RELATIONSHIPS BETWEEN METAL CHEMISTRY, CARBON CONTENT, AND PARTICLE SIZE IN CLASTIC CAVE SEDIMENTS

Caves function as sinks for clastic sediment that is generated both within the cave and transported into the cave from without. To understand the role that sediments play in contaminant storage and in-cave geochemical processes, more detailed studies of the sediment characteristics need to be completed. For this study, nine cave sediment samples were compared for particle size, total organic carbon, total nitrogen, and XRF bulk chemistry. Mineralogy was determined on both the bulk and the clay-fraction of the samples using XRD. All analyses were conducted on aliquots of 4-cm composites taken from core samples. Samples were collected from two caves in Puerto Rico: El Tallonal (TAL) and Cueva Clara (CAM) and from one cave in Virginia: Butler Cave (BTC). BTC contains older sediment; the sediment in CAM was deposited during Hurricane Maria in 2017. The age of sediment in TAL is unknown.

Grain sizes less than 2 mm comprised most of the sediment in the three caves sampled. Some clasts larger than 2 mm were present in BTC and CAM sediment but were not included as part of these analyses. Total carbon ranged from 0.07 to 1.12 wt%; total nitrogen ranged from 0.02 to 0.09 wt%. In BTC and TAL, sediments were classified based on their location relative to the water level; higher concentrations of carbon and nitrogen are present below the water level than above.

A Pearson correlation matrix was used to determine the significance of the relationships between elemental concentrations. Preliminary XRF data on the composites indicate that Si and Al are negatively correlated (p<0.05), supporting that clay mineral and quartz co-exist. XRD data indicated that the sediments in TAL and BTC were composed of 70-80% quartz while the CAM sediments were composed of 50-55% quartz. Previous work collected on a larger set of TAL cave sediments found strong positive relationships between trace metals Cr and Mn and Ti. These trends were not present in BTC or CAM sediments. These data suggest that trends in trace metal chemistry could be indicative of variations in sediment sources.

Additional analyses are underway to determine surface area via BET and to include an additional the full set of analyses on ca. 200 samples.