Geochemical and isotopic evidence for post-glacial palaeoceanographic changes in Saanich Inlet, British Columbia

Coring at site ODP 1033B in Saanich Inlet recovered 59.4 m of mainly lamina ted olive-grey diatom ooze and an underlying 55.15 m of massive grey to oli ve-grey silty clay. Based on AMS radiocarbon dating, the boundary between t he two units is between 11,000 and 13,800 calibrated years BP, and represen ts the Holocene-Pleistocene boundary. The lower unit represents glaciomarin e deposition, whereas deposition of the upper unit began when the modern se mi-restricted physiography of the fjord was established following glacial r ebound and highly productive marine conditions were established. The glacio marine clay is almost entirely terrigenous, whereas the diatom ooze contain s 2-3 wt.% organic C and 20-40 wt.% biogenous silica; CaCO3 contributions a re minor, but there are several peaks in carbonate abundance in the upper u nit. The isotopic composition of organic C and total N suggests that organi c matter in the glaciomarine clay is dominantly terrestrial (delta C-13(org anic) < -25<parts per thousand> and delta N-15(total) = ca. 3 parts per tho usand) and in the diatom oozes it is mainly marine (delta C-13(organic) > - 22 parts per thousand and delta N-15(total) = Ca. 10 parts per thousand). T he heavy delta N-15(total) values probably record a contribution of isotopi cally heavy nitrate to the surface waters of the inlet that is transported to British Columbia (BC) coastal waters from the eastern tropical Pacific b y the California Undercurrent. Major and minor elemental data suggest that the composition of the terrigenous material and its grain-size has changed over the last 15 kyr, and there are marked enrichments in several redox-sen sitive elements in the diatom oozes. Thus, Cu, Mn, Mo, Ni, Pb, V and Zn hav e higher concentrations in the upper unit; Br and I are also enriched becau se of their association with organic matter. Mn is enriched in the anoxic d iatom oozes due to the presence of manganoan carbonate (Mn peaks generally corresponding with carbonate peaks) formed in the sediment when deep water renewal caused precipitation of Mn oxyhydroxides, which dissolved in the an oxic sediment and was precipitated as a diagenetic phase. The remaining met als are enriched because of their removal to the sediment as sulphides (Cu, Mo, Ni, Pb and Zn) or as particle-reactive reduced species (V). Cr enrichm ent is obscured by the presence of Fe-rich chlorite, The lag in the enrichm ent of Mo with respect to organic C in the sediments indicates that anoxia developed some time after marine production increased following the semi-is olation of the fjord. (C) 2001 Elsevier Science B.V. All rights reserved.