CHARACTERIZATION OF LAYER-BOUNDING SURFACES IN A GREAT BASIN STALAGMITE UTILIZING BOTH PETROGRAPHIC AND HIGH-RESOLUTION STABLE ISOTOPE ANALYSES
We conducted a petrographic and high-resolution stable isotopic analysis of the layer-bounding surfaces in stalagmite LMC-1 from Lehman Caves, Nevada. Fourteen 234U-230Th ages show that deposition occurred discontinuously between ~659 – 243 ka, with two hiatuses at ~243 ka and ~387 ka, within error of interglacial periods (MIS 7 and 11, respectively). A third, less well-defined hiatus at ~308 ka may also be the result of arid climate during MIS 9. One additional hiatus with a poorly constrained age (U-series ages show only that it occurred between ~659 – 423 ka) may have occurred during interglacial period MIS 13. Petrographic observations of these hiatuses reveal they are type L layer-bounding surfaces, suggesting arid Great Basin climates similar to the Holocene.
High-resolution (100 µm) stable isotope analyses drilled up to and across each hiatus reveal that δ18O values become progressively more negative towards the termination of each layer-bounding surface. If increases in evaporation or prior calcite precipitation had dominated at these hiatuses, an opposite trend would be expected, with δ18O values becoming progressively more positive. The isotopically light trend is thus interpreted to be the result of a consistent change in the processes that control the δ18O value of drip water in Lehman Cave such as a shift in the dominant sources of precipitation. δ13C values exhibit less consistency at the L surfaces, however, and may reflect multiple effects such as variations in vegetation density, soil water residence times, or variations in the pCO2 of the cave atmosphere. Overall, petrographic and high resolution stable isotope data of LMC-1 may offer an additional method of deciphering climate change that was not possible using coarse resolution stable isotope data.