Diatom responses to long‐term climate and sea‐level rise at a low‐elevation lake in coastal British Columbia, Canada

Karen Neil,Terri Lacourse

doi:10.1002/ecs2.2868

Abstract

AbstractDiatom assemblages from Lake Stowell, a low‐elevation lake in coastal British Columbia, were used to assess limnological responses associated with changes in relative sea level (RSL) and climate over the past 14,500 yr. Marine diatom taxa dominated the early record and disappeared abruptly by 14,000 cal yr BP in response to glacio‐isostatic uplift and an ensuing decrease in RSL. A brackish‐freshwater phase from 14,200 to 13,400 cal yr BP was marked by increases in several diatom taxa with tolerance for low‐to‐moderate salinity, after which assemblages became strictly freshwater. Biostratigraphic changes in both diatom and chironomid assemblages from Lake Stowell occurred more or less simultaneously throughout the record in response to long‐term changes in climate forcings, such as temperature. Increases in cold‐adapted diatom (Staurosirella pinnata, Staurosira construens, Staurosirella dubia) and chironomid (Sergentia) taxa between 12,900 and 11,700 cal yr BP coincided with Younger Dryas cooling, suggesting a direct link with summer temperature. Shifts in aquatic biota also reflected indirect mechanisms through which climate affected the Lake Stowell ecosystem, for example, by altering thermal stratification, disturbance regimes, and nutrient concentrations. For instance, high summer insolation and warm summer temperatures from 11,700 to 8500 cal yr BP were associated with greater abundances of Stephanodiscus hantzschii and especially Aulacoseira subarctica. A concurrent opening of the forest canopy triggered by warming would have exposed the lake to strong winds and enhanced mixing, favoring these more heavily silicified taxa as an indirect consequence of climatic changes. Diatom assemblages appear to have been impacted by the deposition of Mount Mazama tephra at 7600 cal yr BP; a notable increase in Aulacoseira tenella suggests a response to increased silica availability in association with tephra deposition.

Highlights

IntroductionIn southern British Columbia, patterns of postglacial climate have been inferred at local and regional scales from paleo-records of terrestrial pollen (Heusser et al 1985, Pellatt et al 2001, Walker and Pellatt 2003, Galloway et al 2007) and aquatic biota (Palmer et al 2002, Rosenberg et al 2004, Lemmen and Lacourse 2018).Paleoclimate reconstructions show that temperature oscillated over rather broadscales; for instance, a generally cooler late-glacial period culminated in warming during the Bølling/ Allerød interstadial (14,640–12,850 cal yr BP; Kuehn et al 2014), followed by a short-term cooling during the Younger Dryas (12,900– 11,700 cal yr BP; Kienast and McKay 2001, Lemmen and Lacourse 2018)
From 14.6 to 8.5 ka, the unconstrained clusters showed clear distinctions of groups that were similar to the PCAbased zones, despite the analysis being unconstrained stratigraphically; this indicates that diatom assemblages over this period most closely resembled those in adjacent samples
Sea-level history and isolation of Lake Stowell The beginning of the Lake Stowell sediment record corresponds with the Bølling/Allerød interstadial (14,640–12,850 cal yr BP; Kuehn et al 2014), which was a period marked by global-scale warming and regional-scale relative sea level (RSL) changes following deglaciation of the Cordilleran Ice Sheet (Cosma et al 2008, James et al 2009)

Summary

Introduction

In southern British Columbia, patterns of postglacial climate have been inferred at local and regional scales from paleo-records of terrestrial pollen (Heusser et al 1985, Pellatt et al 2001, Walker and Pellatt 2003, Galloway et al 2007) and aquatic biota (Palmer et al 2002, Rosenberg et al 2004, Lemmen and Lacourse 2018).Paleoclimate reconstructions show that temperature oscillated over rather broadscales; for instance, a generally cooler late-glacial period culminated in warming during the Bølling/ Allerød interstadial (14,640–12,850 cal yr BP; Kuehn et al 2014), followed by a short-term cooling during the Younger Dryas (12,900– 11,700 cal yr BP; Kienast and McKay 2001, Lemmen and Lacourse 2018). In addition to long-term changes in climate, sea-level histories of coastal areas in southern British Columbia have been studied closely (Hutchinson et al 2004, James et al 2009, Roe et al 2013, Shugar et al 2014, Fedje et al 2018) These studies offer insights into patterns of basin isolation and lake development following deglaciation of the Cordilleran Ice Sheet, and subsequent isostatic uplift and decreases in relative sea level (RSL) after ~14,000 cal yr BP. In studies from southern British Columbia (Bennett et al 2001, Galloway et al 2007, Michelutti et al 2016) and elsewhere (Rosen et al 2001, Rosen et al 2003, Neil and Gajewski 2017, 2018), multi-proxy approaches incorporating combinations of diatom data with chironomid, pollen, and/or geochemical data have been used to provide detailed and rich histories of environmental change, to assess links between biota from various trophic levels and their responses to external forcings, as well as to support the validity of proxy-based quantitative reconstructions (Battarbee 2000, Birks et al 2000)

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