Reconstructing fluvial incision rates based on palaeo-water tables in chalk karst networks along the Seine valley (Normandy,France)

Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high-level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo-water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest-level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka⁻¹, dropping to ~0.08 m ka⁻¹ between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka⁻¹ from MIS 5 to the present time. © 2020 John Wiley & Sons, Ltd.     

Seasonal changes in phosphatase activities in Toulon Bay (France)

We studied the characteristics of the phosphatase activity (Km and Vmax) in total seawater and in particulate material of the three main plankton classes (0.25-5, 5-90 and >90 microm) in a coastal marine ecosystem of Toulon Bay (French Mediterranean Sea). The measurement of the hydrolysis of sodium paranitrophenylphosphate (pNPP), a substrate of phosphatase, revealed low and high affinity components in unfiltered seawater and in particulate matter. In unfiltered seawater, the low affinity activity was predominant from October to March during phytoplankton development. The high affinity activity dominated from April to June and was significantly correlated with the bacterial abundances. The phosphatase behaviour in the particulate material differs from that in the unfiltered seawater. The activity of the three particulate classes was generally much lower than that of unfiltered seawater, particularly the low affinity activity. The >90 microm size fraction consisted in greater part of zooplankton. In this size class, the activity (nmol l(-1) h(-1)) of the low affinity component was predominant from May to August, when the abundance of the larvae of copepods (copepodites) was highest. Its high specific activity (Activity/Protein concentration as nmol l(-1) h(-1) microg(-1)) was particularly elevated during this period. The 5-90 microm fraction consisted of phytoplankton cells, especially Dinoflagellates. Between September and January, the activity (nmol l(-1) h(-1)) of this size class was mostly supported by the low affinity component. The specific activity (nmol l(-1) h(-1) microg(-1)) of the high affinity component was highest in June and August. No significant correlation was found between phosphatase activities and chlorophyll a or total cell abundance. In return temporary relationships with specific taxa exist in particular with Ceratium spp., Gymnodinium spp. and Protoperidinium spp. The contribution of the 0.25-5 microm size class exceeded rarely 20% of the total particulate activity. Between June and August, high specific activities (nmol l(-1) h(-1) microg(-1)) were observed for its high affinity component. In autumn, strong rainfall increased the phosphate and nitrate concentrations and led to a drop in salinity, which probably explains the low phosphatase activities (nmol l(-1) h(-1)) and cell densities observed during this period.     

Fluxes of selected trace metals from the Seine estuary to the eastern English Channel during the period August 1994 to July 1995

Two sampling cruises conducted in the Seine estuary (France) under low-water and flood conditions produced high resolution profiles for dissolved cadmium, lead, copper, zinc and nickel concentrations versus salinity. The distribution of dissolved trace metals differed depending on hydrologic conditions, partly because of the dilution of upstream inputs during flood periods. Daily fluxes of these dissolved trace metals were estimated for the two sampling periods (September 1994 and February 1995) by extrapolating the dilution lines observed in higher salinity waters to salinity=0 and then multiplying the effective freshwater concentrations thus obtained by the corresponding freshwater flow. Several procedures were subsequently applied to deduce each daily flux for the year studied from data for these two periods. A consensus was found among these procedures, allowing the determination of net fluxes of dissolved trace metals with a precision of 20–35%. The net fluxes thus estimated were 4 T yr⁻¹ for Cd, 4 T yr⁻¹ for Pb, 40 T yr⁻¹ for Cu, 130 T yr⁻¹ for Zn and 50 T yr⁻¹ for Ni.     

The growth of the continent through geological time studied by Nd isotope analysis of shales

We determined the Nd isotopic composition and the Sm/Nd ratios in a series of Australian shales ranging from 0.2 Gy to 3.3 Gy. The first result of this study is the constancy of the Sm/Nd ratio in these shales, as in granitoids. Secondly, the initial (¹⁴³Nd/¹⁴⁴Nd) ratio gives a regular curve decreasing through geological time. Both results confirm that shales are representative samples of the continental crust, when insoluble elements, like REE, are studied.We calculated their Nd model ages of crustal differentiation. The model ages regularly decrease with the stratigraphic ages and after 2 Gy, the curve flattens and tends to an asymptotic value at around 1.8 Gy. The significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin.From these results, we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas. We propose that the results obtained here are representative of the whole Gondwana continent.Having studied the case of the Australian shales, we try to extend our study. First we applied our quantitative approach to the recent results obtained by O'Nions et al. on North Atlantic provinces. The continental growth curves obtained by our inversion procedure are quite distinct from the Australian shales showing the regional character of shales.With these two cases studied we try to compare our results with the already developed model for continental growth. The consideration of the surface conservative versus surface non-conservative models clearly show the non-uniqueness of the geological interpretation of the growth curve. On the other hand, we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the Hurley-Rand province age curve. Such recycling increases with time in agreement with the data obtained on Nd and Sr initial ratios on granitoids.     

Experimental study of the coastal thermocline height variations caused by an impulsively applied wind of limited along-shore extent

On the large rotating platform of Grenoble's University, a channel (8×2×0.6 m) has been built, equipped with a wind entrainment simulation effects mechanism. With a local wind, acting over a two-layered ocean, upwelling or downwelling patterns are induced, which are Kelvin-type height variations of the interface; propagating so that, in the northern hemisphere, they leave the coastline to the right of their direction of propagation, and having a profile exponentially decreasing from the coastline towards the open sea. The agreement between these observations and the recently proposed model of Crepon and Richez (1982) is very good at some distance from the excitation zone.However, this model, for its application, is limited to the situation in which the Ekman layer is of the same order of magnitude as the upper layer. Otherwise the oceanographic observations, as well as our observations, show a cross-shore current in the intermediate layer. Near the boundaries of the excitation zone (i.e., the wind zone), such a current is responsible for vortices which cause local upwelling. The model does not account for such local upwelling which is superimposed on the Kelvin-type height variation of the thermocline. These vortices may be explained by consideration of variations of relative vorticity. The experiments also show that a wind perpendicular to the coast may also induce upwelling or downwelling.     

Benthic primary production, respiration and remineralisation: in situ measurements in the soft-bottom Abra alba community of the western English Channel (North Brittany)

In situ measurements of ammonium and carbon dioxide fluxes were performed using benthic chambers at the end of spring and the end of summer in two soft-bottom Abra alba communities of the western English Channel (North Brittany): the muddy sand community (5 m, about 10% of surface irradiance) and the fine-sand community (19 m, about 1% of surface irradiance). High rates of ammonium regeneration were measured in the two communities at the end of summer (296.03±40.07 and 201.7±62.74 μmolN m⁻² h⁻¹, respectively) as well as high respiration rates (2.60±0.94 and 2.23±0.59 mmolC m⁻² h⁻¹, respectively). Significant benthic gross primary production (up to 6.11 mmolC m⁻² h⁻¹) was measured in the muddy sand community but no benthic primary production was measured in the fine-sand community. It suggests that microphytobenthic production values used in simulations previously published for these two communities were overestimated while values of community respiration were underestimated. The study confirms that this benthic system is heterotrophic and strengthens the idea that an important pelagic-benthic coupling is required for the functioning in such coastal ecosystems.     

A new approach to detecting vegetation and land-use change using high-resolution lipid biomarker records in stalagmites

A hundred-year stalagmite lipid biomarker record from Mechara, southeastern Ethiopia, is presented. The record has been recovered at a 10-yr temporal resolution, marking the first time this has been achieved in stalagmite biomarker work and providing the first opportunity to investigate the relationship between stalagmite lipid records and hydrological transport lags, a vital issue in interpreting palaeoenvironmental signals. Preserved plant-derived n-alkanes and n-alkanols show clear changes in composition over time, relating to known land-use changes in the area, particularly the expansion of agriculture in the early twentieth century. The level of environmental detail provided by this technique, combined with the long-term chronological framework offered by stalagmites, holds significant promise for the investigation of early human environments and their associated climatic and anthropogenic controls.     

Utilisation de dépôts d'argile sur lame de verre pour leur étude en spectroscopie infrarouge

IR spectra of clay samples can be collected in transmission through oriented deposits onto glass slide and are similar to the IR spectra of the same samples obtained by traditional way (KBr pellets). As examples, it is shown that it is possible to differentiate dickite from kaolinite and smectites of various chemistries, as easily by both ways. It is thus possible to use oriented clay deposits onto glass slide for both XRD and IR studies and to easily take benefit of the complementarity of both techniques. To cite this article: S. Petit et al., C. R. Geoscience 335 (2003).     

Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France

Abstract Explosive volcanic activity is recorded in the Upper Jurassic of the Paris Basin and the Subalpine Basin of France by the identification of five bentonite horizons. These layers occur in Lower Oxfordian (cordatum ammonite zone) to Middle Oxfordian (plicatilis zone) clays and silty clays deposited in outer platform environments. In the Paris Basin, a thick bentonite (10–15 cm), identified in boreholes and in outcrop, is dominated by dioctahedral smectite (95%) with trace amounts of kaolinite, illite and chlorite. In contrast, five bentonites identified in the Subalpine Basin, where burial diagenesis and fluid circulation were more important, are composed of a mixture of kaolinite and regular or random illite/smectite mixed-layer clays in variable proportions, indicating a K-bentonite. In the Subalpine Basin, a 2–15 cm thick bentonite underlain by a layer affected by sulphate–carbonate mineralization can be correlated over 2000 km². Euhedral zircon, apatite and biotite crystals have been identified in all the bentonites. The geochemical composition of the bentonites in both basins is characterized by high concentrations of Hf, Nb, Pb, Ta, Th, Ti, U, Y, Zr and low concentrations of Cr, Cs and Rb. Biostratigraphical and geochemical data suggest that the thick bentonite in the Paris Basin correlates with the thickest bentonite in the Subalpine Basin, located 400 km to the south. These horizons indicate that significant explosive volcanic events occurred during the Middle Oxfordian and provide potential long-distance isochronous marker beds. Immobile element discrimination diagrams and rare-earth element characteristics indicate that the original ash compositions of the thickest bentonites correspond to a trachyandesitic source from a within-plate alkaline series that was probably related to North Atlantic rifting.