U-Th dating of lake sediments: Lessons from the 700 kyr sediment record of Lake Junín, Peru

Abstract

*----- NOTE: This is a non-peer reviewed preprint. The manuscript has been submitted as an invited research article to Quaternary Science Reviews and is under peer review. -----* Deep sediment cores from long-lived lake basins are fundamental records of paleoenvironmental history, but the power of these reconstructions has often been 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-long 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 quantities of material, giving us the opportunity to improve sample selection by avoiding detrital contamination, the greatest limiting factor to the success of previous U-Th dating efforts in other lake basins. The dates from 174 analyses on 55 bulk carbonate samples revealed significant scatter that could not 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 d234U initial 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. We find that the greatest impediment to U-Th dating of these sediments is not detrital contamination, but rather post-depositional remobilization of uranium. After 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 the sample's geologic context is equally important as the accuracy and precision of analytical measurements when determining the age of sample materials. 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. As such, this record is uniquely poised to offer new insights on past climate and environmental changes in the tropical Andes, complementing and testing the long but tuned sediment records from Sabana de Bogotá to the north (~5°S; Groot et al., 2011) and Lake Titicaca to the south (~16°S; Fritz et al., 2004, 2007).

Paleomagnetic Constraint of the Brunhes Age Sedimentary Record From Lake Junín, Peru

“…Proyecto Lago Junín (The Lake Junín Project) Lake Junín (11.03 • S, 76.11 • W) is a large (300 km 2 ), but shallow (<15 m water depth), semi-closed basin lake located 4,085 m above sea level in the Peruvian Andes (Seltzer et al, 2000;Rodbell et al, 2014) (Figure 1A). Regional moraine mapping and cosmogenic radionuclide dating indicate that paleoglaciers reached the lake edge, but have not overridden the lake in (Woods et al, 2019;Chen et al, 2020) with 2-sigma uncertainty (blue shading) and calculated mean sedimentation rate (black line). The 79 radiocarbon and 6 U-Th age constraints from the last 50 kyrs are not shown on the age-depth plot for clarity.…”

“…The uppermost sediments were resampled with a Livingstone push corer that better preserves softer, lessconsolidated materials, and a quasi-continuous stratigraphic splice (PLJ-1) was constructed for the upper ∼95 m composite depth (mcd) (Hatfield et al, 2020). Sediment composition alternates between the influx of detrital clastic sediments during glacial periods, accumulation of authigenic calcite during interstadial periods, and organic matter rich peat horizons during lake-level low stands that results in a heterogenous lithostratigraphy that reflects sensitivity to climatic and hydrological variations (Seltzer et al, 2000;Woods et al, 2019;Chen et al, 2020;Hatfield et al, 2020). Following completion of the field season all cores were shipped to the National Lacustrine Core Facility (LacCore) at the University of Minnesota for acquisition of physical property measurements, splitting, imaging, and long term curation.…”

“…However, these approaches are more frequently employed in marine settings than in sedimentologically complex lacustrine environments. Here, we make continuous u-channel based paleomagnetic measurements on sediments recovered during ICDP Drilling of Lake Junín, Peru to augment and refine the existing 14 C and U-Th radiometric-based age model (Woods et al, 2019;Chen et al, 2020). We then integrate all new and existing age-depth constraints and use the age-depth modeling software "Undatable" (Lougheed and Obrochta, 2019) to provide estimates of uncertainty alongside the new higher-density agemodel for the Lake Junín sediment record.…”

Chronostratigraphy of a 1.5±0.1 Ma composite sedimentary record from Colônia basin (SE Brazil): Bayesian modeling based on paleomagnetic, authigenic 10Be/9Be, radiocarbon and luminescence dating

“…The old age together with overall organic-rich and clayey sediments from Colônia basin do not permit using Uranium-series dating approach within the lacustrine unit (e.g. Chen et al, 2019). We did not attempt U-Th dating within the peatland unit (e.g.…”

“…These different settings imply that interpreting chronostratigraphic results from lake is rarely, or never, straightforward (e.g. Colman et al, 2006;Kadereit et al, 2012;Kliem et al, 2013;Shanahan et al, 2013;Nowaczyk et al, 2013;Roberts et al, 2018;Leicher et al, 2016;Chen et al, 2019;Wagner et al, 2019). In this study, we used several dating approaches, i.e.…”