TYLER HUTH

triple oxygen isotopes in speleothems

  • Research
    • triple oxygen isotopes in speleothems
    • laminated soil carbonate rinds
    • Holocene climate from Tibetan lakes
  • Publications and CV
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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.

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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.

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Interested in learning more? Check out my presentation at the 2020 AGU Fall Meeting.
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  • Research
    • triple oxygen isotopes in speleothems
    • laminated soil carbonate rinds
    • Holocene climate from Tibetan lakes
  • Publications and CV