PLANKTON PARADOX · Climate change, hydrography and the paradox of plankton extinctions

Final Activity Report Summary - PLANKTON PARADOX (Climate change, hydrography and the paradox of plankton extinctions)

A fundamental question in palaeoceanography and evolutionary ecology is why so many groups of planktic organisms are apparently so susceptible to extinction despite having a wealth of traits (such as huge population sizes, cosmopolitan distributions, rapid reproduction rates) that classic evolutionary theory suggests should make them practically immune to extinction. Yet many groups of plankton have rates of extinction as high as those of terrestrial mammals. A 'Paradox of the Plankton' is therefore why species that are seemingly so effectively armoured against extinctions should be so susceptible to extermination. This project aimed to use the excellent deep sea fossil record of single-celled planktic foraminifera to unravel this paradox of plankton extinction. This project has discovered that, at any given time, plankton species do not have global distributions, but smaller, more provincial populations that are a function of environmental heterogeneity paced by insolation changes driven by Earth's orbital configurations. This may predispose these species to greater extinction susceptibility. From reconstructing the palaeobiogeographic dynamics of extinction events at high resolution, the sequence of local disappearances leading up to final 'global' extinction has been documented for a series of extinctions between 3.2 and 3.0 Myr ago. These biogeographic constraints provide insight into the mechanisms leading to extinction. The precise timing (within an error of a couple of thousand years) of 17 plankton extinctions in the past 5 million years has now been established. The timing of global extinctions are not random, but occur in clusters. These extinction 'clusters' occur when orbital configurations create persistently weak global environmental gradients (driven by cyclic changes in the axis of Earth's tilt), increasing the chance that the entire habitat of a given species might be eliminated. The project discovered that a number of species of plankton are extremely sensitive to the ventilation state (amount of dissolved oxygen available) of their habitats in the modern oceans. This discovery has been validated in the recent part of the geologic record using independent monitors of ventilation state. This is a major finding to come out of this project, and offers the promise of a quantitative proxy for reconstructing the evolution of the ventilation state of the upper ocean through geologic time. A further exciting outcome of this project has been the challenging of the paradigm view as to the source of carbon driving past extreme climatic warming events ('hyperthermals'). It was shown that, contrary to existing assumptions that large volumes of carbon were repeatedly released from sedimentary reservoirs, these extreme warmings were in fact driven by large-scale releases of carbon dioxide from the ocean abyss in a fashion somewhat analogous to that which drove the much more recent terminations of Pleistocene ice ages. This mechanism for rapid climate change is linked to changes in the rate of ventilation of the world ocean and, although it has been almost entirely overlooked until now, may explain much of the observed instability in palaeoclimate records throughout Earth history.

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