The Canfield Ocean model was proposed by geochemist Donald Canfield to explain the composition of the ocean in the middle to late Proterozoic.
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👉 Canfield ocean in the context of Photosynthesis
Photosynthesis (/ˌfoʊtəˈsɪnθəsɪs/ FOH-tə-SINTH-ə-sis) is a system of biological processes by which photopigment-bearing autotrophic organisms, such as most plants, algae and cyanobacteria, convert light energy — typically from sunlight — into the chemical energy necessary to fuel their metabolism. The term photosynthesis usually refers to oxygenic photosynthesis, a process that releases oxygen as a byproduct of water splitting. Photosynthetic organisms store the converted chemical energy within the bonds of intracellular organic compounds (complex compounds containing carbon), typically carbohydrates like sugars (mainly glucose, fructose and sucrose), starches, phytoglycogen and cellulose. When needing to use this stored energy, an organism's cells then metabolize the organic compounds through cellular respiration. Photosynthesis plays a critical role in producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.
Some organisms also perform anoxygenic photosynthesis, which does not produce oxygen. Some bacteria (e.g. purple bacteria) uses bacteriochlorophyll to split hydrogen sulfide as a reductant instead of water, releasing sulfur instead of oxygen, which was a dominant form of photosynthesis in the euxinic Canfield oceans during the Boring Billion. Archaea such as Halobacterium also perform a type of non-carbon-fixing anoxygenic photosynthesis, where the simpler photopigment retinal and its microbial rhodopsin derivatives are used to absorb green light and produce a proton (hydron) gradient across the cell membrane, and the subsequent ion movement powers transmembrane proton pumps to directly synthesize adenosine triphosphate (ATP), the "energy currency" of cells. Such archaeal photosynthesis might have been the earliest form of photosynthesis that evolved on Earth, as far back as the Paleoarchean, preceding that of cyanobacteria (see Purple Earth hypothesis).
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Canfield ocean in the context of Euxinic
Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified; have an oxic, highly productive, thin surface layer; and have anoxic, sulfidic bottom water. The word "euxinia" is derived from the Greek name for the Black Sea (Εὔξεινος Πόντος (Euxeinos Pontos)) which translates to "hospitable sea". Euxinic deep water is a key component of the Canfield ocean, a model of oceans during part of the Proterozoic eon (a part specifically known as the Boring Billion) proposed by Donald Canfield, an American geologist, in 1998. There is still debate within the scientific community on both the duration and frequency of euxinic conditions in the ancient oceans. Euxinia is relatively rare in modern bodies of water, but does still happen in places like the Black Sea and certain fjords.
Canfield ocean in the context of Boring Billion
The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is an informal geological time period between 1.8 and 0.8 billion years ago (Ga) during the middle Proterozoic eon spanning from the Statherian to the Tonian periods, characterized by more or less tectonic stability, climatic stasis and slow biological evolution. Although it is bordered by two different oxygenation events (the Great Oxygenation Event and Neoproterozoic Oxygenation Event) and two global glacial events (the Huronian and Cryogenian glaciations), the Boring Billion period itself actually had very low oxygen levels and no geological evidence of glaciations.
The oceans during the Boring Billion may have been oxygen-poor, nutrient-poor and sulfidic (euxinia), populated by mainly anoxygenic purple bacteria, a type of bacteriochlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H2S) for carbon fixation instead of water and produces sulfur as a byproduct instead of oxygen. This is known as a Canfield ocean, and such composition may have caused the oceans to be colored black-and-milky-turquoise instead of blue or green as later. (By contrast, during the much earlier Purple Earth phase during the Archean, photosynthesis was performed mostly by archaeal colonies using retinal-based proton pumps that absorb green light, and the oceans would be magenta-purple.)
Canfield ocean in the context of Purple Earth hypothesis
The Purple Earth hypothesis (PEH) is an astrobiological hypothesis, first proposed by molecular biologist Shiladitya DasSarma in 2007, that the earliest photosynthetic life forms of Early Earth were based on the simpler molecule retinal rather than the more complex porphyrin-based chlorophyll, making the surface biosphere appear purplish rather than its current greenish color. It is estimated to have occurred between 3.5 and 2.4 billion years ago during the Archean eon, prior to the Great Oxygenation Event and Huronian glaciation.
Retinal-containing cell membranes exhibit a single light absorption peak centered in the energy-rich green-yellow region of the visible spectrum, but transmit and reflect red and blue light, resulting in a magenta color. Chlorophyll pigments, in contrast, absorb red and blue light, but little or no green light, which results in the characteristic green reflection of plants, cyanobacteria, green algae, and other organisms with chlorophyllic organelles. The simplicity of retinal pigments in comparison to the more complex chlorophyll, their association with isoprenoid lipids in the cell membrane, as well as the discovery of archaeal membrane components in ancient sediments on the Early Earth are consistent with an early appearance of life forms with purple membranes prior to the turquoise of the Canfield ocean and later green photosynthetic organisms.
Canfield ocean in the context of Donald Canfield
Donald Eugene Canfield (born 1957) is an American geochemist and Professor of Ecology at the University of Southern Denmark known for his work on the evolution of Earth's atmosphere and oceans. The Canfield ocean, a sulfidic partially oxic ocean existing during the middle of the Proterozoic eon, is named after him.