Seasonal variations in Sr, Mg and P in modern speleothems (Grotta di Ernesto, Italy)

Sub-annual variations in trace element chemistry and luminescence have rece ntly been demonstrated from speleothems and offer the potential of high-res olution palaeoclimatic proxies. However, no studies have yet examined micro scopic trace element variations in relation to modern cave conditions. In t his study, the spatial variations in trace element (Sr, Mg and P) concentra tions in speleothems (a stalagmite and a soda straw stalactite) from the al pine Ernesto cave (temperature 6.6 +/- 0.1 degreesC) in a forested catchmen t in NE Italy have been studied using secondary ion mass spectrometry (SIMS ) and compared with environmental parameters and waters in the modern cave. An annual lamination exists in the stalagmite and soda straw stalactite in the form of clear calcite with narrow visible layers, which are UV-fluores cent and interpreted to contain soil-derived humic/fulvic acids washed into the cave during autumn rains. Microanalyses were undertaken of seven annua l laminae, probably deposited during the 1960s in the stalagmite, and seven laminae in the 1990s for the stalactite. The analysis results show that Sr consistently has a trough and P. a peak c entred on the inclusion-rich layer. Mg shows mainly a negative covariation with Sr in laminae formed in the 1990s, but a positive covariation in the s talagmite formed in 1960s. The spatial scale of the main geochemical variat ions is the same as that of annual laminae of inclusion-poor and inclusion- rich couplets. Mass balance arguments are used to show that the P is inorga nic in form and presumably occurs as individual phosphate ions within the c alcite, Most drip waters show limited chemical variations. but a summer peak in tra ct elements in 1995 and a decrease in Mg/Ca in the following winter are not able. More pronounced covariations in Mg/Ca and Sr/Ca are shown by a site w ith highly variable drip rates where ratios increase at slow drip rates. Th e strongest seasonal variations are found in pool waters, where ratios incr ease reflecting significant Ca removal from the water into the calcite duri ng the winter in response to seasonal PCO2 variations in cave air. Thus. th e cave waters' compositions tend to reflect climate conditions. such that M g/Ca and Sr/Ca are tentatively interpreted to be higher when climate condit ions are dry. Combining results from the speleothems and cave water along with the behavi our of each trace species, Mg/Ca variations in the speleothems are consider ed to reflect their variation in the cave waters, whereas, Sr incorporation is also dependent on precipitation rate, in this case, mainly controlled b y temporal variations in PCO2 in the cave (and conceivably, also by inhibit ors such as phosphate). P adsorption (a fraction of which is subsequently i ncorporated within calcite) depends on aqueous phosphate concentration and water flux, both of which should increase during the autumn. Therefore, mul tiple trace element profiles in speleothems reflect multiple aspects of env ironment seasonality and conditions, and hence, a calibration against weath er records is desirable to establish their palaeoclimatological meaning. Th e strong annual variation of trace elements, and particularly P, can provid e chronological markers for high-resolution studies of other climate proxie s, such as stable isotopes. (C) 2001 Elsevier Science B.V. All rights reser ved.