Sea salt unfolds past 150 million years geological history

Jun 13, 2023

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Binghamton University researchers have managed to examine changes in seawater chemistry over the last 150 million years.

According to the press release, the detailed examination provided them with insights into geological processes, like plate tectonics and climatic changes over the last million years.

The researchers searched for halite, a sea salt that may preserve geologic history.

Halite originated throughout the last 150 million years in myriad sedimentary basins of the world, including the United States, Europe, Asia, and Africa. Additionally, Halite salt was selected as it includes "tiny droplets" of ancient saltwater.

The researchers used a laser to bore holes into the salt crystals to analyze these tiny drops. Following this, the mass spectrometer technique was employed to test for the existence of various trace elements.

The ocean “is like a giant soup of different elements. Sodium and chloride are the most common ones, but there are dozens of others dissolved in seawater in trace amounts such as lithium,” said Tim Lowenstein in an official release.

The main focus of this study was to identify traces of lithium in the salt samples.

Lithium is described as a trace element that has "sustained a seven-fold decrease" during the last 150 million years, which an increase in magnesium-to-calcium ratios has accompanied.

The authors propose that the fall in lithium levels in seawater is connected to "reduced production of oceanic crust and decreased seafloor hydrothermal activity." Both of these aspects influence the movement of the Earth's tectonic plates.

The results showcased that the reduction in the plate movement activity over the last 150 million years appears to have resulted in less lithium being supplied to the ocean.

While it also resulted in less carbon dioxide being emitted into the atmosphere, eventually leading to global cooling and the ice age.

“There is a close link between ocean chemistry and atmospheric chemistry. Whatever changes happen in the ocean also reflect what’s happening in the atmosphere,” said Mebrahtu Weldeghebriel, the lead author of this study.

The findings have significant implications for understanding the chemistry of ancient oceans and how tectonic plate movement shaped the chemical makeup of our planet's hydrosphere (layer of water) and atmosphere.

Lowenstein added: “Everything is connected.”

These changes, in turn, impact the biology of numerous creatures, for instance, marine organisms with shells made of chemical compound calcium carbonate.

The findings were reported in the journal Science Advances.

Study abstract:

Secular variations in the major ion chemistry and isotopic composition of seawater on multimillion-year time scales are well documented, but the causes of these changes are debated. Fluid inclusions in marine halite indicate that the Li concentration in seawater [Li+]SW declined sevenfold over the past 150 million years (Ma) from ~184 μmol/kg H2O at 150 Ma ago to 27 μmol/kg H2O today. Modeling of the lithium geochemical cycle shows that the decrease in [Li+]SW was controlled chiefly by long-term decreases in ocean crust production rates and mid-ocean ridge and ridge flank hydrothermal fluxes without requiring changes in continental weathering fluxes. The decrease in [Li+]SW parallels the 150 Ma increase in seawater Mg2+/Ca2+ and 87Sr/86Sr, and the change from calcite to aragonite seas, KCl to MgSO4 evaporites, and greenhouse to icehouse climates, all of which point to the importance of plate tectonic activity in regulating the composition of Earth’s hydrosphere and atmosphere.