The large-scale evolution of neodymium isotopic composition in the global modern and Holocene ocean revealed from seawater and archive data

Neodymium isotopic compositions (143Nd/144Nd or εNd) have been used as a tracer of water masses and lithogenic inputs to the ocean. To further evaluate the faithfulness of this tracer, we have updated a global seawater εNd database and combined it with hydrography parameters (temperature, salinity, nutrients and oxygen concentrations), carbon isotopic ratio and radiocarbon of dissolved inorganic carbon. Archive εNd data are also compiled for leachates, foraminiferal tests, deep-sea corals and fish teeth/debris from the Holocene period (< 10,000 years). At water depths ≥ 1500 m, property-property plots show clear correlations between seawater εNd and the other variables, suggesting that large-scale water mass mixing is a primary control of deepwater εNd distribution. At ≥ 200 m, basin-scale seawater T-S-εNd diagrams demonstrate the isotopic evolution of different water masses. Seawater and archive εNd values are compared using property-property plots and T-S-εNd diagrams. Archive values generally agree with corresponding seawater values although they tend to be at the upper limit in the Pacific. Both positive and negative offsets exist in the northern North Atlantic. Applying multiple regression analysis to deep (≥ 1500 m) seawater data, we established empirical equations that predict the main, large-scale, deepwater εNd trends from hydrography parameters. Large offsets from the predicted values are interpreted as a sign of significant local/regional influence. Dominant continental influence on seawater and archive εNd is observed mainly within 1000 km from the continents. Generally, seawater and archive εNd values form gradual latitudinal trend in the Atlantic and Pacific at depths ≥ 600 m, consistent with the idea that Nd isotopes help distinguish between northern/southern sourced water contributions at intermediate and deep water depths

A new breath of life for anoxia.

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Neodymium isotope evolution of NW Tethyan upper ocean waters throughout the Cretaceous

International audienceNeodymium isotope compositions of twenty-four fish teeth, nineteen from the NW Tethys and five from different locations within the Tethys, are interpreted to reflect the evolution of Tethyan upper ocean water composition during the Cretaceous and used to track changes in erosional inputs to the NW Tethys and in oceanic circulation throughout the Cretaceous. The rather high ∑Nd (up to -7.6) of the NW Tethyan upper ocean waters recorded from the Late Berriasian to the Early Aptian and the absence of negative excursions during this interval support the presence of a permanent westward flowing Tethys Circumglobal Current (TCC). This implies that temperature variations during this time period, inferred from the oxygen isotope analysis of fish tooth enamel, were not driven by changes in surface oceanic currents, but rather by global climatic changes. The results presented here represent a significant advance over previously published Cretaceous seawater Nd isotope records. Our newly acquired data now allow the identification of two stages of low eNd values in the NW Tethys, during the Early Albian–Middle Albian interval (down to -10) and the Santonian–Early Campanian (down to -11.4), which alternate with two stages of higher ∑Nd values (up to -9) during the Late Albian–Turonian interval and the Maastrichtian. Used in conjunction with the oxygen isotope record, the fluctuations of ∑Nd values can be related to major climatic, oceanographic, and tectonic events that appeared in the western Tethyan domain

Can crystallinity be used to determine the degree of chemical alteration of biogenic apatites?

International audienceA new Raman spectroscopic technique has been refined to more efficiently previous termdetermine the crystallinitynext term indeces of previous termbiogenicnext term apatites. We investigate the possible relationships between the structure (previous termcrystallinity)next term and geochemistry (rare earth element (REE), δ18O) of previous termbiogenicnext term apatites. A selection of phosphatic remains dated from present to about 510 Ma (Late Cambrian), for most of which either the oxygen isotope compositions or REE patterns are available, has been characterized for previous termcrystallinitynext term using Raman spectroscopy. We define a new previous termcrystallinitynext term index (CIRaman) from the ratio of the full width at half maximum (FWHM) of the intense peak of the PO4 symmetric stretching mode in the sample and a reference magmatic apatite. In order to compare our previous termcrystallinitynext term index with CI previous termusednext term in previously published studies, we also analyzed part of our sample set with FT-IR spectroscopy and X-ray diffractometry. A detailed study of natural samples demonstrates that previous termcrystallinitynext term index is a poor criterion for determining if a sample has been altered since deposition. This result is based on three major observations: (1) independently of the CIRaman, the original geochemical signatures of the previous termbiogenicnext term apatites previous termcannext term be preserved over a long period, (2) strong geochemical perturbations (lowering of δ18O values and of La/Sm ratios) may occur without detectable recrystallization, and (3) previous termalterationnext term by heating, marked by the transformation of organic matter into graphite, produces REE fractionations and limited oxygen isotope exchange with crustal aqueous fluids

Minéralogie (fraction argileuse) de la région stratotypique.

5 pagesNational audienc

Late Cretaceous changes in continental configuration: toward a better-ventilated ocean?

International audienceOceanic anoxic events (OAEs) are large-scale events of oxygen depletion in the deep ocean that happened dur-ing pre-Cenozoic periods of extreme warmth. Last global OAE occurred at the Cenomanian-Turonian boundary(OAE2) prior to the Late Cretaceous long term cooling. Ever since, and despite the occurrence of warming events,Earth no more experienced such large-scale anoxic conditions. Here we explore the role of major continentalconfiguration changes occurring during the Late Cretaceous on oceanic circulation modes through numerical sim-ulations using a General Circulation Model (GCM), that we confront to existing neodymium isotope data ("Nd).Except from a continuous deep-water production in the North Pacific, the simulations at 95 Ma and 70 Ma revealmajor differences marked by a shift in the southern deep-water production sites from South Pacific at 95Ma toSouth Atlantic and Indian oceans at 70Ma. Our modelling results support an intensification of southern Atlanticdeep-water production as well as a reversal of the deep-water fluxes through the Caribbean Seaway as the maincauses of the decrease in"Nd values recorded in the Atlantic and Indian deep waters during the Late Cretaceous.We conclude that the change from a sluggish to a much more active circulation depicted by the model in theAtlantic from 95Ma to 70Ma may have driven the disappearance of OAEs after the Late Cretaceous

Calcareous nannoplankton and palaeoenvironmental changes in the Valanginian (Lower Cretaceous) of the Vocontian Trough, SE France

Riquier Laurent, Mattioli Emanuela, Pittet Bernard, Excoffier Frédéric, Pucéat Emmanuelle. Calcareous nannoplankton and palaeoenvironmental changes in the Valanginian (Lower Cretaceous) of the Vocontian Trough, SE France. In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. p. 197

Nitrogen and carbon cycle perturbations through the Cenomanian-Turonian oceanic anoxic event 2 (~94 Ma) in the Vocontian Basin (SE France)

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