Anorthositic lunar regolith breccia Dhofar 1769—Clear indications for repeated mixing of impact melt lithologies     

Addi Bischoff  Mike Komnik  Jakob Storz  Jasper Berndt 《Meteoritics & planetary science》2023,58(9):1260-1273

The lunar regolith breccia Dhofar 1769, which was found in 2012 as a single 125 g piece in the Zufar desert area of Oman, contains a relatively large, dark-colored impact melt breccia embedded in a fine-grained clastic matrix. The internal texture of the fragment indicates the repeated melt breccia formation on the lunar surface, their repeated brecciation, and mixing in second, third, and fourth generations of brecciated rock types. The chemical and mineralogical data reveal the incorporation of a feldspar-rich subophitic crystalline melt within a feldspar-rich microporphyritic crystalline melt breccia. This lithic paragenesis itself is embedded within a mafic, crystalline melt breccia. The entire breccia with the three different impact melts has been finally incorporated into the whole rock breccia. The three impact melts are mixtures of different source rocks and impact projectiles, based on the obtained minor and trace element compositions (in particular of Ni and the rare earth elements [REE]) of the impact melt lithologies. For all processes of impact melt formation, additional steps of their brecciation and re-lithification require a minimum number of seven impact processes.  相似文献   

Sidescan backscatter variations of cold seeps on the Hikurangi Margin (New Zealand): indications for different stages in seep development     

Ines Dumke  Ingo Klaucke  Christian Berndt  Jörg Bialas 《Geo-Marine Letters》2014,34(2-3):169-184

Cold seeps on the Hikurangi Margin off New Zealand exhibit various seabed morphologies producing different intensity patterns in sidescan backscatter images. Acoustic backscatter characteristics of 25 investigated seep sites fall into four distinct types characterised by variations in backscatter intensity, distribution and inferred structural heights. The types reflect different carbonate morphologies including up to 20-m-high structures (type 1), low-relief crusts (type 2), scattered blocks (type 3) and carbonate-free sites (type 4). Each seep corresponds to a single type; intermediates were not observed. This correlates well with published data on seep fauna at each site, with the four types representing three different faunal habitats of successive stages of seep development. Backscatter signatures in sidescan sonar images of cold seeps may therefore serve as a convenient proxy for variations in faunal habitats.  相似文献   

Gas migration through Opouawe Bank at the Hikurangi margin offshore New Zealand     

Stephanie Koch  Henning Schroeder  Matthias Haeckel  Christian Berndt  Joerg Bialas  Cord Papenberg  Dirk Klaeschen  Andreia Plaza-Faverola 《Geo-Marine Letters》2016,36(3):187-196

This study presents 2D seismic reflection data, seismic velocity analysis, as well as geochemical and isotopic porewater compositions from Opouawe Bank on New Zealand’s Hikurangi subduction margin, providing evidence for essentially pure methane gas seepage. The combination of geochemical information and seismic reflection images is an effective way to investigate the nature of gas migration beneath the seafloor, and to distinguish between water advection and gas ascent. The maximum source depth of the methane that migrates to the seep sites on Opouawe Bank is 1,500–2,100 m below seafloor, generated by low-temperature degradation of organic matter via microbial CO₂ reduction. Seismic velocity analysis enabled identifying a zone of gas accumulation underneath the base of gas hydrate stability (BGHS) below the bank. Besides structurally controlled gas migration along conduits, gas migration also takes place along dipping strata across the BGHS. Gas migration on Opouawe Bank is influenced by anticlinal focusing and by several focusing levels within the gas hydrate stability zone.  相似文献   

Factors affecting the movement and persistence of nitrate and pesticides in the surficial and upper Floridan aquifers in two agricultural areas in the southeastern United States     

B. G. Katz  M. P. Berndt  C. A. Crandall 《Environmental Earth Sciences》2014,71(6):2779-2795

Differences in the degree of confinement, redox conditions, and dissolved organic carbon (DOC) are the main factors that control the persistence of nitrate and pesticides in the Upper Floridan aquifer (UFA) and overlying surficial aquifer beneath two agricultural areas in the southeastern US. Groundwater samples were collected multiple times from 66 wells during 1993–2007 in a study area in southwestern Georgia (ACFB) and from 48 wells in 1997–98 and 2007–08 in a study area in South Carolina (SANT) as part of the US Geological Survey National Water-Quality Assessment Program. In the ACFB study area, where karst features are prevalent, elevated nitrate-N concentrations in the oxic unconfined UFA (median 2.5 mg/L) were significantly (p = 0.03) higher than those in the overlying oxic surficial aquifer (median 1.5 mg/L). Concentrations of atrazine and deethylatrazine (DEA; the most frequently detected pesticide and degradate) were higher in more recent groundwater samples from the ACFB study area than in samples collected prior to 2000. Conversely, in the SANT study area, nitrate-N concentrations in the UFA were mostly <0.06 mg/L, resulting from anoxic conditions and elevated DOC concentrations that favored denitrification. Although most parts of the partially confined UFA in the SANT study area were anoxic or had mixed redox conditions, water from 28 % of the sampled wells was oxic and had low DOC concentrations. Based on the groundwater age information, nitrate concentrations reflect historic fertilizer N usage in both the study areas, but with a lag time of about 15–20 years. Simulated responses to future management scenarios of fertilizer N inputs indicated that elevated nitrate-N concentrations would likely persist in oxic parts of the surficial aquifer and UFA for decades even with substantial decreases in fertilizer N inputs over the next 40 years.  相似文献   

Alteration of crystalline zircon solid solutions: a case study on zircon from an alkaline pegmatite from Zomba–Malosa, Malawi     

Aneesh Soman  Thorsten Geisler  Frank Tomaschek  Marion Grange  Jasper Berndt 《Contributions to Mineralogy and Petrology》2010,160(6):909-930

A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO₃ ²⁻, as suggested by the occurrence of REE carbonates and CO₂-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.  相似文献   

Relations between sinkhole density and anthropogenic contaminants in selected carbonate aquifers in the eastern United States   总被引：2，自引：1，他引：1  

Bruce D. Lindsey  Brian G. Katz  Marian P. Berndt  Ann F. Ardis  Kenneth A. Skach 《Environmental Earth Sciences》2010,60(5):1073-1090

The relation between sinkhole density and water quality was investigated in seven selected carbonate aquifers in the eastern United States. Sinkhole density for these aquifers was grouped into high (>25 sinkholes/100 km²), medium (1–25 sinkholes/100 km²), or low (<1 sinkhole/100 km²) categories using a geographical information system that included four independent databases covering parts of Alabama, Florida, Missouri, Pennsylvania, and Tennessee. Field measurements and concentrations of major ions, nitrate, and selected pesticides in samples from 451 wells and 70 springs were included in the water-quality database. Data were collected as a part of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Areas with high and medium sinkhole density had the greatest well depths and depths to water, the lowest concentrations of total dissolved solids and bicarbonate, the highest concentrations of dissolved oxygen, and the lowest partial pressure of CO₂ compared to areas with low sinkhole density. These chemical indicators are consistent conceptually with a conduit-flow-dominated system in areas with a high density of sinkholes and a diffuse-flow-dominated system in areas with a low density of sinkholes. Higher cave density and spring discharge in Pennsylvania also support the concept that the high sinkhole density areas are dominated by conduit-flow systems. Concentrations of nitrate-N were significantly higher (p < 0.05) in areas with high and medium sinkhole density than in low sinkhole-density areas; when accounting for the variations in land use near the sampling sites, the high sinkhole-density area still had higher concentrations of nitrate-N than the low sinkhole-density area. Detection frequencies of atrazine, simazine, metolachlor, prometon, and the atrazine degradate deethylatrazine indicated a pattern similar to nitrate; highest pesticide detections were associated with high sinkhole-density areas. These patterns generally persisted when analyzing the detection frequency by land-use groups, particularly for agricultural land-use areas where pesticide use would be expected to be higher and more uniform areally compared to urban and forested areas. Although areas with agricultural land use and a high sinkhole density were most vulnerable (median nitrate-N concentration was 3.7 mg/L, 11% of samples exceeded 10 mg/L, and had the highest frequencies of pesticide detection), areas with agricultural land use and low sinkhole density still were vulnerable to contamination (median nitrate-N concentration was 1.5 mg/L, 8% of samples exceeded 10 mg/L, and had some of the highest frequencies of detections of pesticides). This may be due in part to incomplete or missing data regarding karst features (such as buried sinkholes, low-permeability material in bottom of sinkholes) that do not show up at the scales used for regional mapping and to inconsistent methods among states in karst feature delineation.  相似文献   

Identification and significance of ice-flow-transverse subglacial ridges (Rogen moraines) in northern central Ireland     

Jasper Knight  A. Marshall McCabe 《第四纪科学杂志》1997,12(6):519-524

In the Omagh Basin, north central Ireland, subglacial diamict ridges lie transverse to southwestward Late Devensian (ca. 23–13 ka) ice flow. These ridges (0.5–2.5 km long, 100–450 m wide, 15–35 m high), are similar morphologically to Rogen moraines, which have not been described previously from the British Isles. The crests of some transverse ridges are streamlined, cross-cut or overprinted by drumlins, whereas other ridges are unmodified and were not affected by later drumlinisation. At Kilskeery, west–east trending eskers overlying unmodified transverse ridges post-date drumlinisation (17–14 ¹⁴C ka). Esker formation shows that the subglacial thermal regime changed from cold-based, favouring bedform preservation, to warm-based with meltwater flowing through enclosed subglacial channels. Patterns of flow-transverse-ridges and spatial variations in the degree of bedform modification record dynamic changes in regional subglacial environments during the last deglacial cycle. This ice-mass variability cannot be reconciled with current Irish glacial models, which are based on immobile ice centres and ordered stages of ice retreat. In a wider context, these changes in bedform patterns and basal ice regimes have a similar signature to millennial-scale ice-mass oscillations recorded by dated proxy evidence elsewhere in the amphi-North Atlantic. © 1997 John Wiley & Sons, Ltd.  相似文献