Down by the River(side)

Hello this time from the University of California at Riverside! Since my last blog post, we finished up our beamtime at the APS on Tuesday morning and got on a plane to Los Angeles on Wednesday morning. So, not much time to rest. We have lots of new data to analyze from our beam trip, however, and we will be working on that for the next few weeks to months. New data is always exciting because it’s like a story unfolding.

I can’t say I enjoyed the Los Angeles traffic on the drive from L.A. to Riverside, but I am having a good time here in Riverside meeting many new people doing fantastic science. And last night we ended up at a pub for dinner and indulged in some fuel required by geologists: beer.

It is now Friday afternoon here in California and I just finished giving a presentation to Tim Lyons and his group. I was nervous but it’s done and it went well. Part of the job of a scientist is communicating his or her work to other scientists, so we spend a fair amount of time preparing talks, writing papers, and making posters. In fact, on Wednesday I will be giving a poster presentation at the Goldschmidt conference of geochemistry. Next week I will write to all of you about what happens at a scientific conference.

For now I have promised to explain why we are interested in molybdenum (Mo). Tim Lyons and his group have been working on, among other things, using Mo to reconstruct the history of the oxygenation of the earth. Tim and some colleagues published a nice review earlier this year in the journal Nature. In well-oxygenated water, Mo exists as the molybdate anion (MoO₄^2-). Remember that oxygen is used for respiration—you and I gain energy from food by reacting it with oxygen (O₂) to form carbon dioxide (CO₂) and water. Some microbes also perform respiration, but in sediments, oxygen can become depleted and other molecules are subsequently used to oxidize organic matter—in other words, to gain energy. One of the last things to be used is sulfate (SO₄^2-), producing sulfide (S^2-). When MoO₄^2- encounters sulfide, it can become tetrathiomolybdate (MoS₄^2-) and be preserved in sediments and sedimentary rocks. In summary, when we find Mo preserved in very old sedimentary rocks, it indicates a lack of oxygen in the environment at the time of deposition. The lack of oxygen can be due to lots of organic matter being delivered to sediments and/or oxygen-depleted overlying waters. Over the last ten years or so, Mo isotopes have also been developed as proxies for ancient ocean chemistry, indicating whether oxygen or sulfide was present in the ocean. Isotopes are atoms of the same element having different numbers of neutrons.

Significant amounts of oxygen were not present in Earth’s atmosphere until the Great Oxidation Event some 2.4-2.1 billion years ago. The ocean may have been well oxygenated from 2.3-2.1 billion years ago. However, from 2.1 billion years ago until at least 800 million years ago, the ocean remained without oxygen, having instead lots of dissolved iron or dissolved sulfide. Oceanic oxygen concentrations began to rise during the Ediacaran period some 635-541 million years ago. This coincided with the earliest diversification of large animals. A second rise in oxygen concentrations occurred during the Devonian (about 419-359 million years ago), and coincided with the rise of vascular plants and large predatory fish. The idea is that large animals with high metabolic energy requirements could not have evolved without higher oxygen concentrations in the ocean and atmosphere, because the most amount of energy is gained by using oxygen—as opposed to sulfate or some other molecule—to oxidize organic matter. So Mo is helping us to understand the co-evolution of oxygenation and life.

However, current models of how Mo cycles through the earth system do not take into account Mo interactions with organic matter, and this is what our lab is exploring.

I need to run for now—see you next week!