By analysing ancient seafloor rocks, Ross and his Australian, Russian, US and Canadian colleagues were able to show that the slowdown in evolution was tightly linked to low levels of oxygen and biologically-important elements in the oceans.
“We’ve looked at thousands of samples of the mineral pyrite in rocks that formed in the ancient oceans. And by measuring the levels of certain trace elements in the pyrite, using a technique developed in our labs, we’ve found that we can tell an accurate story about how much oxygen and nutrients were around billions of years ago.”
But the puzzle remains. As the USD researchers who studied sponges showed, life forms do not universally need a lot of oxygen. Accustomed as we are to gasping for breath, we forget that oxygen often acts as a volatile and destructive gas, and many cellular processes require machinery to ensure its exclusion. So if life forms such as the sponges can thrive with little oxygen, we might reasonably expect to find fossils of their type, as opposed to no type, in the billion years before the Ediacaran.
We are living on a planet where at last one bacterium “breathes” iron and another sulfur, the bdelloid rotifer dispensed with sex in favour of horizontal gene transfer, and the sea slug just incorporated its plant meals’ chlorophyll factories (chloroplasts). It almost seems as if, whatever the deficiency, a life form will be found somewhere that can cope with it. But come to think of it, would any of the above-mentioned make good fossils? Maybe we just don’t know enough yet to be sure that the famously “boring billion” years was quite so boring.
Abstract Sedimentary pyrite formed in the water column, or during diagenesis in organic muds, provides an accessible proxy for seawater chemistry in the marine rock record. Except for Mo, U, Ni and Cr, surprisingly little is known about trace element trends in the deep time oceans, even though they are critical to developing better models for the evolution of the Earth’s atmosphere and evolutionary pathways of life. Here we introduce a novel approach to simultaneously quantify a suite of trace elements in sedimentary pyrite from marine black shales. These trace element concentrations, at least in a first-order sense, track the primary elemental abundances in coeval seawater. In general, the trace element patterns show significant variation of several orders of magnitude in the Archaean and Phanerozoic, but less variation on longer wavelengths in the Proterozoic. Certain trace elements (e.g., Ni, Co, As, Cr) have generally decreased in the oceans through the Precambrian, other elements (e.g., Mo, Zn, Mn) have generally increased, and a further group initially increased and then decreased (e.g., Se and U). These changes appear to be controlled by many factors, in particular: 1) oxygenation cycles of the Earth’s ocean–atmosphere system, 2) the composition of exposed crustal rocks, 3) long term rates of continental erosion, and 4) cycles of ocean anoxia. We show that Ni and Co content of seawater is affected by global Large Igneous Province events, whereas redox sensitive trace elements such as Se and Mo are affected by atmosphere oxygenation. Positive jumps in Mo and Se concentrations prior to the Great Oxidation Event (GOE1, c. 2500 Ma) suggest pulses of oxygenation may have occurred as early as 2950 Ma. A flat to declining pattern of many biologically important nutrient elements through the mid to late Proterozoic may relate to declining atmosphere O2, and supports previous models of nutrient deficiency inhibiting marine evolution during this period. These trace elements (Mo, Se, U, Cu and Ni) reach a minimum in the mid Cryogenian and rise abruptly toward the end of the Cryogenian marking the position of a second Great Oxidation Event (GOE2). – Ross R. Large, Jacqueline A. Halpin, Leonid V. Danyushevsky, Valeriy V. Maslennikov, Stuart W. Bull, John A. Long, Daniel D. Gregory, Elena Lounejeva, Timothy W. Lyons, Patrick J. Sack, Peter J. McGoldrick, Clive R. Calver. Trace element content of sedimentary pyrite as a new proxy for deep-time ocean–atmosphere evolution. Earth and Planetary Science Letters, 2014; 389: 209 DOI: 10.1016/j.epsl.2013.12.020
After an initial burst of oxygen, the study plots a long decline in oxygen levels during the ‘boring billion’ years before leaping up about 750-550 million years ago. “We think this recovery of oxygen levels led to a significant increase in trace metals in the ocean and triggered the ‘Cambrian explosion of life’.
Add “trace metals” to attempts to account for the Cambrian explosion.
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