Paleohydrology and paleoclimate records in a saline prairie lake core: mineral,isotope and organic indicators

This pilot study examines the potential of obtaining a sedimentary record of paleoenvironmental/climatic/hydrologic conditions for saline Redberry Lake in southern Saskatchewan, Canada. The tools are mineralogy, stable isotopes and pigments. The upper meter of an offshore sediment core contains 10 to 20% by weight aragonite (CaCO₃), which apparently precipitated in the water column. The ¹⁸O and ¹³C of the bulk aragonite (corrected for content of detrital calcite) vary by 4 to 5. Enrichment in ¹⁸O in aragonite is significantly correlated with pigment concentrations (chlorophyll a, phaeophytin). The ¹⁸O and pigment data provide evidence for relatively dry and/or warm conditions and high limnetic productivity for the period 2500 to 1500 yrs B.P. After 1500 B.P., the climate was apparently similar to the present, with two episodes of relatively enhanced productivity, dryness and/or warmth, at around 1000 to 900 and 500 to 200 B.P. During the past century, Redberry Lake has decreased approximately 8 m in depth and its salinity has doubled. No clear sedimentary signal was observed in response to these recent hydrologic trends. These changes have not been associated with a significant climate trend in the region, but may have been induced by land use changes in the catchment.This publication is the third of a series of papers presented at the Conference on Sedimentary and Paleolimnological Records of Saline Lakes. This Conference was held August 13–16, 1991 at the University of Saskatchewan, Saskatoon, Canada. Dr. Evans is serving as Guest Editor for this series.     

δO of carbonate, quartz and phosphate from belemnite guards: implications for the isotopic record of old fossils and the isotopic composition of ancient seawater

Belemnite guards of Cretaceous and Jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum. The oxygen isotopic composition of coexisting carbonate, quartz and phosphate from the same rostrum was measured according to well-established techniques. None of these compounds showed isotopic values in equilibrium with one another. Assuming δ¹⁸O values of the diagenetic water within the range of meteoric waters, the δ¹⁸O(SiO₂) yield temperatures in agreement with the apparent secondary origin of this phase. The δ¹⁸O(CO₃²⁻) range, with a certain continuity, between −10.8 and +0.97 PDB-1 with most of the intermediate values being within the range of the carbonate isotopic values of Mesozoic fossils. The most positive isotopic results obtained from phosphate are close to +23/+24‰ (V-SMOW). They can hardly be related to a secondary origin of the phosphate, or to the presence of diagenetic effects, since these results are among the most positive ever measured on phosphate. As far as we know there is no widespread diagenetic process determining an ¹⁸O enrichment of phosphate. The very low concentration of phosphate did not allow the determination of its mineralogical composition. All the available δ¹⁸O(PO₄³⁻) values from belemnite and non-belemnite fossils of marine origin of Tertiary and Mesozoic age are reported along with the newly measured belemnites. The following conclusions may be drawn from the data reported: (1) the pristine oxygen isotope composition of fossil marine organisms (either carbonate or phosphate) may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water; this process is apparent even in the case of geologically recent fossils; (2) the δ¹⁸O(PO₄³⁻) of belemnite rostra seems to be, at least in the case of the most positive results, in isotopic equilibrium with environmental water because of the similarity between the results from Cretaceous belemnites and the results from Cretaceous and Lower Tertiary pelecypods and fish teeth; 3) if so, the only feasible interpretation that can be suggested for the ¹⁸O enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from Jurassic to recent time.     

The ClBrI composition of 3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry

Fluid inclusion leachates obtained from vug and vein quartz samples from an Archean (3.23 Ga) Fe-oxide hydrothermal deposit in the west-central part of the Barberton greenstone belt, South Africa, were analyzed by ion chromatography for chloride, bromide, and iodide. The deposit, known as the ironstone pods, formed by seafloor hydrothermal activity and fluid discharge. Quartz is dominated by type I liquid-vapor, aqueous inclusions with a bimodal salinity distribution (0–0.25 M_Cl⁻ and 0.9–1.8 M_Cl⁻). Bulk analytical salinities range from 0.45 to 0.99 M_Cl⁻ represent averages of type I inclusions. Bulk fluid inclusion bromide and iodide concentrations are 1.44–3.32 mM and 0.01–0.12 mM, respectively. For comparison, modern seawater has halogen contents of 590 mM chloride, 0.9 mM bromide, and 0.5 μM total iodine. In the fluids from the ironstone pods, bromide and iodide are enriched relative to chloride, when compared with modern seawater.Approximate Br⁻Cl⁻ and I⁻Cl⁻ ratios of 3.2 Ga Barberton seawater are 2.5 × 10⁻³ and 40 × 10⁻⁶, respectively. Dispersion to higher values was caused principally by reaction with organic sediments whose trends are similar to those seen for modern vent fluids at unsedimented and sedimented ridges, relative to modern seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromine and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values.     

桂西地区二叠系合山组铝土岩的沉积环境*

桂西地区二叠系合山组铝土岩的沉积环境存在海相、陆相之争。通过对桂西二叠系铝土岩的区域调查及钻孔样品微量元素地球化学数据的分析,认为桂西二叠系铝土岩产出于陆相环境。从铝土岩分布情况看,桂西二叠纪各孤立碳酸盐岩台地上均广泛分布铝土岩沉积,该区域产出特征证明矿层是在碳酸盐岩台地全部暴露地表的环境下形成的。从铝土岩层与下伏茅口组灰岩的接触关系来看,铝土岩层底部存在茅口组灰岩砾石,在部分剖面尚见下伏茅口组灰岩中充填铝土岩等现象,说明铝土岩矿化过程直接于茅口组灰岩的古喀斯特面上完成。此外,在矿层中发现古植物根茎化石也是铝土岩形成于陆相环境的有力证据。从微量元素地球化学结果来看,来自铝土岩层样品的硼含量均小于10×10^-6,Sr/Ba值范围为0.29~1.61,平均值为0.92,Rb/K值范围为0.0011~0.0025,平均值为0.0017,以上指标结合V/Zr-Zr/Cu图解的判别,结果均指示铝土岩形成于陆相淡水环境。     

Chironomid-based paleosalinity records in southern British Columbia, Canada: a comparison of transfer functions

Chironomid remains from Big Lake, British Columbia were analysed and paleosalinities were estimated using a pre-existing transfer function and several developed using new regression methods. A two component partial-least-squares model (PLS-2) had the highest coefficient of determination (R² _(Jackknifed) = 0.75) and lowest root-mean-squared error-of-prediction (RMSEP). As compared to the pre-existing model, it was also less sensitive to the influence of rare taxa. Nevertheless, the marginally larger R² _(Jackknifed) and lower RMSEP do not clearly identify a single best model. The models were applied to Big, Mahoney and Kilpoola lakes, revealing the sensitivity of paleosalinity inferences to model selection. A synopsis of chironomid-based paleosalinities in British Columbia and their correspondence with other paleoclimatic data are presented and discussed.     

近800a来内蒙古岱海湖水的盐度定量及其气候意义

在干旱半干旱地区 ,封闭湖泊的湖水盐度对气候变化反应敏感。首先通过现生介形类 (Lim nocytherecf.inopinata)壳体与湖水Sr/Ca比值的测定确定了Sr/Ca的分配系数 ;然后测试现代岱海湖水盐度与湖水Sr/Ca比值 ,建立湖水盐度与湖水Sr/Ca比值的函数关系 ;最后利用湖泊沉积剖面中介形类壳体的Sr/Ca比值定量恢复了岱海湖水近 80 0a来的盐度。通过分析岱海湖水盐度变化过程 ,揭示了研究区小冰期前期降水增加的冷湿气候 ,这明显不同于东部其它地区的冷干气候 ;研究区在小冰期中后期以偏干旱气候为主 ,这与东部其它地区干冷气候相一致 ;推测了 2 1世纪初 10～ 2 0a ,岱海地区降水将有所增加 ,干旱趋于缓和。     

沧东—南皮凹陷孔店期古盐度与古环境演化

内陆封闭湖泊的盐度与湖平面具有同步反向变化的特点,通过古盐度研究可推测沉积时期的古气候状况。通过Sr/Ba、Th/U含量比值及C、O同位素的含量变化分析,对沧东—南皮凹陷孔店期各层序古盐度的变化进行系统研究,结合孢粉及岩性组合,推断该区孔店期的水体性质及各层序间古环境的演化。综合认为孔店期整体水体性质为半咸—微咸水,水体深度呈浅→深→浅的变化规律,古气候变化特征为半干旱→湿润→半干旱→干旱气候。     

浅议塔河地区“东河砂岩”三角洲沉积

近年来在塔河地区钻揭了一套典型的河控三角洲相“东河砂岩”,它与东河塘、塔中和哈得逊等地区发现的前滨相和滨面相“东河砂岩”明显不同.钻井岩心上该套三角洲相砂岩表现为板状交错层理、平行层理厚砂体;粒度概率具有河道、河口坝微相特征;平面上地震振幅、反射强度斜率属性异常呈以S99、T728井为中心的“鸟足状”外形;古盐度测试结...     

陕北子洲地区二叠纪山西组沉积环境

针对陕北子洲地区山西组是否受海水影响的争议,对该组岩石进行了微量元素测试。结果显示山西组的古环境以淡水为主,但间或受海水的影响。根据野外露头和岩心观测,依据沉积旋回、岩电特征、沉积构造等,识别出研究区山西组为曲流河三角洲,发育三角洲前缘,以水下分流河道广泛发育为特征,并构成研究区天然气的主要储集砂体,可进一步划分为水下分流河道、分流间湾、间湾沼泽等微相;论述了山23段沉积微相的平面展布。      

从济阳坳陷沙一段古盐度的横向变化看古近纪的海侵方向

在济阳坳陷全区层序地层统一精细划分的基础上,利用古盐度指标B/Ga及济阳坳陷层序Ⅳ（层位大致相当于沙二上-沙一中）的湖侵体系域的古盐度对各凹陷进行了横向对比,发现惠民凹陷、东营凹陷的B/Ga比值较低（3.01～4.16）;车镇凹陷较高（4.08～8.26）;沾化凹陷大致为6～6.95,车镇凹陷东部-沾化凹陷是济阳坳陷此期古盐度最大的区域,很可能代表沙一段时期的海侵通道,这为济阳坳陷古近纪存在海侵的推论提供了一个新的证据.