CARBON EXPORTS IN THE CRITICAL ZONE OF KARST AQUIFERS: THE PĂDUREA CRAIULUI MOUNTAINS OF NORTHWEST ROMANIA

Continuous monitoring data and discrete sample analyses spanning the fall and winter of 2016 provide insight into critical zone processes within the epigenetic karst aquifers of the Pădurea Craiului Mountains of northwestern Romania. This work focuses on meteoric recharge on the Zeze Hotare karst plateau to spring discharge along the Criul Repede River. Results based on rating curves quantitatively evaluate the net export of dissolved solutes including: dissolved inorganic carbon (DIC), total suspended sediments (TSS) including particulate inorganic carbon (PIC), and dissolved organic carbon (DOC). Inorganic carbon isotopes in DIC (δ¹³C_DIC) and in bedrock, specific ultraviolet absorption (SUVA), and carbon to nitrogen (C:N) ratios from the water samples were used to evaluate the relative contribution of carbon sources to the aqueous system. Four key outcomes of this study are summarized as follows:

• Direct meteoric recharge accounts for 4 to 13% of observed discharge; most recharge enters the karst basin through infiltration into dolines and epikarst on the karst plateau.
• Modeled annual DIC flux is 1.37 x 10⁵ to 1.64 x 10⁵ kg/yr from this basin. This scales to a global CO₂ flux of 7.08 x 10¹¹ to 1.06 x 10¹² kg/yr. Adding modeled DOC flux increases the estimates by 12 to 22%.
• During winter, the δ¹³C_DIC of recharge trends toward an atmospheric-based value with lower DOC concentrations that have C:N and SUVA corresponding to less degraded carbon. In aquifer discharge, δ¹³C_DIC values trend more toward a soil-based signal after storm events that bring increased DOC concentrations that have C:N and SUVA corresponding to more degraded carbon.
• The denudation rate computed by DIC alone (5–56.9 mm/ka) is increased by 1.1 to 1.2% by adding the PIC flux and 7.9 to 8.3% by adding the TSS flux.

Results contribute the following key findings to critical zone science in karst aquifers: Mass balance methods are one effective method to estimating carbon flux; PIC and DOC are significant contributions to total flux. Seasonal changes in carbon source are evident and influenced by temperature, recharge, and microbial respiration. Suspended sediment outpaces dissolved solutes during some storm events, demonstrating the relative importance of mechanical versus chemical weathering.