Research paperIn situ cosmogenic 10Be production rate in the High Tropical Andes
Introduction
The growing number of studies based on terrestrial cosmogenic nuclides evidence their importance in modern Earth Surface Sciences. This expanding interest can be explained by the wide fields of application of cosmogenic nuclides, including Cosmic-Ray Exposure (CRE) dating, and estimation of burial ages and denudation rates. CRE dating of glacial landscapes has significantly improved our knowledge of glacial chronologies and has provided tight constraints on the evolution of the continental climate since the Last Glacial Maximum in many settings (e.g. Gosse et al., 1995, Barrows et al., 2011, Blard et al., 2007, Jomelli et al., 2014, Licciardi et al., 2009, Smith et al., 2005). This is of particular interest for the tropical Andes since this region is thought to play a key role in the dynamics of the Atlantic Meridional Overturning Circulation (Leduc et al., 2007). The determination of a CRE age relies on a priori knowledge of the production rate of the measured cosmogenic nuclide in a specific mineral. This production rate depends on spatial parameters, namely the latitude, altitude and depth of the sample, and also varies with time due to temporal fluctuations in the Earth's magnetic field. Several scaling models have therefore been proposed for converting Sea Level High Latitude (SLHL) production rates into production rates for different sampling locations (Desilets et al., 2006, Dunai, 2001, Lal, 1991, Lifton et al., 2005, Stone, 2000). However, discrepancies between the different proposed models are often difficult to assess and biases are introduced that ultimately affect the accuracy of the CRE dating. Even though Lifton et al. (2014) have proposed a new scaling model that aims to reconcile the different published models, the use of production rates calibrated from independently-dated geomorphological surfaces remains necessary. An effective way to overcome scaling discrepancies is to use a “local” calibration site; “local” meaning close, both in space (a few hundreds of km horizontally and less than 1000 m in elevation) and time (a few ka for Pleistocene moraines), to the studied site. In this case, scaling from the Sea Level High Latitude (SLHL) production rate to the local object to be dated is symmetric to the scaling originally used to convert the local production rate to a SLHL production rate (Fig. 1) (Balco et al., 2008, Licciardi et al., 2009). This dramatically reduces the impact of the scaling model on the CRE age.
In this study, we present a new 10Be production rate from a calibration site located at 18.91°S - 66.76°W and 3800 m asl in the tropical Andes, on the Bolivian Altiplano. The motivation for this work is twofold: first, to enrich the worldwide database of production rates and thus improve the accuracy of CRE dating methods, and second, to provide a new local calibration site to chronologically anchor palaeoclimatic and geomorphologic studies in the central Andes. The calibration feature is the Challapata fan-delta, a glaciogenic fan that shows sedimentological evidence for synchronicity between its deposition and the highest level reached by the Paleolake Tauca (Clapperton et al., 1997, Clayton and Clapperton, 1997). Sedimentological evidence and 14C ages constrain this synchronicity and enable emplacement of the fan-delta to be dated. The local in situ 10Be production rate is then derived from measurement of the 10Be concentrations of 15 boulders embedded in the fan. This calibrated local in situ 10Be production rate is then scaled to the Sea Level High Latitude conditions and compared with an updated and homogenized dataset of regional production rates (Blard et al., 2013a; Kelly et al., 2015). The dataset exhibits good internal consistency and can therefore be taken as a reference calibration dataset for in situ 10Be production rates in the High Tropical Andes.
Section snippets
The Altiplano and the Paleolake Tauca shorelines
The Altiplano is a wide intermontane plateau, covering an area of 196 000 km2 and delimited by the eastern and the western Andes cordilleras (Fig. 2). Latitudinally, it spans from 15.5°S (Peru) to 22.5°S (Bolivia), and it ranges in elevation from 3658 masl in the middle of the plateau to 6542 masl at Sajama volcano. Due to its configuration, the Altiplano is an endorheic basin, which is today dry but was covered by large paleolakes (>50 000 km2) during the wettest periods of the Quaternary (
Absolute dating of the Challapata fan-delta
We determined the age of the fan-delta using the minimum and the maximum age limits presented in Section 2.3. Since we established that the 3770-masl Lake Tauca level is synchronous with or younger than the fan-delta build-up, we used the absolute dating of the so-called ‘S1 shoreline’ reported in Blard et al. (2013b) to determine the minimum age (Section 2.3). Four carbonate samples from S1 have been dated with radiocarbon-dating and two of these have also been dated with the U–Th method (
Age of the Challapata fan-delta
Fig. 5 displays the results of the calculation presented in Section 3.1. Graph (A) shows the FMinAge and FMaxAge probability density functions derived from the S1 shoreline and the 14C peat dating, respectively. The two distributions are multimodal and overlap at around 16 ka. Graph (B) presents Ffan-delta, which is the age probability density function of the Challapata fan-delta obtained from Equation (2). The distribution of Ffan-delta is bimodal and the highest mode is centred on 15.83 ka
Accuracy of the Azanaques 10Be production rate
The accuracy of the Azanaques production rate relies both on the determination of the 10Be concentration of the fan-delta and on the fan-delta dating.
The minimum age used to constrain the age of the Challapata fan-delta was not obtained from direct dating of the fan itself but was instead derived from stratigraphic correlation with the highest level reached by the Lake Tauca. Because the Gilbert delta facies belongs to the distal part of the Challapata fan-delta, a hypothesis of progressive
Conclusion
We reported a new production rate for 10Be in quartz in the High Tropical Andes (18.91°S – 66.76°W – 3800m). The new calibration site, the Challapata fan-delta, is located on the Cerro Azanaques massif. The site is a glaciogenic fan that evolves into a Gilbert type delta in its distal part, at the edge of the Paleolake Tauca. The occurrence of delta type facies at the highest altitudes reached by Lake Tauca has enabled us to determine the age of the fan-delta (16.07 ± 0.64 ka, 1σ) using robust
Acknowledgement
This work was funded by the INSU EVE-LEFE program and the ANR Jeunes Chercheurs GALAC project “ANR-11-JS56-011-01”. We greatly appreciated the logistical support of the IRD of La Paz (Bolivia) during our field trips conducted between 2010 and 2013. R. Hemelsdaël, L. Ménabréaz and N. Lifton are thanked for useful discussions about Gilbert delta formation, paleomagnetic reconstructions and the LSD model, respectively. R. Joussemet and the STEVAL crew (GeoRessources, Nancy) are acknowledged for
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2020, Quaternary Science ReviewsCitation Excerpt :Ages were calculated with the assumption of zero surface erosion of boulders with a density of 2.7 g cm−3. We used the reference sea level high latitude (SLHL) 10Be-production rate of 3.74 ± 0.09 atoms g−1 yr−1 reported for the high-elevation Central Andes (Martin et al., 2015). We further used the Lal/Stone time-corrected scaling scheme (Lal, 1991; Stone, 2000; Balco et al., 2008), the standard atmosphere model (National Oceanic and Atmospheric Administration, 1976), and the atmospheric 10Be-based VDM geomagnetic database of Muscheler et al. (2005) and Valet et al. (2005).
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