Methods
October 29, 2022
Monitoring of Cave Without a Name in Central Texas has been ongoing since early 2013. Approximately every 6 weeks, dripwaters, calcite plates, and environmental parameters (drip rate, temperature, relative humidity, and atmospheric CO2 concentration) are collected from two sample sites within Cave Without a Name, Brain Dead and Hole of Wisdom. Dripwaters are collected 125 mL bottles then aliquoted for various analyses. After collecting the dripwaters, sanded glass plates are placed under the drips in order to collect calcite. Upon returning to the cave for the next sampling drip, those plates are collected and weighed.
60 dripwater samples and 60 farmed calcite plates have been chosen for analysis by ICP-MS spanning five years of monitoring. This will provide a comprehensive dataset of 120 samples from which to examine seasonal changes in dripwater and calcite chemistry. This range, spanning from 2013 to 2019, not only contains seasonal temperature and CO2 variations, but also includes a period of intense drought and a period of moderate moisture (2013-2014 and 2015-2016, respectively; Miller et al, 2021). This sample selection will provide a wide range of moisture conditions to hopefully elucidate differences in fractionation of the dripwaters and farmed calcite.
Each glass plate will have 100 micrograms of calcite scraped off the corner most downstream of the drip splatter. This location was chosen in order to promote consistency within the data as well as preserve the majority of each plate of farmed calcite for future studies. The scraped calcite will then be dissolved in 100 microliters of 7 N nitric acid according to the method as described by Tremaine and Froelich (2013). This solution will then be diluted in 18.3 megohm deionized water to 40 ppm [Ca] and analyzed in solution mode on the Agilent 7500ce Quadrupole ICP-MS on pulse mode in order to obtain all desired elemental ratios. This dilution factor has been chosen in order to remain safely in pulse-counting mode (Yu et al., 2005).
In order to preserve water chemistry and prevent cation sorption, dripwater bottles were acidified and sealed upon being aliquoted. All dripwater samples will be diluted using a 2% nitric solution to 40 ppm [Ca] (chosen to remain within limits of pulse-counting mode). They will also be analyzed in solution mode, also in pulse-counting mode, on the ICP-Q-MS.
In order to confirm that the above dilution will be sufficient enough to remain in pulse-counting mode, a complete AMU mass range, semi-quantitative analysis will be performed on four dripwater samples. This technique measures approximate concentrations of elements in order to determine if they have been sufficiently diluted for the given elements. Two measurements will be performed for each drip site – a winter and a summer sample. These samples will be sufficient enough to determine the specific dilution factor that will be used for all 60 dripwater analyses.
In order to evaluate the accuracy of the dripwater data, an external standard (NIST 1643e) will be used for trace elements. To evaluate the accuracy of the plate calcite data, two carbonate reference materials will be measured (USGS MACS-3 and FEBS-1). Each standard will be measured before and after each run in order to evaluate drift. In order to measure all elements in the same dilution, calcium will be measured by a minor isotope, enabling for all measurements to be completed in pulse counting mode, thus eliminating any calibration factors between modes. The elements Mg, Sr, Ba, and Ca will be measured, specifically 46Ca, 24Mg, 86Sr, and 136Ba, in order to create the desired TE/Ca ratios (Yu et al, 2006).