In 1997, an R/V Pt. Lobos/Ventana expedition to the Eel River Basin offshore
Northern California discovered several sites of active gas venting, characterized by
degassing sediments, living chemosynthetic communities, and extensive carbonate
precipitates. Although gas hydrates were not observed during the 1997 expedition, they had
previously been recovered at these sites (Brooks et al.; Marine Geology, 96 (1991),
103-109). Whereas observations of gas hydrates or free gas provide insight into the
short-term variability in areas of fluid venting, carbonate precipitates are good
candidates to provide a long-term record of changing processes at fluid expulsion sites.
In my talk I will focus on the mineralogy and isotope geochemistry of the carbonate
precipitates from these vent sites and what they can tell us about changing geochemical
conditions through time.
The slab- and chimney-shaped precipitates recovered from the seafloor are composed of
carbonate-cemented clay and siltstones. Based on detailed XRD and electron microprobe
analyses, their mineralogy varies from aragonite to high-Mg-calcite and dolomite. At one
site, repeated changes in the geochemical conditions (pore water Mg/Ca ratio, pH, PCO2,
phosphate and sulfate concentrations) during carbonate precipitation seem to have caused
the formation of alternating aragonite and Mg-calcite layers.
The carbon isotopic composition of the carbonates shows an unusually wide range,
varying from +9 to 40 (PDB), which indicates a complex carbon source
from both a 13C-depleted and a residual, 13C-enriched, carbon pool.
The d18O values vary between +3.2 and +6.6
(SMOW), pointing toward a heavy, 18O-enriched oxygen source. Different
carbonate minerals exhibit significant differences in their oxygen isotopic fractionation.
I will discuss the importance of considering the mineralogy of authigenic precipitates
when estimating conditions, i.e. the presence or absence of gas hydrates, during carbonate
Next: Edges and interfaces in the deep sea: "Hot spots" for
Last updated: December 19, 2000