The Arctic has a disproportionately large response to changes in radiative forcing of climate, and glaciers respond sensitively to these changes. Proglacial lake sediments are valuable archives of glacier and climate change because lakes are widespread in the Arctic and their sediments are often continuous and datable. Furthermore, multiple chemical, physical and biological proxies can be extracted from lake sediments and analyzed to provide highly resolved, reliable indicators of past climate. In addition, varve thickness can be used to infer paleo-temperatures. Here, we use varve thickness to expand our understanding of late Holocene temperature changes on northeastern Baffin Island.

We obtained a ¹⁴C- and ^239+240Pu-dated surface core/percussion core pair from proglacial Big Round Lake south of Inugsuin Fjord, 70 km southwest of the hamlet of Clyde River, Baffin Island (Fig. 1). Together these cores span > 8000 years and the sediments are varved, verified by the ^239+240Pu analysis, from 950-2000 AD. Magnetic susceptibility was high during the early Holocene, decreased to near-zero values during the mid-Holocene and increased during the past 2500 years to reach the highest values seen in the record around 1000 years ago. Loss-on-ignition had an opposite trend, with the highest values in the mid-Holocene. Sedimentation rate was constant during most of the Holocene (0.03 cm yr ^-1) and increased during the past 1000 years to 0.05 cm yr ^-1. These parameters indicate that following the absence of an active glacier during the middle Holocene, glacier activity initiated ~2500 years ago and reached peak activity over the last 1000 years.

Thin sections were made of the 34-cm-long surface core and of 23.5-78.5 cm in the piston core and were scanned at high resolution (1600 dpi). Varves were identified and counted in Adobe Illustrator and were measured three times each in ImageJ before being averaged into a composite varve thickness record. Error estimates on the varve chronology were generated by comparing varve counts by different individuals and comparing ~6 cm of overlap between the surface and piston cores. Varve thickness was compared to instrumental weather data from Clyde River (70 km away, 1946-present) and Ilulissat, Greenland (700 km away, 1840-present) and correlated best with summer temperature at both sites, particularly when both datasets were averaged over several years. The r² for annual comparison of varve thickness to Clyde River JAS temperature was 0.21. When both datasets were averaged in two-year intervals, the r² increased to 0.4. Annual comparison of varve thickness to Ilulissat JJA temperature yielded an r² of 0.06, which increased to 0.5 when the datasets were averaged in five-year intervals. We therefore believe that varve thickness is most strongly influenced by summer temperature and can be used to infer pre-instrumental summer temperatures.

Average (25-yr) varve thickness is greatest (>1 mm) in the 16th and 20th centuries (Fig. 2). The high average varve thickness of the 16th century is influenced by large interannual variability and thick (>3 mm) individual varves. On the other hand, the 20th century contains the highest average varve thickness in the record, in part due to an increase in minimum varve thickness (Fig. 2). The thinnest varves (~0.4 mm) with the least interannual variability occur from 1600-1750 AD. These data suggest that there was no manifestation of the Medieval Warm Period (~800-1200 AD; Jones and Mann 2004) on northeastern Baffin Island. The peak of the Little Ice Age and the coldest temperatures in this 1050-year-long record occurred from 1600-1750 AD.

A warming trend beginning ~1800 AD continued through the 20th century, corresponding with positive climate forcings that increased during this interval (IPCC 2007). The 20th century is unique in this record, indicating enhanced warmth during this time compared to the previous millennium. This is similar to other paleoclimate records in the northeastern Canadian Arctic, including the Agassiz and Devon Ice Cap melt-layer records (Fisher 1979; Fisher et al. 1995), the Upper Soper Lake varve thickness record (Hughen et al. 2000), and the Lake CF8 chironomid record (Thomas et al. 2008).