{"id":7378,"date":"2018-01-31T11:10:14","date_gmt":"2018-01-31T10:10:14","guid":{"rendered":"https:\/\/game-csic.com?p=7378\/"},"modified":"2018-02-16T12:41:26","modified_gmt":"2018-02-16T11:41:26","slug":"stable-isotopes-a-tool-for-palaeo-ecological-studies","status":"publish","type":"post","link":"https:\/\/game-csic.com\/en\/stable-isotopes-a-tool-for-palaeo-ecological-studies\/","title":{"rendered":"Stable isotopes: a tool for palaeoecological studies"},"content":{"rendered":"
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Leiva-Due\u00f1as, C.<\/a> \u00a0\u201cThe Posidonia oceanica (L. Delile) mat archive: a tool for palaeo-ecological studies\u201d. M.Sc. in Ecology, Environmental Management and Restoration (University of Barcelona), 2015<\/h4>\n\n\t\t<\/div>\n\t<\/div>\n
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A meaningful reconstruction of past ecosystem variability strongly and critically depends on the quality of palaeo-archives where the information is contained, and on its correct interpretation, which is limited by the tools in paleoecology: the proxies. These are indirect measurable descriptors of desired environmental variables (but unobservable and unmeasurable) contained in the archives to which they are assumed to be related. Proxies can be based on physical, chemical, isotopic or biological features of the biogeochemical archive. In reality, most proxies respond concurrently to many variables, although two or three of these will usually predominate; these predominant variables also often have interacting effects. The use of a single proxy is inherently plagued by uncertainties, as it is a multivariate function of several environmental variables, leading to ambiguous establishment of cause and effect and calibration. Multi-proxy studies may give greater insight; the use of multiple paleo-proxies is a powerful approach very common in current paleo-environmental reconstructions. Thus, multi-proxy techniques do provide more reliable information, although gaps in our knowledge of the relationships of proxies to environmental factors are weaknesses that should not be ignored.<\/p>\n

One example of a proxy is stable isotope ratios. They have become widely used for palaeoenvironmental studies. Stable isotopic composition varies substantially between different compartments because of isotopic fractionation in chemical, physical and metabolic processes. The relative isotopic abundance of an animal or plant tissue, or of biogenic carbonates (shells), can be related, using theoretical knowledge of the fractionation, to the biogeochemical conditions driving biological or geological processes occurring when those isotopes were incorporated. Isotopic composition is reported on a delta scale (\u03b4) which indicates the deviation (in \u2030) of the isotopic composition of a sample from an internationally accepted standard. The carbon isotopic composition (\u03b413<\/sup>C) provides integrative information about a wide range of physiological and geochemical processes. There are two stable carbon isotopes, 12<\/sup>C and 13<\/sup>C. The activity of the enzymes RUBISCO and PEP is responsible for isotopic discrimination against the heavy carbon isotope (13<\/sup>C) during the carboxylation process, but in different proportions depending on the metabolic pathway (C3<\/sub> or C4<\/sub> plants). This is the reason why 13<\/sup>C content in plants is usually lower than atmospheric CO.2<\/sub> they assimilate and why C3<\/sub> and C4<\/sub> plants have \u03b413<\/sup>C values that differ significantly.<\/p>\n

Por lo tanto, el inter\u00e9s en utilizar is\u00f3topos estables de P. oceanica <\/em>The significance for environmental studies lies in the fact that environmental variability alters plant biology, predictably changing the isotopic ratios of plant tissues.<\/p>\n

Seagrasses show substantial variability in \u03b413<\/sup>C values, the average value ranging between -10 and -11%. Carbon stable isotopic ratios in the seagrasses are affected by environmental variability: hydrological conditions, C source for photosynthesis, CO2<\/sub> (or bicarbonate) availability, irradiance, temperature, etc. Positive correlations between irradiance and \u03b413<\/sup>C in C4 and C3 plants have been observed, including for P. oceanica<\/em>A higher irradiance causes a higher photosynthetic demand, which increases plant net productivity. This creates a lower carbon availability relative to lower irradiance conditions, resulting in the incorporation of a higher proportion of the heavy isotope. 13<\/sup>C by the enzymes (less discrimination\/fractionation). The tissues of the plant are therefore said to become enriched in 13<\/sup>C and the \u03b413<\/sup>C values increase (or become less negative). The \u03b413<\/sup>C could therefore be used as a proxy for irradiance and primary production of P. oceanica<\/em>, allowing reconstructions of environmentally-related changes in plant productivity.<\/p>\n\n\t\t<\/div>\n\t<\/div>\n\n\t

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Fig.1.<\/strong> Conceptual outline of the palaeo-ecological reconstruction strategy used. Once a proxy that describes the relationship between environmental (e.g. irradiance) and biotic components (e.g. \u03b413<\/sup>C of P.oceanica<\/em> sheaths) of the ecosystem is identified and calibrated, this relationship can be reconstructed from the changes of the proxy in the paleo-archive.<\/p>\n\n\t\t<\/div>\n\t<\/div>\n<\/div><\/div><\/div><\/div>

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