Northeastern Section - 54th Annual Meeting - 2019

Paper No. 5-2
Presentation Time: 8:25 AM


CHEN, Christine Y.1, WOODS, Arielle2, HATFIELD, Rob3, MCGEE, David1, EDWARDS, Lawrence4, VALERO-GARCES, Blas Lorenzo5, STONER, Joseph3, WEIDHAAS, Nicholas2, TAL, Irit1, TAPIA, Pedro6, BUSH, Mark7, ABBOTT, Mark B.2 and RODBELL, Donald T.6, (1)Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, (2)Department of Geology and Environmental Science, University of Pittsburgh, Pittsburgh, PA 15260, (3)College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, (4)Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, (5)Pyrenean Institute of Ecology, Spanish National Research Council, Avda Montañana 1005, apdo 13034, Zaragoza, 50080, Spain, (6)Geology Department, Union College, Schenectady, NY 12308, (7)Department of Biology, Florida Institute of Technology, Melboune, FL 32901

Paleoenvironmental interpretations of lacustrine sequences are often limited by our ability to develop accurate age-depth relationships. In this presentation, we discuss our efforts to uranium-thorium (U/Th) date sediments from the ~100-m-long sediment core extracted from Lake Junín, Perú, a record unprecedented amongst deep lacustrine records in its dependence on U/Th ages throughout its length.

The sediment core is characterized by alternating packages of glaciogenic siliciclastic sediment and authigenic carbonate marl. We applied U/Th dating to ∼180 bulk carbonate marl samples representing the array of carbonate sedimentological facies observed in the core. With this suite of analyses, we developed a framework for evaluating U/Th dates alongside corresponding sedimentological, elemental, and geochemical data to enhance our ability to make "age" interpretations from "dates" calculated from measurements. As a result of this framework, improved sample selection, and recent advances in analytical techniques, the Lake Junín record is the first continuous, U/Th-dated paleolake record spanning multiple glacial-interglacial cycles in the tropical Andes.

The carbonate marls have high uranium concentrations (0.3-4 ppm) and low detrital content, with ratios of radiogenic 230Th to initial 230Th that are 10-20 times greater than sediments from Lake Titicaca (Fritz et al., 2007) and the Great Salt Lake (Balch et al., 2005). These qualities allow us to date these sediments to within ±200-800 years in the Holocene and ±6000-8000 years between 280 and 400 kyr ago (2-sigma range in uncertainty).

We compare all of our U/Th analyses to corresponding radiocarbon dates, high-resolution X-ray fluorescence (XRF) data, elemental ICP-MS data, magnetic susceptibility measurements, sediment carbon data, and sedimentological facies descriptions to evaluate the robustness of a U/Th date for any given sample. Consequently, we also demonstrate the influence of detrital carbonate (eroded limestone bedrock) on U/Th dating.

When combined with radiocarbon ages and relative paleointensity picks, our geochronological results indicate that the Lake Junín record spans ∼7 glacial-interglacial cycles, complementing the long records from Sabana de Bogotá, Lake Titicaca, and local speleothems and making it well-positioned to yield critical insights on past climate changes in South America.