Seafloor Hydrothermal Systems and Ancient Ocean Chemistry

Category Science

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Dr. Mebrahtu Weldeghebriel and Professor Tim Lowenstein at Princeton and Binghamton University, respectively, used sea salt from sedimentary basins around the world and advanced equipment funded by the National Science Foundation to reconstruct shifts in seawater chemistry spanning 150 million years. They revealed that tectonic plate movement slowed, leading to less lithium in the ocean and less carbon dioxide in the atmosphere, ultimately leading to global cooling and the present ice age.

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Sea salt hides a secret: tiny droplets of the seawater from which it came, preserving geologic history. Utilizing advanced equipment funded by the National Science Foundation grant, Dr. Mebrahtu Weldeghebriel, Ph.D. ’22, a postdoctoral researcher at Princeton University, collaborated with Binghamton University’s Distinguished Professor of Earth Sciences, Tim Lowenstein. Together, they reconstructed shifts in seawater chemistry spanning the last 150 million years, shedding light on corresponding geological events and climatic transformations. Their paper was recently published in the journal Science Advances.

The ancient ocean was seven times as high in lithium concentration compared to the present day ocean.

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.

The tectonic plates have slowed their movement over the past 150 million years.

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.

Nearly all ocean-dwelling creatures build their shells out of calcium carbonate.

"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.

The release of carbon dioxide into the atmosphere is necessary for life on Earth.

"The oceans and atmosphere are connected to one another, and how they change is related," Lowenstein explained. "Everything is connected." .

Reference: "Seafloor hydrothermal systems control long-term changes in seawater [Li+]: Evidence from fluid inclusions" by Mebrahtu F. Weldeghebriel and Tim K. Lowenstein, 26 July 2023, Science Advances. DOI: 10.1126/sciadv.adf1605 .

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