Decreasing inputs of particulate organic matter from wetland and terrestrial habitats into a karst subterranean estuary during the last millennium

The mixing of seawater and terrestrial-borne meteoric waters in the subsurface on carbonate landscapes creates karst subterranean estuaries, a unique coastal environment with poorly understood ecosystem functioning, biogeochemical cycles and impact on submarine groundwater discharge. Networks of flooded caves in these coastal platforms accelerate hydrologic transport and promote material exchange between the marine and terrestrial environments while they can accumulate sediment and record internal environmental change. Using sediment cores from a flooded coastal cave (El Aerolito) on western Cozumel Island (Mexico), we document that deposition of non-marine particulate organic matter within the marine-sector of a karst subterranean estuary decreased over the last 1000 years. Despite the presence of a direct submarine opening, the organic matter archived in the sediment is primarily terrestrial in source (δ¹³C_org values range from −22.5‰ to −27.1‰). These results suggest that the inland opening surrounded by mangroves and dry tropical forest vegetation was a major source of particulate organic matter traveling through the conduit with groundwater flow and, eventually, discharging to the coastal sea. However, all cores show a concomitant decrease in bulk organic matter with an enrichment of δ¹³C_org values over time. The observed pattern is likely a result of changing vegetation in the area - perhaps due to a diminishing mangrove habitat - which decreases the flux of particulate organic matter into cave passages. These results demonstrate that vegetation changes in adjacent aquatic, wetland and terrestrial environments can impact the source and quantity of particulate organic matter that is fluxed into and through the karst subterranean estuary. Ongoing microfossil analysis will allow us to examine the ecological impact of changing organic matter flux on the benthic meiofauna community.