Mineral magnetic study of Late Quaternary South Caspian Sea sediments: palaeoenvironmental implications

Magnetic properties of sediments from a core (10 m long) in the southern basin of the Caspian Sea have been investigated. Varying concentrations of greigite (Fe₃S₄) dominate the magnetic fraction in Late Pleistocene sediments. The synsedimentary formation of greigite indicates that the Late Pleistocene Caspian Sea was a brackish or fresh-water, poorly ventilated basin and suggests a water level higher than at the present. The variation in magnetic parameters, with the detrital magnetite-bearing fraction remaining constant, is interpreted in terms of greigite grain-size variation and related to the slight variation in water salinity. The Holocene sediments are characterized by detrital magnetite. This indicates better ventilation of the basin and suggests lower water levels than in the Late Pleistocene. The gradual change in magnetic properties of the sediments between 90 and ≈60 cm depth, with decreasing quantities of greigite, indicates stepwise establishment of oxic conditions in the Holocene.     

内蒙古嘎顺诺尔湖泊沉积物磁化率与粒度的古环境意义

湖泊沉积物磁化率和粒度已被广泛地应用于古环境和古气候研究,但在不同地区两者的关系不同,一般与磁性矿物来源、种类和赋存粒级有关。查明磁性矿物来源和赋存状态是利用粒度和磁化率恢复古环境的重要基础。通过测定干旱区嘎顺诺尔GXN剖面湖泊沉积物的磁化率与粒度及两者的相关关系,结合代表性样品的岩石磁学特征,揭示此类地区湖泊沉积物磁性矿物来源和赋存状态,探讨它们在干旱区湖泊沉积物中所反映的环境变化信息。结果显示,剖面下部（200~105 cm）和上部（36~0 cm）沉积物磁化率与粗砂含量呈正相关,中部（105~36 cm）与粉砂含量呈正相关,表明磁性矿物主要富集于粗颗粒和较粗颗粒组分中。剖面下部和中部沉积物以顺磁性矿物为主,含少量亚铁磁性矿物（如磁铁矿+磁赤铁矿）,含少量顺磁性矿物;剖面上部沉积物以亚铁磁性矿物为主（如磁赤铁矿+磁铁矿）,含少量顺磁性矿物。根据嘎顺诺尔晚全新世湖泊沉积物剖面的岩性、磁化率和粒度组合特征,可将其划分为河漫滩相沉积（200~105 cm）、浅湖相沉积（105~36 cm）和滨浅湖相沉积（36~0 cm）3个阶段。该湖粒度组分中<4 μm和4~64 μm颗粒主要反映了湖泊水动力的变化,受碎屑物来源和水动力条件的控制;而河漫滩相中>64 μm粗颗粒组分的峰值是风力作用的结果。河漫滩相及滨浅湖相的磁化率高值则指示了湖面较低,水动力较强,有较多的磁性矿物随粗颗粒入湖;浅湖相的磁化率低值则指示该时期湖面较高,水动力较弱,磁性矿物随入湖粗颗粒含量的减少而降低。     

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three-axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X-ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300  nm, possibly in the single-domain state. Greigite clumps of about 3  μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15  nm, indicating that the particles seen in the SEM are polycrystalline.
  GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.     

Mineral magnetic record of Holocene environmental changes in Sägistalsee, Switzerland

The Holocene magnetic signature due to environmental change has been investigated in sediments from Sägistalsee, a small alpine lake in the Bernese Alps, Switzerland. The environmental signal in the mineral magnetic parameters of the sediments was compared with palynological and geochemical data. The types of magnetic minerals and their grain size reflect changes in the lake catchment, vegetation, and degree of erosional input. The concentration of the magnetic minerals, as expressed by isothermal and anhysteretic remanent magnetizations also reflect changes in vegetation, but may also be related to redox conditions during sedimentation. Climate influence on the mineral magnetic record has been recognized and interpreted as a consequence of the production of an authigenic mineral with particularly uniform magnetic properties during warmer stages and the influx of heterogeneous detrital magnetic mineral during cooler stages and under increased human activity.     

Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt

The analysis and interpretation of changes in mineral magnetic signatures from a long (ca. 8.2 m) sedimentary sequence recovered from Lake Qarun, Middle Egypt in 2003 spanning a timescale of approximately the last 2,000 years is reported. A suite of mass specific susceptibility and magnetic remanence measurements were made at irregular intervals downcore on 39 samples. These samples were selected on the basis of trends and abrupt changes in whole-core magnetic susceptibility measured using a Bartington^® MS2E sensor and were analysed for low and high temperature loss on ignition and their particle size distribution. Trends in all mineral magnetic concentration parameters are remarkably similar and were initially used to divide the core into three magnetically distinct zones. The upper and lower sections of the core (0–119 cm and 445–822 cm depth) are characterised by low values for all magnetic concentration parameters. Between 153 and 380 cm depth, concentration parameters are considerably higher, although somewhat variable. The S ratio and percentage loss of remanence after 24 h (IRM_loss) follow a different trend and are inversely related to each other. A low S ratio (<0.7) is associated with a loss of remanence of >4%. On the basis of these parameters, the core can be divided into four zones, and differences in magnetic mineralogy between these four zones were confirmed by measurement of IRM acquisition curves. The major difference between concentration parameters and ratios or percentage loss of IRM lies in the identification of an additional zone below 619 cm depth where the S ratio is high and IRM_loss is low. There is little evidence to suggest that the magnetic signatures are controlled by particle size or by trends in organic matter and/or carbonate content. The signatures appear to be predominantly detrital and show little evidence of post-depositional alteration through dissolution or authigenic addition of bacterial magnetite or greigite. Analysis of Saharan dust deposition rates in Northern Egypt suggests that atmospheric fallout is likely to make only a very minor contribution (<1%) to sedimentation rates in Lake Qarun. The downcore trends therefore appear to reflect major changes in fluvial sediment sources over the ca. 2,000 year time period spanned by this ～8 m core. Preliminary mineral magnetic characterisations of potential local sources suggest that these cannot account for the range of signatures recorded in the Qarun sediments and it is hypothesised that these sediments are derived from Nile river floods.     

科尔沁沙地风沙沉积物磁学特征及其古环境意义初探

选取位于科尔沁沙地东北缘的六户屯剖面风沙沉积物为研究对象,进行系统的磁学特征和粒度分析。结果表明：六户屯剖面磁性矿物含量较少,磁学性质主要受亚铁磁性磁铁矿控制,并含有少量赤铁矿和针铁矿,磁性矿物粒度较细,以稳定单畴和超顺磁颗粒为主。沉积物中磁性矿物的粒度分布受风力大小与成壤强度的影响,单畴颗粒相对含量随风力的增强而增加,随土壤发育程度的增强而减少,χARM/χlf和χARM/SIRM可以指示冬夏季风的相对强弱。六户屯剖面磁性矿物粒度主要受冬季风强度及其动力分选作用的控制,磁性矿物粒度与沉积物总体粒度呈反相关;但在某些层位,夏季风影响下的成土过程对其改造作用不容忽视。成壤过程中形成的超顺磁颗粒是磁化率增强的主要因素,单畴颗粒对土壤发育适宜期磁化率的增加贡献相对较弱。     

北极巴罗AB-67钻孔岩芯的沉积过程分析

对巴罗ＡＢ－６７钻孔岩芯的粒度分析资料和地球化学分析资料进行了系统的多变量沉积过程分析。结果表明,３２～６０ｃｍ岩芯段为陆相沉积,该段沉积环境和沉积过程都较为稳定;２０～３２ｃｍ岩芯段为由陆相向海相转换的过渡时期,此时粗粒的含Ｃｒ磁性矿物由峰值跌入谷值,硅、钙质氧化物减少,钛、铁质氧化物增加,有机质成分增加;０～２０ｃｍ岩芯段为海相沉积,本段沉积粗粒成分很少,氧化物、粘土、有机质和吸附成分波动很大,表层淋溶残余成分迅速增加,物源环境稳定,但动力条件变化较大     

安徽巢湖全新世湖泊沉积物磁化率与粒度组合的变化特征及其环境意义

通过对获取的巢湖湖泊沉积岩芯的磁化率与粒度组合特征的分析,结合孢粉及相邻其它区域的环境考古资料,得出该岩芯柱样所反映的环境变化信息。结果表明:(1)巢湖湖泊沉积物磁化率曲线在117 cm以下总体上比较平稳而略有下降,117 cm以上人类活动所导致的侵蚀作用加剧使得磁化率值异常升高;(2)本岩芯柱样117 cm以下样品的磁化率强度与粘土的百分比含量呈正相关,而与粉砂组分的百分比含量成负相关;(3)根据磁化率与粒度组合变化特征,结合孢粉分析及全新世以来区域气候变化背景,9 870 Cal a B.P.以来的巢湖流域古气候环境演变可以划分6个不同的时期:9 870~7 000 Cal a B.P.,气候呈温和略干的特点;7 000~4 750 Cal a B.P.,气候温暖湿润;4 750~2 170 Cal a B.P.,流域气候温和干燥,巢湖湖盆局部地区可能出露水面以上;2 170~1 040 Cal a B.P.,气候总体上温和湿润,但是冷暖波动明显;1 040~2 00 Cal a B.P.流域处在温凉稍湿的时期,人类活动逐渐加强;200 Cal a B.P.至今,流域总体上处于相对温暖湿润阶段。     

新疆塔城黄土古土壤磁化率特征及其影响因素

 对新疆塔城奴拉厚13 m的黄土-古土壤剖面磁化率、粒度、TOC、CaCO3进行了实验分析。结果表明,奴拉剖面粒度组成以粉砂(63~4 μm)为主,由于该区黄土磁化率受控于原生磁铁矿、磁赤铁矿和赤铁矿等较粗颗粒磁性矿物,而黏粒物质吸附的铁磁性矿物及成土过程中生成的超顺磁颗粒和磁性矿物较少,磁化率与细颗粒物质成负相关,与粗颗粒物质成正相关关系; 受生物成磁、化学作用以及TOC自身磁性的影响,磁化率与TOC表现出很强的正相关性; 对磁化率与CaCO3含量的相关性分析发现,两者关系密切,CaCO3含量对该区磁化率的独特变化有重要影响。     

Greigite (Fe3S4) as an indicator of drought – The 1912–1994 sediment magnetic record from White Rock Lake, Dallas, Texas, USA

Combined magnetic and geochemical studies were conducted on sediments from White Rock Lake, a reservoir in suburban Dallas (USA), to investigate how land use has affected sediment and water quality since the reservoir was filled in 1912. The chronology of a 167-cm-long core is constrained by the recognition of the pre-reservoir surface and by 137Cs results. In the reservoir sediments, magnetic susceptibility (MS) and isothermal remanent magnetization (IRM) are largely carried by detrital titanomagnetite that originally formed in igneous rocks. Titanomagnetite and associated hematite are the dominant iron oxides in a sample from the surficial deposit in the watershed but are absent in the underlying Austin Chalk. Therefore, these minerals were transported by wind into the watershed. After about 1960, systematic decreases in Ti, Fe, and Al suggest diminished input of detrital Fe-Ti oxides from the surficial deposits. MS and IRM remain constant over this interval, however, implying compensation by an increase in strongly magnetic material derived from human activity. Anthropogenic magnetite in rust and ferrite spherules (from fly ash?) are more common in sediment deposited after about 1970 than before and may account for the constant magnetization despite the implied decrease in detrital Fe-Ti oxides. An unexpected finding is the presence of authigenic greigite (Fe3S4), the abundance of which is at least partly controlled by climate. Greigite is common in sediments that predate about 1975, with zones of concentration indicated by relatively high IRM/MS. High greigite contents in sediment deposited during the early to mid-1950s and during the mid-1930s correspond to several-year periods of below-average precipitation and drought from historical records. Relatively long water-residence times in the reservoir during these periods may have led to elevated levels of sulfate available for bacterial sulfate reduction. The sulfate was probably derived via the oxidation of pyrite that is common in the underlying Austin Chalk. These results provide a basis for the paleoenvironmental interpretation of greigite occurrence in older lake sediments. The results also indicate that greigite formed rapidly and imply that it can be preserved in the amounts produced over a short time span (in this lake, only a few years). This finding thus suggests that, in some lacustrine settings, greigite is capable of recording paleomagnetic secular variation.     

粒度在沉积物物源判别中的运用

自然界中沉积物的组成和成因具有多样性。应用粒度方法可以区分、提取和判别沉积物中的各个组分。粒度方法主要有频率曲线和累积曲线分布法、数学函数组分提取法(包括Weibull分布函数拟合法、端元模型分析法、标准偏差法、粒度分维法等)和粒度参数判别公式法。综合应用上述方法可有效地判别出单一沉积物的物质来源或者复杂沉积物中的各个组分,而这些方法被前人广泛地应用于湖泊研究中,提取了过去的环境变化信息,也指导了作者在盐湖沉积中的研究工作。     

Gyroremanent magnetization and the magnetic properties of greigite-bearing clays in southern Sweden

The acquisition of a gyroremanent magnetization (GRM) by single-domain (SD) greigite particles during alternating-field (AF) demagnetization is demonstrated. Previous palaeomagnetic studies failed to identify the presence of authigenic greigite in the glacio-marine clays studied. These clays formed the subject of an earlier debate about the validity of a Late Weichselian geomagnetic excursion (the Gothenburg Flip) in southern Sweden. The greigite carries a stable chemical remanent magnetization (CRM), which coexists with a detrital remanent magnetization (DRM) carried by magnetite. AF demagnetization could not isolate the primary remanence in the sediments where magnetite and greigite coexist, due to the overlapping coercivity spectra of the two minerals and the inability to determine the time lag between sediment deposition and CRM formation. Thermal demagnetization removed the CRM at temperatures below 400 C, but this method was hindered by the unconsolidated nature of the sediments and the formation of secondary magnetic minerals at higher temperatures. The results suggest that the low-coercivity DRM carried by magnetite was mistaken for a 'viscous' component in the earlier studies. Hence the former debate about the record of the Gothenburg Flip may have been based on erroneous palaeomagnetic interpretations or non-reproducible results. AF demagnetization procedures applied to samples suspected of bearing SD magnetic particles (such as greigite) should be carefully selected to recognize and account for GRM acquisition.     

The rock magnetic signal of climate change in the maar lake sequence of Lac St Front (France)

Rock magnetic properties of the maar lake sediments of Lac St Front (Massif Central, France) reflect environmental changes during the last climatic cycle. High magnetic concentrations are measured in the sediments deposited under glacial climatic conditions, while lower concentrations correspond with more temperate climatic periods. Low- and high-temperature measurements indicate that the remanence is carried by (titanium-poor) magnetite. However, some maghemite and haematite is present in sediments deposited under temperate conditions.
Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clearly higher for the sediments deposited during the temperate climatic periods of the Eemian, St Germain I, II and Mid-glacial than for glacial sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possible effects of grain interactions, it is not straightforward to relate this different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with an optical laser granulometer. Dissolution of smaller grains is the most likely explanation for this larger grain size.
Changes in magnetic composition and grain size are extremely limited for the glacial sediments, but magnetic concentration varies considerably. Magnetic concentration maxima in the glacial sediments of Lac St Front correlate with those of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the δ¹⁸O record of the GRIP ice cores (Thouveny et al. 1994) suggests that magnetic concentration maxima may correspond with short cold climatic episodes, associated with Heinrich events.     

Storage diagenesis versus sulphide authigenesis: possible implications in environmental magnetism

Experiments involving the gradual drying out of controlled mixtures of soil and organic lake sediment during storage at room temperature show that this leads to a loss of magnetic susceptibility and isothermal remanence greatly in excess of the initial values for the sediment components of the mixtures. We conclude that during storage in the moist state, soil-derived, fine-grained, ferrimagnetic iron oxides (magnetite and/or maghemite) are transformed to paramagnetic and/or imperfect antiferrimagnetic minerals. The imperfect anti-ferromagnetic component of the initial mixtures, which probably includes goethite, appears to survive and may even increase during storage. The experimental results compare well with the previously documented effects of storing wet sediment from the site, Peckforton Mere, Cheshire, U.K., over a comparable time interval. We conclude that transformation of fine grained ferrimagnets during storage diagenesis may be responsible for many of the examples of loss of magnetic susceptibility and remanence attributed by other authors solely to the oxidation of an iron sulphide such as greigite. Only where greigite is positively identified is it valid to infer a contribution from it to the magnetic properties of lake sediments: loss of susceptibility or remanence during storage is not alone a sufficient basis for such an inference. Early drying of samples will normally avoid the effects of storage diagenesis; and recent sediment samples so treated will, where greigite formation, bacterial magnetite and magnetite dissolution are insignificant, provide a valid basis for source identification on the basis of magnetic properties.     

Environmental magnetic studies of sediment cores from Gonghai Lake: implications for monsoon evolution in North China during the late glacial and Holocene

Environmental magnetic studies were conducted on a 9.42-m-long sediment core from Gonghai Lake, North China. Radiocarbon dating indicates that the record spans the last 15,000 cal year BP. The principal magnetic mineral in the sediments is pseudo-single domain magnetite of detrital origin with minimal post-depositional alteration. Although the variations in the concentration of detrital magnetic minerals and their grain size throughout the core reflect inputs from both soil erosion and eolian dust, it is shown that their climatic and environmental significance changes with time. In the lowermost part of the core, ~15,000–11,500 cal year BP, the magnetic minerals were supplied mainly by bedrock erosion, soil erosion and dust input when climate ameliorated after the cold and dusty last glacial maximum. The increasing magnetic susceptibility (χ) in this interval may indicate a combination of changes in the lake environment together with catchment-surface stabilization and a decreasing proportion of dust input. In the central part of the core, ~11,500–1,000 cal year BP, the detrital magnetic minerals mainly originated from dust inputs from outside the catchment when the lake catchment was covered by forest, and catchment-derived sediment supply (and thus the lake sediment accumulation rate) were minimal. The generally low concentration of magnetic minerals in this part of the core reflects the highest degree of soil stability and the strongest summer monsoon during the Holocene. In the uppermost part of the core, the last ~1,000 years, detrital magnetic minerals mainly originated from erosion of catchment soils when the vegetation cover was sparse and the sediment accumulation rates were high. Within this part of the core the high magnetic susceptibility reflects strong pedogenesis in the lake catchment, and thus a strong summer monsoon. This scenario is similar to that recorded in loess profiles. Overall, the results document three main stages of summer monsoon history with abrupt shifts from one stage to another: an increasing and variable summer monsoon during the last deglacial, a generally strong summer monsoon in the early and middle Holocene and a weak summer monsoon in the late Holocene. The results also suggest that different interpretational models may need to be applied to lake sediment magnetic mineral assemblages corresponding to different stages of environmental evolution.     

青海苏干湖表层沉积物粒度分布模式与大气粉尘记录

位于干旱区柴达木盆地具有年纹层的苏干湖是以地下水补给为主的内陆封闭湖泊, 其表层沉积物粒度具有多组分峰态分布特征, Weibull 函数的拟合结果发现, 其粒度分布全部具 有4 个基本组分: 超细粒组分, 众数粒径在1 μm 上下; 细粒组分, 众数粒径在5~10 μm; 粗粒组分, 众数粒径在50~100 μm; 砂组分, 众数粒径在500 $m 左右。粒度主峰出现在细 粒组分或者粗粒组分。直接来自大气粉尘的冬季湖泊冰面样品及当地尘暴样品的Weibull 函数粒度拟合也显现出了相似的分布模式, 冰面样品的粒度主峰位于众数粒径在15~20 μmm 的 细粒组分, 砂组分(众数粒径452.9μm) 的出现指示出砂粒在冬半年沿冰面而进入湖泊中心。 尘暴降尘样品在尘暴季节(5 月份) 和非尘暴季节(11 月份) 的粒度分布普遍具有3 个组分, 缺少砂组分, 强沙尘暴事件样品则由4 个组分组成, 粒度主峰均位于众数粒径在100~200 μm 的粗粒组分。结果显示, 苏干湖湖泊沉积能够较好记录当地大气粉尘, 砂组分与湖泊周边干旱地表经历的近地面强沙尘活动有关, 粗粒组分记录了区域性的尘暴事件, 细粒组分主要反映 流域径流状况, 超细粒组分代表干旱区的大气背景粉尘和气溶胶。研究表明, 苏干湖内陆湖 泊沉积客观记录了区域大气粉尘和尘暴事件, 具有重建干旱区大气粉尘变化历史的潜在优势。     

The thermal maturity of sedimentary basins as revealed by magnetic mineralogy

The thermal evolution of sedimentary basins is usually constrained by maturity data, which is interpreted from Rock-Eval pyrolysis and vitrinite reflectance analytical results on field or boreholes samples. However, some thermal evolution models may be inaccurate due to the use of elevated maturities measured in samples collected within an undetected metamorphic contact aureole surrounding a magmatic intrusion. In this context, we investigate the maturity and magnetic mineralogy of 16 claystone samples from Disko-Svartenhuk Basin, part of the SE Baffin Bay volcanic margin. Samples were collected within thermal contact metamorphic aureoles near magma intrusions, as well as equivalent reference samples not affected by intrusions. Rock-Eval pyrolysis (T_max), and vitrinite reflectance (Ro) analysis were performed to assess the thermal maturity, which lies in the oil window when 435°C ≤ T_max ≤ 470°C and 0.6%–0.7% ≤ Ro ≤ 1.3%. In addition, we performed low- (<300K) and high-temperature (>300K) investigations of isothermal remanent magnetization to assess the magnetic mineralogy of the selected samples. The maturity results (0.37% ≤ Ro ≤ 2%, 22°C ≤ T_max ≤ 604°C) show a predominance of immature to early mature Type III organic matter, but do not reliably identify the contact aureole when compared to the reference samples. The magnetic assemblage of the immature samples consists of iron sulphide (greigite), goethite and oxidized or non-stoichiometric magnetite. The magnetic assemblage of the early mature to mature samples consists of stoichiometric magnetite and fine-grained pyrrhotite (<1 μm). These results document the disappearance of the iron sulphide (greigite) and increase in content of magnetite during normal burial. On the other hand, magnetite is interpreted to be the dominant magnetic mineral inside the contact aureole surrounding dyke/sill intrusions where palaeotemperatures indicate mature to over-mature state. Interestingly, the iron sulphide (greigite) is still detected in the contact aureole where palaeotemperatures exceeded 130°C. Therefore, the magnetic mineralogy is a sensitive method that can characterize normal burial history, as well as identify hidden metamorphic contact aureoles where the iron sulphide greigite is present at temperatures beyond its stability field.     

Record of Late Pleistocene Glaciation and Deglaciation in the Southern Cascade Range. I. Petrological Evidence from Lacustrine Sediment in Upper Klamath Lake, Southern Oregon

Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter.     

Mineral magnetic and physical properties of surficial sediments and onshore samples from the southern basin of Lake Baikal, Siberia

Results of lithostratigraphic and mineral magnetic analysis of two surficial sediment cores (21 cm and 45 cm in length) collected from the Southern basin of Lake Baikal at a water depth of 1390 m, are presented. The sediments have been measured for a wide range of mineral magnetic parameters in order to assess their value in the identification of turbidite layers. Particle size and geochemical data are also presented and these explain some of the down core variations in magnetic mineralogy. It is suggested that changes in the particle size frequency distributions down core may be related to fossil diatom shells. One of the cores has been dated using ²¹⁰Pb. The sediment cores were cross-correlated using low frequency magnetic susceptibility (f) and these cores can also be correlated with a nearby core collected earlier in 1992. Changes in the magnetic parameters of lf, IRMs and HIRM₂₁₀ suggest that there are significant changes in the concentration of ferrimagnetic minerals in the sediment cores, indicating changing sediment sources and/or increasing concentrations of spheroidal carbonaceous particles and the dissolution of minerals through reduction below the oxidised layer within the sediment core.     

A rock magnetic record from Lama Lake, Taymyr Peninsula, northern Central Siberia

An 11 m long sediment core ftorn Lama Lake, Northern Siberia, has been subjected to intense sedimentological, geochemical and rock magnetic analyses. According to a palynologic investigation the recovered sediments cover the whole Holocene and the late Pleistocene reaching back to about 17 ka. IRM acquisition experiments, hysteresis loop and back field as well as thermomagnetic measurements revealed magnetite in the pseudo-single domain range as the only remanence carrier. Sharp rock magnetic boundaries occur at 20 and 745 cm sub-bottom depth that are clearly linked to shifts in the median grain size of the magnetite. These boundaries are close to the present boundaries that bracket an anoxic zone between the subrecent and a late Pleistocene oxic section of the sediments. Within the anoxic section, magnetites are characterized by significantly larger median grain sizes but within a very narrow grain size range. The shift from fine grained magnetite within the oxic sediments to coarse grained magnetite is interpreted as the result of dissolution of the finest magnetite grains within the anoxic sediments. A significant shift of the Ti/Fe-ratio of the bulk sediment at a sub-bottom depth of 735 cm does not correspond to thermomagnetic properties, i.e. Curie-temperatures do not follow the variable Ti-content of the sediment.