A 5,000-YEAR RECORD OF TRACE ELEMENTS AND STABLE ISOTOPES IN A BELGIAN SPELEOTHEM

Speleothems are able to record past environments through trace element concentrations and isotopic ratios. We investigate these proxies in a stalagmite from the Han cave in Han-sur-Lesse, Belgium. The stalagmite is 34 cm long and was dated >5380 ± 93 years old using U/Th dating. Trace element concentrations were analyzed using laser ablation ICP-MS. Previous studies have shown that higher Sr/Ca, Ba/Ca and Mg/Ca ratios correlate with drier years while lower ratios indicate rainier periods. This trend is likely due to the amount of prior calcite precipitation (PCP) that occurs. Drier periods allow for longer water residence time (WRT) in the rock above the cave allowing for more calcium to be removed from the water during PCP causing elemental ratios to increase. However, Mg/Ca ratios in speleothems are also dependent on temperature, while Sr/Ca and Ba/Ca ratios are less dependent on cave temperature. Differences between Sr/Ca and Mg/Ca are suggested to be due to incongruent carbonate dissolution in the host rock above the cave. In the Han stalagmite, Sr/Ca and Ba/Ca ratios are in phase with each other over much of the speleothem. Both ratios exhibit large variations, but there is a period of low amplitude variations from ~1500 to 3000 years before present (BP), possibly indicating more consistent climatic conditions during this period. Mg/Ca ratios exhibit a steady decline from ~5400 years BP (the base of the stalagmite ) to ~1500 years BP, but do exhibit their lowest values when Sr/Ca and Ba/Ca ratios are also low and less variable. After this time period, all three ratios change with Sr/Ca and Ba/Ca increasing to present and Mg/Ca ratios first increasing and then decreasing. Interpreting trace element data alone is not straightforward; oxygen isotope data will significantly assist the interpretation. Nevertheless, this speleothem shows large variations in elemental ratios, with Sr/Ca, Ba/Ca, and Mg/Ca ratios becoming relatively stable and low between 1500 to 3500 BP, suggesting a drier period in Belgium during this time.