The Frequency of Global Events and “hidden Mass Extinctions” – A Mid-Palaeozoic Perspective

A better understanding of global environmental perturbations, ecosystem crises, and extinctions is essential to understand the past, presence, and future of Earth biosphere and humans’ fate within it. In the public recognition of this important topic, there are two widespread misconceptions. First, based on the classical marine palaeobiodiversity analyses of Raup & Sepkoski, there is the “Big Five” concept, with humans currently causing a “Sixth Mass Extinction”. However, all mid-Palaeozoic specialists know that the Frasnian-Famennian (Kellwasser) and Devonian-Carboniferous (Hangenberg) Crises were completely separate first order mass extinctions with different causations, patterns, and processes. Therefore, we are currently facing a 7th global ecosystem crises, if not further past events qualify for the 1st order criteria: complete extinction of major, common and diverse fossil groups (at least classes and orders), and complete (episodic) loss of ecosystems, such as all metazoan reefs or all forests. The second misconcept is a simplified contrast between times of global crises with mass extinctions and of intervening, longer “calm episodes” with background extinction rates that mostly reflect regional and local ecological change. In fact, 2nd to 5th order global extinctions can be recognized between the 1st order crises and these often share large similarities with specific phases of the latter, such as black shale pulses or isotopic excursions. It is clear that Milankovitch cyclicity and its sealevel and climatic effects, e.g. monsoon or glacial-interglacial cycles, operate globally, preventing long-term ecosystem stability on land and in shallow seas. Therefore, the phrase “background extinctions” is misleading and blurs the recurrent but distinctive global up-and-down of ecological conditions. This leads to the question of the frequency of global events of different magnitude. A detailed review of the Devonian and Lower Carboniferous, spanning ca. 90 Ma, resulted in the identification of ca. 40 individual events, giving an average of one per 2.3 Ma, which is - possibly by chance - close to the long eccentricity cycle. Mass extinctions were often identified based on various taxon counts. However, there are several examples, especially the Annulata Events (basal upper Famennian) and Lower Alum Shale Event (basal middle Tournaisian), that stand out as globally distributed, hypoxic black shale events that had the same intensity and range as the black shale pulses of the Kellwasser and Hangenberg Crises. They do not show prominent in taxon counts, simply because the recoveries after the latter had not much proceeded yet. There were simply no new metazoan reefs, no extensive forests, and only few anoxia-sensitive shallow-water biota that could be affected by the same recurrent processes. But for our understanding of Earth´s biosphere it is important to know how often potential mass extinction conditions can and did manifest as a consequence of abiotic processes. Therefore, widespread and distinctive 2nd/3rd order global events occurring in times of low biodiversity are considered here as “hidden mass extinctions”, times that would have seen 1st order extinctions if normal (fully recovered) shallow marine and terrestrial ecosystems had existed.