To understand our rapidly changing climate, researchers often look back at how Earth’s climate has behaved in the past. Marine sediment cores, like tree rings, can provide a log of former environmental conditions, allowing scientists to infer everything from the temperature and salinity of the oceans to precipitation rates on land.
Precipitation rates affect river flows and thus sediment erosion rates, which means researchers can look at ratios of marine to terrestrial materials in marine sediment cores to reconstruct past precipitation rates. But this and other existing proxies for precipitation, including examining hydrogen isotopes in plant wax compounds in cores, have limitations as they can be affected by other factors and processes.
Here Mendes et al. describe a new proxy for determining past precipitation from marine sediment cores using luminescence signals from feldspar and quartz grains. Together with other reconstructive methods, the new technique can provide a more complete picture of precipitation and continental erosion, the team notes. Researchers have previously used such luminescence signals to study processes on Earth’s surface, such as mountain uplift rates, and in surface exposure dating. But here the team applied the method to sediment cores collected 180 kilometers off the northeastern coast of Brazil, where the Parnaíba River spills into the Atlantic.
The Parnaíba is the main source of land-based sediments where the core was extracted, so the authors reasoned that changes in luminescence sensitivity in the core likely stemmed from changes in the amount of suspended sediments in the river over time.
The cores contained a record of the past 30,000 years. Over that time, the region has gone through three millennium-scale shifts in the Intertropical Convergence Zone, the major driver of precipitation in northeastern Brazil. These shifts triggered periods of increased rainfall over much of tropical South America, making the cores ideal for testing new methods of precipitation reconstruction.
The team compared the optically stimulated luminescence and thermoluminescence signals in the marine core sediments to luminescence measurements from sediment samples collected on land where the core sediments may have originated. Quartz and feldspar luminescence can change on the basis of where these materials come from and their exposure to surface processes, which means scientists can use the signal to trace sediments back to their parent rocks on land.
The team showed that the results from the new proxy generally agree with those from other proxies of precipitation in marine cores, as well as model simulations. The authors also confirmed the occurrence of other, shorter periods of increased precipitation that were suggested in a 2009 analysis of stalagmites in Brazil.
The new study provides an elegant and inexpensive way to bolster reconstructions of Earth’s paleoclimate—critical information for researchers and policy makers planning for the future. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2019PA003691, 2019)
—Kate Wheeling, Freelance Writer
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