In this study, we paired state-of-the-art analytical approaches and global climate simulations to learn more about how seasonal climate changed at key times in earth’s history. The samples we analyzed are cave sediments (e.g. stalagmites) that grew consistently, but slowly, over thousands of years. We examined stalagmites—made of the carbonate mineral calcite—that grew at times when the size of polar ice sheets and global sea level were similar to today, in two places (Israel and Socotra Island, Yemen) where we could learn more about the amount and geographical extent of summer monsoon rainfall.

Caves are excellent natural laboratories where stalagmites can grow in stable, isolated conditions. Furthermore, stalagmites are excellent sources of past climate information because they can be radiometrically dated. For decades, scientists have measured a chemical signature of rainfall that is recorded in calcite stalagmites all around the world. Specifically, the chemical signature is the ratio of two naturally-occurring oxygen isotopes, ¹⁶O and ¹⁸O, that vary in their proportion in rainwater depending on the amount of rainfall, the source of the water vapor, and how much of the original water vapor has rained out. This signature is then transmitted to the cave as rainwater percolates into the soil, and eventually reaches the cave where it drips onto a stalagmite. The calcite layer that forms from the dripwater records that chemical signature such that stalagmites can be used to infer long rainfall histories.

Normally, the ratio of ¹⁶O and ¹⁸O is measured in a series of spots along the vertical growth-axis of a stalagmite by sampling calcite powders with a 1-mm-diameter dental drill. Here, we used state-of-the-art instrumentation (a secondary ion mass spectrometer) to measure the oxygen isotope ratio of calcite in spots that are only 0.01 mm in diameter. This allowed us to interpret rainfall histories at seasonal- instead of decadal-resolution.

In a stalagmite from Socotra Island, off the Horn of Africa, we generated a 1000-year-long record of oxygen isotopes from the end of the most recent deglaciation along with a dozen new radiometric dates. These data allow us to clearly observe three abrupt changes in rainfall between 11,800 and 10,800 years ago, correct the erroneous timescale for these changes suggested by prior workers, and confirm their abruptness as sub-centennial. A climate model suggests that the driving mechanism for these abrupt changes in East African rainfall could be tied to ocean circulation in the North Atlantic, a reminder of how sensitive and intertwined the climate system is.

In the stalagmites we analyzed from Israel, there is evidence in the seasonal-resolution oxygen isotope data for summer monsoon rains at 125,000 and 105,000 years ago. This is important because the tropically-sourced summer monsoon doesn’t get anywhere close to Israel today—in fact, summer is hot and very dry in Israel. Our finding is corroborated by global climate simulations that show: (1) the summer monsoon expanding at 125,000 years ago as far north as Israel, (2) that the summer monsoon rain should be distinct in its oxygen isotope ratio, and (3) that the simulated effect of expanded monsoon rains in East Africa and the Middle East is to narrow the width of the desert across this region.

Together, these results are significant when considering how climate may have played a role in early modern human migration out of Africa. If the hot, arid summers of the Middle East had impeded human migration pathways, then it is important to establish that there were windows of time when summers may not have been so arid. While our findings do not definitively answer the long-standing questions of if and how climate affected early human migration, they do provide critical environmental context and a testable hypothesis that archaeological evidence of migration should coincide with climatic optima.

The PIs for this interdisciplinary project were two early career scientists, who benefitted greatly from the opportunity to plan, lead, and manage a grant while also advising a student and disseminating results to both the scientific and local community. In addition to co-authoring multiple manuscripts describing the work, PIs worked with an undergraduate for 3 years. The student started as an hourly trainee helping with sample preparation and imaging, assisted in the planning, field-testing, and execution of a cave-themed outreach event that engaged >800 people in our community, and finished by completing a senior thesis (co-advised by PI on this grant) that stemmed from their developed interest in stalagmites. The beneficial outcomes of the project clearly include more than just the scientific advances in paleoclimatology and anthropology.

Last Modified: 02/03/2020
Modified by: Ian J Orland