450,000 years of the Global Monsoon: The history of water availability on our planet

The geochemistry underground 

Caves in the Peruvian Andes hold remarkable records of monsoon strength in rocks called stalagmites.  Stalagmites are mineral deposits formed inside caves from cave drip water. These formations record environmental conditions, providing records of changes in cave water chemistry over geologic time, which in turn is related to temperature and rainfall patterns outside the cave.  While caves around the world have these deposits, we were interested in these caves in the Peruvian Andes because previous research by us¹ and others^2,3,4,5 had demonstrated that these systems are sensitive to changes in South American monsoon strength. We sampled rainwater, cave drip water, air, and rocks to determine the seasonality of stalagmite formation

 Decoding the signal in stalagmites

Back in the lab, we analyzed the samples' isotopic composition. The rainwater samples showed a clear annual cycle, with the ratio of heavier to lighter oxygen isotopes changing over the year. Year to year, the strength of this signal changed in relation to the amount of rainfall in the area. This annual rainfall amount is directly related to the strength of the South American monsoon, as ~80% of rain in the area falls during the monsoon season. In the caves, drip water forming the stalagmites occurred only during the monsoon, when the oxygen isotope value was at its lowest. This isotopic signature is then trapped in stalagmites over time as they form from precipitation of calcite (CaCO₃), from the cave drip water. By analyzing the oxygen isotope composition of stalagmite calcite at high resolution across stalagmite layers, we can reconstruct monsoon strength over time. To date the stalagmites, we used uranium-thorium dating which allowed us to build a history of South American monsoon strength over the past 450,000 years. 

Monsoons overtime

With the 450,000-year record of South American Monsoon strength, we then compared our record with previously published work. Lake Junín is located just 30 km from the caves in this study, and previously, co-author Donald Rodbell and others published a 700,000-year record of glaciation in the region using sediment cores from the lake⁶. Since glaciers will grow only if temperatures are cold and there is precipitation (snow), the Junín Lake record is a proxy for changes in local temperature and monsoon precipitation. While the stalagmite oxygen isotope proxy incorporates both local temperature and monsoon precipitation, it also contains information about how much rainfall is lost upwind of our location as the monsoon system travels over the continent.  We tested the local vs upwind effects by subtracting the lake from the cave records to test if the cave was recording the monsoon in the Amazon Basin lowlands before it reaches the high Andes (our caves are located ~3600 meters above sea level).  We then compared this output to a stalagmite record from the lower Amazon basin and found that they matched remarkably well. This approach to separating out the local versus regional processes is something we have not seen done before in this way and could be replicated in other areas in the future. 

Having confirmed that our record captures the strength of the South American monsoon system over time, we then compared our record to a well-known record of the Northern Hemisphere Asian Monsoon⁷.  This comparison tests the interhemispheric monsoon response to the average amount of solar radiation absorbed by each hemisphere. In the past, monsoon systems were thought to be isolated systems driven by local sea-land temperature contrasts.  Recently, however, this view of monsoons has shifted to consider that each monsoon is instead part of the larger 'global monsoon' system.  The 'global monsoon' response is determined by the cumulative amount of solar radiation energy received on a specific area (solar insolation), which drives changes in atmospheric circulation.  On geologic timescales, solar insolation reaching Earth varies because of changes in Earth's orbit, axial tilt, and wobble, collectively referred to as the Milankovitch theory. For example, when the Northern Hemisphere receives more solar insolation than the Southern Hemisphere, the Northern Hemisphere monsoon strengthens, and the Southern Hemisphere receives less solar insolation and weakens. By comparing a Northern Hemisphere monsoon record with a Southern Hemisphere monsoon record, we can compare it with model predictions of the interhemispheric insolation difference driven by changes in solar insolation.  By subtracting the northern and southern hemisphere records, we found that the strength of the interhemispheric monsoon signal closely matches the Milankovitch theory for changes in cross-equatorial solar insolation. This was very satisfying because it proves, for the first time, that the global monsoon behaves over geologic time as a predictable, cohesive system.

The same, but opposite: an unexpected problem

We could have left it there, but the data had a few places where the pattern broke down. Overlaying the Asian and South American monsoon records we noticed times when the signals did not mirror each other.  Rather than showing opposite responses, the two systems sometimes synchronized, with both sites wet or both dry.  These synchronous events occurred several times in the record and especially during each glacial termination (end of ice age). These were unexpected, and we wanted to dig further to see why they might be happening. We found that synchronous drying occurs during each of the four terminations and four warm interglacial/interstadial periods in our record. The synchronous wet events only occur during the penultimate glacial (MIS6).  This response was quite surprising because it shows that water availability in both hemispheres can be strained simultaneously. For the more than 2.7 billion people on Earth who depend on monsoon rains for freshwater, this is not good news. We observed that monsoon rainfall patterns have shifted spatially, at times for thousands of years, resulting in long-term drought. 

Citations

1. Olson, E. et al. Cave monitoring in the Peruvian Andes reveals monsoon climate preserved in speleothem calcite. Chem. Geol. 667, 122315 (2024).

2. Kanner, L. C., Burns, S. J., Cheng, H. & Edwards, R. L. High-Latitude Forcing of the South American Summer Monsoon During the Last Glacial. Science 335, 570–573 (2012).

3. Kanner, L. C., Burns, S. J., Cheng, H., Edwards, R. L. & Vuille, M. High-resolution variability of the South American summer monsoon over the last seven millennia: insights from a speleothem record from the central Peruvian Andes. Quat. Sci. Rev. 75, 1–10 (2013).

4. Burns, S. J., Kanner, L. C., Cheng, H. & Edwards, R. L. A tropical speleothem record of glacial inception, the South American Summer Monsoon from 125 to 115 ka. Clim. Past 11, 931–938 (2015).
5. Burns, S. J., Welsh, L. K., Scroxton, N., Cheng, H. & Edwards, R. L. Millennial and orbital scale variability of the South American Monsoon during the penultimate glacial period. Sci. Rep. 9, 1234 (2019).
6. Rodbell, D. T. et al. 700,000 years of tropical Andean glaciation. Nature 607, 301–306 (2022).
7. Cheng, H. et al. The Asian monsoon over the past 640,000 years and ice age terminations. Nature 534, 640–646 (2016).