A recent study published in the Journal of Geophysical Research: Atmospheres is the first to examine the oxygen stable isotope (δ¹⁸O) records of a collection of four ice cores drilled in 2019 from Earth's highest tropical mountain, Nevado Huascarán, to offer valuable insights into the behavior of δ¹⁸O in the tropical Andes.

The annually resolved δ¹⁸O records, spanning the most recent six decades, from 1960 to 2019, were assessed for statistical relationships with measured meteorological and reanalysis data to gauge whether the δ¹⁸O signals are driven by the same mechanisms on the mountain Col, the lowest point on a mountain ridge between two peaks, at 6,050 meters, and on the mountain Summit, at 6,768 meters.

The study results show that the ice core-derived δ¹⁸O records at both elevations significantly correlate to sea surface temperatures in the NINO3.4 region of the tropical Pacific Ocean. The ice core-derived δ¹⁸O records at both elevations also significantly correlate to rainfall amounts across the Amazon Basin. 

Such findings are consistent with previous studies showing a connection between the El Niño Southern Oscillation (ENSO) and tropical precipitation and provide further evidence that the initial drivers of δ¹⁸O in Peruvian ice cores can be explained by the intensity of rainfall leading up to the deposition of snow on the glaciers.

However, the authors also demonstrated that the relationship between δ¹⁸O and temperature is strengthening at a faster rate in the Summit core records than in the Col core records. From this, it was suggested that contemporary warming trends have an enhanced influence on water isotope fractionation at higher elevations. 

The authors thereby propose the possibility of a mechanistic "switch" between a largely rainfall-driven isotope relationship at lower altitudes and a more temperature-driven relationship at higher altitudes. They explain that local climatic conditions are more reflective of polar conditions at high elevations in the tropics. 

Thus, temperature may play a more significant role in determining the final isotopic composition of the snowfall, though precipitation amount remains the dominant initial control.

Understanding the drivers of δ¹⁸O variability in tropical ice cores is vital for paleoclimatic research. The ice cores used in the study are expected to contain a δ¹⁸O history that extends back 20,000 years or more, meaning that there is a potential for reconstructing the relationship between ENSO and rainfall over the Amazon going back to the Late Glacial Stage. 

The study also gives insights into future modeling studies that seek to predict the behavior of oxygen-stable isotopes in the upper troposphere.

The publication's lead author, Austin Weber, is a Ph.D. student in the School of Earth Sciences at The Ohio State University. Co-authors include the Ice Core Paleoclimatology Group members at the Byrd Polar and Climate Research Center led by Earth Sciences Distinguished University Professor Lonnie Thompson and Geography Distinguished University Professor Ellen Mosley-Thompson.

The National Science Foundation supported this study.

To learn more about this study, visit JGR Atmospheres or download the PDF.

UPDATED: 11-15-23- Visit the Ohio State News to read about the news article titled, "Ice cores from Earth’s highest tropical peak provide insight into climate variability."

The authors of this study acknowledge the contribution of the 2019 field team without whom these ice core records would not have been possible.

Image: The expedition team on Huascarán mountain in 2019. Image provided by the expedition team.