Multi-proxy Holocene palaeoclimatic record from a saline lake in the Canadian Subarctic

Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequen ce at a centennial-scale resolution from an athalassic saline lake in the Y ukon were conducted to infer patterns of Holocene climatic change in the Ca nadian Subarctic, using sediment mineralogy and biostratigraphy (diatoms, p igments). Diatom-inferred quantitative estimates of palaeosalinity were obt ained by use of transfer functions developed from a calibration set of 219 lakes from western North America. The sediment mineralogy and fossil pigmen ts at the base of the core indicated a moderately deep non-stratified lake dominated by clastic influx, probably in a basin fed by glacial meltwater. The early-Holocene history (c. 11 000-8100 C-14 yr BP) was characterized by a relatively deep mesosaline lake with diatom-inferred salinities approxim ating 20 g L-1. The occurrence of both aragonite and dolomite, as well as e levated concentrations of chlorophyll and carotenoid pigments, support the interpretation of deepwater anoxia and possibly strong chemical stratificat ion. High concentrations of the chemically stable beta -carotene suggest th at total algal abundance was particularly high during the early Holocene, w hen planktonic Cyclotella cf. choctawhatcheeana and Chaetoceros muelleri we re the most common diatom taxa. Relatively fresh (2-15 g L-1 ) eutrophic co nditions prevailed during the mid-Holocene period (c. 8000- 2000 yr BP), wi th four periods of alternating fresh and saline conditions. The diatom-infe rred salinity profile reveals significant fluctuations within these cycles, but overall they indicate humid climatic conditions compared to today. Alg al abundance is inferred to have declined three-fold relative to the early Holocene, particularly in the case of eukaryotic algae (e.g., diatoms, cryp tophytes, chlorophytes). The recent history of the lake (about 2000 years B P until the present day) was marked by important changes in ionic compositi on (e.g., occurrence of gypsum and Mg-carbonates) and hydrologic conditions . The lakewater composition during the last two millennia was characterized by hypersaline Mg-SO4 brines. The palaeolimnological evidence from most pr oxies indicates a trend towards drier conditions during the past 2000 years . The various indicators reveal a complex history of frequent and rapid shi fts in palaeosalinity and lake palaeoproductivity during the Holocene, and the effects of the Younger Dryas and 'Little Ice Age' episodes may be recor ded in the palaeoclimate proxy data. The palaeoclimatic interpretation emer ging from this high-latitude lake corroborates existing broad trends based on palynological studies in this region but provides evidence for more dyna mic climatic change during the mid- and late Holocene.