Environments of the Russian Plain during the Middle Valdai Briansk Interstade (33,000–24,000 yr B.P.) Indicated by Fossil Mammals and Plants

Abundant mammal (45 localities) and plant (52 sections) fossil data collected from the Russian Plain deposits of the Middle Valdai Briansk Interstade indicate a period of noticeable warming. These materials were jointly analyzed. The data were organized in the database software PARADOX and then moved to the GIS program ARC/INFO. Mathematical methods as well as traditional research methods were used for the analysis. The reconstructed biogeographical provinces illustrate the specific environmental and climatic conditions of the Briansk Interstade. Analogues of modern natural zones did not exist at this time on the Russian Plain. The Briansk landscapes reflect the moderate-cool climate of this period, when the majority of northern subarctic plant and mammal species extended their ranges considerably to the south to the Central Russian Plain. At the same time, steppe species penetrated farther north and west. This combination shows that the continuous forest zone was destroyed during the Briansk Inter-stade. Five biogeographical provinces have been reconstructed from the north to the south for the Russian Plain during the Briansk warming on the basis of joint analysis of mammalian and botanical data.     

The Magnitude and Proximate Cause of Ice-Sheet Growth Since 35,000 yr B.P.

The magnitude of late Wisconsinan (post-35,000 yr B.P.) ice-sheet growth in the Northern Hemisphere is not well known. Ice volume at 35,000 yr B.P. may have been as little as 20% or as much as 70% of the volume present at the last glacial maximum (LGM). A conservative evaluation of glacial–geologic, sea level, and benthic δ¹⁸O data indicates that ice volume at 35,000 yr B.P. was approximately 50% of that extant at the LGM (20,000 yr B.P.); that is, it doubled in about 15,000 yr. On the basis of literature for the North Atlantic and a sea-surface temperature (SST) data compilation, it appears that this rapid growth may have been forced by low-to-mid-latitude SST warming in both the Atlantic and Pacific Oceans, with attendant increased moisture transport to high latitudes. The SST ice-sheet growth notion also explains the apparent synchroneity of late Wisconsinan mountain glaciation in both hemispheres.     

Late Quaternary Upwelling Intensity and East Asian Monsoon Forcing in the South China Sea

Foraminifera from two cores off eastern Vietnam and the northwestern Philippines, where modern summer and winter monsoon-driven upwelling occurs in the South China Sea, respectively, were analyzed to evaluate the changes in paleoproductivity and upper water structure over the last 220,000 yr. We observed enhanced organic carbon flux and a shoaled thermocline when upwelling intensified off eastern Vietnam during interglacial ages and off the northwestern Philippines during glacial ages. This indicates that the East Asian summer monsoon increased while the winter monsoon decreased during interglacial ages. Particularly, the upwelling reached a maximum off eastern Vietnam during late marine isotopic stage (MIS) 5 and off the northwestern Philippines during MIS 2, implying that the summer monsoon decreased gradually since MIS 5 while the winter monsoon displayed an opposite trend. The variations in upwelling proxies exhibit a distinct cyclicity with frequencies near 41,000 yr and 23,000 yr off eastern Vietnam, in contrast to a strong frequency peak near 100,000 yr off the northwestern Philippines. We suggest that the East Asian summer monsoon has been forced by changes in solar insolation associated with precession and obliquity, while ice-volume forcing is probably a primary factor in determining the strength and timing of the East Asian winter monsoon but with less important insolation forcing.     

Terminal Pleistocene/Early Holocene Environmental Change at the Sunshine Locality, North-Central Nevada, U.S.A.

Sedimentological, faunal, and archaeological investigations at the Sunshine Locality, Long Valley, Nevada reveal a history of human adaptation and environmental change at the last glacial–interglacial transition in North America's north-central Great Basin. The locality contains a suite of lacustrine, alluvial, and eolian deposits associated with fluvially reworked faunal remains and Paleoindian artifacts. Radiocarbon-dated stratigraphy indicates a history of receding pluvial lake levels followed by alluvial downcutting and subsequent valley filling with marsh-like conditions at the end of the Pleistocene. A period of alluvial deposition and shallow water tables (9,800 to 11,000 ¹⁴C yr B.P.) correlates to the Younger Dryas. Subsequent drier conditions and reduced surface runoff mark the early Holocene; sand dunes replace wetlands by 8,000 ¹⁴C yr B.P. The stratigraphy at Sunshine is similar to sites located 400 km south and supports regional climatic synchroneity in the central and southern Great Basin during the terminal Pleistocene/early Holocene. Given regional climate change and recurrent geomorphic settings comparable to Sunshine, we believe that there is a high potential for buried Paleoindian features in primary association with extinct fauna elsewhere in the region yet to be discovered due to limited stratigraphic exposure and consequent low visibility.     

南海北部1144站中更新世浮游有孔虫的千年尺度古气候记录

南海北部ODP1144站中更新世气候转型时期的浮游有孔虫在千年尺度上以高频率变化为主要特征.冰期旋回主要周期在0.9Ma由41ka转变为100ka, 浮游有孔虫组合也随冰期旋回出现大量的冷水种.据转换函数得出的表层水降温高达10℃发生在0.9~0.6Ma间的转型过渡期, 跨越MIS22、20、18、16四大冰期.同时, 温跃层深度呈阶梯式变浅, 在MIS20上升至65m左右.所以, 南海北部上层水体环境在中更新世气候转型期出现比末次冰期更重的δ18O值, 温跃层变浅, 深水种含量降低或消失, 突出了边缘海区南北气候梯度反差和冬季风在冰期增强的讯号.南海南北部的环境差异与东西太平洋的差异, 共同表明低纬过程在气候变化中的重要性.      

A Late Glacial–Holocene Tephrochronology for Glacial Lakes in Southern Ecuador

Despite the presence of numerous active volcanoes in the northern half of Ecuador, few, if any, distal tephras have been previously recognized in the southern one third of the country. In this article, we document the presence of thin (0.1–1.0-cm-thick) distal tephras comprising glass and/or phenocrysts of hornblende and feldspar in sediment cores from five glacial lakes and one bog in Las Cajas National Park (2°40′–3°00′S, 79°00′–79°25′W). The lake cores contain from 5 to 7 tephras, and each has a diagnostic major element geochemistry as determined from electron microprobe analysis of 710 glass shards and 440 phenocrysts of feldspar and hornblende. The loss of sodium with exposure to the electron microbeam causes a 10±7 wt.% (±1σ) reduction in Na content, which we empirically determined and corrected for before correlating tephras among the sediment cores. We use a similarity coefficient to correlate among the sediment cores; pair-wise comparison of all tephras generally yields an unambiguous correlation among the cores. Six tephras can be traced among all or most of the cores, and several tephras are present in only one or two of the cores. Twenty-six accelerator mass spectrometry ¹⁴C dates on macrofossils preserved in the sediment cores provide the basis for establishing a regional tephrochronology. The widespread tephras were deposited 9900, 8800, 7300, 5300, 2500, and 2200 cal yr B.P. The oldest tephras were deposited 15,500 and 15,100 cal yr B.P., but these are not found in all cores. Two of the tephras appear correlative with volcaniclastic strata on the flanks of Volcán Cotopaxi and one tephra may correlate with strata on the flanks of Volcán Ninahuilca; both volcanoes are in central Ecuador. The absence of tephras in sediment cores correlative with the numerous eruptions of active volcanoes of the past two millennia implies that the earlier eruptions, which did deposit tephras in the lakes, must have been either especially voluminous, or southerly winds must have prevailed at the time of the eruption, or both.     

Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Perú

Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10⁻⁵ to 114×10⁻⁵ km³ yr⁻¹ and 112×10⁻⁵ to 247×10⁻⁵ km³ yr⁻¹ for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10⁻⁵ to 220×10⁻⁵ km³ yr⁻¹, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent.     

Climatic Regionalization and the Spatio-Temporal Occurrence of Extreme Single-Year Drought Events (1500–1998) in the Interior Pacific Northwest, USA

Tree-ring records from western juniper (Juniperus occidentalis var. occidentalis Hook.) growing throughout the interior Pacific Northwest identify extreme climatic pointer years (CPYs) (i.e., severe single-year droughts) from 1500–1998. Widespread and extreme CPYs were concentrated in the 16th and early part of the 17th centuries and did not occur again until the early 20th century. The 217-yr absence of extreme CPYs may have occurred during an extended period of low variance in the Pacific Decadal Oscillation. We mapped climatic boundaries for the interior Pacific Northwest based on the location of sites with similar precipitation variability indices. Three regions, the Northwest (based on chronologies from nine sites), the Southwest (four sites), and the East (five sites) were identified. Our results suggest that western juniper radial growth indices have substantial interannual variability within the northwestern range of the species (central Oregon), particularly when compared with western juniper growing in its eastern range (eastern Oregon, southeastern Idaho, and northern Nevada) and southwestern range (southern Oregon and northeast California). We suspect that the substantial differences in the variability of western juniper radial growth indices are linked to the influence of ENSO events on winter/spring precipitation amounts.     

南海北部晚更新世以来的古海洋环境演化特征——来自N-31柱样的沉积记录

对南海北部N-31柱状样沉积物进行了浮游有孔虫定量分析鉴定、氧碳同位素、碳酸盐含量和粒度测试以及AMS14C测年,并采用古生态转换函数计算了表层海水古温度和温跃层深度,揭示了该海区约55 ka以来的古海洋环境特征.转换函数计算的表层海水古温度结果表明,夏季温度的变化范围为27.0～29.6℃,变化幅度为2.6℃,冬季温...     

Impact of the Kuroshio Current on the South China Sea based on a 115 000 year diatom record

We used the tropical oceanic planktonic diatom species Nitzschia marina, Rhizosolenia bergonii and Azpeitia africana/Azpeitia neocrenulata, most commonly found in the surface sediments of the northeasternmost South China Sea (SCS) where the Kuroshio Current enters the SCS through the Bashi Strait north of Luzon, to analyse the influence of the the Kuroshio Current on the SCS. The impact of the Kuroshio Current seems to be relatively strong during major warm periods and, to a lesser degree, in minor warm periods during the last 115 000 years. The strongest influence took place during the main part of the Holocene and during the very late phase of Marine Isotope Stage (MIS) 5e. It is possible to distinguish two magnitudes of change in the impact of the Kuroshio Current on the SCS: large changes occurred at shifts between glacial and interglacial conditions, while smaller changes seem to have recurred in both glacial and interglacial episodes as well as during the Holocene. Climatic/oceanographic changes and sea‐level variations were possibly the two most important mechanisms for the varying influences of the Kuroshio Current on the SCS. The interaction between the Kuroshio Current and monsoon‐related processes may also have played a role. However, because of restricted knowledge of the present‐day Kuroshio Current and the absence of a modern analogue to the ancient SCS due to the marked changes in palaeogeography, this relationship is difficult to establish. Copyright © 2006 John Wiley & Sons, Ltd.     

A prolonged warm and humid interval during marine isotope stage 13–15 as revealed by hydrographic reconstructions from the South China Sea (IMAGES MD972142)

Variations of orbital-driven insolation, ice volume, and greenhouse gas effects have been proposed as major controlling factors in determining the timing and amplitude of Quaternary cyclic climate changes. However, it remains to be determined how the internal feedback in the tropical atmosphere and ocean and the coupling between the low- and high-latitude systems may have produced instability or non-cyclic changes in the long-term climate evolution. Such “abnormalities” have been reported increasingly from paleoclimatic reconstructions in East Asia and the western Pacific for a prolonged warm and humid climate interval during marine isotope stage (MIS) 13–15, ～475–610 kya. To better address the climate abnormality in MIS 13–15 that has been observed in the western Pacific, here we report high-resolution late Quaternary planktic foraminifer faunal abundance and faunal sea surface temperature (SST) records from the International Marine Past Global Change (IMAGES) program core MD972142, which was retrieved from the southeastern South China Sea (SCS). Our results indicate that the faunal assemblages and SSTs in the southeastern SCS express a substantially prolonged, unusual warm interglacial-type climate condition in MIS 13–15. The climate was abnormally warm during the cold MIS 14. Our study also suggests a lowering of sea surface salinity (SSS) during MIS 13–15. While the western Pacific climate experienced a persistently warm and humid period at MIS 14, a “normal” cooling (～2 °C) condition on the surface of the eastern equatorial Pacific existed concomitantly. While assessing possible interpretations of this “abnormal” climate interval in MIS 13–15, our study indicates that an enhanced interhemispheric and/or longitudinal temperature gradient across the basin-wide Pacific cannot be ruled out. A change in the sensitivity of the East Asian Monsoon (EAM) that controlled temperature and precipitation patterns in East Asia and the western Pacific is probably also important or responsible for this climate abnormality. Based on our evidence, we suggest that the tropical dynamics would have played a role in the climate abnormality in MIS 13–15, through maintaining or even increasing the longitudinal SST gradient in the equatorial Pacific, which may have intensified the low-latitude trade winds in the eastern component of the Walker Circulation that drove a longer duration and/or stronger intensity of the summer vs. winter EAM.     

南海西沙海槽6 Ma以来天然气水合物稳定带演化初探

水深、海底温度、地温梯度是影响天然气水合物稳定带的重要因素。文章通过海平面变化、底栖有孔虫氧同位素、构造热演化资料分析了水深、海底温度、地温梯度的变化,进而探讨了南海西沙海槽地区6 Ma以来天然气水合物稳定带的演化。结果表明：南海西沙海槽地区6 Ma以来天然气水合物稳定带经历了稳定、缓慢减薄、快速增厚、小幅度变化和减薄的一个过程。海底温度的变化对天然气水合物稳定带厚度影响最大,热历史的影响次之,海平面变化的影响最小。     

A fission-track and (U–Th)/He thermochronometric study of the northern margin of the South China Sea: An example of a complex passive margin

Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.     

末次冰期以来南海北部下陆坡区沉积有机质地球化学特征及其意义

选取南海北部下陆坡-深海平原过渡带典型沉积柱状样,通过对其沉积有机质的整体有机地球化学特征进行表征,探讨了南海下陆坡-深海平原区沉积有机质在冰期/间冰期旋回中的赋存状态、来源变化以及与古气候环境之间的相应关系。结果表明,总有机碳（TOC）、总氮（TN）和有机质稳定碳同位素（δ13Corg）与冰期/间冰期旋回有明显的对应关系,冰期时对应高值,间冰期时对应低值;而有机质稳定氮同位素（δ15N）表现为全新世时其值偏低,末次冰消期其值偏高,与气候旋回没有相关性; C/N值和δ13Corg值都表明南海北部下陆坡沉积有机质来源是陆相和海相的混源,且在末次冰期/间冰期尺度上主要以海相来源为主。     

南海北部一百万年以来的表层古生产力变化：来自ODP1144站的蛋白石记录

通过南海北部ODP 1144站蛋白石含量测定及其堆积速率的计算,并结合氧同位素记录等相关资料,获得南海北部1050ka以来高分辨率的表层古生产力变化与冰期旋回和东亚季风的关系。约900ka以来,蛋白石含量及其堆积速率较900ka以前明显增加,反映了“中更新世革命”事件之后,全球气候变冷,并导致表层生产力的提高。由于第四纪冰期旋回中的冬、夏季风的加强,加上1144站特殊的地理位置,使该站在冰期时表层生产力增加,间冰期时表层生产力降低。浮游有孔虫氧同位素记录与蛋白石含量及其堆积速率的时间序列频谱分析结果显示,三者均出现了相对应的偏心率周期、斜率周期和岁差周期,说明该站表层生产力的变化主要受地球轨道周期的驱动。     

Intrusion and its implication for subsidence: A case from the Baiyun Sag, on the northern margin of the South China Sea

We develop a series of simple numerical models to explain the anomalous subsidence and deposition phenomena on the northern continental margin of the South China Sea, in particular in the Baiyun Sag. The results suggest that a short-period high rate deposition of around 17 Ma is related to a rapid subsidence event, which may be due to episodic emplacement of a dense intrusion. Necking and gravity models indicate that in the basement of the Baiyun Sag, there is a dense zone that is 100–200 kg/m³ more dense than the surrounding country rock. Considering its high magnetic intensity and regional igneous activity, the dense zone is thought to be related to a phase of basalt intrusion that may have taken place around 17 Ma. Thermal and subsidence models indicate that a hot denser intrusion can cause significant subsidence immediately after the intrusion. The subsidence rate then slows down with cooling, thus becoming consistent with the observed subsidence curves at around 17 Ma. The results also indicate that the lithospheric strength under the Baiyun Sag is negligible, and that the high-velocity layer in the lowermost crust may be not an original part of the pre-rift crust. Instead, it is thought to be underplated intrusion emplaced at around 30 Ma when the continental margin broke up.     

Palynological records of Holocene monsoon change from the Gulf of Tonkin (Beibuwan), northwestern South China Sea

Palynological records in cores C4 and B106 from the Gulf of Tonkin reveal signals of paleo-monsoon and paleoenvironmental change during the late Pleistocene and Holocene. Before ∼ 13.4 cal kyr BP, the Gulf of Tonkin was exposed to the atmosphere and covered by grassland. Starting at ∼ 11.7 cal kyr BP, the Gulf of Tonkin was inundated by brackish water, indicated by the appearance of the brackish algae Cleistosphaeridium, Sentusidinium and Spiniferites, a decrease of herb content, and an increase of Pinus. After Hainan Island was completely separated from the Leizhou Peninsula by Qiongzhou Strait at ∼ 8.5 cal kyr BP, a continuous marine sedimentary environment was found. The current patterns were similar to those of the present, with a general trend of current homogenization reflected by gradually decreasing quantities of Quercus pollen and a narrowing gap between the palynological concentrations of the southern and northern parts of the region. The data suggest that three short periods of strengthened winter monsoons and currents were centered at ∼ 6.0 cal kyr BP, ∼ 2.7 cal kyr BP and ∼ 0.2 cal kyr BP and that two short periods of strengthened summer monsoons and currents were centered at ∼ 7.5 cal kyr BP and ∼ 3.4 cal kyr BP.     

南海西北次海盆新生代构造 沉积特征及伸展模式 探讨

通过对穿越西北次海盆的3条地震测线以及一条深反射地震剖面的解释,对其新生代的构造 沉积特征进行研究,探讨了伸展模型,并进而对其新生代的构造演化过程和动力学机制进行了分析。结果显示：西北次海盆在30 Ma时开始发育,断层的活动期集中在渐新世,并大致以海盆中部的岩浆岩凸起为轴对称分布,对渐新统的沉积起控制作用。海盆扩张东强西弱,西部显示出更多的陆缘裂谷盆地的特征。25 Ma后扩张轴向南跃迁,西北次海盆的海底扩张运动停止,进入裂后沉降阶段。构造展布方向受到其南侧的中-西沙地块的影响,大致沿其北部边界展布。深反射地震剖面所反映的深部地壳结构也显示出大致沿海盆中轴对称的特征,显示研究区很可能为纯剪的变形模式。     

南海神狐海域Site 4B沉积物地球化学特征及其对甲烷渗漏的指示意义

前人对Site 4B站位海底浅表层沉积物的研究显示该站位历史上可能存在甲烷渗漏事件。为研究该站位的甲烷渗漏历史,分析了Site 4B站位柱状沉积物的元素地球化学特征,及沉积物中Mg/Ca和Sr/Ca比随深度的变化规律以及自生碳酸盐矿物的分布特征。元素相关性分析表明:研究区Ca、Sr等碳酸盐相关的元素极有可能来自于与甲烷渗漏有关的自生碳酸盐。结合沉积物的粒度特征,认为Site 4B站位在15.78 ka B.P.之前泥火山活动造成甲烷渗漏,且甲烷渗漏速率具有增大—减小的周期性变化特点。研究站位在24.15 ka B.P.、27.47 ka B.P.、31.05 ka B.P.具有相对较高的甲烷渗漏速率,且在31.05 ka B.P.达到最大值。研究结果对探明南海北部的甲烷渗漏历史具有重要的意义。