Orbitally forced sedimentary rhythms in the stratigraphic record: is there room for tidal forcing?

The imprint of orbital cycles, which result from the varying eccentricity of the Earth’s orbit and changes in the orientation of its axis, have been recognised throughout the Phanerozoic rock record. Variations in insolation and their effect on climate are generally considered to be the sole transfer mechanism between the orbital variables and cyclic sedimentary successions. Common oceanographic principles, however, show that the ocean tide also responds to variations in the orbital parameters. The ocean tide has not yet been considered to be a valid, additional transfer mechanism for the orbital variations. In geological studies of Milankovitch cycles in sedimentary successions the insolation paradigm offers satisfying explanations, and the role of long‐term variations of the ocean tide has not yet been appreciated. Variations in the ocean tide, related to changing eccentricity (at present 0·0165, theoretical maximum 0·0728), affect a variety of oceanographic and sedimentary processes. In addition to the widely accepted paradigm of orbitally forced insolation changes, the tidal transfer of orbital signals may explain certain less well‐understood aspects of orbitally induced cycles in the stratigraphic record related to ocean mixing, organic productivity, and tidal processes in shallow seas and deep water. Variations of the ocean tide in relation to the 18·6 year lunar nodal cycle, which has no insolation counterpart by which they may be obscured, indeed show that these relatively small variations can produce significant effects in sedimentary environments that are sensitive to variations in the strength of the ocean tide. In analogy with the 18·6 year lunar nodal cycle, orbital variations of the tide on Milankovitch time scales are likely to have affected sedimentary systems in the past.     

雾迷山旋回层的费希尔图解及其在定义前寒武纪三级海平面变化中的应用

天津蓟县剖面的中元古界雾迷山组是一套厚 3 3 0 0m的碳酸盐岩地层 ,其中广泛发育叠层石生物层及凝块石生物丘。这些生物层及生物丘与潮坪相白云岩和砂泥质白云岩一起构成具有近似对称相序组构的环潮坪型碳酸盐米级旋回层序 ,将其命名为“雾迷山旋回层”。这些旋回层常由泻湖相白云质泥页岩覆盖 ,在其顶常发育古土壤层 ,由此而表明雾迷山旋回层的界面多为瞬时暴露间断面。根据雾迷山旋回层在长周期三级层序中的有序叠加形式 ,可在雾迷山组中识别出 2 6个Ⅲ级层序 ,它们还可进一步归为 6个Ⅱ级层序。为了进一步研究三级海平面的变化幅度和Ⅲ级层序的大致形成时限 ,在露头发育良好的雾迷山组中上部共识别出 62 6个旋回层 ;考虑到雾迷山组的总体厚度 ( 3 3 0 0m)及其大致的总体形成时限 ( 10 3± 2 0Ma左右 ) ,可假定雾迷山组沉积期的总体地壳沉降速率为 2～ 4m/ 10万a。在上述假定的基础上 ,运用费希尔图解来估算三级海平面的变化幅度。     

高原东部春季降水云层的微物理特征分析

利用人工增雨飞机和机载PMS粒子探测系统，对高原东部地区春季人工增雨主要降水云层的微物理结构进行大量探测研究。根据1995年和1997年共15架次的飞行探测资料，分析研究了高原东部低层大气气溶胶微粒的分布特征及其与大气层结的关系，着重分析了高原东部春季降水云层的云粒子、降水粒子以及冰晶、过冷水的一些初步分布特征。     

高原夏季风指数的定义及其特征分析

基于1958~2002年ECMWF再分析资料,我国160个台站降水和气温资料,从夏季高原季风环流系统特点出发,定义了能较好表征高原夏季风环流变化的特征指数,分析了高原夏季风年际、年代际变化特征,并揭示了高原夏季风强弱异常时的环流特征及其与中国夏季降水和气温的关系,主要结论为:(1)用6~8月600hPa (27.5~30°N,80~100°E)范围内平均的西风分量距平与(35~37.5°N,80~100°E)范围内平均的东风分量距平差定义了高原夏季风指数(PM I)。该指数计算简单,意义清楚,代表性好。(2)1958~2002年高原夏季风整体呈增强趋势,在20世纪60年代中期之前是高原夏季风的强盛期,之后是高原夏季风弱期,在80年代以后又转为季风强期。(3)高原夏季风与中国夏季降水和气温相关很好。将该指数与之前汤懋苍定义的指数进行性能综合比较后,发现该指数对川渝地区的夏季降水及气温有更好的指示意义。      

青海省东北部地区春季空中水资源潜力分析

采用动力气象学原理计算了青海高原东北部春季水汽输送、辐合辐散情况 ,利用微波辐射计观测总水汽含量 (V)、液态水 (L)的情况。经计算 ,青海高原东北部大气水汽状况为输入大于输出 ,且有近 83%的水汽影响该地区后移出青海 ,有较大的潜在水资源 ;从液态水含量情况来看 ,尽管水汽含量比平原地区少 ,但液态水含量却比平原地区偏高。另外还计算了 1997年 3月下旬至 5月上旬的云总凝结水和降水效率 ,大气总凝结水量为 2 84× 10 8t,但实际降水量为 34× 10 \+8t,平均降水效率为 0 .12。如果能提高 1%的降水效率 ,则可增加约 2 .5× 10 8t降水 ,且具有较好的人工增雨潜力。     

A new petrogenetic grid for low-grade metabasites

Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.     

The Occurrence of Garnet in the Granulite-Facies Terrane of the Adirondack Highlands

Based on the presence or absence of almandine in charnockiticand metabasic rocks, Buddington (1963) recognized two garnetisograds in the northwestern Adirondacks which were inferredto represent a general increase in grade of regional metamorphismfrom northwest to southeast and east. ACF plots of analyzed Adirondack rocks demonstrate that thepresence or absence of garnet is primarily controlled by thebulk chemistry of the rocks. In metabasites, garnet forms partof the type assemblage of the almandine-amphibolite facies inwhich it developed. The assemblage becomes unstable in the granulitefacies where quartz is present. Garnet does not occur in themore calcic metabasites. In charnockites, garnet develops concomitantlywith orthopyroxene in the aluminous varieties of this rock typeand not in the calcic varieties. The two may occur intimatelyinterlayered. The present definition of the granulite facies, however, doesnot cover clinopyroxene-almandine-bearing assemblages reportedin rocks of diverse composition from the proximity of anorthositemassifs. Reaction rims prove that within the granulite faciesa reaction occurs in rocks with excess silica by which the typomorphicorthopyroxene-plagioclase association is replaced by the higherdensity clinopyroxene-almandine-quartz association. To accommodate this assemblage a fourfold subdvision of thegranulite facies is proposed. Four subfacies are obtained bytwo intersecting boundaries: (1) PH₂O-T controlled dehydrationreactions separating pyroxene-granulite from hornblende-granulitesubfacies, and (2) the anhydrous, P_load-T controlled reactionwhich separates subfacies with orthopyroxene-plagioclase assemblagesfrom those with clinopyroxene-almandine assemblages.