Paleo-precipitation seasonality
Coupled authigenic clay—soil carbonate oxygen isotopes
Inferring precipitation seasonality from the difference between clay and soil carbonate oxygen isotope ratios is not new—it was first applied over 20 years ago, but it required numerous assumptions and was never widely adopted. However, recent advances are eliminating these assumptions. Carbonate clumped isotope thermometry and numerical modeling are helping constrain the nuanced seasonality of soil carbonate formation, and massive increases in clay data coverage reveal coherent clay-carbonate δ18O trends that align with theoretical predictions.
My work is advancing this coupled proxy approach with new clay data, geochemical modeling, and modern validation to refine interpretations of precipitation seasonality. Ultimately, this system will bring new constraints to the two main drivers of paleo-precipitation seasonality: (1) past atmospheric dynamics, and (2) paleotopography.
Kukla et al., “Drier winters linked to Cenozoic open habitat expansion in North America”. AGU Advances (2022). DOI: 10.1029/2021AV000566. [pdf].
Gao et al., (with Kukla) 2021, “Terrestrial climate in mid-latitude East Asia from the latest Cretaceous to the earliest Paleogene: A multiproxy record from the Songliao Basin in northeastern China”. Earth Science Reviews. [pdf].
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Precipitation seasonality reflects atmospheric dynamics and affects ecosystems
The west-east winter-wet to summer-wet seasonality gradient in the western U.S. is set by topography and competing cold and warm-season moisture sources. This gradient exerts significant influence over the distribution of forests and grasslands. Reconstructing this precipitation seasonality gradient it in Earth history, however, is challenging because few geologic materials record seasonal-scale processes.
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Clay measurements for comparison with soil carbonates
After screening for smectites and kaolinites and removing contaminants, we measure pedogenic clay or altered ash oxygen isotopes using a laser ablation approach following Sharp, 1990 (see also Chamberlain et al., 1999). The MAT 253+ shown here measures both δ18O and δ17O.
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Seasonality of mineral formation constrains rainfall
Soil carbonates tend to form in summer when climate is winter-wet (and spring/fall when summer-wet), while clays form much more slowly, integrating the annual mean. Therefore, given a seasonal cycle of rainfall δ18O, the difference between clay and carbonate δ18O can be used to infer the direction and magnitude of precipitation seasonality. The above example applies to regions with a low-δ18Ο winter (like the mid-latitudes) but the same approach can be applied to monsoon regions (e.g. Stern et al., 1997; Tabor and Montañez, 2002).
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Single-mineral thermometers help disentangle temperature and δ18O effects
Clays and carbonates both have stable isotope single-mineral thermometers (δ18O-δD for clays; clumped isotopes for carbonates). With these analyses, it is possible to disentangle the contributions of formation temperature differences versus soil water δ18O differences to the Δδ18O signal. Thus, this approach can be used to infer rainfall and temperature seasonality and to test clumped-derived interpretations about mineral formation seasonality.