In situ cosmogenic 10Be production rate in the High Tropical Andes

doi:10.1016/j.quageo.2015.06.012

Quaternary Geochronology

Volume 30, Part A, October 2015, Pages 54-68

https://doi.org/10.1016/j.quageo.2015.06.012 Get rights and content

Highlights

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Dating of a glaciogenic fan in the Cerro Azanaques massif (Bolivia, 18.91°S – 66.76°W – 3800 masl).
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Stratigraphic correlation and ¹⁴C dating yields a fan age of 16.07 ± 0.64 ka BP.
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Mean in situ-produced ¹⁰Be concentrations of fan boulders 4.92 ± 0.05 × 10⁵ at g⁻¹.
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Local (18.91°S – 66.76°W – 3800 masl) ¹⁰Be production rate is 30.8 ± 1.3 at g⁻¹ yr⁻¹ (SLHL P₁₀ = 3.76 ± 0.15 at g⁻¹ yr⁻¹ Lm).
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An internally-consistent reference calibration dataset combining three High Tropical Andes calibration studies is proposed.

Abstract

Continental climate change during the late glacial period has now been widely documented thanks to Cosmic-Ray Exposure (CRE) dating of glacial features. The accuracy of these CRE ages mainly relies on a priori knowledge of the production rate of the cosmogenic nuclide that has accumulated in a specific mineral. To produce unequivocal and accurate chronologies of glacier fluctuations during the late glacial period, it is crucial that the cosmogenic nuclide production rates are better constrained, particularly in the high tropics where existing spatial and temporal scaling models show significant discrepancies. Here we report a new production rate established at low latitude (19°S) and high elevation (3800 masl) on the Challapata fan-delta, at the edge of the Paleolake Tauca, on the flank of Cerro Azanaques (Bolivia). Sedimentological evidence for synchronicity with the Tauca Lake highstand along with U–Th and ¹⁴C measurements established that the fan-delta is 16.07 ± 0.64 kyr BP old. In situ-produced ¹⁰Be concentrations measured in 15 boulders lying on the fan-delta yield a mean ¹⁰Be concentration of 4.92 ± 0.05 × 10⁵ at g⁻¹. A local in situ ¹⁰Be production rate of 30.8 ± 1.3 at g⁻¹ yr⁻¹ is thus obtained at 3800 masl and 19°S. Application of the “Lal-modified” scaling scheme to this Azanaques production rate, using a standard atmosphere and the Muscheler et al. (2005) geomagnetic reconstruction, leads to a Sea Level High Latitude (SLHL) in situ ¹⁰Be production rate of 3.76 ± 0.15 at g⁻¹ yr⁻¹ (1σ uncertainty). In addition, we propose a reference in situ ¹⁰Be calibration dataset for the region that combines the production rates of this study with those of Blard et al. (2013b) and Kelly et al. (2015). This dataset of three calibration sites shows a good consistency and yields a regional in situ ¹⁰Be production rate of 3.74 ± 0.09 at g⁻¹ yr⁻¹ using the same scaling.

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 F_MinAge and F_MaxAge 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 F_fan-delta, which is the age probability density function of the Challapata fan-delta obtained from Equation (2). The distribution of F_fan-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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Citation Excerpt :
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¹: M. Arnold, G. Aumaître, D. L. Bourlès, K. Keddadouche.

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Research paperIn situ cosmogenic 10Be production rate in the High Tropical Andes

Highlights

Abstract

Introduction

Section snippets

The Altiplano and the Paleolake Tauca shorelines

Absolute dating of the Challapata fan-delta

Age of the Challapata fan-delta

Accuracy of the Azanaques 10Be production rate

Conclusion

Acknowledgement

Quat. Geochronol.

Quat. Geochronol.

Quat. Sci. Rev.

Earth Planet. Sci. Lett.

Quat. Res.

Earth Planet. Sci. Lett.

Quat. Sci. Rev.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms

Quat.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Quat. Geochronol.

Quat. Sci. Rev.

Earth Planet. Sci. Lett.

Quat. Geochronol.

Earth Planet. Sci. Lett.

Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms

Earth Planet Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet Sci. Lett.

Quat. Int.

Quat. Sci. Rev.

Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms

Quat. Geochronol.

Research paper
In situ cosmogenic ¹⁰Be production rate in the High Tropical Andes

Accuracy of the Azanaques ¹⁰Be production rate