Sedimentology, stratigraphy and chronology of adecantation tank in the sewer network of Orléans (France): new perspectives on the Anthropocene

Abstract : Ongoing debates on the definition of the Anthropocene encourage geologists and palaeoenvironmentaliststo explore more recent time-windows and new territories. Cities are probably the most emblematic socio-ecosystem of the Anthropocene, with a dominant control of human-driven productions and fluxes of uncommon materials, elements and isotopes ratios, justifying a new sphere of biogeochemical exchanges: the " Astysphere ". Sedimentary accumulationsresulting from the production, transport, and sedimentation of materials in sewer networks could thus represent efficientarchivesof the Anthropocene. We studied a decantation tankthat collects wastewaters and rainwatersfrom the sewer network of Orléans, before any water treatment plant. This original sedimentary basin accumulated 17m of sediments since AD1942,and has never been cleaned out, potentially offering a very high-resolution archive of events that occurred within the catchment over the last 70 yrs. In this context, we retrieved 17 gravity cores before (A, February 2015) and after (B, March 2016) clean out. The sediment is stratified with coarse and fine mineral layers regularly interrupted by organic-rich black deposits. Abrupt transitions between each facies suggest a succession of sedimentary events. The presence of 7 Be at the top of cores confirms that top sedimentsare younger than 6 months. This implies that there is no or few time lag in sediment transport within the catchment. Cores B, which record one year of sedimentation after clean out, are half constituted of mineral (top) and organic (bottom) units.The mineral facies is enriched in 7 Be whereas the organic one is not. This suggests that the mineral facies is typical of the autumn-winter season, whereas the organic one could be related to the spring-summer period. Organic/mineral successions observed in all cores could therefore reveal a seasonal signal. 30 postbomb 14 C dates performed on cores Aare logically organized with depth, with oldest sediments (2.25 m depth) dated from AD1980. These first results confirm that urban sedimentary archives, as more natural ones, are constituted of deposits chronologically ordered, probably under a seasonal pattern. This reinforces our assumptions that high-resolution sedimentary archives in urban contexts could help unraveling the precise chronology of the Anthropocene.
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Submitted on : Wednesday, November 30, 2016 - 6:02:23 PM
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Alexandre Thibault, Jérémy Jacob, Anaëlle Simonneau, Claude Le Milbeau, Christian Giovanni, et al.. Sedimentology, stratigraphy and chronology of adecantation tank in the sewer network of Orléans (France): new perspectives on the Anthropocene. Dating the Anthropocene in Environmental Archives - DANTE, Oct 2016, Toulouse, France. ⟨hal-01401864⟩

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