The information stored in clastic cave sediments has been poorly explored relative to the more commonly studied chemical precipitates. Clastic cave sediments have the potential to provide evidence relevant to paleoclimate (Panno et al. 2004, White 2007, van Hengstum et al. 2010), elemental cycling (Vesper and White 2003, Hartland et al. 2012), subterranean ecosystems (Birdwell and Engel 2010, Pipan and Culver 2013, Husic et al. 2017, de Paula et al. 2020), paleohydrology (Granger et al. 2001, McCloskey and Keller 2009, Chess et al. 2010, Polk et al. 2013), and contaminant storage (Vesper and White 2004, Mahler et al. 2007, Torres et al. 2018). Organic carbon is a controlling factor in many of these processes, however, organic carbon concentrations in clastic cave deposits are generally not reported. While spatiotemporal heterogeneity in flow regime, geomorphology, and water chemistry adds complexity, deposited cave sediments experience consistent environmental conditions compared to subaerial fluvial systems (Bull 1981, de Paula et al. 2020). This property makes cave sediments valuable environments to explore organic matter dynamics between systems with differing hydrogeology.
Clastic cave sediments include all materials not precipitated in-situ. These may include detrital matter derived from the weathering of the host rock or surface-derived material (White 1988, Bosch and White 2007). The latter is generally considered to be the dominant source of clastic sediments and its introduction may be associated with collapsed structures such as sinkholes, in response to major storms, or associated with more global climate changes. Surface-derived sediments are transported into a system via gravity or associated with incoming recharge flow (Hart and Schurger 2005). These surface-derived sediments are injected and subsequently deposited in the subsurface where they are relatively protected from the erosive processes occurring at the surface. During speleogenesis, a cave system may undergo cycles of sediment accumulation and erosion in response to external forcing due to climatic events or, on a longer time scale, glacial and inter-glacial cycles (Anthony and Granger 2007, Farrant and Smart 2011, Asanidze et al. 2017). Geomorphological and land use changes may also alter the types and volumes of input from the surface.
Sedimentary deposits within fluvial systems are a known carbon sink, storing organic carbon introduced from upland areas (Martínez-Mena et al. 2019). Terrestrial organic carbon transport within a fluvial system is positively related to the onset of suspended sediment and in turn the erosive processes occurring within the catchment area (Galy et al. 2015). Cave systems are the subterranean equivalent of river systems where materials from upland areas are transported and deposited within a natural sink (Ford and Williams 2007, Nichols 2009). While the sediment carrying capacity of cave systems is significantly less (with some exceptions) than that of fluvial systems, sediment transport is sustained for longer periods of time after a hydrologic event (Husic et al. 2017). Recent works have shown that organic carbon concentrations associated with suspended sediment are a potentially significant pool of carbon in cave systems (McCarthy and McKay 2004, Cruz et al. 2005, Simon et al. 2007, Ban et al. 2008, Hartland et al. 2012, Husic et al. 2017). Deposited sediments are found in nearly all cave systems, and while suspended sediment as a carbon source has been explored, organic carbon associated with deposited cave sediments remains largely unreported.
Sediment input into a cave is episodic and the sediments may continue to persist with limited alteration due to relatively consistent environmental conditions and lack of sunlight. Work by Rossel et al. (2013) revealed that microbial activity in the absence of light played a significant role in organic carbon transformation. While the lack of light eliminates phototropic microbial activity, it has been demonstrated that alternative metabolic processes can flourish in these ambient, oligotrophic conditions (Simon et al. 2003, Engel 2007, Birdwell and Engel 2010, Ortiz et al. 2014, Galassi et al. 2016, Simon 2019, de Paula et al. 2020). The importance of microbial degradation and transformation of organic matter on the surface is well known, however, the role subterranean ecosystems play in carbon cycling within these systems remains poorly quantified (Butman et al. 2007, de Paula et al. 2020). Recent work has shown that initial chemical composition has little control on natural organic matter transformation but rather depends on the depositional environmental conditions (Schmidt et al. 2011, Marín-Spiotta et al. 2014, Martínez-Mena et al. 2019). Sediment organic carbon stored under ambient conditions within a cave could provide important insights into how and why environmental conditions play such an important role in organic matter transformation.
The sediments in this study were collected from the northern karst region of Puerto Rico, where abundant groundwater resources have promoted industrial and urban development, resulting in 25 Superfund sites on the EPA National Priority List (Padilla et al. 2001). Groundwater contamination in this region of Puerto Rico includes chlorinated volatile organic compounds (CVOCs), pesticides, and heavy metals. Although concentrations have decreased over time, contamination has persisted for more than 40 years (Yu et al. 2015, Torres et al. 2019). The concentration of organic matter present in a sediment is one of the primary controls on how organic contaminants are partitioned between the sorbed and dissolved phase (Schwarzenbach et al. 2016). Storage of sediment organic carbon within a karst system has important implications for contaminant transport and storage. Extreme weather events, such as hurricanes, could mobilize previously deposited subsurface sediments and stored contamination, resulting in potential human exposure to harmful compounds. The organic carbon data from cave systems in Puerto Rico reported in this study will help predict the long-term contaminant fate and support potential future remediation efforts.
In this study, organic carbon concentrations were obtained for clastic sediments from two caves in Puerto Rico with different surface connections and hydrological properties. The goal of this study was to determine the range and distribution of organic carbon in cave sediments along with potential correlations between carbon and sedimentological or hydrological properties. Paired nitrogen data were obtained in order to further characterize the sediment organic carbon using total organic carbon (TOC) and total organic nitrogen (TON) ratios. This study demonstrates the presence of carbon in cave sediments has a wider range than concentrations reported from suspended cave sediment and dissolved organic carbon (DOC) within cave streams. Despite the differing hydrologic conditions between the two cave systems, sediment TOC concentrations were comparable. The presence of organic carbon within deposited cave sediments is an important carbon reservoir that future works should consider regarding subsurface carbon cycling and contaminant transport.