Cave air control on dripwater geochemistry, Obir Caves (Austria): Implications for speleothem deposition in dynamically ventilated caves

There are very few process studies that demonstrate the annual variation in cave environments depositing speleothems. Accordingly, we initiated a monitoring program at the Obir Caves, an Austrian dripstone cave system characterized by a seasonally changing air flow that results in a predictable pattern of high pCO₂ during summer and low pCO₂ in winter. Although similar seasonal changes in soil pCO₂ occur, they are not directly connected with the changes in the subsurface since the dripwaters are fed from a well-mixed source showing little seasonal variation. Cold season flushing by relatively CO₂-poor air enhances degassing of CO₂ in the cave and leads to a high degree of supersaturation of dripwater with regard to calcite. Forced calcite deposition during the cold season also gives rise to a pronounced pattern of synchronous seasonal variations in electrical conductivity, alkalinity, pH, Ca and δ¹³C_DIC which parallel variations recorded in δ¹³C_{cave air}. Chemical components unaffected by calcite precipitation (e.g., δD, δ¹⁸O, SiO₂, SO₄) lack a seasonal signal attesting to a long residence in the karst aquifer. Modeling shows that degassing of CO₂ from seepage waters results in kinetically-enhanced C isotopic fractionation, which contrasts with the equilibrium degassing shown from the Soreq cave in Israel. The Obir Caves may serve as a case example of a dripstone cave whose seepage waters (and speleothems) show intra-annual geochemical variability that is primarily due to chemical modification of the groundwater by a dynamic, bidirectional subsurface air circulation.     

云南白水台钙华水池中水化学日变化及其生物控制的发现

为弄清云南白水台泉及其下游钙华水池中水化学的日变化,选取1号泉及其流经的两个钙华水池(6号和10号)作为研究对象并对其水温、pH值和电导率进行了自动监测.根据Ca2+、HCO3-与电导率存在的线性关系,用WATSPAC软件计算了水中方解石的饱和指数和Pco2.监测发现:泉水不存在显著的水化学日动态变化,而两个钙华水池表现出显著的日动态变化.其中10号钙华水池在白天温度较高时水中的CO2大量逸出并通过水下水生植物的光合作用加速了水中碳酸钙的沉积.6号钙华水池水生植物生长茂盛,其叶片和部分枝干露出水面,因而光合作用主要发生在空中,所以此处水化学表现为白天pH值降低和电导率升高的反常现象,即由温度主导的根呼吸作用,在白天释放更多的CO2进入水体而使沉积下来的碳酸钙重新溶解.     

中国中生代木化石研究新进展:多样性变化及古气候波动

作为植物化石的重要组成部分,木化石不仅在探究地质历史时期植物群的组成特征和植物界演化方面发挥着重要作用,而且也是反映陆地生态系统古气候变化和古生态环境重建的重要证据之一。中国中生代木化石不仅数量丰富,而且类型多样,研究成果丰硕。近年来,在四川、重庆、云南、辽西和新疆等三叠纪—白垩纪化石产地和层位陆续发现了一批木化石新材料,进一步丰富了我们对中国中生代木化石的多样性的认识,并在利用木化石揭示三叠纪至白垩纪陆相古气候特征和变化方面取得积极进展。文中重点介绍在四川盆地晚三叠世、辽西侏罗纪燕辽生物群及早白垩世热河生物群中的木化石新发现并分析其古气候指示意义,揭示上述化石所反映的气候降温事件、古气候波动及其与带毛恐龙出现的关系等。通过对中生代木化石多样性演化、时空分布模式以及木材生长轮解剖特征的研究和分析揭示侏罗纪和白垩纪的古气候整体特征,并对中国中生代木化石研究领域存在的问题和未来的研究方向提出展望。     

Seasonal dripwater Mg/Ca and Sr/Ca variations driven by cave ventilation: Implications for and modeling of speleothem paleoclimate records

A 4-year study in a central Texas cave quantifies multiple mechanisms that control dripwater composition and how these mechanisms vary at different drip sites. We monitored cave-air compositions, in situ calcite growth, dripwater composition and drip rate every 4-6 weeks. Three groups of drip sites are delineated (Groups 1-3) based on geochemical variations in dripwater composition. Quantitative modeling of mineral-solution reactions within the host carbonate rock and cave environments is used to identify mechanisms that can account for variations in dripwater compositions. The covariation of Mg/Ca (and Sr/Ca) and Sr isotopes is key in delineating whether Mg/Ca and Sr/Ca variations are dictated by water-rock interaction (i.e., calcite or dolomite recrystallization) or prior calcite precipitation (PCP). Group 1 dripwater compositions reflects a narrow range of the extent of water-rock interaction followed by varying amounts of prior calcite precipitation (PCP). Group 2 dripwater compositions are controlled by varying amounts of water-rock interaction with little to no PCP influence. Group 3 dripwater compositions are dictated by variable extents of both water-rock interaction and PCP. Group 1 drip sites show seasonal variations in dripwater Mg/Ca and Sr/Ca, whereas the other drip sites do not. In contrast to the findings of most previous dripwater Mg/Ca-Sr/Ca studies, these seasonal variations (at Group 1 drip sites) are independent of changes in water flux (i.e., rainfall and/or drip rate), and instead significantly correlate with changes in cave-air CO₂ concentrations. These results are consistent with lower cave-air CO₂, related to cool season ventilation of the cave atmosphere, enhancing calcite precipitation and leading to dripwater geochemical evolution via PCP. Group 1 dripwater Mg/Ca and Sr/Ca seasonality and evidence for PCP as a mechanism that can account for that seasonality, have two implications for many other regions where seasonal ventilation of caves is likely: (1) speleothem trace-element records may provide seasonal signals, and (2) such records may be biased toward recording climate conditions during the season when calcite is depositing. Additionally, we use our results to construct a forward model that illustrates the types of speleothem Mg/Ca and Sr/Ca variations that would result from varying controls on dripwater compositions. The model provides a basis for interpreting paleo-dripwater controls from high frequency Mg/Ca and Sr/Ca variations for speleothems from caves at which long term monitoring studies are not feasible.     

A high‐resolution multi‐proxy stalagmite record from Mechara,Southeastern Ethiopia: palaeohydrological implications for speleothem palaeoclimate reconstruction

An annually laminated stalagmite from Southeastern Ethiopia grew for 443±8 yr starting at 5023 yr BP (±160 yr) as determined by lamina‐tuned U‐Th dating. The speleothem shows three growth phases: (1) an initial phase where the stalagmite has a candlestick shape, deposited by relatively slow rate drip water; (2) a middle growth phase where laminae grow on the flanks, suggesting faster drip rates; and (3) a final growth phase where the speleothem becomes narrower owing to a decrease in drip water supply towards the end of deposition. Morphometry, annual growth rate, fluorescence index, and ²³⁴U/²³⁸U isotope ratio show marked differences between the growth phases, while δ¹³C and δ¹⁸O do not show significant variation, except within the third phase towards the top of the stalagmite, where they have higher values. The study indicates that the multiproxy approach is crucial as the different proxies respond to single climate forcing in different manners. Our results can be widely applied to speleothem studies distinguishing the relative importance of the various proxies in recording cave ‘external’ (climate, hydrology) and cave ‘internal’ (e.g. evaporation) processes. Copyright © 2006 John Wiley & Sons, Ltd.     

Spatial variations of monsoonal rain in eastern China: Instrumental, historic and speleothem records

We select four caves and their nearby cities in the monsoonal region of China for studying the relationships among precipitation, temperature, summer monsoon intensity and stalagmite δ¹⁸O. The instrumental, historic and stalagmite δ¹⁸O records from these areas show strong spatial disparities on annual to decadal scales, so that climatic conditions in a single location cannot represent these of the entire eastern China. On time scales <500 years, stronger summer monsoon may lead to higher rainfall in some locations but not over eastern China. Correlation between the summer monsoon strength and precipitation is not only location-dependent but also changes with time. One may not use the paleoclimatic pattern of cold–dry and warm–wet on glacial/interglacial ages throughout all time scales for climatic conditions in the monsoonal region. On centennial to millennial scales, stalagmite δ¹⁸O variation trend from eastern China resemble solar irradiance with lighter δ¹⁸O corresponding to increased solar irradiance, and vice versa. The similar trends may reflect climatic feedbacks link to solar forcing to cause changes in the summer monsoon intensity and/or in monsoonal circulation. Changes in monsoonal circulation and intensity affect (1) summer rainfall intensity, (2) summer/winter precipitation ratio, or (3) ratio of moisture from Indian/Pacific oceans, or a combination of the three. Thus, a speleothem δ¹⁸O record may not be proper to be used as a proxy of paleo-precipitation amount, especially on short time scales. Based on the four stalagmite δ¹⁸O records during the last 2000 years, EASM strength decreased from AD 200 to AD 500, and from AD 1300 to AD 1600 (the 1st half of the Little Ice Age), whereas EASM strength increased from AD 1600 to AD 1900 (the 2nd half of the Little Ice Age). The EASM strength has weakened since early 1900’s.     

川东北楼房洞洞穴系统水体元素含量季节变化与影响因素

通过对川东北诺水河地区楼房洞洞穴系统水体Ca、Mg、Sr、Ba和U元素含量变化进行为期一年(2011年7月至2012年6月)的监测,结果发现:(1)各监测点的元素含量变化一般具有较明显的季节性,但不同的监测点之间、不同的元素之间季节变化趋势并不一致,反映了在不同的环境下不同的元素含量变化的影响机制存在差异;(2)对于2011年9月的强降水事件,河水的元素含量明显受到稀释作用影响,而洞穴内水体尤其是池水的元素含量变化明显较弱,这反映了洞穴上覆地层对外部降水事件的缓冲作用,特别是洞穴内池水由于存在更复杂的影响机制,其对外界的降水事件的响应最不敏感;(3)在所有监测点,Sr含量和Sr/Ca比值变化均表现出冬春季节相对较高而夏秋季节较低的特点,可能反映了水岩相互作用和大气沉降活动是影响Sr含量和Sr/Ca比值变化的主要机制。这对该地区岩溶洞穴沉积中的Sr含量(或Sr/Ca比值)和87Sr/86Sr比值作为研究大气粉尘活动和冬季风强度变化指标的观点给予了支持。     

Deconstructing the Transvaal Supergroup, South Africa: implications for Palaeoproterozoic palaeoclimate models

Current correlations between the Pretoria and Postmasburg Groups of the Transvaal Supergroup are shown to be invalid. The Postmasburg Group is also demonstrated to be broadly conformable with the underlying Ghaap Group and therefore considerably older (2.4 Ga) than previously supposed. The new stratigraphy documents an extensive (100 Ma) and continuous cold-climate episode with a glacial maximum at the Makganyene Formation diamictite. Iron formations of the underlying Asbesheuwels and Koegas Subgroups and overlying Hotazel Formation have similar origins, related, respectively, to the onset and cessation of the glacial event. This interpretation of the Transvaal Supergroup stratigraphy has significant implications for various Palaeoproterozoic environmental models and for the timing of the development of an oxygenated atmosphere.     

Influences of air CO<Subscript>2</Subscript> on hydrochemistry of drip water and implications for paleoclimate study in a stream-developed cave,SW China

Cave air CO₂ is a vital part of the cave environment. Most studies about cave air CO₂ variations are performed in caves with no streams; there are few studies to date regarding the relationship of cave air CO₂ variations and drip water hydrochemistry in underground stream–developed caves. To study the relationship of underground stream, drip water, and cave air CO₂, monthly and daily monitoring of air CO₂ and of underground stream and drip water was performed in Xueyu Cave from 2012 to 2013. The results revealed that there was marked seasonal variation of air CO₂ and stream hydrochemistry in the cave. Daily variations of cave air CO₂, and of stream and drip water hydrochemistry, were notable during continuous monitoring. A dilution effect was observed by analyzing hydrochemical variations in underground stream and drip water after rainfall. High cave air CO₂ along with low pH and low δ¹³C_DIC in stream and drip water indicated that air CO₂ was one of the dominant factors controlling stream and drip water hydrochemistry on a daily scale. On a seasonal scale, stream flows may promote increased cave air CO₂ in summer; in turn, the higher cave air CO₂ could inhibit degassing of drip water and make calcite δ¹³C more negative. Variation of calcite δ¹³C (precipitated from drip water) was in reverse of monthly temperature, soil CO₂, and cave air CO₂. Therefore, calcite δ¹³C in Xueyu Cave could be used to determine monthly changes outside the cave. However, considering the different precipitation rate of sediment in different seasons, it was difficult to use stalagmites to reconstruct environmental change on a seasonal scale.     

Processes controlling the retention and release of manganese in the organic-rich catchment of Loch Bradan,SW Scotland

Loch Bradan, a drinking water reservoir in SW Scotland, frequently exhibits unacceptably high dissolved Mn concentrations. Both the surrounding catchment and the loch sediments are potential sources of Mn to the loch water. This study focused on the catchment soils, which are peaty, and found that redox cycling was an important process with respect to retention of Mn in the top sections (0–15 cm). Under more reducing conditions, reduction to Mn(II) and subsequent complexation by humic substances was observed at greater depth in some soil profiles. Complexation by humic substances is important because lateral water flow can remove soluble complexes and indeed this study observed that about 50% of Mn was humic-complexed in the stream waters feeding into the loch. It was particularly evident that the soil profile with the lowest Mn inventory exhibited the greatest extent of humification and that the remaining Mn was predominantly in a non-easily reducible form.     

Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation

A well-developed, 1,000 m thick basaltic sequence in the Yangliuping region, northern part of the Emeishan basalt province, includes the Lower and Middle Units of tholeiitic basalts and an Upper Unit of both tholeiites and subalkalic basalts. The basalts contain 42–55 wt% SiO₂ and 4.1–8.3 wt% MgO. Most of these lavas have Gd/Yb > 2.0, Zr/Nb < 12, and ɛ_{Nd(260 Ma)} values from +2.5 to +4.7. The platinum-group elements (PGE) are very mildly depleted in most of the basalts which contain 8–19 ppb Pt and 7–27 ppb Pd. However, a significant proportion of the Middle Unit basalts are strongly depleted in PGE with some samples having concentrations lower than detection limits. They have extremely high Zr/Nb ratios (up to 14.5) and low ɛ_{Nd(260 Ma)} values (+3.21 to +0.65), features of extensive lower crustal contamination. Some samples in this unit have high Ni/Pd (3,965–61,198) and low Pd/Cr (410,000–3,930,000) ratios, indicating sulfide segregation and PGE depletion prior to eruption. The primary magmas were S-undersaturated and derived from partial melting at variable depths in the upper mantle. The early and late stage magmas, as represented by the Lower and Upper Units, underwent AFC processes which induced mild S-saturation and PGE depletion in some of the basalts, whereas the magmas represented by the Middle Unit experienced more extensive crustal contamination resulting in stronger S-saturation and in most cases significant PGE depletion.     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

Petrogenetic implications of mineral chemical data for the Permian Baima igneous complex, SW China

The Late Permian Fe-Ti oxide ore-bearing Baima igneous complex (BIC) is one of three gabbro-granitoid complexes with the Emeishan large igneous province. Mineral compositions are determined for the BIC layered gabbro in order to constrain subsolidus and magma chamber processes. The averaged compositions of cumulus olivine, clinopyroxene and plagioclase within individual samples range from Fo_65-76, Mg# = 75 to 82 and An_49-64 but they are not correlative. The observed mineral compositions are consistent with those modeled using the pHMELTS program. Highly variable magnetite compositions are consistent with extensive subsolidus re-equilibration and exsolution. The occurrence of reversely-zoned granular olivine in Fe-Ti oxide ores is a manifestation of Mg transfer between Fe-Ti oxides and olivine at relatively high (<1150?°C) subsolidus temperatures. The primary oxide is inferred to be an aluminous titanomagnetite. Similar to other layered intrusions in the region, the gabbroic unit of the BIC displays Zr depletion which is consistent with loss of a residual liquid during magma differentiation. If the most Zr-rich syenites of the complex are taken into account, the Zr budgets between the combined gabbro-syenite and the basalts are similar. This indicates that the BIC most likely represents a closed system in terms of magma extraction.     

GIS analysis of cirques on Vestfirðir,northwest Iceland: implications for palaeoclimate

This study presents the first regional analysis of cirques on Vestfirðir, NW Iceland, using a Geographical Information System (GIS). The length, width, elevation of cirque‐floor, latitude and the distance to the modern coastline (both ocean and fjord coastlines) of cirques were quantified using ArcGIS. The topographical analysis revealed a total of 100 cirques on western and northern Vestfirðir. Several additional cirques are present, but they had poorly defined toewalls, making the cirque‐floor difficult to identify. Mean cirque length is 515 m and mean cirque width is 752 m. The modal orientation of the aspect of cirques is northeast, with a strong secondary mode to the northwest. Cirques at low elevations are more abundant close to the ocean, whereas cirques further from the ocean are present at high elevations. Three techniques were used to reconstruct past equilibrium‐line altitudes (ELAs) of cirque glaciers: the cirque‐floor method, the altitude‐ratio method and the accumulation‐area ratio method. The largest range of past ELAs is generated from the cirque‐floor method with values from 40 up to 730 m. Mean past ELA values range from ～395 to 423 m depending on the method used to reconstruct former ELAs. A strong positive relationship is observed between past ELA values and distance to the ocean demonstrating the importance of access to a moisture source for glacier survival. This relationship is stronger than the relationship between former ELAs and latitude. Based on the small size of cirque glaciers, it is likely that even minor fluctuations in the Irminger Current and the East Greenland Current influence cirque glaciation on Vestfirðir.     

Fossil coniferous wood from the Early Cretaceous Jehol Biota in western Liaoning,NE China: New material and palaeoclimate implications

The Early Cretaceous Jehol Biota, one of the most important Mesozoic lagerstätten in East Asia, is especially well-known for occurrences of fossil feathered dinosaurs and early angiosperms. However, the terrestrial biodiversity, especially the fossil wood record, is poorly known. In this study, several structurally preserved coniferous wood specimens from the Lower Cretaceous Yixian Formation are investigated, based on collections from the Heichengzi Basin in Beipiao of western Liaoning, Northeast China. Four species referred to four genera of fossil wood are described, including Taxodioxylon heichengziense sp. nov., Thujoxylon beipiaoense sp. nov., Sciadopityoxylon liaoningense Ding and Protocedroxylon shengjinbeigouense sp. nov. These new records enlarge the fossil wood diversity of the Yixian Formation up to 10 species in 9 genera, and provide further insights into the forest vegetation composition of the Early Cretaceous Jehol Biota. Systematic analysis of the floral constitution indicates that the petrified forests of the Yixian Formation are dominated by conifers, represented by Araucariaceae, Podocarpaceae, Sciadopityaceae, Pinaceae and Cupressaceae in the western Liaoning region. Palaeoclimatical analysis of the fossil wood assemblage implies that the western Liaoning region was dominated by a cool temperate, wet and seasonal climate with variable interannual water supply in the western Liaoning region during the Early Cretaceous.     

The palynological record from Coniacian to lower Campanian continental sequences in the Songliao Basin,northeastern China and its implications for palaeoclimate

A palynological record recovered from successions of Coniacian to early Campanian age (89.1–83.5 Ma) was obtained from the lacustrine sequences of the SK-I south core (SK-Is) in the Songliao Basin, northeastern China. The palynoflora is dominated by bisaccate gymnosperm pollen, followed by spores of pteridophytes, and just minor amounts of angiosperm pollen. Based on the relative abundance of the different spore and pollen taxa through the core, the succession was subdivided into three palynological assemblages. The results indicate two opposite trends for climate change, a minor warming trend (from 89.1 to 85.7 Ma) followed by a rapid cooling trend (85.7–83.5 Ma). The first warming trend reached its maximum at 85.7 Ma, which is inconsistent with results from the marine realm (which instead show a minor cooling trend based on several proxy records). However, the second cooling phase is consistent with global changes from various and abundant palaeoclimate proxies from marine deposits. We interpret the climatic changes within the studied interval (89.1–83.5 Ma) as a consequence of the shifting climate from a hot/super greenhouse to a temperate greenhouse.     

The Younger Dryas glaciation in the southeastern Monadhliath Mountains,Scotland: glacier reconstruction and palaeoclimate implications

Geomorphological mapping of locally nourished glaciers was conducted in four glens in the southeastern Monadhliath Mountains, Scotland. Three glaciers are interpreted to be of Younger Dryas age based on geomorphological similarity to features in other Scottish upland areas known to have been glaciated during the Younger Dryas, and on comparison to adjacent ice‐free areas in the lower glens where landform‐sediment assemblages typically reflect peri/paraglacial readjustment during the stadial. Here we reconstruct Younger Dryas glacier termini based on moraine alignments and associated geomorphological and sedimentological evidence. An adjacent wide plateau area at high altitude may have permitted extensive ice accumulation, but no unequivocal geomorphological signature is evident. To establish upper glacier limits, a series of ice profiles are modelled. The results yield a range of realistic glacier configurations bracketed between two distinct scenarios: a valley glaciation with the glaciers' upper limit on the plateau edge, and a low‐domed icecap centred on the plateau with ice flowing radially into the lower glens. Reconstructed equilibrium‐line altitudes are 795 m a.s.l. for the valley‐glacier scenario and 894 m a.s.l. for the icecap scenario. Calculated mean ablation‐season temperatures at the ELA are 1.2°C and 0.4°C for the valley‐glacier and the icecap scenario, respectively, from which we infer mean annual precipitation rates between 323 and 520 mm a^?1. Palaeoclimate results indicate a stadial climate in central Scotland 65–79% more arid than at present, comparable to that of western Norway for the stadial and to the present‐day Canadian Arctic.     

Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.     

贵州丹寨地区南华系地层地球化学特征及其对古气候、古环境的意义

南华系是地球历史上最重要的成冰时期,为了示踪冰期前后的气候与环境的变化,本文通过对南华系细屑岩的化学成分分析,重塑南华系古气候、古环境的演化过程。分析结果显示铁丝坳组具有明显低的CIA指数以及低Ti/Al比值,说明该时期主要以寒冷干燥气候为主,化学风化程度低,物源供应和沉积速率相对较低,为冰期特征,隆里组和大塘坡组具有高的CIA指数以及相对较高的Ti/Al比值,表明该时期主要以温暖湿润气候为主,化学风化程度相对较高,物源供应和沉积速率相对较高,为间冰期特征。南沱组的CIA指数以及Ti/Al比值具有交替变化特征,说明南沱组沉积时期气候存在冷暖交替变化,反映了冰期与间冰期的相互转换特征。另外,在铁丝坳组顶部和大塘坡组的下部以及南沱组中上部,具有一定程度的正Eu异常,说明在南华系沉积时期,存在热液活动,而这种热液活动可能与冰期的消亡存在着成因联系。