INITIAL INDUCTIVELY COUPLED PLASMA - MASS SPECTROMETRY (ICP-MS) AND SCANNING TRANSMISSION ELECTRON MICROSCOPY - ENERGY DISPERSIVE SPECTROSCOPY (STEM-EDS) INVESTIGATION OF THE GEMELOS CAVE NEAR TORRECILLAS, PUERTO RICO

Study of cave sediments is important to understanding the geological record of materials in karst systems, along with environmental and anthropogenic influences on karst systems. A comparatively understudied karst system in regards to cave sediment is located near Torrecillas, Puerto Rico. It is recognized that there very likely has been an extensive native habitation of these caves that has occurred over at least a few thousand years. Therefore detailed studies on the cave sediments can offer guidelines on the constraints of future archaeological study in the cave as well as insight into the broader environmental record and system. In January of 2024, samples (n= 14) from the upper 2 cm of sediment on the floor of the Gemelos cave in this system were obtained. These were investigated using inductively coupled plasma - mass spectrometry (ICP-MS) and scanning transmission electron microscopy - energy dispersive spectroscopy (STEM-EDS) to establish the nature of their geochemical variation. ICP-MS data indicates that concentration ranges of major elements of interest are Fe₂O₃ (0.99 wt% to 4.51 wt%), Al₂O₃ (1.32 wt% to 6.00 wt%), CaO (0.98 to 2.01 wt%), Na₂O (0.07 to 0.29 wt%), K₂O (0.07 to 0.51 wt.%). Concentrations of Fe₂O₃ and Al₂O₃ show very high correlation with r² = 0.97 and p <0.001. Phosphorus varies from 0.76 to 4.14 wt% and is interpreted as being largely biogenic from mixed guano. These concentrations provide a wide range of chemical weathering index (CIA) values however from 37.5 (low) to 83.2 (high), with the average values reflecting moderate CIA (58.7). Biogenic calcium phosphate and possible past human activity involving wood burning or ash associated with the cave may contribute to variable Ca and K concentrations in the sediment, giving rise to the extremely variable CIA values. Siderophile elements have variable concentrations (V: 31 to 126 ppm; Cr: 38 to 177 ppm; Ni: 17 to 74 ppm; Co: 4 ppm to 37 ppm ) but generally show strong correlations amongst themselves suggesting a single geogenic source for these elements. STEM-EDS data on selected samples show that fine aggregates of Fe-Al oxyhydroxide minerals are most abundant and are intimately mixed throughout the samples. This in part explains the strong correlations between Fe₂O₃, Al₂O₃, and siderophile elements.