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Chemistry and mineralogy of outcrops at Meridiani Planum
No full textAnalyses of outcrops created by the impact craters Endurance, Fram and Eagle reveal the broad lateral continuity of chemical sediments at the Meridiani Planum exploration site on Mars. Approximately ten mineralogical components are implied in these salt-rich silicic sediments, from measurements by instruments on the Opportunity rover. Compositional trends in an apparently intact vertical stratigraphic sequence at the Karatepe West ingress point at Endurance crater are consistent with non-uniform deposition or with subsequent migration of mobile salt components, dominated by sulfates of magnesium. Striking variations in Cl and enrichments of Br, combined with diversity in sulfate species, provide further evidence of episodes during which temperatures, pH, and water to rock ratios underwent significant change. To first order, the sedimentary sequence examined to date is consistent with a uniform reference composition, modified by movement of major sulfates upward and of minor chlorides downward. This reference composition has similarities to martian soils, supplemented by sulfate anion and the alteration products of mafic igneous minerals. Lesser cementation in lower stratigraphic units is reflected in decreased energies for grinding with the Rock Abrasion Tool. Survival of soluble salts in exposed outcrop is most easily explained by absence of episodes of liquid H2O in this region since the time of crater formation
Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas
No full textRe-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187Os/188Os; however, the postorogenic lavas are characterized by radiogenic 187Os/188Os ratios (0.139-0.559). Simple modeling shows that only very large degrees of melt depletion (>25-30) can lower source Re/Os ratios sufficiently to permit time-integrated development of subchondritic 187Os/188Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere
Unexpected earthquake hazard revealed by Holocene rupture on the Kenchreai Fault (central Greece): Implications for weak sub-fault shear zones
No full textHigh-resolution elevation models, palaeoseismic trenching, and Quaternary dating demonstrate that the Kenchreai Fault in the eastern Gulf of Corinth (Greece) has ruptured in the Holocene. Along with the adjacent Pisia and Heraion Faults (which ruptured in 1981), our results indicate the presence of closely-spaced and parallel normal faults that are simultaneously active, but at different rates. Such a configuration allows us to address one of the major questions in understanding the earthquake cycle, specifically what controls the distribution of interseismic strain accumulation? Our results imply that the interseismic loading and subsequent earthquakes on these faults are governed by weak shear zones in the underlying ductile crust. In addition, the identification of significant earthquake slip on a fault that does not dominate the late Quaternary geomorphology or vertical coastal motions in the region provides an important lesson in earthquake hazard assessment
Aerosol forcing of the position of the intertropical convergence zone since ad 1550
No full textThe position of the intertropical convergence zone is an important control on the distribution of low-latitude precipitation. Its position is largely controlled by hemisphere temperature contrasts. The release of aerosols by human activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s (refs 1, 3, 4, 5, 6) by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval1, but this proposed shift remains equivocal. Here we reconstruct monthly rainfall over Belize for the past 456 years from variations in the carbon isotope composition of a well-dated, monthly resolved speleothem. We identify an unprecedented drying trend since ad 1850 that indicates a southward displacement of the intertropical convergence zone. This drying coincides with increasing aerosol emissions in the Northern Hemisphere and also marks a breakdown in the relationship between Northern Hemisphere temperatures and the position of the intertropical convergence zone observed earlier in the record. We also identify nine short-lived drying events since ad 1550 each following a large volcanic eruption in the Northern Hemisphere. We conclude that anthropogenic aerosol emissions have led to a reduction of rainfall in the northern tropics during the twentieth century, and suggest that geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics
Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India
No full textTwo types of El Niño events are distinguished by sea surface temperature (SST) anomalies centered in the central or eastern equatorial Pacific. The Central Pacific El Niño events (CP-El Niño) are more highly correlated with weakening of the central Indian Summer Monsoon and linked to decadal Pacific climate variability. We present a 50 year, subannually resolved speleothem δ18O record from northeast India that exhibits a significant correlation with northern Pacific decadal variability and central equatorial Pacific SSTs. Accordingly, we suggest that δ18O time series in similar northeast Indian speleothems are effective tools for investigating preinstrumental changes in Pacific climate, including changes in El Niño dynamics. In contrast to central India, rainfall amounts in northeast India are relatively unaffected by El Niño. However, back trajectory analysis indicates that during CP-El Niño events moisture transport distance to northeast India is reduced, suggesting that variations in moisture transport primarily control δ18O in the region
Reconstruction MPE colour matched
No full textIllustration from this published paper:
Ediacaran macrofossils from the Avalon Terrane (primarily eastern Newfoundland and the central UK) record some of the earliest large and complex multicellular organisms on Earth. Perhaps the greatest unknown regarding these fossils is their relevance to the early evolutionary history of the Kingdom Animalia. In recent years, new data and discoveries have revealed insights into Ediacaran paleobiology, taxonomic relationships, paleoecology and taphonomy, significantly refining our understanding of Avalonian ecosystems. Here, we summarise recent observational and quantitative studies, and their bearing on the current understanding of Avalonian benthic marine ecosystems. A review of existing knowledge of the biological composition of Avalonian marine assemblages demonstrates that they record densely-populated ecosystems inhabited by a diverse range of organisms, likely representing multiple biological Kingdoms. Appreciation of this diversity, and of the complexities it introduces to paleoecological studies, is vital when considering the relationship between macroevolution and contemporaneous climatic, tectonic and geochemical events. We then summarise current understanding of Avalonian paleoecology. Studies into locomotion, reproduction, feeding strategies, and community structure and succession reveal that these ecosystems were considerably different to Phanerozoic settings. Furthermore, we suggest that Avalonian ecosystems witnessed the appearance of novel nutrient sources, offering new opportunities and niches for benthic organisms. The suggestion that the numerically dominant rangeomorphs were osmotrophic is reviewed and appraised in light of geochemical, morphological, and biological information. Finally, the use of modern ecological metrics in the study of Ediacaran fossil assemblages is assessed. Concerns regarding the interpretation of paleoecological data are outlined in light of current taphonomic and sedimentological understanding, and these cast doubt on previous suggestions that the Avalonian assemblages were largely composed of metazoans. Nevertheless, we emphasise that if treated with necessary caution, paleoecological data can play a significant role in assisting efforts to determine the biological affinities of late Ediacaran macroscopic organisms
The crustal structure of the western Himalayas and Tibet
No full textWe present new, high-resolution, shear velocity models for the western Himalayas and West Tibet from the joint inversion of P receiver functions recorded using seismic stations from four arrays in this region and fundamental mode Rayleigh wave group velocity maps from 5–70 s covering Central and Southern Asia. The Tibetan Plateau is a key locality in understanding large-scale continental dynamics. A large number of investigations has examined the structure and processes in eastern Tibet; however, western Tibet remains relatively understudied. Previous studies in this region indicate that the western part of the Tibetan Plateau is not a simple extension of the eastern part. The areas covered by these arrays include the Karakoram and Altan-Tagh faults, and major terrane boundaries in West Tibet and the Himalayas. The arrays used include broadband data collected by the West Tibet Array, a U.S.-China deployment on the western side of the Tibetan Plateau between 2007 and 2011. We use the shear wave velocity models to obtain estimates of Moho depth. The Moho is deep (68–84 km) throughout West Tibet. We do not observe significant steps within the Moho beneath West Tibet. A large step in Moho depth is observed at the Altyn-Tagh fault, where Moho depths are 20–30 km shallower to the north of the fault compared to those to the south. Beneath the Lhasa Terrane and Tethyan Himalayas, we observe a low-velocity zone in the midcrust. This feature is not interrupted by the Karakoram Fault, suggesting that the Karakoram Fault does not cut through the entire crust
Architecture of North Atlantic Contourite Drifts Modified by Transient Circulation of the Icelandic Mantle Plume
No full textOverflow of Northern Component Water, the precursor of North Atlantic Deep Water, appears to have varied during Neogene times. It has been suggested that this variation is moderated by transient behavior of the Icelandic mantle plume, which has influenced North Atlantic bathymetry through time. Thus pathways and intensities of bottom currents that control deposition of contourite drifts could be affected by mantle processes. Here, we present regional seismic reflection profiles that cross sedimentary accumulations (Björn, Gardar, Eirik and Hatton Drifts). Prominent reflections were mapped and calibrated using a combination of boreholes and legacy seismic profiles. Interpreted seismic profiles were used to reconstruct solid sedimentation rates. Björn Drift began to accumulate in late Miocene times. Its average sedimentation rate decreased at ∼2.5 Ma and increased again at ∼0.75 Ma. In contrast, Eirik Drift started to accumulate in early Miocene times. Its average sedimentation rate increased at ∼5.5 Ma and decreased at ∼2.2 Ma. In both cases, there is a good correlation between sedimentation rates, inferred Northern Component Water overflow, and the variation of Icelandic plume temperature independently obtained from the geometry of diachronous V-shaped ridges. Between 5.5 and 2.5 Ma, the plume cooled, which probably caused subsidence of the Greenland-Iceland-Scotland Ridge, allowing drift accumulation to increase. When the plume became hotter at 2.5 Ma, drift accumulation rate fell. We infer that deep-water current strength is modulated by fluctuating dynamic support of the Greenland-Scotland Ridge. Our results highlight the potential link between mantle convective processes and ocean circulation
Single-step chromatographic isolation of lithium from whole-rock carbonate and clay for isotopic analysis with multi-collector ICP-mass spectrometry
No full textLithium isotope ratios play a key role in tracing weathering processes within the context of paleoclimatologic research. Therefore, accurate and precise determination of the isotopic composition of Li is required for a large variety of complex geological samples with widely different Li contents. This technical note describes the extension and fine-tuning of the existing cation exchange chromatographic procedures for isolation of Li to whole-rock carbonate and clay matrices prior to its isotopic analysis via multi-collector ICP-mass spectrometry. The method developed permits the isolation of ≥20 ng amounts of Li from carbonates and clays, by using a 25 cm long cylindrical column loaded with 8 and 2 mL of AG-50W-X8 cation exchange resin, respectively. A 0.5 M HCl solution is used as eluent. Li recoveries ≥99% were achieved and isotopic analysis using 10–20 ng of isolated Li resulted in accurate and precise δ7Li values – external precision (2SE) of 0.2‰ (N = 15). The analytical procedures developed allow Li isotope ratios to be investigated in parallel in both the clay and carbonate fractions of a geological sample within the context of paleoclimatological research