U-Th dating of lake sediments: Lessons from the 700 ka sediment record of Lake Junín, Peru. (15th September 2020)

• Record Type: Journal Article
• Authors: Chen, Christine Y.; McGee, David; Woods, Arielle; Pérez, Liseth; Hatfield, Robert G.; Edwards, R. Lawrence; Cheng, Hai; Valero-Garcés, Blas L.; Lehmann, Sophie B.; Stoner, Joseph S.; Schwalb, Antje; Tal, Irit; Seltzer, Geoffrey O.; Tapia, Pedro Miguel; Abbott, Mark B.; Rodbell, Donald T.
• Is Part Of: Quaternary science reviews. Volume 244(2020)
• Publication Date: 2020-09-15
• Subjects: Quaternary -- Paleoclimatology -- South America -- U-Th series -- Sedimentology -- Lakes lagoons & swamps -- Geochronology -- ICDP -- Age model; Geology, Stratigraphic -- Quaternary -- Periodicals; Stratigraphie -- Quaternaire -- Périodiques; 551.79
• DOI: 10.1016/j.quascirev.2020.106422
• Languages: English
• ISSNs: 0277-3791

Abstract

Abstract: Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has been often limited by poor age control. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools available for such sediment archives. We present our systematic approach to U-Th date carbonate-rich sediments from the ∼100 m drill core from Lake Junín, Peru. The results form the foundation of an age-depth model spanning ∼700 kyrs. High uranium concentrations (0.3–4 ppm) of these sediments allow us to date smaller amounts of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest factor limiting the success of previous U-Th dating efforts in other lake basins. Despite this advantage, the dates from 174 analyses on 55 bulk carbonate samples reveal significant scatter that cannot be resolved with traditional isochrons, suggesting that at least some of the sediments have not remained closed systems. To understand the source of noise in the geochronological data, we first apply threshold criteria that screen samples by their U/Th ratio, reproducibility, and δ 234 Uinitial value. We then compare these results with facies types, trace element concentrations, carbonate and total organic carbon content, color reflectance, mineralogy, and ostracode shell color to investigate the causes of open system behavior. Alongside simulations of the isotopic evolution of our samples, we find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. Examining U-Th data in these contexts, we identify samples that have likely experienced the least amount of alteration, and use dates from those samples as constraints for the age-depth model. Our work has several lessons for future attempts to U-Th date lake sediments, namely that geologic context is equally as important as the accuracy and precision of analytical measurements. In addition, we caution that significant geologic scatter may remain undetected if not for labor intensive tests of reproducibility achieved through replication. As a result of this work, the deep sediment core from Lake Junín is the only continuous record in the tropical Andes spanning multiple glacial cycles that is constrained entirely by independent radiometric dates. Highlights: An exploration of mechanisms driving scatter in 174 U-Th dates on lake sediments. Uranium remobilization, not detrital contamination, is greatest barrier to success. Single, unreplicated U-Th dates from lake sediments must be considered with caution. Geologic context is critical for interpretation of geochronological data. The 700 ka Lake Junín record is constrained entirely by independent radiometric dates.