科研之友
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摘要
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 2mm 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 delta O-18 and delta C-13 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 pCO(2) and higher delta C-13 values of the CO2 in the cave air. From June to December, the measured factors display an opposite behavior. %26lt;br%26gt;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 delta C-13 values over the summer months. Water recharge of the epikarst in winter diminishes the effect of PCP and as a result the pH and delta C-13 of the drip water gradually decrease. The delta O-18 and delta C-13 signals of fresh calcite precipitated on glass slabs also vary seasonally and are both reflecting equilibrium conditions. Lowest delta O-18 values occur during the summer, when the delta C-13 values are high. The delta O-18 values of the calcite display seasonal variations due to changes in the cave air and water temperature. The delta C-13 values reflect the seasonal variation of the delta C-13(DIC) of the drip water, which is affected by the intensity of PCP. This same anticorrelation of the delta O-18 versus the delta C-13 signals is seen in the monthly resolved speleothem record that covers the period between 1976 and 1985 AD. Dark layers display lower delta O-18 and higher delta C-13 values. The cave system varies seasonally in response to the activity of the vegetation cover and outside air temperature between a %26quot;summer mode%26quot; lasting from June to December and a %26quot;winter mode%26quot; from December to June. The low delta O-18 and high delta C-13 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 delta O-18 signal and the more positive its delta C-13 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.
- 出版日期2014
