Corrosion of cast iron pipelines buried in Fraser River silt subject to climate-induced moisture variations

Acta Geotechnica - Experiments were carried out to characterise the influence of soil water retention properties on the corrosion of a cast iron specimen buried in Fraser River silt. The corrosion...     

Mediterranean drought fluctuation during the last 500 years based on tree-ring data

A 2.5 × 2.5° gridded summer (April–September) drought reconstruction over the larger Mediterranean land area (32.5°/47.5°N, 10°W/50°E; 152 grid points) is described, based on a network of 165 tree-ring series. The drought index used is the self-calibrated Palmer Drought Severity Index, and the period considered is 1500–2000. The reconstruction technique combines an analogue technique for the estimation of missing tree-ring data with an artificial neural network for optimal non-linear calibration, including a bootstrap error assessment. Tests were carried out on the various sources of error in the reconstructions. Errors related to the temporal variations of the number of proxies were tested by comparing four reconstructions calibrated with four different sized regressor datasets, representing the decrease in the number of available proxies over time. Errors related to the heterogeneous spatial density of predictors were tested using pseudo-proxies, provided by the global climate model ECHO-G. Finally the errors related to the imperfect climate signal recorded by tree-ring series were tested by adding white noise to the pseudo-proxies. Reconstructions pass standard cross-validation tests. Nevertheless tests using pseudo-proxies show that the reconstructions are less good in areas where proxies are rare, but that the average reconstruction curve is robust. Finally, the noise added to proxies, which is by definition a high frequency component, has a major effect on the low frequency signal, but not on the medium frequencies. The comparison of the low frequency trends of our mean reconstruction and the GCM simulation indicates that the detrending method used is able to preserve the long-term variations of reconstructed PDSI. The results also highlight similar multi-decadal PDSI variations in the central and western parts of the Mediterranean basin and less clear low frequency changes in the east. The sixteenth and the first part of the seventeenth centuries are characterized by marked dry episodes in the west similar to those observed in the end of the twentieth century. In contrast, the eighteenth and nineteenth centuries (Little Ice Age) are characterized by dominant wet periods. In the eastern part of the Mediterranean basin the observed strong drought period of the end of the twentieth century seems to be the strongest of the last 500 years.     

Sea-level estimates during the last deglaciation based on δO and accelerator mass spectrometry C ages measured in Globigerina bulloides

Coupled measurements of δ¹⁸O and accelerator mass spectrometry (AMS) ¹⁴C in a particular species of planktonic foraminifera may be used to calculate sea-level estimates for the last deglaciation. Of critical importance for this type of study is a knowledge of the seasonality of foraminiferal growth, which can be provided by δ¹⁸O measurements of modern shells (core tops, plankton tows). Isotopic (δ¹⁸O, AMS-¹⁴C dating) and faunal records (transfer function sea surface temperature) were obtained from two cores in the North Atlantic at about 37°N. The locations were chosen to obtain high sedimentation rate records removed from the major ice-melt discharge areas of the last deglaciation. Based upon Globigerina bulloides data, four δ¹⁸O-based sea-level estimates were calculated: −67 ± 7 m at 12,200 yr B.P. and −24 ± 8 m at about 8200 yr B.P. for core SU 81-18; −83 ± 10 m at 12,200 yr B.P. and −13 ± 11 m at about 8500 yr B.P. for core SU 81-14. Using a second working hypothesis concerning the seasonability of G. bulloides growth, it is suggested that the sea-level rose by about 40 m during the millennium which followed 14,500 yr B.P.     

Using a maximum simplicity paleoclimate model to simulate millennial variability during the last four glacial periods

Many studies have documented the existence of millennial-scale variability in the Earth system during the last glacial period. An increasing number of studies document the occurrence of similar millennial variability during glacial periods previous to the last one. Here we use the simplest possible thermal-bipolar seesaw model to consider this variability for the last four glacial periods. We invert this model and use the high-pass filtered Vostok stable isotope records to make a first, tentative, attempt to estimate high-latitude N. Hemisphere temperature variability over the last four glacial periods, beyond the reach of Greenland ice-core records. The model result is compared against the Vostok methane record, which shows rapid variations in parallel to Greenland temperature records during the last glacial period. A further comparison is carried out against the planktonic oxygen isotope of north Atlantic core ODP 980. There is agreement between the records on the existence of similar millennial-scale variability during the last three glacial periods with very similar characteristics to the variability during the last glacial cycle.     

Impact of soil map specifications for European climate simulations

Soil physical characteristics can influence terrestrial hydrology and the energy balance and may thus affect land–atmosphere exchanges. However, only few studies have investigated the importance of soil textures for climate. In this study, we examine the impact of soil texture specification in a regional climate model. We perform climate simulations over Europe using soil maps derived from two different sources: the soil map of the world from the Food and Agricultural Organization and the European Soil Database from the European Commission Joint Research Center. These simulations highlight the importance of the specified soil texture in summer, with differences of up to 2 °C in mean 2-m temperature and 20 % in precipitation resulting from changes in the partitioning of energy at the land surface into sensible and latent heat flux. Furthermore, we perform additional simulations where individual soil parameters are perturbed in order to understand their role for summer climate. These simulations highlight the importance of the vertical profile of soil moisture for evapotranspiration. Parameters affecting the latter are hydraulic diffusivity parameters, field capacity and plant wilting point. Our study highlights the importance of soil properties for climate simulations. Given the uncertainty associated with the geographical distribution of soil texture and the resulting differences between maps from different sources, efforts to improve existing databases are needed. In addition, climate models would benefit from tackling unresolved issues in land-surface modeling related to the high spatial variability in soil parameters, both horizontally and vertically, and to limitations of the concept of soil textural class.     

Use of voltammetric solid-state (micro)electrodes for studying biogeochemical processes: Laboratory measurements to real time measurements with an in situ electrochemical analyzer (ISEA)

Solid-state voltammetric (micro)electrodes have been used in a variety of environments to study biogeochemical processes. Here we show the wealth of information that has been obtained in the study of sediments, microbial mats, cultures and the water column including hydrothermal vents. Voltammetric analyzers have been developed to function with operator guidance and in unattended mode for temporal studies with an in situ electrochemical analyzer (ISEA). The electrodes can detect the presence (or absence) of a host of redox species and trace metals simultaneously. The multi-species capacity of the voltammetric electrode can be used to examine complex heterogeneous environments such as the root zone of salt marsh sediments. The data obtained with these systems clearly show that O₂ and Mn²⁺ profiles in marine sedimentary porewaters and in microbial biofilms on metal surfaces rarely overlap indicating that O₂ is not a direct oxidant for Mn²⁺. This lack of overlap was suggested originally by Joris Gieskes' group. In waters emanating from hydrothermal vents, Fe²⁺, H₂S and soluble molecular FeS clusters (FeS_aq) are detected indicating that the reactants for the pyrite formation reaction are H₂S and soluble molecular FeS clusters. Using the ISEA with electrodes at fixed positions, data collected continuously over three days near a Riftia pachyptila tubeworm field generally show that O₂ and H₂S anti-correlate and that H₂S and temperature generally correlate. Unlike sedimentary environments, the data clearly show that Riftia live in areas where both O₂ and H₂S co-exist so that its endosymbiont bacteria can perform chemosynthesis. However, physical mixing of diffuse flow vent waters with oceanic bottom waters above or to the side of the tubeworm field can dampen these correlations or even reverse them. Voltammetry is a powerful technique because it provides chemical speciation data (e.g.; oxidation state and different elemental compounds/ions) as well as quantitative data. Because (micro)organisms occupy environmental niches due to the system's chemistry, it is necessary to know chemical speciation. Voltammetric methods allow us to study how chemistry drives biology and how biology can affect chemistry for its own benefit.     

Terrigenous input response to glacial/interglacial climatic variations over southern Baja California: a rock magnetic approach

The sediments deposited off south-western Baja California have recorded millennial-scale fluctuations in biogenic compounds and trace metals accumulation during the last glacial period [Ortiz, J.D., O’Connell, S.B., DelViscio, J., Dean, W.E., Carriquiry, J.D., Marchitto, T., Zheng, Y., van Geen, A., 2004. Enhanced marine productivity off western North America during warm climate intervals of the past 52 ky. Geology 32, 521–524; Dean, W.E., Zheng, Y., Ortiz, J.D., van Geen, A., 2006. Sediment Cd and Mo accumulation in the oxygen-minimum zone off western Baja California linked to global climate over the past 52 ka. Paleoceanography 21, PA4209]. Since the variations in trace metals concentration appear to result from dilution with nonbiogenic matter, the variability of the terrigenous sedimentation needs to be addressed. Therefore, we performed rock magnetic and geochemical analysis on a 38 m-long sediment core collected from the slope off Baja California. The temporal framework provided by ¹⁴C dating and identification of palaeoclimatic transitions allows assigning the sequence to the last glacial–interglacial cycle (last 120 ka). The comparison of magnetic and geochemical properties led to retain the hypothesis of a primary modulation of iron oxides quantity and quality by terrigenous input variations, with a secondary diagenetic amplification. Two magnetic mineral input regimes are distinguished: (i) magnetic susceptibility variations reveal changes in titano-magnetite concentration related to fluvial transport of the terrigenous fraction; (ii) coercivity changes allow detecting variations of hematite or goethite concentrations, minerals generally issued from aeolian deflation of weathered rock surfaces. These two regimes are paced by two distinct climatic forcing: the millennial-scale changes in titano-magnetite input are related to the northern hemisphere climatic variability whilst the record of wind-blown magnetic mineral input contains its major power in the precessional frequency band, with higher input during low insolation periods (and conversely). This record highlights the great sensibility of this region to high and low latitudes climatic regimes.     

Tropical sea-surface temperatures during the last glacial period: a view based on alkenones in Indian Ocean sediments

The qualitification of tropical temperatures during the last glacial cycle (0-150 kyr BP) is a controversial issue since different proxies seem to provide conflicting informations. To obtain a complementary point of view, we use the alkenone method to estimate sea-surface temperatures and focus our work on deep-sea sediments recovered from the tropical Indian Ocean. We present alkenone data obtained in two cores which cover in detail the last deglaciation and in about twenty cores distributed between 20°S and 20°N that were chosen to evaluate the temperature contrast of the last glacial-interglacial transition. Our results indicate that Indian Ocean tropical temperatures remained an average within 1.5-2.5°C of their present values during the last glaciation. At 10°N the last deglaciation is characterized by two warming steps which is similar to the classical deglacial chronology observed in the North Atlantic area. At 20°S the deglacial warming occurred at ca. 15 cal kyr BP, lagging significantly (5-4 kyr) behind the Antarctic warming, but in phase with northern hemisphere time series.     

Numerical Treatment of Radiative Transfer

We present a numerical treatment of radiative transfer in three dimensions. The radiation is modeled by the grey moment M ₁ system. The introduced scheme is able to compute accurate numerical solutions over a broad class of regimes from the transport to the diffusive limit. We discuss numerical issues concerning the resolution and the parallelization of this scheme for multi-dimensional simulations. Several numerical results are then presented, which show that this approach is robust and have the correct behavior in both the diffusive and free-streaming limits. We also present a comparison in two dimensions of our code with a Monte-Carlo transfer code.