Loess Units Research Articles

A first paleomagnetic and rock magnetic investigation of calcareous nodules from the Chinese Loess Plateau

Abstract A detailed paleomagnetic and rock magnetic investigation was carried out on calcareous nodule layers embedded in the loess-paleosol sequences near Baoji city, southern Chinese Loess Plateau. The anisotropy of low-field magnetic susceptibility confirms that calcareous nodules retain the primary sedimentary fabric of original deposits. Rock magnetic properties demonstrate that calcareous nodules have the same mineralogy and grain sizes as loess-paleosol deposits. Characteristic remanent magnetizations of calcareous nodules have directions consistent with those of adjacent loess and paleosol deposits. The Punaruu event, previously found from loess deposits in the upper part of loess unit L13 in Weinan, southern Chinese Loess Plateau, is observed at the corresponding stratigraphic level in the calcareous nodule layer. These observations indicate that the calcareous nodule layers preserve primary remanent magnetizations of original deposits before calcification, and that they can contribute to paleomagnetic studies in the Chinese Loess Plateau.

Evidence for the Kamikatsura and Santa Rosa excursions recorded in eolian deposits from the southern Chinese Loess Plateau

A detailed paleomagnetic investigation of a loess‐paleosol sequence in Baoji, southern Chinese Loess Plateau, reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late Matuyama Chron. Rock magnetic experiments demonstrate that the sediments of the excursion intervals, in loess unit L9, have the same magnetic properties as those from surrounding normal and reversed polarity intervals. Assuming constant sedimentation rate between the Jaramillo‐Matuyama and Matuyama‐Brunhes magnetic polarity boundaries, the midpoint ages and durations for these two excursions are estimated at 0.89 Ma and 16 kyr, and 0.92 Ma and 7 kyr, respectively. In addition, these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr. These estimates are supported by paleoclimatologically inferred age constraints. The two excursions are probably correlated with the Kamikatsura and Santa Rosa excursions, respectively.

Mangshan Loess in Central China and the Paleomonsoon Variations since the Last Interglaciation

Abstract The Mangshan Yuan is a loess platform on the southern bank of the Yellow River, which is located in northwestern Zhengzhou of Henan Province, China. The typical Zhaoxiayu section of the Mangshan Yuan preserves stratigraphical loess units above S10 with a total thickness of 172.1 m, which includes 15.7 m of the last interglacial paleosol S1, 77.3 m of the last glacial loess L1 that consist of 41.6 m of the late stade L1LL1, 13.2 m of the interstade L1SS1 and 22.5 m of the early stade L1LL2. Based on the age marking points by correlating magnetic susceptibility of the section with the SPECMAP curve, the timescale of the section was constructed, and the average accumulation rate and the resolution of each loess strata over the S2 were subsequently calculated using the susceptibility age model. The results indicate that strata units developed in the glacial, interglacial stages, stadial and interstadial show substantial differences in grain size, average accumulation rate and time resolution ub the Zhaoxiayu section. Specifically, the average accumulation rate of the loess L1LL1 is 3.45 mm/a, whereas that of paleosol S1 is only 0.28 mm/a. Based on the high‐resolution records of magnetic susceptibility and >45 μm fraction percentage of the loess‐paleosol, the summer and winter monsoon variations as well as their interrelations since the last interglaciation have been discussed, which were correlated with the SPECMAP and the GRIP climate records.

The magnetism of a glacial aeolianite sequence from Lanzarote (Canary Islands): coupling between luvic calcisol formation and Saharan dust trapping processes during wet deposition events off northwestern Sahara

SUMMARY In order to better document the climatic origin of pedogenized loess deposits west of Sahara, rock-magnetic measurements were performed on a Last Glacial coastal sand sequence from La Mala (LM) quarry (Lanzarote, Canary Islands) containing six interbedded loess–palaeosol units. Hysteresis and susceptibility data point to a coarse-grained magnetic enhancement in the coastal sand, which contrasts with the superparamagnetic (SP) to pseudo-single domain (PSD) behaviour of the Saharan loess and palaeosol. High- and low-temperature experiments show that oxidized titanomagnetite contributions dominate in the sand, while fine-grained (SP to PSD) iron oxidation products such as maghemite and goethite are evidenced in the Saharan loess/palaeosol units. At room temperature, the detrital PSD–multidomain titanomagnetite contribution of local origin is tentatively estimated from the AC-field dependence of magnetic susceptibility. Surface oxidation of detrital spinel grains and authigenesis of fine-grained iron oxides (including SP goethite) are proposed to explain the magnetic properties of the primarily fine, Saharan-dust-bearing material. The dry, local climate of the present-day and Late Holocene means that loess deposits are not preserved. The six pedogenized loess units, however, point to dust trapping under semi-arid, wetter conditions, probably illustrating periods of reduced latitudinal temperature gradients and climate variability of the North Atlantic climate, respectively. These findings suggest that both the Canary palaeosol and its content of (ultra)fine iron oxide might be constrained by (wet) deposition and trapping of fine Saharan dust.

Grain sizes of susceptibility and anhysteretic remanent magnetization carriers in Chinese loess/paleosol sequences

Detailed rock magnetic studies show that susceptibility (mass‐specific χ) and anhysteretic remanent magnetization (ARM) of the Chinese loess/paleosol sequences are carried by almost identical magnetic carriers. Therefore the ratio Δχ/χARM (or equivalently χARM/Δχ, where Δχ is defined as χ − χ0, and χ0 is the intercept susceptibility of the plot of χ versus ARM, and χARM is field‐normalized ARM) can be used to quantify the grain size of χ and ARM carriers. By determining this ratio for three Chinese loess/paleosol profiles (Jiuzhoutai, Yuanbao, and Yichuan) characterized by different degrees of environmentally controlled pedogenesis and sedimentation rates, we show that the lower grain‐size limit of aeolian magnetic particles in the less pedogenically altered loess units is about 100–300 nm, in the finer‐grained pseudosingle domain (PSD) grain‐size range. In contrast, the grain sizes of pedogenically produced magnetic particles for mature paleosols dominantly cover both the superparamagnetic (SP) and single‐domain (SD) ranges. On the basis of plots of Δχ/χARM against Δχ, samples can be divided into four regions (I, II, III, and IV). Region I corresponds to the least pedogenically altered primary loess samples, with Δχ/χARM of 0.165–0.24. Samples in region II, a transition zone between the least altered loess and the onset of development of paleosols, have χ values identical to those in region I but have lower Δχ/χARM of 0.09–0.165. With increasing susceptibility in zone III, Δχ/χARM is positively correlated with Δχ, indicating the gradually increasing influence of SP particles. Finally, in zone IV with Δχ higher than ∼6.5 × 10−7 m3 kg−1, Δχ/χARM is independent of the variations in Δχ, suggesting that Δχ/χARM is totally controlled by the pedogenic finest‐grained particles and the size distribution of these particles remains almost constant. The development of soils in the Chinese loess revealed by these three profiles from three sites can be clearly explained by a continuous process of pedogenesis, increasing from zone I to zone IV. The definition of the pedogenic zones can help to improve our understanding of the underlying mechanisms and variability of pedogenesis and thus could enable more successful and accurate separation of the authentic pedogenic signals from the background signal of the aeolian inputs at different loess sites worldwide.

Determining the climatic boundary between the Chinese loess and palaeosol: evidence from aeolian coarse-grained magnetite

This paper proposes a new method to distinguish between interglacial deposits and pedogenically-overprinted glacial loess based on the concentration variations of coarse-grained aeolian (magnetically pseudo-single domain and multidomain) magnetite. We apply the method to a sequence from the upper part of the loess unit L2 (marine isotope stage MIS 6) to the sub-loess unit S1L2 (MIS 5d) at the YuanBo (YB) section in the Chinese Loess Plateau. The method is based on the differences in low temperature properties between the coarse-grained (multidomain and pseudo-single domain) detrital magnetite and the pedogenic magnetic particles including superparamagnetic (SP) particles and relatively larger (>SP) maghemite particles. The former is characterized by a crystallographic Verwey transition around 120 K. In contrast, the magnetization of the latter continuously decreases in intensity with increasing temperatures. The method involves two steps: (1) calculating the first-derivative of the low temperature thermal demagnetization of the saturated isothermal remanent magnetization acquired at 20 K (LT-SIRM), which enhances the behaviour related to the Verwey transition, and (2) fitting a third-order polynomial background to the data between both 50–70 and 150–300 K, and then subtracting this background from the total derivative curves. The area under the background-corrected derivative curves represents the absolute intensity drop associated with the Verwey transition (ΔJTV). This is caused by the aeolian coarse-grained magnetite, which is very sensitive to the changes in the intensity of the winter monsoon, and in turn related to the changes in palaeoclimate. The results show that the sharp drop of ΔJTV at 39.44 m corresponds to the climatic boundary from L2 (MIS 6) to S1S3 (MIS 5e), and not at 39.8 m as thought previously. Thus, the palaeosol deposits just below this boundary are in fact highly altered L2 materials instead of S1S3 accumulations.

Chronological discord between the last interglacial paleosol (S1) and its parent material in the Chinese Loess Plateau

An and Porter (Geology 25 (1997) 603) reported six high dust-influx events of millennial timescales recovered from the last interglacial paleosol S1 and correlated them to six cool events of millennial timescale in the North Atlantic. However, the complexity of soil-forming processes may have made the chronological correlation with the North Atlantic records inadequate. To examine the complexity of the S1 formation, the S1 paleosol was traced laterally and identified based on the preserved characteristics observed in the field and analyzed in the laboratory. Our data show that from the northwest to the southeast, the S1 paleosol gradually converges from three distinctive soil profiles into a single welded profile because the net rate of loess accumulation was attenuated to the southeast and pedogenic development intensified southeastward during the last interglacial. Three soil-forming events within the S1 paleosol (S1S1, S1S2 and S1S3) separated by two loess units (S1L1 and S1L2) in the northwestern part of the Loess Plateau are stratigraphically coeval with a single soil profile in the southeastern margin of the Loess Plateau. In the southeast, the S1 paleosol developed into underlying older loess L2 (e.g., at the Lantian section). The three paleosols (S1S1, S1S2 and S1S3) are partially welded in the central part of the Chinese Loess Plateau (e.g., at the Tianshui section), where the lower portion of S1 paleosol developed in the underlying older loess unit L2. In the northwestern margin of the Chinese Loess Plateau (e.g., at the Lanzhou section), the preservation of the repeating soil–loess sequence (S1S1, S1L1, S1S2, S1L2 and S1S3) continuously documented the climatic events of the last interglacial. Our data also show that the magnetic signatures and particle-size information are more or less acceptable climatic proxies only for the northwestern sections, where the degree of pedogenesis was lower and the rate of eolian influx was greater during the last interglacial. It appears that in all cases investigated, the median grain size and the coarse fraction (>63 μm) content define the upper and lower boundaries of the S1 paleosol reasonably well and can be used to estimate the time-transgressive nature of the S1 paleosol relative to its parent material. Soil welding, bioturbation and material translocation within the S1 soil profiles make it impossible to preserve the detailed and high-resolution information of climate changes in those S1 profiles in the southeastern part (including the popularly called central part) of the Chinese Loess Plateau.

Holocene loess accumulation and soil development at the western edge of the Chinese Loess Plateau: implications for magnetic proxies of palaeorainfall

A high-resolution Holocene sequence of loess, palaeosols and incipient soils at the western edge of the Chinese Loess Plateau, dated using optically stimulated luminescence techniques, is used to identify the respective influence of dust accumulation rate, time and climate on soil magnetic properties. Dust deposition and soil formation have both been quasi-continuous through the Holocene at this site; the soils are thus accretionary in nature. The degree of soil development (as indicated both by geochemical and magnetic properties, which correlate strongly) varied through the Holocene. Compared with the less-weathered loess units, each palaeosol is enriched in nitrogen and organic carbon, depleted of carbonate due to leaching, and displays higher values of pedogenic magnetic susceptibility, frequency-dependent susceptibility (%), remanences and magnetisation. Magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine, of single domain and superparamagnetic dimensions. Sediment accumulation rates were lowest from ∼12 to 2.5 ka, providing ample time (100 s of years) for weathering and soil formation to proceed, yet pedogenesis through this interval was relatively weak. Conversely, soil development was stronger during later intervals—when loess accumulation rates were higher, and soil-forming intervals were correspondingly shorter. In terms of Jenny's (1941) soil-forming equation, accumulation rate (and thus time), sediment source (parent material) and topography show no systematic variation between the loess and the palaeosols. Therefore, climate seems to be the key soil-forming factor which has controlled the geochemical and magnetic properties of these Holocene palaeosols.

Geomagnetic episodes of the last 1.2 Myr recorded in Chinese loess

The Matuyama‐Brunhes (M/B) boundary, termination of Jaramillo normal subchron (J/M) as well as three young geomagnetic excursions has been revealed in the Weinan loess section [Zhu et al., 1994,1998,1999]. Recently, we carried out further paleomagnetic directional studies on the same section and recovered the onset of the Jaramillo normal subchron (M/J) and four geomagnetic excursions, two in the Brunhes and two in the late Matuyama. The fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short‐lived geomagnetic signals could be only recorded in loess units, and may have been fully smoothed or wiped out in paleosol units by strong pedogenesis. Constrained by a new astronomical timescale (HLD00), ages of the geomagnetic excursions and reversal boundaries have been estimated. We tentatively correlated these excursions with previous results, indicating that a large lock‐in model may not be correct for the whole loess profile, although some apparently big positive age discrepancies were observed for the main boundaries.

Variation of Zr/Rb ratios on the Loess Plateau of Central China during the last 130000 years and its implications for winter monsoon

To find a new index of winter monsoon without effects of chemical weathering and pedogenesis is an important task in loess research. Chemical analysis of five grain size fractions (i.e. >32 μm, 32–20 μm, 20–8 μm, 8–2 μm and <2 μm) show that Zr is enriched in coarser grain size, while Rb is concentrated in finer fractions. An excellent correlation occurs between Zr/Rb ratios and mean grain size in loess units (L1LL1 and L1LL2) from Huanxian and Xifeng sections, reflecting that Zr/Rb ratios increase when grain size is coarser. Due to immobility of the two elements during the postdepositional process, Zr/Rb ratios can eliminate the effect of weathering and pedogenesis and serve as an excellent proxy on winter monsoon variation. The Zr/Rb ratio curves of the Huanxian, Xifeng and Luochuan sections during the last 130000 years show that the winter monsoon strength was very weak during MIS 3, which corresponded to the intensity during MIS 5.

Quaternary stratigraphy and optical dating of loess from the east European Plain (Russia)

Optical dating of silt-sized potassium feldspar grains was found to be suitable for dating the formation of key loess units at two sites on the central east European Plain. Loess bracketing the Bryansk palaeosol yielded optical ages in agreement with the well-dated radiocarbon chronology when corrected for anomalous fading. Optical ages from loess bracketing a significant pedogenic unit below the Bryansk palaeosol support our lithostratigraphic interpretation that it is the Mezin palaeosol complex, and its formation during the Last Interglaciation, Oxygen Isotope Stage (OIS) 5. Optical ages from a stratigraphically older loess unit directly underlying a significant (∼10 m thick) till unit, in conjunction with its regional stratigraphic context, suggest that the loess, and the till were deposited during the Dnieper Glaciation, which probably occurred during OIS 6.

Rock magnetic properties of a loess-paleosol couple along an N-S transect in the Chinese Loess Plateau

We have conducted detailed rock magnetic experiments on samples from loess unit 8 (L8) and paleosol unit 8 (S8) in Jingbian, Yichuan and Duanjiapo loess sections along an N-S transect in the Chinese Loess Plateau. Major rock magnetic results are as follows: (ⅰ) An increase of high field susceptibility ( χ h ) in the same level of loess or paleosol from north to south is observed, suggesting an enhancement of pedogenesis. (ⅱ) The low field susceptibility ( χ L ) in loess unit L8 is almost the same in three sections. In contrast, the χ L of paleosol unit S8 in Yichuan is highest, and the χ L of Duanjiapo is lower than that in Yichuan section, suggesting that there is not correlation between the χ L and the degree of pedogenesis in loess-paleosols. (ⅲ) With the increasing of χ L , both the contents of the superparamagnetic (SP) and the ferrimagnetic grains in loess-paleosol increase, however, the enhancement of magnetic susceptibility is probably dependent more on the increase of the ferrimagnetic concentration than on a change in the grain size. (ⅳ) The content of the maghemite in loess unit 8 increases from the northern to the southern part of the Chinese Loess Plateau, and is positively correlated with the pedogenesis of the loess.

Geochemical indicator of original eolian grain size and implications on winter monsoon evolution

Grain size of eolian deposits from the Loess Plateau in China has been widely used to reconstruct the history of the East Asian winter monsoon. However, the grain size of bulk samples is only partially indicative to the strength of the winter monsoon because post-depositional weathering processes have significantly changed the grain size of original eolian particles. Here, non-weathered loess samples were separated into eight different particle fractions, and major chemical elements were determined in order to establish a geochemical indicator of original eolian grain size. The results show that SiO 2 and Al 2 O 2 contents and the SiO 2 /Al 2 O 2 ratio in different fractions vary regularly with grain size, and that a good linear relation exists between the SiO 2 /Al 2 O 2 ratio and grain size for the fractions 2 /Al 2 O 2 ratio, this index can be used to reflect the grain size of original eolian particles. Application of this index in the Weinan and Luochuan loess sections of the last climatic cycle shows that SiO 2 /Al 2 O 2 is in good agreement with median grain size (Md) in the loess units. On the contrary, SiO 2 /Al 2 O 2 has documented a series of fluctuations in the soil units that are not clearly indicated by the grain-size changes of bulk samples.

Luminescence chronology of loess‐paleosol sequences from Canterbury, South Island, New Zealand

The extensive Quaternary loess‐paleosol deposits of South Island, New Zealand, represent one of the major proxy records of paleoclimatic changes in the Southern Hemisphere. We attempted to produce the first numeric chronology of these subaerial sequences in the Canterbury region by using thermoluminescence and infrared‐stimulated luminescence dating methods. We examined five exposures: a 6 m thick section at Cust, north of Christchurch; two thicker (c. 14 m) sequences on Banks Peninsula (Barrys Bay and Onawe sites); farther south, a c. 12 m sequence in Timaru; and a c. 7 m sequence on the coast at the Normanby site near Timaru. Our results are largely based on single experiments per sample, and therefore provide imprecise ages for several of the older samples. The most satisfactory results are those from the youngest site (Cust), for which three samples were dated. Here, phases of maximum loess deposition are dated at 73 ± 13 ka (basal loess‐paleosol unit L3), 41 ± 5 ka (basal L2), and 27 ± 3 ka (basal LI). At Barrys Bay an age of 70 ± 15 ka was obtained in the basal LI, and at Timaru two separate samples in the base of LI also yielded ages of c. 70 ka, thus correlating the entire Cust loess sequence with the LI loess unit at these two other study sites. Only at Barrys Bay were ages (c. 130–250 ka) in stratigraphic order obtained for older samples (units L2 and deeper). At the other sites, some samples in the sub‐L1 units gave age reversals, and some (including the oldest sample at Barrys Bay) yielded poor precision (e.g., 20%). Units L2 at Timaru and Barrys Bay may correlate to all or part of MIS 6; however, the poor precision and some age reversals in other units at these sites and at Normanby and Onawe preclude any unambiguous correlations between sites or to the MIS time‐scale. Nevertheless, in the absence of any prior numeric ages, these first results serve as a basis for more precise future dating of these units. Although these reconnaissance dating results illustrate some of the problems for luminescence dating of such sequences in South Island, they do provide a beginning for a more accurate correlation of terrestrial and terrestrial‐marine sedimentary sequences in this part of the Southern Hemisphere.

Lack of correlation between paleoprecipitation and magnetic susceptibility of Chinese Loess/Paleosol Sequences

We have conducted a detailed mineral magnetic study of loess unit 8 (L8) and paleosol unit 8 (S8) from three localities (Jingbian, Yichuan, and Duanjiapo) along a N‐S transect in the Chinese loess plateau. As expected, the low‐field magnetic susceptibility (χ) has higher values in S8 and lower values in L8. Similarly, superparamagnetic particle concentrations increase in S8 with increasingly humid climates along the N‐S transect, which suggests that pedogenic magnetic enhancement is related to climate. However, in S8 at Duanjiapo, χ is so low that there is no correlation between χ and the degree of pedogenesis in this loess‐paleosol cycle. It therefore appears that χ is not consistently the most suitable indicator of paleoclimate in Chinese loess‐paleosol sequences. Our results indicate that there are complexities in the mineral magnetic response to pedogenesis, which could invalidate interpretation of the χ record of Chinese loess‐paleosol sequences in terms of paleoprecipitation.

Late Pleistocene Environments of the Central Ukraine

The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess–steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe.

Magnetic study of Late Pleistocene loess/palaeosol sections from Siberia: palaeoenvironmental implications

Summary Rock-magnetic properties, including anisotropy of magnetic susceptibility, were investigated in detail for two loess/palaeosol sections in south-western Siberia (Bachat and Kurtak). The results obtained agree with the ‘Alaskan’ type of susceptibility variations for the Kurtak section, showing maximum susceptibility values in loess horizons and minima in palaeosols. For the Bachat section the palaeoenvironmental record, expressed through susceptibility, is not clear, in spite of the geographical proximity of the two sections. Instead, frequency-dependent magnetic susceptibility (FD%) discriminates well between loess and palaeosol units, showing maxima in pedocomplexes and minima in loess units. This suggests certain pedogenic formation of fine ferrimagnetic grains in soils. Studies of anisotropy of magnetic susceptibility (AMS) reveal a well-defined sedimentary magnetic fabric for both sections. Different origins of loess deposition, pure aeolian at Bachat and aeolian affected by secondary processes at Kurtak are reflected in the AMS patterns. The two sections also show different high-temperature behaviour of magnetic susceptibility. While for the Bachat section only one type of thermomagnetic curve was found, the Kurtak section shows different behaviour for loess and palaeosol units. Magnetic properties of samples from the two sections are discussed in terms of palaeoclimatic and depositional conditions.

Clast fabric in relict periglacial colluvium, salamanca re‐entrant, southwestern new york, usa

Samples of macrofabric data obtained from colluvial deposits near the late‐Wisconsinan glacial margin display widely divergent characteristics. Those from highly weathered and jointed sandstone plot as girdle distributions. Samples from red colluvium of early‐Wisconsinan age form moderately strong clusters. Fabrics from shallow depth in medium‐textured brown colluvium of late‐Wisconsinan age generally have modes aligned with the local slope but are weak, and appear to have been modified by frost heaving. Clasts in the basal layers of an overlying loess unit were emplaced by frost heaving from the brown colluvium and have very weak or uniform fabrics. Particle shape exerts an influence on fabric strength in the brown colluvium, with samples composed of more elongated clasts displaying stronger fabrics. Fabrics from near‐surface deposits show generally weaker shape and strength characteristics and higher inclinations than those at depth. Calculated seasonal frost penetration is consistent with the depth to which weaker, more steeply inclined fabrics are found. Sampling in relict periglacial deposits should be performed only below the level of disturbance by seasonal frost.

Pedogenesis affecting the Matuyama-Brunhes polarity transition recorded in Chinese loess?

A detailed record of the Matuyama-Brunhes (M-B) transition has been obtained from the loess unit 8 (L8) at Duanjiapo (34.2°N, 109.2°E), Shannxi Province of China. An investigation of the rock magnetic properties using hysteresis loops, thermomagnetic analyses identifies pseudosingle domain magnetite as the main carrier of the remanence, with a small contribution from maghemite and hematite. The paleo-direction records obtained reveal: (i) The M-B transition was recorded in the middle and lower part of L8, and comprises of five fast reversals. (ii) The duration of the M-B polarity transition related to the directional change is about 4800 a. (iii) The virtual geomagnetic pole (VGP) path during the transition is confined over Africa, peaked 90° away from the sampling site, in contrast with the results obtained from the Weinan loess section. The different VGPs are probably attributed to the pedogenesis.

A high-resolution micromorphological record of changing landscapes and climates on the western Loess Plateau of China during oxygen isotope stage 5

The micromorphological characteristics of a continuous set of block samples from 24–38m depth in a well section on Jiuzhoutai Mountain, Lanzhou, China, are described. The shape of the magnetic susceptibility curve, together with previously published radiocarbon, luminescence and geomagnetic dates, indicates that this sample set encompasses the whole of the last interglacial (S1) palaeosol complex, and extends into the overlying and underlying loess (L1 and L2) units. Seven pedosedimentary units are recognised on the basis of trends in the bulk properties and the key micromorphological indicators (excrements, textural concentrations, shell fragments, and calcite and gypsum features). High rates of loess deposition, with significant cryogenic activity, gypsum precipitation and only weak bioturbation, characterised Oxygen Isotope Stage (OIS) 6 and OIS 4 at Lanzhou. Depositional rates were moderately high in OIS 5d and 5b, with slightly higher bioturbation and no significant cryogenesis or gypsum precipitation. OIS 5e, 5c and 5a were marked by low rates of accretion, more extensive bioturbation and limited leaching. This reconstruction reflects the pedosedimentary responses to fluctuating dominances of monsoonal controls during this period of global climatic change. Comparison of earlier micromorphological results from sites to west and east of Lanzhou along a dry to humid climatic gradient indicate a clear regional pattern with reduced loess accumulation rates and enhanced bioturbation and leaching in the humid east (Tianshui and Xian). Clay translocation occurred only in OIS 5e at Xian, the warmest and most humid site on the transect.