GeoResJ最新文献

High resolution analyses of an aggregate of aragonite crystals in a methane-derived carbonate nodule revealed evolution of interstitial water geochemistry associated with increases in methane flux at the Umitaka Spur gas seep site in the Sea of Japan. Geochemical data were obtained from the aggregate using Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), millimeter-scale powdering, and electron probe micro analysis. Most elements measured by LA-ICP-MS, and carbon and oxygen isotopic compositions (δ¹³C and δ¹⁸O, respectively) of aragonite, have symmetrical patterns in the aggregate. Concentrations of Ba, δ¹³C_CaCO3, and δ¹⁸O_CaCO3 increase monotonically towards the center of the aggregate (δ¹³C_CaCO3; from –12 ‰ to –4 ‰VPDB), while rare earth elements (REEs) and Mn oscillate. Iron peaks are located closest to those of light REEs, and the shale-normalized pattern of a REE peak event is enriched in middle REEs, suggesting dissociation of Fe-oxides as the source of the REEs. The monotonically changing geochemical data and fan-shaped, acicular aragonites growing inwardly from the aggregate rim, suggest aggregate formation from rim to center. Therefore, the peak position of Mn to the interior of that of Fe, suggests an increase in dissolved Mn after the dissociation of Fe-oxides. The isotopic trends continue into the surrounding matrix, where δ¹³C_CaCO3 reaches ∼–20 ‰VPDB, close to the δ¹³C of dissolved inorganic carbon currently found in the sulfate-methane transition (SMT). The trends of δ¹³C_CaCO3 and Ba, and the evidence of dissociation of Fe-oxides suggest upward migration of the SMT during carbonate cementation, which is initiated in the SMT.

Mid-Late Neoproterozoic to Early Paleozoic volcanism in the Qilianshan area, which shows systematic variations in space and time, is the volcanic response to the tectonic evolution of the Qilianshan. The volcanism gradually changed from continental rift-related and continental flood basaltic through MORB-type, island-arc and back-arc to post-collisional rift-related eruptions along with the tectonic evolution of the Qilianshan shifting from rifting and break-up of Rodinia through opening, spreading, subducting and closing of the Early Paleozoic oceans to arc-continent and continent-continent collision. The continental rift-related and flood lavas with ages of 850–604 Ma distribute mainly on the Qilian and Qiadam Blocks. The widespread MORB-type and “island-arc-backarc”-type lavas were generated from ∼550 to 446 Ma in both the North Qilian and the South Qilian ocean-trench-arc-basin systems. In the meantime the intracontinental rift-related volcanism occurred in the central Qilian Block between ∼522 and 442 Ma. The Early Paleozoic oceanic basins were closed at the end of Ordovician (∼446 Ma). Subsequent post-collisional vocanism occurred on the northern margin of the Qilian Block from ∼445 to ∼428 Ma. Such spatial-temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Qilianshan. These processes involve: (1) upwelling of mantle plumes or a mantle superplume and subsequent rifting and break-up of Rodinia and subsequent opening, spreading and subduction of Early Paleozoic oceans followed by island-arc formation, (2) roll-back of the subducted oceanic slabs followed by back-arc extension and back-arc basin formation, (3) ocean closure and slab break-off followed by upwelling of asthenosphere and post-collisional volcanism. Intensive orogenic activities occurred in the Late Silurian and Early Devonian (∼420 - ∼400 Ma) in response to the exhumation of the subducted crustal materials. Mountain collapse and lithosphere extension happened in the priod of ∼400–360 Ma and formed post-collisional granitic intrusions, which marked the end of a complete orogenic cycle.

This study examines gully erosion in northeast Tennessee hillslopes in the Southern Appalachian Valley and Ridge physiographic province, where a thick sequence of red clay Ultisols (Acrisol, according to the World Reference Base for Soil) overlies dolomite and limestone bedrock. The role of freeze-thaw processes in gully erosion was examined weekly from 6/3/2012 to 9/17/2014 using a network of n = 78 erosion pins in three geomorphic areas: channels, interfluves, and sidewalls. Freeze-thaw days were identified using meteorological data collected on site. When freeze-thaw days occurred, erosion and deposition increased and gully conditions were more dynamic. When daily temperature did not plunge below freezing, more stable gully conditions persisted. Ordinary Least Square regression models of erosion pin length using freeze-thaw events explained significant portions of variability in channels (R² = 0.113, p < 0.01), interfluves (R² = 0.141, p < 0.01), and sidewalls (R² = 0.263, p < 0.01). Repeat analysis on only the winter-spring months minimally improved the sidewall model (R² = 0.272, p < 0.01). Erosion in interfluves exhibited a lagged effect, and was best correlated to freeze-thaw events during the prior period while erosion in channels and sidewalls was related to freeze-thaw events in the current week. Of the three geomorphic areas studied, sidewall erosion was best modeled by freeze-thaw events which contribute to widening of gullies through mobilization of sediment and mass wasting. This research demonstrates that freeze-thaw processes are a significant contributor to erosion in gully channels, interfluves, and especially sidewalls, and therefore temperature variability should be considered in erosion studies in similar climates.

Coastal notches are used as paleo-sea level markers and to determine rates of tectonic uplift. This has been especially important in warm, microtidal seas. Modelling was used in this paper to test the hypothesis, developed in the Mediterranean, that the shape of notch profiles provides insights into changes in relative sea level (RSL) and modes of tectonic activity. Variables in the model included local factors such as the gradient of the initial slope, whether notches collapsed or remained stable, and rock strike, dip, and bed resistance to erosion. The main regional-scale variables included climatically induced changes in erosional efficacy and a variety of uniform and episodic, positive and negative changes in RSL. Model results suggest that attempts to use notch profiles to identify changes in climate and RSL must be accompanied by careful field observation and mineralogical analysis in order to extract the obfuscating effects of local factors. Similar notch profiles can be produced by different combinations of local and regional factors and, based on ambiguous field evidence, differentiating the morphological effect of changes in RSL from the effect of these other factors may continue to be problematic, especially where there has been low tectonic activity or stability.

Accurate measurements of volcanic ash morphology are critical to improving both our understanding of fragmentation processes and our ability to predict particle behaviour. In this study, we present new ways to choose and apply shape parameters relevant to volcanic ash characterisation. First, we compare shape measurements from different imaging techniques, including cross-sectional (2-D) and projected area images, and discuss their respective applications. We then focus on specific information that can be obtained from shape analysis of 2-D images. Using cluster analysis as an unbiased method to identify key controls on particle morphology, we find that four shape parameters – solidity, convexity, axial ratio, and form factor – can effectively account for the morphological variance within most ash samples. Importantly, these parameters are scaled to values between 0 and 1, and therefore contribute evenly to discrimination diagrams. In particular, co-variation in convexity and solidity can be used to distinguish different juvenile ash components based on characteristic bubble properties. By reducing observations of natural samples to simplified ash geometries, we quantify morphological changes associated with variations in the relative size and shape of bubbles and particles. Using this relationship, we assess the potential application of size-dependent shape analysis for inferring the underlying bubble size distribution, and thus the pre-fragmentation conditions. Finally, we show that particle shape analysis that includes the full range of available grain sizes can contribute not only measurements of particle size and shape, but also information on size-dependent densities.

Outflow channels on Mars reveal the past presence of water, possibly released from pressurized groundwater reservoirs. We aim to improve our understanding of such outflow systems in order to better constrain past hydrological conditions on Mars. We investigate the morphology of possible pressurized groundwater outflow systems on Mars and compare them to landscape evolution experiments. These experiments show that incised channels, like the classic outflow channels, form in a last, erosional, stage in morphological development. This is preceded by the formation of sedimentary lobes due to rapid water loss by infiltration. On Mars, we observe similar morphologies related to different stages of groundwater outflow in Lunae and Ophir Plana. In the experiments, pits formed by the pressure of the groundwater, whereas the pits in the source regions of the outflow channels relate to the regional tectonic structure and are not formed by groundwater alone. Faulting, subsidence and collapse likely triggered outflow from a pressurized aquifer. This scenario is consistent with the presence of one or several cryosphere-confined aquifers from the Early Hesperian to at least the middle Amazonian. A pronounced spatial trend of larger and further developed outflow systems at lower elevations suggests that features ranging from small lobes to large outflow channels were sourced from a common aquifer or from aquifers with similar pressures. The required cryosphere indicates a cold climate and enables groundwater outflow even under atmospheric conditions unfavorable for sustained presence of liquid water.

The Pacific Ocean is surrounded by subduction zone systems leading to a decreasing surface area as well as sub-surface mantle domain. In contrast, the Atlantic realm is characterized by passive margins and growing in size. To maintain global mass balance, the Caribbean and the Scotia Sea have been proposed as Pacific-to-Atlantic transfer channels for sub-lithospheric shallow mantle. We concentrate on the Caribbean here and test this idea by calculating the present-day regional dynamic topography in search of a gradual decrease from west to east that mirrors the pressure gradient due to the shrinkage of the Pacific. To calculate the dynamic topography, we isostatically correct the observed topography for sediments and crustal thickness variations, and compare the result with those predicted by lithospheric cooling models. The required age-grid was derived from our recently published reconstruction model. Our results confirm previous geochemical and shear-wave splitting studies and suggest some lateral asthenosphere flow away from the Galapagos hotspot. However, they also indicate that this flow is blocked in the Central Caribbean. This observation suggests that rather than through large scale Pacific-to-Atlantic shallow mantle flow, the global mass balance is maintained through some other process, possibly related to the deep mantle underneath Africa.

We study the influence of non-tidal loading by the Baltic Sea on GNSS daily coordinate time series. The momentary sea surface is estimated from hourly tide gauge recordings around the Baltic and the load is convolved with Green’s functions to determine 3-D deformation, gravity, potential and tilt effects at 193 stations around the Baltic. This paper concentrates on 3-D deformation at a small number of continuous GNSS stations. Daily coordinate time series based on both Precise Point Positioning (PPP) and double differences (DD) were used. We find that for the east component of inter-station vectors crossing the Baltic, up to 56% of the variance can be explained by the Baltic loading. In the north and up components the Baltic loading is not well detectable. We think that for the north component this is due to station positions, and for the up component also to interaction with regional atmospheric loading.

We report here on the chemical signature of degassing at Erebus lava lake associated with intermittent explosions and the return to passive conditions. Explosions caused by bubble bursts were frequent during the 2013 field season, providing the first opportunity to observe such activity since 2005–06. Several of the explosions were captured by multiple instruments including an open-path Fourier transform infrared spectrometer. Explosive bubble bursts and other transient degassing events are associated with gas compositions that are distinct from the usual range of passive degassing compositions. We set out to compare the chemical signature of explosive degassing during the 2005–06 and 2013 episodes, and to characterise the chemistry of gases emitted during the period of lake refilling after explosions. We found little change in the explosive gas chemistry between 2005–06 and 2013, suggesting reactivation of a common mechanism of gas segregation. Bubbles can be distinguished by their size and composition, the ranges of which are likely modified during ascent by gas–melt interaction and adiabatic expansion. The proportions of water, SO₂, and HCl in the emitted gas plume increase during the refill of the lake after explosions, as the lake is recharged by a combination of magma that has already partially degassed, and that vesiculates rapidly in response to the drop in magmastatic pressure at the lake.