A multi-proxy reconstruction of temperature and aridity from lake sediments north of the Brooks Range, Alaska for the past 30,000 years

Abstract:

Paleoclimate reconstructions from the northern Alaska tundra since the LGM have mainly relied on paleoecological proxies such as pollen and chironomids. However, lack of trees, non-analogue assemblages, and limited species diversity present significant challenges to quantitative reconstructions. The abnormally warm LGM conditions implied by pollen assemblages, supported by the CCSM3 but contradicted by CCSM4 simulations, have been a source of great confusion. Here we present a synthesis of results from a NSF-funded project, aiming at utilizing a full suite of organic geochemical and paleolimnological proxies, in conjunction with model comparison, to quantitatively reconstruct temperature and moisture changes since the LGM. Compound specific hydrogen isotopic ratios of plant leaf waxes from Lake E5 near Toolik Lake suggest summer temperature at the LGM is ~ 7 degrees colder than the early Holocene and there is a temperature decline of 2-3 degrees over the Holocene, coherent with greenhouse and insolation forcings respectively. A similar LGM to present temperature gradient is found in the alkenone-inferred early summer temperatures, although an expanded Arctic sea ice in the early Holocene significantly suppressed this temperature. The LGM to present temperature gradient is further supported by temperatures inferred by br-GDGTs from our sediment core, which also display an early Holocene to present temperature decline of 2 to 3 degrees. Our paleotemperature data are supported by model results using CCSM4, with its use of newer generation ice sheet configuration, indicating lower LGM temperatures than present. Our paleohydrological reconstructions based on hydrogen isotopic contrast between inferred precipitation and lake water isotopic ratios, and carbon isotopic fractionation of terrestrial plant leaf waxes, suggesting an arid last glacial relative to the Holocene. This presentation will also summarize all geochemical measurements including TOC, bulk C and N isotopic ratios, terrestrial and aquatic biomarker fluxes (with implication for terrestrial and aquatic productivities) from Lake E5. We will also discuss the cause for an unusual observation that the Bølling-Allerød event is highly muted in the study region relative to other Arctic regions, such as Greenland.