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

摘要

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. percent), and interstitial fluids replete in SO_4 (ca. 27 mM), devoid of CH_4 and low in nutrients. This indicates oxic conditions are present, reflecting the open exchange of waters with Ham 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. percent, respectively) over a period of ca. 1000 years. The nutrient concentrations are especially high (TA, NH_4, PO_4 up to 115 meq/l, 20 mM and 400 #mu#M, respectively), SO_4 is exhausted and CH_4 is prolific. Stable carbon isotope ratio measurements of CH_4 and co-existing CO_2 indicate that methanogenesis is via carbonate reduction (#delta# ~(13)C-CH_4 ca. -60 to -70 per thousand, #delta# ~(13)C-CO_2 ca. + 10 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.