Joint luminescence and radiocarbon dating of Holocene fire history and dune activity in Arctic Fennoscandia

Aeolian parabolic dunes in Arctic Fennoscandia are widely scattered across diverse biomes from tundra to mountain birch and pine forest, with many systems in more sparsely vegetated locations still showing signs of current degradation or partial activity. Dune stratigraphy reveals that these dunes have also undergone multiple phases of Holocene activity and reactivation, following initial dune formation immediately post deglaciation. Multiple buried soil, charcoal and homogenous dune sand layers demonstrate that dune stability is repeatedly interrupted by multiple fire events and dune reworking episodes during the Holocene. As such, these systems have great potential as archives of Arctic landscape, climate and fire history in a highly sensitive environment, and over a range of biomes.

However, few of these dunes, especially in Sweden, have been independently dated using modern luminescence and radiocarbon techniques in order to constrain the timing of phases of dune stability, fire activity and reworking. As such, there is limited understanding of whether fire frequency or landscape degradation episodes have changed over the Holocene, whether there are wider patterns in these events over Arctic Fennoscandia, and if so what the climatic or anthropogenic drivers of such changes may be. Luminescence dating of aeolian sands constrains the timing of dune sand movement (or rather the cessation of movement), allowing direct dating of reworking events, while radiocarbon dating of charcoal constrains the timing of fires that may have marked the end of periods of stability, and the initiation of dune degradation and reworking.

Here we address this by applying detailed quartz optical stimulated luminescence (OSL) and feldspar post infrared - infrared stimulated luminescence (pIR-IRSL) dating to cross bedded (initial dune form) and homogenous sand (reactivation layers) units in multiple dunes in Arctic Sweden and Finland over a diverse range of biomes. We also apply detailed AMS ¹⁴C dating to multiple charcoal fragments from charcoal bands exposed in the stratigraphy of these dunes, which often separate dune reactivation sand units. In this way, not only can we cross check multiple independent chronological techniques, but we can also construct detailed local dune development and reworking histories across a wide area of Arctic Europe. Combining these histories allows testing of potential wider climatic and human drivers for fire and landscape changes in Arctic Fennoscandia over the Holocene. Our results show that the choice of best luminescence dosimeter mineral varies substantially by location, and that scattered charcoal fragments exposed in dune sands are often reworked from previous fires. However, our detailed, multi-technique approach allows development of detailed fire and reactivation histories for these dunes, and reveals substantial differences in these histories across biome type and location. Despite these differences, some wider trends are also apparent, notably a substantial increase in fire and dune activity after the Holocene Optimum and in recent centuries. We discuss possible causes for these changes.