High Resolution Electrical Resistivity Tomography (ERT) in a Transition Zone Environment: Application for Detailed Internal Architecture and Infilling Processes Study of a Rhône River Paleo-channel

Electrical Resistivity Tomography (ERT) was applied in a sand-infilled paleo-channel located in the Rhône Delta, in Southern France. The detailed pattern and sedimentological evolution of the channel fill deposits – know from both historical and geological sources – made it the ideal site to test the ERT method. A geoelectrical survey was performed, using the ABEM SAS-4000 multi-electrode array system in March 2003. Very low electrical resistivity values were obtained, ranging from 0.3 to 10 Ohm m, consistent with the high salinity measured in situ (the pore water conductivity was found to range from 0.9 to 1.2 S/m at 25 °C). The electrical resistivity profiles reflect mainly salinity variations. Indeed, in this case, salinity is so high that surface conductivity associated with clay minerals can be safely neglected. ERT provided valuable high-resolution information that complemented other exiting data such as historical information, bathymetric, geological, and lithostratigraphic data, which allowed the architecture of the channel to be defined. The ERT was used to determine the infilling dynamics of the Pégoulier Channel, which opens new perspectives in terms of paleoenvironmental reconstruction and paleodynamic studies.     

Temporal study of estrogenic responses of mussel (Mytilus galloprovinciallis) extracts applied to reporter cell lines

In vitro bioassays would facilitate monitoring of estrogen-like compounds in mussels (Mytilus galloprovincialis) since they tend to accumulate lipophilic compounds in their fat and muscle tissue. However, estradiol (E2) steroid (already identified in mussels) could induce false positive responses. This study focuses on temporal variability in estrogenic responses of mussel extracts and the possible relation of this response with E2 levels. The reproductive cycle and growth were thus monitored for six months in sexed mussels. E2 levels were similar between genders and these levels varied with reproductive stages. human estrogen receptor (hER) activities were similar at all stages except in February. E2 present in male and female extracts accounted for part of the hER activation observed, but no correlation was found between E2 levels and hER activities.     

Sources of particulate and dissolved organic carbon in the St Lawrence River: isotopic approach

We investigate sources of both dissolved and particulate organic carbon in the St Lawrence River from its source (the Great Lakes outlet) to its estuary, as well as in two of its tributaries. Special attention is given to seasonal interannual patterns by using data collected on a bi‐monthly basis from mid‐1998 to mid‐2003. δ¹³C measurements in dissolved inorganic carbon, dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as molar C : N in particulate organic matter (POM), are used to bring insight into the dynamic between aquatic versus terrigenous sources. In addition, ¹⁴C activities of DOC were measured at the outlet of the St Lawrence River to its estuary to assess a mean age of the DOC exported to the estuary. In the St Lawrence River itself, aquatically produced POC dominates terrestrially derived POC and is depleted in ¹³C by approximately 12‰ versus dissolved CO₂. In the Ottawa River, the St Lawrence River's most important tributary, the present dataset did not allow for convincing deciphering of POC sources. In a small tributary of the St Lawrence River, aquatically produced POC dominates in summer and terrestrially derived POC dominates in winter. DOC seems to be dominated by terrestrially derived organic matter at all sampling sites, with some influence of DOC derived from aquatically produced POC in summer in the St Lawrence River at the outlet of the Great Lakes and in one of its small tributaries. The overall bulk DOC is relatively recent (¹⁴C generally exceeding 100% modern carbon) in the St Lawrence River at its outlet to the estuary, suggesting that it derives mainly from recent organic matter from topsoils in the watershed. Copyright © 2005 John Wiley & Sons, Ltd.     

A detailed study of the distribution of heat flow and radioactivity in New Hampshire (U.S.A.)

We present a set of 39 new determinations of heat flow and radiogenic heat production for several different geological environments in the State of New Hampshire (U.S.A.). With the extensive data set now available for the state, the linear relation of heat flow and heat production appears as a very useful generalization for the study of the heat flow field of a geological province. Our measurements indicate that the vertical distribution of radiogenic heat production is similar in plutonic and metasedimentary rocks. Our data are compatible with the linear relationship established earlier by F. Birch and his co-workers in 1968. Young granites are markedly enriched in radioactive elements and those which do not outcrop are revealed by anomalies in the general relation of heat flow versus radioactivity.Heat flow is high for plutons by low elsewhere. The mean heat flow through metasedimentary formations is 1.15 μcal/cm² s (48 mW/m²), a value near the mean heat flow for old cratons. The lowest heat flow measured is 0.76 μcal/cm² s (32 mW/m²) for a unit poor in radioactivity. The heat flow field grades smoothly into the low heat flow regions of the Canadian Shield.The New Hampshire region is in thermal equilibrium: its heat flow is in secular equilibrium with the heat generated by crustal sources and that supplied from the mantle. In this area, the thermal perturbations due to orogenic events decrease below the detection level in less than 200–275 Ma. The thickness of the layer which is thermally affected during continent-continent collision-type orogenies cannot be greater than about 190 km.     

The lead isotope geochemistry and geochronology of late-kinematic intrusives from the Abitibi greenstone belt,and the implications for late Archaean crustal evolution

Pb isotope abundances are reported for six late-kinematic granitoid intrusives from the Quebec sector of the Abitibi greenstone belt. Leaching experiments on K-feldspar separates reveal the presence of radiogenic Pb, attributed to in situ decay of U and Th. Pb-Pb mineral isochrons were constructed with the K-feldspar data plus results obtained on the total-rock, sphene, apatite and other mineral phases; five localities show no evidence of post-emplacement disturbance and yield ages ranging from 2616 ± 19 to 2718 ± 12 Ma. These ages, which are corroborated by U-Pb dating of small populations of sphene, imply that the orogenic events in the Abitibi belt were terminated 2700–2710 Ma ago, and followed by a period of granitization which lasted for 80 to 100 Ma.The initial Pb isotope composition of the magmas shows that their source regions were isotopically heterogeneous; the time integrated ${}^{238}\text{U}{}^{204}\text{Pb}$ values for the source regions vary from 7.62 to 7.92 and the K-feldspar data indicate that similar heterogeneities were present at the scale of a single intrusion. The range of isotopic composition spans the compositional domain of the mantle, defined by sulfides associated with komatiites and some galenas, and that of the continental crust, defined by sulfides associated with Abitibi iron-formations. Consequently, the granitoid magmas are interpreted as partial melts of a continental crust comprising juvenile, mantle-derived rocks and non-negligible amounts of earlier formed sialic material. The Pb isotope data for the latter are consistent with the presence in the area of 3.0 to 3.4 Ga old sialic crust. The episode of crustal anatexis occurred as a consequence to the orogenic events which resulted in burial of altered supracrustal rocks rich in water and heat-producing elements.     

New constraints on the delivery of cometary water and nitrogen to Earth from the N/N isotopic ratio

New independent constraints on the amount of water delivered to Earth by comets are derived using the ¹⁵N/¹⁴N isotopic ratio, measured to be roughly twice as high in cometary CN and HCN as in the present Earth. Under reasonable assumptions, we find that no more than a few percent of Earth’s water can be attributed to comets, in agreement with the constraints derived from D/H. Our results also suggest that a significant part of Earth’s atmospheric nitrogen might come from comets. Since the ¹⁵N/¹⁴N isotopic ratio is not different in Oort-cloud and Kuiper-belt comets, our estimates apply to the contribution of both types of objects.     

Cosmogenic neon in ultramafic nodules from Asia and in quartzite from Antarctica

We have performed systematic analyses of both cosmogenic ³He (³He_c) and cosmogenic ²¹Ne (²¹Ne_c) in ultramafic xenoliths from Central Asia and in a quartz sample from Antarctica. Five xenoliths, which show no or insignificant ²¹Ne_c excesses, were used to estimate the initial ⁴He/³He ratio of 90,470 in the subcontinental lithospheric mantle under the Baikal extension zone. Seven xenoliths show large ²¹Ne/²²Ne anomalies ranging up to 0.204 and ⁴He/³He down to 31,000, due to the presence of cosmogenic ²¹Ne and ³He. The (³He/²¹Ne)_c ratio is 1.41 ± 0.22 in the xenoliths and 2.76 in the quartzite. This difference is due to the dependence of the ²¹Ne_c production rate on the elemental composition of the target material. We estimated the ³He_c and ²¹Ne_c production rates at different locations worldwide and calculated the ³He_c and ²¹Ne_c exposure ages. These ages range between 7100 and 28,000 years for the xenoliths, and we determined their relative positions within the volcanic tuff layer. The mean ³He_c and ²¹Ne_c exposure ages of the quartz sample are 1.35 ± 0.07 and 2.21 ± 0.12 Ma, respectively. This difference is most probably related to ³He_c diffusive losses from the quartz mineral grains, even at low temperatures, due to the relatively high diffusion coefficient for cosmogenic ³He.