Lithium concentrations in the oceans have decreased seven-fold in 150 million years research
Analysis of salt samples revealed that lithium concentrations in seawater have decreased seven-fold over the past 150 million years. Previously, researchers assumed that lithium concentration was constant, but the new findings may help explain larger geological influences, including links to climate and the carbon cycle.
“Going back the clock by 150 million years, the Earth was warmer, with more carbon dioxide in the atmosphere and more lithium in the sea,” explains geologist Mebrato Wildegbrell of Princeton University in the US, who led the new research. “The ocean is like a giant soup of elements, (and) it is important to understand how they respond to major changes to (infer) the relationship between ocean chemistry and atmospheric chemistry.”
Wildegbrell and his colleague Tim Lowenstein have now revealed that, despite previous assumptions, lithium concentrations in seawater have decreased significantly.
To date, the concentration of different lithium isotopes in seawater has been reconstructed from fossils of foraminifera – a type of unicellular planktonic organism. However, this data did not give a complete picture. “There were no robust records of lithium concentration in seawater,” says Samboda Misra, a geochemist who specializes in reconstructing past seawater chemistry and its links to climate. “We only had one piece of this global puzzle.” Indian Institute of Science, Bangalore, India. “This article provides the first quantitative reconstruction of lithium concentration in seawater over the past 150 million years,” he adds.
Wildegbrell explains that he and Loewenstein reconstructed historical lithium concentrations, by measuring samples of ancient seawater that remained trapped in “tiny pockets of halite crystals.” They then used state-of-the-art mass spectrometry equipment to identify and quantify lithium and magnesium. It is the ratio between these two cations that provides a detailed picture of seawater content going back in time.
“We assume that the ratios of major ions are constant in the ocean,” explains Erik Achterberg, a chemical oceanographer at the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany. “The major ions are present in high concentrations because they are not reactive and therefore have very long residence times in the oceans,” he adds. “(Analysis of) marine halite samples from Mesozoic and Cenozoic marine evaporation basins, in combination with modeling, identified a seven-fold decrease in ocean lithium concentration.”
“This is a huge challenge,” Misra says. “[They also]took great care to exclude specimens that showed evidence of non-marine origin.”
The researchers explain that the concentration of lithium in seawater could be linked to global climate impacts, such as the recent “global cooling” of the current geological era. Since hydrothermal vents — cracks in the seafloor that drain geothermally hot water — are a common source of lithium, Wildegbrell believes there is a strong link between long-term changes in lithium concentration and tectonic activity.
“The decrease in lithium concentration in seawater is mainly related to the decrease in oceanic crust production and (…) hydrothermal activity – both of which are affected by tectonic plate movements,” says Wildegbrell. “The slowdown in plate activity over the past 150 million years released less lithium (into seawater) and (less) carbon dioxide into the atmosphere, ultimately (creating) the current ice age.”
For this reason, Wildegbrell points out, improving our understanding of seawater chemistry over the long term will ultimately provide important insights into “the global carbon cycle, as well as the sources and sinks of carbon dioxide.”