Photochemical fractionation of fluvial stable carbon isotopes: A meta-analysis

Fluvial systems annually export large amounts of C to the oceans. The stable C isotope ratio (δ¹³C) of fluvial dissolved organic matter (typical δ¹³C values around -28 ‰) is vastly different from that of oceanic dissolved organic matter (around -22 ‰). The broadly accepted view for the apparent disappearance of the -28 ‰ signature is that terrestrial organic matter is oxidized by the time it reaches the ocean, with quantitative flux of this C into the atmosphere. Instead, it is replaced by material from algae fixing atmospheric CO₂ and producing oceanic C having an algal δ¹³C value. However, an alternative proposed explanation for the vastly different δ¹³C values between rivers and ocean is that photochemical irradiation leads to isotopic fractionation. This could conceal much more significant terrestrial C fluxes to the ocean, with photochemically bleached organic C becoming marine-like in its δ¹³C value. While numerous studies have tested this alternative proposition, the evidence has been inconclusive. Here, we perform a meta-analysis of all studies that are known to us on this subject. We observe a strong relationship between δ¹³C fractionation (i.e., increase in δ¹³C) and C loss (r = 0.87), with cumulative δ¹³C fractionation reaching ~6 ‰ when 80+% of C is photo-bleached. These results indicate that terrestrial fluxes of C to the oceans are likely underestimated, and thus the fluvial and oceanic cycles of organic C are coupled. This requires revisiting the global biogeochemical cycles and quantitative estimates of terrestrial-to-marine transfers of organic matter. 

Presentation Preference: Oral

Primary Presenter: Alex Goranov, Old Dominion University (aleksandar.i.goranov@gmail.com)

Authors:

Susan Carter, Harvard University  (sjcarter@fas.harvard.edu)

Ann Pearson, Harvard University  (apearson@eps.harvard.edu)

Patrick Hatcher, Old Dominion University  (phatcher@odu.edu)