As drops of water squeeze through fissures in limestone cave ceilings and drip one by one to the floor like a slow rain, each drop leaves behind dissolved minerals carried from Earth’s surface above. And as these minerals solidify, they create a record of the planet’s climate in the stone.
Because cave mineral deposits called flowstones often preserve such detailed records, they offer unparalleled tools with which scientists can track Earth’s climate history. Climate information is stored in the isotopic ratios of elements like oxygen deposited in the flowstones. But many factors can influence oxygen isotope ratios in precipitation and surface waters, so their interpretation is hotly debated.
In a new study, Hu et al. try to pin down how scientists can make the best use of these records. Focusing on caves in eastern China, they used a recently developed model to explore which contributing factors have the strongest effect on oxygen isotope ratios in cave formations.
It is well known that over long timescales, isotope ratios measured in Chinese caves correlate to the amount of solar radiation the area above receives each summer. Among other processes that affect this amount, the wobble of Earth’s axis induces a cyclic fluctuation every 23,000 years. At times in this cycle when the Northern Hemisphere is closest to the Sun, the belt of heavy precipitation in East Asia’s monsoon season moves faster and farther north. As it does, it draws less moisture from areas in the Pacific with high oxygen-18 to oxygen-16 ratios and more moisture from the Indian Ocean, where this ratio is lower. The air also has farther to travel and drops more rain on the way to eastern China, so the moisture becomes increasingly depleted in oxygen-18, and the ratio becomes more negative.
Thus, the researchers found that the isotope ratios scientists observe in caves over these long timescales are linked primarily to patterns of monsoon movement rather than to rainfall amounts at cave sites, which has been the prevailing interpretation. And when they zoomed in on a seasonal timescale, they found that a similar interpretation of the isotope ratios in the caves held true.
The new study suggests that oxygen isotope records do provide key constraints on the water cycle in monsoon regions, but they do not offer the straightforward picture of monsoon intensity as assumed. More observations and more modeling are needed to further illuminate the complexities of isotope ratio data from cave formations and the timescales over which these data are most useful, as well as to discern influences on the ratios from other factors such as local atmospheric convection and cave processes. In the meantime, the new research reinforces the notion that conventional scientific understanding often needs challenging. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2019PA003741, 2019)
—Elizabeth Thompson, Science Writer