CRONUS-Earth cosmogenic 36Cl calibration

doi:10.1016/j.quageo.2015.10.002

Quaternary Geochronology

Volume 31, February 2016, Pages 199-219

https://doi.org/10.1016/j.quageo.2015.10.002 Get rights and content

Highlights

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CRONUS-Earth production rates for ³⁶Cl, including low-energy production.
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Results presented for 7 scaling methods, including Lifton et al. (2014).
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New empirical method to calculate production rate uncertainties.
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Some unresolved variability, but parameters fit validation dataset well.

Abstract

Chlorine-36 production rates obtained from different geological calibration studies (e.g. Evans et al., 1997; Phillips et al., 2001; Schimmelpfennig et al., 2011; Stone et al., 1996; Swanson and Caffee, 2001) vary significantly, principally because of the many reactions contributing to the production of this nuclide. The CRONUS-Earth Project has provided high-quality geological calibration sites, including Lake Bonneville, Peru, and Scotland, for a large-scale calibration of ³⁶Cl production rates. Three sites were used to calibrate the K and Ca spallation pathways for ³⁶Cl production yielding production rates of 56.0 ± 4.1 at ³⁶Cl (g Ca)⁻¹ yr⁻¹ and 155 ± 11 at ³⁶Cl (g K)⁻¹ yr⁻¹ respectively, using Lifton-Sato-Dunai scaling (LSDn). The low-energy production parameter, P_f(0), was calibrated separately using CRONUS-Earth data from the Bonneville and Baboon Lakes sites where Cl concentrations were higher, and yielded a value of 759 ± 180 neutrons (g air)⁻¹ yr⁻¹. There is significant uncertainty associated with this pathway due to the sensitivity of this reaction to environmental conditions. The uncertainties associated with the calibrated production parameters were estimated based on the variance of calculated ages from independent ages for an independent secondary dataset.

Introduction

A premier use of cosmic-ray-produced (cosmogenic) nuclides is the determination of exposure ages and erosion rates, both of which require precise knowledge of production rates. The CRONUS-Earth (Cosmic-Ray prOduced NUclide Systematics on Earth) Project was funded to improve our knowledge of cosmogenic nuclide systematics. A significant outcome of the CRONUS-Earth Project was the calibration of terrestrial cosmogenic nuclide production rates using geological calibration locations. This paper describes the CRONUS-Earth geological calibration effort for ³⁶Cl.

Of the most commonly measured cosmogenic radionuclides – ¹⁰Be, ²⁶Al, and ³⁶Cl – the ³⁶Cl production rate is the most difficult to calibrate owing to the many production reactions that must be accounted for – spallation, low-energy neutron absorption, and muon reactions – and the numerous possible target elements: K, Ca, Cl, Ti, and Fe. Table 1 presents a summary of 20 years of calibration studies. There are glaring discrepancies and many mechanisms for these discrepancies have been proposed: problematic independent ages and/or erosion (Schimmelpfennig et al., 2014, Swanson and Caffee, 2001); incorrect scaling models (Swanson and Caffee, 2001); improper partitioning of reaction mechanisms (Alfimov and Ivy-Ochs, 2009, Schimmelpfennig et al., 2009); and sensitivity to hydrogen (Dunai et al., 2014, Licciardi et al., 2008, Swanson and Caffee, 2001).

The calibrated nuclear reactions (and production parameters) considered in this study are: spallation production from Ca (P_s,Ca) and K (P_s,K); muon production from Ca (σ_0,Ca and $f_{C a}^{*}$ ) and K (σ_0,K and $f_{K}^{*}$ ); and production from low-energy neutron absorption by ³⁵Cl (P_f(0)). The calibrated production rate parameters are discussed in terms of previous calibrations and are validated against a secondary dataset, with extensive discussion regarding the uncertainties, limitations, and sensitivity of sample ages to various inputs.

Section snippets

Scaling models

Scaling models allow site-specific production rates to be extended to other geographic locations or to other times. Numerous scaling models have been proposed to correct for latitude and longitude, elevation, atmospheric pressure anomalies, geomagnetic field changes, dipole and non-dipole effects, and solar modulation. Production rates based on geological calibrations are completely dependent on the scaling model used to produce the results. Previous ³⁶Cl production rate calibrations used the

Calibration sites

The CRONUS-Earth sampling sites are classified as either ‘primary’ or ‘secondary’ calibration sites. Primary sites have robust independent age control, continuous exposure to cosmic rays, simple geometry, and minimal or no erosion. Sites that did not meet the strict primary site guidelines were labelled ‘secondary’ sites. Additional high-quality sites from the literature, mostly previous calibration efforts, were included in the secondary dataset as well. These secondary calibration datasets

Laboratory methods

The samples used in this study were processed by four different labs and then measured at two different Accelerator Mass Spectromety (AMS) facilities. The four processing labs were University of Washington (UW), New Mexico Tech (NMT), Dalhousie University (DAL), and PRIME Lab (PRIME). In general, the procedures follow the same steps for sample preparation, but with slightly varying lab-specific details. After any necessary mineral separation (typically performed using heavy liquids), the

Spallation and low-energy production rates

The final spallation and low-energy production rates from this study are shown in Table 3. Small differences between the spallation production parameters from Borchers et al. (2015) and the values reported here are because of the inclusion of P_f(0) in the calibration in this study, whereas it was a fixed parameter in Borchers et al. (2015), as well as a small correction to the attenuation lengths used for the samples. The differences between the spallation parameters in the two studies range

Comparison to previously published production rates

The production rates for this study for all three main pathways are plotted in Fig. 5 and compared to those from the previous studies listed in Table 1. Note that the St (and not LSDn) production rates are plotted so as to be consistent with previous calibrations. The K production rate produced by this study (150 ± 15 at ³⁶Cl (g K)⁻¹ yr⁻¹) agrees with the recent production rate study by Schimmelpfennig et al. (2014) of 148 ± 8 at ³⁶Cl (g K) ⁻¹ yr⁻¹ as well as those of Phillips et al. (2001) and

Conclusions

This study completes the calibration presented in Borchers et al. (2015) by including the muon reparameterization, calibration of P_f(0), and determination of the uncertainties associated with the production-rate parameters, thus providing a single, coherent set of production-rate parameters for ³⁶Cl from the CRONUS-Earth dataset. Based on the increasing body of evidence that variations in water content can cause significant variations in the production rates of ³⁶Cl from low-energy reaction

Statement of contributions

Shasta Marrero participated in field sampling and sample preparation and analysis of samples from Tabernacle Hill, Scotland, and Copper Canyon, sample preparation of the Quelccaya samples, co-wrote the data-reduction software, and performed the actual calibration. Fred Phillips oversaw the project, participated in sampling at Tabernacle Hill, Scotland, and Copper Canyon, and led the sampling effort at Quelccaya. Marc Caffee participated in sampling at Tabernacle Hill and Scotland and prepared

Acknowledgements

This research was supported by the National Science Foundation through the Cosmic-Ray-Produced Nuclides on Earth (CRONUS-Earth) Project, through grant EAR-0345949. Thanks to Guang Yang, who performed the chemistry at Dalhousie Geochronology Centre, with funding from NSERC-DG and NSERC-MRS to JCG, and support from the NSF-CRONUS-Earth grant to FP. MWC would like to acknowledge support from NSF awards EAR-1153689 and EAR-0345820. Thanks to John Stone for the mineral separate data contributed for

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Research paperCRONUS-Earth cosmogenic 36Cl calibration

Highlights

Abstract

Introduction

Section snippets

Scaling models

Calibration sites

Laboratory methods

Spallation and low-energy production rates

Comparison to previously published production rates

Conclusions

Statement of contributions

Acknowledgements

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Research paper
CRONUS-Earth cosmogenic ³⁶Cl calibration