A Holocene record of diatom and silicoflagellate microfossils in sediments of Saanich Inlet, ODP Leg 169S

Abstract

ODP Leg 169S laminated sediment cores provided a unique opportunity to examine changes in the siliceous phytoplankton community of Saanich Inlet. These laminated sediments are rich in diatom and silicoflagellate remains as a result of the inlet's high production and excellent preservation. Our results suggest that species composition and abundance of diatoms and silicoflagellates in the inlet have changed very little since the inlet's formation. Carbon and opal data indicate an increase in primary productivity with time which is not apparent in the fossil data. However, the discrepancy between data sets may be due to changes in grazer activity which could have reduced the number of identifiable fossils. Individual species were useful for identifying changes in the inlet. Some planktonic diatoms suggest that nutrient supply was highest in the early–mid Holocene. Increases in the abundance of a partially benthic diatom, Paralia sulcata , follow variations in sea level and availability of benthic habitat in the inlet. A large peak of the silicoflagellate, Dictyocha fibula , corresponds to a catastrophic flood event in the early Holocene, which probably washed terrestrial material and low salinity water from the British Columbia mainland into Saanich Inlet. Fossil data were also analyzed to infer past changes in the environment using a weighted averaging calibration and regression technique. These data suggest that only minor variations in sea surface temperature have occurred.