Speleothems provide paleoclimate information on multi-millennial to decadal scales in the Holocene. However seasonal or even monthly resolved records remain scarce. They require fast growing stalagmites and a good understanding of the proxy transfer function on very short time scales. The Proserpine stalagmite from the Han-sur-Less cave (Belgium) displays seasonal layers of 0.5 to 2 mm thickness that reconstruct paleoclimates at a monthly scale. Through a regular cave monitoring, we acquired a

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... ood understanding of how δ¹⁸O and δ¹³C signals in modern calcite reflect climate variations on sub-seasonal scale. Cave parameters vary seasonally in response to the activity of the vegetation cover and outside air temperature. From December to June, the cave remains in "winter-mode". Outside temperatures are cold inducing low cave air and water temperatures. Bio-productivity in the soil is limited leading to low <i>p</i>CO₂, higher δ¹³C composition of the CO₂ in the cave air and high discharge due to the inactivity of the plant coverage. From June to December, the cave switches to "summer-mode" and the measured factors display an opposite behavior. The δ¹⁸O and δ¹³C signals of fresh calcite precipitated on glass slabs vary seasonally. Lowest δ¹⁸O values occur during the summer-mode when the δ¹³C values are high. The δ¹⁸O composition of the calcite is in equilibrium with the drip water δ¹⁸O and display seasonal variations due to changes in the cave air and water temperature. In contrast to the δ¹⁸O signal, δ¹³C values of the calcite precipitated on the glass slabs do not reflect equilibrium conditions. Highest δ¹³C values occur during summer, when discharge rates are low increasing the evaporation effect on the thin water film covering the stalagmite. This same antithetical behavior of the δ¹⁸O vs. the δ¹³C signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers are formed during summer, while light layers are formed during winter when calcite deposition occurs fast. The darker the color of a layer, the more compact its calcite structure, the more negative its δ¹⁸O signal and the more positive its δ¹³C signal.