Authigenic carbonates from methane seeps of the northern Congo fan: Microbial formation mechanism Dong Feng a , Duofu Chen a, * , Jo ¨ rn Peckmann b , Gerhard Bohrmann b a CAS Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China b MARUM, University of Bremen, Post Box 330 440, D-28334 Bremen, Germany article info Article history: Received 15 May 2009 Received in revised form 25 July 2009 Accepted 17 August 2009 Available online 22 August 2009 Keywords: Methane Seep Carbonate Isotopes Rare earth elements Trace elements Seepage rate Congo fan abstract Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The d 13 C values of the Hydrate Hole carbonates range from 62.5& to 46.3& PDB, while the d 13 C values of the Diapir Field carbonate are somewhat higher, ranging from 40.7& to 30.7& PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of 18 O(d 18 O values as high as 5.2& PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO 3 -treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in d 13 C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Carbonate precipitation is a widely observed phenomenon in modern and ancient marine seep environments (e.g. Roberts and Aharon, 1994; Peckmann and Thiel, 2004; Campbell, 2006; Naehr et al., 2007). It is a product of anaerobic oxidation of methane (AOM), which is mediated by consortia of methane-oxidizing archaea (MOA) and sulfate-reducing bacteria (SRB; Hinrichs et al., 1999; Boetius et al., 2000). Bicarbonate released during AOM increases alkalinity and results in the precipitation of authigenic carbonate (cf. Berner, 1980). Accordingly, the study of stable isotopes, lipid biomarkers, and biogenic fabrics of seep carbonates provides further insight into microbially mediated carbonate formation (e.g. Roberts and Aharon, 1994; Thiel et al., 1999; Boetius et al., 2000; Peckmann et al., 2001; Peckmann and Thiel, 2004; Pape et al., 2005; Chen et al., 2005, 2006, 2007; Campbell, 2006; Birgel and Peckmann, 2008; Feng et al., 2008). Furthermore, stable carbon and oxygen isotopic compositions of carbonates provide information pertaining to the composition and temperature of seep fluids from which carbonates precipitated (e.g. Naehr et al., 2000, 2007; Peckmann and Thiel, 2004). The relative upward fluid flux at seep sites may change over time (Roberts and Carney, 1997; Roberts, 2001; Hovland, 2002; Chen et al., 2004; Lapham et al., 2008) and this temporal variability can be expressed by different geochemical and mineralogical signatures archived in authigenic seep carbonates (Leo ´ n et al., 2007; Feng et al., 2009a,b). For example, De Boever et al. (2006a,b) pointed out that carbon isotopes can be used to assess the control of seepage rates on the formation of ancient seep carbonates. However, only relatively few studies have used such an approach due to the scarcity of appropriate samples. During AOM, sulfate in pore water is reduced to hydrogen sulfide (Valentine and Reeburgh, 2000). As a consequence of sulfate- dependent methane oxidation, redox conditions in the sediment at seeps change. Cerium is a sensitive indicator of redox conditions in sedimentary environments. Under oxidizing conditions Ce 3þ is oxidized to Ce 4þ , and precipitates as CeO 2 , resulting in a negative Ce anomaly, which is therefore an effective indicator of oxic conditions (McArthur and Walsh, 1984; Wright et al., 1987). Negative Ce anomalies in marine carbonates have been shown to reflect seawater oxygenation (McArthur and Walsh, 1984; Sholkovitz and * Corresponding author. Tel.: þ86 20 85290286; fax: þ86 20 85290130. E-mail addresses: fd@gig.ac.cn (D. Feng), cdf@gig.ac.cn (D. Chen), peckmann@ uni-bremen.de (J. Peckmann), gbohrmann@uni-bremen.de (G. Bohrmann). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.08.006 Marine and Petroleum Geology 27 (2010) 748–756