Caribbean hydrological variability during the Holocene as reconstructed from crater lakes on the island of Grenada

Contemporary precipitation patterns in the Caribbean region are spatially variable, and the small number of Holocene paleoclimatic records may not adequately capture patterns of variation in the past. The hydrological history of Grenada was inferred from paleolimnological analyses of sediment cores from two crater lakes on the island. The basins were formed by volcanic activity some time during the Last Termination, but were dry between ca. 13 000 and ca. 7200 cal. a BP. After filling, the lakes were initially very shallow, and sedimentation was interrupted by a hiatus ca. 6300–5500 cal. a BP, followed by deposition of a thick tephra in both sites. After 5500 cal. a BP, lake level shows considerable multi‐centennial variability, superimposed upon a long‐term trend of generally higher lake level after 3200 cal. a BP. The pattern of lake‐level variation in Grenada shows some similarity with other Caribbean paleoclimatic records in terms of the timing of transitions, but differs from several classic studies in the sign of inferred precipitation change. The differences among records may reflect spatially variable precipitation patterns in the past in response to the position of the Intertropical Convergence Zone and to sea surface temperature influences on the trade winds and Caribbean low‐level jet. Copyright © 2011 John Wiley & Sons, Ltd.     

The current refugial rainforests of Sundaland are unrepresentative of their biogeographic past and highly vulnerable to disturbance

Understanding the historical dynamics of forest communities is a critical element for accurate prediction of their response to future change. Here, we examine evergreen rainforest distribution in the Sunda Shelf region at the last glacial maximum (LGM), using a spatially explicit model incorporating geographic, paleoclimatic, and geologic evidence. Results indicate that at the LGM, Sundaland rainforests covered a substantially larger area than currently present. Extrapolation of the model over the past million years demonstrates that the current “island archipelago” setting in Sundaland is extremely unusual given the majority of its history and the dramatic biogeographic transitions caused by global deglaciation were rapid and brief. Compared with dominant glacial conditions, lowland forests were probably reduced from approximately 1.3 to 0.8 × 10⁶ km² while upland forests were probably reduced by half, from approximately 2.0 to 1.0 × 10⁵ km². Coastal mangrove and swamp forests experienced the most dramatic change during deglaciations, going through a complete and major biogeographic relocation. The Sundaland forest dynamics of fragmentation and contraction and subsequent expansion, driven by glacial cycles, occur in the opposite phase as those in the northern hemisphere and equatorial Africa, indicating that Sundaland evergreen rainforest communities are currently in a refugial stage. Widespread human-mediated reduction and conversion of these forests in their refugial stage, when most species are passing through significant population bottlenecks, strongly emphasizes the urgency of conservation and management efforts. Further research into the natural process of fragmentation and contraction during deglaciation is necessary to understand the long-term effect of human activity on forest species.     

乌海盆地内末次冰期砂楔/冰楔铸型的发现及古气候意义

砂楔/冰楔铸型等具有定量古环境意义的冰缘现象在中国北方已有多处报道,但多集中在末次冰期晚冰阶。报道了首次发现于乌海盆地的冰缘现象,从楔体形态特征、填充物粒度特征和光释光测年等方面对其成因和发育年代进行了深入研究。楔体和填充物剖面穿插关系、物质组成和形态特征分析等指示其形成过程大致可分为两个阶段:早期原生砂楔发育与楔壁近平行的充填-挤压构造;晚期冰楔铸型(包括冰楔形成、融化和填充)在砂楔之上进行叠加。冰楔切穿早期砂楔,导致楔壁物质发生强烈挤压变形,结构扰动,楔体表现为弯曲的不规则形态。光释光测年结果显示,原生砂楔发育的年代主要集中在(62.69±6.81)~(66.45±7.08)ka B.P.,即末次冰期早冰阶,对应于MIS4阶段,并可能部分延续至MIS3b阶段;冰楔铸型填充物年代为(5.95±0.61)~(6.62±0.73)ka B.P.,对应于全新世中期。由于冰楔铸型的填充晚于冰楔发育年代,推测冰楔发育年代可能为末次冰期晚冰阶。基于砂楔/冰楔铸型的发育过程及对环境条件的要求,初步估算出砂楔和冰楔发育时期年均气温比现在低12.1 ℃~13.6 ℃,砂楔发育时期(即末次冰期早冰阶)年降水量与现在相当。乌海盆地砂楔/冰楔铸型的发现为研究中国北方末次冰期尤其是末次冰期早冰阶纬度多年冻土分布及古气候环境提供了新的依据。     

西藏札达盆地上新世—早更新世气候变迁与湖泊演化

通过孢粉组合分析, 结合河湖相地层岩性特征和古地磁及电子自旋共振(ESR)法年龄测定结果, 探讨了西藏札达盆地上新世—早更新世的古气候变迁与札达古湖泊演化的关系.研究表明, 札达盆地古湖泊演化可划分为早(湖泊形成期)、中(稳定发展期)、晚(湖泊消亡期)三期.早期(距今5.41~4.40 Ma), 札达盆地为温凉而干旱的疏林草原植被气候, 随后转变为温暖稍湿的森林草原植被气候, 最后转变为温暖潮湿的亚热带针阔叶混交林气候, 这一时期古湖开始形成; 中期(距今4.40~2.57 Ma), 古气候进入寒温期, 古植被表现为先由暖温带针阔叶混交林带向山地寒温带暗针叶林带过渡, 再由山地暖温带针阔叶混交林→山地寒温带暗针叶林交替出现的过程, 古湖泊进入发育期; 晚期(距今2.57~1.36 Ma), 湖区古气候环境进入寒冷期, 古植被为山地寒温带暗针叶林→山地暗针叶林向低矮灌木→干冷草原的变化, 古湖泊进入消亡阶段.古湖泊演化与古植被、古气候演变有很好的相关性, 高原隆升控制了古气候环境的变化, 进而影响湖泊水量的变化.      

阿西克剖面记录的西天山地区黄土磁学性质及古气候意义初探

选取阿西克剖面作为研究对象,对伊犁地区末次间冰期以来的沉积的黄土地层的磁学特征进行分析。结果发现,伊犁地区黄土地层的磁性矿物有磁铁矿、磁赤铁矿、赤铁矿、针铁矿,磁学性质受亚铁磁性的磁铁矿（和磁赤铁矿）控制;MD和PSD是地层中亚铁磁性矿物的主要磁畤状态,随着成壤强度增加,地层中的SP和SD颗粒相对含量逐渐增加;亚铁磁性矿物含量与地层并不完全对应。对磁学参数控制机制的分析表明,本剖面磁学性质受控因素比较复杂,磁性矿物含量主要受控于物源的磁学性质,成壤作用对地层中细粒磁性矿物含量具有很大影响,还原作用可能对底部古土壤层的磁学性质造成一定影响,但对整个剖面的磁学性质影响相对较小。     

冀中坳陷饶阳凹陷中北部古近纪沙河街组古生态分析

基于对冀中坳陷饶阳凹陷中北部42口井孢粉化石的鉴定,讨论了该区沙河街组孢粉植物群的古生态分类,建立了一个Quercoidites-Ulmipollenites高比例组合和5个亚组合:Ephedripites-Ulmoideipites亚组合,Pinaceae-Ephedripites-Podocarpidites亚组合,Ephedripites-Pinaceae-Labitricolpites亚组合,Pinaceae-Podocarpidites亚组合和Piceaepollenites-Ephedripites-Cedripites亚组合。通过与黄骅坳陷滩海沙河街组孢粉组合对比,根据研究区各时期孢粉组合的不同特征,探讨了饶阳凹陷中北部古近纪沙河街组古生态的变化。结果表明:研究区各孢粉组合中生组分比例稳定,平均70%以上,而旱生和湿生孢粉组分比例变化细微;沙四段晚期植被类型以常绿—落叶阔叶林为主,属半干旱暖温带气候;沙三段植被类型以针叶林为主,阔叶林混生,属半湿润亚热带气候;沙二段植被类型是针叶混交林,属半干旱亚热带气候;沙一段植被类型以针叶混交林为主,湿生草本植被增多,属湿润亚热带气候。     

湖北神农架地区南华系大塘坡组元素地球化学特征

湖北神农架地区南华系大塘坡组属于间冰期沉积,目前对其沉积地层及古环境等方面的认识有限。在神农架地区南华系大塘坡组野外调查的基础上,利用元素地球化学分析测试资料,系统研究了该地区大塘坡组古环境特征及其对油气勘探的意义。CaO/(CaO+Fe)、Sr/Ba、Rb/K、K/Na值表明神农架地区大塘坡组沉积水体盐度整体为中—低盐度,其中大九湖剖面沉积水体盐度比宋洛剖面沉积水体高。氧化-还原性敏感元素相关参数V/(V+Ni)、Ni/Co、Ce_anom值研究表明,神农架地区南华系大塘坡组沉积于弱氧化—弱还原水体。CIA、ICV、Sr/Cu、Sr/Ba、Rb/Sr、Mg/Ca值和Sr含量等研究表明,神农架地区大塘坡组沉积期属于温暖湿润气候环境。总有机碳(TOC)、古生物资料以及Cu和Ba含量综合分析表明神农架地区大塘坡组沉积期温暖湿润气候以及合适的水体条件,为藻类的大量繁殖提供了优越的环境,并为大塘坡组优质烃源岩的发育提供了充足的有机质来源。     

豫西宜阳地区晚二叠世晚期—早三叠世沉积记录及其对古气候变化的响应

晚二叠世-早三叠世宜阳地区位于东特提斯低纬度地区的华北板块南缘,发育一套连续且沉积特征显著的陆相碎屑 岩相沉积,是研究陆相晚二叠世-早三叠世古环境与古气候变化的理想地层。本文在前人研究与野外实测的基础上,采用 岩石学、沉积学、古生物学方法综合分析研究区的沉积记录,识别出宜阳地区在该时期由陆源近海的滨湖沉积环境向河流 沉积环境过渡的古环境,重建了晚二叠世晚期-早三叠世宜阳地区的古地理面貌。沉积特征显示,此期整体气候条件为炎 热-半干旱,并且在长兴期末期与奥伦尼克期早期出现了极端干旱气候,可能是受到当时全球性巨型季风环流的影响。这 一时期的古气候与古环境的变化可能对二叠纪末陆地大灭绝事件起到了推动作用,并抑制了早三叠世生物复苏的进展。     

江西九江红土生物标志化合物的分布特征及其古气候环境意义

为了探求江西九江红土地层中所记录的古气候环境信息,论文在对江西九江红土剖面的野外地貌、岩性和相关沉积环境调查分析基础上,利用气相色谱-质谱联用仪对正构烷烃、一元正脂肪酸等生物标志化合物进行了研究,着重分析了正构烷烃和一元正脂肪酸的来源及其碳数分布特征。结果显示:江西九江红土剖面上部均质红土中正构烷烃的CPI、∑C_(21)~–/∑C_(22)~+及一元正脂肪酸的CPI_L值均大于其下部的网纹红土,且波动较为明显,表明网纹红土的形成气候比其上的均质红土形成时期气候暖湿,反映了中更新世中期(600～300kaBP)的极端暖湿期;正构烷烃的∑C_(21)~–/∑C_(22)~+、(C_(15)+C_(17)+C_(19))/(C_(27)+C_(29)+C_(31))、(C_(16)+C_(17)+C_(18))/(C_(29)+C_(31)+C_(33))、C_(17)/C_(31)在剖面上表现出一致的旋回性变化,反应了江西九江中更新世以来气候的冷暖旋回变化。结果表明红土地层中的生物标志化合物具有很好的古气候环境意义。     

吐哈盆地硫磺山铜多金属矿床表生黄钾铁矾~(40)Ar/~(39)Ar定年及对风化、地貌演化及古气候的启示

黄钾铁矾是干旱—半干旱地区硫化物矿床氧化带中一种常见表生含钾硫酸盐矿物,对其进行~(40)Ar/~(39)Ar年代学研究,不仅可以直接限定硫化物矿床次生富集的时间,还可以为区域地貌形成演化和古气候演变等重大地质事件提供关键的年代学信息。本文对中国新疆吐哈盆地硫磺山铜多金属矿床氧化带中三个表生黄钾铁矾样品进行了~(40)Ar/~(39)Ar定年,第一次获得了始新世的风化矿物的年龄。结合已发表的年龄数据显示:在吐哈盆地,海拔较高的矿床氧化带剖面(如硫磺山矿床)记录了基岩的持续和较完整的区域风化事件,这些老的风化年龄的存在证明了剥蚀并不是均匀的,即其地貌演化遵循的是斜坡后退模型。对于吐哈盆地的古气候,这些表生硫酸盐的年龄表明:始新世以来吐哈盆地曾多次出现过有利于化学风化和硫化物矿床次生富集的干旱—半干旱气候,且主要集中在以下几个时期:27.7~23.3 Ma、16.4~14.7 Ma、11~7.8 Ma,之后气候开始向极端干旱气候转变,4.1~3.3 Ma之后吐哈盆地可能已经流行大规模的极端干旱气候。