Ultra-high resolution sediment analysis and diatom paleoecology from Effingham Inlet, British Columbia, Canada: Implications for late Holocene environmental change

Late Holocene laminated sediments from Effingham Inlet, a small anoxic fjord on the southwest coast of Vancouver Island, Canada, were analyzed at ultra-high resolution (subannual to annual) time scales in order to detect short-term environmental shifts from modern times to 5500 yr BP. Four topics were examined in detail. (1) Nature of laminae. A core dating from ∼600–5500 yr BP was examined using 52 thin sections from ten ∼60-year long sediment slabs. The sediments consist of alternating diatom-rich and silt-rich laminae, where a diatom/silt couplet represents one year of deposition. A distinct seasonal succession of diatoms was determined. (2) Modern-day sediments. Fifty-five sediment trap samples taken over a 16-month period from 1999–2000 were compared to environmental records. A seasonal diatom succession was determined where Skeletonema costatum, Thalassioisira spp., Minidiscus chilensis and Chaeloceros spp. resting spores were common during spring and summer. Benthic taxa and terrigenous debris were common in autumn and winter. Detrended canonical correspondence analysis (DCCA) identified daylength, sea surface temperature (SST), precipitation, and wind stress as important variables. (3) Late 20th century sediments. Forty-two samples from a freeze core spanning from 1947–1993 were examined for diatom abundance and sedimentation patterns and compared with environmental data. Diatom abundance and the environment were influenced by the 1976–1977 climate regime shift from cooler to warmer conditions. DCCA indicated that SST, sea surface salinity and the Aleutian Low Pressure Index were important variables. Cyclostratigraphy revealed that sedimentation patterns and total diatom abundance had significant 2–5 year El Niño cycles and ∼11-year sunspot cycles, suggesting that solar forcing on time scales other than the annual cycle were important for influencing primary productivity. (4) Mid-Holocene sediments. Diatoms from 67 samples were examined from a 15-cm long sediment slab encompassing at least 62 years of deposition and dated at ca. 4450 yr BP. Sedimentary couplets show a thinning-upward pattern. Laminae in thicker couplets are abundant in S. costatum. Benthic species dominate the thinner, siltier couplets. This sequence indicates a shift from strong coastal upwelling to weakened upwelling and increased rainfall washing benthic diatoms and silt into the inlet. Cyclostratigraphy detected the 2–5 year El Niño cycle, and a 9–15 year cycle which may correspond to the sunspot cycle. This research illustrates how laminated sediments can be used as an ultra-high resolution paleoenvironmental archive, and how past environmental conditions and diatom assemblages have changed throughout the late Holocene in coastal British Columbia.