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 ∼100m 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 δ234Uinitial 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.
Bibliographical noteFunding Information:
This research was supported by a grant from the International Continental Scientific Drilling Program (ICDP) and grants from the U.S. National Science Foundation (McGee, EAR-1404414 ; Rodbell, EAR-1402076 ; Abbott, EAR-1404113 ; Stoner, EAR-1400903 ) and the National Science Foundation of China (Cheng, #41888101 ). CYC was supported by an NSF Graduate Research Fellowship during the execution of this research. We are grateful to Lake Junín Drilling Project members for their contributions to fieldwork and data collection, the ICDP for financial and logistical support, the authorities of the Reserva Nacional Junín for their assistance, and DOSECC Exploration Services (USA) and GEOTEC (Peru) for drilling expertise. We thank LacCore for access to facilities, core curation, color reflectance data, and data management. We thank Doug Schnurrenberger, Amy Myrbo, Shane Loeffler, Darren Larsen, Mark Shapley, Bryan G. Valencia, Angela Rozas Davila, James Bartle, and Cecilia Oballe for their logistical assistance during drilling; Kristina Brady and Anders Noren for their work managing and coordinating this project; Liam Glennon for assisting with XRD analyses; Ben Hardt for providing lab assistance at MIT; and Sarah Katz and Naomi Levin for collecting modern water samples used for uranium isotope analysis. CYC also thanks resources for making graphics accessible to those with color vision deficiencies by Wong (2011) and Martin Krzywinski ( http://mkweb.bcgsc.ca/colorblind ); and P. Donut for companionship and support with revisions. We also gratefully acknowledge Jay Quade, Tim Lowenstein, Kristin Bergmann, C. Brenhin Keller, Adam Maloof, Noah Anderson, Marjorie Cantine, Adam Jost, Christopher Kinsley, Gabi Serrato Marks, Josh Murray, and Ruth Tweedy for reading the initial draft and providing helpful comments. We also gratefully acknowledge two anonymous reviewers for their constructive feedback, Ana Moreno for refereeing our manuscript, and Antje Voelker for the invitation to submit this paper to Quaternary Science Reviews.
- Age model
- Lakes lagoons & swamps
- South America
- U-Th series