Mass extinctions are rapid global decreases in Earth’s biodiversity, with five key events identified over the planet’s history, arguably the most famous of which occurred ~66 million years ago during the Cretaceous, which brought the reign of dinosaurs to an end. However, the largest mass extinction is attributed to the Permian, during which it is estimated that >95% of all life on Earth was eradicated.
New research published in Chemical Geology lends further support to the latter theory. Yu Wang, of Nanjing University, and colleagues conducted geochemical experiments on sediment samples obtained from Zhigao Quarry, in the region of Upper Yangtze River, China. Within the samples, the scientists identified an important mercury anomaly, hosted in organic matter, associated with a peak in carbon isotopes.
This negative carbon-13 isotope excursion is interpreted as being caused by massive releases of carbon into the atmosphere, likely due to volcanic eruptions, and these events are also a major source of mercury in nature. Indeed, the timing of this excursion coincides with known volcanic activity of the Emeishan Traps, Sichuan, a vast area (>250,000 km2) of flood basalts creating a large igneous province.
Trace elements such as molybdenum, uranium and vanadium within the sediment samples are susceptible to reducing and oxidizing conditions, with three noticeable spikes indicative of anoxic conditions when the oceans were depleted in dissolved oxygen. In addition to this, significant global sea level fall during the Capitanian (~264–259 million years ago) is evidenced by terrestrial-derived organic carbon, while lower ratios of the elements cadmium and molybdenum suggest weakened or seasonal upwelling of cold, nutrient-rich waters.
The geochemical data and decline in calcareous algae and fusulinacean foraminifera in the chert, mudstone and limestone samples indicates marine anoxia and more pronounced stratification of the water column. In such conditions, oxygen minimum zones likely developed, with persistent areas of oxygen-poor conditions that would have inhibited the survival of organisms.
2023-12-30 22:00:03
Article from phys.org rnrn