Factors affecting the diagenesis of Quaternary sediments at ODP Leg 172 sites in western North Atlantic: evidence from pore water and sediment geochemistry

Abstract

Pore waters and Quaternary sediments at ODP Leg 172 sites on the Carolina Slope (CS; Site 1054), Blake Outer Ridge (BOR; Sites 1057 and 1060), Bahama Outer Ridge (BAOR; 1062) and Bermuda Rise (BR, Site 1063) were studied. The sediments are mainly clayey and silty mud with intercalations of nannofossil-rich and lutite-rich beds towards the top. Sedimentation rate ranges from 4.2 cm/ky at Site 1054 to 23 cm/ky at Site 1060. Average total organic carbon (TOC) contents of the upper sediment units range from 0.40% to 0.58% at Sites 1057, 1060, 1062 and 1063, and from 0.86% to 1.25% at Site 1054. The organic matter at all the sites is mostly degraded, except at Site 1054. Sediments at Sites 1062 and 1063 have lower total sulfur (TS) (0.00–0.62%; average: 0.06%) than those at the other sites. Site 1054 has the highest TS (0.8–1.4%) and the lowest reactive iron (0.003–0.074%) contents among all the sites. The deep-water Sites 1062 and 1063 have concave-down pore-water sulfate concentration/depth profiles, relatively deep sulfate/methane interfaces (SMIs) (65 and 38 meters below sea floor (mbsf), respectively) and steep methane gradients at the base of the SMI, whereas the shallow-water Sites 1054, 1057 and 1060 have near-linear sulfate and relatively shallow methane gradients. The near-linear sulfate/depth profiles and shallow SMIs (17 and 14 mbsf, respectively) at Sites 1057 and 1060 are primarily due to high sulfate consumption by anaerobic methane oxidation (AMO) at the SMI. Concave-down profiles and deep SMIs at Sites 1062 and 1063 are the result of low rates of sulfate consumption through organic matter degradation, due to a limitation of metabolizable organic matter. Site 1054 is unusual in having an 8-m thick oxic–suboxic zone, a 48-m deep SMI and a linear sulfate/depth profile. Despite the high organic matter content of sediments at this site, the thick oxic–suboxic zone is mainly due to intense bioturbation, while the linear sulfate/depth profile and the deep SMI are likely caused by low sedimentation rate and by “reactive” iron limitation. Pore-water magnesium, calcium and strontium profiles indicate that carbonate precipitation has occurred near the SMI. This is confirmed by the presence of authigenic dolomite rhombs and nodules especially at and below SMI at all sites.