Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46°S and 33°S). (November 2016)

• Record Type: Journal Article
• Authors: Muñoz, Práxedes; Cárdenas, Lissette J.; Garbe-Schönberg, Dieter; Sellanes, Javier; Dezileau, Laurent; Melville, Ives; Mendes, Stephanie D.
• Is Part Of: Progress in oceanography. Volume 148(2016:Nov.)
• Page Start: 26
• Page End: 43
• Publication Date: 2016-11
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1016/j.pocean.2016.09.002
• Languages: English
• ISSNs: 0079-6611

Abstract

Graphical abstract: Highlights: At the Chilean Triple Junction, major element concentrations in the pore water and some metal enrichment in the sediments are caused by diluted seeping fluid and element precipitation; these occur in a reduced environment under oxygenated waters. At the Central Chilean margin (EQSS), the presence of the OMZ and organic matter diagenesis produce suboxic bottom waters and reduced conditions in the sediments, promoting fluxes of reduced elements to the bottom water. The reduced condition in the sediments promotes the enrichment of elements such as Cd, Pb, U, and in some degree Mo. These elements, except for U, are maintained within the sediments in equilibrium with precipitated sulfide compounds. Abstract: 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), δ 13 DIC, 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). 13 C-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 cold-seep areas; and (b) correspond to clay mineral dehydration, as reported in plate subduction systems. The reducing conditions established at the CTJ conduct the Cd enrichment at a similar magnitude of that seen at the shallower suboxic site (EQSS). Evidence of chimney or vent fauna was not observed. At the EQSS, higher TOC and total sulfide contents were consistent with enhanced deposition of organic matter and reducing conditions developed in the OMZ, favoring the authigenic enrichment of Cd, U and Pb. The geochemical evidence, based only on methane concentrations and δ 13 DIC, is insufficient to establish the presence of methane seeps, as previously reported.