Importance of suspended sediments and dissolved organic carbon to carbon exports in karst – The Vadu Crişului karst basin in the Pădurea Craiului Mountains, Romania

Abstract

Exports of dissolved inorganic carbon (DIC) from karst aquifers are a significant contribution to the global carbon cycle. Contributions of particulate inorganic carbon (PIC) and dissolved organic carbon (DOC) in karst groundwater are often unaccounted for in carbon budgets. This investigation considers field chemistry, discrete samples, and continuous monitoring data collected between October 2016 and December 2016 to quantitatively evaluate the net export of dissolved solutes, total suspended sediments (TSS), and DOC within the Vadu Crişului karst basin in the Pădurea Craiului Mountains of northwest Romania. Measurements of carbon isotopes (δ13C) in DIC and in bedrock, specific ultraviolet absorption (SUVA), and carbon to nitrogen (C:N) ratios were used to evaluate the relative contribution of different sources of carbon to the aqueous system. δ13CDIC at the aquifer input trend toward an atmospheric signal in winter with lower DOC concentrations and SUVA values and C:N ratios corresponding to degraded carbon sources, illustrating the reduction in microbial respiration in soil with cold temperatures. At the spring, δ13CDIC values trend more toward a soil-based signal after storm events that bring increased DOC concentrations with SUVA values corresponding to significantly degraded carbon.Dissolution in this karst basin is principally driven by the carbonate equilibrium reactions, and direct meteoric recharge only accounts for 4 to 13% of observed discharge. Most recharge enters the karst basin through infiltration into dolines and epikarst on the Zece Hotare karst plateau, and this results in a broadly stable chemical signature of spring water. Storm events do have a significant impact on mechanical erosion and chemical processes operating in the karst basin; the addition of PIC and TSS flux increase landscape erosion rates by 1.1 to 1.2% and 7.9 to 8.3%, respectively, above the denudation rate computed by dissolved solutes (36.5–56.9 mm/ka). The modeled annual flux of DIC from this karst basin, 1.37 × 105 to 1.64 × 105 kg/yr, scales to a global CO2 flux of 7.08 × 1011 to 1.06 × 1012 kg/yr. Adding a modeled flux of DOC increases the estimates of CO2 flux by 12% to 22%, a significant addition to the global carbon cycle.