High resolution palaeoceanography and palaeoclimatology from late Holocene laminated sediments, Saanich Inlet, British Columbia
Late Holocene (c. 2100 yr BP) varved sediments recovered from Saanich Inlet, Ocean Drilling Program Leg 169S, have been analyzed using high resolution scanning electron microscope techniques. The individual centimetre-scale varves comprise two segments: a biogenic varvesegment which consists of up to 19 alternating diatom ooze and diatomaceous mud laminae, which were deposited during spring through autumn; and a terrigenous varve-segment containing up to 6 laminae dominated by silty clay, which were deposited during winter. The laminae within the varves record individual flux events of a few weeks to months duration. Diatom flux typically reached the fjord floor as a mixture of whole/fragmented aggregates and Zooplankton faecal pellets. The diatom succession within the biogenic varve-segment typically comprises: an early spring Thalassiosira spp. bloom, followed by a late spring Chaetoceros spp. -dominated bloom, a late spring/early summer Skeletonema costatum bloom and a series of one to several summer/autumn Chaetoceros spp. -dominated blooms. Interannual variability in diatom sediment flux is typically recorded by the presence/absence of Thalassiosira spp., S. costatum and the number of summer/autumn Chaetoceros spp. - dominated blooms. The varved sediment documents a major increase in the sedimentation rate at approximately 2100 varve yr BP. This represents a transition from a colder/drier climate during the Tiedmann glacial advance when sediment flux was lower, to a relatively warmer/wetter late Holocene climate when sediment flux increased. As glaciers receded, fluvial input increased throughout the year as a result of increased rainfall during winter months and glacial meltwater during spring through autumn months. This increased fluvial input introduced additional terrigenous sediment throughout the year and additional nutrients enhancing diatom production during spring through autumn. The varved sequence is punctuated by centimetre-scale massive intervals originating from low density turbidity currents initiated above the anoxic zone. A tephra laminae containing volcanic glass has also been observed and might originate from Mount St Helens. Spectral analyses of inter-varve variability in sediment fluxes have shown strong periodicities within El Nino Southern Oscillation (ENSO), Quasi-Biennial Oscillation (QBO) and North Pacific Oscillation (NPO) bands. Thalassiosira spp. flourished in the more stable conditions associated with strong La Nina-type phases of the ENSO cycle. S. costatum blooms were associated with El Nino events when winter storms increased levels of ground water storage, which subsequently increased fluvial input of nutrients during the late spring/early summer. The summer/autumn Chaetoceros spp. blooms were dominated by the NPO, as successive years of increased rainfall/snowfall associated with the colder/wetter NPO regime increased long term mean levels of ground water storage and accumulation of snow, which subsequently increased fluvial input of nutrients during summer and autumn months. Winter terrigenous flux was dominated by NPO at Site 1033 and ENSO at Site 1034. Site 1033 which is the most distal to the terrigenous source, records increased/decreased fluvial input associated with the wetter/dryer regimes of NPO. Site 1034, records the effects of El Nino storms which rework nearshore sediments and deposit them in the deeper offshore regions. QBO frequencies are recorded in the overall sediment flux and biogenic sediment flux at Site 1033 and may represent the alternation of a relatively weaker/s"tronger Aleutian Low Pressure System causing consecutive years to be relatively wetter and drier, affecting sediment fluxes. ENSO operated at approximately 3.5 years, with stronger events occurring at approximately 9 year intervals; QBO operated at approximately 2.4 years; and NPO operated at approximately 15 years. Multi-decadal periodicities of 31 and 42 years are also recorded and might represent stronger NPO regimes.