Sedimentary evolution since the late Last Deglaciation in the western North Yellow Sea

To decipher the sedimentary evolution and environmental changes since the late Last Deglaciation, two gravity cores were analyzed from the western North Yellow Sea (NYS). The two cores (B-L44 and B-U35) were sampled for grain size, clay minerals, detrital minerals, and 14C dating. They are comparable in lithofaies, and the observed succession was divided into four depositional units based on lithology and mineral assemblages, which recorded the postglacial transgression. Depositional unit 4 (DU 4) (before 11.5 ka) was characterized with enrichment in sand, and was interpreted as nearshore deposits in shallow water during the Younger Dryas Event. DU 3 (11.5-9.6 ka) displayed a fining-upward succession composed of sediments from local rivers, such as the Huanghe (Yellow) River, and from coastal erosion, which clearly were related to the Early Holocene transgression. Stable muddy deposition (DU 2) in NYS began to form at about 9.6 ka, which received direct supply of fine materials from the Shandong subaqueous clinoform. It is believed that the Yellow Sea circulation system played a major role in controlling the formation of fine sediment deposition in DU 1 (after 6.4 ka) after the sea level maximum.     

萨拉乌苏河流域MGS4层段记录的末次冰期早冰阶气候波动

萨拉乌苏河流域米浪沟湾末次冰期早冰阶时期记录了5.5个由古沙丘砂与河流相、湖沼相构成的沉积旋回。对各沉积旋回的粒度及微量元素分析表明,河流相和湖沼相的平均粒径Mz、分选系数σ和SC/D的值明显高于古沙丘砂,河湖相中的微量元素Mn、P、Rb、Zr、V、Ni含量普遍较古沙丘砂含量高,而Ba、Co等则呈现相反的变化;这些气候代用指标的波动构成了与沉积旋回相对应的5.5个粒度及微量元素变化旋回,指示了该地末次冰期早冰阶至少经历了6次冷干和5次暖湿的气候波动,其中若干次气候回暖事件和冷干事件可与格陵兰冰芯和葫芦洞石笋等记录的D/O事件和Heinrich事件相对应。米浪沟湾剖面地层记录的千年尺度气候波动既是对东亚季风环流演变历史的体现,同时也是对全球气候变化的响应。     

A 17,900-year multi-proxy lacustrine record of Lago Puyehue (Chilean Lake District): introduction

This paper introduces the background and main results of a research project aimed at unravelling the paleolimnological and paleoclimatological history of Lago Puyehue (40° S, Lake District, Chile) since the Last Glacial Maximum (LGM), based on the study of several sediment cores from the lake and on extensive fieldwork in the lake catchment. The longest record was obtained in an 11-m-long piston core. An age-depth model was established by AMS ¹⁴C dating, ²¹⁰Pb and ²³⁷Cs measurements, identification of event-deposits, and varve-counting for the past 600 years. The core extends back to 17,915 cal. yr. BP, and the seismic data indicate that an open-lake sedimentary environment already existed several thousands of years before that. The core was submitted to a multi-proxy analysis, including sedimentology, mineralogy, grain-size, major geochemistry and organic geochemistry (C/N ratio, δ¹³C), loss-on-ignition, magnetic susceptibility, diatom analysis and palynology. Along-core variations in sediment composition reveal that the area of Lago Puyehue was characterized since the LGM by a series of rapid climate fluctuations superimposed on a long-term warming trend. Identified climate fluctuations confirm a.o. the existence of a Late-Glacial cold reversal predating the northern-hemisphere Younger Dryas cold period by 500–1,000 years, as well as the existence of an early southern-hemisphere Holocene climatic optimum. Varve-thickness analyses over the past 600 years reveal periodicities similar to those associated with the El Niño Southern Oscillation and the Pacific Decadal Oscillation, as well as intervals with increased precipitation, related to an intensification of the El Niño impact during the southern-hemisphere equivalent of the Little Ice Age.     

Late Quaternary climatic changes in southern Chile, as recorded in a diatom sequence of Lago Puyehue (40°40′ S)

A late Quaternary diatom stratigraphy of Lago Puyehue (40°40′ S, 72°28′ W) was examined in order to infer past limnological and climatic changes in the South-Chilean Lake District. The diatom assemblages were well preserved in a 1,122 cm long, ¹⁴C-dated sediment core spanning the last 17,900 years, and were in support of an early deglaciation of Lago Puyehue. The presence of a short cold spell in South Chile, equivalent to the Younger Dryas event in the Northern Hemisphere, the Antarctic Cold Reversal in Antarctica, or the Huelmo-Mascardi event in southern South America, was not clearly evidenced in the diatom data, although some climate instability may have occurred between 13,400 and 11,700 cal. yr. BP, and a relatively long period (between 16,850 and 12,810 cal. yr. BP) with low absolute abundances and biovolumes could be tentatively interpreted as a period of low rainfall and/or temperatures. An increase in the moisture supply to the lake was tentatively inferred at 12,810 cal. yr. BP. After 9,550 cal. yr. BP, inferred stronger and longer persisting summer stratification, may have been the result of the higher temperatures associated with an early-Holocene thermal optimum. The mid-Holocene appeared to be characterized by a decrease in precipitation, culminating around 5,000 cal. yr. BP, and rising again after 3,000 cal. yr. BP, likely associated with a previously documented lowered frequency and amplitude of El Niño events. An increase in precipitation during the late Holocene (3,000 cal. yr. BP–present) might have marked subsequent increased frequency of El Niño occurrences, leading to drier summers and slightly moister winters in the area.     

Climate oscillations evidenced by spectral analysis of Southern Chilean lacustrine sediments: the assessment of ENSO over the last 600 years

The Chilean Lake District (38–42°S) is strongly influenced by Southern westerlies-driven precipitations. At 40°S Lago Puyehue provides high resolution sedimentation rates (∼1–2 mm/yr) suitable for annual climate reconstruction. Several short and long sediment cores were collected in this lake. Their analysis aim at a better understanding of climate mechanisms related to ENSO in this part of the world. The recognition of ENSO related periodicities and their stability is studied through the analysis of two short varved cores collected from underflow and interflow key sites. According to varve chronology controlled by ¹³⁷Cs and ²¹⁰Pb profiles and chronostratigraphical markers, the short core from underflow site (PU-I) spans 294 ± 18 years and the core in the interflow site (PU-II) covers 592 ± 9 years. Several methods of spectral analysis were applied on the total varve thickness to identify potential periodicities in the signal. Blackman–Tuckey, Maximum Entropy, Multi-Taper Methods (MTM) and singular spectrum analysis were applied on the whole record. In addition, evolutive MTM and wavelet analyses allow to identify temporal influence of some periodicities. In the PU-I studied interval (AD 1700–2000), a period at ∼3.0 years appears in a large part of the interval, mostly in the recent part. Periods at ∼5.2 and ∼23 years also show up. PU-II record (AD 1400–2000) displays the most robust periodicities at around 15, 9, 4.4, 3.2 and 2.4 years. These periodicities are in good agreement with the sub-decadal periods identified by Dean and Kemp (2004) and linked to the El Nino Southern Oscillation and the Pacific Decadal Oscillation. Differences in the recorded periodicities between PU-I and PU-II sites are consistent with different sedimentation processes in the lake. According to climate instrumental data for the last 20 years, varves in PU-I site are mostly related to fluvial dynamics and regional climate factors, i.e., precipitation, temperature and wind. In PU-II site, varves increment is related to both regional and global climate forcing factors, i.e., El Nino Southern Oscillation. The evolutive MTM analysis and the wavelet analysis suggest a striking break in the periodicities at around AD 1820. Finally relationships between El Nino and longer term climate phase like the Little Ice Age (LIA) are also assessed. This is the seventh in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.     

A 450-ka long record of glaciation in Northern Mongolia based on studies at Lake Khubsugul: high-resolution reflection seismic data and grain-size variations in cored sediments

We use high-resolution reflection seismic data and detailed grain-size analysis of a drill core (KDP-01) from Lake Khubsugul (northern Mongolia) to provide an improved reconstruction of the glacial history of the area for the last 450 ka. Grain-size analysis of suspended sediment load in modern rivers draining into the lake and of moraine material from the northern part of the catchment shows that the silt fraction is transported to the central part of the lake mainly by river suspension, whereas the clay fraction is mainly transported by glacial meltwater during deglaciation. The changes in of the clay/silt ratio in Lake Khubsugul sediments correlates well with the standard global paleoclimate records: low clay/silt ratios indicate warm climates, while a high clay/silt ratio reflects glacial erosion and cold climates. Pulses of clay input into the lake occur at the final stages of glacial periods (i.e., glacial maxima and subsequent onsets of deglaciation). The periodicity in glacial clay input in Lake Khubsugul is in tune with global periods of deglaciation, but there are differences in the intensity of the deglacial events for MIS-12 and MIS-2. These differences are attributed to specific conditions in regional distribution of moisture during glaciation, glacial ice volumes, and solar insolation intensity at the onset of deglaciation. Deglaciation of the Khubsugul glaciers occurred in response to an increase in summer solar insolation above a threshold value of 490 W/m². Two types of deglaciation can be distinguished: (1) slow melting during several tens of 1,000 years during weak increases in summer insolation, and (2) short and fast melting during several thousands of years in response to strong increases in summer insolation. The maximum ice volume in the area of Lake Khubsugul during the past 450 ka occurred during the period of 373–350 ka BP (MIS 11a-10) and was caused by high levels of moisture in the region, whereas the MIS-2 and MIS-12 glacial periods were characterized by minima in ice volume, due to the strong aridity in the region.     

巽他海峡BAR9442岩心30 kaBP以来的沉积记录与古季风活动

为重建巽他海峡区古季风活动历史,通过对BAR9442岩心氧同位素、AMS14C测年、陆源碎屑标准偏差区间粒径分布,以及上升流种Globigerina bulloides等变化特点分析,并结合前人对该岩心粘土矿物的研究,获得如下认识:末次冰期30~17 kaBP,海平面下降,陆源粉砂物质输入增加; 南半球夏季日照率高,然而受冰期热带辐聚带位置影响,西北季风受阻,东南信风持续吹过研究海域形成类似东南季风的效应,气候冷干; 受印度东北季风影响,伊利石含量最高,上升流活动不发育, 古生产力降低.17~8 kaBP,海平面迅速上升,出现较强的表层穿越流活动,粘土物质成为沉积物的主要组分; 此时东南与西北季风都有增强,说明当时研究区气候有着更强的季节性.全新世8 kaBP以来,全球海平面上升使穿越流活动加强,沉积物粘土组分含量最大,西北季风带来温暖湿润的气候条件.      

Geological controls on the geometry of incised‐valley fills: Insights from a global dataset of late‐Quaternary examples

Incised valleys that develop due to relative sea‐level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised‐valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database‐driven statistical analysis of 151 late‐Quaternary incised‐valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised‐valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised‐valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised‐valley fills, probably through effects on relative sea‐level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley‐fill geometry is positively correlated with the associated drainage‐basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley‐fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley‐margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised‐valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley‐fill thickness and present‐day coastal‐prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of ‘source to sink’ sediment route‐ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs.     

中国东部（105o E以东）第四纪冰川研究回顾、进展及展望

中国东部（105^o E以东）第四纪冰川及其演化可为中国东部乃至东亚地区古气候环境重建提供基础信息及关键证据,正确认知冰川发育条件及正确运用冰川遗迹判别标准又是重建中国东部第四纪冰川及其演化的关键。中国东部中低山地第四纪期间冰川发育与否,即泛冰川论与有限冰川论的争议由来已久。近年来,山东与周边地区中低山及沿海丘陵带的“第四纪冰川”研究波澜再起。在系统地回顾中国东部第四纪冰川研究历史及新近出现争议的基础上,结合冰川发育条件、山地冰川地貌组合、判别原则与识别依据、中国东部数座中高山地存在无争议且形态清晰的冰川地形及其测年资料以及东亚古气候环境演化记录等得出：中国东部仅有台湾山地、长白山、贺兰山与太白山等数座主峰海拔超过2 500 m的中高山地保存有晚更新世以来的冰川作用地形。其余山地及海拔更低的丘陵带在第四纪期间任何一次冰川作用中均未达到与当时冰期气候相耦合的高度,不存在泛冰川作用。已报道的“冰川遗迹”皆为各种形似地质地貌现象的系统误判,其确切成因及环境意义有待深入探讨。     

青藏高原东部当子沟末次冰期冰川演化光释光测年

青藏高原东部横断山脉沙鲁里山北支雀儿山北麓的当子沟, 保留了4 组末次冰期冰碛垄, 每组冰碛垄由若干道小冰碛垄组成, 是末次冰期多次冰川波动的理想地貌证据。为恢复该地末次冰期冰川演化历史, 从这4 组冰碛垄中采集了7 个光释光测年样品进行年代测定。等效剂量采用单片再生法(SAR)和标准生长曲线法(SGC)进行测试。年代结果显示:第1~3 组冰碛垄形成于22.4-16.5 ka BP, 属深海氧同位素2 阶段(MIS-2), 大致与全球末次盛冰期(LGM)相当。第4 组冰碛垄形成于MIS-3。MIS-3 冰川规模大于MIS-2。第1~3 组9 道次一级小冰碛垄表明, 约在22.4-16.5 ka BP期间当子沟冰川经历了9 次小波动。冰川在稍早于22.4 ka BP达到MIS-2 的最大范围, 此后规模在波动中逐渐萎缩, 可能是冰川对MIS-2 后期太阳辐射增强、气温回升的响应。最里侧冰碛垄形成于16.5 kaBP, 可能标志着该地冰消期的开始。此后, 当子沟冰川大幅快速退缩。冰川融水在最里侧冰碛垄里侧低洼谷地汇集并被阻塞形成当子错。