An oxygen-free ocean from bottom to surface is probably the worstscenario that marine higher life can experience. Are processes andfeedbacks linking the atmosphere to the deep ocean capable to cause arapid change from an oxygen-rich to an oxygen-free deep ocean? And whatare the consequences for the global carbon cycle that ultimately drivemarine 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 haveshown how vulnerable mankind is in the face of nature. Professor TomWagner of Newcastle University, England, led a cross-disciplinary studyof geological records combined with climate modeling to shed new lighton the mechanisms and processes that led to repetitive rapid climaticchange with major impact on the ocean during past greenhouseconditions.
By analysing sediments laid down on the ocean floor about 85myears ago in the Cretaceous, the research team found evidence thatCretaceous greenhouse climate was highly variable and repeatedlyresulted in major changes in ocean chemistry and deep circulationcausing disastrous consequences for marine ecosystems. These extremeconditions fostered massive burial of dead organic matter from marinespecies, such as algae and plankton, at the sea floor, leading to theformation of distinct sediments, "marine black shale", also well knownas the world's primary source for oil and gas.
Professor Wagner and colleagues uncovered evidence of themechanisms that drove rapid and repetitive climate change by studyingthe quantity and content of proxy parameters in black shale in a coreof sedimentary rock drilled out of the ocean bed, off Africa's IvoryCoast, and comparing these results with data from a global climatemodel.
The model data were used to quantify the freshwater run-offfrom tropical Africa into the equatorial Atlantic, where the core hasbeen drilled, and to specify the role of orbital configuration and thewater cycle on climate and oceanographic variation. With these data, itwas possible to explain the formation of the sedimentary succession ofblack shale and carbonate-rich sediments, indicating alternationbetween oxygen-depleted and oxygen-rich conditions in the deep ocean.All life other than simple organisms like bacteria would have beenseriously depleted in the deeper ocean as oxygen became progressivelyscarce. On land, the climate variability would cause strong regionalcontrasts, with widespread deserts at mid-latitudes and extremely humidareas in the tropics.
Processes in the atmosphere driven by cyclic changes in theamount of energy from the sun entering the top of the atmosphere(insolation) have been identified to be the cause for the observeddramatic changes in ocean chemistry that resulted in the formation ofblack shale. This contributes to the current discussion on whether theatmosphere drives the oceans or vice-versa.
Higher rainfall would have caused increased amounts of freshwater running off the land, carrying large quantities of nutrients intothe oceans, resulting in an increase in marine productivity andsupporting oxygen depletion and a change in circulation patterns in thedeep ocean.
Climate modeling identified that specific periods of extremelyhigh river discharge occurred during maxima in seasonal contrasts whenthe northern equinox (when the sun is directly over the earth'sequator) coincided with perihelon (when the earth passes closest to thesun). It was only during this specific orbital configuration thatfreshwater run-off exceeded a certain threshold, finally to result in arapid change to ocean anoxia.
The findings, reported in Nature, the international weeklyjournal of science, suggest that variations in the water cycle, oncethey have exceeded a certain threshold, are capable of inducing majorenvironmental change in the oceans.
The researchers conclude: 'The results of this studydemonstrate how sensitively and rapidly tropical marine areas close tocontinental margins react to even relatively moderate increases incontinental freshwater discharge.
'The freshwater threshold required to shift sheltered andsemi-enclosed areas of the modern ocean into an anoxic mode are unknownbut the progressive emission of greenhouse gases to the modernatmosphere is gradually shifting Earth towards a greenhouse mode withan accelerated hydrological cycle.'
'At present it is hardly possible to estimate where we are onthe long-term climate trend but once the freshwater threshold ispassed, a substantial impact on biochemical cycling of continentalmargins may be expected.'
Commenting on the Nature paper, Professor Wagner said that themajority of the world's population live in coastal areas, which werethe most vulnerable to natural catastrophes as recorded in thegeological record.
'Understanding the processes and feedbacks controlling carbonand nutrient cycling in the modern world and during past periods ofextreme warmth is therefore critical to separate human impact onclimate from natural variability and underpins the ability to adapt tofuture conditions,' he said.
Professor Wagner, of theInstitute for Research on Environment and Sustainability at NewcastleUniversity, England, worked with colleagues from the Universities ofBremen and Cologne and the GEOMAR Leibniz Institute of Marine Sciencesat Kiel, in Germany, and the Royal Netherlands Institute for SeaResearch (NIOZ) at Texel, Netherlands.
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