A 15,000-year reconstruction of paleolimnological change through diatom assemblage biostratigraphy of Greyling Lake, Chugach Range, South-Central Alaska

This thesis presents a 15 ka record of Holocene paleolimnological change from Greyling Lake (61.4° N, 145.7° W, 1015 m asl), a proglacial lake in the Chugach Mountains of south-central Alaska. Fluctuations in paleolimnological conditions were reconstructed through the analysis of fossil diatom assemblages preserved within a sediment core taken from the central lake basin. The composition of diatom communities, together with physical and chemical proxies (magnetic susceptibility, bulk density, grain size, and organic matter), were used in the reconstruction. As a proglacial lake, Greyling Lake provides a record of paleoclimate change in southern Alaska through the erosive characteristics of nearby glaciers. Changes in the erosive input of the glacier cause responses in the biological, physical, and chemical properties.;A 3.5 m sediment core retrieved from Greyling Lake, dated through AMS 14C methods, and previously examined for sedimentological proxies was analyzed in this thesis using diatom biostratigraphy. The diatom assemblage record shows large variations in the community dynamics indicating significant paleolimnological changes during the Holocene. Several layers of the core are bereft of diatom valves (15,000--14,000; 2,250--1,160 cal. yr BP) indicating conditions not suitable for aquatic diatom productivity. Other layers (14,000--12,500; 3,250--2,250; 500 cal. yr BP--present) show very low diatom concentrations that also indicate low productivity. These low productivity signals correspond to Holocene climate events such as the Onset of Neoglaciation around 4,000 cal. yr BP, the First Millennial Advance cooling period (FMA) around 2,000 cal. yr BP, and the Little Ice Age (LIA) from 500--200 cal. yr BP. The use of a regional diatom-based transfer function allows us to compare fossil diatom taxa from Greyling Lake along environmental gradients from modern samples to indicate paleolimnological changes in depth, conductivity, and turbidity that are possibly linked to paleoclimate change.;These combined paleolimnological and paleoclimatological inferences allow us to reconstruct that conditions from the late Pleistocene-Early Holocene (12,000 cal. yr BP) through to 4,000 cal. yr BP were sufficient for a wide variety of diatom taxa to exist, with sufficient depth for multiple niches of benthic and planktonic species to co-exist. This could possibly give indication of a Holocene Thermal Maximum (HTM) signal. Diatom concentrations decrease after 4,000 cal. yr BP which could be linked to the Onset of Neoglacation and a return of extensive ice cover. As diatoms reappear in the past 1,000 years, different assemblages with a dominance of planktonic taxa indicate much deeper lake level. This is consistent with recent climate warming as indicated by the instrumental record of the 20th century.