Climate shift at 4400 years BP: Evidence from high-resolution diatom stratigraphy, Effingham Inlet, British Columbia, Canada

Diatom paleoecology and climatic interpretations were assessed from a 15-cm long laminated sediment slab extracted from an anoxic fjord in southwest British Columbia. The slab spans at least 62 years of deposition, determined from counting varves, and is dated at approximately 4400 years BP. The slab shows a sedimentation pattern where thick diatom-rich varves at the bottom become thinner and more silty toward the top. Thin section analysis reveals that the thicker varves contain a distinct succession of diatoms, representing seasonal deposition throughout each year. Annual-scale subsampling shows that the abundance of coastal marine diatoms, namely a weakly silicified form of Skeletonema costatum, decreased over the 62-year period, while benthic and brackish water diatoms, such as Planothidium delicatulum and Achnanthes minutissima, increased with the concomitant increase in silt. The increase in such benthic species and silt, along with the presence of ∼1 cm thick nonlaminated intervals, is interpreted to represent deposition during progressively increasing precipitation over time. These sedimentation patterns and changes in diatom assemblages may signify a change in the relative intensities of the Aleutian Low (AL) and North Pacific High (NPH) atmospheric pressure systems. Thicker diatomaceous varves at the bottom of the slab reflect a stronger NPH system with associated coastal upwelling and enhanced diatom production. The thinner silty varves at the top of the slab suggest that the AL system was prevalent, resulting in greater amounts of precipitation and reduced upwelling. The findings of this study show that significant natural environmental change can occur within a twenty-year time frame, and can provide a basis for the study of modern change in the ocean-atmosphere system over the northeast Pacific Ocean.