By José Tadeu Arantes | Agência FAPESP – The analysis of marine sediments has become a powerful research method in paleoclimatology. The composition of the sediments carried by rivers from the mainland to the ocean can be used as a basis for calculating variables such as temperature, precipitation and marine salinity. In the context of ongoing global climate change, the study of the past is fundamental to validating the accuracy of the climate models used to make predictions (read more at: agencia.fapesp.br/25845 and agencia.fapesp.br/23134).
A novel method of sediment analysis was proposed by Vinícius Ribau Mendes, a professor at the Federal University of São Paulo (UNIFESP), in Brazil, in an article published in Paleoceanography and Paleoclimatology, a journal of the American Geophysical Union (AGU). As a tribute to its originality, the study was also featured in the section “Research Spotlights” of EOS: Earth & Space Science News, another AGU publication.
The study was conducted during Mendes’s PhD research with the support of a scholarship from FAPESP and was supervised by Paulo César Fonseca Giannini. The study also received funding via a Young Investigator Grant awarded to Cristiano Mazur Chiessi, a co-author of the article, and a research grant under FAPESP’s Multiuser Equipment Program awarded to Giannini.
Mendes and collaborators analyzed marine sediment cores collected off Brazil’s northeastern coast. A marine sediment core is a tubular sample of mud extracted from the sea bed to capture stratigraphic layers while preserving the depositional sequence; younger sediments are at the top, and older sediments are at the bottom.
The cores analyzed by the researchers contained clay minerals, quartz and feldspar transported from the continent by the Parnaíba, the most important river in the region. Feldspars, a group of aluminosilicate minerals containing calcium, sodium and potassium, make up more than half of the Earth's crust.
Any substantial change undergone by the river, such as variations in the rainfall regime, is potentially reflected by these sediments. “We propose a new method of reconstructing the changes that have occurred in continental precipitation during the past 30,000 years based on the luminescence of quartz and feldspar grains. This luminescence varies according to the geological constitution of the areas in which the crystals originated and the length of time they were exposed to surface processes before reaching the ocean bottom. A crystal from the headwaters of the river has a specific signature that differs from the signature of a crystal from the middle or lower reaches of the river. In the case of the Parnaíba, an increase in rainfall has a significant impact on its source, so a larger proportion of grains from this area in a marine sediment core reflects a rise in precipitation,” Mendes told Agência FAPESP.
Luminescence is light emitted by materials that have been exposed to ionizing radiation and are subjected to a stimulus such as heat or light. Other studies have discovered that for quartz, the intensity of light emission or sensitivity depends on the geological medium in which the grain was formed and the duration of its exposure to sunlight during successive transportation events down the river until it reached the ocean. “We proved this empirically by collecting quartz and feldspar samples along the Parnaíba. We found significant differences between samples from the headwaters and from the lower reaches of the river,” Mendes said.
The physicochemical basis for these differences is not fully understood. Quartz is a simple material that is more abundant in sediments than feldspar. Quartz mainly consists of silica (silicon dioxide, SiO2) but may contain defects formed by the incorporation of chemical elements other than silicon and oxygen or due to the absence of these elements in the crystal lattice (“vacancies”). Luminescence results from these alterations in the lattice; the alterations depend on the chemical composition of the medium in which the rock was formed. Once formed and transformed into the sediment, the mineral is influenced by other factors, such as fragmentation, which changes the way it interacts with ionizing radiation in the environment, exposure to sunlight, and a high incidence of cosmic rays.
Exactly how these factors influence the capacity of defects to create luminescence is unknown, but a study conducted in the Amazon River basin by André Oliveira Sawakuchi, a professor at the University of São Paulo’s Institute of Geosciences (IGc-USP), and collaborators showed that the longer quartz remains exposed to terrestrial surface processes, the greater its luminescence.
“These physicochemical hypotheses about the differences are still being studied,” Mendes said. “We don’t have a definitive explanation. What we can say for sure, because we have the empirical data, is that crystals from various sites along the course of the river are different. So when we found grains of these crystals in marine sediment cores, we were able to measure their luminescence to determine where they were formed and map the process that took them from the continent to the ocean.”
Luminescence was measured in samples taken every 2 cm along the core. All samples were exposed to the same type and quantity of ionizing radiation and then stimulated by heat or light.
“Our novel method complements two others that are well established in the scientific literature. One produces precise and accurate information about rainfall and is based on measuring the proportion of hydrogen isotopes present in highly resistant molecules of terrestrial plants transported to the ocean. These isotopes, which the plants incorporate into their molecules, come from rainwater,” Mendes explained.
The stable isotopes of hydrogen are protium, with one proton and one electron, and deuterium (heavy hydrogen), with one proton, one neutron and one electron. Protium, the most abundant isotope, combines with oxygen to form ordinary water (H2O), and deuterium combines with oxygen to form heavy water (D2O).
Rainclouds in Brazil’s Northeast region travel from the ocean to the continent, so the more it rains, the less deuterium falls on the land because heavy water is denser and tends to fall first. The proportions of deuterium and protium in plants deposited at different depths of marine sediments therefore provide very precise information on variations in the amount of precipitation over time.
“Despite its precision, however, this method is very laborious and costly. It also depends on the preservation of the organic matter in the sediment, which isn’t always the case,” Mendes said.
The second method is based on analyses of the chemical composition of sediments using X-ray fluorescence. The technique entails exciting the sample with X-rays and analyzing the characteristics of the electromagnetic signal emitted in response. Each chemical element emits information in a specific frequency band, and the elements present in the sample can be identified based on this information. The goal is to determine the proportions of titanium and iron, which are continental elements, and that of calcium, which is from the shells of marine animals. Increases in the percentages of titanium and iron indicate that more material was transported from the land into the ocean due to more intense rainfall in a given period compared to the conditions in previous periods.
This second method is simple, fast and inexpensive. Although determining the percentages of hydrogen isotopes takes several months of analysis in a laboratory, samples can be scanned for titanium, iron and calcium in a day. The problem is a lack of precision compared with the first method. Variations in the percentages of titanium and iron indicate an increase or decrease in precipitation in a given period, but the variations are not proportional to the amount of rainfall because they are also affected by rising and falling sea levels. If the sea level falls, the mouth of the river moves closer to the area from which the sediment is collected. If the sea rises, the distance to the river mouth increases. In this case, any increase or decrease in the proportions of titanium and iron in the sediment core may not be caused by rain variations. Thus, the lack of precision of this method offsets its advantages in terms of speed and cost.
“Our new method lies somewhere between the two,” Mendes said. “It is less precise than the first and more precise than the second. It is also intermediate in terms of difficulty and cost. Additionally, quartz is a highly resistant material, so there is no risk it will degrade in the sediment, as can happen with organic molecules in the first method. Because the new method is based on the intrinsic properties of these crystals, there is also no risk they will be affected by external factors such as rising sea levels, as there is with the second method. We do not propose substitution of one method for another. They’re complementary and should be used as appropriate in each case.”
The researchers are also assessing the possibility of constructing a luminescence scanner for marine sediment cores.
“The equipment used to measure luminescence sensitivity is sophisticated, but the laboratory procedure involved is relatively simple and fast, and the targeted minerals are more common in sediments coming from the continent. The method developed by Vinícius Mendes therefore promises to become routine from now on in the study of paleoprecipitation based on marine sediments in the areas of influence of major rivers,” Giannini said.
The article “Thermoluminescence and optically stimulated luminescence measured in marine sediments indicate precipitation changes over Northeastern Brazil” can be retrieved from: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019PA003691.
The Research Spotlight article on the study, “A New Proxy for Past Precipitation”, published in EOS can be read at: eos.org/research-spotlights/a-new-proxy-for-past-precipitation.