Composition of Ancient Seawater

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LLO Researcher Finds Clues to the Early Earth in Indonesian Lake


UMD's Sergei Katsev
Sergei Katsev in his office at UMD's Large Lakes Observatory

UMD’s Sergei Katsev’s research on the composition of ancient seawater was published in Science Magazine on November 7, 2014.

Katsev, associate professor and researcher at UMD’s Large Lakes Observatory, and a team of researchers around the world have made a connection between the modern Indonesian Lake Matano and the composition of seawater on the planet during the Archaen Eon, about 3 billion years ago. The connection is strong. Lake Matano's deep waters are warm, lacking oxygen, rich in dissolved iron, and habitable to microbial communities similar to those that might have dominated the oceans at that time. The team's research on the signature of sulfate and sulfide isotopes within these waters implies a significant role for sulfur as an element that may have limited the biological productivity on the Early Earth.

Imagine the Archaen Ocean
Think back to the time when the Earth's crust had cooled and the first oceans had formed. The oceans had their own chemical composition which included sulfur. Sulfur is a vital element on Earth. Organisms require it as a nutrient, and it plays a central role in regulating atmospheric chemistry and global climate. Sulfur is a key constituent of proteins and is an important energy source for many microbes.

During the Archaen Eon, the earth couldn’t support animal or plant life, but a complex set of chemical reactions in these early oceans developed, which transformed carbon-containing molecules into simple, single-celled life forms. When the right conditions developed, some of these microorganisms, cyanobacteria (blue-green algae), began to produce minuscule amounts of oxygen by photosynthesis. Most of the primitive marine life at that time, however, survived on other metabolisms that are exceptionally rare today. The research of Katsev and his colleagues are helping the scientific community determine the conditions under which those early organisms evolved and persisted.

All life today is descended from these simple organisms. By the end of the Archean, the first oxygen-generating photosynthesizing organisms had evolved and begun to produce oxygen, which was released into the oceans and atmosphere, dramatically changing life on Earth.

Lake Matano
This ferry was outfitted with equipment to take samples from the deepest part of the lake.
Lake Matano
Lake Matano, in Sulawesi, Indonesia, is the seventh deepest lake in the world.

Connecting Lake Matano in Indonesia to Early Earth
Since 2006, Katsev has been studying Lake Matano, twice through visits to collect samples, and regularly to analyze data collected by other teams.

Lake Matano has properties similar to Early Oceans. The deep waters are warm and lack oxygen. They are rich in dissolved iron and habitable to microbial communities similar to those that might have dominated oceans 3 billion years ago.

At almost 600 meters deep, Lake Matano is the seventh deepest lake on the planet. Only the upper, extremely transparent, 100 meters can exchange substances like oxygen with the atmosphere, so the water below 100 meters is permanently anoxic (lacking oxygen). The water is stratified in horizontal layers and vertical mixing below 100 meters is slow.

Fractionation Inside the Vertical Water Column
In Lake Matano's oxygen-free waters and sediments, microbially-catalyzed reactions take place. Like their ancient ancestors, Lake Matano's microbes convert oxidized sulfur (sulfate) into its reduced form (sulfide) and the iron-sulfide minerals are deposited at the bottom of the lake. In that conversion, the microbes separate or fractionate the sulfur isotopes. It is this microbial reaction Katsev and his colleagues have studied. The reaction generates a characteristic signature in the distribution of sulfur isotopes, which becomes preserved in the iron-sulfide minerals that form as a result. This signature of isotopic fractionation holds the most significant new evidence about the composition of ancient seawater.

Katsev zeroed in on data about stable sulfur isotopes and created a numerical model to make inferences about the composition of the ancient ocean seawater. The research team’s findings are significant. They have determined that the level of sulfate was two orders of magnitude smaller than presently believed for that time in the Earth history and 10,000 times lower than in the modern ocean. At these trace amounts, sulfate would have been poorly mixed and short-lived in the oceans—and this sulfate scarcity would have shaped the nature, activity and evolution of early life on Earth.

Katsev's Research Makes the News
Katsev is one of the authors of a paper published on November 7, 2014 in Science magazine, one of the top scientific journals in the world. The paper is titled, "Sulfate was a trace constituent of Archean seawater." The lead authors are Katsev’s collaborators Sean Crowe from the University of British Columbia and Guillaume Paris from CalTech. 

By Cheryl Reitan, November 2014.

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Cheryl Reitan,

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