Magnetic anisotropy produced by magma flow: theoretical model and experimental data from Ferrar dolerite sills (Antarctica)

Volcanic rocks forming sills, dykes or lava flows may display a magnetic anisotropy derived from the viscous flow during their emplacement. We model a sill as a steady-state flow of a Bingham fluid, driven by a pressure gradient in a horizontal conduit. The magma velocity as a function of depth is calculated from the motion and constitutive equations. Vorticity and strain rate are determined for a reference system moving with the fluid. The angular velocity and the orientation of an ellipsoidal magnetic grain immersed in the fluid are calculated as functions of time or strain. Magnetic susceptibility is then calculated for a large number of grains with a uniform distribution of initial orientations. It is shown that the magnetic lineation oscillates in the vertical plane through the magma flow direction, and that the magnetic foliation plane changes periodically from horizontal to vertical. The results are compared with the magnetic fabric of Ferrar dolerite sills (Victoria Land, East Antarctica) derived from low-field susceptibility measurements.     

TERRESTRIAL RECORD OF POST-EOCENE CLIMATE HISTORY IN MARIE BYRD LAND, WEST ANTARCTICA

ABSTRACT. Hydrovolcanic deposits, interbedded tills and recycled microfossils, together with erosion anomalies in the Marie Byrd Land (MBL) landscape, each provide a portion of the record of glaciation and deglaciation events from late Oligocene to the present. We have attempted to synthesize these data sources to provide a more complete record, and to reconcile them with climatic events recorded elsewhere in Antarctica and the deep sea. The MBL data suggest that the late Oligocene was marked by the development of an ice cap at Mount Petras, where the MBL dome was beginning to rise from a near-sea-level position. Furthermore, unusually advanced cirque development in the dome crest area is difficult to explain unless there was a period of effective cirque erosion in that area between c. 25 and 15 Ma BP. These inferences are consistent with evidence from the Ross Sea for an expansion of the West Antarctic Ice Sheet (WAIS) in mid-Miocene (15–17 Ma BP) time.
The deep sea oxygen isotope proxy record has been interpreted to show the inception of West Antarctic glaciation around 6 Ma BP. This can perhaps be reconciled with the terrestrial record if one considers (1) the observation that large volume changes in the WAIS cannot produce a ω¹⁸O signal that is significantly outside limits of error, and (2) that the landscape in West Antarctica has evolved from very low regional relief in the Oligocene, to more than 2 km of local relief in the present day, as a result of dome uplift since c. 27 Ma BP, and the growth of large volcanoes since c. 19 Ma BP.     

Rock Weathering Processes in Antarctica: A Comparison of Some Recent Studies with Those from the Northern Hemisphere

A review of some recent studies on rock weathering in Antarctica has revealed that Antarctic weathering research has a significant contribution to make to the ongoing debate about rock weathering in cold climates. Largely conducted in the field rather than the laboratory as in the Northern Hemisphere, it demonstrates that whilst all weathering processes can occur in Antarctica this is highly localised and dependent on the particular micro‐environment. Freeze‐thaw, for example, is not the most dominant process in many parts of Antarctica. The right combination of rock temperature and moisture can mean that salt, insolation, hydration or even chemical weathering can predominate.     

Sedimentary and thermal evolution of the Eocene‐Oligocene mudrocks from the southwestern Thrace Basin (NE Greece)

Paleothermal indicators based on clay mineral and organic matter analyses, were integrated with mudrock geochemistry and stratigraphic data to define the sedimentary evolution of the southwestern Thrace Basin during the Eocene to Oligocene. This multi‐method approach allowed us to reconstruct the burial evolution of the basin in Eocene and Oligocene times and to study the mudrock composition and relate this to their provenance and source area weathering. The studied mudrocks show similar chemical variations. The distribution of some major and trace elements for the studied samples reflect heterogeneous source areas containing both felsic to mafic rocks. In particular, the Light Rare Earth Elements/Transition elements (LREEs/TEs) ratios are very high for the Avdira and Organi samples (on the average between 1.5 and 2.2 for (La + Ce)/Cr and 3.5–8 for (La + Ce)/Ni), suggesting a felsic source(s), and very low for the Samothraki, Limnos, Paterma and Iasmos samples (on the average between 0.4 and 0.6 for (La + Ce)/Cr and 0.6–1 for (La + Ce)/Ni), suggesting a mainly basic source(s). The mineralogical composition coupled with the A‐CN‐K and A‐N‐K plots suggest a complex evolution. The clay mineral data (illite percentage in I/S and the stacking order R and the Kübler Index) coupled to vitrinite reflectance analysis indicate a high to intermediate diagenetic grade for the Middle to Upper Eocene samples (from Iasmos, Gratini, Organi, Paterma, Esimi and Samotraki sections) and a low diagenetic grade for the Upper Eocene to Oligocene samples (from Limnos and Avdira sections). These data helped in interpreting the geodynamic evolution of the studied basins where the magmatic activity plays an important role. In particular, Middle to Upper Eocene sediments show high to intermediate diagenetic grade since they are located in a portion of the basin dominated by Eocene to Oligocene magmatic activity and intrusion of granitoids, whereas, the Upper Eocene to Oligocene sediments are not involved in important magmatic activity and intrusion of granitoids and, thus, show low diagenetic grade. Furthermore, Middle to Upper Eocene sediments experienced deeper burial processes caused by lithostatic load, rather than the uppermost Eocene and Oligocene sediments, in relation of their position along the stratigraphic succession. These data suggest a burial depth of at least 3–4 km with a tectonic exhumation mainly related to the extensional phases of the Miocene age.     

Characteristics of the mineral phase constituents of lacustrine deposits from the Fildes Peninsula of King George Island, Antarctica and their environmental implication

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Morphology and origin of major Cenozoic sequence boundaries in the eastern North Sea Basin: top Eocene,near‐top Oligocene and the mid‐Miocene unconformity

Unconformities in sedimentary successions (i.e. sequence boundaries) form in response to the interplay between a variety of factors such as eustasy, climate, tectonics and basin physiography. Unravelling the origin of sequence boundaries is thus one of the most pertinent questions in the analysis of sedimentary basins. We address this question by focusing on three of the most marked physical discontinuities (sequence boundaries) in the Cenozoic North Sea Basin: top Eocene, near‐top Oligocene and the mid‐Miocene unconformity. The Eocene/Oligocene transition is characterized by an abrupt increase in sediment supply from southern Norway and by minor erosion of the basin floor. The near‐top Oligocene and the mid‐Miocene unconformity are characterized by major changes in sediment input directions and by widespread erosion along their clinoform breakpoints. The mid‐Miocene shift in input direction was followed by a marked increase in sediment supply to the southern and central North Sea Basin. Correlation with global δ¹⁸O records suggests that top Eocene correlates with a major long‐term δ¹⁸O increase (inferred climatic cooling and eustatic fall). Near‐top Oligocene does not correlate with any major δ¹⁸O events, while the mid‐Miocene unconformity correlates with a gradual decrease followed by a major long‐term increase in δ¹⁸O values The abrupt increases in sediment supply in post‐Eocene and post‐middle Miocene time correlate with similar changes worldwide and with major δ¹⁸O increases, suggesting a global control (i.e. climate and eustasy) of the post‐Eocene sedimentation in the North Sea Basin. Erosional features observed at near‐top Oligocene and at the mid‐Miocene unconformity are parallel to the clinoform breakpoints and resemble scarps formed by mass wasting. Incised valleys have not been observed, indicating that sea level never fell significantly below the clinoform breakpoint during the Oligocene to middle Miocene.     

Biotic weathering of rocks by lichens in Antarctica

１ＩｎｔｒｏｄｕｃｔｉｏｎＡｓｔｈｅｅａｒｌｉｅｓｔｃｏｌｏｎｉｚｅｒｓｏｆｔｅｒｅｓｔｒｉａｌｈａｂｉｔａｔｓｏｎｔｈｅｅａｒｔｈ,ｔｈｅｏｃｕｒｅｎｃｅｏｆｌｉｃｈｅｎｓｍａｙｂｅｄａｔｅｄｆｒｏｍｔｈｅＥａｒｌｙＤｅｖｏｎｉａｎ（Ｔａｙｌｏｒｅ...     

泥河湾盆地马圈沟剖面早更新世矿物变化特征及其古环境意义

在野外考察采样的基础上,对泥河湾盆地马圈沟剖面更新世河湖相沉积序列进行了XRD矿物组分和磁性矿物分析。研究发现,剖面沉积矿物主要由碎屑矿物和碳酸盐矿物组成,碎屑矿物主要为石英、钠长石、白云母和斜绿泥石,其平均含量分别为38.77%、17.16%、29.63%和4.11%,是剖面沉积矿物组成主体,且各矿物之间基本没有相关性;相对于碎屑矿物,碳酸盐矿物在整个剖面中含量较低,主要为方解石和白云石,其平均含量分别为4.72%、3.59%,文石在剖面中零星出现。对剖面典型样品进行热磁分析,确定了沉积物中磁性矿物的种类以及加热过程中的变化和产物,并对其来源进行了探讨。同时发现,含绿泥石较高的灰绿色粉砂样品加热后磁化率的升幅比灰黄色样品高,而绿泥石的相对含量与该地区化学风化密切相关,进一步说明κ-T曲线的变化特征可以用来反应盆地化学风化作用强度的变化,具有重要的古环境意义。     

Geochemical constraints on the Palaeocene–Miocene evolution of eastern Azerbaijan,with implications for the South Caspian basin and eastern Paratethys

Fine‐grained Palaeogene–early Neogene strata of the South Caspian basin, specifically the Oligocene–Lower Miocene Maikop Series, are responsible for the bulk of hydrocarbon generation in the region. Despite the magnitude of oil and gas currently attributed to the source interval offshore, geochemical evaluation of 376 outcrop samples from the northern edge of the Kura basin (onshore eastern Azerbaijan) indicates that depositional conditions in these proximal strata along the basin margins were dominantly oxic to mildly suboxic/anoxic throughout three major depositional stages: the Palaeocene–Eocene, Oligocene–early Middle Miocene and late Middle–Late Miocene. Palaeocene–Eocene samples have low average total organic carbon (TOC) values (0.3%), with higher total inorganic carbon (TIC) values (average=2.6%), extremely low sulphur content (0.2%) and relatively high detrital input as indicated by Fe/Al and Ti/Al ratios. C–S–Fe associations, along with relatively lower concentrations of redox‐sensitive trace elements (e.g. V, Ni, Mo, U) indicate dominantly oxic environments of deposition during much of the Palaeocene–Eocene. A pronounced geochemical shift occurred near the Eocene–Oligocene boundary, and continued through the Early Miocene. Specifically, this interval is characterized by a distinct increase in TOC (ranging from 0.1 to 6.3% with an average of 1.5%), C–S–Fe associations that reveal an abrupt relative increase of carbon and sulphur with respect to iron‐dominated Palaeocene–Eocene samples, and higher concentrations of redox‐sensitive trace metals. These changes suggest that a shift away from unrestricted marine conditions and towards more variable salinity conditions occurred coincident with the initial collision of the Arabian plate and partial closure of the Paratethys ocean. Despite periodic basin restriction, the majority of Upper Eocene–Lower Miocene strata in the northern Kura basin record oxic to slightly dysoxic conditions.     

囊谦盆地贡觉组含膏盐地层粘土矿物组合特征及气候环境意义

囊谦盆地晚始新世贡觉组厚层红色含膏盐粉砂质泥岩是研究青藏高原始新世古气候演化和含膏盐沉积环境的良好素材。采集囊谦盆地贡觉组约257 m含膏盐地层的碎屑沉积物,研究其粘土矿物成分和组合特征及气候与沉积环境变化。结果显示,粘土矿物主要以伊利石(52.1%)为主,其次为绿泥石(8.1%)和高岭石(3.5%)。根据岩性特征及粘土矿物组合变化,剖面自下而上可划分为3个阶段,分别指示了暖湿—低盐度、冷干—高盐度、较暖湿—低盐度的气候和沉积环境。贡觉组气候与沉积环境受到青藏高原地区干湿交替变化大背景的影响。     

Surface Moisture Availability and Rock Weathering in Cold Climates

The importance of moisture in determining which rock weathering processes may operate, as well as the rate at which rock weathering occurs, has long been recognised. However, little is known about how often or when moisture is available to the rock surface, or for how long the surface may be wetted. This paper gives some initial results from current research on granite rocks in Antarctica. Recordings of surface moisture, for two aspects and three locations in Victoria Land, show that moisture reaches the rock surface more frequently than might initially have been expected in this dry climate. The number of occasions the surface is wetted, and the quantity of moisture the surface received, varied with aspect and time of year. Rock surfaces could be wet for up to 40% of the time during the summer months. This has important implications for rock weathering where some processes are deemed not to operate, or to operate in very restricted locations, due to the apparent absence of moisture.     

Environmental magnetic studies of sediment cores from Gonghai Lake: implications for monsoon evolution in North China during the late glacial and Holocene

Environmental magnetic studies were conducted on a 9.42-m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal year BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000–11,500 cal year BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility (χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500–1,000 cal year BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply (and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1,000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. The results also suggest that different interpretational models may need to be applied to lake sediment magnetic mineral assemblages corresponding to different stages of environmental evolution.     

Record of Late Pleistocene Glaciation and Deglaciation in the Southern Cascade Range. I. Petrological Evidence from Lacustrine Sediment in Upper Klamath Lake, Southern Oregon

Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter.     

Patterns of Cenozoic sediment flux from western Scandinavia

The significance of variations in the sediment flux from western Scandinavia during the Cenozoic has been a matter of debate for decades. Here we compile the sediment flux using seismic data, boreholes and results from other publications and discuss the relative importance of causal agents such as tectonism, climate and climate change. Western Scandinavia, the northern British Isles and the Faeroe‐Shetland Platform were significant sediment sources during the Paleocene, which is well founded in tectonic causes related to the opening of the North Atlantic. From the Eocene and onward, variations in the sediment flux from western Scandinavia correlate better with climate and climate change. During the Eocene, sediment production was low. From the late Eocene onward, increased seasonality may have contributed to stimulating the sediment flux. Significant climatic cooling episodes correlate with Oligocene deposits in the North Sea, the post‐mid‐Miocene Molo and Kai Formations of the Norwegian Shelf, the southern North Sea delta system and large volumes of the Late Pliocene‐Holocene Naust Formation. The sediment flux from Scandinavia during the Cenozoic is in general agreement with the detrital flux to the world's oceans. Furthermore, the large variations in the size of sediment catchment areas as well as the possibility of submarine and glacial erosion must be incorporated to understand regional variations in climate driven sediment flux.     

MELTWATER FEATURES THAT SUGGEST MIOCENE ICE-SHEET OVERRIDING OF THE TRANSANTARCTIC MOUNTAINS IN VICTORIA LAND, ANTARCTICA

ABSTRACT. We illustrate here spectacular meltwater features associated with outburst floods beneath an ice sheet that overrode the Transantarctic Mountains in southern Victoria Land. Because of long-term hyperarid polar climate, these features are part of an ancient landscape preserved for about 14 million years. Some channels are associated with areal scouring of basement rocks extending from sea level to as much as 1200–2100 m elevation in coastal regions. Scablands with scallops, potholes and plunge pools are cut in Beacon Super group sandstones and Ferrar Dolerite and cover wide areas of high western plateaus near the mountain crest. Subglacial channel systems commonly originate near divides and converge downhill toward the northeast. We argue that the landforms were created beneath a major Antarctic Ice Sheet that submerged the whole area, with the possible exception of the high peaks of the Royal Society Range, as it flowed northeastward toward the outer Antarctic continental shelf. Areal scouring, associated with warm-based regimes, is restricted to the lower slopes close to the coast. In the higher terra in, meltwater channels and scabland alongside preserved patches of regolith are best explained by the breaching of cold-based ice on the mountain rim by subglacial melt water outbursts. Melt from warm-based ice, along with subglacial lakes trapped upstream of the mountain rim, are possible sources of the meltwater necessary to form the channel systems and scablands.     

南极乔治王岛海军湾亨内克角早第三纪晚期孢粉化石及其地层学意义

根据乔治王岛海军湾亨内克角群上部凝灰质砂泥岩夹层中的孢粉化石组合特征,讨论其时代归属和沉积环境。研究表明亨内克角群上部火山沉积岩中孢粉化石共４０余种,主要成分为冈瓦纳早第三纪群落,属Ｗｅｄｄｅｌｉａｎ生物地理区,Ｎｏｔｈｏｆａｇｉｄｉｔｅｓ为代表分子,其中Ｎ．ｃｆ．ｓａｒａｅｎｓｉｓ和Ｎ．ｃｆ．ｆｌｅｍｉｎｇｉ的存在以及Ｐｒｏｔｅａｃｉｄｉｔｅｓｓｐｐ．的缺失,提示含化石地层时代可能属渐新世,沉积环境为近中低山区的湖沼环境,气候温暖潮湿。     

南极罗斯海盆地油气地质条件及资源潜力研究

罗斯海位于罗斯海湾北部、南极太平洋扇形区,该区域的罗斯海盆地是南极最具资源潜力的盆地之一,可进一步分为维多利亚地盆地、北部盆地、中央海槽和东部盆地四个次级单元。在对区域地层特征、地震地层特征进行总结归纳后,也分析了盆地的构造、沉积、温压及烃类地化条件,认为罗斯海盆地具有较好的油气地质条件。以二维地震、钻孔资料为基础,对盆地成熟度及生烃量进行了模拟。研究认为:东部盆地与维多利亚地盆地油气生成区均较广,但后者的源岩热演化程度较前者稍高,而中央海槽与北部盆地在油气生成区域及热演化程度方面均较差;根据生烃量模拟结果,同时选择合理的排聚系数,推测罗斯海盆地的地质资源量约为91.5亿吨。     

南极半岛东北部海域碎屑矿物特征与物源分析

基于南极半岛东北部海域15个表层碎屑沉积物样品和1个柱状样的碎屑矿物学分析,对该区的沉积环境与物质来源进行了相关分析。结果显示,该区碎屑矿物共有31种,其中,重矿物19种,以石榴子石、紫苏辉石、绿帘石、普通角闪石为主;轻矿物12种,以石英、斜长石、火山玻璃(褐色、无色)为主。根据矿物组合分布特征,研究区可以划分为2个碎屑物沉积区:Ⅰ区为辉石-磁铁矿-火山玻璃型,主要物质来源为南设得兰群岛及南极半岛北段火山岩与火山喷发物,冰川为其主要搬运介质和动力来源;Ⅱ区为石榴子石-绿帘石-角闪石-石英型,南奥克尼群岛物质、南极绕极环流搬运南极半岛及其附近岛屿的物质对该区沉积物均有贡献。     

Drainage response to Arabia–Eurasia collision: Insights from provenance examination of the Cyprian Kythrea flysch (Eastern Mediterranean Basin)

The Cenozoic geodynamics of the north‐eastern Mediterranean Basin have been dominated by the subduction of the African Plate under Eurasia. A trench‐parallel crustal‐scale thrust system (Misis–Kyrenia Thrust System) dissects the southern margin of the overriding plate and forms the structural grain and surface expression of northern Cyprus. Late Eocene to Miocene flysch of the Kythrea (De?irmenlik) Group is exposed throughout northern Cyprus, both at the hanging‐wall and foot‐wall of the thrust system, permitting access to an extensive Cenozoic sedimentary record of the basin. We report the results of a combined examination of detrital zircon and rutile U–Pb geochronology (572 concordant ages), coupled with Th/U ratios, Hf isotopic data and quantitative assessment of grain morphology of detrital zircon from four formations (5 samples) from the Kythrea flysch. These data provide a line of independent evidence for the existence of two different sediment transportation systems that discharged detritus into the basin between the late Eocene and late Miocene. Unique characteristics of each transport system are defined and a sediment unmixing calculation is demonstrated and explained. The first system transported almost exclusively North Gondwana‐type, Precambrian‐aged detrital zircon sourced from siliciclastic rock units in southern Anatolia. A different drainage system is revealed by the middle to late Miocene flysch sequence that is dominated by Late Cretaceous–Cenozoic‐aged detrital zircon, whose age range is consistent with the magmatic episodicity of southeast Anatolia, along the Arabia–Eurasia suture zone. Deposition of these late Miocene strata took place thereupon closure of the Tethyan Seaway and African–Eurasian faunal exchange, and overlap in time with a pronounced uplift of eastern Anatolia. Our analytical data indicate the onset of prominent suture‐parallel sediment transport from the collision zone of south‐eastern Anatolia into the Kyrenia Range of northern Cyprus, marking the drainage response to the continental collision between Arabia and Eurasia.     

南极洲泥盆系概况及其与中国泥盆系之比较

南极洲的泥盆系主要分布于横贯南极山脉的麦克默多和俄亥俄岭－埃尔斯沃思山等两个沉积盆地中。前一个盆地的泥盆系代表从海岸泻湖－河流三角洲到近岸冲积平原的层序；后一个盆地的彭萨科拉山的泥盆系较厚，从非海相冲积扇－冲积平原－浅海相，最后又恢复到非海相沉积环境，但在俄亥俄岭却沉积了厚度不大的浅海相地层，含Ｍａｌｖｉｎｏｋａｆｒｉｃ生物地理大区的海相双壳类、腹足类、三叶虫、竹节石和鱼类等化石。除了上述两个沉积盆地外，在罗斯海两边却出露了火山岩，说明该地当时处于俯冲带附近的火山弧中。中国华南的曲靖型和西北的祁连山型泥盆系也属于滨海相和非海相沉积，它们与南极洲的泥盆系可资比较，但两者的生物地理区系并不相同