Fossilized Mollusks used to determine Cenozoic climate and elevation of the Himalayan-Tibetan Plateau

Clumped isotope thermometry of modern and fossil snail shells from the Himalayan-Tibetan Plateau: Implications for paleoclimate and Paleo-elevation reconstruction

Yang Wang, Benjamin Passey, Rupsa Roy, Tao Deng, Shijun Jiang, Chance Hannold, Xiaoming Wang, Eric Lochner, and Aradhna Tripati

Summarized by Brynn Crocker, pursuing a master’s in teaching at Binghamton University with a bachelors in Geology. 

What data were used: Both fossilized and modern aragonite mollusk shells were collected from seven different lakes within the Tibetan Plateau. The fossils collected from these sites were dated to be of Cenozoic age. Clumps of carbon 13 and oxygen 18 (isotopes of carbon and oxygen) were measured to determine paleo-temperatures. The formation of the Himalayan Mountains is thought to have had a large impact on the regional climate during the time. Mollusk fossils are great archives for determining paleoclimate.

Methods: This study used X-ray diffraction to determine the values of C13 and 18O bonds (clumps) within the shells. These clumps help determine paleo temperatures and elevations. Modern shells both alive and dead were collected from the lakes in the Tibetan Plateau. The fossil mollusks were collected from fine grained sandstone, indicating that they were not transported there but lived in the freshwater lakes. These shells were then analyzed to find their clump values, which were then compared to modern temperatures. Intact Cenozoic fossil shells were then collected and analyzed to find their clump values. Intact shells were used to avoid using shells that have gone through any diagenetic alteration (changes to the fossils through heat,  pressure, and chemistry). Shells that contain calcite indicate diagenesis. Trace amounts of calcite yield temperatures of an average ~10º C lower than those with no calcite from the same strata. Shells were cleaned using HCl (hydrochloric acid) solution and then rinsed with distilled water. Modern shells were soaked in 30% H2O2 (hydrogen peroxide) to remove any organic matter. The isotope clump data was reduced using both the Henkes calibration and the Eagle calibration.

Black and white map showing the location of the study areas.
This figure shows the region of study and the basins within the area.


Results: After analyzing the 13C-18O clumps it was determined that southwest Tibet was warmer 4-5 Ma than today and paleo-elevation was similar to today. Using the Henkes calibration of temperatures calculated from the clump values, the temperature of the Himalayan-Tibetan Plateau ranges from 1ºC to 17ºC, with a mean of 10ºC. Using Eagles calibration, the temperature values range from 8ºC to 21ºC, averaging 16ºC. The Henkes calibration is better used for freshwater shells. There were no former long term temperature records for the lakes within the Tibetan plateau. The difference in the modern shell clump values and the fossil clump values can be explained by a change in global climate. The temperature difference between fossil shells and modern shells, after adjusting for temperature change due to sampling elevation difference, is similar to the change in the global mean temperature since the Pliocene warm period. This result tells us that the elevation during the Cenozoic was similar to today. These findings have important implications for paleoclimate and paleo-elevation reconstructions using clumped isotope data from aragonite fossil shells.

Why is this study important? This study provides additional paleo-temperature data that can be used for future paleoclimate research. The affect that tectonic events have on our climate can be significant and the significance of the Himalayan Orogeny on the climate is still disputed. This study can provide more insight on the temperatures of the surrounding areas during that time. Understanding the paleoclimate of our planet can help us better understand how it will react to things in the future.

Chart with water oxygen 18 values on the y-axis and study sites on the x-axis.
This figure represents the calculated d18O values using the Henkes calibration vs the Eagles calibration vs the actual d18O values of the water from the lake sites. The error bars indicate 1 standard deviation.

Citation: Wang, Y., Passey, B., Roy, R., Deng, T., Jiang, S., Hannold, C., … & Tripati, A. (2021). Clumped isotope thermometry of modern and fossil snail shells from the Himalayan-Tibetan Plateau: Implications for paleoclimate and paleoelevation reconstructions. GSA Bulletin133(7-8), 1370-1380.

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