Bypassing the Suess-effect: Age determination of charcoal kiln remains using OSL dating

Highlights

• Quartz-OSL dating is applicable to heated sediments associated with relict charcoal kilns.

• Good agreement between OSL and ¹⁴C dates for pre-1650 CE features confirms accuracy.

• Post-1650 CE features can be dated with significantly higher precision.

• Large datasets can be generated using a simplified procedure, not affecting precision or accuracy.

• Potential pitfalls and possibilities are highlighted.

Abstract

Ancient forests all over Europe often preserve remains of (pre)historic charcoal production (kilns), informing on past forest composition and anthropogenic woodland exploitation. So far, the chronology of these features has been entirely derived from ¹⁴C-dating of associated charcoal. Though generally successful, ¹⁴C-dating cannot provide meaningful age information for post-1650 CE features, while most of the archaeological remains of charcoal kilns date to this period. Here, we investigate the potential of quartz-based optically stimulated luminescence (OSL) dating of sediments that were heated during charcoal production as a more precise alternative tool for age determination of these kilns.

Using a set of 14 samples collected from both relict charcoal kilns and the underlying parent material, we first document the quartz OSL characteristics in terms of the procedural tests commonly used to assess the appropriateness of the single-aliquot regenerative-dose (SAR) protocol. The results suggest that the measurement procedure allows determining equivalent doses both accurately and precisely. We then demonstrate how the linear part of the dose-response curve can be exploited to examine completeness of resetting and/or post-depositional disturbance without compromising instrument time, precision and laboratory accuracy. Optical ages derived from these distributions are consistent with the available independent (¹⁴C) age information for the charcoal kilns. We conclude that OSL can provide an accurate and viable alternative to ¹⁴C and will be especially useful to constrain post-1650 CE features in time. Our results also suggest that the approach may allow establishing relative chronologies with a time resolution of 2–4%.

Introduction

Remains of former charcoal production sites (charcoal kilns) can be found in many forested areas in Europe (Nelle, 2003; Ludemann, 2010; Carrari et al., 2017; Paradis-Grenouillet and Dufraisse, 2018; Schneider et al., 2020) and North America (Hart et al., 2008; Potter et al., 2013). Two major types of charcoal kilns have been described, i.e. pit kilns, which generally date from the Iron Age till 1200 CE, where charcoal has been produced in small rectangular or circular pits (e.g. Groenewoudt, 2007; Deforce et al., 2018; Drailly and Deforce, 2019), and mound kilns, dating between 1300 CE and the 20th century CE, where large, above-ground dome-shaped stacks of wood have been carbonized (e.g. Ludemann, 2003, 2010; Nelle, 2003). Archaeological remains of these charcoal kilns are frequently used in palaeoecological studies, as they are an important source of information on former forest composition and past human woodland exploitation and fuel production (e.g. Ludemann, 2003, 2010; Nelle, 2003; Deforce et al., 2013, 2018). More recently, charcoal kiln remains are also studied to identify the role of biochar in relation to soil properties (e.g. Hardy et al., 2016, 2017a; Hirsch et al., 2017, 2018), carbon sequestration (e.g. Hardy et al., 2017b; Hirsch et al., 2017; Mastrolonardo et al., 2018) and composition, biodiversity and growth rate of the vegetation. All these studies require a chronological framework for the original kilns, which is generally established using radiocarbon (¹⁴C) dating (Carrari et al., 2016a, b; Buras et al., 2020). In exceptional cases, dendrochronology can be applied when charcoal fragments from specific taxa containing a large number of growth rings can be recovered (e.g. Raab et al., 2015), or when radiocarbon dating and dendrochronology of old trees growing on top of the kiln sites can be combined (e.g. Deforce et al., 2013).

The majority of the charcoal kilns found in Europe (e.g. Ludemann, 2010; Raab et al., 2015; Carrari et al., 2017) and the Americas (Raab et al., 2017;Patzlaff et al., 2018) are mound kilns dating between the seventeenth and nineteenth century CE. Age determination of post-1650 CE archaeological features using radiocarbon dating is problematic because of plateaus in the calibration curve (De Vries and Suess-effects), caused by variations in sunspot activities and the dilution of atmospheric ¹⁴CO₂ by fossil fuel derived CO₂ which does not contain ¹⁴C (Stuiver, 1961; Tans et al., 1979). This is a major constraint for studies using former charcoal production sites and an alternative dating method would therefore be of great value.

At present, luminescence dating is widely used for establishing sediment deposition chronologies by determining when the constituent mineral grains (such as quartz and feldspar) were last exposed to sunlight (e.g. Duller, 2004, 2008; Rhodes, 2011). Sufficient exposure to heat, however, also resets the luminescence ‘clock’ allowing application to a range of fired materials such as pottery, bricks, hearths and burnt sediments (e.g. Aitken, 1985; Wagner, 1998; Benea et al., 2007; Armitage and King, 2013). Based on measurements of magnetic susceptibility and soil organic matter, Hirsch et al. (2018) recently suggested that the heat generated during charcoal production should be sufficient to reset the optically stimulated luminescence (OSL) signal from quartz in sandy sediments underlying the kilns up to depths of about 5 cm.

In this paper, we report on an exploratory investigation into the potential of quartz-OSL signals for dating heated sediments associated with archaeological remains of charcoal kilns. Our approach is empirical in that we test the applicability of OSL dating procedures that are widely adopted by luminescence geochronologists around the world. To that respect, the underlying rationale is straightforward: as reliable chronometric information is required for a vast amount of charcoal kilns, it is desirable to have a dating tool that is also workable in practice (i.e. readily accessible instrumentation, a cost and time-effective workflow for laboratory analysis, etc.). We document our experimental design and the general quartz-OSL characteristics for samples collected from both pre and post-1650 CE features. For each of these features, we then evaluate the accuracy of the resulting OSL ages through comparison with independent ¹⁴C ages. The entire dataset is then used to discuss sampling strategies, dose rate issues, resetting and precision.