排序方式： 共有18条查询结果，搜索用时 15 毫秒
1.
The spatial variability of Holocene (past 10,000 years) sediment accumulation in Santa Monica Bay (California) was examined to identify controls sediment trapping in a bathymetrically complex coastal embayment and to provide geologic context for the post-industrial sedimentary record and associated pollution gradients. Sediment chronologies based on downcore AMS 14C dates were used to quantify long-term (millennia) accumulation rates in an effort to elucidate particle-transport pathways and sinks. Sediment accumulation rates for the full range of bayfloor environments (50-630 m water depths) range from 22 to 102 mg/cm(2)/year (15-88 mm/100 year), have an overall mean of 51+/-21 mg/cm(2)/year (1 sigma, n=11), and are comparable to rates reported for adjacent borderland basins. Maximal accumulation rates on the Malibu shelf and within a reentrant to Redondo canyon are interpreted to reflect (1) proximity to sediment sources and (2) localized oceanographic and topographic conditions conducive to sediment trapping and deposition. The 14C-derived accumulation rates are 2-10 times lower than rates determined through (210)Pb geochronology for the same sites in a related study, revealing that Holocene sediment accumulation has been non-steady-state. Santa Monica Bay is an important sink for suspended matter; averaged over the past several millennia a mass of sediment equivalent to 10-80% of the modern annual river supply is sequestered yearly. Net influx of suspended matter derived from the adjacent Palos Verdes shelf is evinced by a concentration gradient of p,p'-DDE in bayfloor sediments, whereas the distribution of anthropogenic silver suggests transport from Santa Monica shelf to the southeastern boundary of the bay. The results of this study provide new insight to the long-term fates of particulate matter in Los Angeles coastal waters. 相似文献
2.
Susanne M. Moskalski Christopher K. Sommerfield Kuo-Chuin Wong 《Estuaries and Coasts》2011,34(4):800-813
Influences of tides, freshwater discharge, and winds on water properties in the St. Jones River estuary (USA), a Delaware
National Estuarine Research Reserve, were investigated using multiyear records of sea level, salinity, and turbidity, supplemented
by a current profiler time series in 2007. Results demonstrate that instantaneous properties fluctuate with semidiurnal tides
and resonant overtides, whereas tidal mean variations are forced by seasonal freshwater inflow and offshore winds. Mean sea
level and salinity are highest in summer and vary with seasonal water temperature and rainfall, whereas sea level variability
and turbidity are highest in winter on account of storm effects. Salinity and discharge modeling suggest that much (43–65%)
of the freshwater resident in the estuary is derived from non-point sources below the head of tide. This diffuse freshwater
inflow produces a seaward surface slope and weak mean current, which temporarily reverses under the influence of storm–wind
setup within Delaware Bay. 相似文献
3.
ROBERT LAGERBÄCK 《Boreas: An International Journal of Quaternary Research》1988,17(4):469-486
The Veikimoraines in northernmost Sweden display a very conspicuous distribution pattern, sharply demarcated to the east and successively decreasing to the south, west and north. The sharp demarcation to the east is thought to reflect the front of a stagnant ice sheet. The downwasting of this glacier was retarded by the insulation of a thick superglacial debris cover and subarctic vegetation invaded at least parts of the slowly collapsing ice. Radiocarbon datings of organic matter deposited in connection with the formation of the Veiki moraine, lithostratigraphical evidence and the relation to other glacial features prove the Veiki moraine landscape to date from the deg laciation of the first Weichselian ice sheet, i.e. the Peräpohjola Interstadial. The good preservation of the features implies that in extensive areas of northern sweden the Early Weichselian glacial landscape escaped significant erosion despite being overrun by two later glaciers. Previous interpretations of the Late Weichselian/Holocene deglaciation are largely based on an Early Weichselian deglaciation pattern. 相似文献
4.
阿尔泰南缘克朗盆地铁木尔特花岗岩体年龄、地球化学特征及成因 总被引：19，自引：6，他引：13
铁木尔特黑云母花岗岩出露于阿尔泰造山带南缘克朗盆地北西侧,岩体主要为黑云母花岗岩。锆石的SHRIMPU-Pb定年结果表明该岩体结晶年龄为459±4.9Ma。岩石具有高的SiO_2(71.92%～76.58%)和Al_2O_3(12.92%～14.55%)含量,富碱(K_2O+Na_2O=7.09%～8.11%)、高钾(K_2O/Na_2O=1.1～1.62)、低P_2O_5(0.14%)和MgO+FeO(5%)含量,强过铝质(ACNK=1.13～1.34);富集LREE和Th、U、Pb,贫Ba、P、Sr、Ti、Nb,呈现明显的Eu负异常(δEu=0.33～0.53);并具有低的Al_2O_3/TiO_2(100)比值和高的CaO/Na_2O(0.3)比值。以上特征表明该岩体属高钾钙碱性高温型强过铝花岗岩。同时岩石具有低的Sr初始值(~(87)Sr/~(86)Sr=0.701727～0.709951)和相对高的ε_(Nd)(459Ma)(-1.43～-0.98)值,两阶段Nd模式年龄较为均一(t_(2DM)=1.2～1.3Ga)。岩体的原始岩浆可能是富含白云母和黑云母的变泥质岩在大于875℃的条件下脱水熔融的产物,在部分熔融过程中有磷灰石、斜长石、钛铁矿等矿物的残留,并混入少量的幔源物质。结合阿尔泰南缘的区域地质背景综合分析,推测岩体形成于俯冲作用的初期,可能是在挤压背景下由于局部伸展减压以及异常地幔和深部热流不均匀上升提供的热影响发生的部分熔融产生。 相似文献
5.
新疆阿尔泰铁木尔特铅锌矿床流体包裹体研究及地质意义 总被引：6，自引：3，他引：3
铁木尔特中型铅锌矿是阿尔泰山南缘克兰盆地内的重要VMS型矿床。矿床赋存于上志留统-下泥盆统康布铁堡组上亚组第二岩性段,容矿岩石为大理岩、绿泥石英片岩、变钙质粉砂岩、夕卡岩。矿体呈似层状和透镜状。矿床的形成经历了喷流沉积期、叠加改造期和表生期。石英、长石、方解石和石榴子石中包裹体类型主要为液体包裹体,在石英中另出现了气体包裹体、纯气体包裹体、含子矿物多相包裹体、含液体CO2的三相包裹体和两相CO2包裹体。喷流沉积期成矿流体均一温度变化于150～330℃,其峰值是165℃和285℃,成矿流体盐度(NaCleq)为4%～16%,流体密度为0.77～0.97g/cm3,流体阳离子主要以Na+为主,次之为K+,阴离子以Cl-为主,其次是SO42-,气相成分主要是H2O和CO2。叠加改造期均一温度范围是150～480℃,峰值为285℃,盐度(NaCleq)为2.2%～17.08%和33.93%～47.2%,流体密度变化于0.61～1.03g/cm3之间,流体阳离子主要以Na+为主,次为K+、Mg2+、Ca2+,阴离子以Cl-为主,其次是SO42-,气相成分主要是H2O和CO2,其次为N2、CH4,含有少量C2H6。 相似文献
6.
John Crusius Michael H. Bothner Christopher K. Sommerfield 《Estuarine, Coastal and Shelf Science》2004,61(4):643-655
Profiles of 210Pb and 239 + 240Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36–192 m) are interpreted with the aid of a numerical sediment-mixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25–35 cm. Roughly half the cores have 210Pb and 239 + 240Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from 0.7 to 40 cm2 yr−1. As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of 0.3 cm yr−1) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210Pb and 239 + 240Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234Th data suggest that in this half of the cores, the vast majority of the present-day flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few centimeters below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition. 相似文献
7.
8.
MARKKU KULMALA VELI-MATTI KERMINEN ARI LAAKSONEN ILONA RIIPINEN MIKKO SIPILÄ TAINA M. RUUSKANEN LARISA SOGACHEVA PERTTI HARI JAANA BÄCK KARI E. J. LEHTINEN YRJÖ VIISANEN MERETE BILDE BIRGITTA SVENNINGSSON MIHALIS LAZARIDIS KJETIL TØRSETH PETER TUNVED E. DOUGLAS NILSSON SARA PRYOR LISE-LOTTE SØRENSEN URMAS HÕRRAK PAUL M. WINKLER ERIK SWIETLICKI MARJA-LIISA RIEKKOLA RADOVAN KREJCI CHRISTOPHER HOYLE ØYSTEIN HOV GUNNAR MYHRE HANS-CHRISTEN HANSSON 《Tellus. Series B, Chemical and physical meteorology》2008,60(3):300-317
9.
Robert J. Chant Christopher K. Sommerfield Stefan A. Talke 《Estuaries and Coasts》2018,41(6):1587-1600
Deepening of estuarine channels is a common practice to ensure navigation. Here, we investigate whether such deepening impacts physical processes such as the strength of the estuarine exchange flow, the horizontal salinity gradient, and tidal dynamics. We analyze recent and historical hydrodynamic observations in Newark Bay, New Jersey, to assess the effect of channel deepening on tides, circulation, and salinity. The Bay’s navigational channel has undergone significant deepening, from 3 to 10 m in the nineteenth century to ~16 m today. Observations presented here include sea-level data from the nineteenth, twentieth, and twenty-first century, and moored Doppler current data and bottom salinity measurements made over the past 20 years. Results show a doubling of the estuarine exchange flow, a slight increase in salinity and in the horizontal salinity gradient, a decrease in tidal current amplitude, and a spatially variable change in the tidal range. The doubling of the exchange flow is consistent with the Hansen and Rattray scaling provided that the horizontal salinity gradient is unable to fully adjust landward because the dredging is limited to a short reach of the estuary. However, uncertainty in channel depth leaves open the possibility that the exchange flow is also augmented by an increase in the horizontal salinity gradient and/or a reduction in vertical mixing. Nevertheless, results demonstrate that a relatively small (15%) increase in depth appears to have doubled the exchange flow. We believe that this result is relevant to other systems where dredging is limited to a short reach of an estuary. 相似文献
10.
Periglacial phenomena in the wooded areas of Northern Sweden - relicts from the Tärendö Interstadial
ROBERT LAGERBÄCK 《Boreas: An International Journal of Quaternary Research》1988,17(4):487-499
Frost-shattered bedrock and ventifacts interpreted to be abraded by drifting snow or ice particles occur frequently in the wooded areas of northernmost Sweden. Ice-wedge casts and periglacial involutions are encountered more sporadically. The phenomena cannot be explained by the present or the Holocene climate and demonstrably pre-date the last deglaciation. The relation to glacial and non-glacial stratigraphy, and to ice flow during the latest glacial stade, strongly suggests that the periglacial activity dates from the local Tärendö Interstadial, tentatively correlated with Odderade. The presence of the periglacial features implies not only that different morphological formations survived the erosive impact of the last continental ice sheet but also that in wide areas the very ground surface, including ventifacts in primary position, is the same today as during the interstadial. A similar situation appears to prevail in northernmost Dalecarlia and parts of Härjedalen and Jämtland in central Sweden. 相似文献