GROWTH HISTORY AND COMPOSITION OF A LACUSTRINE TUFA DOME FROM WINNEMUCCA DRY LAKE, NV, USA

Numerous tufa domes and towers are observed around the margins of Winnemucca Dry Lake, NV, USA, a desiccated subbasin of pluvial Lake Lahontan. A 2.5-meter-wide concentrically-layered tufa mound was sampled at high-resolution; ¹⁴C, isotope, and minor elemental data record a 7750-year depositional record that includes the Last Glacial Maximum and most recent Lahontan highstand. Eight radiocarbon dates obtained along the growth axis were used to develop an age model for tufa deposition. The deposit built radially outward from a central nucleation point, with decimeter-scale layers defined by variations in texture. Two distinct tufa types are observed; the inner section is composed of thinolite, with a dense core comprised of very small crystals (<0.25 cm) and an outer layer composed of larger, ~3 cm-long crystals that grade into dense calcite fans. The outer section consists of two distinct layers of thrombolitic calcite tufas with a dendritic mesofabric. Deposition of thinolites began during the LGM (~21 ka), and continued until ~17 ka. A sharp boundary between the thinolite and thrombolitic tufa may indicate a hiatus in deposition; thrombolitic tufa deposition began ~16 ka and continued until 13 ka. Thinolites are pseudomorphs after ikaite, indicating deposition in a near-freezing, anoxic environment. Thrombolitic textures, however, indicate a possible microbial substrate and warmer, oxygenated waters. The δ¹³C and δ¹⁸O compositions do not covary, nor are they consistent with published lake level models, indicating that tufas deposited in lake bottom mixing environments may not accurately reflect levels of lake fill and evaporation cycles. However, thrombolitic tufas demonstrate increased growth rate, Mg/Ca, Sr/Ca, δ¹³C_org, and TOC compared to thinolitic tufas, supporting an interpretation of warming conditions, and also indicating a potential increase in microbial biomass. These observations support the hypothesis that thinolite tufa growth is dominated by physico-chemical mixing processes, whereas thrombolitic tufas grow via more complex, microbially-influenced mechanisms. Specific environments of tufa deposition can vary considerably within the same deposit, and these environmental changes need to be considered carefully when assessing tufas for records of paleolimnological change.