An ocean–ice coupled response during the last glacial: a view from a marine isotopic stage 3 record south of the Faeroe Shetland Gateway

150 μm) lithic fraction (grain concentration) and the analysis of selected biogenic proxies (assemblages and stable isotope ratio of calcareous planktonic foraminifera, dinoflagellate cyst – e.g. dinocyst – assemblages). Results presented here are focussed on the dinocyst response, this proxy providing the reconstruction of past sea-surface hydrological conditions, qualitatively as well as quantitatively (e.g. transfer function sensu lato). Our study documents a very coherent and sensitive oceanic response to the MIS3 rapid climatic variability: strong fluctuations, matching those of stadial/interstadial climatic oscillations as depicted by Greenland ice cores, are recorded in the MD99-2281 archive. Proxies of terrigeneous and detritical material suggest increases in continental advection during Greenland Stadials (including Heinrich events), the latter corresponding also to southward migrations of polar waters. At the opposite, milder sea-surface conditions seem to develop during Greenland Interstadials. After 30 ka, reconstructed paleohydrological conditions evidence strong shifts in SST: this increasing variability seems consistent with the hypothesised coalescence of the British and Fennoscandian ice sheets at that time, which could have directly influenced sea-surface environments in the vicinity of core MD99-2281

Regional seesaw between the North Atlantic and Nordic Seas during the last glacial abrupt climate events

Dansgaard–Oeschger oscillations constitute one of the most enigmatic features of the last glacial cycle. Their cold atmospheric phases have been commonly associated with cold sea-surface temperatures and expansion of sea ice in the North Atlantic and adjacent seas. Here, based on dinocyst analyses from the 48–30 ka interval of four sediment cores from the northern Northeast Atlantic and southern Norwegian Sea, we provide direct and quantitative evidence of a regional paradoxical seesaw pattern: cold Greenland and North Atlantic phases coincide with warmer sea-surface conditions and shorter seasonal sea-ice cover durations in the Norwegian Sea as compared to warm phases. Combined with additional palaeorecords and multi-model hosing simulations, our results suggest that during cold Greenland phases, reduced Atlantic meridional overturning circulation and cold North Atlantic sea-surface conditions were accompanied by the subsurface propagation of warm Atlantic waters that reemerged in the Nordic Seas and provided moisture towards Greenland summit

Stratification of surface waters during the last glacial millennial climatic events: a key factor in subsurface and deep-water mass dynamics

The last glacial period was punctuated by abrupt climatic events with extrema known as Heinrich and Dansgaard–Oeschger events. These millennial events have been the subject of many paleoreconstructions and model experiments in the past decades, but yet the hydrological processes involved remain elusive. In the present work, high-resolution analyses were conducted on the 12–42 ka BP section of core MD99-2281 retrieved southwest of the Faeroe Islands, and combined with analyses conducted in two previous studies (Zumaque et al., 2012; Caulle et al., 2013). Such a multiproxy approach, coupling micropaleontological, geochemical and sedimentological analyses, allows us to track surface, subsurface, and deep hydrological processes occurring during these rapid climatic changes. Records indicate that the coldest episodes of the studied period (Greenland stadials and Heinrich stadials) were characterized by a strong stratification of surface waters. This surface stratification seems to have played a key role in the dynamics of subsurface and deep-water masses. Indeed, periods of high surface stratification are marked by a coupling of subsurface and deep circulations which sharply weaken at the beginning of stadials, while surface conditions progressively deteriorate throughout these cold episodes; conversely, periods of decreasing surface stratification (Greenland interstadials) are characterized by a coupling of surface and deep hydrological processes, with progressively milder surface conditions and gradual intensification of the deep circulation, while the vigor of the subsurface northward Atlantic flow remains constantly high. Our results also reveal different and atypical hydrological signatures during Heinrich stadials (HSs): while HS1 and HS4 exhibit a "usual" scheme with reduced overturning circulation, a relatively active North Atlantic circulation seems to have prevailed during HS2, and HS3 seems to have experienced a re-intensification of this circulation during the middle of the event. Our findings thus bring valuable information to better understand hydrological processes occurring in a key area during the abrupt climatic shifts of the last glacial period

Last Glacial Period Cryptotephra Deposits in an Eastern North Atlantic Marine Sequence: Exploring Linkages to the Greenland Ice-Cores

The establishment of a tephra framework for the Greenland ice-cores spanning the last glacial period, particularly between 25 and 45 ka b2k, provides strong potential for precisely correlating other palaeoclimatic records to these key archives. Tephra-based synchronisation allows the relative timing of past climatic changes recorded within different depositional environments and potential causal mechanisms to be assessed. Recent studies of North Atlantic marine records have demonstrated the potential of tracing cryptotephra horizons in these sequences and the development of protocols now allows a careful assessment of the isochronous nature of such horizons. Here we report on tephrochronological investigations of a marine sequence retrieved from the Goban Spur, Eastern North Atlantic, covering ?25–60 ka b2k. Density and magnetic separation techniques and an assessment of potential transport and depositional mechanisms have identified three previously unknown isochronous tephra horizons along with deposits of the widespread North Atlantic Ash Zone II and Faroe Marine Ash Zone III. Correlations between the new horizons and the Greenland ice-core tephra framework are explored and despite no tie-lines being identified the key roles that high-resolution climatostratigraphy and shard-specific trace element analysis can play within the assessment of correlations is demonstrated. The previously unknown horizons are new additions to the overall North Atlantic tephra framework for the last glacial period and could be key horizons for future correlations

Radiation-induced melting in coherent X-ray diffractive imaging at the nanoscale

Coherent X-ray diffraction techniques play an increasingly significant role in imaging nanoscale structures which range from metallic and semiconductor samples to biological objects. The conventional knowledge about radiation damage effects caused by ever higher brilliance X-ray sources has to be critically revised while studying nanostructured materials

Les reconstitutions paléo-hydrographiques de surface dans le Golfe de Gascogne au cours de l'Holocène

La carotte MD95-2002, prélevée dans le Golfe de Gascogne (47°27'N; 08 °32'W, 2174m) a fait l'objet d'analyses micropaléontologiques afin de « documenter » les changements dans les conditions palée-hydrographiques de surface et de la colonne d'eau sous-jacente au cours de l'Holocène dans l'Atlantique nord-est. Les reconstructions quantitatives basées sur les assemblages de kystes de dinoflagellés (dinokystes) et les compositions isotopiques de foraminifères planctoniques ont permis de mettre en évidence des tendances à long terme auxquelles se superpose une importante variabilité climatique. Cette dernière se traduit principalement par des fortes variations des températures d'eau de surface (Sea-Surface Temperatures - SSTs) à l'Holocène inférieur, avec des maxima allant jusqu'à 25°C. Les faibles salinités d'eau de surface (Sea-Surface salinities – SSSs) enregistrées à ce moment-là témoignent du démantèlement de la calotte laurentidienne rémanente, et expliquent partiellement les conditions instables qui y prévalaient. Un optimum climatique tardif et atténué est enregistré entre 7,5 et 6 ka, comme dans nombre de carottes en Atlantique Nord ne se trouvant pas le long de la banche principale de la Dérive Nord Atlantique (DNA). Le forçage atmosphérique sous la forme des variations de l'Oscillation Nord Atlantique (indice NAO) semble jouer un rôle prépondérant, tant au niveau de l'expression tardive de l'Hypsithermal, qu'au niveau des variations qui ponctuent l'Holocène. En effet les variations des SSTs dans la carotte MD95-2002 pourraient être le résultat des variations d'influence entre la branche est de la DNA, et le courant de pente (Slope Current - SC) plus chaud et plus salé au niveau du site, ce dernier étant plus important lors d'indices NAO négatifs. Ainsi, l'Holocène inférieur aurait-il été marqué par de fortes variations de l'indice NAO, de façon concomitante à l'instabilité du système océan-atmosphère-cryosphère jusque vers 7,5 ka suivi d'une stabilisation en mode négatif dominant durant l'Holocène moyen et, enfin, par un indice positif dominant à l'Holocène supérieur qui accompagne la chute des SSTs à partir de 4 ka