Late Quaternary climate variability and terrestrial carbon cycling in tropical South America
Fornace, Kyrstin L.
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Characterizing global and regional climate variability and climate-carbon cycle interactions in the past provides critical context for evaluating present and future climate trends. In this thesis, I use stable isotope and radiocarbon analysis of vascular plant biomarkers in lacustrine and marine sediment cores to explore late Quaternary climate variability and connections between past climate change and terrestrial carbon cycling in tropical South America. I investigate temporal and spatial trends in South American Summer Monsoon precipitation by reconstructing hydrologic variability over the past 50,000 years at two sites: the Lake Titicaca drainage basin in the Central Andes and the Pantanal wetlands in the interior lowlands. Diverging hydrologic trends at these two sites during the last glacial period suggest altered monsoon circulation patterns under glacial conditions, while changes in summer insolation appear to be an important control of precipitation at both sites during the Holocene. I next assess the relationship between climate change and the age structure of terrestrial biospheric carbon exported from two tropical catchments over the past 20,000 years. Radiocarbon dating of leaf waxes in Cariaco Basin and Lake Titicaca sediment records indicates that waxes preserved in sediments are likely composed of a fresh component transported to sediments within decades of production by vegetation and an old component derived from aged soil organic matter with an average age on the order of millennia at time of deposition. Results from both sites show that past hydrologic variability had a significant impact on the mobilization and export of different pools of terrestrial biospheric carbon. In particular, results from Cariaco Basin suggest that wetter conditions in the past resulted in increased export of fresh biospheric carbon to the ocean, representing a potentially important climate feedback mechanism on geologic timescales.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2016
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