August 7, 2015 report
End-Permian mass extinction may have been driven by an ocean teeming with life
(Phys.org)—The Permian geologic period that ended the Paleozoic era climaxed around 252 million years ago with a sweeping global mass extinction event in which 90 to 95 percent of marine life became extinct. It would take 30 million years for planetary biodiversity to recover. Understanding the contributing factors of the end-Permian mass extinction is critical to understanding and perhaps mitigating the current anthropogenic climate change.
Scientists have speculated that during the Permian period, venting of oceanic hydrogen sulfide gas killed off most eukaryotes and allowed oceanic prokaryotes to flourish. An international collaborative of researchers has conducted an analysis of the chemistry embedded in oceanic geologic formations, providing new geochemical evidence for this theory. The study, published in the Proceedings of the National Academy of Sciences, links the end-Permian mass extinction event with climate change, geologic weathering, and widespread marine anoxia as a result of biogeochemical sulfur and carbon cycles.
The reduction of sulfates in the ocean by microbes is an important pillar of the sulfur cycle. Most of the hydrogen sulfide in the ocean is reoxidized, with a fraction buried in sediment. Release from the sediment is another important factor, affecting sulfide formation and reactive iron availability. A notable increase in hydrogen sulfide would have affected the oceanic environment in a host of complex ways, creating major changes in biodiversity.
The current study demonstrates successive enhanced organic matter degradation by microbial sulfate reduction. This produces hydrogen sulfide, which is toxic for most eukaryotes, killing them by interfering with mitochondrial energy production. Rather than a so-called "Strangelove" ocean in which all life dies off, the study proposes a huge reduction of species richness—a decline in marine biodiversity of around 80 percent.
Carbonate-associated sulfate isotope data compiled by the researchers demonstrates that widespread euxenic zones resulted in sulfide toxicity, driving the marine biodiversity loss during this period. Under low competition pressure, prokaryotic life rapidly occupied the resulting vacant ecospaces. It's a remarkable portrait of biological productivity that contradicts the idea of an entirely dead ocean.
The researchers note that this scenario also reinforces the idea that life forms influence seawater chemistry. The authors write, "This study also emphasizes that, besides the property of organisms to construct a habitable planet, they can also act as a catalyst for destruction," noting that marine life would have been quite different with the flourishing of prokaryotes.
Geologic data from the Early Triassic that followed records increased sequestration of sulfur sourced from pyrite owing to the lack of eukaryotic organisms that would have irrigated oceanic sediments with O2 via burrowing. These and other processes generated a negative feedback loop of the carbon cycle in which enhanced production and sequestration of organic carbon was stimulated by global warming and rates of chemical weathering. "The prolonged disturbance after the end-Permian mass extinction contradicts a fast return (<100 ky) to predisturbance climate and carbon cycle," the authors note.
The authors conclude that post-extinction marine prokaryote domination, particularly the sulfate-reducing microbes, might have exerted enormous influence on the carbon cycle in the Early Triassic, affecting marine conditions and global climate, preventing the planet's recovery for a long period of time.
The end-Permian mass extinction, the most severe biotic crisis in the Phanerozoic, was accompanied by climate change and expansion of oceanic anoxic zones. The partitioning of sulfur among different exogenic reservoirs by biological and physical processes was of importance for this biodiversity crisis, but the exact role of bioessential sulfur in the mass extinction is still unclear. Here we show that globally increased production of organic matter affected the seawater sulfate sulfur and oxygen isotope signature that has been recorded in carbonate rock spanning the Permian−Triassic boundary. A bifurcating temporal trend is observed for the strata spanning the marine mass extinction with carbonate-associated sulfate sulfur and oxygen isotope excursions toward decreased and increased values, respectively. By coupling these results to a box model, we show that increased marine productivity and successive enhanced microbial sulfate reduction is the most likely scenario to explain these temporal trends. The new data demonstrate that worldwide expansion of euxinic and anoxic zones are symptoms of increased biological carbon recycling in the marine realm initiated by global warming. The spatial distribution of sulfidic water column conditions in shallow seafloor environments is dictated by the severity and geographic patterns of nutrient fluxes and serves as an adequate model to explain the scale of the marine biodiversity crisis. Our results provide evidence that the major biodiversity crises in Earth's history do not necessarily implicate an ocean stripped of (most) life but rather the demise of certain eukaryotic organisms, leading to a decline in species richness.
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