Vertical chemical profiles collected across haloclines in the water column of the Ox Bel Ha cave network within the coastal aquifer of the Yucatan Peninsula in January 2015 and January 2016

Discrete water samples were collected from Cenote Bang during four sampling events: January 15 and January 16 of 2015, and January 22 and January 23 of 2016. Cave samples were collected by scuba divers and followed protocols established by the American Academy of Underwater Sciences and the National Speleological Society Cave Diving Section. Water samples for geochemical analysis of dissolved materials were collected near sonde profile locations in plastic 60 ml syringes fitted with 3-way stopcocks. The syringes were loaded into the sampler-rack prior to deployment.
The samplers used to collect the water samples were the OctoPiPi (OPP) consisting of a rack of ten 60 ml plastic syringes mounted and evenly distributed along an aluminum frame with a mechanical, spring-powered trigger-system that fills syringes when activated. An OPP with fixed 12 cm syringe spacing and another with 2.5 cm syringe spacing (the mini-OPP) were deployed during this study. The OPPs were mounted vertically across interfaces and left in place for at least 24 hours to allow the water column to re-stratify before being triggered. The top-mount of the OPP was connected to either an inflated dive lift bag or elastic bands carefully attached to the cave ceiling. The bottom-mount was connected to 1/8” braided nylon line deployed by dive reel and attached to at least 5 kg of lead for stability. When triggered, the OPP slowly and simultaneously filled the syringes with sample water.
Following OPP recovery, the syringes were transported on ice to the field lab, where they were processed within eight hours. The 60-ml water sample was divided into subsamples for geochemical analyses of dissolved constituents, including methane, dissolved inorganic carbon, dissolved organic carbon, inorganic nutrients and ions. Samples for ion analysis were filtered through a 0.45 micrometer (µm) Acrodisc syringe filter and collected in a 2-ml plastic screw-top vial. 30-ml serum vials for methane water samples were prepared prior to sample collection by adding 0.5 ml 8M NaOH into the empty vial as a preservative, sealing the container with 1 cm thick butyl septa, and vacating the vial of air with a vacuum pump. An unfiltered 20-ml water sample was then transferred from the syringe into the serum vial by piercing the septum with a 20-gauge syringe needle and regulating water flow with the stopcock. Water samples were stored in the fridge at 7 degree C.
Processing took place after each deployment period in 2015 and 2016.

Analysis of the samples was performed at the Woods Hole Oceanographic Institution (WHOI) and U.S. Geological Survey (USGS) in Woods Hole MA, USA. Sulfate and chloride concentrations were determined using a Metrohm 881 Compact Plus ion chromatograph (IC) equipped with a Metrosep A Supp 5-250 anion column. Chloride concentrations millimol (mM) were converted to milligram per liter (mg l-1) and multiplied by 0.0018066 to determine salinity (psu). The analytical error for chloride and sulfate was +/- 3.5% of the IAPSO standard sulfate and chloride values.
Dissolved inorganic carbon (DIC) concentrations were determined with a Model 5011 UIC coulometer and quantified relative to a seawater certified reference material (CRM) with a relative standard deviation of 4% relative to the CRM value (2.2 millimolar, mM). The stable carbon isotopic content of the DIC was measured with a Thermo-Finnigan DELTAplus XP IRMS with an analytical error (1-sigma) of 1.1 per mil.
Headspace methane concentrations were determined using a Shimadzu 14-B gas chromatograph (GC) equipped with a flame ionization detector (FID). Headspace concentrations were converted to dissolved concentrations using the method of Magen and others, 2014 and are reported with a relative standard deviation of 2%.
The stable carbon isotope composition of methane from the headspace of the serum vials was determined using a Thermo-Finnigan DELTAPlus XL isotope ratio mass spectrometer (IRMS) coupled to an Agilent 6890 Gas Chromatograph (GC) via a Finnigan GCCIII combustion interface with 1-sigma deviation of 1.6 per mil.
DOC concentration and d13C were analyzed by high-temperature combustion-isotope ratio mass spectrometry (HTC-IRMS) at the USGS-WHOI Dissolved Carbon Isotope Lab (DCIL). The DCIL HTC-IRMS system consists of an OI 1030C total carbon analyzer and a Graden molecular sieve trap interfaced to a Thermo-Finnigan DELTA-PLUS XL IRMS. DOC concentrations are reported with relative standard deviation (RSD) of less than 5% of the measured value. Stable carbon isotope ratios for DIC, methane, and DOC are reported in the standard delta-notation relative to VPDB (Vienna Pee Dee Belemnite) with a 1-sigma deviation of less than 0.5 per mil.
All sample analyses were performed by David Brankovits under the supervision of John Pohlman. Full details of the laboratory analysis of samples is described in the Methods section and Supplementary Materials of Brankovits, D., Pohlman, J.W., Hiemann, H., Leigh, M.B., Leewis, M.C., Becker, K.W., Iliffe, T.M., Alvarez, F., Lehmann, M.F., and Phillips, B., 2017, Methane- and dissolved organic carbon-fueled microbial loop supports a tropical subterranean estuary ecosystem: Nature Communications, vol. 8, 1835, https://doi.org/10.1038/s41467-017-01776-x. Sample processing occurred between January 2015 and September 2016.

The data were organized into a Microsoft Excel spreadsheet (O365). The first three header rows of that Excel file were deleted, and the resulting header row and data were exported as an MS-DOS CSV file so the data are available in both XLSX and CSV formats.

Added keywords section with USGS persistent identifier as theme keyword.