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

Sites 1033 and 1034 of ODP Leg 169S in Saanich Inlet have an unusual diagenetic system, that has the appearance of being depth reversed, i.e, a bacterial methane accumulation zone underlain by a sulphate reduction zone. During the late Pleistocene grey, undifferentiated, glacio-marine clays were deposited 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 oxic 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 restricting water circulation? and leading to the presence of anoxic bottom waters. 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 period 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 exhausted and CH4 is prolific. Stable carbon isotope ratio measurements of CH4 and 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 parts per thousand). At the sulphate-methane interfaces, both at the near-surface and at 50 mbsf (Site 1033) and 80 mbsf (Site 1034) methane consumption by sulphate reducing bacteria is intensive, (C) 2001 Elsevier Science B.V. All rights reserved.