Palaeomagnetism of the loess/palaeosol sequence in Viatovo (NE Bulgaria) in the Danube basin

The results of a palaeomagnetic investigation of a 27 m thick loess/palaeosol sequence in Viatovo (NE Bulgaria) are presented in this paper. The sequence consists of topsoil S₀, seven loess horizons (L₁–L₇) and six interbedded palaeosols (S₁–S₆) overlying a red clay (terra rossa) complex. Magnetic viscosity experiments, IRM acquisition, AMS analysis and NRM stepwise alternating and thermal demagnetisation experiments of pilot samples were implemented for precise determination of the characteristic remanence and construction of a reliable magnetostratigraphical scheme. Analysis of IRM acquisition curves using the expectation – maximization algorithm of Heslop et al. [Heslop, D., Dekkers, M., Kruiver, P., van Oorschot, H., 2002. Analysis of isothermal remanent magnetization acquisition curves using the expectation – maximization algorithm. Geophys. J. Int., 148, 58–64] suggests that the best fitting is obtained by three coercivity components. Component 1 corresponds to SD maghemite/magnetite, while component 2 is probably related to the presence of oxidised detrital magnetites. The third component shows varying coercivities depending on the degree of pedogenic alteration of the samples and probably reflects the presence of detrital magnetite grains oxidised at different degree.

The relevance of the Viatovo section as a key representative sequence for the loess cover in the Danube basin is confirmed by the presence of geomagnetic polarity changes in the lower part of the sequence. The youngest one recorded in the seventh loess unit L₇ can be identified as corresponding to the Matuyama/Brunhes palaeomagnetic polarity transition. Two normal magnetozones were found in the red clay complex, probably corresponding to the Jaramillo and Olduvai subchronozones of the Matuyama chron.     

The characteristics,origins, and geodynamic settings of supergiant gold metallogenic provinces

An investigation of the rock magnetic properties using stepwise isothermal remanence (IRM) acquisition, thermomagnetic analysis and temperature-dependent susceptibility history, identifies magnetite as the carrier of the main fraction of the remanence, associated with maghemite and hematite in Malan loess (L1), Holocene soil (S0) and last-glacial paleosol (S1). The presence of short-lived direction fluctuations indicates that no significant smoothing occurs in L1 when its remanence is locked, and thus L1 is capable of recording the geomagnetic secular variation (PSV), while the PSV has been severely smoothed or wiped out by pedogenic processes during S1 formation. It has been suggested that the Mono Lake and Laschamp excursions are two independent geomagnetic events based on this study.     

三门峡盆地晚新生代沉积物磁性载体类型

对三门峡盆地晚新生代沉积岩样品进行岩石磁学研究,通过三轴饱和等温剩磁和剩磁矫顽力实验、交变退磁和热退磁实验及磁化率测定,发现黄土-古土壤、河湖相灰绿层和冲洪积层3种不同岩性的磁载体存在显著差异.即黄土-古土壤以磁铁矿为主,赤铁矿和磁赤铁矿很少;河湖相灰绿色沉积磁性矿物含量较低,主要为赤铁矿和磁铁矿,但磁铁矿较多,此外还有一些不稳定磁性矿物(如针铁矿、菱铁矿等);冲洪积物以磁铁矿和赤铁矿为主,磁铁矿相对较多.     

三门峡盆地晚新生代沉积物磁性载体类型

对三门峡盆地晚新生代沉积岩样品进行岩石磁学研究,通过三轴饱和等温剩磁和剩磁矫顽力实验、交变退磁和热退磁实验及磁化率测定,发现黄土-古土壤、河湖相灰绿层和冲洪积层3种不同岩性的磁载体存在显著差异.即黄土-古土壤以磁铁矿为主,赤铁矿和磁赤铁矿很少;河湖相灰绿色沉积磁性矿物含量较低,主要为赤铁矿和磁铁矿,但磁铁矿较多,此外还有一些不稳定磁性矿物(如针铁矿、菱铁矿等);冲洪积物以磁铁矿和赤铁矿为主,磁铁矿相对较多.     

中中新世秦安黄土-古土壤序列磁性增强机制及其环境意义

中中新世黄土高原风尘堆积序列磁化率显著增高,在区域内已成为地层对比标志,然而其磁性增强机制依然不清楚.本文对秦安地区QA-I剖面约14~17.1 Ma期间的黄土-古土壤样品进行了系统的环境磁学、岩石磁学（热磁性质、磁滞性质、非磁滞剩磁与饱和等温剩磁比值χ_ARM/SIRM等）和色度指标分析.结果表明,QA-I剖面黄土-古土壤序列样品中磁性矿物为准单畴（PSD）磁铁矿、超顺磁/单畴（SP/SD）磁赤铁矿和赤铁矿,古土壤中细粒磁赤铁矿含量高于上下相邻黄土层.CBD（Citrate-Bicarbonate-Dithionite）处理方法显示,中中新世风尘序列中碎屑成因的PSD磁铁矿含量变化不大,约14.5~16.0 Ma期间成壤成因SP/SD磁赤铁矿含量的增多,是导致该时期风尘堆积序列磁化率增强的主要原因;这与该时期较低的土壤发育程度并不一致,表明CBD处理方法虽然可有效获取成壤成因SP/SD磁赤铁矿信息,却无法区分其为当地还是源区成壤成因.低温磁学研究表明,QA-I剖面约14.5~16.0 Ma期间SP/SD磁赤铁矿可能为碎屑磁铁矿颗粒的风化外壳,其含量高低变化与风尘样品中值粒径粗细变化一致,揭示出此类磁性颗粒含量受控于风力强弱,可能为源区风化成因.尽管中中新世古土壤样品的磁化率与上下黄土层相比可反映其成壤强度,但约14.5~16.0 Ma期间SP/SD磁赤铁矿含量变化可能同时受到当地成壤强度以及源区风化的影响,导致磁化率与当地成壤强度之间关系较为复杂.因此,中新世风尘堆积序列长尺度的磁化率趋势并不反映东亚夏季风强度的变化.

     

Rock magnetic properties of Nihewan sediments at Xujiayao

Rock magnetic measurements of Nihewan sediments from Xujiayao section demonstrate that magnetite, hematite and maghemite are dominant remanent magnetization carriers. Monitoring the variations of magnetic susceptibility (MS) and saturating isothermal remanent magnetization (SIRM) at low temperature are the attractive ways of detecting the presence of magnetite, maghemitization and superparamagnetic grain sizes. Low-temperature MS investigations suggest that susceptibility enhancement for Xujiayao samples is mainly due to the remarkable presence of SD/MD magnetite to some degree though some magnetite grains have been partially oxidized at some depths. It is tentatively concluded that both SD/MD magnetite and hematite are of detrital origin and carry a characteristic remanent magnetization (ChRM), whereas maghemite can be attributed to be chemical origin, overprinting a reversed polarity component of Matuyama age.     

Blake极性事件与古土壤层S1的沉积环境

对黄土高原定边、西峰、渭南三个剖面中古土壤层S1的定向连续古地磁样品作了详细的岩石磁学和古地磁学研究,均未发现Blake极性事件．在对其成因进行了分析研究后,提出如下看法：（1）六盘山以东黄土高原北部边缘的风成沉积可能存在局部的不连续性,因而没有记录Blake极性事件;（2）黄土沉积物的主要载磁矿物力磁铁矿,次生磁性矿物对剩磁的贡献可能大于原生磁铁矿,故在六盘山以东黄土高原北部边缘没有发现Blake极性事件;（3）六盘山以东黄土高原中部和南部地区,强烈的成土作用掩盖了持续时间较短的Blake极性事件;（4）六盘山以西的黄土高原具高速稳定沉积的特点,且成土作用弱,可记录Blake极性事件．     

Blake极性事件与古土壤层S1的沉积环境

对黄土高原定边、西峰、渭南三个剖面中古土壤层S1的定向连续古地磁样品作了详细的岩石磁学和古地磁学研究，均未发现Blake极性事件．在对其成因进行了分析研究后，提出如下看法：（1）六盘山以东黄土高原北部边缘的风成沉积可能存在局部的不连续性，因而没有记录Blake极性事件；（2）黄土沉积物的主要载磁矿物力磁铁矿，次生磁性矿物对剩磁的贡献可能大于原生磁铁矿，故在六盘山以东黄土高原北部边缘没有发现Blake极性事件；（3）六盘山以东黄土高原中部和南部地区，强烈的成土作用掩盖了持续时间较短的Blake极性事件；（4）六盘山以西的黄土高原具高速稳定沉积的特点，且成土作用弱，可记录Blake极性事件．     

Characteristics and genesis of maghemite in Chinese loess and paleosols: Mechanism for magnetic susceptibility enhancement in paleosols

Morphological characteristics and microstructures of magnetic minerals extracted from Chinese loess and paleosols were investigated using powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Our results indicate that maghemite in loess–paleosol sequences was transformed from magnetite through oxidation of magnetite. Maghemite transformed from eolian magnetite during chemical weathering has low-angle grain boundaries among maghemite nano-crystals. Some nano-crystalline maghemites with nanoporous texture resulted from microbe-induced precipitation of magnetite or transformation of poorly crystalline ferric Fe (oxy)hydroxides in presence of Fe-reducing bacteria. Aggregates of euhedral maghemite nano-crystals were transformed from magnetite magnetosomes. Both microbe-induced nanoporous magnetite and microbe-produced magnetite magnetosomes are directly related to microbial activities and pedogenesis of the paleosols. It is proposed that the formation of nano-crystalline maghemite with superparamagnetic property in paleosol results in the enhancement of magnetic susceptibility, although the total amount (weight percent) of magnetic minerals in both paleosol and loess units is similar. Our results also show that nano-crystalline and nanoporous magnetite grains prefer to transform into maghemite in semi-arid soil environments instead of hematite, although hematite is a thermodynamically stable phase. This result also indicates that a decrease in crystal size will increase stability of maghemite. It is also inferred that surface energy of maghemite is lower than that of hematite.     

黄土高原西北缘末次冰期晚期以来黄土沉积物的岩石磁学性质

以黄土高原西北缘的靖远和古浪剖面(包含黄土层L1上部和古土壤层S0)作为研究对象,选取代表性样品进行磁化率、频率磁化率、热磁曲线、等温剩磁获得曲线和磁滞回线等测定.结果表明,靖远和古浪L1黄土和S0古土壤具有相似的岩石磁学特征.磁性矿物含量相对较低,载磁矿物均以磁铁矿为主,同时含有磁赤铁矿和赤铁矿,且S0古土壤中的磁赤铁矿含量要高于L1黄土.靖远和古浪剖面L1黄土和S0古土壤的亚铁磁性矿物平均粒度都分布在准单畴(PSD)范围,并且L1黄土平均粒径比S0古土壤更靠近多畴(MD)范围.通过与黄土高原东部黄土剖面L1黄土和S0古土壤的磁化率对比分析,认为导致靖远和古浪S0古土壤磁化率增强的主要因素是气候,但是,研究区受物源影响较大,来自粉尘源区的粗颗粒磁性矿物对磁化率贡献不可忽略.     

The application of high resolution IRM acquisition to the discrimination of remanence carriers in Chinese loess

Summary High resolution isothermal remanent magnetisation (IRM) acquisition was performed on forty-one Chinese loess samples with the aim of investigating the effects of weathering on the remanence carrying mineralogy. Magnetic susceptibility was taken as a measure of the degree of weathering. This is a valid assumption based on previous studies which have demonstrated a close relationship between climate and magnetic susceptibility in the Chinese loess. Detailed analysis, based on fitting accumulative log-gaussian curves to IRM acquisition data, reveals up to four distinct coercivity components. The remanences of the two harder coercivity components are found to be almost independent of magnetic susceptibility. On the other hand, the remanences of the two softer coercivity components are found to be positively correlated with magnetic susceptibility. Most probably the two harder components are associated with detrital iron-oxides in the primary dustfall, whereas the softer components were created during pedogenesis.     

Magnetic Anisotropy as an Aid to Identifying CRM and DRM in Red Sedimentary Rocks

To further evaluate the potential of magnetic anisotropy techniques for determining the origin of the natural remanent magnetization (NRM) in sedimentary rocks, several new remanence anisotropy measurement techniques were explored. An accurate separation of the remanence anisotropy of magnetite and hematite in the same sedimentary rock sample was the goal.In one technique, Tertiary red and grey sedimentary rock samples from the Orera section (Spain) were exposed to 13 T fields in 9 different orientations. In each orientation, alternating field (af) demagnetization was used to separate the magnetite and hematite contributions of the high field isothermal remanent magnetization (IRM). Tensor subtraction was used to calculate the magnetite and hematite anisotropy tensors. Geologically interpretable fabrics did not result, probably because of the presence of goethite which contributes to the IRM. In the second technique, also applied to samples from Orera, an anisotropy of anhysteretic remanence (AAR) was applied in af fields up to 240 mT to directly measure the fabric of the magnetite in the sample. IRMs applied in 2 T fields followed by 240 mT af demagnetization, and thermal demagnetization at 90°C to remove the goethite contribution, were used to independently measure the hematite fabric in the same samples. This approach gave geologically interpretable results with minimum principal axes perpendicular to bedding, suggesting that the hematite and magnetite grains in the Orera samples both carry a depositional remanent magnetization (DRM). In a third experiment, IRMs applied in 13 T fields were used to measure the magnetic fabric of samples from the Dome de Barrot area (France). These samples had been demonstrated to have hematite as their only magnetic mineral. The fabrics that resulted were geologically interpretable, showing a strong NW-SE horizontal lineation consistent with AMS fabrics measured in the same samples. These fabrics suggest that the rock's remanence may have been affected by strain and could have originated as a DRM or a CRM.Our work shows that it is important to account for the presence of goethite when using high field IRMs to measure the remanence anisotropy of hematite-bearing sedimentary rocks. It also shows that very high magnetic fields (>10 T) may be used to measure the magnetic fabric of sedimentary rocks with highly coercive magnetic minerals without complete demagnetization between each position, provided that the field magnetically saturates the rock.     

河西走廊早新生代红层次生剩磁及其对原生剩磁影响评价

系统研究了河西走廊火烧沟组陆相红层200个采点岩石的热退磁行为和17个代表性样品的岩石磁学特征,结果表明有102个采点可以分离出A、B、C三个剩磁分量,有82个采点只有一个分量(C分量),16个采点只能分离出A、B分量.A、B分量分别由针铁矿和磁赤铁矿携带,C分量在砂岩中由磁铁矿携带,在泥质砂岩和泥岩中由磁铁矿和赤铁矿共同携带.A、B分量剩磁方向随机分布,不能获得置信水平的古地磁平均方向,为次生剩磁.它们的存在并不影响岩石原生剩磁的分离,也不影响原生剩磁信号的稳定性和获得的古地磁数据的可靠性.16个砂岩采点中不能分离出C分量,是由于后生的赤铁矿彻底改变了由磁铁矿携带的原生剩磁组分,在高密度采样的情况下剔除这些采点并不影响古地磁极性柱的构建和解释.     

Transformation relationship among different magnetic minerals within loess-paleosol sediments of the Chinese Loess Plateau

The dominant magnetic minerals and carriers of magnetic signals within the Chinese Loess Plateau are magnetite, maghemite, hematite, and goethite. In this study, we investigated the provenance and evolution of magnetic minerals during loess pedogenesis, using X-ray diffraction (XRD) and optical and electron microscopy, including field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Our results reveal that single- and multiphase mineral assemblages among magnetic minerals in the loess-paleosol sequence have been formed. Partial oxidation of coarse eolian magnetite has occurred in the desert source area and the oxidation degree is enhanced after deposition of the dust upon the Chinese Loess Plateau. This mode of origin resulted in a microtexture consisting of an inner magnetite core surrounded by a hematite rim, and strongly affected the magnetic characteristics of the loess. Goethite coexists with hematite in the loess and paleosol, and nanometer-scale hematite is formed upon goethite rims via dehydration. Our study provides direct mineralogical evidence of the magnetic record and paleoclimatic implications of the loess-paleosol sequence of the Chinese Loess Plateau. Supported by National Natural Science Foundation of China (Grant Nos. 40772032 and 40573054) and National Basic Research Program (Grant No. 2007CB815603)     

一个变泥质岩包体样品的系统热磁试验及其矿物学意义

利用一件采自河北汉诺坝周坝地区变泥质岩包体样品,结合系统的低温和高温磁性测量结果,探讨了应用热磁实验鉴别样品中所含原生磁性矿物的多解性问题. 结果表明, 饱和等温剩余磁化强度（SIRM）在室温～250℃以及280℃～380℃的降低分别由高钛钛磁铁矿的剩磁解阻过程（一种物理过程）以及由磁赤铁矿转换成赤铁矿（一种矿物相变）引起.样品在500℃以后磁化率的升高则是由磁铁矿从钛磁赤铁矿中出溶所致.因此,κ-T（即磁化率随温度变化）曲线中呈现约580℃居里点是由加热过程中次生的磁铁矿引起,而并非代表原始（即加热前）样品中的磁铁矿成分.     

CBD方法对天然样品磁性矿物影响

本研究选择川西高原、天山和西伯利亚Kurtak剖面的黄土古土壤样品及亚热带非风成样品进行CBD处理,系统测量并对比处理前后的磁学参数,包括低频磁化率、频率磁化率、非磁滞剩磁、饱和等温剩磁、剩磁矫顽力和热磁曲线(J-T曲线),分析处理前后磁性矿物种类、含量和磁畴的变化.结果表明,CBD方法对于磁性矿物的溶解并无明显的选择性,在温度与反应时间一定的条件下,磁性矿物的溶出量主要受控于其粒径分布.CBD方法可以非常有效地去除具有更大比表面积的细粒(< 1 μm)磁性矿物,同时溶解粗粒(>1 μm)磁性矿物外缘,使其粒径变细.CBD处理后磁化率变化存在多种可能,对于成土作用较强的古土壤,CBD方法可以较为准确地提取成土成因的磁性信息;而干旱和过度湿润条件下的风积黄土,不宜使用CBD方法区分原生与次生磁性矿物.     

CBD方法对天然样品磁性矿物影响

本研究选择川西高原、天山和西伯利亚Kurtak剖面的黄土古土壤样品及亚热带非风成样品进行CBD处理,系统测量并对比处理前后的磁学参数,包括低频磁化率、频率磁化率、非磁滞剩磁、饱和等温剩磁、剩磁矫顽力和热磁曲线(J-T曲线),分析处理前后磁性矿物种类、含量和磁畴的变化.结果表明,CBD方法对于磁性矿物的溶解并无明显的选择性,在温度与反应时间一定的条件下,磁性矿物的溶出量主要受控于其粒径分布.CBD方法可以非常有效地去除具有更大比表面积的细粒(1 μm)磁性矿物外缘,使其粒径变细.CBD处理后磁化率变化存在多种可能,对于成土作用较强的古土壤,CBD方法可以较为准确地提取成土成因的磁性信息;而干旱和过度湿润条件下的风积黄土,不宜使用CBD方法区分原生与次生磁性矿物.     

Two pedogenic models for paleoclimatic records of magnetic susceptibility from Chinese and Siberian loess

The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.     

Identification of aeolian loess deposits on the Indo-Gangetic Plain(India) and their significance

Aeolian loess deposits contain abundant information about the evolution of the paleoenvironment.For example,paleoclimate changes recorded in Chinese loess area obtained significant achievement in the past few decades.Compared to Chinese loess,research on Indian loess is lacking.Currently,most studies focus on the Kashmir area located in the southern Himalayas,and studies on other areas are rare.However,field observations demonstrate that the sediments around the New Delhi-Agra-Jaipur Plain are similar to Chinese loess-paleosol sequences.For example,the boundary between two strata is transitional and without horizontal bedding.Moreover,obvious pedogenic horizons developed among sediment sequences,probably indicating unrecognised aeolian deposits in the Indo-Gangetic Plain(IGP).To confirm this,pilot samples were obtained from the IGP and detailed indoor measurements conducted.The results indicate that the distribution patterns of particle size and rare earth elements(REE) of the pilot samples are similar to Chinese loess.Furthermore,the scanning electronic microscopy(SEM) images of pilot samples show obvious conchoidal fractures,dash-shaped concavities,and abundant small pits that usually form through mechanical impact.These are typical characteristics of aeolian particles.In addition,environmental and rock magnetic measurements indicate that the dominant magnetic minerals in the pilot samples are magnetite and maghemite,and that they likely contain small amounts of hematite.Furthermore,conventional magnetic parameters are comparable with Chinese loess.Based on this,aeolian loess deposits are widely distributed in the IGP,which may have promoted the development of Indian farming and contributed towards the prosperity of ancient Indian civilisation.This study also provides a new and valuable record for the research on paleoclimate changes in the study area in the future.     

Palaeomagnetism of the Torridonian of Rhum, Scotland: evidence for limited uplift of the central intrusive complex

The Tertiary Central Intrusive of Rhum was emplaced into the Torridonian Sandstone, causing a heat influx into the sediment. Detailed progressive demagnetisation of sandstones from the thermal aureole of the intrusion show that they reached maximum temperatures which decrease systematically with distance in a manner indicative of conduction-dominated cooling. Temperatures reached near to the intrusion are higher than would be predicted theoretically, probably as a result of the presence of smaller plugs of magma at depth which are only rarely exposed at the surface. The shape and magnitude of the thermal aureole suggest that upward movement of the Central Intrusive along the bounding fault was limited.Primary remanence in Torridonian sandstones unaffected by the intrusion is carried by both magnetite and haematite, often with both occurring in the same specimen. It is suspected that only magnetite carries a truly primary (detrital?) remanence; the haematite probably formed diagenetically (largely by oxidation of detrital magnetite) at some time shortly after deposition.