Arctic lake ontogeny across multiple interglaciations

Abstract

Few lakes in the Arctic preserve sediments older than Holocene age because of pervasive glacial scour during the last ice age. Here we present sediment diatom and geochemical records from a lake on east-central Baffin Island (CF8, Nunavut, Canada) that captures three successive interglacial periods within the last 200,000 years: a portion of marine isotope stage (MIS) 7; the last interglacial (MIS 5e); and the Holocene (MIS 1). An additional unit of diatom-rich organic sediment occurs between the latter two intervals, and is ascribed to MIS 5a interstadial conditions. The Lake CF8 paleolimnological record reveals similar ontogenetic trends within each interglacial. Early postglacial environments in both the Holocene and MIS 5e were characterized by a dominance of colonial benthic fragilarioid diatoms that thrived in relatively alkaline waters. These species shifts do not coincide with major changes in base cation delivery to the sediments, suggesting that shifts in diatom assemblages are controlled mainly by climate-driven pH dynamics. Diatom assemblages then transitioned into dominance by tychoplanktonic Aulacoseira species, likely in response to climate-driven pH dynamics. The highest sustained abundances of Aulacoseira occur in sediments ascribed to MIS 5e, and this is the only interval in which thalassiosiroid centric taxa also occur (e.g. Discostella and Puncticulata spp.). Given that these planktonic taxa typically reach high abundances during extended periods of open-water conditions, we surmise that MIS 5e was the warmest interval recorded. The overall similarity of lake ontogenetic trajectories recorded within each interglacial period suggests that climatic and edaphic factors drive a complex succession of environmental changes through indirect effects on lake ice cover, habitat availability, and lake-water pH. In recent decades (past ∼50 yrs), an unprecedented increase in periphytic Eunotia taxa is evident; these diatoms were only present intermittently during previous glacial–interglacial cycles. The expansion of Eunotia is attributed to increased habitat availability associated with declining ice cover during recent 20 th century warming. By integrating data from several distinct interglacials, the CF8 diatom record considerably extends our understanding of Arctic lake ontogeny, and provides an unparalleled natural archive with which to compare recent changes associated with anthropogenic climate warming.