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The Baffin Island region in the eastern Canadian Arctic has recently experienced a rapid warming, possibly unprecedented in millennia. To investigate the response of freshwater environments to this warming and place it in a secular perspective, we analyzed a 90-cm-long sediment core from Nettilling Lake, the largest lake of the Canadian Arctic Archipelago. The core was taken from a part of the lake basin that receives meltwater and sediment inputs from the nearby Penny Ice Cap. The core time scale, established using 137Cs and palaeomagnetic techniques, spans an estimated 600 years. A multi-proxy approach was used to document changes in the physical, chemical, and biological properties of the sediments. We found evidence for a relatively warm period (mid/late 15th century to mid/late 16th century) during the early part of the ‘Little Ice Age’ (LIA), characterized by high sedimentation rates and laminations. This was followed by colder, drier, and windier conditions corresponding to the coldest phase of LIA and coinciding with the latest and most extensive period of regional ice cap expansion (early 16th to late 19th centuries). A rapid warming occurred at the beginning of the 20th century. Variations in titanium (Ti) content in the core, a proxy for detrital sediment inputs, showed good agreement with reconstructed secular variations in summer melt rates on Penny Ice Cap between the mid-14th century and the present-day, providing supporting evidence for a climatic–hydrological connection between the ice cap and Nettilling Lake.

This study documents the past ~7000 years of Holocene climatic history for Labrador and Nunatsiavut, using a sedimentary sequence of more than 8 m retrieved in Nachvak fjord, one of the northernmost fjords of Nunatsiavut. Using a multi-proxy approach combining a solid Accelerator Mass Spectrometry (AMS)-14C chronology and the fossil assemblages of pollen grains and dinoflagellate cysts (dinocysts), we were able to compare terrestrial and marine records in an effort to obtain a better understanding of the mid- to late-Holocene climate history of the Nunatsiavut. Records begin at the end of the deglaciation and showed a general delay in the sequence of climate events which followed, both in terrestrial and marine realms. The presence of Pentapharsodinium dalei in great abundance in Nachvak Fjord revealed a strong influence of the North Atlantic Ocean and the Labrador Sea until ~3000 yr BP. Afterward, its rather fast disappearance marked the increased influence of Arctic waters. The last 1000 years show climate stability in the region over the marine realm and a cooling trend over terrestrial landscapes.

Paleo surface wind for southern Quebec was inferred quantitatively for the past 9500 years from diatom assemblages archived in the sediment of the shallow mountain Lac du Sommet using biweekly sediment trap samples along an elevation gradient in the study area. The wind reconstruction was compared with diatom-inferred dissolved organic carbon concentration, chironomid-inferred mean August air temperature, pollen, grain size and loss-on-ignition. Increased lake circulation, interpreted as indicating stronger surface winds, was inferred from diatoms around 8700, 4500, 3000 to 2000 cal. yr BP and during the past 250 years. Increased lake stratification was diatom-inferred from 7500 to 5000 cal. yr BP and between 1500 and 500 cal. yr BP. Diatom-inferred paleowinds were congruent with the regional fire history. In general, diatom production was significantly correlated with diatom-inferred lake circulation. Diatom-inferred lake circulation and diatom production were not correlated with the pollen assemblage changes, diatom-inferred dissolved organic carbon and chironomid-inferred August air temperature, which were highly intercorrelated. After the disappearance of the meltwater in the St Lawrence River valley, the chironomids reflected a warming trend that lasted until about 5000 cal. yr BP, trees replaced shrubs and diatom-inferred dissolved organic carbon increased from 4 mg/l to 6 mg/l. Diatom-inferred lake circulation exhibited periodicities of 200 and 900 years, whereas chironomid-inferred August air temperatures indicated a distinct (significant) 200 year periodicity.

Concerns about the effects of global warming on Arctic environments have stimulated multidisciplinary research into the history of their long-term climatic and environmental variability to improve future predictions of climate in these remote areas. Here we present the first palaeolimnological study for Southampton Island using analyses of chironomids supported by sedimentological analyses, carried out on a 1 m long core retrieved from a lake located in the northeastern part of the island. This core was made up of marine sediments underneath 65 cm of freshwater lake sediments. A marine shell, humic-acids and chironomid head capsules were used to date this sequence. The Holocene environmental history of the lake consisted of two major contrasting periods. The first one, between about 5570 and 4360 cal. yr BP, was climatically unstable, with common postglacial chironomid taxa such as Corynocera oliveri-type, Paracladius and Microspectra radialis-type. This period also corresponded to the highest chironomid-inferred August air temperature (10°C) for the whole record and to significant increases in major chemical elements as detected by x-ray fluorescence. During the second period, which lasted from about 3570 cal. yr BP until the present, limnological conditions seemed to stabilize after a change to cold oligotrophic chironomid taxa, such as Heterotrissocladius subpilosus-group, with no major variations in the abundance of chemical elements. Inferred August air temperatures ranged between 8 and 9°C. This study provided unique information on the timing of the Holocene Thermal Maximum in the Foxe Basin area, a region with very little information available on long-term climate change. This region showed, so far, relatively few signs of recent climatic change, as opposed to other regions in the High Arctic.

Analyses of sedimentary diatoms in two New England ponds reveal limnological changes during the past 2500 years that are related to climatic change, anthropogenic activities and natural disturbance. Deforestation in the lake catchments during the eighteenth and nineteenth centuries temporarily affected diatom assemblages, with subsequent recovery. However, algal communities did not return completely to presettlement conditions as a result of long-term trends in climatic change, small-scale natural disturbances and delayed watershed recovery from acidification. A short-term rise in diatominferred dissolved organic carbon (DOC) in Levi Pond was related to partial removal of vegetation by logging in the catchment, and a similar DOC increase at North Round Pond is correlated with a hurricane and increased aquatic productivity. At Levi Pond, increasing diatom-inferred DOC concentrations during the past c. 2000 years likely reflect a long-term increase in allochthonous organic matter and peat development in the watershed related to moister conditions, supported by corresponding patterns in the stable-isotope, chironomid and pollen records. These results correspond with moisture patterns in adjacent areas inferred from pollen and sediment analyses, suggesting that diatoms in Levi Pond recorded a larger regional trend in increasing moisture. This is the first study in a temperate region that links diatom-inferred DOC concentration to past changes in moisture balance, suggesting that fossil diatoms may be a promising proxy for future palaeohydrological studies in temperate regions. However, more studies are necessary to separate the effects of peatland growth and allochthonous organic-matter input on lake-DOC concentrations.

Two diatom-based transfer functions for the reconstruction of past levels of dissolved organic carbon (DOC) and water colour were applied to fossil diatom species assemblages from a coastal isolation basin on the eastern shore of Hudson Bay. Diatom stratigraphic changes and the diatom-inferred patterns of limnological change following the retreat of Holocene marine waters of the Tyrrell Sea revealed a highly variable Holocene lake trajectory associated with successional shifts in lake catchment vegetation and soil development. The main trends observed in the Holocene history of Lake Kachishayoot are: (1) a progressive loss of alkalinity over time; and (2) abrupt increases in DOC and water colour that coincide with the arrival of spruce (Picea mariana) in the catchment. Reconstructed DOC allowed the estimation of past depths of ultraviolet (UV) penetration in the water-column of Lake Kachishayoot. Past variations in biological UV exposure were inferred using optical models based on DOC concentrations and the response curves for DNA damage and UV photoinhibition of photosynthesis. The palaeoecological analyses revealed large changes in the underwater photobiological environment over the course of the Holocene period, from extremely high UV exposure after the initial formation of the lake and its isolation from the sea, to an order-of-magnitude lower biological UV exposure under the present conditions of catchment vegetation.

Multi-proxy palaeolimnological analyses of a postglacial sedimentary sequence at a centennial-scale resolution from an athalassic saline lake in the Yukon were conducted to infer patterns of Holocene climatic change in the Canadian Subarctic, using sediment mineralogy and biostratigraphy (diatoms, pigments). Diatominferred quantitative estimates of palaeosalinity were obtained by use of transfer functions developed from a calibration set of 219 lakes from western North America. The sediment mineralogy and fossil pigments 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 14C yr BP) was characterized by a relatively deep mesosaline lake with diatom-inferred salinities approximating 20 g L-1. The occurrence of both aragonite and dolomite, as well as elevated concentrations of chlorophyll and carotenoid pigments, support the interpretation of deepwater anoxia and possibly strong chemical stratification. High concentrations of the chemically stable b-carotene suggest that total algal abundance was particularly high during the early Holocene, when planktonic Cyclotella cf. choctawhatcheeana and Chaetoceros muelleri were the most common diatom taxa. Relatively fresh (2–15 g L-1) eutrophic conditions prevailed during the mid-Holocene period ( c. 8000– 2000 yr BP), with four periods of alternating fresh and saline conditions. The diatom-inferred salinity profile reveals significant fluctuations within these cycles, but overall they indicate humid climatic conditions compared to today. Algal abundance is inferred to have declined three-fold relative to the early Holocene, particularly in the case of eukaryotic algae (e.g., diatoms, cryptophytes, chlorophytes). The recent history of the lake (about 2000 years BP until the present day) was marked by important changes in ionic composition (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 proxies indicates a trend towards drier conditions during the past 2000 years. The various indicators reveal a complex history of frequent and rapid shifts in palaeosalinity and lake palaeoproductivity during the Holocene, and the effects of the Younger Dryas and ‘Little Ice Age’ episodes may be recorded in the palaeoclimate proxy data. The palaeoclimatic interpretation emerging from this high-latitude lake corroborates existing broad trends based on palynological studies in this region but provides evidence for more dynamic climatic change during the mid- and late Holocene.