CHARACTERIZATION OF NATURAL ORGANIC MATTER (NOM) WITHIN VOLCANIC (LAVA TUBE) CAVES IN A POST-FIRE REGIME

Wildfires are natural climatic events that are increasing in intensity and frequency in the western United States (US) consistently with climate trends expected from anthropogenic climate change. The increasing aridity of fuel sources is escalating fire danger throughout the western US, with the probability of extreme fire years predicted to increase. Wildfires are known to impact soils, vegetation and hydrological processes, but the impact that high-magnitude fire events have on subsurface cave ecosystems, particularly lava tube volcanic caves, is poorly understood. The high magnitude Caldwell (2020) and Antelope (2021) fires burned 97% of the surface area at Lava Beds National Monument (LABE), a national monument protecting numerous volcanic caves. Post-fire soil, and cave water samples were collected from both unburned and burned cave sites at LABE. Pre-fire data collected from these caves between 2017 and 2019 have been compared with the new post-fire samples to determine the impact of high-magnitude fire on these environments. 70 cave water samples from six caves were collected four, nine, and thirteen months after the Antelope fire along with twenty-two soil samples collected from the surface directly above the caves. Water samples from both caves and leached soil waters were analyzed for pH, specific conductance, major inorganic ions, trace elements, and dissolved organic carbon (DOC). All water samples were analyzed using Jobin Yvon Aqualog Fluorometer to characterize the DOM with corrected three-dimensional fluorescence excitation emission matrix (EEM) data fitted to a parallel factor (PARAFAC) model. Three major organic matter (OM) constituents were present in cave waters post-fire: two humic-like components and one protein-like component. One-year post-fire, the humic like components trended slightly upward in total composition in cave waters, while the protein-like component trended downward. In one cave analyzed, the humic-like component #1 changed from 44.1% of the average OM composition to 47.7% of the average OM composition one-year post-fire. The humic-like component #2 changed slightly from 2% of the average OM composition to 2.7%. The protein-like component changed from 35.9% to 24%. These findings suggest that high magnitude wildfires could have a potential impact on the organic matter composition within cave waters over time post-fire.