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

摘要

Coring at site ODP 1033B in Saanich Inlet recovered 59.4 m of mainly laminated olive-grey diatom ooze and an underlying 55.15 m of massive grey to olive-grey silty clay. Based on AMS radiocarbon dating, die boundary between the two units is between 11,000 and 13,800 calibrated years BP, and represents the Holocene-Pleistocene boundary. The lower unit represents glaciomarine deposition, whereas deposition of the upper unit began when the modern semi-restricted physiography of the fjord was established following glacial rebound and highly productive marine conditions were established. The glaciomarine clay is almost entirely terrigenous, whereas the diatom ooze contains 2-3 wt. percent organic C and 20-40 wt. percent biogenous silica; CaCO_3 contributions are minor, but there are several peaks in carbonate abundance in the upper unit. The isotopic composition of organic C and total N suggests that organic matter in the glaciomarine clay is dominantly terrestrial (#delta# ~(13)C_(organic) < -25 per thousand and #delta# ~(15)N_(total)= ca. 3 per thousand) and in the diatom oozes it is mainly marine (#delta# ~(13)C_(organic) < -22 per thousand and #delta# ~(15)N_(total)= ca. 10 per thousand). The heavy #delta# ~(15)N_(total) values probably record a contribution of isotopically heavy nitrate to the surface waters of the inlet that is transported to British Columbia (BC) coastal waters from the eastern tropical Pacific by 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-sensitive elements in the diatom oozes. Thus, Cu, Mn, Mo, Ni, Pb, V and Zn have higher concentrations in the upper unit; Br and I are also enriched because of their association with organic matter. Mn is enriched in the anoxic diatom 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 anoxic sediment and was precipitated as a diagenetic phase. The remaining metals are enriched because of their removal to die sediment as sulphides (Cu, Mo, Ni, Pb and Zn) or as particle-reactive reduced species (V). Cr enrichment is obscured by the presence of Fe-rich chlorite. The lag in the enrichment of Mo with respect to organic C in the sediments indicates that anoxia developed some time after marine production increased following the semi-isolation of the fjord.