Impacts of point source and diffuse metal and nutrient loading on three northern boreal lakes

A geochemical–paleolimnological study was conducted to investigate human influences on three lakes (Nuasjärvi, Jormasjärvi and Kolmisoppi) located in an area with high background levels of metals and sulphur in bedrock and till overburden. Accordingly, background concentrations of Co, Fe, Mg, Mn, Ni and Zn were above average in sediments of the study lakes.The land use-related erosion and transport of particulate matter into Lake Nuasjärvi started as early as in the seventeenth century, while increased inputs to Lake Jormasjärvi date to the eighteenth century and Lake Kolmisoppi to the 1970s and 1980s. Local tills and fine-grained sediments are the source of the particles and hydraulic sorting has resulted in elevated sediment concentrations of Ca, Cr, Cu, K, Mg, Na and Ni. At the same time, sedimentation of carbon and autochthonous phases has decreased, leading to low concentrations of Co, Fe and Mn in the erosion-associated sediment layers in Nuasjärvi and Kolmisoppi. Despite the geochemical changes, no marked eutrophication of the lakes could be detected with diatom-based nutrient reconstructions during the early land-use period.Elevated amounts of sulphur and chalcophilic elements were deposited throughout the study area during the 1970s and 1980s. In Lake Nuasjärvi combination of the elements is associated with the Lahnaslampi mine and the difference in the sulphide degradation rate is reflected as a sequence of element mobility and sedimentation in the order 1) S, and 2) Co, Ni, Cu, Zn, Cd and Sb. Sulphur, Cu, Ni, Pb and Zn enrichment in the other two lakes is related to other land uses and to atmospheric deposition. In addition to sulphur and metals, these most recent inputs from the catchment have caused nutrient enrichment in all three study lakes, most notably in the lowest-lying Lake Nuasjärvi. This lake has the highest percentage of fine-grained soils in its catchment and also suffers from point source nutrient inputs. In addition, metals (Ni) had a signal in diatom assemblages in Lake Nuasjärvi that was statistically independent of eutrophication (N:C) but inseparable from mineral matter inputs (K).     

Comparing X-ray densitometry and BSE-image analysis of thin section in varved sediments

A 10-cm long section from varved sequence of Lake Nautajärvi, Finland, has been analysed using both X-ray densitometry and image analysis of Backscattered Scanning Electron microscope Images (BSEI) of thin sections. Lake Nautajärvi's clastic-organi c varves appear in X-ray images as a succession of dark and pale stripes, pale ones being the spring detrital minerogenic layers and dark ones organic matter laid down during the time of summer, autumn and winter ice cover. Owing to the sharp contrast of the minerogenic spring layer, a semiautomatic tree-ring width and density measurement DendroScan system was applied to obtain seasonal/annual data of relative X-ray density. Mean density of BSEI was also acquired. Then BSEI were processed to produce black and white images, where white pixels represent the clay-rich sedimentary matrix and black pixels the particles in the matrix: silt- or sand-sized, terrigenous grains, authigenic particles and diatoms. Measurements of the size of the detrital particles forming the varves were obtained. For each varve, mean grey-level values in BSEI correlated well with the mean grey-level values of X-ray images, but grain size obtained on BSEI did not correlate with any parameters computed from X-ray radiography.     

Preservation potential of seasonal laminated deposits as a useful tool for environmental analysis in mesotrophic Lake Kizaki,central Japan

In this paper discusses the aquatic environment at Lake Kizaki, central Japan, during the last ca. 60 years and describes the importance of laminated sediments in environmental analyses. The flux of total algal carbon increased between 1965 and 1996 while the BSi flux, which can be used in diatom production, showed a larger increase between 1960 and 1998 than typically occurs on average. The flux of dinosterol, an indicator of dinoflagellate production, also increased from 1966 to 1998, and the sedimentary dinoflagellate record correlated with observations of freshwater red tides caused by dinoflagellate blooms. Parallel laminated sediment was determined to have appeared between 1960 and 1995, corresponding to a period of high total algal carbon levels and BSi fluxes. For lamina formation and its preservation process, the thickness of the oxygen-poor layer depended on excessive total algal production during stagnant periods and appeared to be important because nutrients were diffused and accumulated in the oxygen-poor layer from bottom sediments. This could result in diatom blooms at the next overturn period. The development of the thick oxygen-poor layer appeared to be triggered primarily by the presence of excessive nutrients generated by human activities. In general, the occurrence of laminated sediments can provide a useful tool for understanding the eutrophic state in Lake Kizaki.     

Long varve series in Finland

Thinly varved 'postglacial' organic sediments are quite common in Finnish lakes, and attempts have been made to locate a site where they could be connected with the varved 'lateglacial' clays in order to he able to count the entire time since the ice retreat within one varved sequence. One promising site is Lake Valkiajärvi near Ruovesi in the Lake Region. The known sedimentary record covers nearly 9,500 years, which means that it starts very close to the time of the deglaciation of the Valkiajärvi area, around 9,600 B.P., or + 600 according to the original Finnish varve chronology (of Sauramo).     

白洋淀近70年生态环境演化过程及影响因素

基于白洋淀北部沉积岩心210Pb和137Cs测年、重金属元素、营养元素、粒度和硅藻优势属种分析,结合水文、气候以及保定市和安新县社会经济统计资料,重建了白洋淀近70年以来的生态环境演化历史,并探讨了影响硅藻优势属种演替的主要驱动因素.结果表明:1960年之前,重金属和营养元素处于较低的水平,硅藻属种以Aulacoseira granulata为主,反映湖泊生态环境处于自然演化阶段;1960年前后,由于受到上游水文调控的影响,白洋淀TP和TN含量开始缓慢增加,富营养指示种Cyclotella meneghiniana明显增加;1960~1990年,TP和TN含量明显升高,反映快速农业化进程中营养物质输入增加,但重金属含量仍保持在较低的水平,硅藻属种以A.granulata和C.meneghiniana为优势组合;1990年以来,流域工农业活动的快速发展导致重金属和营养盐富集,尤其是2000年以后,重金属和营养盐富集进一步加剧,此时硅藻组合也发生了重大转变,由1990年前的A.granulata优势种转变为1990年后的C.meneghiniana优势种,重金属耐受种Nitzschia palea也自2000年以来开始持续增加.冗余分析表明沉积物重金属、TP、TN、温度、风速、入淀水量和水位是影响白洋淀近70年以来3个硅藻优势属种演替的显著因子,其中重金属和营养是硅藻组合演替的主要驱动因素,水文过程、温度和风速是硅藻组合演替的重要诱因.      

云龙天池湖泊水深与硅藻生物多样性的关系

以云龙天池湖泊为研究对象, 通过分析不同深度表层硅藻样品的生物多样性以及100年来硅藻生物多样性的变化特征, 探讨硅藻生物多样性与水深变化之间的关系。虽然硅藻组合随水深发生了明显的变化, 浅水区底栖硅藻占优势, 深水区浮游硅藻占优势, 但是不同深度表层样品中硅藻的属种数(简单分异度)基本相同, 硅藻种群的生物多样性也没有发生显著的趋势性变化。这主要是由于硅藻各属种在不同深度的权重互补, 导致云龙天池硅藻的多样性在湖泊的不同深度变化不大。近百年来的湖泊沉积速率显示, 自1960年前后, 湖泊水位在波动中逐步升高, 但是硅藻种群的生物多样性并没有发生相应的变化。以上研究结果说明, 云龙天池硅藻种群的生物多样性, 不管是在时间上, 还是空间上, 与水深变化均无明显相关性。     

Historical eutrophication in Lake Taihu: evidence from biogenic silica and total phosphorus accumulation in sediments from northern part of Lake Taihu

Sediment and water from the Meiliang Bay of Lake Taihu were analyzed to examine the historical relation between the accumulation of biogenic silica (BSi) and total phosphorus (TP). The results indicate that BSi accumulation in the northern part of Lake Taihu had been controlled by diatom production and phosphorus loading since the 1950s. BSi accumulation increased with the growing agricultural activity since the 1950s, up to a maximum level in the 1960s. After that, BSi accumulation decreased due to the diatom dissolution till the 1980s, and then the diatom biomass decreased with BSi accumulation increased. Lake Taihu came into an accelerated eutrophication periods since the 1990s, while BSi accumulation began to increase but the proportion of diatom decreased. Although the onset of silica depletion cannot be confirmed in the present work, it is clear that BSi accumulation was restrained by the input of TP.     

Magnetic mineralogy of the sediments in Lake Ådran, eastern Sweden, and an interpretation of early Holocene water level changes

A number of magnetic parameters have been measured on samples from a master sequence in Lake Adran, eastern Sweden. Based on the results, frequency dependent magnetic susceptibility, high isothermal rem-anence magnetization and different magnetic ratios have been calculated. The results from the master sequence have also been compared with magnetically analysed soil samples from the lake catchment. The results of the magnetic analyses show that the sediments in Lake Ådran originate from two sources, one regional and one local. The different sources can be related to water level changes during the Ancylus and Litorina stages. Sediments originating from the regional source are characterized by high ferrimagnetic to antiferromagnetic ratios (high S-ratios) and are assumed to have been deposited when the water level was well above the threshold. Sediments derived from the catchment, having lower S-ratios, are assumed to have been deposited when the basin was cut off from the regional water circulation. On the basis of the magnetic results, combined with pollen and diatom analyses, tentative water level changes for the Lake Adran basin are compiled.     

Diatom record from Lake Hovsgol, Mongolia, during the last 1 Ma: The results from the HDP-04 drill core

Quantitative diatom abundance and species composition was studied in the longest 81-m Lake Hovsgol drill core, HDP-04. Lake Hovsgol diatom biostratigraphy of the past 1 Ma includes 14 local diatom assemblage zones (LDAZ). Planktonic diatom flora of Lake Hovsgol is dominated by members of Stephanodiscus and Cyclotella. A large-scale shift from Stephanodiscus-dominated to Cyclotella-dominated flora occurred in the mid-Brunhes chron. Significant morphological variability of Cyclotella ocellata complex and Cyclotella cf. minuta, as well as the presence of new extinct endemic taxa of genus Stephanodiscus, suggest long speciation and/or adaptation history of diatom flora. The recognition of taxa morphologically similar to the extinct endemic species of Lake Baikal suggests potential correlation ties between Lake Hovsgol and Lake Baikal diatom biostratigraphies around the MIS 17, MIS 25, and, possibly, MIS 15 interglacial intervals. Narrow peaks of diatom abundance in the Lake Hovsgol sedimentary record are episodic and short-lived. These peaks may represent humid optima of past interglacials and/or interstadials, characterized by elevated surface runoff in the Hovsgol basin and relative highstands of the lake.     

Climatic and volcanic forcing revealed in a 50,000-year diatom record from Lake Massoko, Tanzania

The interactions between climatic and volcanic forcing on diatom communities contained in a 50,000-year sedimentary sequence from Lake Massoko, Tanzania, were examined. At the century scale, 19 discrete tephra inputs to the lake isolated the sedimentary nutrient supply and shifted the diatom communities to those tolerant of low phosphorus levels, whereas at the millennial scale, diatom-inferred shifts in precipitation–evaporation based on conductivity optima and diatom life-form ratios were broadly similar to lake-level reconstructions from Lake Rukwa, Lake Malawi, and others in the region. Some fluctuations of Lake Massoko are consistent with the precession-driven changes in insolation, but the major climate shifts do not relate directly to orbital forcing of summer insolation south of the equator and show more consistency with records from the equatorial and northern tropics that receive rainfall from the passing of the intertropical convergence zone. Sea surface temperatures are strongly correlated to multimillennial-scale climate patterns over this region of Africa.     

Holocene history of Lake Soldatskoje (Kola Peninsula, Russia) inferred from sedimentary diatom assemblages

A sediment core from Lake Soldatskoje, a small tundra lake located in the northern coastal area of the Kola Peninsula and surrounded by numerous archaeological dwelling sites, was analysed for diatom species. The core covers the entire Holocene, i.e. 10000 radiocarbon years. The diatom record has similarities with studies made earlier from tundra lakes of northern Russia and from Arctic lakes in general. The genus Fragilaria was dominant and many other small, benthic, nordic, cold-water diatom species typical of Arctic lakes were common, including Achnanthes minutissima Kützing, A. pusilla (Grunow) De Toni, Cyclotella tripartita Håkansson and Navicula absoluta Hustedt. Multivariable ordinations were used to characterize the changes in the diatom flora. Diatom-based pH and total phosphorus inferences indicate that the lake has become progressively more acidic and poorer in nutrients. Disturbances in the diatom stratigraphy of Lake Soldatskoje around 4000–5500 ¹⁴C yr BP may be related to human activity.     

Diatom responses to limnological and climatic changes at Ribains Maar (French Massif Central) during the Eemian and Early Würm

High-resolution diatom analysis was carried out to assess the limnological and climatic changes that took place at Ribains maar (French Massif Central) during the Late Pleistocene (∼131–∼105 ka BP), with a focus on the Eemian interglacial in particular. Numerical analyses were used to show that most of the variability in the fossil diatom assemblages was due to climate independently from the changes in the lake catchment vegetation (as represented by pollen data). Diatom-based quantitative reconstructions of the past limnological conditions, as well as a comprehensive literature review on the auto-ecological requirements for the principal diatom taxa, were used to interpret the record. An absolute time-scale for the sequence was derived by matching the major pollen shifts with the radiometrically dated changes in oxygen isotopes observed in Italian stalagmites. This study shows that at Ribains maar, the transition from the Riss (=Saalian) Glacial to the Eemian interglacial was marked by a gradual increase in the contribution of spring-blooming diatom species, indicating a longer growing season and milder winter/spring conditions at that time. A short cooling event interrupts this trend and may correspond to a stadial. At the start of the Eemian a peak in benthic taxa and the suppression of spring-blooming flora probably reflects the effects of deglaciation on the catchment. During the Eemian interglacial itself three main phases were distinguished within the diatom record. The first phase (∼8000 years in duration) was dominated by Stephanodiscus minutulus, which suggests that intense mixing in the water-column took place during spring. The pollen record was simultaneously dominated by Quercus and Corylus that typify this phase as the climatic optimum of the Eemian. The second phase, almost equal in duration to the first phase (∼7000 years), is generally dominated by Cyclotella taxa and suggests a less productive lake and much reduced period of spring mixing compared with the first phase. In the pollen diagram this corresponds to an interval dominated by Carpinus–Picea–Abies that indicates a cooler and wetter climate. The third and last phase of the Eemian, ∼2000 year long, saw the return to Stephanodiscus-dominated assemblages, indicating a warming that may correspond to the Dansgaard–Oeschger event 25 identified in the Greenland ice-core record. In the early stage of the Würm Glacial (=Weichselian), assemblages in the Melisey I stadial (∼3000 year long) were dominated by either Aulacoseira subarctica or Asterionella formosa, which suggest colder spring conditions than during the late Eemian, but not as cold as the ones indicated by the pollen record. Stephanodiscus spp. again dominate during the Saint-Germain Ia interstadial (∼5000 year long) suggesting a return to the conditions that prevailed before the Melisey stadial, in agreement with the pollen record. The record ends with the Montaigu cold event, which is characterised by a Pinus peak in the pollen record, and corresponds to a large abundance of A. subarctica in the diatom sequence. Throughout the Eemian the abundance of Stephanodiscus spp., which is thought to be driven by winter conditions, show cyclic fluctuations that most likely match the cooling events identified in a pollen record from Germany. Variation in insolation throughout the Eemian may have been the driving factor behind the species succession observed in the diatom sequence. While this study demonstrates that diatom analysis of lake sediment can provide very detailed information on long-term climate change, a review of the few other diatom investigations published on European Eemian deposits shows that this technique has been so far seldom used to its full potential in this context in central and southern Europe.     

A 1000-yr record of environmental change in NE China indicated by diatom assemblages from maar lake Erlongwan

Past environmental changes based on diatom relative abundances have been inferred from the maar Lake Erlongwan in northeast China. The limnology of Lake Erlongwan is affected by the strongly seasonal regional climate. The composition of diatom assemblages, in turn, responds to changes in the seasonal duration of ice cover in winter, water-column turnover in spring and autumn, and thermal stratification in summer. Statistical analysis of the sedimentary diatom assemblages reveals three significant stratigraphic zones over the past 1000 yr. The highest abundance of the planktonic species Discostella species occurs between AD 1050 and 1400 and suggests an annual ice-free period of long duration and well-developed summer stratification of the water column. This planktonic diatom peak between ca. AD 1150 and 1200 suggests that this period was the warmest over the past 1000 yr. The interval between AD 1400 and 1800 is marked by a decline in planktonic diatoms and suggests shorter duration of the ice-free season, weaker water stratification and possibly generally cold conditions. After AD 1800 relative abundances of planktonic diatoms, including Puncticulata praetermissa and Asterionella formosa, increase again, which indicates lengthening of the duration of the annual ice-free period and a stronger overturn of the water column. All these data imply that the pattern of the seasons is different between the MWP and the 20th century.     

Holocene freshwater diatoms: palaeoenvironmental implications from south Kamchatka,Russia

Holocene palaeolimnological conditions were reconstructed by analysing fossil diatom assemblages within a lacustrine sediment core from Lake Sokoch, southern Kamchatka (Russia). Sediments of this proglacial lake cover the past 9400 years and hence represent almost the whole Holocene history. The biosiliceous muddy sample material was analysed for several geochemical and biological parameters, such as the total organic carbon and biogenic silica content, and the diatom community (quantitative and qualitative changes). Based on changes in the relative abundances of the most frequent species Aulacoseira subarctica, Staurosira martyi and Stephanodiscus alpinus and a depth‐constrained cluster analyses (CONISS), five diatom assemblage zones could be identified. The oldest stage recovered lies between 9400 and 9000 cal. a BP and reflects the initial lake stage after the retreat of local glaciers, with a high detrital sediment supply, shallow‐water conditions and a high diatom diversity. The next zone (9000–6200 cal. a BP) shows a more mature lake system with accumulating biogenic remains and higher water levels during climate amelioration. This is followed by the most obvious change in the diatom assemblage, delineated by an occurrence of S. alpinus, between 6200 and 2700 cal. a BP. Wet conditions in spring probably led to an enhanced fluvial runoff and eutrophic to hypertrophic conditions. The end of this period might reflect climate deterioration related to the Neoglacial epoch of the Holocene. Between 2700 and 1600 cal. a BP the sediments of Lake Sokoch reveal oligotrophic water conditions in a windy high‐energy environment. The youngest interval, between 1600 cal. a BP and the Present, indicates shallow‐water conditions and a very short growing season, which might reflect the Little Ice Age. The results may offer a baseline for the interpretation of Holocene palaeoenvironmental changes in Kamchatka and their relation to regional climate change from a palaeoecological perspective.     

Subfossil littoral Cladocera as indicators of brackish-water Littorina transgression of the Baltic Basin in a small lake in Finland

The study aimed to investigate the value of freshwater littoral Cladocera (chydorids, Ophryoxus gracilis and Sida crystallina ) in stratigraphical studies of shore displacement of the Baltic Sea. Diatoms and Cladocera were analysed from a sediment core from Lake Ruokolampi (S Finland, 60°34'N, 27°26'E), where a brackish-water Baltic transgression (Littorina Sea) is clearly expressed by changes in lithology. The diatom flora indicates a development of the waterbody from an Ancylus Lake bay to a small lake that was subject to a brackish Littorina transgression followed by another small-lake stage. There was a rich chydorid fauna in the Ancylus Lake bay and the following small lake. Littoral Cladoceran diversity fell sharply at the onset of the brackish transgression; however, three species ( Alona rectangula, Alona affinis and Chydorus sphaericus s.l. ) appear to have been tolerant of the saline conditions. Concentrations of all species rose rapidly after the transgression. Littoral Cladocera appear to react to the inflow of brackish water as sensitively as the diatoms and may provide a valuable additional method for pinpointing Baltic Sea transgression and isolation events, especially in cases where the diatom and biostratigraphical evidence is not as clear as in the Ruokolampi sequence.     

硅藻生物多样性对千年尺度气候突变的快速响应:以云南云龙天池湖泊末次冰消期记录为例

湖泊是全球生态系统的重要组成部分。尽管湖泊初级生产力的生物多样性在湖泊生态系统中发挥着非常重要的作用,但对其如何在千年时间尺度上对气候变化做出响应却知之甚少,而千年时间尺度与预测未来变化最为相关。本文以云南云龙天池湖泊为研究对象,以湖泊重要的初级生产力硅藻为研究手段,分析了末次冰消期期间硅藻生物多样性对千年尺度上气候变化的响应。云龙天池硅藻生物多样性表现为暖期高、冷期低。随着全球温度的快速变化,硅藻生物多样性亦对应的快速响应:在转暖时(Bolling/Allerod暖期)快速增加,在转冷时(Herinrich 1和Younger Dryas)快速降低。这些变化主要与温度变化驱动的湖泊环境条件的变化(比如冰封期长短、边岸带水生植被的变化等)有关。研究结果还表明,在末次冰消期期间,云龙天池湖泊硅藻生物多样性与千年尺度的气候变化同步,而且在长时间尺度上,气候变暖对高山湖泊生物多样性可能是有利的。     

白洋淀近80年生态环境变化的硅藻记录

白洋淀是雄安新区实现"以水定城"理念的关键,了解其生态变化轨迹及特征有望为白洋淀的治理提供科学参考依据。本文基于取自白洋淀东南部湖区的一根长79 cm的沉积岩芯,分析该岩芯的硅藻记录并结合沉积物地球物理和化学指标、气象监测数据及人类活动指标,重建了该湖区近80年的硅藻属种组合及多样性变化过程和特征并探讨其变化机制。研究结果表明,Achnanthidium minutissimum、Amphora copulata、Puncticulata praetermissa、Pantocsekiella ocellata和Cyclotella distinguenda是沉积硅藻群落的优势种。本研究分析的物种多样性指数包括物种丰富度、均匀度和β多样性指数（βsor）。其中,物种丰富度和均匀度呈总体上升趋势,βsor呈总体下降趋势,说明过去80年硅藻属种数量净获得量增加,但硅藻群落结构变化趋于均匀化。冗余分析（RDA）和变差分解（VPA）显示气候和人类活动因子对硅藻属种组合和多样性变化产生了不同程度的影响,其中年均温、湖水位和沉积物P元素含量的贡献较大。年均温对硅藻属种组合变化的独立解释百分比最大,说明未来气候持续变暖的条件下,气温对白洋淀生态系统的影响可能会增强。湖水位对硅藻多样性变化的贡献最大,说明维持生态需水量对保护白洋淀生物多样性的重要性。基于普通最小二乘法残差的累积和（OLS-CUSUM）未检测出硅藻属种组合和多样性存在突变点,说明过去几十年人类活动和气候变化尚未造成该湖区生态系统稳态的转化。因此,减轻环境压力有望保持该湖区生态系统的恢复力和稳定性。但对于重污染湖区,其硅藻优势种和属种组合变化与本研究湖区存在很大差异,需进一步研究以获得更具针对性的水环境治理对策。     

Records of land use and medieval settlement history in Vuonninen,Dvina Karelia,Northwest Russia

Although the documented history of Dvina Karelia (northern Russian Karelia) does not begin until the second half of the 16th century, individual finds of prehistoric artifacts point to earlier human occupation. The present paleoecological study of the Vuonninen area is the first of its kind concerning prehistoric land use in Dvina Karelia, a large region extending from Finland to the White Sea. It is based on the pollen analysis of sediments from Lake Ylä‐Kuittijärvi near the old Karelian village of Vuonninen. In its general features, the picture of the early stages of cultivation and human activity in the village of Vuonninen is comparable to results from East Finland, the Karelian Isthmus, and the northern shores of Lake Ladoga. The first, minor indications of human influence appear in the sediments of the 4th–6th centuries A.D. in Lake Kuittijärvi. Distinct and unambiguous signs of human activity appear in the pollen record in the 12th century. This is particularly evident in the decreasing proportion of Picea, caused by the clearance of spruce forest. The beginning of cultivation is dated according to Secale pollen to the beginning of the 15th century. Intensive arable farming emerges in the early 19th century in the form of regular occurrences of Secale and Hordeum pollen. © 2010 Wiley Periodicals, Inc.     

Late Cretaceous seasonal palaeoclimatology and diatom palaeoecology from laminated sediments

Laminated diatom-rich marine sediments from California and the Arctic Ocean provide a window into the seasonal climate and oceanography of the mid- and high-latitude Late Cretaceous. These remarkable shallow-buried sediments constitute palaeo-sediment traps that record exceptionally well-preserved sequential biogenic and lithogenic flux events. Many of the diatom laminae are composed of a few dominant taxa, and we use a species-based approach for palaeoecological interpretation. Contrary to many earlier interpretations, results indicate that both sites preserve a major flux of taxa adapted to exploit a strongly stratified ocean. The uppermost Maastrichtian Marca Shale of California records a seasonal cycle initiated with a spring bloom flux of diatom resting spores followed by a summer lithogenic sediment input likely driven by monsoonal storms providing river runoff and aeolian input. This is followed by a dominant diatom flux of species that thrived in the summer stratification in deep chlorophyll maxima and were sedimented in the fall when this stratification broke down. Dominant taxa comprising this group include Hemiaulus, Stephanopyxis, Stellarima and Rhizosolenia. The uppermost Campanian CESAR 6 core from the Arctic Alpha Ridge, records a spring bloom flux of resting spores followed by a more dominant summer and fall flux of diatom vegetative cells which were likely concentrated in subsurface summer blooms that generated a “summer export pulse” and by the breakdown of stratification in the fall that gave massive flux of deep chlorophyll maxima species in the fall dump. The dominance and diversity of Hemiaulus in the CESAR 6 core together with widespread evidence of N₂-fixation may indicate that some diatom blooms were powered by intracellular N₂-fixing cyanobacteria as in the modern oligotrophic ocean. Thin lenses of fine lithogenic sediment that occur mostly in the spring layer represent rafting by winter sea ice and support other evidence that suggests intermittent winter freezing in the Late Cretaceous Arctic.     

西藏麻米错富硼锂盐湖成矿物质平衡计算与物质来源

麻米错盐湖是西藏境内发育的特大型锂矿床之一,目前对该盐湖的Li、B来源仍存在一定争议。文章系统采集麻米错盐湖水及周边补给水体样品,分析其主量元素特征,并使用多种水化学手段进行讨论,同时对盐湖水、河水及地热水进行蒸发模拟。结果表明,研究区河水及泉水中的主要离子受控于流域内岩石风化作用,其中碳酸盐岩矿物与蒸发岩矿物贡献最大,其次是硅酸盐矿物。文章对补给水体输入盐湖的Li、B资源进行定量计算,得出单纯通过河水和泉水的输入无法形成盐湖目前的探明储量。选择西藏地区最为富Li的地热水同补给水体按照一定比例混合后进行蒸发模拟,发现其析盐序列与盐湖水的析盐序列存在很大程度上的重叠,以上均证明麻米错盐湖中的Li、B资源除受到周边河水及泉水的补给外,还受到地热水的补给。