Evidence for an abrupt climate fluctuation c. 8,400 years ago in a new laser ablation δ¹⁸O record from S W Ireland

Evaluating the significance of Holocene submillennial δ 18 O variability in the Greenland ice cores is crucial for understanding how natural climate oscillations may modulate future anthropogenic warming. A high-resolution oxygen isotope record from a speleothem in southwestern Ireland provides evidence for centennial-scale δ 18 O variations that correlate with subtle δ 18 O changes in the Greenland ice cores, indicating regionally coherent variability in the early Holocene. Evidence for previously undetected early Holocene cooling events is presented, but mid- to late-Holocene ice rafting in the North Atlantic appears to have had little impact on δ 18 O at this ocean margin site. </jats:p

Isotopic relationships of volatile and lithophile trace elements in continental ultramafic xenoliths

The concentrations and isotopic compositions of Sr, Nd, Pb, He and C have been determined for suites of xenoliths from Bullenmerri (Australia), Ichinomegata (Japan), Geronimo (Arizona), and East Africa. The wehrlites and pyroxenites from Bullenmerri have Sr, Nd and Pb isotopic compositions that are generally similar to those found for alkali basalts in the region. The spinel lherzolites, in contrast, have higher 87Sr/86Sr and 206Pb/204Pb and lower 143Nd/144Nd ratios. Whereas the isotopic compositions of He are generally within the range of mid-ocean-ridge basalts (MORB) and do not covary with those of other trace elements, there is an apparent correlation between the 13C/12C and 143Nd/144Nd ratios for each of the two petrologic groups. These relationships, if substantiated for other xenolith suites, greatly limit the possible mechanisms for generating lithophile and volatile isotopic variations in the continental lithosphere. The helium isotopic compositions for all of the xenoliths fall within the range for MORB. This includes those from Ichinomegata, suggesting that the lower 3He/4He ratios found for He sampled at the surface at subduction zones result from mixing mantle He with near-surface crustal He rather than with subducted radiogenic He. Measured C isotopic compositions (relative to Peedee belemnite) for the Ichinomegata xenoliths {Mathematical expression} include values that are both lighter and heavier than those in MORB, and are compatible with contributions from subducted carbon. The Nd and Sr isotopic compositions of the Ichinomegata xenoliths exhibit a correlation over a substantially greater range of values than typically observed for other light-rare-earth-element (LREE)-depleted xenoliths, and include more radiogenic Sr and less radiogenic Nd compositions. The carbon isotopic compositions found for the East African and Geronimo xenoliths extend to values that are lighter than those typically found for MORB. © 1992 Springer-Verlag

The nature of the lithospheric mantle near the Tancheng-Lujiang fault, China: An integration of texture, chemistry and O-isotopes

A unique suite of basalt-borne sheared and granular spinel Iherzolites from the Yitong graben provides an insight into the nature of the lithospheric mantle adjacent to the Tancheng-Lujiang fault, northeastern China. Data obtained by ion microprobe (multi-elemental) and laser-fluorination (delta(18)O ratios) techniques have been integrated with thermobarometric data. Shallow (25-35 km), cool (742-967 degrees C), sheared peridotites contain clinopyroxenes with LREE-depleted profiles and minor enrichments in La, A marked difference in the delta(18)O ratios of porphyroclast and neoblast olivine in these sheared peridotites may be due to thermal re-equilibration or recent fluid activity. Deeper (50-60 km), hotter (950-1073 degrees C), granular peridotites contain LREE-depleted and enriched clinopyroxenes, the latter depleted in Ti and Zr. Melts in equilibrium with these LREE-enriched and Ti-Zr-depleted pyroxenes are very similar to potassic glasses in wehrlite xenoliths from the Yitong graben. On the basis of these data it is proposed that toward the base of the lithosphere hot, granular 'refractory' peridotites predominate and that they have been variably enriched by intergranular migration of small volume potassic melts. It is believed that the cold, sheared, peridotites from near the Moho were derived from a hot, granular, refractory precursor but that movement on the Tancheng-Lujiang fault, a major intracontinental shear zone, generated a 'sheared' texture and vertical displacement relocated these mantle rocks in a cooler part of the lithosphere

Accurate dating of stalagmites from low seasonal contrast tropical Pacific climate using Sr 2D maps, fabrics and annual hydrological cycles

Tropical Pacific stalagmites are commonly affected by dating uncertainties because of their low U concentration and/or elevated initial ²³⁰Th content. This poses problems in establishing reliable trends and periodicities for droughts and pluvial episodes in a region vulnerable to climate change. Here we constrain the chronology of a Cook Islands stalagmite using synchrotron µXRF two-dimensional mapping of Sr concentrations coupled with growth laminae optical imaging constrained by in situ monitoring. Unidimensional LA-ICP-MS-generated Mg, Sr, Ba and Na variability series were anchored to the 2D Sr and optical maps. The annual hydrological significance of Mg, Sr, Ba and Na was tested by principal component analysis, which revealed that Mg and Na are related to dry-season, wind-transported marine aerosols, similar to the host-rock derived Sr and Ba signatures. Trace element annual banding was then used to generate a calendar-year master chronology with a dating uncertainty maximum of ± 15 years over 336 years. Our approach demonstrates that accurate chronologies and coupled hydroclimate proxies can be obtained from speleothems formed in tropical settings where low seasonality and problematic U–Th dating would discourage the use of high-resolution climate proxies datasets

Accurate dating of stalagmites from low seasonal contrast tropical Pacific climate using Sr 2D maps, fabrics and annual hydrological cycles

Tropical Pacific stalagmites are commonly affected by dating uncertainties because of their low U concentration and/or elevated initial 230Th content. This poses problems in establishing reliable trends and periodicities for droughts and pluvial episodes in a region vulnerable to climate change. Here we constrain the chronology of a Cook Islands stalagmite using synchrotron µXRF two-dimensional mapping of Sr concentrations coupled with growth laminae optical imaging constrained by in situ monitoring. Unidimensional LA-ICP-MS-generated Mg, Sr, Ba and Na variability series were anchored to the 2D Sr and optical maps. The annual hydrological significance of Mg, Sr, Ba and Na was tested by principal component analysis, which revealed that Mg and Na are related to dry-season, wind-transported marine aerosols, similar to the host-rock derived Sr and Ba signatures. Trace element annual banding was then used to generate a calendar-year master chronology with a dating uncertainty maximum of ± 15 years over 336 years. Our approach demonstrates that accurate chronologies and coupled hydroclimate proxies can be obtained from speleothems formed in tropical settings where low seasonality and problematic U-Th dating would discourage the use of high-resolution climate proxies datasets

Hydrothermal deposits and associated basement rocks from the Galapagos spreading center

Relatively thick (up to 15 m) hydrothermal deposits from the southern flank of the Galapagos spreading center were drilled on a crust of about 0.62 myr old (magnetic age), during Leg 54 of the "Glomar Challenger" in the central Eastern Pacific Ocean. A stratigraphic sequence of ferrobasalt, pelagic oozes, and hydrothermal deposits of green clay-rich material and Fe-Mn concretions were recovered. The green clay-rich material (Fe-Si type of smectite) is also found interlayered between the pelagic ooze and filling veins, veinlets and vesicles in the drilled basement rocks. The low transitional metal content (Cu < 150 ppm). Ni < 100 ppm and Co < 200 ppm) and the nature of the green clay-rich material from the Galapagos deposits make them comparable to that of other hydrothermal products discovered from other oceanic ridges. In general, the basement rock has a higher transitional metal content than the overlying hydrothermal material. Hydrothermal fluids percolating from the basement through the sedimentary cover or precipitating from solutions emmanating from localized vents are proposed models for the origin of the hydrothermal layersLa découverte de couches relativement épaisses (jusqu'à environ 15 m) de matériel hydrothermal a pu être réalisée à l'aide de plusieurs forages profonds dans une croût de 0.62 ma pendant le Leg 54 du "Glomar Challenger" dans l'Océan Pacifique oriental. Ces forages ont permis de mettre en évidence une séquence stratigraphique allant du socle basaltique au sédiment pélagique et a des produits hydrothermaux faits de produit vert riche en argile et de concrétions de Fe-Mn. Les produits verts riches en argile (type de smectite riche en Fe et Si) sont interstratifiés dans du sédiment pélagique et existent aussi comme matériel de remplissage de veines et de vacuoles dans le socle basaltique. La faible teneur en éléments métalliques (Cu inf 150 ppm, Ni inf 100 ppm et Co inf 200 ppm) ainsi que le type d'argile rencontré dans les dépôts des Galapagos les rend semblables aux produits hydrothermaux découverts sur d'autres dorsales d'accrétion. Chaque trou atteignant le socle a révélé l'existence de basalte riche en Fe et Ti ayant une teneur d'éléments métalliques plus importante que celle trouvée dans les dépôts hydrothermaux qui les recouvrent. L'infiltration de fluide hydrothermal à travers la couverture sédimentaire ou la précipitation de solutions émanantes par des conduits localisés sont les modèles proposés pour expliquer les différents niveaux de produits hydrothermaux. [OCR NON CONTRÔLE