CARBON INFLOWS AND FORMATION OF SHALLOW SALINE ENVIRONMENTS IN ARID CLIMATE BASINS

In dry climate regions, shallow hydrologic systems containing magmatic carbon dioxide (CO₂) manifest by their high concentration of dissolved inorganic carbon (DIC) and carbon isotopic contents (d¹³C) that deviate from biogenic and other continental sources. The carbon dynamics of shallow groundwater brines from the northern part of Sua Pan, one of the Makgadikgadi Pans complexes of northeastern Botswana, are unique and require a substantial source of CO₂ of uniform origin. Values of DIC as high as 5grams of carbon per liter (gC/L) have been measured and together with the d¹³C_DIC approximating -1.6±0.2‰, the compositions are compatible with the results that would be obtained because of isotopic equilibrium with CO₂ gas from geothermal systems. Spatial patterns in DIC concentrations and fluid conductivity show a linear anomaly to the northeast that may be correlated with variations in the geological setting where faults communicate saline magmatic fluids into the pans. The halogen content (ratio Cl/Br=470 to 695) is consistent with studies of fluid-inclusion brines from magmatic rocks and indicates that they are products of magmatic processes. Up to 52% of the water has been calculated to derive from a magmatic source. The sodium inventory on the other hand suggests an addition of secondary Na in the west that is likely coming from intense weathering of silicate rocks.