provide anchorage. These systems broke up the upper layers of bedrock and stabilized a deep layer of soil, which would have been of the order of metres thick. In contrast, early Devonian plants bore only rhizoids and rhizomes that could penetrate no more than a few centimeters. The mobilization of a large portion of soil had a huge effect: soil promotes weathering, the chemical breakdown of rocks, releasing ions which are nutrients for plants and algae. The relatively sudden input of nutrients into river water as rooted plants expanded into upland regions may have caused eutrophication and subsequent anoxia. For example, during an algal bloom, organic material formed at the surface can sink at such a rate that decomposing organisms use up all available oxygen by decaying them, creating anoxic conditions and suffocating bottom-dwelling fish. The fossil reefs of the Frasnian were dominated by stromatoporoids and (to a lesser degree) corals—organisms which only thrive in low-nutrient conditions. Therefore, the postulated influx of high levels of nutrients may have caused an extinction. Anoxic conditions correlate better with biotic crises than phases of cooling, suggesting anoxia may have played the dominant role in extinction. Evidence exists of a rapid increase in the rate of organic carbon burial and for widespread anoxia in oceanic bottom waters. Signs of anoxia in shallow waters have also been described from a variety of localities. Good evidence has been found for high-frequency sea-level changes around the Frasnian–Famennian Kellwasser event, with one sea-level rise associated with the onset of anoxic deposits. The timing, magnitude, and causes of Kellwasser anoxia remain poorly understood. Anoxia was not omnipresent across the globe; in some regions, such as South China, the Frasnian-Famennian boundary instead shows evidence of increased oxygenation of the seafloor. Trace metal proxies in black shales from New York state point to anoxic conditions only occurring intermittently, being interrupted by oxic intervals, further indicating that anoxia was not globally synchronous, a finding also supported by the prevalence of cyanobacterial mats in the Holy Cross Mountains in horizons encompassing the Kellwasser event. Evidence from various European sections reveals that Kellwasser anoxia was relegated to epicontinental seas and developed as a result of upwelling of poorly oxygenated waters within ocean basins into shallow waters rather than a global oceanic anoxic event that intruded into epicontinental seas. Global cooling A positive oxygen isotope excursion in phosphates is known from the Frasnian-Famennian boundary, corresponding