Phosphogenesis in the Bonarelli Level from northwestern Sicily, Italy: petrographic evidence of microbial mediation and related REE behaviour G. Scopelliti * , A. Bellanca, R. Neri, N. Sabatino Dipartimento di Chimica e Fisica della Terra ed Applicazioni (CFTA), Universita ` di Palermo, Via Archirafi 36, 90123 Palermo, Italy article info Article history: Received 30 July 2009 Accepted in revised form 19 November 2009 Available online 26 November 2009 Keywords: OAE2 Carbonate-fluorapatite Phosphogenesis Rare earth elements Iron redox cycle Carbon cycle abstract Phosphogenesis at the base of the Bonarelli Level from the Calabianca stratigraphic section (north- western Sicily) was investigated using X-ray diffraction, scanning electron microscopy, X-ray fluores- cence, and stable-isotope mass spectrometry. The anoxic event onset is marked by high P 2 O 5tot concentrations related to the presence of authigenic carbonate-fluorapatite. This mineral is poorly crystallized and occurs in three different forms: (i) fish debris locally showing evidence of re-crystalli- zation, (ii) phosphatically cemented layers, and (iii) dense aggregates of microcrystals. Petrographic features are indicative of a microbial genesis for the carbonate-fluorapatite. Conversely, this evidence is lacking in phosphate from the upper part of the section, where lower P 2 O 5tot concentrations are linked to the presence of sparse unaltered hydroxy-apatite fish debris. Distribution patterns of rare earth elements (REE) throughout the Calabianca section, the Ce anomaly and V/(V þ Ni) values suggest fluctuating redox conditions during phosphogenesis. These conditions triggered the iron redox cycle that operated as phosphate sink-switching mechanism. Depth profiles of P mass accumulation rate and C-isotope compositions highlight the decoupling of P and C-org cycles, whose global implication is confirmed by correlation with the same proxies from other well known stratigraphic sections, which embody sedi- mentary expressions of the Oceanic Anoxic Event 2 (OAE2). Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Most sedimentary rocks contain P concentrations less than 1%. The formation of rocks highly enriched in P 2 O 5 requires specific sedimentary and/or diagenetic conditions to be met. The main mineral phase in phosphate-rich rocks is apatite, more specifically the carbonate-fluoride (CFA) variety known as francolite. Both structure and chemical composition of CFA have been the subject of extensive research and debate (Benmore et al., 1983; Jahnke, 1984; McArthur, 1990; Van Cappellen and Berner, 1991; Knubovets, 1994; Krajewski et al., 1994; Soudry and Nathan, 2001). Phosphorites are found in sediments of all the geological ages, from the Proterozoic to present, but their distribution in space and time does not seem to be random. Cook and McElhinny (1979) have shown that the phosphorites are confined to specific epochs during which they formed phosphogenic provinces. Phosphorites occur in character- istic lithologic associations, frequently with chert and organic-rich deposits (Strakhov, 1970; Kolodny, 1980; Pacey, 1985; Ingall and Van Cappellen, 1990; Ingall and Jahnke, 1997; Hein et al., 1999; Kholodov, 2008; Reid et al., 2008; Yan et al., 2008). Phosphatic nodules and micronodules, pellets and lenses have been commonly found in brown or black shales or in carbonates whose origin is connected with enhanced upwelling and/or high-stand sea-level (e.g., McManus et al., 1997; Coleman and Holland, 2000; Wallmann, 2003). In general, the P 2 O 5 contents of such rocks do not exceed a few percent, whereas economic phosphorites usually exhibit clear evidence of clastic reworking, winnowing and enrichment. In addition to a purely inorganic formation, many apatite pellets and cements have been identified as biological precipitates. Williams and Reimers (1983) identified sulphur-oxidizing bacteria of the genus Beggiatoa in sediments of California, and a sequence of studies by Lucas and Pre ´ vo ˆt (e.g., 1985) experimentally demon- strated a bacterial mediation process involved in the replacement of Ca-carbonate by apatite. Phosphogenesis is a biogeochemical process governed by microbially mediated Eh and pH of bottom and pore waters, and by dissolved chemical species and sedimen- tation rates (Fo ¨llmi et al., 1991). It characterizes marine settings from three geochemical zones: sub-oxic zone, sulphate reduction zone, and microbial methanogenesis zone (Benmore et al., 1983; Coleman, 1985; Morad and Al-Aasm, 1994). Preferential settings are the continental margins at slow to intermediate sedimentation rates, particularly in suboxic to oxic bottom waters (Van Cappellen and Ingall, 1996; Schuffert et al., 1998). * Corresponding author. E-mail address: giovanna.scopelliti@unipa.it (G. Scopelliti). Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2009.11.004 Cretaceous Research 31 (2010) 237–248