Arkansas Field Workshop

The Biogeochemistry working group developed two specific science questions that the members of the group plan to address over the final year of the RCN.  Both questions relate to changes in biogeochemical reactions across two gradients including the silicate-carbonate mineral continuum and salinity.  The questions are: (1) What are the magnitudes of redox gradients within the carbonate critical zone and their impacts on biogeochemical reaction products such as nutrient concentrations, greenhouse gas production and consumption, and trace metal cycling and (2) What are the impacts of variations in organic carbon loading to the carbonate critical zone particularly related to energy transport through the carbonate critical zone?  These questions will be addressed through compilation, syntheses, and analyses of legacy data depending on their availability at targeted locations.  Locations were chosen based on differing salinity and lithological characteristics and where WG members had prior knowledge of the region.

The Rates and Time Working Group developed two related hypotheses.  These include: (1) The relative contributions of matrix and conduit flow are influenced by the type of karst. High frequency nitrate data from contrasting karst types in Florida and Missouri are being examined using discharge hysteresis plots. (2) When carbonate systems respond to land use and climate change, a threshold exists where transport, weathering, and related processes are faster in carbonate systems compared with silicate-dominated systems.  The second hypothesis focused much of the Working Group activities over the past year and included collecting, synthesizing, and evaluated 50-year discharge data from 36 European springs and 30 to 50-year nitrate concentration data from 11 US springs.  The discharge data were classified by their variability and compared with surrounding land use.   Both increasing nitrate concentration and stable concentrations were observed in each karst region but the data did not show a distinct pattern with karst type, which might have been expected if storage was a key factor.  One of the lessons learned, which may be the focus of a future paper, is that long-term time series data are sparse but necessary for cross site comparisons.

The Boundaries and Scales Working Group switched their focus from their original question of what controls the bottom boundary of the carbonate critical zone to one related to the location of erosion within the carbonate critical zone.  The primary question this group plans to address is: What controls the partitioning of erosional fluxes between flow across the surface compared with flow through karst conduits?  Multiple potential variables that may control the flux including: climate (temperature, precipitation, PET), rock properties across the carbonate-silicate spectrum (silicate content and distribution), permeability/porosity, tectonic setting (relief/slope), soil type, and land use/vegetation.  An evaluation of this problem will use the karstification index, which is derived from the fraction of landscape that drains into sinkholes.  This index will be calculated from LIDAR derived land surface DEMs and two published data sets of sinkhole distribution in the US.

All working groups have been and will continue to use available legacy data and the ad hoc data committee focuses on archiving the data synthesis. To that end, the group has created and are continuing to develop a public Group called “Carbonate Critical Zone RCN” on Hydroshare that will be a repository for compiled data sets related to the carbonate critical zone.  The Hydroshare site is supplemented by the RCN website and both sites have information that includes Collab notebooks, field trip videos, GIS files, and instructions on using Hydroshare.