Sources and transfers of particulate organic matter in a tropical reservoir (Petit Saut, French Guiana): a multi-tracers analysis using d¹³C, C/N ratio and pigments

International audienceCarbon cycling and organic matter transfers in the tropical Sinnamary river system (French Guiana), including a mid-stream reservoir (Petit Saut) and its estuary on the Atlantic coast, were studied during the dry season by analyzing the organic carbon isotopic ratios (d13C-OC), C/N ratios and pigments contents of suspended matter, sediments, sediments traps and epiphytic and epilithic biofilms. In the River upstream as well as in surface sediments at the entrance of the reservoir and at the littoral zone of the reservoir, particulate organic matter (POM) was in majority of terrestrial origin, with a d13C-C/N signature close to the one of soil and litter collected in the surrounding forest and with high OC/total pigments ratios. High concentrations of Pheopigments a and b in these surface sediments showed that this terrestrial POM, either carried by the river and eolian transport or present in the soil before flooding, undergoes intense degradation. Deeper in the sediment, the d13C profile showed a decreasing trend with depth typical of what is found in soils, showing that the flooded soil still remains present at the reservoir bottom 10 years after flooding. At the center of the reservoir, POM in the water column, in sediment traps and in surface sediments was in majority of aquatic origin with low C/N and OC/total pigments ratios. In the oxic epilimnion at 3 m depth, Chl a, Chl b and Lutein showed the predominance of Chlorophyceae to the phytoplankton community. At this depth, a C/N ratio of 21 suggests a large contribution of transparent exopolymeric particles to the bulk POM, which, in addition, was 13C-depleted due to a significant contribution of methanotrophic bacteria. At 7 m depth, below the oxicline, high concentrations of BChl d and occasionally BChl c revealed the presence of anoxygenic phototrophic bacteria, namely Chlorobiaceae. In the sediment traps, Chl a, Chl b, Lutein and BChl c and BChl d confirmed the contribution of plankton to the sedimentary POM. This material was undergoing intense degradation as revealed by high concentration of pheopigments and by an increase in C/N ratio and an increase in d13C-OC with trap depth. Scytonemin was found in a biofilm developed on tree trunks at the reservoir surface and in all sediment traps. Other tracers showed however that the contribution of the biofilm to the sedimentary POM was minor compared to the planktonic source. In the Sinnamary downstream of the dam, POM became more 13C-depleted showing a larger contribution of methanotrophic bacteria. Chl b, Lutein and BChl c + BChl d originating from the reservoir progressively decreased downstream as the result of mineralization. At the estuarine mouth, fucoxanthin showed the presence of diatoms and the d13C-C/N signature matched the one of POM carried by the Amazonian coastal mobile mud belt

Anaerobic decomposition of tropical soils and plant material : implication for the CO2 and CH4 budget of the Petit Saut Reservoir

Tropical hydroelectric reservoirs contribute significantly to atmospheric CH4 and CO2 emissions. To evaluate the contribution of the mineralization of the flooded soils and biomass to these atmospheric gas emissions, field and laboratory experiments were conducted. Cores were retrieved inform the littoral zone of the Petit Saut Reservoir (French Guiana), flooded 10 a prior to sampling, and different soils and plant material in the tropical forest Surrounding the reservoir. All the samples were flooded and incubated in anoxic conditions in the dark at 30 degrees C. The potential CH4 and CO2 production rates were determined. Soils and plant material from the tropical forest were incubated over one year and the production measurements were performed at a frequency of 1-5 months. Methane and CO2 production rates of soils and littoral sediments were linearly correlated to the Organic C (OC) content of the slurries. The slopes of the relationships were 2.6 x 10(2) +/- 5.6 x 10(1) nmol (CH4) g(OC(dry))(-1) h(1) and 3.5 x 10(2) +/- 6.9 x 10(1) nmol (CO2) g(OC(dry))(-1) h(-1). For plant material, no relationship between the production rates and the OC content was found and on average over the year of incubation, production rates were 2.4 x 10(3) +/- 1 x 10(3) nmol (CH4) g(OC(dry))(-1) h(-1) and 3.9 x 10(3) +/- 5 x 10(3) nmol (CO2) g(OC(dry))(-1) h(-1), which is one order of magnitude higher than the mineralization of the soils and sediments. Extrapolated at the scale of the Petit Saut Reservoir over 10 a, these results show that the mineralization of the soil and the plant biomass initially flooded Contributed to 75-95% of the total C emissions to the atmosphere since the flooding of the reservoir. Methane: CO2 molar ratios were 3 times higher for anaerobic decomposition than atmospheric emissions, quantitatively consistent with aerobic CH4 oxidation

1400 ans d'interactions homme-milieu : évolution des sols sur le Saint-Mont (Remiremont, Vosges)

International audienceThe SolHoM(a) project aims at determining the degree of landscape anthropisation in the Fossard massif (Remiremont, Vosges, France) by estimating the impact of past human occupations on current soils. Nowadays, a forest covers this relatively inhospitable mountain (hard climate, steep slopes, acid soils ...), but some remains suggest human presence for at least 1400 years.The Saint-Mont is a variscan orogenic granitic summit (height 672m) with valuable preserved archaeological relics, located south of the Fossard. At the top was founded the monasterium Habendum (from the 7th century to the French Revolution), which potentially comes after a late-antic castrum.Furthermore, monumental enclosure dry-stones walls, not yet clearly linked with the monastery, crosses the massif.Our methodology is first experienced on the Saint-Mont, well documented by recent archaeological research. It features:- sampling soil transects presenting a clear gradient from highly to poorly human impacted areas. According to those transects, soils are sampled and characterised (physico-chemistry, soil micromorphology, analysis of organic matter, dating); this should highlight traces of past human activity;- a development of algorithms to detect geometric structures in the raw LiDAR data provided by the PCR AGER(b) (Ch. Kraemer, dir). This research might help archaeologists to determine areas influenced or not by men.Once tested on the Saint-Mont, this approach will be applied to another archaeologically less investigated area of the Fossard.First field works highlight two soil sequences:- the first one, near a probable merovingian wall close to the monastery, reveals a very black 80cm deep unit,- the second one, far-off the monastery and beside the enclosure dry-stone wall, is less thick but have a thin black unit buried under 20 cm of pedogenised sediment.Physico-chemical and soil micromorphological analyses are in progress. The coming study of profiles/sequences more distant from archaeological sites will contribute to estimate the degree of soils anthropisation in the Saint-Mont.(a) Interactions Sol-Hommes-Milieux(b) ArchéoGEographie du premier Remiremont et de ses abord

1400 ans d'interactions homme-milieu : évolution des sols sur le Saint-Mont (Remiremont, Vosges)

International audienceThe SolHoM(a) project aims at determining the degree of landscape anthropisation in the Fossard massif (Remiremont, Vosges, France) by estimating the impact of past human occupations on current soils. Nowadays, a forest covers this relatively inhospitable mountain (hard climate, steep slopes, acid soils ...), but some remains suggest human presence for at least 1400 years.The Saint-Mont is a variscan orogenic granitic summit (height 672m) with valuable preserved archaeological relics, located south of the Fossard. At the top was founded the monasterium Habendum (from the 7th century to the French Revolution), which potentially comes after a late-antic castrum.Furthermore, monumental enclosure dry-stones walls, not yet clearly linked with the monastery, crosses the massif.Our methodology is first experienced on the Saint-Mont, well documented by recent archaeological research. It features:- sampling soil transects presenting a clear gradient from highly to poorly human impacted areas. According to those transects, soils are sampled and characterised (physico-chemistry, soil micromorphology, analysis of organic matter, dating); this should highlight traces of past human activity;- a development of algorithms to detect geometric structures in the raw LiDAR data provided by the PCR AGER(b) (Ch. Kraemer, dir). This research might help archaeologists to determine areas influenced or not by men.Once tested on the Saint-Mont, this approach will be applied to another archaeologically less investigated area of the Fossard.First field works highlight two soil sequences:- the first one, near a probable merovingian wall close to the monastery, reveals a very black 80cm deep unit,- the second one, far-off the monastery and beside the enclosure dry-stone wall, is less thick but have a thin black unit buried under 20 cm of pedogenised sediment.Physico-chemical and soil micromorphological analyses are in progress. The coming study of profiles/sequences more distant from archaeological sites will contribute to estimate the degree of soils anthropisation in the Saint-Mont.(a) Interactions Sol-Hommes-Milieux(b) ArchéoGEographie du premier Remiremont et de ses abord