Isotopic Signature (δ¹³C, ∆¹⁴C) of DIC in Sediment Pore Waters: An Example from the Rhone River Delta

Abstract

A better understanding of the dynamics of different particulate organic matter (OM) pools in the coastal carbon budget is a key issue for quantifying the role of the coastal ocean in the global carbon cycle. To elucidate the benthic component of this carbon cycle at the land-sea interface, we investigated the carbon isotope signatures (δ13C and ∆14C) in the sediment pore waters dissolved inorganic carbon (DIC) in addition to the sediment OM to constrain the origin of the OM mineralized in sediments. The study site is located at the outlet of the Rhône River (Mediterranean Sea), which was chosen because this river is one of the most nuclearized rivers in Europe and nuclear 14C can serve as a tracer to follow the fate of the OM discharged by the river to the coastal sea. The ∆14C results found in the pore waters DIC show a general offset between buried and mineralized OM following a preferential mineralization model of young and fresh particles. For example, we found that the sediment OM has values with a mean ∆14C=–33‰ at sampling stations near the river mouth whereas enriched ∆14C values around +523‰ and +667‰ respectively were found for the pore waters DIC. This indicates complete mineralization of a riverine fraction of OM enriched in 14C in the river conduit during in-stream photosynthesis. In shelf sediments, the ∆14C of pore waters DIC is slightly enriched (+57‰) with sediment OM reaching –570‰. A mixing model shows that particles mineralized near the river mouth are certainly of riverine phytoplanktonic origin whereas OM mineralized on the shelf is of marine origin. This work highlights the fact that pore waters provide additional information compared to sediments alone and it seems essential to work on both pools to study the carbon budget in river prodelta.

Benthic alkalinity and dissolved inorganic carbon fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes

“…This temporal evolution yields similar diagenetic signatures from mid-spring to end of summer, including almost complete sulfate reduction, large concentrations of DIC and alkalinity (30-40 mM), 500-800 µM of dissolved iron, and no dissolved sulfide in the pore waters (Rassmann et al, 2016;Pastor et al, 2011). This pattern was observed consistently over several sampling campaigns, including April 2007 (Pastor et al, 2011), April 2013 (Dumoulin et al, 2018), May 2014 (Rassmann et al, 2016), September 2015 (this paper), and May 2018 (Christophe Rabouille, unpublished data). Altogether, the pore water data collected over the years in the Rhône prodelta system are consistent and indicate that biogeochemical processes in the critical proximal zone reach a reproducible state on a yearly basis due to the regularity of flood deposition in late fall and maturation of the system in spring and summer.…”

Comment on bg-2020-435

“…Despite that (and somewhat surprisingly), previous investigations at the Rhone delta found similar biogeochemical trends on an interannual basis, i.e. data collected along the years in this system are consistent (Dumoulin et al, 2018;Pastor et al, 2011;Rassmann et al, 2016Rassmann et al, , 2020. As such, it is likely that the majority of sediment load and OM are delivered during late fall and early winter flood events, and the diagenetic system close to the river mouth reaches a mature state during early spring when the typical porewater and benthic-pelagic fluxes have established.…”

FESDIA (v1.0): exploring temporal variations of sediment biogeochemistry under the influence of flood events using numerical modelling

“…This idea is derived from the mathematical theory that describes chaos in the real world (Strogatz, 2018;Ghil, 2019). The existence of a "biogeochemical attractor" may explain why multiple temporal data sets in the Rhône River prodelta show a similar diagenetic signature from spring to summer (Rassmann et al, 2016;Dumoulin et al, 2018). Our timescale analysis estimates that such rapid system restoration is indeed plausible and of the correct order of magnitude, based on the range of uncertainty reported here.…”