Phosphorus cycling, marine primary productivity, and redox conditions during mass extinction events
Phosphorkreislauf, marine Primärproduktion und Redoxbedingungen während Massenaussterbeereignissen
Creators: name: Müller, Johann
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: paleoenvironment, sedimentology, geochemistry, phosphorus cycle, mass extinction

description: A decrease in marine primary productivity and in the dissolved oxygen inventory due to the effects of anthropogenic global warming, in particular enhanced water column stratification, has been observed over the last five decades. Moreover, modeling studies predict that these trends will continue under scenarios of ongoing greenhouse gas emissions. In the geological past, multiple episodes of climate change occurred, and investigating the associated biogeochemical changes in the sedimentary records of the ancient oceans can give important insights. Some of the warming phases and one cooling phase led to mass extinction events through a cascade of environmental perturbations. While global warming due to greenhouse gas emissions from the Siberian Trap Large Igneous Province is well established for the end-Permian mass extinction, the concomitant chain of effects is a matter of debate. In particular, there are two controversial questions: 1. Did marine primary productivity increase due to enhanced phosphorus input from the continents or decrease (or even collapse) due to inefficient phosphorus recycling in a stratified water column? 2. What was the driver of the observed ocean deoxygenation during the latest Permian? In principle, marine primary productivity and water column redox conditions are closely related through two mechanisms. One is that enhanced primary productivity results in a large organic matter sinking flux and therefore high rates of respiratory oxygen consumption. The other is that both depend on ocean circulation strength, upwelling intensity, mixing efficiency, and the degree of water column stratification. In a stagnant ocean, ventilation of the water column is weak, and nutrient recycling in upwelling regions becomes inefficient. Since the latest Permian warming is thought to have resulted in a stagnating ocean, the following hypothesis was developed: “In distal regions of the Tethys Ocean, primary productivity decreased from the latest Permian into the Early Triassic due to inefficient phosphorus recycling, and the water column was deoxygenated due to weak ventilation of deep waters and reduced oxygen solubility in warm surface waters.” In order to test this, productivity and redox proxies were measured on samples collected from two Tethyan, distal, low-latitude carbonate sections spanning the Permian-Triassic transition. These are the Chibi section (China) and Chanakhchi section (Armenia), the former representing a deep slope setting on the northern margin of the South China craton in the eastern Tethys and the latter a carbonate ramp far from land in the western equatorial Tethys. Both sections exhibit a decrease in total organic carbon contents, reactive phosphorus, and nickel concentrations in the latest Permian, suggesting a productivity decline. At Chibi, C/Porg and C/Preact ratios indicate a switch from inefficient P-burial with high P-regeneration in the late Permian to efficient burial in the Early Triassic. In parallel to the productivity decline, cerium-anomalies, measured on the carbonate fraction, shift from negative to positive values indicative of water column deoxygenation. In the Early Triassic interval of both sections, the proxies consistently signal weak productivity under low-oxygen conditions. Hence, a eutrophication scenario can be confidently ruled out for the study sites, and the presented hypothesis is verified. Distal regions of the Tethys Ocean probably switched to an oligotrophic-anoxic state in the course of latest Permian global warming due to a stably stratified water column with high sea surface temperatures. In this scenario, food shortage for higher trophic levels and anoxia are both potential killing mechanism with a common trigger. The late-Ordovician mass extinction is in stark contrast to the end-Permian mass extinction. It is preceded by a long-term cooling trend through the Ordovician and is commonly seen in the context of the large-scale glaciation of Gondwana in the Hirnantian. A major sea level drop occurred at the base of the M. persculptus graptolite zone, leading to the exposure of epeiric and shelf seas. Published local and global redox proxy records point towards an expansion of anoxic conditions in the open ocean at that time, while shallow settings remained oxygenated. During times of high sea level, shelf environments host the bulk of marine primary productivity. For the modern ocean, it is estimated that about 70% of the delivered phosphorus is retained in shelf sediments through the deposition of organic matter and the diagenetic stabilization of phosphorus into authigenic apatite minerals. Based on this, the following hypothesis was developed to explain the open ocean anoxia in the Hirnantian: “Due to the sea level fall during the glacial maximum in the Hirnantian shelf environments lost their function as a nutrient filter and the open ocean received a large excess in bioavailable phosphorus which fueled primary productivity and led to a high organic matter sinking flux and enhanced respiratory oxygen consumption.” The hypothesis was tested by measuring reactive phosphorus on samples from two tropical shallow-water carbonate sections covering the Ordovician-Silurian transition. These are the Ruisseau aux Algues section on Anticosti Island (Canada) and the Valga-10 core section from Estonia. Decreasing concentrations towards and a minimum at the Hirnantian glacial maximum were found. Using an estimate for the global shelf area and its reduction due to the sea level fall, phosphorus burial fluxes were calculated. According to this, the burial flux in shelf environments was approximately halved, and the open ocean would have received the excess phosphorus. The resulting high primary productivity led to an elevated organic matter sinking flux and strong respiratory oxygen consumption. This scenario is able to explain widespread open ocean anoxia during a cold climatic state, with sea level being the overarching control.