Nadine Tisnérat-Laborde

Radiocarbon calibration by means of mass spectrometric 230Th/234U and 14C ages of corals: an updated database including samples from Barbados, Mururoa and Tahiti

As first shown by Bard et al. (1990a), high-precision 230Th-234U ages can be used successfully to calibrate the radiocarbon time scale beyond the high-precision tree-ring calibration that now reaches 11,900 cal BP (Kromer and Spurk 1998). Using mass spectrometric techniques, we measured 14C and 230Th ages on new samples collected from boreholes drilled off the islands of Tahiti and Mururoa (French Polynesia) in order to complement the database previously obtained on Barbados corals (Bard et al. 1990a, 1993).

New chronology and organic matter paleoclimatic significance of Nußloch loess sequence (Rhine Valley, Germany)

Abstract Due to its high accumulation rate, the loess sequence of Nusloch (Rhine Valley, Germany) is well suited to a high-resolution study of continental paleoenvironmental changes over the last climatic cycle for northwestern Europe. As carbon isotope ratios of plants depend on environmental variations, δ 13 C of loess-derived organic matter constitute reliable paleoclimatic proxies. We present here a new chronology of Nusloch loess sequence based on several 14 C and TL-dates. Our data show high correlations with global climate effects (Vostok-[CO2]) or semi-global (GISP2- δ 18 O ) and thus offer, in this way, a new approach to establish accurate chronologies in continental sequences. Furthermore, using a deconvolution of the δ 13 C signal would provide a quantitative proxy of moisture supply and permit paleoprecipitation reconstructions.

Context and dating of Aurignacian vulvar representations from Abri Castanet, France

We report here on the 2007 discovery, in perfect archaeological context, of part of the engraved and ocre-stained undersurface of the collapsed rockshelter ceiling from Abri Castanet, Dordogne, France. The decorated surface of the 1.5-t roof-collapse block was in direct contact with the exposed archaeological surface onto which it fell. Because there was no sedimentation between the engraved surface and the archaeological layer upon which it collapsed, it is clear that the Early Aurignacian occupants of the shelter were the authors of the ceiling imagery. This discovery contributes an important dimension to our understanding of the earliest graphic representation in southwestern France, almost all of which was discovered before modern methods of archaeological excavation and analysis. Comparison of the dates for the Castanet ceiling and those directly obtained from the Chauvet paintings reveal that the “vulvar” representations from southwestern France are as old or older than the very different wall images from Chauvet.

AMS Radiocarbon Dating of Bones at LSCE

In this paper, we explain our routine pretreatment of bone for radiocarbon dating by accelerator mass spectrometry (AMS), based on the specific reaction between amino acids and ninhydrin described by Nelson (1991). The values and uncertainties of the total system background are presented as a function of the carbon sample mass and the reliability of this method is discussed.

Radiocarbon calibration by means of varves versus (super 14) C ages of terrestrial macrofossils from Lake Gosciaz and Lake Perespilno, Poland.

This paper presents radiocarbon dates of terrestrial macrofossils from Lakes Gosciaz and Perespilno, Poland. These data agree very well with most of the German pine calibration curve. In the Late Glacial, they generally agree with the data from Lake Suigetsu, Japan, and indicate constant or even increasing (super 14) C age between 12.9 and 12.7 ka BP, rapid decline of (super 14) C age around 12.6 ka BP, and a long plateau 10,400 (super 14) C BP around 12 ka BP. Correlation with corals and data from the Cariaco basin seems to support the concept of site-speficic, constant values of reservoir correction, in contradiction to those introduced in the INTCAL98 calibration. Around the Allerod/Younger Dryas boundary our data strongly disagree with those from the Cariaco basin, which reflects large discrepancy between calendar chronologies at that period. The older sequence from Lake Perespilno indicates two periods of rapid decline in (super 14) C age, around 14.2 and 13.9 ka BP.

PAIRED 14C AND 230Th/U DATING OF SURFACE CORALS FROM THE MARQUESAS AND VANUATU (SUB-EQUATORIAL PACIFIC) IN THE 3000 TO 15,000 CAL YR INTERVAL

Paired radiocarbon and 230Th/U dating was performed on 13 surface corals from submerged reefs in the Marquesas and from raised terraces in Vanuatu. The absolute ages of the corals analyzed ranged from 3000 to 15,000 cal yr. Estimates of the difference between the absolute and 14C ages of these corals are in agreement with previous determinations up until 11,500 cal yr. The resulting mean sea surface reservoir age R is determined at 390 60 yr for the Marquesas region (9S), which is slightly higher than the R value at 280 50 yr for the Tahiti Islands (18S). Multiple 14C analyses of 2 corals from the Marquesas present scattered 14C ages at ~12,000 and ~15,100 cal yr. This could be attributed to rapid changes of the 14C content of surface waters around the Marquesas Islands or to a subtle submarine diagenesis.

Oceanic Radiocarbon Between Antarctica and South Africa Along Woce Section 16 at 30°E

Accelerator mass spectrometry (AMS) radiocarbon measurements were made on 120 samples collected between Antarctica and South Africa along 30 degrees E during the WOCE-France CIVA1 campaign in February 1993. Our principal objective was to complement the Southern Oceans sparse existing data set in order to improve the (super 14) C benchmark used for validating ocean carbon-cycle models, which disagree considerably in this region. Measured (super 14) C is consistent with the theta -S characteristics of CIVA1. Antarctic Intermediate Water (AAIW) forming north of the Polar Front (PF) is rich in (super 14) C, whereas surface waters south of the PF are depleted in (super 14) C. A distinct old (super 14) C signal was found for the contribution of the Pacific Deep Water (PDW) to the return flow of Circumpolar Deep Waters (CDW). Comparison to previous measurements shows a (super 14) C decrease in surface waters, consistent with northward displacement of surface waters, replacement by old deep waters upwelled at the Antarctic Divergence, and atmospheric decline in (super 14) C. Conversely, an increase was found in deeper layers, in the AAIW. Large uncertainties, associated with previous methods for separating natural and bomb (super 14) C when in the Southern Ocean south of 45 degrees S, motivated us to develop a new approach that relies on a simple mixing model and on chlorofluorocarbon (CFC) measurements also taken during CIVA1. This approach leads to inventories for CIVA1 that are equal to or higher than those calculated with previous methods. Differences between old and new methods are especially high south of approximately 55 degrees S, where bomb (super 14) C inventories are relatively modest.

Oceanic rock art. First direct dating of prehistoric stencils and paintings from New Caledonia (Southern Melanesia)

Rock art in an inland cave on the island of Lifou, New Caledonia, has been radiocarbon dated. A cluster of early paintings date to 2500 years ago, soon after the arrival of the first settlers, who must have quickly gone inland probably in pursuit of fresh water, available near the cave. They left their mark on the cave in the form of numerous hand stencils. During the first millennium AD, later generations of artists used the same cave, drawing birds and a circular sign for water still recognised by the present community.

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

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. 1 Scientific context and objectives Understanding the distribution, sources, and sinks of trace elements and isotopes (TEIs) will improve our ability to understand past and present marine environments. Some TEIs are toxic (e.g. Hg), while others are essential micronutrients involved in many metabolic processes of marine organisms (e.g. Fe, Mn). The availability of TEIs therefore constrains the ocean carbon cycle and affects a range of other biogeochemical processes in the Earth system, whilst responding to and influencing global change (de Baar et al., 2005; Blain et al., 2007; Boyd et al., 2007; Pollard et al., 2007). Moreover, TEI interactions with the marine food web strongly depend on their physical (particulate/dissolved/colloidal/soluble) and chemical (organic and redox) forms. In addition, some TEIs are diagnostic in allowing the quantification of specific mechanisms in the marine environment that are challenging to measure directly. A few examples include (i) atmospheric deposition (e.g. 210Pb, Al, Mn, Th isotopes, 7Be; Baker et al., 2016; Hsieh et al., 2011; Measures and Brown, 1996); (ii) mixing rates of deep waters or shelf-to-open ocean (e.g. 231Pa/230Th,114C, Ra isotopes, 129I, 236U; van Beek et al., 2008; Casacuberta et al., 2016; Key et al., 2004); (iii) boundary exchange processes (e.g. εNd, Jeandel et al., 2011; Lacan and Jeandel, 2001, 2005); and (iv) downward flux of organic carbon and/or remineralization in deep waters (e.g. 234Th/238U, 210Pb/210Po, Baxs; Buesseler et al., 2004; Dehairs et al., 1997; Roca-Martí et al., 2016). In such settings, TEIs provide chemical constraints and allow the estimation of fluxes which was not possible before the development of their analyses. Finally, paleoceanographers are wholly dependent on the development of tracers, many of which are based on TEIs used as proxies, in order to reconstruct past environmental conditions (e.g. ocean productivity, patterns and rates of ocean circulation, ecosystem structures, ocean anoxia; Henderson, 2002). Such reconstruction efforts are essential to assess the processes involved in regulating the global climate system, and possible future climate change variability. Despite all these major implications, the distribution, sources, sinks, and internal cycling of TEIs in the oceans are still largely unknown due to the lack of appropriate clean sampling approaches and insufficient sensitivity and selectivity of the analytical measurement techniques until recently. This last point has improved very quickly as significant improvements in the instrumental techniques now allow the measurements of concentrations, speciation (physical and chemical forms), and isotopic compositions for most of the elements of the periodic table which have been identified either as relevant tracers or key nutrients in the marine environment. These recent advances provide the marine geochemistry community with a significant opportunity to make subBiogeosciences, 15, 7097–7109, 2018 www.biogeosciences.net/15/7097/2018/ G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) 7099 Figure 1. Schematic diagram of the mean large-scale circulation adapted from Daniault et al. (2016) and Zunino et al. (2017). Bathymetry is plotted in color with color changes at 100 and 1000 m and every 1000 m below 1000 m. Black dots represent the Short station, yellow stars the Large ones, orange stars the XLarge ones, and red stars the Super ones. The main water masses are indicated: Denmark Strait Overflow Water (DSOW), Iceland–Scotland Overflow Water (ISOW), Labrador Sea Water (LSW), Mediterranean Water (MW), and lower North East Atlantic Deep Water (LNEADW). stantial contributions to a better understanding of the marine environment. In this general context, the aim of the international GEOTRACES programme is to characterize TEI distributions on a global scale, consisting of ocean sections, and regional process studies, using a multi-proxy approach. The GEOVIDE section is the French contribution to this global survey in the North Atlantic Ocean along the OVIDE section and in the Labrador Sea (Fig. 1) and complements a range of other international cruises in the North Atlantic. GEOVIDE leans on the knowledge gained by the OVIDE project during which the Portugal–Greenland section has been carried out biennially since 2002, gathering physical and biogeochemical data from the surface to the bottom (Mercier et al., 2015; Pérez et al., 2018). Rationale for the GEOVIDE section i. The North Atlantic Ocean plays a key role in mediating the climate of the Earth. It represents a key region of the Meridional Overturning Circulation (MOC) and a major sink of anthropogenic carbon (Cant) (Pérez et al., 2013; Sabine et al., 2004; Seager et al., 2002). Since 2002, the OVIDE project has contributed to the observation of both the circulation and water mass properties of the North Atlantic Ocean. Despite the importance of the MOC on global climate, it is still challenging to assess its strength within a reasonable uncertainty (Kanzow et al., 2010; Lherminier et al., 2010). The MOC strength estimated from in situ measurements on OVIDE cruises has thus helped to validate a time series for the amplitude of the MOC (based on altimetry and ARGO float array data) that exhibits a drop of 2.5± 1.4 Sv (95 % confidence interval) between 1993 and 2010 (Mercier et al., 2015), consistent with other modelling studies (Xu et al., 2013). This time series, along with the in situ data, shows a recovery of the MOC amplitude in 2014 at a value similar to those of the mid1990s, confirming the importance of the decadal variability in the subpolar gyre. During OVIDE, the contributions of the most relevant currents, water masses, and biogeochemical provinces were localized and quantified. This knowledge was crucial for the establishment of the best strategy to sample TEIs in this specific region. In addition to the OVIDE section, the Labrador Sea section offered a unique opportunity to complement the MOC estimate, to analyse the propagation of anomalies in temperature and salinity (Reverdin et al., 1994), and to study the distribution of TEIs along the boundary current of the subpolar gyre, coupling both observations and modelling. Moreover, recent results provided evidence that CO2 uptake in the North Atlantic was reduced by the weakening of the MOC (Pérez et al., 2013). The most significant finding of this study was that the uptake of Cant occurred almost exclusively in the subtropical gyre, while natural CO2 uptake dominated in the subpolar gyre. In light of these new results, one issue to be addressed was the coupling between the Cant and the transport of water, with the aim to understand how the changes in the ventilation and in the circulation of water masses affect the Cant uptake and its storage capacity in the various identified provinces (Fröb et al., 2018). Finally, as the subpolar North Atlantic forms the starting point for the global ocean conveyor belt, it is of particular interest to investigate how TEIs are transferred to the deep ocean through both ventilation and particle sinking, and how deep convection processes impact the TEI distributions in this key region. ii. A better assessment of the factors that control organic production and export of carbon in the productive North Atlantic Ocean together with a better understanding of the role played by TEIs in these processes is research priorities. Pronounced phytoplankton blooms occur in the North Atlantic in spring in response to upwelling and water column destratification (Bury et al., 2001; Henson et al., 2009; Savidge et al., 1995). Such www.biogeosciences.net/15/7097/2018/ Biogeosciences, 15, 7097–7109, 2018 7100 G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) blooms are known to trigger substantial export of fastsinking particles (Lampitt, 1985), and can represent a major removal mechanism for particulate organic carbon, macronutrients, and TEIs to the deep ocean. iii. In the North Atlantic, TEI distributions are influenced by a variety of sources including, most importantly, the atmosphere and the margins (Iberian, Greenland, and Labrador margins). 1. Atmosphere. Atmospheric inputs (e.g. mineral dust, anthropogenic emission aerosols) are an important source of TEIs to the North Atlantic Ocean due to the combined effects of anthropogenic emissions from industrial/agricultural sources and mineral dust mobilized from the arid regions of North Africa (Duce et al., 2008; Jickells et al., 2005). Model and satellite data for the GEOVIDE section suggested that an approximately 10fold decrease in the atmospheric concentrations of mineral dust was expected from south to north (Mahowald et al., 2005). As there had been relatively few aerosol TEI studies in the northern North Atlantic compared to the tropical and subtropical North Atlantic prior to GEOVIDE, constraining atmospheric deposition fluxes to this region had been identified as a research priority (de Leeuw et al., 2014). During the GEOVIDE campaign, a multi-proxy approach (e.g. aerosol trace element concentrations, dissolved and particulate Al and Mn, seawater 210Pb, Fe, Nd, and Th isotopes, 7Be) was taken to achieve the objective of better constraining the atmospheric deposition fluxes of key trace elements. 2. Margins. The continental shelves can act

Temporal and Spatial Variations of Freshwater Reservoir Ages in the Loire River Watershed

In order to map the freshwater reservoir effect (FRE) variability of the Loire River and its tributaries, spatial and temporal carbon isotope ( 13 C and 14 C) analyses of the dissolved inorganic carbon (DIC) were conducted. Sites were selected to represent the diversity of geological settings, soil type, and land use. Results show a large spatial variability of 14 C FRE ranging between 135 and 2251±30 yr, objectively correlated to DIC contents and alkalinity. Deeper investigations of the relationship between 14 C activity of DIC and environmental variables show that the geological substrate is the dominant factor in the 14 C reservoir effect, and far more influential than the river flow discharge.