Abstract
Abstract. Dripwater hydrology and hydrogeochemistry is particularly useful in constraining the meaning of speleothem palaeoclimate archives, for example using δ18O signatures. Here, we calibrate the relationship between δ18O in precipitation, percolation waters and contemporary calcite deposits, at Tartair cave, Sutherland, NW Scotland, an Atlantic site sensitive to regional changes both of temperature and precipitation. Monthly precipitation displayed a 7.1‰ range in δ18O, a negative linear relationship with rainfall amount, and no correlation with temperature. Autogenically-derived cave percolation waters show little variation in δ18O during the same period and their annual weighted mean is the same as that of the local precipitation. This evidence together with hydrological data and electroconductivity values indicates that percolation waters are well mixed and dominated by stored water. Calculated values of δ18O of calcite deposited in this cave environment indicate that the cave deposits are forming close to isotopic equilibrium and kinetic effects are negligible. Comparison of a high-resolution δ18O stalagmite record with the instrumental record of climate indicates that isotopically heavy values are reflective of relatively cold, dry conditions (and vice-versa for warm, wet condition) and hence that stalagmite oxygen isotopes provide an appropriate means of investigating the palaeoclimate in this location.
Highlights
The hydrological behaviour of small water volumes becomes important when they feed, and control, the composition of growing calcareous deposits in karstic cavities
The annual amount-weighted δ18O value for Inchnadamph precipitation is −7.1‰. This is 1.7‰ heavier than the amount weighted value of δ18O of the local precipitation (δ18Oppt) calculated for the nearest Global Network of Isotopes in Precipitation (GNIP) site at Altnabreac, Caithness (Fig. 1), which existed for a 13 month period from 1980 to 1981 (Kay et al, 1984)
A strong linear relationship exists between δ18O and δ2H in Inchnadamph monthly precipitation (r=0.98), demonstrated by the linear regression shown in Fig. 3, which represents the local meteoric water line (LMWL)
Summary
The hydrological behaviour of small water volumes becomes important when they feed, and control, the composition of growing calcareous deposits (speleothems) in karstic cavities. Speleothems are recognised as important continental archives of palaeoenvironmental information: they can be accurately dated using U-series techniques and their subterranean location means that they can accumulate undisturbed for thousands of years. Their 18O/16O ratios can be used to reconstruct climate, the rationale used can sometimes be complex and in need of further testing (McDermott, 2004; Fairchild et al, 2006a). There are two major forcing factors which can change the 18O/16O ratio of calcite deposits: a change in the isotopic composition of the precipitation feeding the speleothem, or a change in cave temperature. The western seaboard of Europe displays complex controls such that changes in δ18O over time cannot be predicted from first principles because the relationship between isotopic composition, synoptic weather
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