A speleothem from a Nevada cave

Cave deposits, also known as speleothems, form as a result of steep chemical gradients that exist in the near-surface. When rainwater hits the ground and begins to infiltrate, it goes from a low CO₂ environment (the atmosphere is ~400 ppm) to a high CO₂ environment (1000s ppm) caused by biological respiration. This increases the water’s acidity and causes soil minerals to dissolve. However, as the mineral-rich water infiltrates into the cave environment, it encounters a lower CO₂ environment as caves can exchange air with the atmosphere – cave air CO₂ can even approach atmospheric levels. This forces CO₂ to rapidly degas as it drips onto the cave floor and ultimately makes mineral deposits like stalagmites.

Speleothems have proved a useful archive for reconstructing ancient environments of the Earth's recent past because they carry a "memory" of the original formation water in their oxygen isotope composition. Traditionally, oxygen isotope composition has been measured as the relative abundance of the ¹⁸O and ¹⁶O isotopes (i.e., ¹⁸O/¹⁶O, which is reported in "δ" notation as δ¹⁸O). Because the isotopes have different masses, they behave slightly different in chemical reactions. As a result, a sample's δ¹⁸O is responsive to regional climate processes like the conditions at the original oceanic moisture source, amount of rainout, rainfall seasonality, and temperature. Local evaporation and “cave kinetic” processes (e.g., the rate of degassing, prior calcite precipitation above the cave) can also influence δ¹⁸O.

Speleothem paleoclimatologists have widely used stalagmite δ¹⁸O to constrain the rate and timing of hydrologic change through the late Quaternary (the last ~1 million years). However, rigorously demonstrating which environmental parameters drive a sample's δ¹⁸O is more difficult, which is where my work comes in. As a postdoc at the University of Michigan, I have been exploring how we can use the rarest stable isotope of oxygen ¹⁷O to provide an additional constraint - this is done through the Δ′¹⁷O parameter ("cap prime 17 O"). The plot to the side shows how different processes should affect the isotope systems. Even though all of the process cause a similar positive change in δ¹⁸O, they have different effects on Δ′¹⁷O parameter. We should therefore be able to use triple oxygen isotopes to distinguish isotopic drivers better than δ¹⁸O alone.

Summary of triple oxygen isotope drivers relevant to speleothem triple oxygen isotopes in δ¹⁸O vs. Δ′¹⁷O space. Processes influence a hypothetical endmember isotope composition (red star). The cave kinetics, Rayleigh distillation (i.e., degree of rainout), mineralization temperature (T, shortest gray arrow) and evaporation/seasonality trends have characteristic slopes of +7, 0, -5, and ≤ -2 per meg/‰, respectively. "Seasonality" refers to precipitation or infiltration seasonality. The mineralization temperature trend is shown for a 4 °C change (an ~1 ‰ δ¹⁸O change), but its potential influence can be constrained by expected temperature variability (e.g., do samples span a deglaciation or are they contained entirely within an interglacial period?).

Here’s an example of how it works in real samples. The data show below are from two Nevada stalagmites – Lehman and Leviathan Caves. The two samples have qualitatively similar δ¹⁸O patterns, which is expected given that they should both experience about the same climate. However, the absolute values are different. The Lehman Caves data are a highly exaggerated version of the Leviathan Cave data, suggesting it has been affected by local processes like variable cave kinetic effects, evaporation, or infiltration seasonality. The Δ′¹⁷O data are able to give us greater insight into which of these effects is most important. On the right, you can see that the combined dataset falls between the local evaporation and infiltration seasonality lines, suggesting that some combination of these is likely to have affected the Lehman Caves sample.

Interested in learning more? Check out my presentation at the 2020 AGU Fall Meeting.