CHARACTERIZATION OF LAYER-BOUNDING SURFACES IN A GREAT BASIN STALAGMITE UTILIZING BOTH PETROGRAPHIC AND HIGH-RESOLUTION STABLE ISOTOPE ANALYSES

Layer-bounding surfaces in stalagmites represent hiatuses in growth due to either erosion during wet climate periods (type E layer-bounding surfaces), or a period of lesser deposition when climate is relatively arid (type L layer-bounding surfaces; Railsback et al., 2013, Int. J. Spel., 42, 167). Accounting for layer-bounding surfaces not only offers an additional method of tracking past climate change, but can also be useful when constructing stalagmite chronologies.

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.