Rock varnish microlamination dating of late Quaternary geomorphic features in the drylands of western USA

Varnish microlamination (VML) dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts. In this study, we establish a generalized late Quaternary (i.e., 0–300 ka) varnish layering sequence for the drylands of western USA and tentatively correlate it with the SPECMAP oxygen isotope record. We then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late Quaternary geomorphic features in the study region. VML dating of alluvial fan deposits in Death Valley of eastern California indicates that, during the mid to late Pleistocene, 5–15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation. During the Holocene interglacial time, however, 0.5–1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate. VML dating of alluvial desert pavements in Death Valley and the Mojave Desert reveals that pavements can be developed rapidly (< 10 ky) during the Holocene (and probably late Pleistocene) in the arid lowlands (< 800 m msl) of these regions; but once formed, they may survive for 74–85 ky or even longer without being significantly disturbed by geomorphic processes operative at the pavement surface. Data from this study also support the currently accepted, “being born at the surface” model of desert pavement formation. VML dating of colluvial boulder deposits on the west slope of Yucca Mountain, southern Nevada, yields a minimum age of 46 ka for the emplacement of these deposits on the slope, suggesting that they were probably formed during the early phase of the last glaciation or before. These results, combined with those from our previous studies, demonstrate that VML dating has great potential to yield numerical age estimates for various late Quaternary geomorphic features in the western USA drylands.     

Foraminiferal depth palaeoecology of Late Pliocene shelf sequences and systems tracts, Wanganui Basin, New Zealand

High-resolution foraminiferal census of benthic taxa was undertaken on 113 closely spaced samples drawn from the Late Pliocene (ca. 2.6−1.7 Ma) cyclothemic marine strata of the Rangitikei Group, eastern Wanganui Basin, New Zealand. These strata comprise a ca. 1 km thick progradational stack of twenty, sixth-order, depositional sequences that accumulated in shelf and shoreline palaeoenvironments. The sequences are correlated with δ¹⁸O Stages 100−58, and each 41 ka glaciallinterglacial stage couplet is represented by an individual sequence comprising transgressive (TST), highstand (HST), and regressive (RST) systems tracts.

Statistical analysis of the census data identifies thirteen foraminiferal associations within the cyclothemic strata, that are grouped into seven depth- and lithology-related biofacies spanning the entire range of marginal marine to outer shelf palaeoenvironments. Foraminiferal palaeobathymetric analysis of the Rangitikei Group sequences reveals cyclical changes in water-depth of ca. 100–200 m amplitude with frequencies corresponding to the 41 ka obliquity orbital rhythm. Water-depth changes of this magnitude are consistent with a glacio-eustatic origin for the cyclothems, which correspond to an interval of Earth's history when successive continental glaciations of the Northern Hemisphere are known to have occurred. Furthermore the derived water-depth changes are also consistent with lithofacies and sequence stratigraphic inferences regarding palaeodepth of the sequences.

Individual sequences display a clear deepening-upward trend from shoreline to mid-shelf water-depths within TSTs. The level of resolution provided by the microfaunal analysis was insufficient to resolve the precise position of the maximum flooding surface (MFS) and its relationship to the downlap surface (DLS). However, the turn around from rising to falling relative sea level (maximum water-depth) corresponds to a < 5 m interval of section spanning the top of TSTs and lower portions of HST's. A progressive shoaling trend to shoreline and marginal marine environments is indicated for the overlying RSTs.

The amplitudes of water-depth changes for asymmetrical sequences, Rangitikei_nt motif (nondepositional transgression) (100–200 m), are somewhat greater than glacio-eustatic sea-level changes derived from the deep-sea δ¹⁸O record (50–100 m). This implies a significant subsidence contribution to relative sea-level changes. Notwithstanding the effect of subsidence and sedimentation on relative sea level, fluctuations in glacio-eustatic sea level are regarded as the primary factor controlling relative sea-level changes recorded in the Late Pliocene Wanganui Basin succession. Foraminifer-derived palaeobathymetric cycles within sequences display the same frequency, relative magnitude and symmetry as their correlative cycles on the δ¹⁸O sea-level curve.     

Late Pleistocene-Holocene paleolimnology of three north-western Russian lakes

The vegetation history and development of three different types of lakes, lakes Valday, Kubenskoye and Vishnevskoye (northwest of the East European Plain) were reconstructed using paleolimnological techniques. Watershed vegetation demonstrates a close connection with climate fluctuations: gradual expansion of the southern broad-leaved trees to the North during the Holocene with the maximum extent during the climate optimum (8000–5000 BP); and their subsequent retreat afterwards; followed by the extension of spruce during the cold and dry Subboreal time; and dominance of pine-spruce-birch forests in the Subatlantic time. The Late Pleistocene and Holocene climate changes resulted in lake-level fluctuations and other ecosystem changes. Valday Lake was formed ca. 12,500 BP as an oligotrophic, deep water basin. The lake level decreased during the dry Boreal, then increased again during the humid Atlantic period. The large shallow Kubenskoye Lake was formerly a part of an ice margin lake, which was then separated (ca. 13,000 BP) and developed into the Sukhona Basin with an outflow to the northwest. During the Atlantic, the outflow direction changed to the east. As a result, the ancient Sukhona Lake disappeared and Kubenskoye Lake formed in its modern size and shape. Vishnevskoye Lake, on the Karelian Isthmus, was formed at the beginning of the Preboreal after the disappearance of the Baltic Ice Lake. It was flooded by waters of the Boreal Ancylus transgression of the Baltic Basin and had become a small eutrophic lake by the time.     

Late holocene environmental change in an upper Montane area of the snowy mountains,New South Wales

This study examines environmental change in the upper montane zone of the Australian Eastern Highlands during the late Holocene, by analysing vegetation, fire and erosion records contained within a small fen located in a frost hollow. Differences in environmental parameters across the prehistoric—historic boundary were particularly investigated in an attempt to characterise better the changes associated with the imposition of European land‐use practices. Decreases in arboreal pollen and an increased charcoal concentration near the base of the analysed sequence, interpreted to be about 1600 y BP until about 1300 y BP, are suggestive of reduced moisture availability. After this, a period of relative stability continued to the close of the prehistoric period. The arrival of Europeans in the region triggered changes in the sediment record, including an increase in the accumulation of sediment by an order of magnitude, and changes in the surrounding vegetation. Saturated isothermal remnant magnetism (SIRM) was found to be significantly higher in the historic period compared to the analysed prehistoric period, suggesting an alteration in the erosional processes within the catchment. The concentration of charcoal was comparable between the prehistoric and historic periods; however, the increased sedimentation rate of the historic period infers an increased accumulation of charcoal. Fire did not appear to be related to the vegetation changes evident in the historic period, perhaps due to the use of cool fires by the pastoralists.     

MIS3晚期查格勒布鲁剖面微量元素揭示的千年尺度气候变化

通过对巴丹吉林查格勒布鲁剖面时代上属MIS3晚期的地层层位(CGS3a层段)中14种微量元素的分析发现:除Co、Ba含量呈相反变化外,P、V、Cr、Mn、Ni、Zn、Pb、Y、Zr、Nb、Cu和As含量在风成沙丘砂中基本上表现为谷值,而在不同层位沙丘砂上覆的湖相中则相对为峰,构成与沉积旋回相应的元素变化旋回。元素在沙丘砂中呈现的谷值,主要揭示了干冷气候条件下东亚冬季风的侵蚀、搬运和堆积作用;在湖相中的相对富集,则与东亚夏季风盛行时暖湿气候背景下该地所处的特殊洼地地貌位置有关。元素旋回实际上代表的是东亚冬夏季风的气候变化旋回。分析结果显示:MIS3晚期剖面所在地至少经历了5次冬夏季风交替变化的气候旋回,反映了该时期我国沙漠地区的千年尺度气候变化。     

Post-collisional rapid exhumation and erosion during continental sedimentation: the example of the late Variscan Salvan-Dorénaz basin (Western Alps)

The Salvan-Dorénaz intramontane basin formed between ca. 308–293 Ma as an asymmetric graben along crustal-scale transtensional fracture zones within the Aiguilles-Rouges crystalline massif (Western Alps) and represents a feature of the post-collisional evolution of the Variscan orogens. It contains 1.5–1.7 km of continental clastic deposits which were eroded from granitic, volcanic, and metamorphic rocks. Textural and compositional immaturity of the sandstones, and the numerous lithic fragments with low chemical and physical stability suggest only short-range transport. ⁴⁰Ar/³⁹Ar analyses of detrital muscovite are interpreted to represent cooling of the crystalline basement below the respective closure temperatures. Ages from detrital muscovite range between ca. 280–330 Ma. ⁴⁰Ar/³⁹Ar white mica plateau ages from granitic boulders range between 301–312 Ma and suggest rapid cooling. The very short time interval recorded between the ⁴⁰Ar/³⁹Ar cooling ages and the stratigraphic age of the host sediment suggests that considerable portions of the upper crust were removed prior to the formation of the basin. Late Variscan granitic boulders document surface exposure and erosion of Late Carboniferous granites during early stages of the infilling of the basin. Therefore, unroofing of basement units, magmatic activity, and formation of the fault bounded Salvan-Dorénaz basin were acting concomitantly, and are highly suggestive of extensional tectonics. When compared with other orogens, this situation seems specific to the Variscan, especially the exclusively young ages of detrital material, however, modern analogous may exist.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.     

内蒙古额济纳旗少斜沟钨铜钼矿成矿特征

额济纳旗少斜沟钨铜钼矿赋存于石炭纪-二叠纪花岗岩类岩体与下石炭统绿条山组的接触带附近及岩体内. 矿化蚀变分带总体呈环状, 最外带为碳酸盐化、绿帘石化、绿泥石化; 次外带为夕卡岩化、角岩化、白钨矿化; 中带为硅化、云英岩化、辉钼矿化; 内带为硅化、孔雀石化、黄铜矿化. 由外向内成矿元素从高温到低温富集, 形成钨矿→钼矿→铜矿. 另外, 结合矿体特征、含矿岩体的矿石地球化学特征和锆石U-Pb测年资料等讨论了矿床的成因类型和成矿时代, 认为该矿床成因类型为斑岩型, 成矿时间为305.1±1.9 Ma, 为晚石炭世.     

河北阳原晚白垩世沉积环境分析

对河北阳原晚白垩世地层的沉积环境进行了系统地分析，初步认为它为一套湖北和扇三角洲沉积，并对沉积环境的基本特征和盆地充填过程进行了详细论述，通过对恐龙化石产出层位的沉积环境分析，将对恐龙的埋藏与生态环境的研究具有重要的参考意义。