Oxygene - The Ocean
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The ratio (relative amount) of these two types of oxygen in water changes with the climate. By determining how the ratio of heavy and light oxygen in marine sediments, ice cores, or fossils is different from a universally accepted standard, scientists can learn something about climate changes that have occurred in the past. The standard scientists use for comparison is based on the ratio of oxygen isotopes in ocean water at a depth of 200-500 meters.
Photosynthesizers have been in the ocean for a long time. Land plants start appearing in the fossil record 470 million years ago, before dinosaurs roamed the earth. But the ocean was producing oxygen for billions of years before that. The oldest known fossil is from a marine cyanobacterium, a tiny-blue green photosynthesizer that was releasing oxygen 3.5 billion years ago. In a way, we owe the ocean for all of the oxygen that comes from land plants as well, because land plants evolved from green marine algae. If there were a race to put oxygen in the atmosphere, the ocean would have one heck of a head start.
While they may be good fodder for speeches, these claims misrepresent where the oxygen we breathe actually comes from, and in doing so, mislead the public as to why we should step up our role as ocean custodians.
If we were to cut or burn all forests and oxidise all organic carbon stored in vegetation and top soils worldwide, it would only lead to a small depletion in atmospheric oxygen. If photosynthesis in the ocean and on land stopped producing oxygen, we could continue breathing for millennia, though we would certainly have other problems.
This oxygen loss is primarily due to increasing ocean stratification. In this process, the mixing of the surface ocean, which becomes warmer and lighter, with the deeper and denser ocean layers is less efficient, restricting the penetration of oxygen. The activity of enzymes, including those involved in respiration, also generally increases with temperature. So, oxygen consumption by ocean creatures increases as the ocean warms.
While it is incorrect to say that the ocean provides 50% of the oxygen we breathe, it is correct to say that, over geological time scales, the ocean has provided a large fraction of the oxygen we take in today. It is also perfectly correct to say that the ocean is responsible for 50% of primary production on Earth, sustaining our food system.
And while we should not worry about the future supply of oxygen for humans to breathe in the future, we should worry about fish being increasingly displaced from expanding ocean areas that are depleted in oxygen.
Plankton are not just one species of sea creature but, rather, a large variety of tiny organisms. Algae, bacteria, crustaceans, mollusks, and more are all considered plankton. What sets them apart from other organisms is how they move. Their extremely small size precludes them from swimming against ocean currents, so they drift.
Pennisi, E. Meet the obscure microbe that influences climate, ocean ecosystems, and perhaps even evolution. Science. March 9, 2017. -obscure-microbe-influences-climate-ocean-ecosystems-and-perhaps-even-evolution
If life as we know it exists in the ocean, there needs to be a way for oxygen to get to it. According to Hesse, the most plausible scenario based on the available evidence is for the oxygen to be carried by salt water, or brine.
Humans and virtually all animal life on Earth require oxygen to breathe. (Yes, even fish breathe oxygen.) Scientists around the world have found that the concentration of oxygen in our water and air is actually declining, says Prof. Andrew Babbin, an oceanographer and professor of chemical oceanography and marine microbiology at MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS).
How serious is all this Recent models estimate a decline of global oxygen concentrations in the ocean to be between 1% and 7% by the year 2100.3 This can lead to serious ecosystem impacts, including reducing biodiversity, disrupting fishery resources, and increasing algal blooms.
Until now, it has been assumed that the oxygenation of the oceans over geological timescales has mainly been driven by atmospheric oxygen levels. However, a new study published in Nature on June 27 2022 suggests otherwise. Work by scientists at the Biogeosciences Laboratory (CNRS/UBFC), together with their colleagues at the University of California's Department of Earth and Planetary Sciences, shows that the movement of tectonic plates has probably contributed to ocean oxygenation. To demonstrate this, the scientists used a three-dimensional climate model to recreate conditions on Earth from 540 million years ago to the present day, in particular taking into account ocean circulation currents. In their model, the scientists modified the position of the continents while keeping the atmospheric oxygen concentration constant. The result was that the oxygen concentration in the oceans increased, despite a constant level of oxygen in the atmosphere. The new paper thus shows for the first time that atmospheric and oceanic oxygen levels are largely independent of each other. Since oxygen is vital to marine life, these findings reveal the hitherto underestimated role played by plate tectonics in the evolution of biodiversity in the oceans over geological time scales.
Continental configuration controls ocean oxygenation during the Phanerozoic, Alexandre Pohl, Andy R. Ridgwell, Richard G.Stockey, Christophe Thomazo, Andrew Keane, Emmanuelle Vennin, Christopher R. Scotese. Nature, June 27 2022. DOI: 10.1038/s41586-022-05018-z
Depending on the climate, the two types of oxygen (16O and 18O) vary in water. Scientists compare the ratio of the heavy (18O) and light (16O) isotopes in ice cores, sediments, or fossils to reconstruct past climates. They compare this ratio to a standard ratio of oxygen isotopes found in ocean water at a depth of 200 to 500 meters. The ratio of the heavy to light oxygen isotopes is influenced mainly by the processes involved in the water or hydrologic cycle.
More evaporation occurs in warmer regions of the ocean, and water containing the lighter 16O isotope evaporates more quickly than water containing the heavier 18O. Water vapor containing the heavier 18O, however, will condense and precipitate more quickly than water vapor containing the lighter 16O. As water evaporates in warmer regions, it is moved with air by convection toward the polar regions.
Ocean-floor sediments can also be used to determine past climate. They reflect the oxygen isotope of the ocean water, because the oxygen in the calcium carbonate shells that are deposited on the ocean floor records the oxygen isotope variations in the ocean at the time of formation.
The table explains how the oxygen isotope ratio can be used to reconstruct the type of past climate. The table explains the oxygen isotope ratio for ice cores and ocean water/ocean floor sediments during a colder climate or glacial period.
The second way to determine past temperatures is by calculating the deuterium to hydrogen ratio in the ice core samples. The water molecule contains two different isotopes of hydrogen (1H and 2H). 1H contains one proton and no neutrons and 2H, known as deuterium or D, contains one proton and one neutron. The ratio of deuterium to hydrogen in the ice core is compared to the ratio of deuterium to hydrogen in standard mean ocean water. The ice cores contain slightly less of the heavier isotopes of oxygen (18O) and deuterium (2H).
In the last 50 years, oxygen-deficient zones in the open ocean have increased. Scientists have attributed this development to rising global temperatures: Less oxygen dissolves in warmer water, and the tropical ocean's layers can become more stratified. googletag.cmd.push(function() { googletag.display('div-gpt-ad-1449240174198-2'); }); But now, contrary to widespread expectations, an international team of scientists led by researchers from the Max Planck Institute for Chemistry and Princeton University has discovered that oxygen-deficient zones shrank during long warm periods in the past.
During their lifetime, these zooplankton absorbed chemical elements such as nitrogen, whose isotope ratio in turn depended on environmental conditions: Under oxygen-deficient conditions, a process called bacterial denitrification occurs, in which bacteria convert the nutrient nitrate to molecular nitrogen. These bacteria prefer to absorb light isotopes of nitrogen instead of heavy ones, so the ratio shifts in periods when the bacteria were active in the oceans. Scientists can measure this to determine the extent of earlier oxygen-deficient zones.
\"We've worked for decades to develop the methods that allowed for these findings,\" said Sigman. \"And right away, the results are altering our view of the relationship between climate and the ocean's oxygen conditions.\"
It is not yet clear, however, what this means for the current expansion of the oxygen-deficient open ocean zones, said Auderset. \"Unfortunately, we don't yet know whether our finding of shrinking marine oxygen-deficient zones is applicable to the coming decades or only to the much longer term,\" she said. \"This is because we have to resolve whether short- or long-term processes were responsible for the change.\"
This chain of events can occur relatively quickly. Thus, if a similar change applies to human-driven global warming as well, then there could be a decline in the extent of open ocean oxygen deficiency in the coming decades.
Alternatively, the cause may lie in the Southern Ocean, thousands of kilometers away. During past prolonged warm periods, the exchange water between Southern Ocean surface waters and the deep ocean (\"deep ocean overturning\") may have accelerated, leading to higher oxygen in the ocean interior as a whole and thus shrinking the low-oxygen zones. If stronger Southern Ocean-driven deep ocean overturning was the main cause of the shrunken tropical oxygen-deficient zones, then this effect would take more than a hundred years at earliest to come into play. 59ce067264
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