Beginning with Victorian science and progressing through the onset of modern deep-sea biology, the dominant paradigm was that the deep sea was a stable ecosystem. Organisms, and the communities that contained them, were unchanging because the deep ocean was buffered against climatic variability. Move head to the 1980’s and beyond and the picture has changed considerably. Work led by John Gage, Paul Tyler, Craig Young demonstrates that many species exhibit seasonal reproduction. Work by Ken Smith and Henry Ruhl here at MBARI reveals an ecosystem where biodiversity shifts are triggered by El Nino events. The paradigm now is one of a dynamic system intimately related to seasonal, annual, and decadal changes in surface production and ocean currents. A system that is not buffered against climate alterations at our hands.
Yet this picture of the deep sea is based on a relatively small snapshot of geologic time and thus lacks a vital component. We know little of how deep-sea systems respond on geologic timescales. Work this week in Proceedings of the National Academy of Science by Yasuhara et al. details a violent history of ecological collapses on the deep-sea floor.
Utilizing fossilized ostracods (small bivalved Crustaceans) from cores taken in the Atlantic, they are able to recreate the last 20,000 years. During major oceanographic changes up to 50% of ostracod species were replaced. Imagine half the species in your favorite dive or hiking spot being replaced with totally new species. These biodiversity shifts occurred during centennial-millennial scale cooling events that altered oceanographic currents and surface production of plankton. This sinking plankton is the major food source for most deep-sea organisms. The largest disruptions occurred during the Heinrich Event 1(1), the Inter-Allerod Cold Period (2), the Younger Dryas (3), and several Holocene Bond events (4). The absolute largest occurred during the Younger Dryas/Inter-Allerod Cold Period that required over 8,000 years for the ostracods to fully recover.
(1) episode during the last glacial period where several glaciers cleaved a vast number of icebergs that traversed the North Atlantic, distinguished by 6 separate events (1-6) H1 occurred between 16,800-14,000 years ago
(2) the Allerod Period occurs at the end of the last glaciation when temperatures in the North Atlantic rose close to present day level, a cooling period ensues afterward lasting about 200 years
(3) A period after the Inter-Allerod Cold Period known as the Big Freeze occurring between 12,700-11,500 years before present
(4) Quasi-1500 year climate cycles occurring in the North Atlantic
Yasuhara, M., Cronin, T.M., deMenocal, P.B., Okahashi, H., Linsley, B.K. (2008). From the Cover: Abrupt climate change and collapse of deep-sea ecosystems. Proceedings of the National Academy of Sciences, 105(5), 1556-1560. DOI: 10.1073/pnas.0705486105