Plant biomarkers in aerosols record isotopic discrimination of terrestrial photosynthesis

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Abstract
Carbon uptake by the oceans and by the terrestrial biosphere can be partitioned using changes in the 12C/13C isotopic ratio (δ13C) of atmospheric carbon dioxide1,2,3,4, because terrestrial photosynthesis strongly discriminates against 13CO2, whereas ocean uptake does not. This approach depends on accurate estimates of the carbon isotopic discrimination of terrestrial photosynthesis (Δ; ref. 5) at large regional scales6, yet terrestrial ecosystem heterogeneity7 makes such estimates problematic. Here we show that ablated plant wax compounds in continental air masses can be used to estimate Δ over large spatial scales and at less than monthly temporal resolution. We measured plant waxes in continental air masses advected to Bermuda, which are mainly of North American origin, and used the wax isotopic composition to estimate Δ simply. Our estimates indicate a large (5–6‰) seasonal variation in Δ of the temperate North American biosphere, with maximum discrimination occurring in late spring, coincident with the onset of production. We suggest that the observed seasonality arises from several factors, including seasonal shifts in the proportions of production by C3 and C4 plants, and environmentally controlled adjustments in the photosynthetic discrimination of C3-plant-dominated ecosystems.