Seasonal variability of soil phosphate stable oxygen isotopes in rainfall manipulation experiments

Phosphorus (P) availability limits productivity in many ecosystems worldwide. As a result, improved understanding of P cycling through soil and plants is much desirable. The use of the oxygen isotopes associated to phosphate can be used to study the cycle of P in terrestrial systems. However, changes with time in the oxygen isotopes associated to available P have not yet been evaluated under field conditions. Here we present the variations in available-P oxygen isotopes, based on resin extractions, in a semi-arid site that included plots in which the amount of rainfall reaching the soil was modified. In addition, the oxygen isotopes in the less dynamic fraction which is extractable by HCl, were also measured. The δ¹⁸O of the HCl-extractable phosphate shows no seasonal pattern and corresponds to the average value of the available phosphate of 16.5‰. This value is in the expected range for equilibration with soil water at the prevailing temperatures in the site. The δ¹⁸O values of resin-extractable P showed a range of 14.5-19.1‰ (SMOW), and evidence of seasonal variability, as well as variability induced by rainfall manipulation experiments. We present a framework for analyzing the isotopic ratios in soil phosphate and explain the variability as mainly driven by phosphate equilibration with soil water, and by the isotopic effects associated with extracellular mineralization. Additional isotopic effects result from fractionation in uptake, and the input to the soil of phosphate equilibrated in leaves. These results suggest that the δ¹⁸O of resin-extractable P is an interesting marker for the rate of biological P transformations in soil systems.     

Palaeoenvironmental significance of the mammalian faunas of Italy since the Pliocene

The evolution of mammalian communities is a significant tool for reconstructing past environments and climate. In this paper, a palaeoecological and palaeoenvironmental reconstruction based on the study of the Italian mammal faunas ranging from Middle Pliocene to the Holocene is presented using the cenogram method and quantification of temperatures based on arvicolid species richness. These analyses reveal open and arid conditions during glacial periods and less open and more humid conditions during interglacials, with some differences between north and central–south Italy. Northern and Southern communities reflect similar trends in the evolution of palaeoenvironments and in the variation of the temperatures, but differences in estimated temperatures, which indicate a more temperate climate for southern Italy throughout the period, are considered here. Copyright © 2002 John Wiley & Sons, Ltd.     

Dysaerobic conditions during Heinrich events 4 and 5: Evidence from phosphorus distribution in a North Atlantic deep-sea core

Reactive phosphorus undergoes diagenetic transformation once transferred into marine sediments. The degree of regeneration and redistribution of phosphorus depends on early diagenetic and environmental conditions, which may be linked to larger scale phenomena, such as bottom water circulation, water column ventilation, and organic carbon flux. Phosphorus phases of the <50-μm-sized fraction of deep-sea sediments from core SU 90-09 (North Atlantic, 43°31′N, 30°24′W, 3375 m below sea level) have been analyzed using a sequential extraction technique (SEDEX method) to reconstruct phosphorus geochemistry during Heinrich events 4 and 5. Comparison with Holocene samples from the same site indicates that postdeposition diagenetic transformation has not affected phosphorus distribution in the deep part of the sediments. Total and reactive phosphorus average 0.40 ± 0.04 mg/g and 0.30 ± 0.05 mg/g, respectively, and are comparable to values found in analog deep-sea environments in the North Atlantic. Detrital phosphorus, the phase linked to igneous- and metamorphic-derived material, sharply increases during Heinrich events and covaries with the ice-rafted debris record, whereas authigenic and Fe-bound phosphorus phases, both influenced by redox conditions, decrease or even disappear. These findings suggest that during the deposition of Heinrich layers (HLs), environmental parameters hampered the precipitation of these phases. Large freshwater discharges in relation to iceberg surges may have provoked a temporary stratification of the water column. Accordingly, dysaerobic conditions in the sediments may have fostered the loss of dissolved phosphorus from the sediments to the water column, in a direct and rapid response to the changed conditions. Decreasing trends in organic matter elemental ratios (total organic carbon/organic phosphorus) and Rock-Eval oxygen index values, along with the presence of partly authigenic dolomite and ankerite within HLs, also support this assumption.     

Non-linear crustal corrections in high-resolution regional waveform seismic tomography

We compare 3-D upper mantle anisotropic structures beneath the North American continent obtained using standard and improved crustal corrections in the framework of Non-linear Asymptotic Coupling Theory (NACT) applied to long period three component fundamental and higher mode surface waveform data. Our improved approach to correct for crustal structure in high-resolution regional waveform tomographic models goes beyond the linear perturbation approximation, and is therefore more accurate in accounting for large variations in Moho topography within short distances as observed, for instance, at ocean–continent margins. This improved methodology decomposes the shallow-layer correction into a linear and non-linear part and makes use of 1-D sensitivity kernels defined according to local tectonic structure, both for the forward computation and for the computation of sensitivity kernels for inversion. The comparison of the 3-D upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected. However, significant variations are observed in the retrieved 3-D perturbations. The largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle, as would be expected. We suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow-layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity.     

Seismic response of multi‐supported structures by proper orthogonal decomposition

The seismic analysis of structures is usually carried out considering the ground motion as fully‐correlated in space and determining the structural response by pseudo‐deterministic methods such as the response spectrum technique. Actually, the partial correlation of the seismic acceleration may influence heavily the behaviour of spatially extended structures, such as bridges, viaducts or pipelines. In order to take its partial correlation into account, the seismic ground motion is schematized as a stochastic process dependent on time and on space; the hypotheses of stationarity and homogeneity are used to obtain simple and general results. The influence of the partial correlation of the seismic ground motion on the structural response is investigated by introducing suitable Equivalent Spectra. The acceleration of the support‐points of the structure is represented by the Proper Orthogonal Decomposition (POD), defining the modes of the earthquake. The method is formulated for any kind of multi‐degree‐of‐freedom system and is applied, as a case study, to an ideal single‐storey multi‐supported frame with an axially rigid beam. In the case of two supports, the POD decouples the pseudo‐static and the dynamic contributions to the structural response. This property is preserved for structural systems with many supports, where only the lower modes of the earthquake, usually the first two POD modes, are responsible for the structural response. Copyright © 2003 John Wiley & Sons, Ltd.     

Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanc?k region in NW Turkey and discuss their petrogenetic–tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the ?zmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al₂O₃ contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al₂O₃ contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanc?k peridotites formed by different degrees of partial melting (~%10–27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4–11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanc?k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanc?k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the ?zmir-Ankara-Erzincan suture zone.     

Degassing, crystallization and eruption dynamics at Stromboli: trace element and lithium isotopic evidence from 2003 ashes

During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity, Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards. Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses, along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower δ⁷Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ⁷Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined with a degassing model based on open-system Rayleigh distillation and on the assumption that ^melt/fluidD_Li > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre- and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system.