An oxygen-free ocean from bottom to surface is probably the worst scenario that marine higher life can experience. Are processes and feedbacks linking the atmosphere to the deep ocean capable to cause a rapid change from an oxygen-rich to an oxygen-free deep ocean? And what are the consequences for the global carbon cycle that ultimately drive marine and terrestrial ecosystems and climate variation?
These are fundamental and burning questions on the society's agenda. Hurricane Katrina and other natural catastrophes in recent years have shown how vulnerable mankind is in the face of nature. Professor Tom Wagner of Newcastle University, England, led a cross-disciplinary study of geological records combined with climate modeling to shed new light on the mechanisms and processes that led to repetitive rapid climatic change with major impact on the ocean during past greenhouse conditions.
By analysing sediments laid down on the ocean floor about 85m years ago in the Cretaceous, the research team found evidence that Cretaceous greenhouse climate was highly variable and repeatedly resulted in major changes in ocean chemistry and deep circulation causing disastrous consequences for marine ecosystems. These extreme conditions fostered massive burial of dead organic matter from marine species, such as algae and plankton, at the sea floor, leading to the formation of distinct sediments, "marine black shale", also well known as the world's primary source for oil and gas.
Professor Wagner and colleagues uncovered evidence of the mechanisms that drove rapid and repetitive climate change by studying the quantity and content of proxy parameters in black shale in a core of sedimentary rock drilled out of the ocean bed, off Africa's Ivory Coast, and comparing these results with data from a global climate model.
The model data were used to quantify the freshwater run-off from tropical Africa into the equatorial Atlantic, where the core has been drilled, and to specify the role of orbital configuration and the water cycle on climate and oceanographic variation. With these data, it was possible to explain the formation of the sedimentary succession of black shale and carbonate-rich sediments, indicating alternation between oxygen-depleted and oxygen-rich conditions in the deep ocean. All life other than simple organisms like bacteria would have been seriously depleted in the deeper ocean as oxygen became progressively scarce. On land, the climate variability would cause strong regional contrasts, with widespread deserts at mid-latitudes and extremely humid areas in the tropics.
Processes in the atmosphere driven by cyclic changes in the amount of energy from the sun entering the top of the atmosphere (insolation) have been identified to be the cause for the observed dramatic changes in ocean chemistry that resulted in the formation of black shale. This contributes to the current discussion on whether the atmosphere drives the oceans or vice-versa.
Higher rainfall would have caused increased amounts of fresh water running off the land, carrying large quantities of nutrients into the oceans, resulting in an increase in marine productivity and supporting oxygen depletion and a change in circulation patterns in the deep ocean.
Climate modeling identified that specific periods of extremely high river discharge occurred during maxima in seasonal contrasts when the northern equinox (when the sun is directly over the earth's equator) coincided with perihelon (when the earth passes closest to the sun). It was only during this specific orbital configuration that freshwater run-off exceeded a certain threshold, finally to result in a rapid change to ocean anoxia.
The findings, reported in Nature, the international weekly journal of science, suggest that variations in the water cycle, once they have exceeded a certain threshold, are capable of inducing major environmental change in the oceans.
The researchers conclude: 'The results of this study demonstrate how sensitively and rapidly tropical marine areas close to continental margins react to even relatively moderate increases in continental freshwater discharge.
'The freshwater threshold required to shift sheltered and semi-enclosed areas of the modern ocean into an anoxic mode are unknown but the progressive emission of greenhouse gases to the modern atmosphere is gradually shifting Earth towards a greenhouse mode with an accelerated hydrological cycle.'
'At present it is hardly possible to estimate where we are on the long-term climate trend but once the freshwater threshold is passed, a substantial impact on biochemical cycling of continental margins may be expected.'
Commenting on the Nature paper, Professor Wagner said that the majority of the world's population live in coastal areas, which were the most vulnerable to natural catastrophes as recorded in the geological record.
'Understanding the processes and feedbacks controlling carbon and nutrient cycling in the modern world and during past periods of extreme warmth is therefore critical to separate human impact on climate from natural variability and underpins the ability to adapt to future conditions,' he said.
Professor Wagner, of the Institute for Research on Environment and Sustainability at Newcastle University, England, worked with colleagues from the Universities of Bremen and Cologne and the GEOMAR Leibniz Institute of Marine Sciences at Kiel, in Germany, and the Royal Netherlands Institute for Sea Research (NIOZ) at Texel, Netherlands.
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