Paleoenvironmental implications of early diagenetic siderites of the Paraíba do Sul Deltaic Complex, eastern Brazil Amanda Goulart Rodrigues a, ⁎, Luiz Fernando De Ros b , Reiner Neumann c , Leonardo Borghi a a Laboratório de Geologia Sedimentar, Instituto de Geociências, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 274, s/J1-011 Rio de Janeiro, RJ, Brazil b Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500 Porto Alegre, RS, Brazil c Centro de Tecnologia Mineral, Av. Pedro Calmon, 900 – Cidade Universitária, Rio de Janeiro, RJ, Brazil abstract article info Article history: Received 30 January 2015 Received in revised form 9 April 2015 Accepted 10 April 2015 Available online 23 April 2015 Keywords: Diagenesis Siderite Quaternary Campos Basin Sedimentary petrography Abundant early diagenetic siderites occur as spherulites and rhombohedral microcrystalline and macrocrystalline crystals in the cores of the 2-MU-1-RJ well, drilled in the Paraíba do Sul Deltaic Complex, Rio de Janeiro (Brazil). The host sediments of the siderites are siliciclastic, hybrid, and carbonate deposits. Intense pedogenetic processes affected the siliciclastic sediments immediately after deposition, comprising clay illuviation, plants bioturbation, feldspar dissolution, and iron oxide/hydroxide precipitation. Siderite and pyrite are the main diagenetic constit- uents. The other diagenetic products are kaolinite, smectite, argillaceous and carbonate pseudomatrix, quartz overgrowths, diagenetic titanium minerals, jarosite, and iron oxides/hydroxides. Early diagenetic siderites were separated into four groups based on their elemental and stable isotopic composition, as well as on their paragenetic relationships with the other constituents and with the host sediments. Spherulitic to macrocrystalline siderites from group 1 are almost pure (average: 94.7 mol% FeCO 3 ; 1.2 mol% MgCO 3 ; 2.3 mol% CaCO 3 ; 1.8 mol% MnCO 3 ) and precipitated from meteoric porewaters in continental siliciclastic rocks under suboxic conditions (δ 18 O vpdb values range in -10.28 to -5.57‰ and the δ 13 C vpdb values in -12.68 to -4.33‰). Microcrystalline rhombohedral siderites from group 2 have zonation due to substantial Ca and Mg substitution (core average: 78.5 mol% FeCO 3 ; 4.2 mol% MgCO 3 ; 15.7 mol% CaCO 3 ; 1.6 mol% MnCO 3 ; edge average: 74.0 mol% FeCO 3 ; 9.2 mol% MgCO 3 ; 15.6 mol% CaCO 3 ; 1.1 mol% MnCO 3 ), and δ 13 C vpdb and δ 18 O vpdb values of +0.17‰ and -1.96‰, precipitated from marine porewaters in packstones/wackestones under methanogenic conditions. The group 3 is represented by irregular spherulitic siderites with moderate Ca and Mg substitutions (average: 80.2 mol% FeCO 3 ; 7.9 mol% MgCO 3 ; 11.3 mol% CaCO 3 ; 0.6 mol% MnCO 3 ), with δ 18 O vpdb values ranging from -5.96 to -7.61‰ and δ 13 C vpdb values ranging from -5.15 to -10.41‰. The group 4 microcrystalline siderites are magnesium-rich (average: 57.3 mol% FeCO 3 ; 31.4 mol% MgCO 3 ; 9.6 mol% CaCO 3 ; 1.7 mol% MnCO 3 ; δ 13 C vpdb +1.43‰ and δ 18 O vpdb -14.09‰). The group 3 and 4 siderites were formed from brackish porewater under suboxic conditions in hybrid and siliciclastic rocks. These variations in siderites are probably related to the Paraíba do Sul River dynamics, to sea level changes and to climatic variations that took place during the Quaternary. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Diagenesis comprises a wide spectrum of post-depositional physical, chemical, and biological processes that are governed by temperature, pressure, and the chemistry of interstitial porewaters. The eodiagenesis stage includes all of the processes that occur under the direct influence of depositional fluids, at shallow depths and low temperatures (b 2 km, b 70 °C; Morad et al., 2000). The principal factors controlling these parameters during eodiagenesis include the depositional setting (e.g. rate of deposition, porewater composition, hydrogeology, climate, lati- tude and sea-level fluctuation), the organic matter content, and the texture and detrital composition of the host sediments, which are directly or indirectly related to the depositional environment (Morad, 1998; Stonecipher, 2000). Porewaters imprisoned within sediments undergo systematic changes in chemical and isotopic compositions during initial burial, which is deeply linked to organic matter diagenesis and the action of specific microorganisms. This leads to a succession of processes of or- ganic matter decomposition through oxidation; the reduction of nitrate, Mn, Fe, and sulfate, and methanogenic fermentation in oxic, suboxic, sulfidic, and methanic environments (Froelich et al., 1979; Berner, 1981). The recognition that the same microbial processes operate Sedimentary Geology 323 (2015) 15–30 ⁎ Corresponding author. E-mail address: rodriguesag.geo@gmail.com (A.G. Rodrigues). http://dx.doi.org/10.1016/j.sedgeo.2015.04.005 0037-0738/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo