Jean-Carlos Montero-Serrano

Nd isotopic composition of water masses and dilution of the Mediterranean outflow along the southwest European margin

Nd isotopic compositions (epsilon Nd) of seawater profiles and deep-sea corals collected off the coast of Iberia and from the Bay of Biscay were measured (1) to constrain the Nd isotopic composition of water masses along the southwest European margin, (2) to track the Mediterranean Outflow Water (MOW) during its northward propagation, and (3) to establish hydrological changes during the last 1500 years. The Eastern North Atlantic Central Water (ENACW) is characterized by Nd isotopic composition of around -12.0. Mediterranean Sea Water (MSW) is collected from 800 and 1200 m depth and is characterized by epsilon Nd values ranging from -10.9, off the coast of Iberia, to -11.6 in the Bay of Biscay. These epsilon Nd results suggest a strong dilution of the pure MOW at the Strait of Gibraltar (epsilon Nd -9.4) of approximately 40% and 30% along its northward circulation pathway essentially with a contribution from ENACW. At around 2000 m depth, epsilon Nd water profiles display the occurrence of a nonradiogenic water mass (epsilon Nd -13), originating from the Labrador Sea (Labrador Sea Water). Fossil deep-sea corals, dated between 84 and 1500 years, display Nd isotopic compositions that vary moderately from present-day seawater values, suggesting a weaker influence of MOW in the formation of MSW during the Dark Ages and the Little Ice Age. These recent cold events seem to be associated with a reduction in the northward penetration of MSW, which may result from a greater eastward extension of the middepth subpolar gyre and/or a reduction of MSW formation, likely tied to a variation in deep Mediterranean water production.

Chronostratigraphy and spatial distribution of magnetic sediments in the Chukchi and Beaufort seas since the last deglaciation

Palaeomagnetic investigation of three sediment cores from the Chukchi and Beaufort Sea margins was performed to better constrain the regional chronostratigraphy and to gain insights into sediment magnetic properties at the North American Arctic margin during the Holocene and the preceding deglaciation. Palaeomagnetic analyses reveal that the sediments under study are characterized by low‐coercivity ferrimagnetic minerals (magnetite), mostly in the pseudo‐single domain grain‐size range, and by a strong, stable, well‐defined remanent magnetization (MAD <5°). Age models for these sediment cores were constrained by comparing their palaeomagnetic secular variations (inclination, declination and relative palaeointensity) with previously published and independently dated sedimentary marine records from the study area. The magnetostratigraphical age models were verified by AMS radiocarbon dating tie points, tephrochronology and 210Pb‐based sedimentation rate estimate. The analysed cores 01JPC, 03PC and 02PC span c. 6000, 10 500 and 13 500 cal. a BP, respectively. The estimated sedimentation rates were stable and relatively high since the deglaciation in cores 01JPC (60 cm ka−1) and 03PC (40–70 cm ka−1). Core 02PC shows much lower Holocene sedimentation rates with a strong decrease after the deglaciation from ~60 to 10–20 cm ka−1. Overall, this study illustrates the usefulness of palaeomagnetism to improve the dating of late Quaternary sedimentary records in the Arctic Ocean.

Mineralogical, geochemical, and magnetic signatures of surface sediments from the Canadian Beaufort Shelf and Amundsen Gulf (Canadian Arctic)

Mineralogical, geochemical, magnetic, and siliciclastic grain‐size signatures of 34 surface sediment samples from the Mackenzie‐Beaufort Sea Slope and Amundsen Gulf were studied in order to better constrain the redox status, detrital particle provenance, and sediment dynamics in the western Canadian Arctic. Redox‐sensitive elements (Mn, Fe, V, Cr, Zn) indicate that modern sedimentary deposition within the Mackenzie‐Beaufort Sea Slope and Amundsen Gulf took place under oxic bottom‐water conditions, with more turbulent mixing conditions and thus a well‐oxygenated water column prevailing within the Amundsen Gulf. The analytical data obtained, combined with multivariate statistical (notably, principal component and fuzzy c‐means clustering analyses) and spatial analyses, allowed the division of the study area into four provinces with distinct sedimentary compositions: (1) the Mackenzie Trough‐Canadian Beaufort Shelf with high phyllosilicate‐Fe oxide‐magnetite and Al‐K‐Ti‐Fe‐Cr‐V‐Zn‐P contents; (2) Southwestern Banks Island, characterized by high dolomite‐K‐feldspar and Ca‐Mg‐LOI contents; (3) the Central Amundsen Gulf, a transitional zone typified by intermediate phyllosilicate‐magnetite‐K‐feldspar‐dolomite and Al‐K‐Ti‐Fe‐Mn‐V‐Zn‐Sr‐Ca‐Mg‐LOI contents; and (4) mud volcanoes on the Canadian Beaufort Shelf distinguished by poorly sorted coarse‐silt with high quartz‐plagioclase‐authigenic carbonate and Si‐Zr contents, as well as high magnetic susceptibility. Our results also confirm that the present‐day sedimentary dynamics on the Canadian Beaufort Shelf is mainly controlled by sediment supply from the Mackenzie River. Overall, these insights provide a basis for future studies using mineralogical, geochemical, and magnetic signatures of Canadian Arctic sediments in order to reconstruct past variations in sediment inputs and transport pathways related to late Quaternary climate and oceanographic changes. -- Keywords : mineralogy ; elemental geochemistry ; magnetic properties ; surface sediment ; sediment provenance ; redox condition ; Canadian Beaufort Shelf ; Amundsen Gulf.

South Atlantic intermediate water advances into the North‐east Atlantic with reduced Atlantic meridional overturning circulation during the last glacial period

The Nd isotopic composition (eNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cadiz and the Alboran Sea, at a depth of 280–827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cadiz over the past 40 ka. eNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cadiz reveals relatively constant values (−8.6 to −9.0 and −9.5 to −10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; eNd ∼ −9.4) and Mediterranean Sea Water (MSW; eNd ∼ −9.9). In contrast, glacial eNd values for CWC collected at thermocline depth (550–827 m) in the southern Gulf of Cadiz display a higher average value (−8.9 ± 0.4) compared to the present-day value (−11.7 ± 0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic eNd values (∼ −8) at 19, 23–24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW eNd values remained stable during the last glacial period, it is suggested that these radiogenic eNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.

Comportamiento geoquímico del molibdeno y sus isótopos en el ambiente sedimentario: Un resumen bibliográfico

Son numerosas las investigaciones dedicadas al estudio de metales traza y sus aplicaciones como indicadores de procesos geoquimicos en ambientes sedimentarios antiguos. Entre estos metales, el molibdeno (Mo) es frecuentemente citado como un marcador efectivo de condiciones redox. En la literatura estan propuestos diversos mecanismos que permiten explicar la movilizacion de Mo en los oceanos y su posterior concentracion en los sedimentos. Debido a la capacidad de formar enlaces de tipo covalente y a sus propiedades atomicas, el Mo disuelto en los oceanos (MoO42-), bajo condiciones anoxicas-euxinicas (ricas en H2S), reacciona para formar tiomolibdatos que son posteriormente adsorbidos por moleculas organicas ricas en sulfuros, oxidos de Fe-Mn (nodulos de hierro-manganeso) y por sulfuros de hierro (pirita). Esta absorcion produce un fraccionamiento en los isotopos de Mo, que es marcada en los δ98/95Mo con un promedio de -0,7‰ en condiciones oxicas y un promedio de 2,3‰ en condiciones anoxicas-euxinicas. El uso de la concentracion de Mo como marcador de condiciones paleoredox necesita ser complementado con otros marcadores independientes de condiciones redox (e.g. U, V, TOC, Re, Os, Fe). La composicion isotopica del Mo parece ser un poderoso marcador para reconstruir y detectar cambios en las condiciones de paleoxigenacion en ambientes sedimentarios marinos.

Sediment Provenance Changes in the Western Arctic Ocean in Response to Ice Rafting, Sea Level, and Oceanic Circulation Variations Since the Last Deglaciation

Two sediment piston cores were recovered from the Chukchi‐Alaskan (05JPC) and Canadian Beaufort (02PC) margins to investigate grain‐size, geochemical, and mineralogical compositions. This allowed the reconstruction of changes in detrital sediment provenance and transport related to climate variability since the last deglaciation. The end‐member modeling analyses of grain size indicate that sea ice and nepheloid transport and the Mackenzie River sediment plume are major factors influencing sedimentation in the Chukchi‐Alaskan and Canadian Beaufort margins, respectively. Unmixing of the sediment composition indicates that detrital sediments in core 02PC are derived mainly from the Mackenzie River, whereas sediments from core 05JPC are derived mainly from the Mackenzie River during the deglaciation and include a mixture of Holocene sediments from the Bering Strait, Mackenzie River, and Eurasian margin. The dolomite‐rich ice‐rafted debris recorded in both cores could be related to the different phases of iceberg discharges from the Amundsen Gulf Ice Stream. Quartz and feldspar‐rich ice‐rafted debris dated at 13 and 10.6 ka cal BP (before present) are related to the Lake Agassiz outburst in core 02PC and meltwater discharge from the Brooks Range glaciers in core 05JPC. Detrital proxies in core 02PC support the hypothesis that large meltwater and iceberg discharges from the Lake Agassiz outburst to the Arctic Ocean and Amundsen Gulf ice stream may have triggered the Younger Dryas. Finally, similar trends observed between sea level curves and our detrital proxy suggest that the sea level changes in the western Arctic Ocean have an important influence on the sediment dynamic during the early to middle Holocene. -- Keywords : bulk and clay mineralogy ; elemental geochemistry ; grain‐size distribution ; sediment provenance ; Canadian Beaufort Sea ; Chukchi Sea.