Extrapolating feedback processes from the present to the past

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Abstract
Extant terrestrial vegetation alters its physical environment via its albedo, and its influence on immediate temperature via stomatal and boundary–layer influences of energy dissipation as sensible and latent heat; aquatic vegetation also controls albedo (e.g. coccolithophorids) and, by competing with water for electromagnetic energy absorption, the depth of the mixed layer and hence the quantity of nutrients trapped for the spring bloom. Both aquatic and terrestrial vegetation have had, together with microbial and geological processes, an influence on O2and CO2levels, and hence on the availability and biological functioning of Fe, Mn, Cu, Zn, Se and P, and the relative competitive advantage of C3versus C4, crassulacean acid metabolism (CAM) and carbon concentration mechanism (CCM) organisms. Less directly, changes in primary productivity impact on the production of CH4and N2O which, like CO2, are greenhouse gases, while some (marine) primary producers yield dimethyl sulphide (and hence cloud condensation nuclei, with effects on cloudiness) and halocarbons (via, in part, O2–dependent processes), partly negating the O3attenuation of UV–B radiation. These effects can be related to the terrestrial embryophytic vegetation back to ca. 450 Ma, and to eukaryotic marine vegetation back to at least 1.7, and probably 2.1 Ga, with implications for inter alia C3versus C4, CAM and CCM photosynthesis, and Fe acquisition mechanisms. Even earlier (3.8 Ga onwards) prokaryotes may have influenced CO2levels and hence controlled (as they did later) surface temperature. By producing O2, they led to decreasing availability of Fe, Mn and P (and utility of Se?), and increasing availability of Cu (and Zn?) that shaped the biochemistry on which later biogeochemistry was based.