently exist in this marine system. The Black Sea sediments have been of interest for many decades for their unique depositional history (Ross et al., 1970; Mitropolsky et al., 1982). Ross et al. (1970) identified the three depositional patterns in the uppermost several meters of the anoxic basin sediments: Unit 1, comprising about the uppermost 30 cm of the sediment column, is rich in coccolith derived carbonates and has relatively low levels of organic carbon. This layer has light micro- laminated layers of high carbonate with intervening darker, organic rich layers. Below this layer is Unit 2, about 40 cm thick and with high levels of organic carbon (up to 20%) and low carbonate content. Below these layers are Unit 3 sediments, very low in organic C (b 1%). Unit 1, which can be sampled with box or multicorers and therefore are most intensely studied among these three types, has been investigated with respect to reactive iron enrichment and pyrite formation in a number of studies (Volkov, 1964; Volkov and Fomina, 1974; Rozanov et al. 1974; Berner 1974; Lyons 1991; Lyons and Berner 1992; Canfield et al., 1996; Hurtgen et al., 1999; Wijsman et al. 2001a; Anderson and Raiswell, 2004). These authors generally observed pyrite enrichment Chemical Geology 269 (2010) 364–375 ⁎ Corresponding author. Tel.: +1 302 645 4008; fax: +1 302 645 4007. E-mail addresses: myucel@udel.edu (M. Yücel), sergey_konovalov@yahoo.com (S.K. Konovalov), tommy.moore@umontana.edu (T.S. Moore), Janzen@Susqu.edu (C.P. Janzen), luther@udel.edu (G.W. Luther). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.10.010 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo