INTRODUCTION
A better understanding of the carbon cycle at the land-ocean interface is crucial to assess the influence of terrestrial inputs and the anthropic impact on the coastal seas (Bauer et al. Reference Bauer, Cai, Raymond, Bianchi, Hopkinson and Regnier2013; Bianchi and Allison Reference Bianchi and Allison2009). Deltas are particularly active in biogeochemical cycles because, despite their small surface areas compared to the oceans (0.3%), they represent nearly half of all the carbon buried in oceanic sediments (Berner Reference Berner1982). These large accumulations of organic matter (OM) are accompanied by high mineralization rates and are a major source of CO2 production, representing one fourth of the CO2 absorbed by the whole ocean (Cai Reference Cai2011). It is thus essential to properly constrain and understand the characteristics of OM deposited in deltaic sediments: its origin (land, river, sea), its nature (soils, vegetation, algae, phytoplankton) and even its age and reactivity (old and refractory or labile and energetic). The study of combined isotopic signatures (δ 13C and Δ14C) of DIC in pore waters is a powerful tool to assess the sources and the mineralization mechanisms of OM deposited in deltas (Bauer et al. Reference Bauer, Reimers, Druffel and Willimas1995; Aller et al. Reference Aller, Blair and Brunskill2008). The δ 13C signatures of the dissolved organic carbon (DIC) of sediment pore waters provide information about the origin of the OM mineralized in the sediment, with different signatures for terrestrial sources (δ 13C around –25‰) and marine sources (δ 13C around –20‰) (Goñi et al. Reference Goñi, Ruttenberg and Eglinton1998; Burdige Reference Burdige2006). The Δ14C signatures provide information about the age of the different OM but can also be used as a tracer for the freshly produced OM of the river (Aller et al. Reference Aller, Blair and Brunskill2008; Aller and Blair Reference Aller and Blair2004). The anthropogenic activities and the presence of nuclear plants on the Rhône river can lead to enriched Δ14C-OM signatures (Eyrolle et al. Reference Eyrolle, Antonelli, Renaud and Tournieux2015) which can be used to follow the fate of the OM in the different pools of the system originating from the river, delta, or ocean (Pozzato et al. Reference Pozzato, Rassmann, Lansard, Dumoulin, van Brugel and Rabouille2018; Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018).
In these previous studies we showed that sedimentation and mineralization of terrestrial OM is very high in the proximal zone near the river mouth, while further offshore on the continental shelf, the organic matter sedimentation is dominated by the mineralization of marine OM. However, the data for the continental shelf were limited in these first studies, and a new sampling cruise, “MissRhoDia II”, was carried out on the RV Tethys II in May 2018. Here we report on these new results from the continental shelf sediments and investigate OM mineralization over a longer period of time (one century) in this zone under the influence of the river plume dominated by autochthonous production.
MATERIALS AND METHODS
Study Site
The Rhône river is one of the largest French rivers and provides the major input of freshwater and OM to the western Mediterranean Sea. The sources of riverine inputs discharged to the delta depend on the hydrological regime which can go from 700 m3 s-1 during low flow periods to over 3000 m3 s-1 during flood periods. Soils of various ages, vegetation debris and river phyto-planktonic production are mixed and carried by the river to the prodelta (Harmelin-Vivien et al. Reference Harmelin-Vivien, Dierking, Banaru, Fontaine and Arlhac2010; Cathalot et al. Reference Cathalot, Rabouille, Tisnerat-Laborde, Toussaint, Kerherve, Buscail, Loftis, Sun, Tronczynski and Azoury2013; Copard et al. Reference Copard, Eyrolle, Radakovitch, Poirel, Raimbault, Gairoard and Di-Giovanni2018). Large amounts of sediment accumulate in the river mouth at a very fast sedimentation rate of around 30 to 40 cm y-1 (Charmasson et al. Reference Charmasson, Bouisset, Radakovitch, Pruchon and Arnaud1998) whereas further from the coast, on the continental shelf, the sedimentation rate is less than <0.3 cm y-1 (Miralles et al. Reference Miralles, Radakovitch and Aloisi2005).
The 14C activities in the Rhône river have varied over the last century with two main periods (Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018). During the pre-nuclear period, before 1960, the Rhone river had a freshwater reservoir effect (FRE) for radiocarbon in dissolved inorganic carbon (DIC) because of its multiple tributaries and the dissolution of old carbonates from its drainage basin. The FRE was evaluated at Δ14C = –114‰, close to the value found for its main tributary, the Durance river which is known to be depleted in Δ14C with a Δ14C = –138‰ (Jean-Baptiste et al. Reference Jean-Baptiste, Fontugne, Fourré, Marang, Antonelli, Charmasson and Siclet2018). The river OM signature was also impacted by this FRE. On the marine side, the DIC and consequently the marine OM, such as phytoplankton, showed a value of Δ14C = –50‰ to Δ14C = –60‰ due to the marine reservoir effect in this zone (Siani et al. Reference Siani, Paterne, Arnold, Bard, Métivier, Tisnérat and Bassinot2000; Tisnérat-Laborde et al. Reference Tisnérat-Laborde, Montagna, Frank, Siani, Silenzi and Paterne2013). Then, during the last 50 years, after the atmospheric nuclear bomb tests and the penetration of the bomb radiocarbon into the river and the ocean, enriched values were measured in the Mediterranean Sea surface waters with Δ14C = +90‰ around 1970 and a decrease to Δ14C = +60‰ in 2010 (Tisnérat-Laborde et al. Reference Tisnérat-Laborde, Montagna, Frank, Siani, Silenzi and Paterne2013; Ayache et al. Reference Ayache, Dutay, Mouchet, Tisnérat-Laborde, Montagna, Tanhua, Siani and Jean-Baptiste2017). After 1980, nuclear energy developed in France. Today, the Rhône River is one of the most nuclearized rivers in Europe. Four nuclear plants are located along the Rhône and legally discharge cooling water in the river (Eyrolle et al. Reference Eyrolle, Antonelli, Renaud and Tournieux2015; Jean-Baptiste et al. Reference Jean-Baptiste, Fontugne, Fourré, Marang, Antonelli, Charmasson and Siclet2018). It has already been reported that DIC from the river can contain enriched 14C values which can reach a maximum of Δ14C = +393‰ with an average value of Δ14C = 60‰ (Jean-Baptiste et al. Reference Jean-Baptiste, Fontugne, Fourré, Marang, Antonelli, Charmasson and Siclet2018). As a consequence, 14C can be used as a tracer of the fresh water DIC and the freshly produced OM such as phytoplankton in the ecosystem of the prodelta and the continental shelf (Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018).
In this study, we focused on station E (Figure 1), located in the distal zone of the continental shelf (43°13’12N 4°41’54E, 75 m depth at 17 Km from the River mouth, Cathalot et al. Reference Cathalot, Rabouille, Tisnerat-Laborde, Toussaint, Kerherve, Buscail, Loftis, Sun, Tronczynski and Azoury2013). This station is further from the coast than station C (8.5 km) or D (13 km) studied in our previous work (Pozzato et al. Reference Pozzato, Rassmann, Lansard, Dumoulin, van Brugel and Rabouille2018; Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018). Station C provided interesting but scarce data on pore water isotopic signatures. It was important to better constrain the distal zone to refine our conclusions for the continental shelf. At this station, we chose one sediment core and collected pore waters at different depths to analyze the evolution of DIC, δ 13C-DIC and Δ14C-DIC.
Table 1 DIC, δ13C-DIC and Δ14C-DIC of pore waters at different depths. BW represents the bottom water sample. NA indicates samples which were not measurable. Uncertainties are 0.1‰ for δ13C and ± 3‰ for Δ14C.
Sampling
Sampling was carried out in May 2018. Interface sediment cores of nearly 40 cm were collected with a gravity corer (Uwitec) using PVC transparent tubes. At station E, with the sedimentation rate of the distal zone around 0.3 cm y-1 a core represents between 100 or 150 years of deposition (Miralles et al. Reference Miralles, Radakovitch and Aloisi2005). Pore waters were extracted using 0.2 µm rhizons (Rhizosphere Research Products, Seeberg-Elverfeldt et al. Reference Seeberg-Elverfeldt, Schlüter, Feseker and Kölling2005) with a 20 mL rubber free syringe. The pore water samples were then sealed in a 15 mL Pyrex glass ampoules and stored frozen at −20°C until 14C analysis. A small aliquot (2mL) was placed in an individual vial with mercuric chloride (HgCl2) for 13C analysis. DIC was measured by the calibrated gas pressure gauge at the end of the CO2 extraction-purification line. A second core was sampled to collect pore waters for more precise DIC measurements. These were also poisoned using mercuric chloride.
Bottom water samples were collected at around one meter from the bottom using a Niskin bottle, poisoned with mercuric chloride (HgCl2) and stored in a fridge.
Measurements
DIC Analysis
Inorganic carbon measured in pore waters and bottom waters refers to dissolved inorganic carbon (DIC) because during extraction from the sediment, the water passed through the 0.2 µm ceramic filter of the rhizon and the solid particles were removed. The DIC analysis was carried out with an Apollo Scitech Dissolved Inorganic Carbon Analyzer with a LI-COR CO2 detector (Rassmann et al. Reference Rassmann, Lansard, Pozzato and Rabouille2016). The uncertainty of this analysis is ±10 µmol/L which is around 0.5%.
13C Analysis
The 13C aliquot was measured at GEOTOP-UQAM using an Isoprime 100-DI with a microgas system in continuous flow for both pore waters and bottom waters. Samples were acidified and heated at 60°C for 1 hour to ensure complete separation of CO2, which was then introduced in the IRMS for isotope ratio measurements. The precision of the isotopic measurements is ± 0.1‰.
14C Analysis
The CO2 extraction from the pore water and bottom water DIC was performed with the CO2 extraction line installed at the LMC14 as described in Dumoulin et al. (Reference Dumoulin, Caffy, Comby-Zerbino, Delque-Kolic, Hain, Massault, Moreau, Quiles, Setti, Souprayen, Tannau, Thellier and Vincent2013). The water samples are introduced through an air-tight septum in the line and 2 mL of 85% phosphoric acid (H3PO4) is added to react with the water DIC and produce CO2. A helium flow is used to push the CO2 through the line and two water traps at −78°C remove the water from the gas. A liquid nitrogen trap at −190°C is used to collect the sample in a sealed tube before the graphitization step.
The 14C activities were calculated with the standard of “oxalic acid II”. Radiocarbon calculations were performed using the Mook and van der Plicht method (1999). The background correction was made with C1 AIEA samples and the Δ14C uncertainty values are ± 3‰ at a confidence interval of 1sigma. The 14C results are given in Δ14C and recalculated to 1950 with the correction of the delay between sampling year and measurement year (Mook and van der Plicht Reference Mook and van der Plicht1999).
Graphitization and AMS Measurements
The CO2 was reduced to graphite at the LMC14 laboratory using hydrogen and iron powder at 600°C (Vogel et al. Reference Vogel, Southon, Nelson and Brown1984; Dumoulin et al. Reference Dumoulin, Comby-Zerbino, Delqué-Količ, Moreau, Caffy, Hain, Perron, Thellier, Setti, Berthier and Beck2017), then pressed into an aluminum cathode and finally loaded into the ion source of the ARTEMIS facility for the AMS measurement (Moreau et al. Reference Moreau, Caffy, Comby, Delqué-Količ, Dumoulin, Hain, Quiles, Setti, Souprayen and Thellier2013, Reference Moreau2020).
Mixing Model
At each depth in the sediment, pore water DIC (DICpore) is a mix of bottom water DIC (DICBW, the original water trapped in the sediment) and DIC originating from the mineralized OM. To calculate the original isotopic signature of the organic matter mineralized in the sediment (δ 13COM, Δ14COM) a mixing model similar to that of Bauer et al. (Reference Bauer, Reimers, Druffel and Willimas1995) was applied. The calculations were detailed in our previous studies (Pozzato et al. Reference Pozzato, Rassmann, Lansard, Dumoulin, van Brugel and Rabouille2018; Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018).
$$\eqalign{{\delta ^{13}}{{\rm{C}}_{{\rm{pore}}}} = \rm{x}*{\rm{\delta} ^{13}}{{\rm{C}}_{{\rm{BW}}}} + \left( {1 - x} \right)*{\delta ^{13}}{{\rm{C}}_{{\rm{OM}}}}\,\ \ {\rm{with \ \ \ \ x}} = {\rm{DI}}{{\rm{C}}_{{\rm{BW}}}}/{\rm{DI}}{{\rm{C}}_{{\rm{pore}}}} \cr \& \hskip 135pt\Downarrow \cr {\delta ^{13}}{{\rm{C}}_{{\rm{pore}}}}*{\rm{DI}}{{\rm{C}}_{{\rm{pore}}}} = {\delta ^{13}}{{\rm{C}}_{{\rm{OM}}}}*{\rm{DI}}{{\rm{C}}_{{\rm{pore}}}} + {\rm{DI}}{{\rm{C}}_{{\rm{BW}}}}*{\delta ^{13}}{{\rm{C}}_{{\rm{BW}}}} - {\rm{DI}}{{\rm{C}}_{{\rm{BW}}}}*{\delta ^{13}}{{\rm{C}}_{{\rm{OM}}}}\cr}$$
And similarly:
$${\Delta ^{14}}{{\rm{C}}_{{\rm{pore}}}}*{\rm{DI}}{{\rm{C}}_{{\rm{pore}}}} = {\Delta ^{14}}{{\rm{C}}_{{\rm{OM}}}}*{\rm{DI}}{{\rm{C}}_{{\rm{pore}}}} + {\rm{DI}}{{\rm{C}}_{{\rm{BW}}}}*{\Delta ^{14}}{{\rm{C}}_{{\rm{BW}}}}-{\rm{DI}}{{\rm{C}}_{{\rm{BW}}}}*{\Delta ^{14}}{{\rm{C}}_{{\rm{OM}}}}$$
With this equation, the slope of Δ14Cpore * DICpore versus DICpore provides the isotopic signature of the OM mineralized in the sediment Δ14COM and similarly for δ 13COM with the δ 13C*DIC versus DICpore). The reported uncertainty on the OM signature was calculated as the uncertainty on the slope due to linear regression.
RESULTS
Sediment Pore Waters
The DIC concentration increases with depth (Table 1) with nearly 100 µmol/L per cm, reaching twice the DIC concentration of the bottom water at 25 cm (from 2.38 mmol/L to 4.96 mmol/L).
The δ13C-DIC values follow the opposite trend with a large decrease with depth from –0.3‰ for the bottom water to –8.52‰ at 34 cm. Unfortunately, two results are missing because two vials were broken before the δ13C analysis.
The Δ14C-DIC in the pore waters follows two trends (Figure 2). In the first 15 cm, the activity increases from 30‰ to 55‰ but deeper down, it decreases from 55‰ to –30‰.
Original isotopic signature of the organic matter mineralized in sediment pore waters:
Figure 3 shows the mixing model and the isotopic signature of the OM mineralized in the sediment.
Figure 1 Map of the Rhône prodelta with the sampling station E.
Figure 2 Evolution of the pore water DIC, its δ13C and Δ14C signature with depth in the sediment at station E. The bottom water value is reported above the sediment-water interface (horizontal line at 0 cm).
Figure 3 Mixing model for Δ14C and δ 13C. The slope of the mixing curve provides the isotopic signature of the mineralized OM. For the Δ14C mixing model, the blue part (closed symbols) corresponds to the first 15 cm (nuclear period, see text) and the brown part (open symbols) corresponds to depths below 15 cm in the pre-nuclear period. (Please see online version for color figures.)
With the growth of DIC concentration in the pore water, the δ 13Cpore*DICpore decreases. The slope of the mixing line for δ 13C gives a δ 13COM signature around –16.2 ± 3‰ for the OM mineralized in the sediment. For the Δ14C mixing model, the plot shows two different trends corresponding to different gradients observed on the Δ14C-DIC distribution. For the first part of the graph, the slope of the mixing model indicates Δ14COM = +99.7 ± 17‰ and for the second part of the graph, the slope of indicates Δ14COM = –171.7 ± 60‰.
DISCUSSION
Mineralization of the Organic Matter and DIC Increase
The measured DIC concentrations in the pore waters of continental shelf sediments at station E double from the surface sediment to the bottom of the core at 34 cm. This DIC concentration increase with depth is the result of oxic and anoxic OM mineralization in the sediment conducted by microorganisms (Rassmann et al. Reference Rassmann, Lansard, Pozzato and Rabouille2016, Reference Rassmann, Eitel, Cathalot, Brandily, Lansard, Taillefert and Rabouille2020). On the continental shelf, the low concentration of nitrate and the low production of reduced metals in the pore waters highlight that oxic mineralization and sulfate reduction are the major organic carbon mineralization pathways (Pastor et al. Reference Pastor, Cathalot, Deflandre, Viollier, Soetaert, Meysman, Ulses, Metzger and Rabouille2011; Ait Ballagh et al. Reference Ait Ballagh, Rabouille, Andrieux-Loyer, Soetaert, Lansard, Bombled, Monvoisin, Elkalay and Khalil2021). It follows the equation (Rassmann et al. Reference Rassmann, Lansard, Pozzato and Rabouille2016, Reference Rassmann, Eitel, Cathalot, Brandily, Lansard, Taillefert and Rabouille2020):
$${\rm{C}}{{\rm{H}}_2}{\rm{O}} + {{\rm{O}}_2} \to {\rm{HC}}{{\rm{O}}_3}^ - + {{\rm{H}}^ + }\,{\rm{and}}\,2{\rm{C}}{{\rm{H}}_2}{\rm{O}} + {\rm{S}}{{\rm{O}}_4}^{2 - } \to 2{\rm{HC}}{{\rm{O}}_3}^ - + {{\rm{H}}_2}{\rm{S}}$$
For the anoxic part of the sediments (directly after the first centimeters), carbonate dissolution can be excluded as a source of DIC since in this zone, carbonate minerals (calcite and aragonite) are supersaturated and cannot therefore dissolve (Rassmann et al. Reference Rassmann, Lansard, Pozzato and Rabouille2016).
Isotopic signature of the mineralized OM
The slope of Figure 3 provides a δ 13COM = –16 ± 3‰ for the signature of the OM mineralized at station E, providing information about its origin. This result is in the lower range of the δ 13C signature corresponding to the mineralization of a marine substrate such as marine phytoplankton which commonly have a δ 13C signature between –19 to –21‰ (Harmelin-Vivien et al. Reference Harmelin-Vivien, Dierking, Banaru, Fontaine and Arlhac2010). This information confirms our previous results (Lansard et al. Reference Lansard, Rabouille, Denis and Grenz2009; Cathalot et al. Reference Cathalot, Rabouille, Tisnerat-Laborde, Toussaint, Kerherve, Buscail, Loftis, Sun, Tronczynski and Azoury2013; Pozzato et al. Reference Pozzato, Rassmann, Lansard, Dumoulin, van Brugel and Rabouille2018; Dumoulin et al. Reference Dumoulin, Pozzato, Rassman, Toussaint, Fontugne, Tisnérat-Laborde, Beck, Caffy, Delqué-Količ, Moreau and Rabouille2018) indicating the limited amount of terrestrial organic matter in continental shelf sediments. The majority of OM delivered by the river settles near the river mouth, in the proximal zone. Further out at sea on the continental shelf, local marine OM is predominant. It is noteworthy that the input of C4 plants which have a heavier δ 13C signature of around –15‰ can be ruled out based on the δ 13C data of lignin phenols previously obtained (Cathalot et al. Reference Cathalot, Rabouille, Tisnerat-Laborde, Toussaint, Kerherve, Buscail, Loftis, Sun, Tronczynski and Azoury2013). The absence of a large delivery of river particles to the continental shelf also explains why the sedimentation rates drastically decrease in this zone compared to those in the prodelta which receives large amounts of terrestrial particles and OM from the continent.
The Δ14C isotopic signatures of the OM mineralized in the sediment (Figure 3) show two different trends. Line #1, in the first 15 cm, has a slope which gives a Δ14COM = 100 ± 17‰. This value of Δ14COM indicates modern organic matter with a slight Δ14C enrichment and fits well with the post bomb period expected for the top of the core. A sedimentation rate of less than 0.3 cm/yr (Miralles et al. Reference Miralles, Radakovitch and Aloisi2005) represents around 50–60 years for the first 15 cm and encompasses the 14C bomb peak period and the initiation of nuclear activity in the Rhône area which started in the 1980s. This Δ14COM result is probably linked to the deposition of enriched phytoplankton grown in marine surface waters during the nuclear period which is mineralized in the sediment and which transfers its isotopic signature to the pore water DIC. Evidence for this is the fact that the Δ14C values obtained in the NW Mediterranean Sea surface were around +90‰ in the 1970s and decreased to 60‰ in 2010 (Tisnérat-Laborde et al. Reference Tisnérat-Laborde, Montagna, Frank, Siani, Silenzi and Paterne2013; Ayache et al. Reference Ayache, Dutay, Mouchet, Tisnérat-Laborde, Montagna, Tanhua, Siani and Jean-Baptiste2017), which is consistent with the signature found in the OM for the nuclear period. As our results are slightly higher than the values of the surface ocean, another possible source of enriched DIC originating from the river plume is the impact of the nuclear power plants along the Rhône during the last 40 years. DIC of the river plume with peaks of Δ14C = +393‰ and an average of 60‰ (Jean-Baptiste et al. Reference Jean-Baptiste, Fontugne, Fourré, Marang, Antonelli, Charmasson and Siclet2018) can be mixed with DIC of the surface waters and can be used by marine phytoplankton together with plume dissolved nutrients, creating an Δ14C enrichment of marine OM.
A different trend is shown by line #2 for the signature of the organic matter mineralized under 15 cm. The slope of the mixing model gives a strongly depleted result with Δ14COM = 172 ± 60‰. These sediment layers under 15 cm correspond to a period of time spanning from before 1900 to 1960 as estimated from the sedimentation rate (Miralles et al. Reference Miralles, Radakovitch and Aloisi2005), i.e., the pre-nuclear period. Before the anthropogenic nuclear activities of the last 60 years, the Δ14C signature of marine OM was directly linked to the local marine reservoir effect of the Mediterranean Sea evaluated around –50 to –60‰ (Siani et al. Reference Siani, Paterne, Arnold, Bard, Métivier, Tisnérat and Bassinot2000; Tisnérat-Laborde et al. Reference Tisnérat-Laborde, Montagna, Frank, Siani, Silenzi and Paterne2013). The value found for marine plankton degrading in these sediments is lower than these DIC signatures of the Mediterranean Sea, so it certainly indicates a larger influence of 14C depleted waters. As indicated above, the river plume brings a large amount of fresh water and dissolved nutrients to the Mediterranean Sea, modifying the DIC of the surface water. During the pre-nuclear period, the freshwater reservoir effect (FRE) for radiocarbon in DIC was strong in the Rhone River basin. It was generated by the dissolution of old carbonates from the drainage basin which created a freshwater reservoir effect and an apparent 14C aging of DIC (see for the Loire River, Coularis et al. Reference Coularis, Tisnérat-Laborde, Pastor, Siclet and Fontugne2016). The FRE measured on the Rhône river DIC was evaluated at Δ14C = –114‰, a value indicating the strong influence of one tributary, the Durance river which is known to be depleted in Δ14C as indicated by a value obtained in May 2014 (Δ14C = –138‰; Jean-Baptiste et al. Reference Jean-Baptiste, Fontugne, Fourré, Marang, Antonelli, Charmasson and Siclet2018). As the DIC signature of the river is partly transferred to the river plume, the local marine OM could have been more depleted than the average marine surface waters. Given the large uncertainty of the result found for Δ14COM (–171.7 ± 60‰), there is an overall compatibility of the mineralized OM signature with the possible signature of river-influenced marine surface waters.
A final possibility to explain such a depleted result for the OM mineralized in the sediments below 15 cm is a possible “priming effect” (Bianchi Reference Bianchi2011 and references therein). This phenomenon occurs when substrates with different labilities co-exist in the sediment. The labile OM is used by the micro-, meio- and macro-organisms to co-metabolize more refractory substrates. At station E in the distal zone, this “priming effect” is possible with the mixture of different sources of substrates. If the sources of energetic OM are limited, the fauna and the different organisms have to mineralize more refractory OM. Cathalot et al. (Reference Cathalot, Rabouille, Tisnerat-Laborde, Toussaint, Kerherve, Buscail, Loftis, Sun, Tronczynski and Azoury2013) described a dominant aged OM fraction in shelf sediments including station E, with a high content of black carbon (up to 50%) and a limited contribution of fresh marine OM, providing an old Δ14C signature of particulate OM (Δ14C = –312‰). Therefore, the mineralization of a small part of this refractory material could explain part of the Δ14COM depleted results in the deeper layers of pore waters.
CONCLUSION
This new study of the continental shelf of the Rhône River prodelta has shed light on the origin and reactivity of organic matter present in the sediments and the coastal carbon cycle in river-dominated ocean margins. The large increase in pore water DIC concentration indicates active OM mineralization by micro and macro fauna over a period of 100–150 yr. This process transfers the OM signature to the pore water DIC. The δ 13C signature of the pore water DIC indicates that mostly marine organic matter is mineralized in these sediments. It also confirms that a limited particulate substrate from the continent is mineralized in continental shelf sediments, confirming its efficient trapping in the proximal zone. The Δ14C signature indicates two behaviors that coincide with the post-bomb/pre-bomb periods. Mineralized OM in the first half of the 20th century (below 15 cm) shows a depleted 14C composition in line with low Δ14C marine DIC values corresponding to the local marine reservoir effect and additions of river plume waters with lower Δ14C-DIC values impacted by a freshwater reservoir effect. Contrarily, mineralized OM of the top of the core is enriched and Δ14C results fit well with the expected values for the nuclear period impacted first by the penetration of bomb radiocarbon in the ocean and later by the anthropic nuclear activities present along the Rhône River. The larger contrast between the Δ14C signature of mineralized OM between the two periods (–172 ± 60‰ to +100 ± 17‰) and that of the marine surface waters (–60‰ to 90‰; Tisnérat-Laborde et al. Reference Tisnérat-Laborde, Montagna, Frank, Siani, Silenzi and Paterne2013) could be explained by a significant influence of the river plume waters and its DIC on marine phytoplankton production. This could have contributed a more depleted DIC during the pre-nuclear period and a more enriched DIC during the nuclear period.
Overall, the δ 13C and Δ14C signature of pore water DIC confirms our previous findings that reactive organic matter mineralized in sediments imprints its signature on the pore waters. We show here that this signature can be maintained on timescales of the order of a century.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the captains and crews of the RV Thetys II for their help during the MissRhoDia II sampling campaign. The authors thank A. Villedieu and L. Brethous who provided technical help during the expedition and in the laboratory. This work was supported by the INSU/EC2CO-MissRhoDia project, the Sulfat_IsoMic project funded by INSU/TELLUS Interrvie and IPSL, the BioGeoMethane project funded by Emergence-SPU-Paris Saclay and the French State program “Investissement d’avenir” run by the National Research Agency (AMORAD project ANR-11-RSNR-0002). The LMC14 is funded by five French organizations: CEA, CNRS, IRD, IRSN, and MCC. This is LSCE contribution number 7813.
Open access
