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What Drives Carbon Isotope Fractionation by the Terrestrial Biosphere?
Author(s) -
Still Christopher,
Rastogi Bharat
Publication year - 2017
Publication title -
journal of geophysical research: biogeosciences
Language(s) - English
Resource type - Journals
eISSN - 2169-8961
pISSN - 2169-8953
DOI - 10.1002/2017jg004155
Subject(s) - photosynthesis , isotopes of carbon , autotroph , photorespiration , rubisco , carbon dioxide , carbon fibers , carbon fixation , carbon cycle , environmental chemistry , botany , chemistry , ecosystem , ecology , biology , total organic carbon , paleontology , materials science , bacteria , composite material , composite number
During photosynthesis, terrestrial plants preferentially assimilate the lighter and much more abundant form of carbon, 12 C, which accounts for roughly 99% of naturally occurring forms of this element. This photosynthetic preference for lighter carbon is driven principally by differences in molecular diffusion of carbon dioxide with differing 13 C/ 12 C across stomatal pores on leaves, followed by differences in carboxylation rates by the Rubisco enzyme that is central to the process of photosynthesis. As a result of these slight preferences, which work out to about a 2% difference in the fixation rates of 12 CO 2 versus 13 CO 2 by C 3 vegetation, plant tissues are depleted in the heavier form of carbon ( 13 C) relative to atmospheric CO 2 . This difference has been exploited in a wide range of scientific applications, as the photosynthetic isotope signature is passed to ecosystem carbon pools and through ecological food webs. What is less appreciated is the signature that terrestrial carbon exchanges leave on atmospheric CO 2 , as the net uptake of carbon by land plants during their growing season not only draws down the local CO 2 concentration, it also leaves behind relatively more CO 2 molecules containing 13 C. The converse happens outside the growing season, when autotrophic and heterotrophic respiration predominate. During these periods, atmospheric CO 2 concentration increases and its corresponding carbon isotope composition becomes relatively depleted in 13 C as the products of photosynthesis are respired, along with some small isotope fractionation that happen downstream of the initial photosynthetic assimilation. Similar phenomena were first observed at shorter time scales by the eminent carbon cycle scientist, Charles (Dave) Keeling. Keeling collected samples of air in glass flasks from sites along the Big Sur coast that he later measured for CO 2 concentration and carbon isotope composition (δ 13 C) in his lab (Keeling, 1998). From these samples, Keeling observed increasing CO 2 concentrations at night compared to the day, along with corresponding depletions in their δ 13 C. These phenomena were understood at the time to be driven by interactions between ecosystem carbon exchanges and vertical movements of the atmospheric boundary layer (Keeling, 1958).