Pronounced summer warming in northwest Greenland during the Holocene and Last Interglacial

Abstract

Projections of future rates of mass loss from the Greenland Ice Sheet are highly uncertain because its sensitivity to warming is unclear. Geologic reconstructions of Quaternary interglacials can illustrate how the ice sheet responded during past warm periods, providing insights into ice sheet behavior and important tests for data-model comparisons. However, paleoclimate records from Greenland are limited: Early Holocene peak warmth has been quantified at only a few sites, and terrestrial sedimentary records of prior interglacials are exceptionally rare due to glacial erosion during the last glacial period. Here, we discuss findings from a lacustrine archive that records both the Holocene and the Last Interglacial (LIG) from Greenland, allowing for direct comparison between two interglacials. Sedimentary chironomid assemblages indicate peak July temperatures 4.0 to 7.0 °C warmer than modern during the Early Holocene maximum in summer insolation. Chaoborus and chironomids in LIG sediments indicate July temperatures at least 5.5 to 8.5 °C warmer than modern. These estimates indicate pronounced warming in northwest Greenland during both interglacials. This helps explain dramatic ice sheet thinning at Camp Century in northwest Greenland during the Early Holocene and, for the LIG, aligns with controversial estimates of Eemian warming from ice core data retrieved in northern Greenland. Converging geologic evidence for strong LIG warming is challenging to reconcile with inferred Greenland Ice Sheet extent during the LIG, and the two appear incompatible in many models of ice sheet evolution. An increase in LIG snowfall could help resolve this problem, pointing to the need for hydroclimate reconstructions from the region.

Keywords: Eemian; Greenland; Holocene thermal maximum; Last Interglacial; paleotemperature.

Fig. 1.

(A) Location of WLL (yellow star) relative to modern July 10 °C isotherm (solid black line) and boreal tree line (dashed black line) (58). Lakes in two North American midge training sets (35, 41) with (red circles) and without (blue circles) Chaoborus in modern sediments. (B) Percentages of Chaoborus in modern sediments (red circles) vs. WLL LIG assemblages (yellow circle). Circles are scaled to represent percent of midge assemblage in each lake, with northernmost red circle <1%, largest red circle 13.5%, and yellow circle 6% Chaoborus. Locations of ice core archives (black diamonds) and lacustrine archives (black squares) are discussed in the text. A, Agassiz; C, Camp Century; D, Dye 3; G, GISP2; J, Jamesonland; N, NGRIP; Nm, NEEM; R, Renland; T, Thule; 1, Last Chance Lake; 2, Lake CF8; 3, Lake CF3; 4, Fog Lake; 5, Brother of Fog Lake; 6, Amorak Lake.

Fig. 2.

Stratigraphy of three WLL cores, with calibrated ¹⁴C ages (cal ky BP), density (Den.), and magnetic susceptibility (MS). *, locations of nonfinite ¹⁴C samples.

Fig. 3.

Midge-inferred July air temperature at WLL as (A) anomalies relative to modern and (B) absolute temperatures. Yellow line marks anomaly of +7.0 °C. Gray points indicate samples from discrete glaciolacustrine units where midge assemblages may not accurately reflect air temperatures (SI Appendix). Error bars are model root mean square error of prediction ±1.7 °C. (C) Percent of each midge type in subfossil assemblage, with colors corresponding to taxon names in D and modern optima and tolerances of taxa in the Francis et al. (35) training set, as shown in E. The apex of each diamond in E indicates optimum while length of diamond indicates tolerance.