Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California

Abstract

Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in 13 C (to ؊19‰ ␦ 13 C), the most 13 C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The ␦ 13 C values are highly variable, but most are within the range ؊12‰ to ؊19‰. This variability may be diagnostic for identifying cold-seep-hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to ϳ1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in 13 C (␦ 13 C ‫؍‬ ؊46‰ to ؊58‰), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane.Keywords: 13 C-depleted bivalve shells, cold seep, methane hydrate, mud volcano, California margin basin. Figure 1. Bathymetry of California borderland and location of seep-hydrate site at 800 m water depth on northern slope of Santa Monica Basin and control site at 550 m depth; 100 m contour interval. ODP-Ocean Drilling Program.

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain)

“…Present-day cold-seep areas are colonized by chemosynthetic marine organisms and by microbial mats (Sibuet and Olu, 1998;Levin, 2005;Vanreusel et al, 2009). These chemosynthetic communities are characterized by low diversity (with only a few species dominating) but high-density populations Sibuet and Olu, 1998;Levin, 2005;Hein et al, 2006;MacDonald et al, 2010). The El Alcor carbonates are densely packed and almost exclusively formed by small mytilids, in agreement with recent cold-seep communities.…”

“…Both skeletal and authigenic non-skeletal carbonates in present-day as well as in fossil cold seeps can display a wide range of  13 C values, from normal marine to very negative (Paull et al, 1985(Paull et al, , 1989(Paull et al, , 1992Kulm et al, 1986;Río et al, 1986Río et al, , 1992Campbell, 1992;Gaillard et al, 1992;CoBabe, 1998;Hein et al, 2006;Campbell, 2006;Lietard and Pierre, 2009). We have analysed isotopes in bulk samples as well as carbonate shells of bivalves (small mytilids, Neopycnodonte, and pectinids), tubes of Ditrupa and benthic foraminiferal tests.…”

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

“…Whilst general 13 C enrichment can be explained with the incorporation of heavy C derived from methanogenesis the enrichment of 18 O is more complicated. The 18 O enrichment in carbonates has previously been explained by different processes occurring in the sedimentary environment, as follows: 1) dissociation of gas hydrates that releases 18 O-rich water in the sediments (e.g., Matsumoto, 1989;Bohrmann et al, 1998;Aloisi et al, 2000;Maekawa, 2004;Hein et al, 2006); 2) interaction with hydrothermal fluids that may yield values as high as 6.5‰ (Clayton and Epstein, 1961); 3) dehydration of clay minerals at great burial depths (D€ ahlmann and de Lange, 2003); 4) crustal/igneous CO 2 circulation (Muehlenbachs and Hodges, 1978;Clayton and Epstein, 1961;Cocker et al, 1982); 5) precipitation of carbonates during glacial times when lower temperatures and isotopically heavier seawater caused a 3e4‰ d 18 O-shift of carbonates in the Mediterranean Sea (e.g., Vergnaud-Grazzini, 1971). Other sources of heavy oxygen can be due to the circulation of deep crustal water (e.g., Lecuyer and Allemand, 1999).…”

“…Generally, siderite formation is favored by low sulfide activity under anoxic conditions (Taylor and Curtis, 1995;Rovere et al, 2015) and therefore occurs outside the sulfate reduction zone, where organic matter fermentation (i.e. methanogenic zone) occurs (Matsumoto, 1989;Carothers et al, 1988;Hicks et al, 1996;Hein et al, 2006;Wittkop et al, 2014). The methanogenic zone provides conditions suitable for preferential precipitation of siderite (e.g., Berner, 1981;Mozley and Wersin, 1992) with respect to that of pyrite (e.g., Rovere et al, 2015).…”

“…In such areas, faults may act as plumbing systems, favoring the uprising of fluids from deep-seated sources, which can affect at different degrees the diagenesis of marine sediments (e.g., Hein et al, 2006). The fluids often sustain complex ecosystems with a high degree of biodiversity (e.g., Levin et al, 2016) and significant chemosynthetic biomass, which may drive anaerobic oxidation of methane (AOM) and trigger carbonate mineral precipitation (Boetius and Suess, 2004).…”