Abstract:
Methane derived authigenic carbonate (MDAC) precipitation is ubiquitous in regions of the seafloor where microbial communities thrive using methane as an energy source and where tectonic fluid expulsion rapidly changes the pressure regime of migrating fluids. Geochemical and isotopic analysis of these carbonates can provide clues as to the carbon source (d13C), temperature (d18O), and fluid source (87/86Sr) from which MDAC precipitation occurs. In this study we have conducted the above analyses on MDACs recovered from IODP Site 1329C and nearby Barkley Canyon, a known thermogenic gas-rich province offshore Vancouver Island. These results have been compared with MDACs sampled from Oligocene and Pliocene sedimentary formations of the uplifted Coast Range forearc in Washington State, USA in order to link modern fluid flow regimes to the past. Four MDACs from Barkley Canyon carry an 87/86Sr signal indicating formation in modern seawater (87/86Sr= approximately .7092). However, one sample indicates formation from fluids that have interacted with an 87/86Sr-depleted system (87/86Sr = .7064). In this region, the only known source of 87/86Sr-depleted fluids involves interaction with the oceanic crust (87/86Sr = approximately .703). The 87/86Sr of this sample falls in line with MDACs recovered from high-permeability regions of IODP Site 1329C (lowest 87/86Sr=.7053, 190 mbsf), which also indicate interaction with a deep fluid in agreement with published pore water 87/86Sr. These anomalous MDACs also show elevated [Mn] and [Fe], likely indicating contribution of a deep diagenetic fluid source. In addition, the d18O of these carbonates range from a low of -12ppm PDB to a high of +7.4 PDB. The d18O-depleted values indicate formation at elevated fluid temperatures, whereas the enriched values likely indicate fluids influenced by clay mineral dehydration at depth. Terrestrial paleoseep samples collected from the Oligocene Pysht/Sooke Fm.'s (n=4) and the Pliocene Quinault Fm. (n=4) also carry distinct geochemical and isotopic signals. The 87/86Sr of all Quinault Fm. samples indicate formation in Pliocene bottom waters. The d13C (-28 to -14ppm PDB) and d18O (+1.0 to +1.6ppm PDB) of these samples indicate formation from a thermogenic carbon source in ambient Pliocene bottom water temperatures. By contrast, the 87/86Sr of only 2 Pysht/Sooke Fm. samples indicate formation in Late Oligocene seawater. The other 2 samples carry a much stronger basaltic signal (87/86Sr = .705), similar to the anomalous modern offshore MDACs. These samples represent fracture fill precipitates that cross-cut surrounding bedding and indicate formation from a deep, warm source fluid (d18O as low as -11.63ppm PDB) and a residual, methanogenic carbon source (d13C = +5.98 and +7.36). The Crescent Volcanic Terrane directly underlies both the paleoseep formations and the modern offshore accretionary wedge. We use the above data to infer that deep fluids interacted with this terrane and sourced MDAC precipitates with anomalously low 87/86Sr and enriched d13C through migration within high permeability horizons in both modern and paleoseep carbonates.