Monitoring of a fast-growing speleothem site from the Han-sur-Lesse cave, Belgium, indicates equilibrium deposition of the seasonal δ¹⁸O and δ¹³C signals in the calcite

Abstract. Speleothems provide paleoclimate information on multimillennial to decadal scales in the Holocene. However, seasonal or even monthly resolved records remain scarce. Such records require fast-growing stalagmites and a good understanding of the proxy system on very short timescales. The Proserpine stalagmite from the Han-sur-Less cave (Belgium) displays well-defined/clearly visible darker and lighter seasonal layers of 0.5 to 2 mm thickness per single layer, which allows a measuring resolution at a monthly scale. Through a regular cave monitoring, we acquired a good understanding of how δ¹⁸O and δ¹³C signals in modern calcite reflect climate variations on the seasonal scale. From December to June, outside temperatures are cold, inducing low cave air and water temperature, and bio-productivity in the soil is limited, leading to lower pCO₂ and higher δ¹³C values of the CO₂ in the cave air. From June to December, the measured factors display an opposite behavior.

The absence of epikarst water recharge between May and October increases prior calcite precipitation (PCP) in the vadose zone, causing drip water to display increasing pH and δ¹³C values over the summer months. Water recharge of the epikarst in winter diminishes the effect of PCP and as a result the pH and δ¹³C of the drip water gradually decrease. The δ¹⁸O and δ¹³C signals of fresh calcite precipitated on glass slabs also vary seasonally and are both reflecting equilibrium conditions. Lowest δ¹⁸O values occur during the summer, when the δ¹³C values are high. The δ¹⁸O values of the calcite display seasonal variations due to changes in the cave air and water temperature. The δ¹³C values reflect the seasonal variation of the δ¹³C_DIC of the drip water, which is affected by the intensity of PCP. This same anticorrelation of the δ¹⁸O versus the δ¹³C signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers display lower δ¹⁸O and higher δ¹³C values. The cave system varies seasonally in response to the activity of the vegetation cover and outside air temperature between a "summer mode" lasting from June to December and a "winter mode" from December to June. The low δ¹⁸O and high δ¹³C values of the darker speleothem layers indicate that they are formed during summer, while light layers are formed during winter. The darker the color of a layer, the more compact its calcite structure is, and the more negative its δ¹⁸O signal and the more positive its δ¹³C signal are. Darker layers deposited from summer drip water affected by PCP are suggested to contain lower Ca²⁺ concentration. If indeed the calcite saturation represents the main factor driving the Proserpine growth rate, the dark layers should grow slower than the white layers.