Deep-sea collapse

The effects of human-caused climate change might eventually reach one of the least explored realms of the planet: the bottom of the ocean. A new analysis of miniscule marine fossils from the last 20,000 years shows that during past periods of global cooling, changes in ocean circulation led to the collapse of deep-sea ecosystems.

Moriaki Yasuhara, then with the US Geological Survey, and colleagues studied a core of ocean floor sediment drilled in the northwest Atlantic, identifying fossil ostracodes — bivalved crustaceans less than two millimetres long — in each layer. Because their shells fossilize so well, diverse ostracode remains represent a vibrant deep-sea community overall. Layers deposited during hundreds or thousands of years of natural cooling revealed dramatic drops in the diversity of ostracode species. During these episodes, 'opportunistic' species, able to thrive amid decay, predominated. After one particularly vicious cooling cycle ended, species diversity took thousands of years to recover, probably because of a persistent change in deep ocean currents.

Deep-sea ecosystem collapses, the authors argue, could have arisen from both altered circulation and changing populations of the ostracodes' main food source, surface algae — which might result from anthropogenic warming as well as natural cooling. A rapidly changing climate could disrupt even deep-sea life, they conclude.

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