Long‐Term Storage and Age‐Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland

Abstract

Terrestrial organic carbon (OC) plays an important role in the carbon cycle, but questions remain regarding the controls and timescale(s) over which atmospheric CO 2 remains sequestered as particulate OC (POC). Motivated by observations that terrestrial POC is physically stored within soils and other shallow sedimentary deposits, we examined the role that sediment storage plays in the terrestrial OC cycle. Specifically, we tested the hypothesis that sediment storage impacts the age of terrestrial POC. We focused on the Efri Haukadalsá River catchment in Iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon-based determinations of POC storage duration by supplying pre-aged "petrogenic" POC.Our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial-scale storage times. Comparison between the sample types (suspended and deposits) suggested an age offset between transported (suspended sediments) and stored (deposits) POC at the time of sampling, which is predicted by theory for the sediment age distribution in floodplains. We also observed that POC in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data, which suggested an additional role for OC cycling. Consistent with this, we observed interparticle heterogeneity in the composition of POC by imaging our samples at the microscale using X-ray absorption spectroscopy. Specifically, we found that particles within individual samples differed in their sulfur oxidation state, which is indicative of multiple origins and/or diagenetic histories. Altogether, our results support recent coupled sediment storage and OC cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling.

Fluvial organic carbon cycling regulated by sediment transit time and mineral protection

“…This may reflect mass loss of 14 C-depleted sediment through selective deposition of coarse sand (including lithic fragments and petrogenic POC) and/or fresh organic matter addition. From river km 420 to 1,220, where v sed is low, δ 13 C DI values increased from -26.4 ± 0.3‰ to -25.5 ± 0.3‰, and F 14 C DI values decreased from 0.92 to 0.86 (~510 years 14 C-age increase). These changes probably reflect entrainment of aged, 13 C-enriched POC from floodplain deposits.…”

“…In agreement, F 14 C alk values were generally lower for surface samples. These compositional differences reflect hydrodynamic sorting 33,37 , where water-logged plant debris dominated POC at depth, and surface-water samples were concentrated in 13 C-enriched POC associated with low-density mineral phases 25,26,38 . With increasing transit distance along the meandering segment, surface-water POC became consistently more 13 C enriched and 14 C depleted, reflecting progressive addition of aged, mineral-associated POC.…”

“…These compositional differences reflect hydrodynamic sorting 33,37 , where water-logged plant debris dominated POC at depth, and surface-water samples were concentrated in 13 C-enriched POC associated with low-density mineral phases 25,26,38 . With increasing transit distance along the meandering segment, surface-water POC became consistently more 13 C enriched and 14 C depleted, reflecting progressive addition of aged, mineral-associated POC. Bedload samples had low POC concentrations (Supplementary Table 1), suggesting that vertical exchange between bedload and suspended load has negligible effects on depth-integrated POC composition.…”

“…In coastal mountains with short (10 1 -10 2 km), steep rivers linking to the ocean, fluvial POC may be efficiently transferred to depocentres 8,10 . In continental settings, fluvial transit lengths and times can exceed 10 3 km and 20 kyr [11][12][13] , respectively, and many large rivers have been characterized as net CO 2 sources due to degassing from channels and floodplains [14][15][16][17][18] . Where measured, 14 C ages of exported fluvial POC are an order of magnitude younger than clastic sediment transit times, indicating substantial POC turnover during lowland transit 19,20 .…”

Impact of River Channel Lateral Migration on Microbial Communities across a Discontinuous Permafrost Floodplain

“…[13][14][15][16]. However, many organic-rich permafrost deposits are located in the floodplains of large rivers, where fluvial processes control the transport of carbon and sediment and the resulting architecture of the deposits (17)(18)(19). In particular, river channel migration may introduce additional variability to the permafrost soil microbiome by eroding the active layer and upper tens of meters of underlying permafrost, and by building new deposits elsewhere on the floodplain (20).…”