Alice S. Chang

Seasonal Sediment and Diatom Record from Late Holocene Laminated Sediments, Effingham Inlet, British Columbia, Canada

Abstract Laminated diatomaceous sediments from Effingham Inlet, British Columbia, are described and classified in this study. Analyses were made from ten 15-cm long sediment slabs, spanning the last 5500 years, and 52 thin sections from which 408 sedimentary couplets were identified. Microfossil analysis and radiocarbon dating of the sediments reveal that the laminae are annually deposited (i.e., varves), with couplets containing a terrigenous and diatomaceous lamina pair. Terrigenous laminae, averaging 0.56 mm in thickness, consist of silt, organic debris, and robust diatoms, and are deposited during the winter months. Diatomaceous laminae, with a mean thickness of 1.85 mm, can be divided into three component laminae of differing compositions that reflect changing seasonal conditions during the spring, summer, and autumn months. This seasonal succession is seen in 76% of the couplets examined, recurring year after year with variations in couplet thickness and species occurrence. Couplets lacking the succession may represent deposition during periods of low diatom production or years with low seasonality (e.g., El Niño). Variability in couplet styles corroborates climate trends derived from pollen and Neoglacial studies. Sediments older than 4000 yr BP (calibrated radiocarbon dates) contain couplets with a distinct annual succession, and are interpreted to have been deposited during conditions that were warmer than today. Sediments deposited between 2000 to 4000 yr BP also contain couplets with an annual succession, but the laminated intervals are interrupted by brief nonlaminated intervals. The sediments were likely deposited during cooler and wetter conditions than today. Sediments younger than 2000 yr BP were deposited during modern conditions. This study illustrates the effective utility of an ultra high-resolution analysis of laminated sediment records, once proxy indicators are defined, and is important for understanding post-glacial climate evolution along the coast of British Columbia and throughout the northeast Pacific Ocean during the late Holocene.