Laboratory Tests on Thin Spray-On Liner Penetrated Rock Joints in Direct Shear     

Qiuqiu Qiao  Jan Nemcik  Ian Porter  Ernest Baafi 《Rock Mechanics and Rock Engineering》2015,48(5):2173-2177

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An improved glacial isostatic adjustment model for the British Isles     

Sarah L. Bradley  Glenn A. Milne  Ian Shennan  Robin Edwards 《第四纪科学杂志》2011,26(5):541-552

The glacial isostatic adjustment (GIA) of the British Isles is of interest due to the constraints that can be provided on key model parameters such as the global meltwater signal, local ice sheet history and viscoelastic earth structure. A number of recent studies have modelled relative sea‐level (RSL) data from this region to constrain model parameters. As indicated in these studies, the sensitivity of these data to both local and global parameters results in a highly non‐unique problem. This study aims to address this inherent non‐uniqueness by combining a previously published British–Irish ice model that is based on the most recent geomorphological data with a new global ice sheet model that provides an accurate prediction of eustatic sea‐level change. In addition, constraints from Global Positioning System (GPS) measurements of present‐day vertical land motion are considered alongside the entirety of RSL data from both Great Britain and Ireland. A model solution is found that provides a high‐quality fit to both the RSL data and the GPS data. Within the range of earth viscosity values considered, the optimal data model fits were achieved with a relatively thin lithosphere (71 km), upper mantle viscosities in the range 4–6 × 10²⁰ Pa s and lower mantle viscosities ≥ 3 × 10²² Pa s. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Integrated measurements of acoustical and optical thin layers I: Vertical scales of association     

Kelly J. Benoit-Bird  Mark A. Moline  Chad M. Waluk  Ian C. Robbins 《Continental Shelf Research》2010,30(1):17-28

This study combined measurements from multiple platforms with acoustic instruments on moorings and on a ship and optics on a profiler and an autonomous underwater vehicle (AUV) to examine the relationships between fluorescent, bioluminescent, and acoustically scattering layers in Monterey Bay during nighttime hours in July and August of 2006 and May of 2008. We identified thin bioluminescent layers that were strongly correlated with acoustic scattering at the same depth but were part of vertically broad acoustic features, suggesting layers of unique composition inside larger biomass features. These compositional thin layers nested inside larger biomass features may be a common ecosystem component and are likely to have significant ecological impacts but are extremely difficult to identify as most approaches capable of the vertical scales of measurement necessary for the identification of sub-meter scale patterns assess bulk properties rather than specific layer composition. Measurements of multiple types of thin layers showed that the depth offset between thin phytoplankton and zooplankton layers was highly variable with some layers found at the same depth but others found up to 16 m apart. The vertical offset between phytoplankton and zooplankton thin layers was strongly predicted by the fraction of the water column fluorescence contained within a thin phytoplankton layer. Thin zooplankton layers were only vertically associated with thin phytoplankton layers when the phytoplankton in a layer accounted for more than about 18–20% of the water column chlorophyll. Trophic interactions were likely occurring between phytoplankton and zooplankton thin layers but phytoplankton thin layers were exploited by zooplankton only when they represented a large fraction of the available phytoplankton, suggesting zooplankton have some knowledge of the available food over the entire water column. The horizontal extent of phytoplankton layers, discussed in the second paper in this series, is likely an important factor contributing to this selective exploitation by zooplankton. The pattern of vertical offset between phytoplankton and zooplankton layers was consistent between studies in different years and using different combinations of platforms, indicating the importance of the relationship between zooplankton layers and the fraction of phytoplankton within a layer at night within Monterey Bay. These results highlight the value of integrating measurements of various types of organisms to understand thin layers processes and the importance of assessing ecological interactions in plankton thin layers within the context of the properties of the entire water column, like the animals themselves do.  相似文献   

Reply: Postglacial relative sea‐level observations from Ireland and their role in glacial rebound modelling     

Robin Edwards  Antony Brooks  Ian Shennan  Glenn Milne  Sarah Bradley 《第四纪科学杂志》2008,23(8):821-825

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Variable water column structure of the South Atlantic on glacial–interglacial time scales     

Gema Martínez-Méndez  Elizabeth G. Molyneux  Ian R. Hall  Rainer Zahn 《Quaternary Science Reviews》2009,28(27-28):3379-3387

The structure of the glacial ocean was significantly different to that of the present day with intermediate to mid-depth waters being notably more δ¹³C enriched than deep waters. This contrast was especially pronounced in the South Atlantic suggesting the development of a sharp chemical divide, or ‘chemocline’, at around 2500 m water depth between upper and lower layers, with implications for deep-ocean carbon storage [Hodell et al., 2003. Pleistocene vertical carbon isotope and carbonate gradients in the South Atlantic sector of the Southern Ocean. Geochemistry, Geophysics, Geosystems, 4(1): doi: 1004 10.1029/2002GC000367.]. We evaluate existing benthic foraminiferal δ¹³C, Cd/Ca and derived carbon isotope air–sea exchange signature (δ¹³C_as) data sets for the Atlantic during the Last Glacial Maximum (LGM), and Marine Isotope Stages (MIS) 6 and 8 in order to examine the regional extent of the chemocline in the South Atlantic. Benthic δ¹³C data north of the approximate latitude of the LGM Subantarctic Front (LGM-SAF, 43°S) linearly decrease with water depth, indicative of mixing between upper ‘well’ and lower ‘poorly’ ventilated water masses, with little evidence of the sharp chemical divide. Conversely, benthic δ¹³C data south of the LGM-SAF below 2500 m water depth are uniformly around ?0.8‰. The apparent δ¹³C gradient across the LGM-SAF suggests enhanced mid-depth ventilation north of the SAF and reduced ventilation to the south. From this pattern we conclude that the regional chemocline in the South Atlantic constituted a dominantly meridional, rather than a vertical gradient, and was developed during at least the past three glacial periods. Benthic Cd/Ca data indicate that the gradient was not nutrient related, although further data from the South Atlantic are needed for a better assessment of this suggestion. The combined benthic δ¹³C and Cd/Ca data indicate the source of well-ventilated upper waters in the South Atlantic changed from Northern Component Water (NCW) during early glacial phases to Upper Southern Component Water (USCW) during mid-to-late glacial phases when the Southern Ocean may have become isolated. USCW maintained a positive δ¹³C and δ¹³C_as signature simulating a North Atlantic origin that has been implicated in previous studies. The data demonstrate that secular imprints on δ¹³C must be taken into consideration when assessing the implications of the vertical δ¹³C gradient. This data also supports a variable water column architecture and modes of water mass formation as primary means to draw down atmospheric CO₂ and storage in the abyssal ocean by involving processes occurring on either side of the SAF in the glacial Southern Ocean.  相似文献   

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada     

Thomas James  Evan J. Gowan  Ian Hutchinson  John J. Clague  J. Vaughn Barrie  Kim W. Conway 《Quaternary Science Reviews》2009,28(13-14):1200-1216

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.  相似文献   

Climatic control of the surge periodicity of an Icelandic outlet glacier     

Johan Striberger  Svante Björck  Ívar Örn Benediktsson  Ian Snowball  Cintia B. Uvo  Ólafur Ingólfsson  Kurt H. Kjær 《第四纪科学杂志》2011,26(6):561-565

Surging outlet glaciers are important in draining large ice caps, but the mechanisms controlling surge periodicities are poorly known. We investigated a sediment sequence from the glacier‐fed Lake Lögurinn in eastern Iceland, and our unique annually resolved data, based on sedimentary varves, imply that Eyjabakkajökull, an outlet glacier of Vatnajökull, began surging about 2200 cal a BP (before 1950 AD). Approximately 1700 cal a BP, the glacier started to surge at a uniform 34‐ to 38‐year periodicity that prevailed until the coldest part of the Little Ice Age when the periodicity almost halved to 21–23 years. Since the late 1800 s the surge periodicity of Eyjabakkajökull has returned to a longer period of 35–40 years. We suggest that surge periodicities of Eyjabakkajökull are forced by climatically driven mass balance changes, which may be a common forcing factor for similar surge‐type outlet glaciers. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Ages (U–Pb SHRIMP and LA ICPMS) and stratigraphic evolution of the Neoproterozoic volcano-sedimentary successions from the extensional Camaquã Basin,Southern Brazil     

Liliane Janikian  Renato Paes de Almeida  Antonio Romalino Santos Fragoso-Cesar  Veridiana Teixeira de Souza Martins  Elton Luiz Dantas  Eric Tohver  Ian McReath  Manoel Souza D'Agrella-Filho 《Gondwana Research》2012,21(2-3):466-482

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation, which are best preserved in the 3000 km² Camaquã Basin. The interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In this contribution, we report new U–Pb and Sm–Nd geochronological constraints for the 605 to 580 Ma Bom Jardim Group, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositional patterns of these units reveal the transition from a restricted, fault-bounded basin into a wide, shallow basin. The expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm–Nd isotopic composition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement. Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas only the older Paleoproterozoic signature can be discerned in the eastern part of the basin.  相似文献   

Constraining groundwater flow,residence times,inter-aquifer mixing,and aquifer properties using environmental isotopes in the southeast Murray Basin,Australia   总被引：1，自引：0，他引：1  

Ian Cartwright  Tamie R. Weaver  Dioni I. Cendón  L. Keith Fifield  Sarah O. Tweed  Ben Petrides  Ian Swane 《Applied Geochemistry》2012

Environmental isotopes (particularly δ¹⁸O, δ²H, and δ¹³C values, ⁸⁷Sr/⁸⁶Sr ratios, and a¹⁴C) constrain geochemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province of the southern Murray Basin. Due to methanogenesis and the variable δ¹³C values of matrix calcite, δ¹³C values are highly variable and it is difficult to correct ¹⁴C ages using δ¹³C values alone. In catchments where δ¹³C values, ⁸⁷Sr/⁸⁶Sr ratios, and major ion geochemistry yield similar a¹⁴C corrections, ∼15% of the C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be used to correct ages in other catchments. Most groundwater has a¹⁴C above background (∼2 pMC) implying that residence times are <30 ka. Catchments containing saline groundwater generally record older ¹⁴C ages compared to catchments that contain lower salinity groundwater, which is consistent with evapotranspiration being the major hydrogeochemical process. However, some low salinity groundwater in the west of the Riverine Province has residence times of >30 ka probably resulting from episodic recharge during infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in ¹⁴C ages being poorly correlated with distance from the basin margins in many catchments; however, groundwater flow in palaeovalleys where the deeper Calivil–Renmark Formation is coarser grained and has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing. Despite the range of ages, δ¹⁸O and δ²H values of groundwater in the Riverine Province do not preserve a record of changing climate; this is probably due to the absence of extreme climatic variations, such as glaciations, and the fact that the area is not significantly impacted by monsoonal systems.  相似文献   

运城盆地地下水同位素年龄特征及其演化过程和可持续利用   总被引：1，自引：0，他引：1  

侯金武 《水文地质工程地质》2012,39(6):1-5

通过分析运城盆地地下水的碳同位素组成,结合水化学特征,揭示了盆地深层承压地下水的补给期为22～3 ka BP （现代碳百分比(a¹⁴C) 6~38 pmC）。浅层地下水（71~89 pmC）由现代水或现代水和老水混合组成。深层地下水氢氧同位素组成特征（δ¹⁸O~-10‰; δ²H~-70‰）表明地下老水在气候较冷的环境下受到补给,而浅层地下水的氢氧同位素组成（δ¹⁸O~-8‰; δ²H～-51‰）特征与现代西安降水组成相似。浅层地下水NO^-₃平均含量（31mg/L）比深层地下水（1.8 mg/L）高,硝酸盐的δ¹⁵N－δ¹⁸O_NO3 组成 (0‰～5‰）揭示了硝酸盐的主要来源为综合肥料。此外,浅层地下水的TDS由于蒸散发、矿物溶解,可达8.5 g/L（平均2.0 g/L）,深层地下老水TDS可达1.8 g/L（平均1.1g/L）水质相对较好。研究区目前主要开采深层地下水,受断裂带影响,浅层地下水已经侵入中深层地下水并与之发生混合,严重影响了中深层地下水的水质。如果发生大规模的浅层地下水与中深层地下水混合,会造成中深层地下老水的NO^-₃、TDS等含量越来越高。  相似文献