Glacial advances and ESR chronology of the Pochengzi Glaciation,Tianshan Mountains,China

The Pochengzi Glaciation is a typical glaciation in Quaternary in the Tianshan Mountains. The glacial landforms comprise several integrated end moraines, like a fan spreading from the north to the south at the mouth of the Muzhaerte River valley and on the piedmont on the southeastern slope of the Tumur Peak, the largest center of modern glaciation in the Tianshan Mountains. The landforms recorded a complex history of the ancient glacier change and contained considerable information of the glacial landscape evolution, and dating these landforms helps us understand the temporal and spatial shifts of the past cryosphere in this valley and reconstruct the paleoenvironment in this region. Electron spin resonance (ESR) dating of the glacial tills in the upper stratum from a well-exposed section, end moraines, and associated outwashes was carried out using Ge centers in quartz grains, which are sensitive to the sunlight and grinding. The results could be divided into three clusters, 13.6–25.3, 39.5–40.4 and 64.2–71.7 ka. Based on the principle of geomorphology and stratigraphy and the available paleoen- vironmental data from northwestern China, the end moraines were determined to deposit in the Last Glaciation. The landforms and the three clusters of ages demonstrate that at least three large glacial advances occurred during the Pochengzi Glaciation, which are corresponding to marine oxygen isotope stage 4 (MIS4), MIS3b and MIS2. The landforms also indicate that the gla- ciers were compound valley glacier in MIS2 and MIS3b and piedmont glacier in MIS4, and the ancient Muzhaerte glacier were 94, 95 and 99 km at their maximum extensions in these three glacial advances.     

In situ cosmogenic ~(10)Be dating of the Quaternary glaciations in the southern Shaluli Mountain on the Southeastern Tibetan Plateau

It is generally considered that four-times ice age happened during the Quaternary epoch on the Tibetan Plateau. However, the research on the chronology of the four-times ice age is far from enough. The Shaluli Mountain on the Southeastern Tibetan Plateau is an ideal place for plaeo-glacier study, because there are abundant Quaternary glacial remains there. This paper discusses the ages of the Quaternary glaciations, based on the exposure dating of roche moutonnée, moraines and gla- cial erosion surfaces using in situ cosmogenic isotopes 10Be. It is found that the exposure age of the roche moutonnée at Tuershan is 15 ka, corresponding to Stage 2 of the deep-sea oxygen isotope, suggesting that the roche moutonnée at Tuershan is formed in the last glacial maximum. The expo- sure age of glacial erosion surface at Laolinkou is 130―160 ka, corresponding to Stage 6 of the deep-sea oxygen isotope. The oldest end moraine at Kuzhaori may form at 421―766 kaBP, corre- sponding to Stages 12―18 of the deep-sea oxygen isotope. In accordance with the climate charac- teristic of stages 12,14,16 and 18 reflected by the deep-sea oxygen isotope, polar ice cores and loess sequence, the oldest end moraine at Kuzhaori may form at stage 12 or stage 16, the latter is more possible.     

10Be dating of boulders on moraines from the last glacial period in the Nyainqentanglha mountains,Tibet

Chronologies of glacial advances during the last glacial period in the Nyainqentanglha mountain range may provide constraints on the past climate in a transition zone of the Asian monsoon.We present 15 new10Be exposure ages from two moraines in the Payuwang valley,on the north slope of the range.The inner moraine has exposure ages ranging from 18.0±1.7 to 30.6±2.8 ka(n=10),with a mean age of 23.8±4.0 ka,corresponding to the global Last Glacial Maximum(LGM).The outer moraine yields exposure ages ranging from 18.0±1.6 to 39.9±3.7 ka(n=5).Evidence for weathering leads us to view the oldest age as a minimum age,placing moraine formation during MIS3.Chronologies from the last glacial period from south slope of the Nyainqentanglha support this interpretation.Thus,there appears to have been a local LGM(LLGM)during MIS3 and a more limited glacial advance during the global LGM.Glacial advances during MIS3 in the Nyainqentanglha may correlate with millennial-scale climate change(Heinrich events).     

Late Quaternary Activity and Paleoearthquakes along the Nanyukou Segment of the Northern Piedmont Fault of Wutai Mountain

The northern piedmont fault of Wutai Mountain is located at the north of the Shanxi Graben system,which is the dominating fault of the south boundary of the Fanshi-Daixian depression. This paper discusses the fault activity and paleoearthquakes around the Nanyukou segment of the northern piedmont fault of Wutai Mountain during the late Quaternary through field investigation along the fault,measuring geomorphic deformation and excavating trenches at some important sites. From Nanyukou to the southwest of Shanhui,we find obviously dislocated alluvial fans,with strong neotectonic movement at these sites. Since nearly 20ka,the vertical average slip rate is 1. 55mm /a to 2. 0mm /a. However,since nearly 6ka,it has reached as high as 2. 3mm /a,which is twice that on other segments. 2 trenches were excavated around Nanyukou with 6 events discovered. The referenced ages of the events are before 7600a,6700a ～ 7600a,5321a ～ 5575a,4400a ～ 5400a,4200a ～ 4400a and after 1600a B. P. with approximate recurrence interval 1400a. The latest event is likely to be the earthquake occurring at 512 A. D. ,so it is necessary to do further work to verify this in the future.     

Global correlation for strontium isotope curve in the Late Cretaceous of Tibet and dating marine sediments

87Sr/86Sr ratios of marine carbonate samples collected from a sedimentary section of the Late Cretaceous in the south of Tibet were measured. Based on the absence of cathodo-luminescence and a very low Mn/Sr ratio (average 0.06) of the samples, it is thought that they contain information on the original seawater strontium isotope composition. The strontium isotope evolution curve of the Late Cretaceous in Tibet we established here, is consistent with other coeval curves from Europe, North America and Antarctica, supports the notion that the strontium isotope composition of seawater is governed by global events, which provides a new approach for the inter-continental and inter-basinal correlations of Late Cretaceous in the area and is a complementarity for biostratigraphy. In addition, we attempt to determine the age of the boundaries for Campanian/Santonian and Maastrichtian/Campanian by 87Sr/86Sr ratios for Gamba section in southern Tibet. The two boundaries are located in the thickness of 217 m (83.5 M     

A Preliminary Study on the Datong Fault Belt

The Datong fault belt is a NE trending fault in the northern Qinghai-Xizang （Tibet） Plateau and controls the boundary of the Xining Basin and Datong Basin. It consists of the Maziying- Miaogou （F1） fault and the Laoye Mountain-Nanmenxia fault （F2）. There is obvious displacement in vertical direction along the belt. The field investigation results show that this belt has long-term activity. There are several meters long crushed zones and veins along the fault side in the basement rock. On the fault section, the Cambria system thrusts over the red- brick-colored Quaternary Period gravel, and there is a fault gouge of several centimeters thick developed on the fault plane. The fault gouge date （ESR） on the fault plane is 610 ± 61ka. The covering deluvial loess is not dislocated, and the OSL result is 14.6 ± 1.5ka. So it can be concluded that the fault belt was active in the middle Pleistocene, but inactive in the late Pleistocene according to the age data and geomorphologic features. Interior formations of the Datong basin features fold with the major axis orienting northwest. According to the relation of fault and fold deformation, Datong fault is a trausversal tear, which is due to uneven compression of the folds in different parts and NNE trending regional compressive stress. It is common among the NE trending faults in the northeast of Qinghai-Xizang （Tibet） Plateau. These NE trending faults aren＇t large, and most are located in the active plate. They are all nearly vertical to the axis of the folds and compressive basins.     

“Tali Glaciation” on Massif Diancang

The term of "Tali Glaciation" is nominated from Massif Diancang in Yunnan Province. The confusing process of the term's being put forward is confirmed through literature checking. Based on several times of field trip, the glacial landforms in this region are studied in detail, and the magnitude of the glaciation was determined. According to the numerical ages from TL, AMS 14C and OSL dating, the ear-liest glacial advance on Massif Diancang occurred at 30―40 ka BP, followed by the advances at the last stage of the last glaciation, the late glacial and neoglations in turn, until glaciers vanished at 1.2―1.5 ca. a BP in this region. It can be concluded that glaciers were limited above 3600 m a.s.l., and no glaciers existed on the lower part and adjacent mountains since the last glaciation.     

Late Quaternary glacial chronology on the eastern slope of Gongga Mountain,eastern Tibetan Plateau,China

The Gongga Mountain is the largest area of modern glaciation in the Hengduan Mountains and,with a summit elevation of 7556 m,is the highest mountain on the eastern margin of the Tibetan Plateau.During the Quaternary glacial-interglacial cycles the Gongga Mountain was extensively and repeatedly glaciated,and glacial landforms and outwash deposits from multiple glaciations are well-preserved in valleys,in basins,and on piedmonts.To constrain the glacial chronology of the eastern slope of Gongga Mountain,sample sites were selected based on the distribution and weathering of glacial tills,relationships among glacial deposits,and soil development on moraines.Dating of the tills and glaciofluvial deposits was undertaken with electron spin resonance(ESR) and optically stimulated luminescence(OSL).The ages of the glacial deposits can be divided into four clusters:2.2±0.5,11.9±0.6,35.9±2.7-58.0±6.3 and 119.2±15.9-194.2±32.8 ka.Five glacial advances in this region have been identified,which are equivalent in age to the Little Ice Age(LIA),Neoglaciation,marine oxygen isotope stage(MIS) 2,mid-MIS3,and MIS6.The largest local last glacial maximum(LGML) occurred on Gongga Mountain during mid-MIS3,characterized by a cold-humid climate,rather than the global Last Glacial Maximum(LGMG) of MIS2.The Gongga,Nanmenguangou(NMGG) and Yajiageng Glaciations occurred during the late part of the last glacial cycle,the middle of the last glacial cycle and the penultimate glacial cycle,respectively.On the basis of geomorphological,sedimentological,and compositional characteristics,landforms of the Moxi Platform and terraces can be grouped by facies and geochronology.In combination with the dating results,this analysis indicates the basal part of the Moxi Platform between Xinxin and the Moxi Hotel is correlative with the till of the Nanguanmen Glaciation(mid-MIS3).This basal unit has occasional lenses of glaciofluvial sandy gravel and lacustrine sediments.The remainder of the Moxi Platform and the terraces beside the platform are glaciofluvial deposits occasionally mixed with debris flow deposits and range in age from MIS3 to Holocene.     

A Preliminary Study on the Use of NCEP Temperature Images and Additive Tectonic Stress from Astro- Tidal- Triggering to Forecast Short-Impending Earthquakes

Taking the three earthquakes which occurred in Tibet, China during the period of July 12 to August 25, 2004 as an example,the paper analyses the M_S≥6.0 earthquakes that occurred in China and M_S≥7.0 earthquakes that occurred overseas since May of 2003 by combining the image data from the National Center for Environmental Prediction of America(NCEP)with the additive tectonic stress from astro-tidal-triggering (ATSA) and makes the following conclusions: The abnormal temperature image data of NCEP can better reflect the spatial-temporal evolution process of tectonic earthquake activity; The ATSA has an evident triggering effect on the activity of a fault when the terra stress is in critical status; using the NCEP images and the ATSA to forecast short-impending earthquake is a new concept; The three earthquakes occurred during the same phase of the respective ATSA cycle, i.e. that occurred at the time when the ATSA reached the relatively steady end of a peak, rather than at the time when the variation rate was maximal. In addition, the author discovered that the occurrence time of other earthquake cases during 2003～2004 in Tibet was also in the same phase of the above-mentioned cycles, and therefore, further study of this feature is needed with more earthquake cases in other areas over longer periods of time.     

Scientific Train of Thought and Methodological Innovation in the Intelligent Decision Support System for Earthquake Prediction in China

The level of present understanding of earthquake prediction of seismologists at home and abroad is very different. This is because China has opened up a special path of earthquake prediction research that has not been explored by other countries, with its own advantages and potentialities.Therefore, we considered that it is the most practical way to use the advantages and potentialities for raising the earthquake prediction level. For this purpose, we have developed a set of intelligent decision support system for earthquake prediction, with the analysis of cluster anomalies process at the core. The facts show that it can obviously raise the level of synthetic earthquake prediction.     

Quaternary biogenic opal records in the South China Sea: Linkages to East Asian monsoon, global ice volume and orbital forcing

Particulate fluxes investigated in the central South China Sea (SCS) during 1993―1996 indicate that opal flux can be used to show primary productivity change, which provides a foundation for tracing the evolutionary relationship between the surface productivity and East Asian monsoon in the SCS during the late Quaternary glacial and interglacial periods. Based on the studies of opal % and their mass accumulation rates (MAR) at the six sites recovered from the SCS during the “Resolution” ODP Leg 184 and “Sonne” 95 cruise of the Sino-Germany cooperation, opal % and their MARs increased evidently in the northern sites since 470―900 ka, and they enhanced and reduced, respectively, during the glacial and interglacial periods. Whereas they increased obviously in the southern sites since 420―450 ka, and they augmented and declined, respectively, during the interglacial and glacial periods. The vari- ability in opal % and their MARs in the late Quaternary glacial cyclicity indicate the “seesaw” pattern of surface productivity in the SCS. The winter monsoon intensified during the glacial periods, surface productivity increased and decreased, respectively, in the northern and southern SCS. The summer monsoon strengthened during the interglacial periods, surface productivity increased and decreased, respectively, in the southern and northern SCS. The cross spectral analyses between the opal % in the northern and southern SCS during the Quaternary and global ice volume (δ 18O) and orbital forcing (ETP) indicate that the East Asian winter and summer monsoons could be ascribed to the different drive mechanisms. On the orbital time scale, the global ice volume change could be a dominant factor for the winter monsoon intension and temporal variations. As compared with the winter monsoon, the correlative summer solar radiation with the obliquity and precession in the Northern Hemisphere could be a mostly controlling factor for the summer monsoon intension and temporal variations.     

A Study on the Influencing Factors on Ground Motion in a Valley Site

A trapezoid valley site is chosen as a research site, and according to numerical models based on orthogonal design, the factors influencing ground motion in the valley site are studied with two-dimensional finite difference method. The influencing factors are ranked, and then the calculation results are verified by ground motion analysis. The conclusions are as follows: there are four factors that have important effects on ground motion of trapezoid valley sites, but the effects are different as the location of sites changes, the influencing factors rank differently with different site locations;The ranking of the influencing factors is the same for all the sites located within a distance of 40m from the valley''s side, among them, the most effective one is the valley slope angle ranks,followed by depth-to-width ratio, overburden thickness, at last the input ground motion intensity. The impact of the factors on surface ground motion is roughly the same in the valley sites within a certain distance to the valley side,and the geometric parameters of the valley terrain play a greater part in influencing ground motion. With the increase of distance away from the valley''s side, the ranking of the influencing factors also changes, the rating of slope angle moves backward, the ranking of the input ground motion and overburden thickness move ahead. The effect of valley geometric parameters on ground motions is gradually weakened, but the effect of other two influence factors are gradually increased, similar to cases of a horizontal layered site. Strong motion records in Anning River valley site were analyzed,and the results show that the valley topography has a significant amplification effect on ground motion, and that the numerical results of this paper are credible.     

A~200 ka pollen record from Okinawa Trough:Paleoenvironment reconstruction of glacial-interglacial cycles

Pollen analysis was carried out on the Core MD982194 of 29.78 m retrieved from the Middle Okinawa Trough which was dated as old as~200 ka BP.The results revealed that pollen assemblages mainly presented an alternation of coniferous and herb pollen.The coniferous saccate pollen,principally Pinus and Tsuga,predominated in most parts of the core,especially highlighted in the interstadial stages including MIS 1,3,5 and MIS7,whereas the herb pollen significantly increased in the glacial periods.Thus the pollen flora and their percentages showed the sensitive changes under the influence of ice volume during the glacial and interglacial periods.Our record from this core has first documented that the percentage of Cyperaceae was extremely high in the glacial stages with a notable increase in Artemisia,Gramineae,Asteraceae,Chenopodiaceae,and freshwater algae,which can be used as a proxy for sea-level change at the study site because of their close negative correlation of the orbital-scale changes in sea level.The distance between the continental coastline and the Okinawa Trough has deeply shortened due to the sea-level drop in the LGM.As a result,the sediment materials from Yangtze River were extensively deposited on the flat,exposed continental shelf owing to the rapid decline of river flow speed,leading to that pollen grains from Okinawa Trough are derived mainly from the flat coastal vegetation of exposed continental shelf at glacial stages.Changes of pollen assemblage were consistent with the variation of temperature and humidity,which showed that the percentage of arboreal pollen was highly augmented at MIS 7,5 and MIS 1,corresponding to the strengthening of the East Asian summer monsoon and increasing of rainfall.Moreover,the peak of Pinus percentage in MIS 5.3,5.1 and MIS 3.3 may be closely linked by orbital and sub-orbital cycles of solar radiation and monsoonal variability.The present study of core MD981294 implied lower temperature and precipitation during the lowest sea-level stage(LGM),and more visibly testified that the vegetation of the flat plain on the exposed continental shelf was dominated by intrazonal communities such as halophyte grasslands and freshwater wetlands instead of zonal steppe or semi-arid desert.All above evidence demonstrated that the fundamental changes of pollen assemblage and their origins in Okinawa Trough since~200 ka BP were affected by combine factors including the coastline position and climate fluctuation.Moreover,the substantial shortening of distance between shoreline and the Okinawa Trough driven by orbital insolation cycles was clearly indicated by the pollen spectra,whereas the source-area climate signal of the pollen record was largely weakened.     

Bouguer gravity study of middle section of Tanlu fault

Although Tanlu fault is one of the most important tectonic fault zones and active earthquake belts in eastern China, little is known about its deep structure. In this study, we use the existing Bouguer gravity data to study the middle section of the Tanlu fault zone, which is also known as the Yishu fault zone. Our gravity inversion results indicate that the Moho has an abrupt offset in depth at the Tanlu fault zone and it has a relatively smooth variation away from the fault zone. The crustal structures on both sides are different from each other. Sediment is thin on the west side with an average thickness of less than 5 km, while it is as thick as 6 km on the east side. The thinnest sediment （3-4 km） is at the fault zone. Moho depth increases from 33 to 34 km on east side and from 36 to 38 km on west side. Tanlu fault zone is shown as a wide zone of linear gradient in the Bouguer gravity anomaly.     

Newly-generated Daliangshan Fault Zone—Shortcutting on the central section of Xianshuihe-Xiaojiang fault system

The Daliangshan fault zone is the eastern branch in the central section of Xianshuihe-Xiaojiang fault system. It has been neglected for a long time, partly because of no destructive earthquake records along this fault zone. On the other hand, it is located on the remote and inaccessible plateau. So far it was excluded as part of the Xianshuihe-Xiaojiang fault system. Based on the interpretation of aerophotographs and field investigations, we document this fault zone in detail, and give an estimation of strike-slip rate about 3 mm/a in Late Quaternary together with age dating data. The results suggest that the Daliangshan fault zone is a newly-generated fault zone resulted from shortcutting in the central section of Xianshuihe-Xiaojiang fault system because of the clockwise rotation of the Southeastern Tibetan Crustal Block, which is bounded by the Xianshuihe-Xiaojiang fault system. Moreover, the shortcutting may make the Daliangshan fault zone replace the Anninghe and Zemuhe fault zones gradually, and finally, the later two fault zones will probably die out with the continuous clockwise rotation.     

Discussion on the Relationship between Different Earthquake Magnitude Scales and the Effect of Seismic Station Sites on Magnitude Estimation

Based on the earthquake catalog reported by the Chinese digital seismic network in recent years, we select the earthquakes with both surface wave magnitude and local magnitude and fit them into a relationship between the two magnitudes. The systematic difference is found from the formula which has been used for 30 years. Because of a large dynamic range and wide frequency range of the current digital observation system, in addition to a larger number of stations and earthquakes being used compared to before, the relation obtained in this paper seems more reliable. Our calculation shows that there is no significant difference before and after magnitude conversion so we suggest the abandonment of magnitude conversion. The site response of a station consists of amplification at different frequencies. The amplification is equal to about 1 and changes little with frequency at stations located on basement rock, and it is greater than 1 at low frequency ranges and less than 1 at high frequency ranges at stations located on sediment layers. The difference between magnitudes from single station located on sediment layer and the average magnitude from the whole network increases from negative to positive with period. It seems that there is no fixed station correction factor and the station correction method does not work to improve the accuracy and magnitude estimates.     

Estimation of clear-sky longwave downward radiation from HJ-1B thermal data

To satisfy the requirement of surface energy budget research on the meso-and micro-scale,a parameterization is developed to calculate high spatial resolution,clear-sky downward longwave radiation(DLR)from HJ-1B thermal data.The DLR algorithm is established based on extensive radiative transfer simulation and statistical analysis.To address the problem that HJ-1B has a single thermal channel and lacks atmospheric information,the brightness temperature of HJ-1B and water vapor content are used in the algorithm.An accuracy evaluation and error analysis for the algorithm is conducted using a simulated radiation dataset.The result shows that the algorithm performs well in most circumstances,but there is obvious underestimation when water vapor content is greater than 4g/cm2 .Error analysis indicates the accuracy of estimated DLRs is affected by uncertainties in input parameters,including water vapor content and top-of-atmosphere radiance.It is also affected by the difference between ground and near-surface air temperature.The algorithm is applied to actual HJ-1B data,and validated by ground data from six stations in the Heihe River and Haihe River basins.The estimated DLRs have good consistency with measured data except at Huazhaizi,and root mean square errors at most sites are around 20W/m2 ,which is slightly better than the result of MODIS. There is significant overestimation of DLR at Huazhaizi during summer,which is mainly produced by the large ground-air temperature difference.A correction process based on temperature difference is proposed and applied at Huazhaizi.The result shows that the positive bias is largely diminished after correction.     

Late Quaternary surface deformation and rupture behavior of strong earthquake on the segment north of Mianning of the Anninghe fault

The Anninghe fault is an important active fault along the eastern boundary of Sichuan-Yunnan active tectonic block, and the study of its surface deformation and rupture behavior during strong earthquake in the late Quaternary is of fundamental importance for understanding the future seismic risk of the fault zone or even the entire western Sichuan region. Using the methods of detailed geomorphic and geological survey, digital image analysis, total station instrument survey, excavation of combined trench and dating, we analyze the geomorphologic sequences of the offset strata at several sites where the late Quaternary deformation remnants are fairly well preserved and obtain some new results as follows: Strong earthquake events with left-lateral displacements of about 3 m occurred at the two sites of Zimakua and Yejitong at 1634-1811, 1030-1050 and 280-550 a BP, respectively, and the recurrence interval is 520-660 a; The youngest event in the area of Dahaizi-Ganhaizi should be the earthquake of 1536, other events are at 1768-1826, 2755-4108 and 4108-6593 a BP, respectively, with a recurrence interval of 1300-1900 a. The strong earthquake activity shows a clustering character. The possibility of occurrence of a strong earthquake exists on the north segment of the Anninghe fault sometime in the future.     

郯庐断裂带赤山段中晚更新世之交的史前地震遗迹

Chishan is located in Sixian County of Anhui Province, and the west branch fault of Tancheng-Lujiang fault zone passes through here. According to previous research, the Chishan segment of Tancheng-Lujiang fault zone has been obviously active since the Quaternary. Trenches excavated perpendicular to the Chishan segment for this study have revealed many prehistoric earthquake ruins-the multi-phase reverse faulting colluvial wedge, which is represented as the western brick-red sandstone of the late Cretaceous or maize gravel stratum of the mid Pleistocene of the hanging wall of the fault overlapping eastward the mid-late Pleistocene brown clay. In the base of the wedges, steep NW-dipping faults were found, and the steep fault planes turned upward to gently dipping collapse planes. As revealed by the trenches, the connection line of the breaking points strikes NNE in general. Heaving landforms are preserved at most parts of the tailing edge of the hanging wall where the fault passes through, and some EW-trending gullies were offset by right-lateral faulting. The two walls of several trenches have consistently shown that the collapse of traces have been pushed by a west-to-east force. Among them, Tc1~Tc4 show that the brick red limestone(K2)overthrust and collapsed on the yellow-brown clay containing ferro-manganese nodules (Q2-3P); Tc5 reveals that the yellowish-white gravel (Q2P) and the sandstone (K2) and overthrust and collapsed on the aforementioned clay. Reverse faulting colluvial wedges are found on both walls of each of the 8 trenches, but the number of wedges revealed in different trenches is different: there is 1 wedge, and 2 wedges in Tc1 and Tc3. 3 wedges in Tc2, Tc4 and Tc5, and in individual trenches, few wedges are revealed. This may be related to the trench’s location, depth and height of the remaining denudation. From the analysis of the trenches and the thermoluminescence dating results, we can preliminarily conclude that multiple large-scale reverse faulting stick events have taken place on the Chishan segment of the Tancheng-Lujiang fault zone during the mid-late Pleistocene, that is to say, many pre-historic strong earthquakes have occurred.     

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.