Controls on trace element (Sr–Mg) compositions of carbonate cave waters: implications for speleothem climatic records

• Published in: Chemical geology 2000-05, Vol.166 (3), p.255-269
• Main Authors: Fairchild, Ian J, Borsato, Andrea, Tooth, Anna F, Frisia, Silvia, Hawkesworth, Christopher J, Huang, Yiming, McDermott, Frank, Spiro, Baruch
• Format: Article
• Language: English
• Subjects: Aqueous geochemistry; Karst; Palaeohydrology; Weathering
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(99)00216-8

At two caves (Clamouse, S France and Ernesto, NE Italy), cave drip and pool waters were collected and sampled at intervals over a 2–3 year period. Mg/Ca and Sr/Ca concentration ratios, corrected for marine aerosols, are compared with those of bedrocks and, in some cases, aqueous leachates of soils and weathered bedrocks. Cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate, but typically show enhanced and covarying Mg/Ca and Sr/Ca. Four factors are considered as controlling processes. (1) The much faster dissolution rate of calcite than dolomite allows for the possibility of increase of Mg/Ca if water–rock contact times are increased during drier conditions. A theoretical model is shown to be comparable to experimental leachates. (2) Prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying Mg/Ca and Sr/Ca ratios. This mechanism requires the solution to lose CO 2 into pores or caverns. (3) Incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation. (4) selective leaching of Mg and Sr with respect to Ca is shown to be important in leachates from Ernesto where it appears to be a phenomenon of calcite dissolution. In general selective leaching can occur whenever Ca is sequestered into precipitates due to freezing or drying of soils, or if there is derivation of excess Sr and Mg from non-carbonate species. The Ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year-round constant P CO 2 of around 10 −2.4 and no evidence of calcite precipitation in the karst above the cave. Two distinct, but overlying trends of enhanced and covarying Mg/Ca and Sr/Ca away from the locus of bedrock compositions are due to calcite precipitation within the cave and, at a variable drip site, due to enhanced selective leaching at slow drip rates. Mg-enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times. The Clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation, both in the cave and in overlying porous dolomite/dedolomitized limestone bedrock. Initial P CO 2 values as high as 10 −1 are inferred. Experimental incubations of Clamouse soils which generated enhanced P CO 2 and precipitated CaCO 3 had compositions similar to the karst waters. Calcite precipitatio