ORGANIC CARBON CONCENTRATION IN CAVE SEDIMENTS AS A TRACER OF INPUT FROM HURRICANE MARIA

Caves are unique sediment sinks with the ability to collect and store allogenic sediment and organic carbon for long periods of time. Organic carbon associated with the allogenic sediments plays a critical role in controlling contaminant fate and transport in the karst system, influencing the redox conditions of the sediments, and supporting cave ecosystems. The source of the carbon can be either from ecosystems within the cave or be injected from the land surface via sinkholes and similar direct means of recharge. In 2017, Hurricane Maria injected massive amounts of sediments into many of the Puerto Rican caves. El Tallonal Cave, located in Puerto Rico’s northern karst belt, is one such cave. El Tallonal is an active stream cave known to connect to upstream sinkholes. Surface sediment samples in the cave were collected in July 2018 for comparison with samples collected prior to Hurricane Maria in 2017. The samples were analyzed for total carbon and organic carbon using an Elementar CHS analyzer. The samples analysed for organic carbon were first treated with acid to remove any carbonates and then analyzed for total carbon.

Sets of pre/post Maria samples were collected from a small passage at the back of the cave (upstream), a wider passage in the middle of the cave, and near the exit of the cave where a dam and weir system typically create a pond used for water supply. At the most upstream location, the organic carbon (expressed as the fraction of organic carbon - a percent of total sediment mass) decreased after the storm (0.47 to 0.18%). In contrast, the further downstream locations had significant pre/post Maria increases in organic carbon (0.31 to 0.88% in the middle of the cave; 0.16 to 0.97% near the front of the cave). These results show that changes in the organic carbon concentrations within the cave can vary with location after a major storm event. The changes in the carbon may be influenced by proximity to an injection site, how the configuration of the cave passage controls zones of deposition and erosion, and how much influence the storm surge had on the specific sediment deposits. Additional data will be obtained from sediment cores to better understand the range of organic carbon concentration as well as to quantify textural and detailed chemical properties.