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991.
992.
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 (δ13Cas) 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 δ13Cas 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   
993.
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   
994.
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   
995.
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   
996.
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   
997.
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-δ18ONO3 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   
998.
New conventional and sensitive high-resolution ion microprobe zircon U-Pb dating has led to a new understanding of the subdivision and evolution of the Amazon Craton during Precambrian time, with major improvements and changes made to the previous Rb-Sr based model. The interpretation of U-Pb and Sm-Nd isotopic data identifies eight main Precambrian tectonic provinces in the Craton, with ages ranging from 3.1 to 0.99 Ga. Some of the provinces were generated by accretional, arc-related processes (Carajás, Transamazonic, Tapajós-Parima and Rondônia-Juruena) and others by recycling of continental crust (Central Amazon, Rio Negro and Sunsas). The exposed Archean crust is restricted to the east (Carajás and south Amapá in Brazil) and north (Imataca in Venezuela) of the craton, indicating that the Amazon Craton is largely a Proterozoic crust. The Carajás-Imataca (3.10–2.53 Ga) and Transamazonian (2.25–2.00 Ga) Provinces are composed predominantly of granite-greenstone terranes. The Tapajós-Parima (2.10–1.87 Ga) and Rondônia-Juruena (1.75–1.47 Ga) Provinces represent new crust added as orogenic belts, while the Rio Negro (1.86–1.52 Ga) and Sunsas (1.33–0.99 Ga) Provinces originated mainly by magmatic-tectonic recycling of the above two orogenic belts. The only zone with a prominent northeast trend is the poorly known K'Mudku Shear Belt, characterized by a 1.20 Ga shear zone which deforms the rocks of at least three different provinces (Rio Negro, Tapajós-Parima and Transamazonic). The Central Amazon Province comprises mostly Orosirian volcano-plutonic rocks (Uatumã Magmatism) and is a terrane in which the exposed crustal structure and deformation are pluton-related. The Sm-Nd TDM model ages and Nd suggest that the Central Amazon Province was generated by the partial melting of Archean continental crust (Carajás Province?), perhaps related to underplating that began at the end of the Tapajós-Parima Orogeny (1.88–1.86 Ga).  相似文献   
999.
Spectral properties, magnetic fields, and dust transport at lunar swirls   总被引:1,自引:0,他引:1  
Lunar swirls are albedo anomalies associated with strong crustal magnetic fields. Swirls exhibit distinctive spectral properties at both highland and mare locations that are plausibly explained by fine-grained dust sorting. The sorting may result from two processes that are fairly well established on the Moon, but have not been previously considered together. The first process is the vertical electrostatic lofting of charged fine dust. The second process is the development of electrostatic potentials at magnetic anomalies as solar wind protons penetrate more deeply into the magnetic field than electrons. The electrostatic potential can attract or repel charged fine-grained dust that has been lofted. Since the finest fraction of the lunar soil is bright and contributes significantly to the spectral properties of the lunar regolith, the horizontal accumulation or removal of fine dust can change a surface’s spectral properties. This mechanism can explain some of the spectral properties of swirls, accommodates their association with magnetic fields, and permits aspects of weathering by micrometeoroids and the solar wind.  相似文献   
1000.
Space weathering is now commonly accepted to modify the optical and magnetic properties of airless body regoliths throughout the Solar System. Although the precise formation processes are not well understood, the presence of ubiquitous sub-microscopic metallic iron (SMFe) grains in lunar soils and corresponding spectral analyses have explained both the unique optical and magnetic properties of such soils. More recently, a variety of ion irradiation, laser melting and vaporisation and impact experiments have been shown to reproduce these effects in the laboratory. Such experiments are crucial to the study of the formation of SMFe under controlled conditions. To date, more emphasis has been placed on optical analyses of laboratory samples, as these address directly the mineralogical interpretation of remote sensing data. However, the magnetic analyses performed on the Apollo and Luna samples have provided useful qualitative and quantitative evaluation of regolith metallic iron content. These techniques are reviewed here, demonstrated on pulsed laser irradiated olivine powder, and their utility for determining the quantity and size distribution of this metallic iron discussed. Ferromagnetic resonance, multi-frequency magnetic susceptibility, vibrating sample magnetometry and thermomagnetic measurements were carried out. Each showed trends expected for the conversion of paramagnetic Fe2+ in olivine to fine-grained Fe0, with some grains in the superparamagnetic size range. Although evidence for superparamagnetic iron was found, the quantity of sub-microscopic metallic iron produced in these experiments proved insufficient to make conclusive measurements of either the quantity or size distribution of this iron. Improvements to both the experimental and analytical procedures are discussed to better enable such measurements in the future.  相似文献   
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