Question

In paleoclimatology, stalagmite trace element ratios such as magnesium-to-calcium ($Mg/Ca$) serve as high-resolution proxy records for past rainfall variability. While oxygen isotope ratios ($\delta^{18}O$) in cave carbonate deposits are influenced by both temperature fluctuations and atmospheric moisture trajectories, trace element partitioning is primarily governed by Prior Calcite Precipitation ($PCP$). During dry climate intervals, slow rainwater percolation through the karst aquifer above the cave leads to prolonged degassing of carbon dioxide in groundwater cavities. Consequently, calcite precipitates prematurely along aquifer pore walls before drip water reaches the cave ceiling. Because calcium is preferentially incorporated into this early-stage calcite relative to magnesium, the remaining cave drip water becomes enriched in magnesium. Thus, elevated $Mg/Ca$ ratios in stalagmite growth layers directly record periods of decreased regional effective moisture rather than temperature changes.

However, geochemist Dr. Elena Vance demonstrated that this relationship can be distorted in aquifers containing dolomite ($CaMg(CO_3)_2$) bedrock. In such lithologies, kinetic dissolution of dolomite yields preferential leaching of magnesium independent of hydrological residence time, artificially inflating $Mg/Ca$ ratios during high-rainfall periods. To isolate genuine $PCP$ signals from bedrock dissolution artifacts, Vance established that researchers must co-analyze trace ratios with strontium-to-calcium ($Sr/Ca$) values. Because strontium partitioning in calcite mirrors magnesium during $PCP$ but remains uninfluenced by dolomite weathering, a synchronous covariation between $Mg/Ca$ and $Sr/Ca$ unequivocally confirms $PCP$ driven by aridity, whereas a divergence indicates bedrock-lithology anomalies.

According to the passage, Dr. Elena Vance identified which of the following as a factor that can cause magnesium-to-calcium ($Mg/Ca$) ratios in cave drip water to increase independently of Prior Calcite Precipitation?