Influence of a turbidite deposit on the extent of pyritization of iron, manganese and trace metals in sediments from the Guaymas Basin, Gulf of California (Mexico) Xose´ Luis Otero a , Miguel Angel Huerta-Diaz b, *, Felipe Macı ´ as a a Dep. Edafoloxı´a e Quı´mica Agrı´cola, Facultade de Bioloxı´a, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain b Instituto de Investigaciones Oceanolo ´gicas, Universidad Auto ´noma de Baja California, PMB-133, PO Box 189003, Coronado, CA 92178-9003, USA Received 10 January 2002; accepted 30 August 2002 Editorial handling by J.S. Herman Abstract A core collected in the Guaymas Basin contained an organic-poor, Mn oxide-rich and (relatively) Fe oxide-rich turbidite layer that affected the distribution of Fe, Mn, C, S and trace metals. Results indicate that sediments not influenced by the turbidite layer achieved a 100% degree of pyritization and, by extension, that pyrite production is Fe- limited in these sediments. In contrast, the mud slide layer apparently supplied enough reactive Fe to transfer essen- tially 98% of the total S present at the base of the turbidite (17–19 cm) to the pyrite reservoir. C/S ratios showed rapid decreases with depth, from a high of 38 close to the sediment-water interface, to minimum values of 2.8 at the lower limit of the turbidite layer, a ratio equal to the average C/S value of normal marine modern sediments, where con- centrations of organic C and pyrite supposedly have attained quasi-steady values. A significant part of the reactive Mn was associated with carbonates (41  12%) and, to a much lower degree, with pyrite (2.7  1.2%). The turbidite layer is currently showing a depletion of Mn relative to the host sediment. It is possible that Mn, a major metal constituent in these sediments, was initially present in high concentrations in the mud slide, but was eventually mobilized and trans- ferred either to the water column or to the sediments immediately below the turbidite layer. Metals associated with this element probably followed the same path, affecting their incorporation into pyrite. The turbidite layer apparently affected the distribution of most of the trace metals associated with pyrite, except maybe Cd, Pb and, to a certain, extent Cr. However, Cu, Cr, Zn, Ni and Co were all found to be highly pyritized ( > 80%) in the sediments of the Guaymas Basin. # 2003 Elsevier Science Ltd. All rights reserved. 1. Introduction In recent years a number of studies have been carried out on the geochemistry of S and Fe in different sedi- mentary environments, with special attention to the formation of metal sulfides. Berner (1970) established the geochemical mechanisms of the formation of sedi- mentary pyrite in marine environments, whereas sub- sequent studies involved investigation of the formation of this mineral on the ocean bed (e.g. Boesen and Postma, 1988; Passier et al., 1996; Wilkin and Barnes, 1997) and in intertidal areas such as salt marshes and mangrove swamps (Postma, 1982; Boulegue et al., 1982; Howarth and Merkel, 1984; Holmer et al., 1994; Kostka and Luther, 1995). Although there are several studies on the association of trace metals with sulfide minerals (e.g., Huerta-Diaz and Morse, 1992; Larsen and Postma, 1997; Otero et al., 2000), there is little 0883-2927/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0883-2927(02)00190-7 Applied Geochemistry 18 (2003) 1149–1163 www.elsevier.com/locate/apgeochem * Corresponding author. E-mail address: mhuerta@uabc.mx (M.A. Huerta-Diaz).