Mapping paleo-lacustrine environments and long-term climate trends using groundwater chemistry

Groundwater chemical and isotopic variability are used to identify and map temporal changes in the paleo-lacustrine environment and the long-term climate trends in the Closed Basin of the San Luis Valley, Colorado, USA. Total dissolved solids, water color, δ¹³C, SO₄²⁻, Cl⁻, and Na⁺ excess are used to identify chemical reaction zones. The reaction zones include methanogenic zones, which represent organic-rich reducing shallow lake conditions; gypsum and halite dissolution zones, which represent saline and playa lakes; and ion exchange zones, which represent organic-rich lakes. The reaction zone maps and vertical stratification depict both long-term and rapid changes in the paleo-lacustrine environmental conditions and changes in the climate regime of the Closed Basin. Based on the reaction zone mapping, the general scheme of the late Pleistocene-Holocene environmental regime of the Closed Basin has been worked out. The upper portion of the confined aquifer was deposited in the organic-rich late Pleistocene Lake Sipapu, which filled the Closed Basin after the draining of the Plio-Pleistocene Lake Alamosa. As the organic-rich lake shrank in size due to the drying climate, the unconfined aquifer sediments were deposited as fluvial sediments and in a series of smaller string lakes. In the string lakes, organic-rich sediments were deposited, and gypsum and halite precipitated as the lakes fluctuated between swamps, fresh and saline lakes, and playas.