Abbott, M.B., 1995, Holocene climatic variability for lake sites in the Bolivian Andes and interior Alaska based on sedimentology and radiocarbon dating by accelerator mass spectrometry: University of Minnesota, Minneapolis, Ph.D. dissertation, 117 p., illust., maps.
The objective of this dissertation is to use lake core studies to identify changes in the regional precipitation-evaporation balance at a century time-scale. The study locations were selected in opposite hemispheres as part of a long-term goal of determining if climatic changes occur synchronously in the Northern and Southern Hemispheres. In interior Alaska seismic profiles provided an effective tool for small-shallow lakes to identify transgression-regression sequences and erosion surfaces formed during lake-level changes. The acoustic stratigraphy of Birch Lake documents a closed-basin lake 15 m below its present water level prior to 12,300 14C yr BP. Between 12,300 and 10,000 14C yr BP, lake level rose rapidly to near its present level. The lake attained its overflow level by 8,780 +/-60 14C yr. Five periods of low lake-stands were identified during the past 3,500 Cal 14C yr. The earliest occurred during a prolonged mid-Holocene dry phase ending after 3500 Cal 14C yr BP. All of the cores collected in Lago Winaymarka for this study penetrate into previously exposed sediments, indicating Lake Titicaca was >15 m BOL prior to 3,500 Cal 14C yr BP. Four other low stands ended after 2,900, 2,200, 1,650, and 700 Cal 14C yr BP. The final low lake-stand is coincident with the decline of raised-field agriculture and collapse of Tiwanaku culture. The sediment records from Lago Taypi Chaka Kkota and Laguna Viscachani contain striking similarities in the pattern and timing of geochemical, biological, and sedimentological changes. Cirque glaciers disappeared between 9,700 and 8,900 14C yr BP. High accumulation rates of organic carbon and biogenic-silica occur from 9,700 to 5,400 14C yr indicate more favorable environmental conditions and higher lake productivity. Between 5,400 and 2,300 14C yr BP macrophytes are absent accumulation rates are very low. After 2,300 14C yr BP sediment characteristics become increasingly similar to the glacial sediments of the latest Pleistocene, indicating the return of cirque glaciers to watersheds with headwalls of 5,500 m a.s.l. with intense neoglaciation occurring after 1,400 14C yr BP.
Theses and Dissertations
