Causes of evolution and global extinction in the deep sea

Collaborators: Ellen Thomas (Yale University, USA); Ann Holbourn (University of Kiel, Germany); Shungo Kawagata (Yokohama National University, Japan)

The floor of the oceans is regarded as one of the most stable habitats on earth and this is generally reflected in the slow turnover rates (extinctions and appearances) of the organisms that live there. By far the best fossil record of organisms that live on and in the sediment on the floor of the deep sea is provided by the shells of benthic foraminifera, which have an estimated background turnover rate of 2% per million years. This is significantly less than the overall Cenozoic rates for planktic (c.11% per million years) or shallow-water benthic foraminifera (c.5% per million years).

Three major extinction episodes have occurred in the marine realm in the last 100 million years:

1. The Cretaceous/Tertiary extinction (which killed off the dinosaurs 65 million years ago), caused by a bolide, killed off 80% of life forms in terrestrial, shallow marine and pelagic realms, but had little impact on the biota of the deep sea.

2. The latest Paleocene extinction (55 million years ago) occurred in just a few thousand years during the warmest period on Earth in the last 100 million years, and killed off 30-50% of the deep-sea benthic foraminifera.

3. The last global extinction event (LGE) in the oceans occurred during the Late Pliocene to mid Pleistocene climate transition (MPT), just 3 million to 600,000 years ago, as the Ice Age climate cycles rapidly increased in severity and amplitude. Thus it differs dramatically from the other two. Unlike the three earlier periods of enhanced turnover, the LGE was highly selective of taxa, targeting elongate, cylindrical and uniserial morphologies with small, specialised apertures, killing-off two complete families (Stilostomellidae, and Pleurostomellidae), one subfamily (Plectofrondiculariinae), 19 genera, and 89 species (c. 25% of deep-sea foraminiferal diversity).

Our recent studies elucidate the architecture of the LGE during the MPT, with pulsed glacial declines and partial interglacial recoveries before final extinction of the group around 0.6 myrs ago. Disappearance of the group began with their withdrawal from deep bottom waters (>3000 m depth) and their survival for several hundred thousand years longer in intermediate waters (600-1500 m depth). At these depths they disappeared earlier in sites closer to locations where the intermediate waters originated at the sea surface and last at the most distant sites. This suggests that their extinction was related to progressive changes in the properties of the deep water masses as they were formed during successively more severe glacial cycles. This increased cold or increased oxygen could have impacted directly upon the foraminifera or on their specific food.

Most of the modern deep-sea foraminiferal biota has evolved since the end of the Greenhouse period and is strongly reliant on the seasonal rain of phytoplankton-derived food. Because they survived the Cretaceous-Tertiary extinction it has been suggested that Greenhouse foraminifera may have been less dependent on plankton and fed on warmth-loving chemosynthetic prokaryotes that lived in the dark bottom waters or on the ocean floor. We speculate that the LGE may have killed off the last of the Greenhouse deep-sea fauna (the two families comprised 50-75% of the fauna towards the end of Greenhouse times), because it was unable to adjust to the increasingly cold and fluctuating deep oceanic Icehouse environment.

Our major goal in this study is to increase understanding of the causes of global evolution and extinction in the deep sea, as we still do not really know how species evolve and mass extinctions occur in the world’s largest environment. Our recent work on the Last Global Extinction (LGE) in the deep sea leads us to the following major question: What type of change could be so all-encompassing and targeted, that specific families of cosmopolitan deep-sea foraminifera would suffer extinction, having progressively died back over several hundreds of thousands of years and been unable to re-immigrate from refugia? Deep-ocean biota live in the dark, in near-freezing temperatures, under high pressures, at constant salinities, with little food, mainly derived from surface primary productivity 100s to 1000s of metres above. In the deep oceans, conditions do not usually change rapidly or permanently over the whole habitat, and there are no large-scale, isolating barriers. Through the existence of recently recognised motile propagules, benthic foraminifera have been shown to be able to move from one region to another, and quickly recolonise disturbed regions.

We hypothesise that the LGE wiped out the remnants of the Greenhouse biota, when they or their food supply were unable to cope with the large and rapid changes in the deep-sea environment that accompanied the pulsed expansion of the northern hemisphere ice cap, especially the colder, more oxygenated conditions during increasingly severe glacials.


To test this hypothesis we have been funded to undertake the following research, 2007-2009:

a. Document the global Cenozoic record of the group of cylindrical and uniserial foraminifera that largely disappeared during the Pleistocene (Stilostomellidae, Pleursotomellidae, Plectofrondiculariinae, Chrysalogonium, Glandulonodosaria, Orthomorphina) to determine their biogeographic and paleobathymetric histories, timings of species originations and extinctions.

b. To achieve (a), we need to complete the task we have undertaken on the Pliocene-Pleistocene record (last 5 myrs), by unravelling the complex taxonomic chaos surrounding these little-studied, but highly distinctive, extinct foraminiferal families and genera back through the rest of the Cenozoic (back to 65 myrs). Many taxa have been identified under a wide variety of names around the globe and identifications have been inconsistent between workers.

c. Relate identified migrational, speciation and extinction events to possible environmental causes through use of geochemical and other proxies for temperature, quantity and character of food supply, dissolved oxygen, acidity, etc., especially during times of known major global climate change in the late Paleocene-early Eocene (warm episodes), late Eocene-early Oligocene (major cooling) and middle Miocene (rapid cooling).