Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46 degrees S and 33 degrees S)

• Published in: Progress in Oceanography
• Main Authors: Munoz, Praxedes, Cardenas, Lissette J., Garbe-Schoenberg, Dieter, Sellanes, Javier, Dezileau, Laurent, Melville, Ives, Mendes, Stephanie D.
• Other Authors: Universidad Católica del Norte, Coquimbo, Christian-Albrechts-Universität zu Kiel (CAU), Géosciences Montpellier, Institut national des sciences de l'Univers (INSU - CNRS)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA), Department of Earth Science Santa Barbara), University of California Santa Barbara (UC Santa Barbara), University of California (UC)-University of California (UC)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2016
• Subjects: [SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry; Krylova; Pacific; Methane hydrate
• Online Access: https://hal.archives-ouvertes.fr/hal-01412700; https://doi.org/10.1016/j.pocean.2016.09.002

International audience This study presents the geochemical composition of superficial sediment under oxic and suboxic bottom water conditions along the Chilean continental margin (SE Pacific), where evidence for benthic chemosynthetic activity associated with diffuse seeping of chemically reduced fluids has been reported. The exploration was carried out at: (1) the Chilean Triple Junction (CTJ), at a water depth of ∼2900 m, with the additional indication of hydrothermal activity near a methane-rich cold-seep area (46°S) (German et al., 2010); and (2) the El Quisco methane seep site (EQSS), at ∼340 m water depth (33°S) (Melo et al., 2007; Krylova et al., 2014). While the deeper CTJ is located within an oxic environment (dissolved oxygen in the bottom waters: 164 μM), the shallower EQSS lies within a suboxic environment (dissolved oxygen in bottom water: 23 μM), located within the lower limit of the SE Pacific oxygen minimum zone (OMZ).Pore water from short cores was analyzed for dissolved major, minor, and trace elements (Cl, Na, Mg, K, Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), δ13DIC, sulfide, sulfate, and methane. The solid sediment fraction was likewise analyzed for total organic carbon (TOC), metals, and redox potential. Elevated sediment temperatures were found in superficial sediments (5–13 °C) at the CTJ site, which could be due to warm fluids associated with the proximity of the ridge, where hydrothermal vents may occur. Reduced fluids were also present here, indicated by higher Mn fluxes toward the water column even in oxidized sediments (RPD > 8 cm), which contrasted with the lower fluxes in reduced sediments of the EQSS site (RPD ∼ 2 cm). 13C-depleted DIC, anomalously low pore water Cl (∼15 ppb), and low concentrations of other major elements may be the result of dilution by fluid seeping and precipitation of major elements, producing authigenic enrichment (Ca, Mg, Sr). The fluid could also: (a) be diluted by pure water produced during methane hydrate dissociation, as observed in other ...