Photosynthesis atmospheric oxygen from

Heavy isotopes endow the compounds in which they appear with slightly greater masses than their unlabeled counterparts. These compounds can be separated and quantitated by mass spectrometry (or density gradient centrifugation, if they are macromolecules). For example, O was used in separate experiments as a tracer of the fate of the oxygen atoms in water and carbon dioxide to determine whether the atmospheric oxygen produced in photosynthesis arose from HgO, COg, or both ... 

Three processes that take place in living organisms - respiration in animals and plants, photosynthesis only in plants, and the precipitation of solids by some aquatic animals - have altered the primeval composition of the outer solid, liquid, and gaseous layers of the earth. Respiration consumes oxygen from the atmosphere and creates carbon dioxide. Photosynthesis, which does the opposite (consumes carbon dioxide and releases oxygen), has... 

Photosynthesis is the process by which plants use energy from sunlight to convert carbon dioxide (CO2) and water (H2O) to carbohydrates. Both the carbon dioxide and water come from the environment. Photosynthesis is the main process that removes carbon dioxide from the atmosphere. Oxygen and water vapor are released into the atmosphere as by-products of the reactions of photosynthesis. 

I he formation of food begins with photosynthesis, the biochemical process lused by plants to create carbohydrates and oxygen from solar energy, water, and atmospheric carbon dioxide ... 

Figure 11.2 Photosynthesis and oxygen atmosphere evolution on Earth. (Adapted from Frausto Da Silva et al. [I])...

Geochemical evidence suggests that there were delays of several hundred million years between the rise of oxygenic photosynthesis, the oxygenation of the atmosphere, and the oxygenation of the deep ocean. Photosynthesis (evidenced by cyanobacterial microfossils and biomarkers) rose as early as 3.5 Ga (billion years before present (Schopf, 1993)) and had been sofidly established by 2.7—2.5 Ga (Brocks et al, 1999 KnoU, 1996 Schopf, 1993 Summons et al, 1999). Data from red beds, detrital mineral deposits, and sulfur isotopes indicate the rise of atmospheric oxygen around 2.4 Ga (Bekker et al, 2004 Chandler, 1980 Des Marais et al, 1992 ... 

The build-up of the original, photosynthetically-generated oxygenated atmosphere resulted from an evolutionary lag between the event of photosynthesis and that of the oxidative respiration that followed. Its maintenance, however, throughout the latter eons was essential to keep the organic cycle operative. Because the residence time of O2 in the present atmosphere is relatively short in the geological sense (6000 y Holland,... 

It has been inferred from carbon and sulfur dating that the concentration of oxygen in the Earth s atmosphere was less than I part per million volume (ppmv) prior to around 2.4 billion (Ga) years ago, whereas methane would have been present at levels of around 10 to 10 ppmv (compared with its present value of around 1.7 ppmv). Methane is generated in signihcant amounts by the anaerobic decomposition of organic matter in modem marine sediments, but is oxidised by sulfate under the ocean sea floor and never reaches the atmosphere. Prior to the watershed constituted by the appearance of oxygenic photosynthesis, the ocean had little sulfate to support anaerobic oxidation of methane, but as atmospheric oxygen and seawater sulfate levels rose, the consequent anaerobic oxidation of methane would have steadily reduced the net release of methane. 

The total amount of oxygen in the atmosphere is about 1.2 X lO g(O). The processes affecting atmospheric oxygen in the short-term are photosynthesis and the combination of respiration by living organisms and decomposition of fixed carbon by abiotic processes. The discussion of the control of atmospheric oxygen is taken from the treatment by Holland (1978) and the reader should refer to this work for a more detailed discussion. We will represent photosynthesis by the following reaction ... 

X 10 g(O)/year. Most of this oxygen is removed rather quickly by respiration/decomposition and thus photosynthesis by itself does not account for the net production of oxygen. The production of oxygen results from the physical removal of some of the reduced carbon from contact with oxygen before it has a chance to decompose. This process is known as carbon burial and it represents the difference between photosynthesis and respiration/decomposition. Carbon burial is the removal of fixed carbon to anaerobic sediments where reaction with atmospheric oxygen does not occur until the sediments are returned to the surface by tectonic processes. The rate of carbon burial corresponds to 3.2 x 10 g(O)/ year, less than 1% of the amount of oxygen formed by photosynthesis. 

The major Carboniferous—Permian icehouse episode at c.300 Ma is associated with elevated atmospheric oxygen concentrations (Fig. 6.14d) and so has been attributed to elevated levels of photosynthesis, which caused atmospheric 02 levels to increase while C02 was drawn out of the atmosphere and locked away in the geochemical C cycle (Berner 1990 Crowell 1999). At the time, the land mass was assembled into a supercontinent, Pangaea, which formed a continuous meridional belt almost from pole to pole (Fig. 6.19). In the equatorial belt, where the areas now represented by Europe... 

---