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Reexposure and advection of 14C‐depleted organic carbon from old deposits at the upper continental slope Public Deposited

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  • Outcrops of old strata at the shelf edge resulting from erosive gravity‐driven flows have been globally described on continental margins. The reexposure of old strata allows for the reintroduction of aged organic carbon (OC), sequestered in marine sediments for thousands of years, into the modern carbon cycle. This pool of reworked material represents an additional source of 14C‐depleted organic carbon supplied to the ocean, in parallel with the weathering of fossil organic carbon delivered by rivers from land. To understand the dynamics and implications of this reexposure at the shelf edge, a biogeochemical study was carried out in the Gulf of Lions (Mediterranean Sea) where erosive processes, driven by shelf dense water cascading, are currently shaping the seafloor at the canyon heads. Mooring lines equipped with sediment traps and current meters were deployed during the cascading season in the southwestern canyon heads, whereas sediment cores were collected along the sediment dispersal system from the prodelta regions down to the canyon heads. Evidence from grain‐size, X‐radiographs and 210Pb activity indicate the presence in the upper slope of a shelly‐coarse surface stratum overlying a consolidated deposit. This erosive discontinuity was interpreted as being a result of dense water cascading that is able to generate sufficient shear stress at the canyon heads to mobilize the coarse surface layer, eroding the basal strata. As a result, a pool of aged organic carbon (D14C = −944.5 ± 24.7‰; mean age 23,650 ± 3,321 ybp) outcrops at the modern seafloor and is reexposed to the contemporary carbon cycle. This basal deposit was found to have relatively high terrigenous organic carbon (lignin = 1.48 ± 0.14 mg/100 mg OC), suggesting that this material was deposited during the last low sea‐level stand. A few sediment trap samples showed anomalously depleted radiocarbon concentrations (D14C = −704.4 ± 62.5‰) relative to inner shelf (D14C = −293.4 ± 134.0‰), mid‐shelf (D14C = −366.6 ± 51.1‰), and outer shelf (D14C = −384 ± 47.8‰) surface sediments. Therefore, although the major source of particulate material during the cascading season is resuspended shelf deposits, there is evidence that this aged pool of organic carbon can be eroded and laterally advected downslope.
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  • Tesi, T., et al. (2010), Reexposure and advection of 14C‐depleted organic carbon from old deposits at the upper continental slope, Global Biogeochem. Cycles, 24, GB4002, doi:10.1029/2009GB003745.
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  • This study was supported by EUROSTRATAFORM (ref. EVK3‐CT‐2002‐0079), HERMES (ref. GOCE‐CT‐2005‐511234‐1), CONSOLIDER GRACCIE (ref. CSD2007‐00067), PROMETEO (ref. CTM2007‐31164‐E/MAR), and HERMIONE (ref. 226354) research projects. Research funds were also provided by the ONR EUROSTRATAFORM Program (contract no. N00014‐03‐01‐0154 to S. Miserocchi, contract no. N00014‐04‐1‐0379 to P. Puig, and contract no. N00014‐99‐ 1‐0028, to C. A. Nittrouer). Funding for M. A. Goñi was provided by grant no. 0628487 from the National Science Foundation. M. Canals and A. M. Calafat acknowledge support from Generalitat de Catalunya through its funding program for excellence research groups (ref. 2005 SGR‐00152). This is contribution no. 1691 of ISMAR‐CNR Sede di Bologna.
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