Drops of seawater contain traces of an ancient world

Sea salt hides a secret: tiny droplets of the seawater from which it came, preserving geologic history.

Using specializing equipment obtained from National Science Foundation grant funds, Mebrahtu Weldeghebriel, PhD '22, a postdoctoral fellow at Princeton University, and Binghamton University Distinguished Professor of Earth Sciences Tim Lowenstein were able to reconstruct changes in seawater chemistry over the last 150 million years, also gaining insight into related geological processes and climate changes. Their article, "Seafloor Hydrothermal Systems Control Long-Term Changes in Seawater [Li+]: Evidence from Fluid Inclusions," was recently published in the journal Science Advances.

The ocean "is like a giant soup of different elements," Lowenstein explained. "Sodium and chloride are the most common ones, but there are dozens of others dissolved in seawater in trace amounts such as lithium."

They looked at sea salt (halite) formed at various times over the past 150 million years in geographically diverse sedimentary basins in the United States, Europe, Asia and Africa. Within the salt samples were tiny pockets containing a bit of ancient seawater.

To access the tiny droplets, the researchers used a laser to drill holes into the salt crystals and then a mass spectrometer to analyze the different trace elements present. In this research, they focused specifically on the concentration of lithium, a trace element that sustained a seven-fold decrease over the past 150 million years, paralleled by a rise in magnesium to calcium ratios.

But why?

The cause for the long-term variations in seawater composition has been debated for the past two decades. The researchers proposed that the decline in lithium concentration in seawater is mainly associated with reduced production of oceanic crust and decreased seafloor hydrothermal activity, both of which are influenced by the movements of tectonic plates. The slowdown in plate activity over the past 150 million years led to less lithium being added to the ocean and reduced amounts of carbon dioxide released into the atmosphere, which ultimately led to global cooling and the present ice age. Turning back the clock 150 million years, the earth was a warmer place with more carbon dioxide in the atmosphere and more lithium in the sea.

"There is a close link between ocean chemistry and atmospheric chemistry," Weldeghebriel said. "Whatever changes happen in the ocean also reflect what's happening in the atmosphere."

Overall, Weldeghebriel and Lowenstein's research has made a significant advance in understanding the chemistry of Earth's ancient oceans and how the movement of tectonic plates has influenced the composition of our Earth's hydrosphere and atmosphere. Such chemical changes impact biology, as well, such as the marine creatures that build their shells out of calcium carbonate.

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Martian meteorite contains large diversity of organic compounds

The Martian meteorite Tissint contains a huge diversity of organic compounds, found an international team of researchers led by Technical University of Munich and Helmholtz Munich's Philippe Schmitt-Kopplin and including Carnegie's Andrew Steele. Their work is published in Science Advances.

Tissint, which crash landed in Morocco more than 11 years ago, is one of only five Martian meteorites that have been observed as they fell to Earth. Pieces of it were found scattered around the desert about 30 miles from the town after which it is named.

This sample of Martian rock was formed hundreds of millions of years ago on our next-door planetary neighbor and was launched into space by a violent event. Unraveling the origin stories of the Tissint meteorite's organic compounds can help scientists understand whether the Red Planet ever hosted life, as well as Earth's geologic history.

"Mars and Earth share many aspects of their evolution," said lead author Schmitt-Kopplin. "And while life arose and thrived on our home planet, the question of whether it ever existed on Mars is a very hot research topic that requires deeper knowledge of our neighboring planet's water, organic molecules, and reactive surfaces."

Organic molecules contain carbon, hydrogen, oxygen, nitrogen, sulfur, and sometimes other elements. Organic compounds are commonly associated with life, although previous Martian meteorite research demonstrated that they can be created by non-biological processes, referred to as abiotic organic chemistry.

"Understanding the processes and sequence of events that shaped this rich organic bounty will reveal new details about Mars' habitability and potentially about the reactions that could lead to the formation of life," added Steele, who has done extensive research on organic material in Martian meteorites, including Tissint, and is a member of both the Perseverance and Curiosity rovers' science teams.

The researchers were able to thoroughly analyze the meteorite's organic inventory, revealing a link between the type and diversity of organic molecules and specific mineralogy. Their efforts resulted in the most comprehensive catalog ever made of the diversity of organic compounds found in a Martian meteorite or in a sample collected and analyzed by a rover. This work uncovered details about how the processes occurring in Mars' mantle and crust evolved, especially with regard to abiotic organics that formed from water-rock interactions.

Of particular interest was the abundance of organic magnesium compounds, a suite of organic molecules not previously seen on Mars, which offer new insights about the high-pressure, high-temperature geochemistry that shaped the Red Planet's deep interior and indicate a connection between its carbon cycle and its mineral evolution.

The researchers say that samples returned from Mars by future missions should provide an unprecedented amount of information about the formation, stability and dynamics of organic compounds in real Martian environments.

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