Effects of prior calcite and prior aragonite precipitation on cave dripwater compositions

Cave monitoring programs have provided a wealth of information on processes that affect speleothem proxy systems such as stable isotopes and trace elements. Prior carbonate precipitation (PCarbP), i.e. the precipitation of a carbonate mineral from supersaturated water occurring in the epikarst or at the ceiling of the cave, is one of the dominant processes that controls the dripwater d¹³C, Mg/Ca, Sr/Ca, Ba/Ca as well as U/Ca. Enhanced PCarbP is associated with reduced infiltration or a decrease in CO₂ partial pressures of the infiltrating water. Often this process can be linked to the precipitation - evaporation balance (P-E) or rainfall variability. The type of carbonate polymorph that precipitates, calcite or aragonite, determines how dripwater trace element to Ca (Me/Ca) ratios evolve as a consequence of PCarbP. This has led to distinguish prior calcite precipitation (PCP) from prior aragonite precipitation (PAP) since calcite trace element partitioning coefficients (D_Me) differ from aragonite D_Me. Determining accurate calcite and aragonite D_Mefor Mg, Sr, Ba, and U is crucial to identify the PCarbP mode (i.e., PCP or PAP) and increase our confidence in interpreting speleothem trace element variability.

Using an autosampler we were able to collect dripwater samples from Grotte de Piste, Morocco at unprecedented 4-day resolution covering the transition from the wet winter to the dry summer season. The drip site has high drip rates during the wet winter but progressively dries out during summer. We observe a strong significant positive correlation between dripwater Ca concentrations and drip rate (r=0.996, p<0.01) indicating that PCarbP is the dominant process affecting dripwater Ca concentrations. By studying the evolution of dripwater Ca and trace element concentrations we show that a single drip site can be affected by both PCP as well as PAP. From the dripwater element compositions we determine new aragonite D_Me values. As opposed to D_Mg, D_Sr and D_U, it appears that aragonite D_Ba is very similar to calcite D_Ba. Barium thus seems least sensitive to mineral phase changes associated with PC_CarbP compared to Mg, Sr, U. However, calcite or aragonite D_Me values should not be considered constants and may vary with solution composition, dripwater saturation index and temperature. Aragonite D_Ba for example increases with dripwater saturation index. It is therefore possible that D_Ba might differ within the epikarst as compared to the cave ceiling or the speleothem surface unless the dripwater saturation index remains low at the speleothem surface. Barium concentrations in aragonite speleothems with low growth rates may be most suitable to trace combined PCP and PAP variations as they precipitate from dripwater with a low saturation index.