Seasonally Distinct Reconstructions of Northern Alaskan Temperature Variability Since the Last Glacial Maximum

Abstract:

Paleoclimate reconstructions have provided little consensus on how continental temperatures in Eastern Beringia changed from the Last Glacial Maximum (LGM) to the present. Reconstructions show regional differences in LGM severity, the timing of deglacial warming, and Holocene temperature variability. Currently, arctic temperatures are increasing at the fastest rates on the planet, highlighting the need to identify the sensitivities of arctic systems to various climate forcings. This cannot be done without resolving the complex climate history of Eastern Beringia.

Here, we present two new organic geochemical temperature reconstructions from Lake E5, north central Alaska that span the LGM, last glacial termination and Holocene. The proxies (alkenones and brGDGTs) record seasonally distinct temperatures, allowing for the attribution of different forcings to each proxy. The alkenone-based U^K₃₇ reconstruction records spring/early summer lake temperatures and indicates a 4 ^oC abrupt warming at 13.1 ka and a relatively warm late Holocene, which peaks at 2.4 ka and exhibits a cooling trend from 2.4 to 0.1 ka. The brGDGT reconstruction is calibrated to mean annual air temperature and interpreted here as exhibiting a strong warm season bias. BrGDGTs show an abrupt 4.5 ^oC warming at 14 ka, and show evidence for an early Holocene Thermal Maximum (HTM), which cools by 3 ^oC after 8.4 ka.

Because U^K₃₇ temperatures do not exhibit an early HTM, we hypothesize that summer insolation had a minimal effect on spring/early summer lake temperatures. Instead, the U^K₃₇ reconstruction agrees with sea ice and sea surface temperature reconstructions from the Beaufort and Chukchi Seas and northeast Pacific Ocean. We hypothesize that forcings associated with sea ice concentration and changes in atmospheric circulation had stronger affects on spring/early summer lake temperatures and we present modern observational data in support of this hypothesis. By contrast, the summer-biased brGDGT reconstruction suggests a strong and relatively direct temperature response to summer insolation forcing. Together, these records suggest that both internal and external forcings significantly affected LGM to present temperature variability in Eastern Beringia, with different seasonal biases.