<p><strong>Abstract.</strong> 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

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... ows 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 <i>p</i>CO₂ and higher δ¹³C values of the CO₂ in the cave air. From June to December, the measured factors display an opposite behavior. <br><br> 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 &amp;delta;¹⁸O 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.</p>