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The Late Glacial (14 700–11 700 cal BP) is a key climate period marked by rapid but contrasted changes in the Northern Hemisphere. Indeed, regional climate differences have been evidenced during the Late Glacial in Europe and the northern Mediterranean. However, past climate patterns are still debated since temperature and precipitation changes are poorly investigated towards the lower European latitudes. Lake Matese in southern Italy is a key site in the central Mediterranean to investigate climate patterns during the Late Glacial. This study aims to reconstruct climate changes and their impacts at Matese using a multi-proxy approach including magnetic susceptibility, geochemistry (XRF core scanning), pollen data and molecular biomarkers like branched glycerol dialkyl glycerol tetraethers (brGDGTs). Paleotemperatures and paleo-precipitation patterns are quantitatively inferred from pollen assemblages (multi-method approach: modern analogue technique, weighted averaging partial least-squares regression, random forest and boosted regression trees) and brGDGT calibrations. The results are compared to a latitudinal selection of regional climate reconstructions in Italy to better understand climate processes in Europe and in the circum-Mediterranean region. A warm Bølling–Allerød and a marked cold Younger Dryas are revealed in all climate reconstructions inferred from various proxies (chironomids, ostracods, speleothems, pollen, brGDGTs), showing no latitudinal differences in terms of temperatures across Italy. During the Bølling–Allerød, no significant changes in terms of precipitation are recorded; however, a contrasted pattern is visible during the Younger Dryas. Slightly wetter conditions are recorded south of 42 ∘ N, whereas dry conditions are recorded north of 42 ∘ N. During the Younger Dryas, cold conditions can be attributed to the southward position of North Atlantic sea ice and of the polar frontal jet stream, whereas the increase in precipitation in southern Italy seems to be linked to relocation of Atlantic storm tracks into the Mediterranean, induced by the Fennoscandian ice sheet and the North European Plain. By contrast, warm conditions during the Bølling–Allerød can be linked to the northward position of North Atlantic sea ice and of the polar frontal jet stream. [ABSTRACT FROM AUTHOR]

The most profound consequences of the presence of Ca–Mg carbonates (CaCO 3 –MgCO 3) in the Earth's upper mantle may be to lower the melting temperatures of the mantle and control the melt composition. Low-degree partial melting of a carbonate-bearing mantle produces CO 2 -rich, silica-poor melts compositionally imposed by the melting relations of carbonates. Thus, understanding the melting relations in the CaCO 3 –MgCO 3 system facilitates the interpretation of natural carbonate-bearing silicate systems. We report the melting relations of the CaCO 3 –MgCO 3 system and the partition coefficient of trace elements between carbonates and carbonate melt from experiments at high pressure (6 and 9 GPa) and temperature (1300–1800 ∘ C) using a rocking multi-anvil press. In the absence of water, Ca–Mg carbonates are stable along geothermal gradients typical of subducting slabs. Ca–Mg carbonates (∼ Mg 0.1–0.9 Ca 0.9–0.1 CO 3) partially melt beneath mid-ocean ridges and in plume settings. Ca–Mg carbonates melt incongruently, forming periclase crystals and carbonate melt between 4 and 9 GPa. Furthermore, we show that the rare earth element (REE) signature of Group-I kimberlites, namely strong REE fractionation and depletion of heavy REE relative to the primitive mantle, is resembled by carbonate melt in equilibrium with Ca-bearing magnesite and periclase at 6 and 9 GPa. This suggests that the dolomite–magnesite join of the CaCO 3 –MgCO 3 system might be useful to approximate the REE signature of carbonate-rich melts parental to kimberlites. [ABSTRACT FROM AUTHOR]

The Lateglacial (14,700–11,700 cal BP) is a key climate period marked by rapid but contrasted changes in the Northern Hemisphere. Indeed, regional climate differences have been evidenced during the Lateglacial in Europe and the Northern Mediterranean areas. However, past climate patterns are still debated since temperature and precipitation changes are poorly investigated towards the lower European latitudes. Lake Matese in Southern Italy is a key site in the Central Mediterranean to investigate climate patterns during the Lateglacial. This study uses a multi-proxy approach including magnetic susceptibility, geochemistry (XRF core scanning), pollen data and molecular biomarkers like branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) to reconstruct climate changes and their impacts at Matese. Palaeotemperatures and -precipitation patterns are quantitatively inferred from pollen assemblages (multi-method approach: Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, and Boosted Regression Trees) and brGDGTs calibrations. The results are compared to a latitudinal selection of regional climate reconstructions in Italy to better understand climate processes in Europe and in the circum-Mediterranean region. A warm Bølling–Allerød and a marked cold Younger Dryas are revealed in all climate reconstructions inferred from various proxies (chironomids, ostracods, speleothems, pollen, brGDGTs), showing no latitudinal differences in terms of temperatures across Italy. During the Bølling–Allerød, no significant changes in terms of precipitation are recorded, however, a contrasted pattern is visible during the Younger Dryas. Slightly wet conditions are recorded south of latitude 42° N whereas dry conditions are recorded north of latitude 42° N. During the Younger Dryas, cold conditions can be attributed to the southward position of North Atlantic sea-ice and of the Polar Frontal JetStream whereas the increase of precipitation is Southern Italy seems to be linked to relocation of Atlantic storm tracks into the Mediterranean, induced by the Fennoscandian ice sheet and the North European Plain. On the contrary, during the Bølling–Allerød warm conditions can be linked to the northward position of North Atlantic sea-ice and of the Polar Frontal JetStream. [ABSTRACT FROM AUTHOR]

The dehydration reaction of phase A + high P clinoenstatite to forsterite + water was experimentally investigated at water-saturated conditions in the pressure range between 7.0 and 10.0 GPa by in situ reversal runs in a multi-anvil press at the synchrotron source of PETRA III in Hamburg. By using closed watertight X-ray transparent Ti capsules, its position is determined by reversal brackets at 8.3 GPa (700–760 ∘ C), 8.6 GPa (700–740 ∘ C), and 9.8 GPa (750–800 ∘ C); thus, the equilibrium of the reaction corresponds ideally to the data reported by Wunder (1998). Optical investigations of the quenched product phases show strong grain coarsening of phase A and clinoenstatite, whereas nucleated forsterite from the breakdown of the aforementioned phases is very fine grained. This corresponds to recent experimental observations that the grain size of phases formed in hydration reactions are significantly larger than those from dehydration reactions. In addition, we performed three time-dependent in situ experiments at 9–10 GPa and 800–870 ∘ C and monitored the reaction progress every 10 min to determine the kinetics of the forsterite formation from phase A + high P clinoenstatite. The growth of forsterite at these P – T conditions, already visible after 10 min, confirms the results of the bracketing experiments. However, the reaction is extremely slow, and even after more than 3 h, significant amounts of phase A and high P clinoenstatite are still present. This is in contradiction to other dehydration reactions of former experimental studies, e.g. the fast dehydration of serpentine, which completely dehydrates within 3 h, even at much lower temperatures, closely overstepping serpentine stability. Despite its reaction sluggishness, which would contradict the concept of earthquake initiation, the observed formation of nano-sized forsterite as a dehydration product may still indicate the potential of this reaction to cause mechanical instabilities and, thus, seismicity within cold subduction zones at depths of the Earth's mantle. Additionally, at depths exceeding serpentine dehydration, the phase A + high P /low P clinoenstatite breakdown to forsterite + water might induce geochemical and geophysical processes, including the formation of low-velocity zones within the overlying mantle wedge from the large amounts of fluid liberated by this water line reaction. After the breakdown of antigorite, the assemblage phase A + clinoenstatite might act as a bridge to transport water to larger depths during cold subduction, followed by the formation of other hydrous high P phases. [ABSTRACT FROM AUTHOR]

The reaction of 3.65 Å phase <=> clinoenstatite + water was investigated in five experiments at 10GPa, 470-600 °C, using a rotating multi-anvil press. Under these P/T conditions, clinoenstatite exists in its high-pressure modification, which, however, is not quenchable to ambient conditions but transforms back to low-pressure clinoenstatite. The quenched run products were characterized by electron microprobe analyses (EMPA), powder X-ray diffraction (XRD), Raman spectroscopy and by high-resolution transmission electron microscopy (HRTEM) on focused ion beam (FIB)-cut foils. We bracketed the reaction in the T range 470 to 510 °C (at 10 GPa). The hydration of clinoenstatite to the 3.65 Å phase at 470 °C was very sluggish and incomplete even after 96 h. Clinoenstatites range in size from less than 1 to up to 50 µm. Usually clinoenstatite has a very small grain size and shows many cracks. In sub-micron-sized broken clinoenstatite, an amorphous phase (0.91Mg : 1.04Si, with about 20 wt % H2O) was observed, which further transformed with increasing reaction time into the 3.65 Å phase (1Mg : 1Si, with 34 wt % H2O). Thus, the sub-micron-sized fractured clinoenstatite transformed via an amorphous water-bearing precursor phase to the 3.65 Å phase. The dehydration to clinoenstatite was faster but still incomplete after 72 h at 600 °C. From the backscattered electron images of the recovered sample of the dehydration experiment, it is obvious that there is a high porosity due to dehydration of the 3.65 À phase. Again, the grain size of clinoenstatite ranges from less than 1 up to 50 µm. There are still some clinoenstatite crystals from the starting material present, which can clearly be distinguished from newly formed sub-micron-sized clinoenstatite. Additionally, we observe a water-rich crystalline phase, which does not represent the 3.65 Å phase. Its Raman spectra show the double peaks around 700 and 1000 cm-1 characteristic for enstatite and strong water bands at 3700 and 3680 cm-1. The Mg : Si ratio of 0.90 : 1.04 was determined by EMPA, totalling to 81 wt %, in accordance with its high water content. Diffraction patterns from high-resolution images (fast Fourier transform - FFT) are in agreement with an orthoenstatite crystal structure (Pbca). The surprising observation of this study is that, in both directions of the investigated simple reaction, additional metastable phases occur which are amorphous in the hydration and crystalline in the dehydration reaction. Both additional phases are water rich and slightly deviate in composition from the stable products 3.65 À phase and clinoenstatite, respectively. Thus, as a general remark, conventional investigations on reaction progress should be complemented by nanoscale investigations of the experimental products because these might reveal unpredicted findings relevant for the understanding of mantle processes. The extreme reduction in grain size observed in the dehydration experiments due to the formation of nanocrystalline clinoenstatite rather than the slowly released fluids might cause mechanical instabilities in the Earth's mantle and, finally, induce earthquakes. [ABSTRACT FROM AUTHOR]