Embedded Files
Research Team
Research Team
First author: Phoebe Bathje
First author: Phoebe Bathje
Mentor: Dr. Mark Krekeler
Mentor: Dr. Mark Krekeler
Co-authors: Abigail Evans, Jack Gugino, Ethan Belak, Angel A. Acosta-Colon, Bianca Maldonado-Arroyo, Pedro Rivera-Ramos, Sydnie Vazquez-Mendez, Iris Escobar
Abstract/Introduction
Abstract/Introduction
Cave sediment offers insight into the nature of past soil processes and can contain a record of human and biological activity. Cave sediment is comparatively understudied in a system located near Barahona, Puerto Rico. The Gemelos Cave is in Las Cabachuelas NatureReserve between Morovis and Ciales, PR and was declared as a preserved site in February 2012 due to its natural and historical value. Although many aspects of the cave have been explored and studied, one aspect that has not been investigated in detail is the cave floor sediment. The mineralogy and geochemistry of cave sediment can provide specific understanding of sediment source(s) to Gemelos Cave and potential geochemical processes. An initial understanding of the surface sediment can provide context for understanding variation of the sediment at depth.
Additionally, Las Cabachuelas Nature Reserve has been one of the most significant places for understanding the first inhabitants of Puerto Rico, as well for the study of regional paleoenvironments in the Caribbean. Recent evidence from radiocarbon dating has identified cave pictography and pyrography dating to 400 CE. (Rodrriguez-Ramos et al. 2021).
A detailed mineralogical and geochemical understanding of sediment may aid preservation efforts and archiving of any artifacts excavated in the future. The cave overall is not impacted by heavy use, is well conserved, and serves as an excellent study site for future geological and archaeological work.
Research Questions
Research Questions
What is the nature and variability of bulk mineralogy of sediments from Gemelos Cave?
Can oxyhydroxide minerals in solid solution, phosphate minerals, and igneous or volcanic input be identified with powder XRD?
Methods
Methods
Sediment samples of approximately 100 grams from the upper ~2 cm of Gemelos Cave were acquired along a transect from an entrance area to a wall for initial study to provide a geochemical and mineralogical framework for future geology and archaeological investigations.
Basic powder XRD analysis was performed on 14 bulk powder samples using front-loading pack mounts to identify major mineral phases in the sample material using a Bruker D8 Advance ECO Powder X-Ray Diffractometer with a LYNXEYE XE-T detector and using a CuKα tube with an operating voltage of 40 kV and a current of 25 mA at Miami University. Powdered bulk samples were prepared using a SPEX minimill and were analyzed with a 2θ range of 4-65°, a step size of 0.010°, and a dwell time of 0.10 s.
Samples 7 and 13 were size fractioned and treated with weak acetic acid to remove CaCO3 to investigate the clay mineral fraction of sediment. These were prepared in-house at Miami University following the methods of Moore and Reynolds (1997) and USGS OFR 01-041 (Poppe et al., 2001). These samples were mounted as smear slides and analyzed with a 2θ range of 4-35°, a step size of 0.010°, and a dwell time of 0.10 s.
All samples were analyzed at room temperature. Minerals were identified using the Crystallography Open Database (COD, rev. 266484; Downs and Hall-Wallace, 2003; Gražulis et al., 2009; 2012; 2015; Merkys et al., 2016; Quirós et al., 2018; Vaitkus et al., 2021; Merkys et al., 2023; Vaitkus et al., 2023). The RRUFF Database and mindat.org were also used to support X-ray identifications.
For transmission electron microscopy (TEM), samples were prepared from ethanol suspensions as simple grain mounts on holey carbon gold copper grids. Data was acquired at the University of Cincinnati using a Talos F200i (Thermofisher) instrument equipped with a 200 kV field emission gun. The instrument was operated in scanning transmission electron microscopy (STEM) mode and high-angle annular dark-field (HAADF), and was equipped with EDS. For high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) solutions were prepared at Miami University in-house using a lab SOP, available upon request. Samples, standards, and total procedural blanks were analyzed for concentrations using a Thermo iCap ICP-MS coupled to a CETAC ASX-560 autosampler at the University of Arkansas.
Results/Findings and Discussion
Results/Findings and Discussion
Image 1. Basic powder XRD results with major reflections for calcite, quartz, gypsum and oxyhydroxide labelled. Minor reflections are present and are consistent with feldspar minerals. Calcite is the dominant mineral in the sediment with gypsum being the second most abundant mineral. Peaks consistent with oxyhydroxide indicate a minor amount occurs. These are samples that are located near the entrance to the cave system.
Image 2. Basic powder XRD results with major reflections for calcite, quartz, gypsum and oxyhydroxide labelled. Minor reflections are present and are consistent with feldspar minerals and hydroxyapatite. Calcite is the dominant mineral in the sediment followed by gypsum. Peaks are consistent with minor amounts of oxyhydroxide and hydroxyapatite. These are samples that are located near the middle of the transect of the cave system.
Image 3. Basic powder XRD results with major reflections for calcite, quartz, gypsum and oxyhydroxide labelled. Minor reflections are present and are consistent with feldspar minerals and hydroxyapatite. Calcite is the dominant mineral in the sediment followed by gypsum. Peaks are consistent with minor amounts of oxyhydroxide and hydroxyapatite. These are samples that are located near the middle of the transect of the cave system.
Image 4. Basic powder XRD results with major reflections for calcite, quartz, gypsum, and hydroxyapatite. Minor reflections are present and are consistent with oxyhydroxide. Calcite is the dominant mineral in the sediment followed by gypsum. However, GEM-13 has a lesser amount of gypsum. Hydroxyapatite is most abundant in this sample set. These are samples that are located near the middle of the transect of the cave system (upper image of Figure 1).
Image 5. The < 2 μm size-fractionated clay XRD results under 4 conditions (air-dried, ethylene glycol, 400° C for 1 hour and 500° C for 30 minutes) for sample 7. Collapse and expansion of 10 Å peaks are consistent with a minor amount of montmorillonite (expansion to 17 Å under ethylene glycol followed by collapses to 10 Å on heating) and kaolinite (no 7Å peak shifts in ethylene glycol or 400° C heating, but the 7Å peak is destroyed at 550° C). A minor 14 Å peak is interpreted as chlorite (001).
Image 6. The < 2 μm size-fractionated clay XRD results under 4 conditions (air-dried, ethylene glycol, 400° C for 1 hour and 500°C for 30 minutes) for sample 13. Collapse and expansion of 10 Å peaks are consistent with montmorillonite (expansion to 17 Å under ethylene glycol followed by collapses to 10 Å on heating) and kaolinite (no 7Å peak shifts in ethylene glycol or 400° C heating, but the 7Å peak is destroyed at 550° C). The14 Å peak is interpreted as chlorite (001) as it does not expand.
Conclusions and Future Study
Conclusions and Future Study
Cave sediments contain significant amounts of gypsum and calcium.
Notable presence of gypsum may impact archaeological preservation and interpretation.
Gypsum is highly soluble and may dissolve and reprecipitate within sediment under a variety of potential conditions.
There are three general sediment sample types based on the dominant mineralogy observed:
Gypsum + calcite + minor quartz +/- feldspars
Calcite + quartz +/- feldspars
Gypsum + quartz + hydroxyapatite
Poorly-crystalline peaks broadly consistent with Fe-Al oxyhydroxides were observed in all powdered samples.
Previous TEM demonstrated high crystallinity of Fe-Al oxyhydroxides, but amorphous oxyhydroxides and other phases may exist.
Limited XRD results on two clay size fractions treated with weak acetic acid show that kaolinite and variable amounts of montmorillonite and minor amounts chlorite occur and Fe-Al oxyhydroxides are not observed in this size fraction for these samples.
Montmorillonite may indicate some degree of volcanic ash input directly or inidirectly into the system.
The presence of kaolinite is consistent with tropical soil minerals and is likely soil derived.
Chlorite may ultimately be derived from volcanic clastic sediments.
Hydroxyapatite-bearing samples are consistent with samples of high bulk P content.
Apatite content may be attributed to teeth and bone fragments.
Calcium and phosphate concentrations are likely influenced by biological accumulations (guano).
Analytical discrepancies between HR-ICP-MS and new XRD data may be due to Ca not being properly calibrated.
Bulk sample mineralogy is consistent with some aspects of previous major element chemistry, however large amounts of calcite and gypsum are inconsistent with the small concentrations of Ca observed in previous bulk chemistry and this could be due to differences in sampling scales of microscopic used in HR-ICP-MS, and relatively larger amounts used in XRD.
Sediment source is likely autogenic.
The large amounts of CaCO3 are interpreted to originate from the surrounding karst limestone.
These basic XRD results place new constraints and context on an understudied karst system.
In a protected cave system like this, sediment studies are efficient ways to investigate mineralogy.
Future Work
Run more clay-fractioned samples
Write up results to submit for a peer-reviewed publication
The following is an image of poster presented at the 2026 Undergraduate Research Forum
Acknowledgements
Acknowledgements
We thank the Department of Geology and Environmental Earth Sciences for additional support in attending this meeting. Phoebe Bathje was supported by a Miami University Honors College Fellowship and the Miami University Mineralogy and Petrology Career Development Fund.
We thank Dr. Melodie Fickenscher (U. of Cincinnati) for assistance with electron microscopy data collection.
We thank Dr. Barry Shaulis (U. Arkansas) for ICP-MS data collection.
We thank the Krekeler lab group for analytical and moral support!
References
References
Lafuente, B., Downs, R. T., Yang, H., & Stone, N. (2015). The power of databases: the RRUFF project. Highlights in Mineralogical Crystallography, T Armbruster and R M Danisi, Eds., Berlin, Germany, W. De Gruyter, 1–30.
Moore, D.M. and Reynolds, R.C. (1997) X-ray diffraction and identification and analysis of clay minerals. 2nd Edition, Oxford University Press, New York.
Monroe, W.H., (1980). Geology of the middle Tertiary formations of Puerto Rico: U.S. Geological Survey Professional Paper 953, Report: 93 p.; 1 Plate: 53.00 x 38.00 inches, https://doi.org/10.3133/pp953.
Poppe, L., Paskevich, V., Hathaway, J., and Blackwood, D. (2001). A laboratory manual for X-ray powder diffraction: U.S. Geological Survey Open-File Report 2001-41, 1 CD-ROM, https://doi.org/10.3133/ofr0141.
Price, J. R., & Velbel, M. A. (2003). Chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks. Chemical geology, 202(3-4), 397-416
Rodríguez Ramos, Reniel; Acosta-Colón, Angel A.; Pérez Reyes, Roberto. (2021). Una aproximación a la temporalidad absoluta del arte rupestre pictográfico de Puerto Rico Ciencia y Sociedad, vol. 46, núm. 3
NACE Career Readiness Competencies
NACE Career Readiness Competencies
Communication: clearly and professionally communicated the goals, results, and implications of this project during a poster presentation session at the Triple Joint GSA section meeting in Memphis, Tennessee in March, 2026.
Teamwork: collaborated with graduated research assistants and peer lab mates to collect, analyze, and interpret XRD data.
Technology: got trained to operate the Bruker D8 AdvanceECO Powder XRD in Shideler Hall
Research Compliance Protocols
Research Compliance Protocols
Standard chemistry lab safety
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