Younger Dryas Climate in South Greenland Inferred from Chironomid and Moss Chemistry at Lake N14

Abstract:

The Younger Dryas (YD) was an iconic abrupt climate change during the transition from the last ice age, characterized by a centuries-long climate reversal to cool and dry conditions in many regions. However, the extent and seasonality of climatic change during this time are unclear and spatially variable, and it has long been hypothesized that YD climatic change was highly seasonal and indeed minimal over Greenland during summer (Bjorck et al., 2002, Geology, 30(5), 427-430; Denton et al., 2005, Quaternary Science Reviews, 24(10-11), 1159-1182). New records, using established and novel proxy measurements, are needed to address these issues.

We collected sediment cores from Lake N14 in South Greenland, where previous work documented the only YD aged sediment record reported from Greenland (Bjorck et al., 2002). Radiocarbon dating of plant macrofossils from our sediment cores confirm preservation of YD sediment, supporting evidence for early deglaciation and isostatic uplift of this portion of Greenland’s coast. Past measurements from this lake, including diatoms, pollen, and biogenic silica, suggest that Lake N14 experienced relativity warm summers and cold winters during the YD (Bjorck et al., 2002). A proposed mechanism for this enhanced seasonality is high Northern Hemisphere summer insolation, combined with extended winter sea ice cover promoted by freshwater inputs to the North Atlantic and slowed AMOC. However, there is conflicting evidence regarding the extent of enhanced seasonality and there is no quantitative terrestrial temperature record from Greenland beyond the ice cores.

Here we report progress on developing temperature reconstructions (from ~13-10 ka) from oxygen isotope measurements on chironomid head capsules and from chironomid assemblage data. Additionally, we investigate oxygen and hydrogen stable isotope measurements on aquatic mosses. Combined, these measurements will allow us to improve our understanding of the magnitude and seasonality of temperature changes through the YD at Lake N14 – and to directly test the hypothesis that YD cooling over Greenland occurred primarily in winter. With this information, we can better infer which mechanisms drove variations in YD climate, provide usable data for global temperature syntheses, and inform climate models.