Organic geochemistry of Saanich Inlet, BC, during the Holocene as revealedby Ocean Drilling Program Leg 169S

Sites 1033 and 1034 of ODP Leg 169S in Saanich Inlet have an unusual diagen etic system, that has the appearance of being depth reversed, i.e, a bacter ial methane accumulation zone underlain by a sulphate reduction zone. Durin g the late Pleistocene grey, undifferentiated, glacio-marine clays were dep osited with low C-org contents (<0.4 wt.%), and interstitial fluids replete in SO4 (ca. 27 mM), devoid of CH4 and low in nutrients. This indicates oxi c conditions are present, reflecting the open exchange of waters with Hare Strait during the Pleistocene before the Saanich Peninsula emerged. In the earliest Holocene (ca. 11,000 years BP) the inlet was formed, severely rest ricting water circulation? and leading to the presence of anoxic bottom wat ers. The sediments are laminated and show a dramatic rise to high C-org, N- org and S-tot contents (up to 2.5, 0.4, 1.4 wt.%, respectively) over a peri od of ca. 1000 years. The nutrient concentrations are especially high (TA, NH4, PO4 up to 115 meq/l, 20 mM and 400 <mu>M, respectively), SO4 is exhaus ted and CH4 is prolific. Stable carbon isotope ratio measurements of CH4 an d co-existing CO2 indicate that methanogenesis is via carbonate reduction ( delta C-13-CH4 ca. -60 to -70 parts per thousand, delta C-13-CO2 ca. + 10 p arts per thousand). At the sulphate-methane interfaces, both at the near-su rface and at 50 mbsf (Site 1033) and 80 mbsf (Site 1034) methane consumptio n by sulphate reducing bacteria is intensive, (C) 2001 Elsevier Science B.V . All rights reserved.