Introduction to the French GEOTRACES North Atlantic transect (GA01): GEOVIDE cruise

© 2018 Author(s). The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special-issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

Downcore variations of sedimentary detrital (238U/232Th) ratio of sediment core SU90-08

Excess 231 Pa and 230 Th ( 231 Pa xs and 230 Th xs ) can be used to reconstruct past oceanic sedimentation ( 230 Th-normalized flux) and circulation changes (( 231 Pa/ 230 Th) xs,0 , hereafter Pa/Th). These quantities are determined by computing the detrital and authigenic contributions from bulk sediment measurement. The method relies on the use of a chosen constant value of the detrital ( 238 U/ 232 Th) activity ratio (hereafter (U/Th) det ). In this study, we have extracted the detrital fraction of the sediments from North Atlantic deep-sea core SU90-08 (43°03'1N, 30°02'5W, 3080m) and determined its (U/Th) det value over the last 40 ky. We find that (U/Th) det varied significantly through time with a minimum value of 0.4 during the Holocene and a maximum value of 0.7 during the Last Glacial Maximum (LGM). The sensitivity of sedimentary 230 Th-normalized flux and Pa/Th is tested for our study site and for other North-Atlantic sites. We show that the sensitivity is highly dependent on the core location and its terrigenous material supply. The 230 Th-normalized flux and Pa/Th signals are very robust in cores with low detrital contributions, whereas they are very sensitive to (U/Th) det changes in cores with higher detrital contribution. In the latter case, changes in 230 Th-normalized flux and Pa/Th due to the choice of a constant (U/Th) det can largely exceed the uncertainty on the 230 Th-normalized flux and Pa/Th, inducing potential biases in the amplitude and temporal variability of reconstructed sedimentation and ocean circulation changes

Contribution de l’ESR/U-Th à la datation de sites du Pléistocène supérieur : exemple de la Grotte des Barasses II (Balazuc, France)

International audienc

Thorium isotopes in the Southeast Atlantic Ocean: Tracking scavenging during water mass mixing along neutral density surfaces

International audienceThe distributions of dissolved and particulate thorium isotopes (230 Th and 232 Th) were established in samples from the BONUS GoodHope (BGH) IPY-GEOTRACES cruise in the SE Atlantic sector of the Southern Ocean (36°S-13°E to 57°S-0°, Feb.-Mar. 2008). The distribution of total (dissolved+particulate) 232 Th is dominated by the inputs from continental margins. The non-linear profiles of dissolved 230 Th are interpreted as due to the southward upwelling of the isopycnal surfaces. However, total 230 Th and 232 Th versus salinity plots illustrate departures from binary mixing and provides evidence for non-conservative behavior of both isotopes along the section. We propose a model for total 230 Th and 232 Th scavenging and mixing along isopycnal surfaces. We use this model to estimate particle settling speeds and isopycnal eddy diffusion coefficients along the BGH section. Data-model comparison suggests particle settling velocities in the range of 400-700 m/y and isopycnal eddy diffusivity of the order of 2000 m 2 /s