Muddying the river-ocean carbon budget: synthesis of previous work on Guianas Mudbanks organic carbon in preparation for a new expedition

Abstract:

The application of novel geochemical analyses allows us to revisit long-standing questions regarding organic carbon (OC) cycling through source-to-sink systems. Continental margins, in particular shelf-deltaic regions, represent the largest sink of both terrestrial and marine OC in the global ocean. Terrestrial organic matter is considered highly degraded and is expected to undergo minimal respiration in the ocean, whereas organic matter deriving from marine primary production is presumably more reactive and should be preferentially oxidized. However, the apparent reactivities of both terrestrial and marine organic matter are contradicted by observations of their burial efficiencies. We show that a simple two end-member mixing model using bulk measurements cannot accurately explain the ¹⁴C and stable isotope composition of OC in Amazon shelf sediments. To reconcile this conundrum, we demonstrate how ramped pyrolysis/oxidation coupled with stable isotope and radiocarbon analysis allows us to break down these biased bulk measurements and establish a continuum of OC reactivity with discrete estimates of source and age. Building on this foundation, our upcoming expedition will aim to substantiate this idea that terrestrial OC in marine sediments of the Amazon River Delta and Guianas Mudbanks consists of both refractory and labile components scaling in reactivity, such that the more recalcitrant constituents are efficiently preserved while the labile constituents are rapidly remineralized. By characterizing the degradation potential of complex OC mixtures, we can more accurately quantify the preservation of organic matter in marine sediments. With this approach, we expect to show that terrestrial OC burial efficiency offshore the Amazon River has been underestimated by 50-100% in previous studies. This has significant implications for our understanding of terrestrial OC cycling in the ocean as well as estimates of atmospheric CO₂ over geologic time scales.