长江河口北槽弯道横向次生流、混合与层化

2013年2月25至26日(枯季/大潮)、7月23至24日(洪季/大潮)分别在长江河口北槽弯道沿着3条横向测线CS6、CSW和CS3(每条测线上有北、中、南3个站位)测得水位、流速、盐度和含沙量的时间序列资料。通过这些资料的定量计算、分析,理解弯道横向次生流、混合与层化的时、空变化和各种物理机制及其相对重要性。3条横向测线均存在横向次生流,且横向测线CS3还出现横向次生环流。枯、洪季,仅在横向测线CS6、CS3出现环状欧拉余流。枯、洪季,沿着横向测线CS3,3个站位的横向斜压梯度比离心加速度和科氏加速度都大1~2个数量级,而后两项大小接近且数量级都是10-4,罗斯贝数在1左右。这些表明:横向次生流受横向斜压梯度、离心加速度和科氏加速度共同驱动,前一项相对于后两项更加重要。沿着3条横向测线:1)枯、洪季大潮,平均势能差异分别约为54.23、66.56 J/m3,表明洪季层化强于枯季;2)枯季涨潮,平均的势能差异普遍小于落潮,而洪季涨潮,平均的势能差异普遍大于落潮,表明枯、洪季湍流混合均存在潮汐不对称性;3)枯季,由横向、纵向水深平均应变(ΦS-y、ΦS-x)引起的势能差异变化率的范围分别是-67×10~(-3)~37×10~(-3)、-7×10~(-3)~11×10~(-3)W/m~3,而洪季,相应的范围分别是-45×10~(-3)~30×10~(-3)、-14×10~(-3)~13×10~(-3)W/m~3,表明枯、洪季差异不明显,横向水深平均应变(ΦS-y)均大于纵向水深平均应变(ΦS-x),前项对水体混合与层化的影响更大;4)枯季大潮,纵向平流(ΦA-x)、横向平流(ΦA-y)、纵向水深平均应变(ΦS-x)和横向水深平均应变(ΦS-y)的潮汐平均绝对值占四项总和之比例分别为26%、33%、18%和23%,而洪季大潮,相应的值的比例分别为13%、9%、22%和56%,表明枯季,平流项(ΦA-y最大)对混合与层化的控制可能占主导地位;洪季,应变项(ΦS-y最大)可能占主导地位。无量纲数(m)被用于判别横向平流(ΦA-y)、横向水深-平均应变(ΦS-y)的相对重要性。一个概念性模式被用于显示层化与横向次生流/环流的相互关系。     

Dynamics and facies model of a macrotidal sand-bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)

The 40-km-long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high-tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel-shaped geometry is developed, and by erosional or non-depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single-channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20-m-thick, upward-fining succession, the lower 1/2–2/3 of which will consist of cross-bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb-dominated portion of this unit will be finer grained than the flood-dominated part, and will contain trough crossbedding produced by 3-D megaripples; the flood-dominated areas, by contrast, will consist mainly of compound cross-bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb- and flood-dominated areas) will create packages of ebb cross-bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral-accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper-flow-regime parallel lamination. The succession will be capped by a 4-m-thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.     

關於西域及西蜀之古氣候與古地理

西域一名,起自前漢。有廣狹二義,從廣義言,奄有今天山南北路及葱嶺外中央亞西亚、印度、高加索及黑海以北一帶之地。從狹義言,僅指天山、葱嶺及崑崙三山間之塔里木盆地是也。本文所論之西域,為狹義之西域,即今新彊省天山南路之塔里木盆地也。至題中不言四川而言西蜀,則因大部論證皆以川西為言故也。在未入正文之前,請先略述本文研究之經過。作者於二十八年五月隨氣象研究所遷北碚,是年秋冬。雨澤稀少。九月上半月且極燠熱,有似盛夏。同事薛君鐵虎(二十九年四月死難於民用輪)適任繪裂天氣圖事,就予叩其故。予初亦茫然,經數次商討後,遂引起研究四川天氣及氣候之興趣。二十九年初寫成‘控制四川雨量的三個主因’一文(註1),其中一个主因,予創其名为‘天漏’。此篇中國氣象上特有之新例。後根據此理引申之而有‘巴山夜雨’之作(註2)。‘天漏’之根據為西藏高原,高原面積之大,世無其匹,西蜀天氣一部分既受此大高原之影響,則此高原上之氣象情形,自為一極有興味之問题。予遂因此而有西藏高原上雨量與氣壓等之研究(註     

Active ice-sheet deglaciation and ice-dammed lakes in the northern Cairngorm Mountains, Scotland

Mapping of ice-marginal and glaciolacustrine deposits in the northern Cairngorm Mountains allows the nature of deglaciation following the Last Glacial Maximum (c.18 000 BP) to be reinterpreted. Two ice-dammed lakes were ponded between the Glenmore lobe of the Scottish ice sheet and local glaciers draining northwards from the Cairngorm Mountains. Delta progradation from the southern end of each lake reflects dominant meltwater sources and glacio-hydrological gradients. Sediment facies representing subaqueous mass-flow deposits, lakebottom rhythmites, lower and upper foresets and topsets are associated with prograding delta fronts. Moraines show that the lakes were ice dammed at both ends, evidence that active glaciers existed in the Cairngorm Mountains while ice was retreating from Strath Spey, and that deglaciation was punctuated by readvances of the ice margin. These results indicate that an ice-stagnation model of deglaciation is invalid for most of the duration of ice wastage, but instead support an active-retreat hypothesis with multiple, climatically forced readvances.     

Mineralogy of arc-derived sediment: siliciclastic sediment on the insular shelf of Puerto Rico

Siliciclastic sediments on the insular shelf of Puerto Rico are derived from a volcanic arc. The sands are feldspatholithic and subquartzose. They confirm predictions about sandstones of arc derivation in the following ways. The quartz content is low, usually less than 25 %. Feldspar is more abundant than quartz, but less so than lithic fragments. If mud rip-up clasts are excluded from the lithic suite, the ratio V/L (volcanic lithic grains to total unstable lithic grains) assumes values near 0.75. Most of the volcanic lithic grains have textures suggesting intermediate volcanics as source rocks. Ratios of plagioclase to total feldspar are high. Values of the parameter C/Q (stable lithic fragments to total lithic grains) are higher than expected for sandstones of volcanic provenance. Four compositionally and areally distinct assemblages of clay minerals are recognized. Clay suites characterized by major concentrations of both halloysite and smectite are found off the north-west coast. Sediments off the north-central and north-east coasts contain major concentrations of halloysite and lesser amounts of smectite, illite and chlorite. Major concentrations of kaolinite are present off the south-east coast. An assemblage of poorly crystallized smectite with lesser amounts of illite and chlorite occurs off the south-west and south-central coasts. Variation in the mineralogic composition of clays around the perimeter of the island reflects changes in the intensity of chemical weathering and the type of rock exposed in the immediate source area and, in instances, either early diagenetic alteration or, more likely, sorting within the clay fraction by currents on the shelf.     

Time Markers for the Evolution and Exhumation History of a Late Palaeozoic Paired Metamorphic Belt in North-Central Chile (34{degrees}-35{degrees}30'S)

A multi-method geochronological approach is applied to unravelthe dynamics of a paired metamorphic belt in the Coastal Cordilleraof central Chile. This is represented by high-pressure–low-temperaturerocks of an accretionary prism (Western Series), and a low-pressure–high-temperatureoverprint in the retro-wedge with less deformed metagreywackes(Eastern Series) intruded by magmas of the coeval arc. A pervasivetransposition foliation formed in metagreywackes and interlayeredoceanic crust of the Western Series during basal accretion nearmetamorphic peak conditions (350–400°C, 7–11kbar) at 292–319 Ma (⁴⁰Ar/³⁹Ar phengite plateau ages).⁴⁰Ar/³⁹Ar UV laser ablation ages of phengite record strain-freegrain growth and recrystallization with a duration of 31–41Myr during a pressure release of 3–4 kbar. During earlyaccretion the main intrusion in the arc occurred at 305 Ma (Pb–Pbevaporation; zircon) and the Eastern Series was overprintedby a short high-temperature metamorphism at 3 kbar, 296–301Ma (⁴⁰Ar/³⁹Ar muscovite plateau ages). Fission-track ages ofzircon (206–232 Ma) and of apatite (80–113 Ma) aresimilar in both series, indicating synchronous cooling duringdistinct periods of exhumation. Early exhumation (period I)during continuing basal accretion proceeded with mean ratesof 0·19–0·56 mm/yr, suggesting that erosionin a tectonically active area was an important unroofing mechanism.At the same time mean rates were 0·03–0·05mm/yr in the Eastern Series, where crustal thickening was minor.A shallow granite intruded into the Western Series at 224 Ma,at the end of basal accretion activity, when exhumation ratesdecreased to 0·04–0·06 mm/yr in both seriesduring period II (100–225 Ma). Major extension, basinformation and local bimodal dyke intrusion at 138 Ma were accompaniedby mean cooling rates of 1–2°C/Myr. Accelerated coolingof 3–5°C/Myr at 80–113 Ma suggests a mid-Cretaceousconvergence event (period III). After 80 Ma cooling rates decreasedto 1–2°C/Myr (period IV). The pressure–temperature–deformation–timeinformation for subduction, basal accretion and exhumation inthe accretionary wedge of central Chile illustrates that theseprocesses reflect a continuous cyclic mass flow that lastednearly 100 Myr, while the retro-wedge remained stable. Afterthe cessation of accretion activity a similarly long periodof retreat of the subducting slab occurred; this ended withrenewed convergence and shortening of the continental margin. KEY WORDS: exhumation rates; Ar/Ar geochronology; fission-track geochronology; Chile; paired metamorphic belt     

Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway

Sedimentary successions in small coastal lakes situated from 0 to 11 m above the 7000 year BP shoreline along the western coast of Norway, contain a distinctive deposit, very different from the sediments above and below. The deposit is interpreted to be the result of a tsunami inundating the coastal lakes. An erosional unconformity underlies the tsunami facies and is traced throughout the basins, with most erosion found at the seaward portion of the lakes. The lowermost tsunami facies is a graded or massive sand that locally contains marine fossils. The sand thins and decreases in grain size in a landward direction. Above follows coarse organic detritus with rip-up clasts, here termed ‘organic conglomerate’, and finer organic detritus. The tsunami unit generally fines and thins upwards. The higher basins (6–11 m above the 7000 year shoreline) show one sand bed, whereas basins closer to the sea level 7000 years ago, may show several sand beds separated by organic detritus. These alternations in the lower basins may reflect repeated waves of sea water entering the lakes. In basins that were some few metres below sea level at 7000 years BP, the tsunami deposit is more minerogenic and commonly present as graded sand beds, but also in some of these shallow marine basins organic-rich facies occur between the sand beds. The total thickness of the tsunami deposit is 20–100 cm in most studied sites. An erosional and depositional model of the tsunami facies is developed.     

Geochemical and Isotopic Evolution of Loihi Volcano, Hawaii

A 680m thick section from the deeply dissected east flank ofLoihi Volcano was sampled using the Pisces V submersible toevaluate the volcano's geochemical evolution. Three types oflavas were recovered: tholeiitic, weakly alkalic and stronglyalkalic. The ratio of alkalic to tholeiitic lavas varies systematicallywith depth, from predominantly alkalic at the base of the sectionto tholeiitic at the top. Glasses from these rocks have similarratios of highly incompatible elements and Pb, Sr and Nd isotopes,but distinct ratios of highly to moderately incompatible elements.Partial melting modeling indicates that these tholeiitic andalkalic lavas could be derived by variable degrees of partialmelting of a slightly heterogeneous source. Many distinct parentalmagmas were generated for each rock type during the 100–150k.y. that the east flank section was formed. Crystal fractionationand olivine accumulation were the dominant processes controllingcompositional variation among lavas of the same rock type. Magmamixing features were observed in only a few of the lavas collected. Loihi typifies the preshield stage of Hawaiian volcanism whenthe volcano drifts closer to the focus of the hotspot. The compositionalvariation in Loihi's east flank section, which may represent40% of the volcano's extrusive history, is consistent with thepredicted increase in partial melting during this drift. Thetransition from dominantly alkalic to tholeiitic volcanism onLoihi was fitful but relatively rapid and is now nearly complete.This transition is the opposite of that which occurs duringthe post-shield stage of Hawaiian volcanism as the volcano migratesaway from the hotspot focus. Loihi's tholeiitic lavas overlap in ratios of incompatible traceelements and Pb, Sr and Nd isotopes with lavas from its moreactive neighbor, Kilauea. The small differences in major elementcontents between lavas from these adjacent volcanoes can beexplained by high-pressure orthopyroxene fractionation of Loihimagmas, which may be a consequence of a low magma-supply rate,or by slightly shallower depths of melt segregation for Kilaueamagmas. KEY WORDS: Loihi volcano; Hawaii; geochemistry; Sr-Nd-Pb isotopes