Late Pleistocene glaciation in the Central Andes: Temperature versus humidity control A case study from the eastern Bolivian Andes (17°S) and regional synthesis

A glacier–climate model was used to calculate climatic conditions in a test site on the east Andean slope around Cochabamba (17°S, Bolivia) for the time of the maximum Late Pleistocene glaciation. Results suggest a massive temperature reduction of about − 6.4 °C (+ 1.4/− 1.3 °C), combined with annual precipitation rates of about 1100 mm (+ 570 mm/− 280 mm). This implies no major change in annual precipitation compared with today. Summer precipitation was the source for the humidity in the past, as is the case today. This climate scenario argues for a maximum advance of the paleo-glaciers in the eastern cordillera during the global Last Glacial Maximum (LGM, 20 ka BP), which is confirmed by exposure age dates. In a synthesized view over the central Andes, the results point to an increased summer precipitation-driven Late Glacial (15–10 ka BP) maximum advance in the western part of the Altiplano (18°S–23°S), a temperature-driven maximum advance during full glacial times (LGM) in the eastern cordillera, and a pre- and post-LGM (32 ka BP/14 ka BP) maximum advance around 30°S related to increased precipitation and reduced temperature on the western slope of the Andes. The results indicate the importance of understanding the seasonality and details of the mass balance–climate interaction in order to disentangle drivers for the observed regionally asynchronous past glaciations in the central Andes.

COMPARISON OF NON-HEATING PALEOINTENSITY TECHNIQUES USING BASALTS FROM LEMPTÉGY VOLCANO, FRANCE AND SYNTHETIC MAGNETITE-BEARING SAMPLES

Data of the strength of Earth’s magnetic field (paleointensity) in the geological past are crucial for understanding the geodynamo. Conventional paleointensity determination methods require heating a sample to a high temperature in one or more steps. Consequently, many rocks are unsuitable for these methods due to a heating-induced experimental alteration. Alternative non-heating paleointensity methods are investigated to assess their effectiveness and reliability using both natural samples from Lemptégy Volcano, France, and synthetic samples. Paleointensity was measured from the natural and synthetic samples using the Pseudo-Thellier, ARM, REM, REMc, REM’, and Preisach methods. For the natural samples, only the Pseudo-Thellier method was able to produce a reasonable paleointensity estimate consistent with previous paleointensity data. The synthetic samples yielded more successful estimates using all the methods, with the Pseudo-Thellier and ARM methods producing the most accurate results. The Pseudo-Thellier method appears to be the best alternative to the heating-based paleointensity methods.

Foraminifera from the Colorado Shale of North-Central Montana

The use of foraminifera in the determination of geologic age, and in the correlation of strata, is one of the most important techniques in oil field stratigraphic studies. The petroleum industry in many regions relies on these microscopic life forms to determine the positions of oil-bearing horizons and to determine the tops of beds. In northern Montana the Colorado group of strata, a series of about 2,000 feet of dense, dark similar shales, is known to contain foraminifers.

Age modeling of young non-varved lake sediments: methods and limits. Examples from two lakes in Central Chile

High-resolution and highly precise age models for recent lake sediments (last 100–150 years) are essential for quantitative paleoclimate research. These are particularly important for sedimentological and geochemical proxies, where transfer functions cannot be established and calibration must be based upon the relation of sedimentary records to instrumental data. High-precision dating for the calibration period is most critical as it determines directly the quality of the calibration statistics. Here, as an example, we compare radionuclide age models obtained on two high-elevation glacial lakes in the Central Chilean Andes (Laguna Negra: 33°38′S/70°08′W, 2,680 m a.s.l. and Laguna El Ocho: 34°02′S/70°19′W, 3,250 m a.s.l.). We show the different numerical models that produce accurate age-depth chronologies based on 210Pb profiles, and we explain how to obtain reduced age-error bars at the bottom part of the profiles, i.e., typically around the end of the 19th century. In order to constrain the age models, we propose a method with five steps: (i) sampling at irregularly-spaced intervals for 226Ra, 210Pb and 137Cs depending on the stratigraphy and microfacies, (ii) a systematic comparison of numerical models for the calculation of 210Pb-based age models: constant flux constant sedimentation (CFCS), constant initial concentration (CIC), constant rate of supply (CRS) and sediment isotope tomography (SIT), (iii) numerical constraining of the CRS and SIT models with the 137Cs chronomarker of AD 1964 and, (iv) step-wise cross-validation with independent diagnostic environmental stratigraphic markers of known age (e.g., volcanic ash layer, historical flood and earthquakes). In both examples, we also use airborne pollutants such as spheroidal carbonaceous particles (reflecting the history of fossil fuel emissions), excess atmospheric Cu deposition (reflecting the production history of a large local Cu mine), and turbidites related to historical earthquakes. Our results show that the SIT model constrained with the 137Cs AD 1964 peak performs best over the entire chronological profile (last 100–150 years) and yields the smallest standard deviations for the sediment ages. Such precision is critical for the calibration statistics, and ultimately, for the quality of the quantitative paleoclimate reconstruction. The systematic comparison of CRS and SIT models also helps to validate the robustness of the chronologies in different sections of the profile. Although surprisingly poorly known and under-explored in paleolimnological research, the SIT model has a great potential in paleoclimatological reconstructions based on lake sediments