PRELIMINARY EXPERIMENTAL RESULTS FROM FLUORESCENCE SPECTROSCOPY OF ORGANIC CARBON IN CAVE SEDIMENTS FROM PUERTO RICO

Sediments deposited in caves can provide a record of past conditions as well as playing an important role in carbon storage in karst aquifers. They may also be a useful proxy to understand the storage of organic contaminants in the aquifer. Clastic cave sediments were collected from two caves in the northern karst belt of Puerto Rico; El Tallonal and Cueva Clara (part of the Rio Camuy system) and from Butler Cave in Virginia. The samples were composited in ca. 4 cm sections based on sedimentology, then dried and sieved to <2 mm.

The sieved sediments were analyzed for grain size and total organic carbon (TOC). The TOC concentrations are generally lowest in Butler Cave and highest in Tallonel Cave. The TOC concentrations did not correlate with mean particle size, % silt or % clay.

The samples were also analyzed for fluorescence characteristics to describe the molecular parameters. A 1:10 sediment to 0.1M NaOH solution was used for extraction before the filtered solution was run on a Horiba Jobin Yvon Fluorolog-3 Spectrofluorometer for excitation and emission and a BioTek Epoch Spectrophotometer for absorption. The fluorescence data are a combination of excitation, emission, and intensity called the emission-excitation matrix (EEM) graphed on a XYZ plot and are characterized using PARAFAC statistical analysis. The analysis also provides fluorescent indices (FI), the BIX (biological index), and the HI (humification index), which can differentiate between terrestrial (FI less than 1.4) and microbial-derived (FI greater than 1.9) organic carbon. The samples were analyzed along with background water samples; standards of humic acid, tyrosine, tryptophan; and a filter-comparison study. The EEM plots have considerable overlap in all caves with subtle distinctions between the systems.