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The (234U/238) equilibrium state of borehole core samples from the deep, low-permeability limestone formations surrounding the target argilite layer of the Meuse/Haute-Marne experimental site of the French agency for nuclear waste management -ANDRA- (Agence nationale pour la gestion des déchets radioactifs) was examined to improve understanding of naturally occurring radionuclide behaviour in such geological settings. Highly precise, accurate MC-ICP-MS measurements of the (234U/238U) activity ratio show that limestone samples characterised by pressure dissolution structures (stylolites or dissolution seams) display systematic (234U/238U) disequilibria, while the pristine carbonate samples remain in the secular equilibrium state. The systematic feature is observed throughout the zones marked by pressure dissolution structures: (i) the material within the seams shows a deficit of 234U over 238U ((234U/238U) down to 0.80) and (ii) the surrounding carbonate matrix is characterised by an activity ratio greater than unity (up to 1.05). These results highlight a centimetric-scale uranium remobilisation in the limestone formations along these sub-horizontal seams. Although their nature and modalities are not fully understood, the driving processes responsible for these disequilibria were active during the last 1–2 Ma. Keywords: uranium isotopes, multiple-collector ICP-MS, waste management, remobilisation, migration

Submitted by Cristina Veiga-Pires (cvpires@ualg.pt) on 2017-03-30T00:10:41Z No. of bitstreams: 1 POnte-EGU2017-919.pdf: 38617 bytes, checksum: 9e3419643d0dc59bf30e9369676ab854 (MD5) Approved for entry into archive by Sofia Lopes (silopes@ualg.pt) on 2017-03-31T20:15:13Z (GMT) No. of bitstreams: 2 1260637804816963.zip: 36796 bytes, checksum: 89be3219b046e1633ea89e81e9fb0957 (MD5) POnte-EGU2017-919.pdf: 38617 bytes, checksum: 9e3419643d0dc59bf30e9369676ab854 (MD5) Made available in DSpace on 2017-03-31T20:15:13Z (GMT). No. of bitstreams: 2 1260637804816963.zip: 36796 bytes, checksum: 89be3219b046e1633ea89e81e9fb0957 (MD5) POnte-EGU2017-919.pdf: 38617 bytes, checksum: 9e3419643d0dc59bf30e9369676ab854 (MD5) Previous issue date: 2017 info:eu-repo/semantics/publishedVersion

International audience; Merging the late Quaternary Arctic paleoceanography into the Earth's global climate history remains challenging due to the lack of robust marine chronostratigraphies. Over ridges notably, low and variable sedimentation rates, scarce biogenic remains ensuing from low productivity and/or poor preservation, and oxygen isotope and paleomagnetic records differing from global stacks represent major impediments. However, as illustrate here based on consistent records from Mendeleev‐Alpha and Lomonosov Ridges, disequilibria between U‐series isotopes can provide benchmark ages. In such settings, fluxes of the particle‐reactive U‐daughter isotopes 230Th and 231Pa from the water column, are not unequivocally linked to sedimentation rates, but rather to sea‐ice rafting and brine production histories, thus to the development of sea‐ice factories over shelves during intervals of high relative sea level. The excesses in 230Th and 231Pa over fractions supported by their parent U‐isotopes, collapse down sedimentary sequences, due to radioactive decay, and provide radiometric benchmark ages of approximately 300 and 140 ka, respectively. These “extinction ages” point to mean sedimentation rates of ∼4.3 and ∼1.7 mm/ka, respectively, over the Lomonosov and Mendeleev Ridges, which are significantly lower than assumed in most recent studies, thus highlighting the need for revisiting current interpretations of Arctic lithostratigraphies in relation to the global‐scale late Quaternary climatostratigraphy.

Trabajo presentado en el XI Convención Internacional sobre Medio Ambiente y Desarrolllo, celebrado en la Habana (Cuba) del 3 al 7 de julio de 2017.

Authigenic Be-10/Be-9 ratios and Be-10-fluxes reconstructed using the Th-230(xs), normalization, proxies of the cosmogenic radionuclide Be-10 production rate in the atmosphere, have been measured in a sedimentary core from Baffin Bay (North Atlantic) spanning the last 136 ka BP. The normalization applied on the exchangeable (authigenic) Be-10 concentrations using the authigenic Be-9 isotope and Th-230(xs) methods yield equivalent results strongly correlated with sedimentological parameters (grain-size and mineralogy). Lower authigenic beryllium (Be) concentrations and Be-10/Be-9 ratios are associated with coarse grained carbonate-rich layers, while higher authigenic Be values are related to fine-grained felspar rich sediments. This variability is due to: i) sediment composition control over beryllium-scavenging efficiency and, ii) glacial history that contributed to modify the Be-10 concentration in Baffin Bay by input and boundary scavenging condition changes. Most paleo-denudation rates inferred from the Be-10/Be-9 ratio vary weakly around 220 +/- 76 tons.km(-2).yr(-1) (0.09 +/- 0.03 mm.yr(-1)) corresponding to relatively steady weathering fluxes over the last glacial cycle except for six brief intervals characterized by sharp increases of the denudation rate. These intervals are related to ice-surging episodes coeval with Heinrich events and the last deglaciation period. An average freshwater flux of 180.6 km(3).yr(-1) (0.006 Sv), consistent with recent models, has been calculated in order to sustain glacially-derived Be-10 inputs into Baffin Bay. It is concluded that in such environments, the authigenic Be-10 measured mainly depends on climatic effects related to the glacial dynamics, which masks the Be-10 production variation modulated by geomagnetic field changes. Altogether, these results challenge the simple interpretation of Be-10-concentration variation as a proxy of Interglacial/Glacial (interstadial/stadial) cycles in Arctic and sub-Arctic regions. They rather suggest the effect of higher-frequency paleoclimatic changes and local glacial dynamics on Be-10 signature.

Submitted by Cristina Veiga-Pires (cvpires@ualg.pt) on 2017-03-30T00:05:08Z No. of bitstreams: 1 Ponte-EGU2016-7199.pdf: 34129 bytes, checksum: be53a434e81aa83ce560499830f629c0 (MD5) Approved for entry into archive by Sofia Lopes (silopes@ualg.pt) on 2017-03-31T20:14:49Z (GMT) No. of bitstreams: 2 1260637804816963.zip: 33091 bytes, checksum: 5879b262a6363bc05c62c47fc0d7886e (MD5) Ponte-EGU2016-7199.pdf: 34129 bytes, checksum: be53a434e81aa83ce560499830f629c0 (MD5) Made available in DSpace on 2017-03-31T20:14:49Z (GMT). No. of bitstreams: 2 1260637804816963.zip: 33091 bytes, checksum: 5879b262a6363bc05c62c47fc0d7886e (MD5) Ponte-EGU2016-7199.pdf: 34129 bytes, checksum: be53a434e81aa83ce560499830f629c0 (MD5) Previous issue date: 2016 info:eu-repo/semantics/publishedVersion