In situ 10Be production-rate calibration from a 14C-dated late-glacial moraine belt in Rannoch Moor, central Scottish Highlands
Introduction
Knowing the rates at which cosmogenic nuclides are produced in situ beneath exposed rock surfaces is essential for the calculation of surface-exposure ages and erosion rates used in studies of landform chronologies and Earth-surface processes. A challenge remains to improve the precision and accuracy of cosmogenic nuclide production rates for the purpose of developing more accurate surface-exposure chronologies. A leading approach has been to determine production rates empirically by targeting geological calibration sites in which cosmogenic nuclide concentrations can be measured in situ from rock surfaces associated with landforms of independently known age (e.g., Balco et al., 2009; Balco et al., 2008; Borchers et al., 2016; Goehring et al., 2010). Empirically determined site-specific production rates are then scaled to other locations using models that account for spatial changes in nuclide production with atmospheric pressure, geomagnetic latitude (Lal, 1991; Stone, 2000), and, in some cases, temporal variations in the strength of Earth's magnetic field and solar wind (Balco et al., 2008; Borchers et al., 2016; Lifton et al., 2005, 2008, 2014; Pigati and Lifton, 2004). Production rates have been conventionally scaled to a nominal value at sea-level and high latitude (SLHL) in order to facilitate comparison among calibration sites in disparate locations (Balco, 2011; Balco et al., 2008, 2009; Borchers et al., 2016; Goehring et al., 2010; Kaplan et al., 2011; Putnam et al., 2010b). A community-wide effort devoted to developing a network of geological calibration sites, distributed across diverse latitudes and altitudes, has improved understanding of cosmogenic nuclide production rates on a global basis and has in turn helped to hone scaling methods (Balco et al., 2008; Borchers et al., 2016; Heyman, 2014; Phillips, 2015; Phillips et al., 2016).
There is particular interest in constraining in situ 10Be spallation production rates and scaling protocols. Because of the comparatively uncomplicated production systematics of this relatively long-lived nuclide [e.g., half-life = 1.4 Myrs (Chmeleff et al., 2010; Korschinek et al., 2009; Nishiizumi et al., 2007)] in the abundant mineral quartz, 10Be has become a commonly used geochronological tool. Improvements in the precision of 10Be analyses has led to transformational progress in the development of landform chronologies (Balco, 2011). Challenges remain, however, especially as answers to emerging scientific questions demand ever-greater chronological accuracy. For example, dispersion among existing SLHL 10Be production-rate estimates indicates remaining uncertainties attending the geological calibration sites themselves and lingering imperfections in scaling models (Borchers et al., 2016; Phillips et al., 2016). This dispersion serves as a source of systematic uncertainty for landform chronologies, especially for regions with no nearby calibration sites. Furthermore, of the available published geological 10Be calibration sites, relatively few are anchored by landforms underpinned directly, at the site, by absolute chronologies. Many sites instead depend upon indirect associations among target landforms and other distal paleoclimatic/stratigraphic signatures (e.g., Ballantyne and Stone, 2012; Borchers et al., 2016; Goehring et al., 2012; Small and Fabel, 2015; Stroeven et al., 2015). Any incorrect assumptions incorporated into production-rates calibrated in this way could accidentally mislead attempts at evaluating and improving scaling protocols (Phillips et al., 2016). Further development of geological 10Be production-rate calibration sites based upon landforms with direct and absolute-dated chronological constraints will help to sharpen empirical estimates of cosmogenic nuclide production rates and aid in improving scaling protocols.
Here, we present a 10Be production-rate calibration data set based on a 14C-dated late-glacial moraine belt located at Rannoch Moor, central Scottish Highlands. Although there are now four published 10Be production-rate calibration sites in Scotland (e.g., Ballantyne and Stone, 2012; Borchers et al., 2016; Small and Fabel, 2015), none is based on landforms that have been directly dated with absolute radiometric techniques (Phillips et al., 2016). Instead, landform ages have been assessed based on assumed correlations to distal biological and/or ice-core-inferred paleoclimatic signatures (Balco et al., 2008; Ballantyne and Stone, 2012; Borchers et al., 2016; Phillips et al., 2016; Stone et al., 1998), or else tentative correlations to distal and undated lacustrine sediments and tephrostratigraphy (Small and Fabel, 2015). Consequently, the reference SLHL production-rate values from these sites exhibit deviation from published production-rate calibration data sets from elsewhere. This has led to the question of whether problems with scaling models, or the calibration sites themselves, are responsible for the discrepancy among Scottish calibration data sets and data sets based on directly dated landforms from father afield (Phillips et al., 2016).
The age of the Rannoch Moor moraine belt is bracketed by maximum- and minimum-limiting 14C ages, and thus affords minimum- and maximum-limiting bounds, respectively, on the regional in situ production rate of 10Be. We (1) present a new geological 10Be calibration data set for the central Scottish Highlands; (2) discuss the fit to distal calibration data sets, with implications for available scaling models; (3) evaluate which previously published production-rate estimates would produce 10Be surface-exposure ages that are compatible with the bracketing 14C chronology; and (4) address previously published 10Be data sets from Rannoch Moor in the context of the results presented here.
Section snippets
Prior work
Four published calibration data sets exist for the Scottish Highlands. These data are from Coire Mhic Fearchair, Maol Chean Dearg, Corie nan Arr (Ballantyne and Stone, 2012; Borchers et al., 2016), and from Glen Roy (Small and Fabel, 2015). Data from Coire Mhic Fearchair, Maol Chean Dearg, and Corie nan Arr are included in the primary global calibration data set of Borchers et al. (2016).
The three studies at Coire Mhic Fearchair (57.2°N, 5.97°W), Maol Chean Dearg (57.49°N, 5.45°W), and Corie
Rannoch Moor calibration site: setting and basis for independent age assignment
Rannoch Moor (56.63°N, 4.77°W; ∼310–330 m a.s.l.; Fig. 1) is an extensive, peat-covered moorland surrounded by high-relief glacially-molded peaks of the southern Grampian Mountains. By most glaciological reconstructions, Rannoch Moor lay near the center of the West Highland ice field (WHIF) during late-glacial time and was likely one of the last lowland regions in Scotland to become deglaciated (Fig. 1, Fig. 2; Golledge, 2010; Golledge et al., 2007; Lowe and Walker, 1976; Sissons, 1976). The
Methods
Our field and laboratory procedures for obtaining in situ 10Be concentrations for production-rate determination followed those reported in Schaefer et al. (2009), Putnam et al. (2010b) and Kaplan et al. (2011), and are described online at http://www.ldeo.columbia.edu/tcn. Methods for developing the 14C chronology of the WHIF and Rannoch Moor deglaciation are reported in Bromley et al. (2018, 2014).
10Be data
Measured 10Be concentrations exhibit tight internal consistency and form an approximately normal distribution when corrected for thickness and topographic shielding (Fig. 6). Uncorrected concentrations range from 6.66 ± 0.22 to 7.70 ± 0.15 [x104] at g−1 (Table 2). Only one measurement (RM-10-01; 7.70 ± 0.15 [x104] at g-1) has a distinguishably different (i.e., higher) concentration from the rest of the population. If treated as a surface-exposure age (as a means of normalizing the measurements
Discussion
Here we discuss the calibration data set presented in this study within the context of: (i) distal calibration data sets based on landforms with direct independent age control, (ii) previously published calibration data sets from indirectly dated landforms in the Scottish Highlands, and (iii) previously published 10Be data sets from the Rannoch Moor region.
Conclusions
- 1)
We present a geological 10Be production-rate calibration based on the Rannoch Moor moraine belt of the central Scottish Highlands (56.63°N, 4.77°W; ∼310–330 m a.s.l.).
- 2)
The landforms targeted for production-rate calibration are bracketed by twenty-seven maximum- and twenty minimum-limiting 14C ages. This 14C chronology indicates that the Rannoch Moor moraines were formed no earlier than 12,700 ± 100 cal. yrs BP, and no later than 12,480 ± 110 cal yrs BP. On the basis of these bracketing ages, we
Data Availability
The calibration data set related to this article can be found at the open-source online data repository hosted at Mendeley Data.
Acknowledgements
The authors wish to thank M.R. Kaplan, G.H. Denton, B.L. Hall, T.V. Lowell, J. Stone, C. Fenton, and many others for insightful discussions over the years. We are particularly grateful to G. Balco for sharing his knowledge and providing advice on the technical aspects of production-rate calculation. G. Balco's ‘cosmognosis’ blog has also been very helpful. D. Fabel and D. Small provided helpful discussion regarding 10Be data from Rannoch Moor. M. Kelly and R. Braucher provided constructive
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