Loess-paleosol Section Research Articles

Changes in mineralogy of loess–paleosol sections across the Chinese Loess Plateau

Quantitative mineralogical analysis of bulk samples and single particles was carried out on three loess sections of different local climate settings on the Chinese Loess Plateau (CLP). Mineralogy, geochemistry, and single-particle petrography of loess and paleosols are relatively uniform over the CLP. However, in detail, there are mineralogical changes related to eolian process and chemical weathering. Particle-size sorting eastward from western sources led to an eastward increase of the total phyllosilicate contents enriched in fine illitic clay minerals. After deposition, detrital minerals susceptible to chemical weathering were sequentially altered in a progressive fashion with increasing precipitation in the order of calcite, dolomite, biotite, illite, chlorite, amphibole, and plagioclase. The weathering of biotite, chlorite, and illite resulted in a significant increase of expandable phyllosilicate contents. The sequential weathering of the minerals is reflected chemically in the decrease of Na and Mg and the increase of iron oxidation. Mineralogy of the Chinese loess at individual sites reflects the effects of size fractionation during eolian transportation and progressive sequential weathering along the climatic gradient, and it is essential to consider both effects when using mineralogical and dependent chemical data in the paleoclimatic reconstruction of the CLP.

Preliminary malacological investigation on the loess profile at Zmajevac, Croatia

Abstract More than 5 500 specimens of 37 terrestrial snail species were collected from a loess paleosol section at Zmajevac village, Croatia. The presence of Ena montana, Mastus bielzi, Cochlodina laminata, Macrogastra ventricosa, Clausilia pumila, Clausilia dubia, Pseudofusulus varians, Trichia unidentata and Trichia edentula species from the lower paleosol layer suggests that this malacological zone formed during the initial phase of a Middle Pleistocene interglacial cycle.

Reconstruction of 130-kyr Relative Geomagnetic Intensities from 10Be in Two Chinese Loess Sections

Efforts to extract weak geomagnetic excursion signals from Chinese loess-paleosol 10Be have generally been unsuccessful due to the complexities of its accumulation, because the geomagnetic and climate (precipitation and dust) signals contained in loess-paleosol sequence are tightly overprinted. Here, we present a reconstruction of geomagnetic relative paleointensities for the past 130 kyr from 10Be records in 2 Chinese loess-paleosol sections using a correction based on the correlation of 10Be with magnetic susceptibility (SUS) to remove the climatic contamination. Both these records reveal the Laschamp and Blake events, which lie in the loess and paleosol (L1SS1 and S1SS3) horizons corresponding to mid-MIS 3 and 5e, respectively. The good agreement between our results and other geomagnetic intensities reconstructions from Atlantic and Pacific sediments indicates that our method is robust. Our study suggests the potential application of loess-paleosol 10Be for reconstructing geomagnetic intensity variations spanning the whole Quaternary.

A mineral tracer toward high-resolution dust provenance on the Chinese Loess Plateau: SEM, TEM, and sulfur isotopes of sulfate inclusions in biotite

The most important terrestrial archive of the Quaternary climate is the Chinese Loess Plateau (CLP) distributed over a vast region peripheral to the deserts of Inner Asia. Numerous paleoclimatic studies have examined magnetic, chemical, and isotopic proxies from deep loess-paleosol sections. However, the highly uniform mineralogy of the CLP has made it difficult to track the provenance, transportation, and deposition of dust particles. Here, we report that the micrometer-scale mineralogical heterogeneity of the loess particles has a significant potential in determining high-resolution loess provenance. Scanning and transmission electron microscopy of the Chinese loess reveals that biotite particles commonly have microinclusions of Ba-Sr sulfates displacing the micaceous layers along cleavages. Chemical compositions varied widely from barite to celestine within the same biotite grain. The Ba-rich sulfates precipitated first, followed by an overgrowth of Sr-rich sulfates. The frequency of sulfate-bearing biotite varied greatly over several loess sections, implying a specific source and sedimentation process. Their abundance on the western CLP and sulfur isotopic composition support their derivation from porous sedimentary sequences reacted with hypersaline solutions, e.g., arkosic sandstone, probably located west of the CLP covering the Qilian Mountains, Qaidam Basin, and surrounding mountains of the Tibetan-Qinghai Plateau. The sulfate-bearing biotite is the first microscopic mineral tracer in the CLP, provoking further search for other microscopic heterogeneities of the Chinese loess and their equivalents in the source regions.

Chronology of the Holocene loess-paleosol section and its deposition and pedogenesis on the south of Chinese Loess Plateau

Optically stimulated luminescence (OSL) dating for polymineral fine-grained loess samples, collected in Laoguantai (LGT) section on the south of the Chinese Loess Plateau, was made by application of single-aliquot regenerative-dose (SAR) protocol. A ‘Double-SAR’ procedure in which aliquots are subjected to both infrared (IR) and blue stimulations was used, and two sets of equivalent dose (De) determinations were produced and assumed to relate predominantly to feldspathic and quartz fine grain populations respectively. The OSL ages estimated from IRSL signals are smaller than those estimated from [post-IR] OSL signals due to the anomalous fading of feldspar IR signals, based on fading experiment. The young ages of the samples near ground surface may be originated from the post-depositional disturbance by the intensifying humanity’s cultivation since 3.0 ka BP in the Guanzhong Basin, south of the Chinese Loess Plateau. Based on OSL dating, as well as field observations and stratigraphic correlation, we determine the chronology of the LGT loess-paleosol sequence. In combination with climate proxy records, it is indicated that aeolian loess deposition and pedogenesis underwent polyphase changes during the Holocene, likely to have been driven by shifts in the East Asian monsoon. This suggests that aeolian loess deposition is episodic and highly variable, with contributions from non-aeolian processes such as alluvial deposition found in the area.

Stratigraphy, and spatial and temporal paleoclimatic trends in Southeastern/Eastern European loess–paleosol sequences

The loess–paleosol sections Batajnica/Stari Slankamen (Serbia), Mircea Voda (Romania) and Stary Kaydaky (Ukraine) are presently located in areas of different types of steppe, and are highly sensitive for recording climatic changes. A stratigraphy for these three Southeastern/Eastern European sections is presented, based on pedostratigraphy and correlation of recently obtained susceptibility records with susceptibility data of other sections of the area (Koriten, Mostistea, Vyazivok), of the Chinese Loess Plateau and with the benthic δ 18O record of ODP 677. Six pedocomplexes were studied at the Serbian and Romanian sections and five at the Ukrainian section. The oldest are related to Marine Isotope Stage (MIS) 17 (Batajnica/Stari Slankamen and Mircea Voda) and MIS 13–15 (Stary Kaydaky). Some points concerning the existing loess chronostratigraphies of Bulgaria, Ukraine and China are discussed. Comparative study of the profiles allows the tracing of paleoclimatic and paleoenvironmental changes in Southeastern/Eastern Europe in time and space. Reconstruction of paleoprecipitation based on susceptibility–rainfall relationships, and calculations of sedimentation rates are evaluated.

Quantitative bulk and single-particle mineralogy of a thick Chinese loess–paleosol section: implications for loess provenance and weathering

A 40 m loess–paleosol section (L1, S1, and upper L2) at the semi-arid western margin of the Chinese Loess Plateau was investigated by quantitative mineralogical, microscopic, and geochemical analyses of bulk samples and single particles. The average ( N=16) mineralogical composition of loess in the studied section is 34.8% quartz, 15.3% plagioclase, 5.0% K-feldspar, 25.1% phyllosilicates, 11.7% calcite, 3.3% dolomite, and 1.9% amphibole. Ratios of plagioclase, K-feldspar, and amphibole to quartz do not change throughout both the loess and paleosol units, while the ratio of total phyllosilicate to quartz is enhanced in the paleosol where there is significant variation in calcite and dolomite contents. Illite and interstratified illite–smectite are the most abundant (84±4%) clay minerals in the clay fraction of both loess and paleosol samples. The calcite/dolomite ratio, Mg/Si ratio, iron oxidation degree, and organic carbon contents are positively correlated to magnetic susceptibility. Weathering and elemental mobilization in paleosol samples was indicated by calcite–dolomite dissolution, nanofiber–calcite reprecipitation, and biotite–hydrobiotite transformation, while there was no notable evidence for weathering observed in loess samples. The essentially pristine state of loess samples with respect to weathering and the minimal alteration of minerals in the paleosol indicate relatively poor weathering conditions at both the depositional and source sites. Analysis of single particles show that most plagioclase is albitic, calcite occurs frequently in particles with dolomite but also with silicates, biotite is associated with barite–celestine inclusions, iron oxide occurs as a pseudomorph after framboidal pyrite, and rock fragments may be siliciclastic sedimentary, metasedimentary, and granitic. These observations indicate that the lithology of source regions must have been dominated by largely siliciclastic and carbonate sedimentary rocks and their low-grade metamorphic equivalents with contributions from granitoids partly affected by hydrothermal alterations and possibly high-grade metamorphic rocks. The mineralogical analysis of loess and paleosols opens new ground for refining the established climatic proxies, and for tracking source lithology and weathering in the source and depositional regions.

Holocene vegetation cover in Qin’an area of western Chinese Loess Plateau revealed by n-alkane

Previous studies have found that wetlands prevailed in western Chinese Loess Plateau and pine pollen could reach up to 80% in Qin’an area of the plateau during middle Holocene. It was then deduced that forest vegetation covered Qin’an area in a warm and wet climate during middle Holocene. The proxies of molecule biomarkers from two Holocene sections, a swamp-alluvial loess section and a typical loess-paleosol section are used to reconstruct regional vegetation history. It is found that the heavy-molecular-weight (HMW) homologues of the n-alkanes in all samples exhibit a pronounced odd-over-even predominance, maximizing at C31 and the abundance of the nC27-alkanes is the lowest in nC27, nC29, nC31, i.e. C27<C29<C31, a typical grass n-alkanes model. The results are the same as those of model surface soil samples under grass cover but different from both forest cover and modern pine leaves, which yield preferentially nC29-alkanes peak. It could be inferred that the area was dominated by grass cover but not forest cover during whole Holocene epoch. The study shows that combining both molecule biomarkers and pollen analysis can avoid the disadvantage of pollen analysis in reconstruction of regional vegetation cover.

An erosional hiatus in Chinese loess sequences revealed by closely spaced optical dating

Chinese loess deposits are some of the best continental records of palaeoclimatic and palaeoenvironmental changes in the late Cenozoic. Many investigations have shown that the long-time climate variations recorded by the loess-paleosol sequences can be correlated with global environmental changes. However, there are still some foundational questions concerning loess deposition that are not well answered, casting uncertainty on many previous conclusions, especially high-resolution palaeoclimatic records. In this study, three typical loess-paleosol sections spanning the last glacial cycle were closely sampled for optically stimulated luminescence (OSL) dating and palaeoclimate interpretation along a northwest-southeast transect in Chinese Loess Plateau. Results show that there is a 4–5 ka hiatus in the typical loess deposits, and as a consequence suggests that the loess record is not continuous over millennial time scales. As such, previous conclusions on climatic and environmental changes at millennial-centennial time scale, reconstructed through the loess record, should be reinvestigated. The erosional hiatus may be forced by a period with strengthened East Asian winter monsoon and the less vegetation cover in the investigated sites.

A multi-proxy study of Argentina loess: Marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite

A 12 m loess–paleosol section (La Angostura) from Tafi del Valle, Tucumán, Argentina was studied using rock magnetic measurements, geochemistry, Mössbauer effect, particle size and Nd isotopes in order to elucidate changes in paleoenvironment at the site. Combining the results of these experiments allowed us to separate the effects of the two major controls (parent material and pedogenesis) on the measured parameters. Two behavior regimes were identified in the sediment: 1) in the lower half of the section pedogenic processes, though weak, dominate the variation of the sediment's physical properties, and 2) for the upper half of the section subtle changes in parent material control the majority of the measured parameters, even those typically controlled by pedogenesis, e.g., clay fraction. Applying rock magnetic, Mössbauer, and bulk geochemistry measurements in concert we calculated an iron budget for the loess. ∼80% of the iron in the sediment is in paramagnetic minerals; ∼20% of the iron is in hematite (∼1.6% of the sediment's mass), and ≪ 1% of the total iron is present as magnetite/maghemite (∼0.1% of the sediment's total mass). Neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment. Throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration, and it is further surmised to be sensitive to the available moisture at the site. Finally we present a curiosity: within the error of our chronology, the concentration of hematite seems to correlate with the coeval climate record from Antarctica (Vostok). It is possible that the sedimentary–magnetic record from La Angostura is a proxy for the strength of the South American summer monsoon.

Zr/Rb ratio in the Chinese loess sequences and its implication for changes in the East Asian winter monsoon strength

The long-term dust accumulation sequences on the Chinese Loess Plateau (CLP) provide a valuable opportunity to study the evolution of East Asian winter monsoon strength. Grain size of bulk samples of loess deposits, though widely used, is a modified measure of the strength of winter monsoon wind due to modification by post-depositional weathering and pedogenesis. This study proposes a new and reliable geochemical climate proxy that records the variability of the East Asian winter monsoon on the CLP. Six loess–paleosol sections on the Plateau were selected for measuring concentrations of the trace elements Zr and Rb as well as grain size. Variations of the Zr/Rb ratio in all the sections encompassing the last 130 ka display a generally similar pattern to that of mean grain size of bulk samples (MGSB). Though a positive correlation exists between the Zr/Rb ratio and MGSB, the correlation coefficient decreases in southern and eastern sections where intense pedogenesis occurred in the relatively warm and humid climates. Long-term Zr/Rb variation in the Lingtai section exhibits amplitudes and frequencies similar to those in MGSB and the mean grain size of quartz particles (MGSQ, accepted as a more reliable proxy than MGSB) in the upper loess–paleosol sequence over the past 2.6 Ma. However, for the underlying Red Clay formation, in the interval from about 7–2.6 Ma B.P., the MGSB record is relatively stable, whereas both the Zr/Rb ratio and MGSQ show distinct variability and display amplitudes similar to those observed in the overlying loess–paleosol sequence. These results demonstrate that the Zr/Rb ratio reflects original eolian grain size and may serve as a reliable index of the strength of East Asian winter monsoon winds.

On the performances of the single-aliquot regenerative-dose (SAR) protocol for Chinese loess: fine quartz and polymineral grains

This paper presents the results of a study which assesses the performances of the single-aliquot regenerative-dose (SAR) protocol in fine quartz and polymineral grains (4– 11 μ m ) obtained from the Luochuan loess–paleosol section on the central Loess Plateau, China. Fine quartz was isolated from polymineral grains, with the grain mineralogy confirmed by infrared (IR) stimulation detection. The fine quartz optically stimulated luminescence (OSL) results obtained yielded a stratigraphically consistent equivalent dose ( D e ) distribution for the Luochuan loess–paleosol sequence deposited during the last glacial–interglacial. The performances of the infrared stimulated luminescence (IRSL) signal and the optically stimulated luminescence signal after infrared exposure ([post-IR] OSL) for polymineral grains were estimated by a comparison with that of fine quartz. Our results suggest that the IRSL D e value is underestimated in relatively old samples (about > 50 Gy ) probably due to anomalous fading of the natural IRSL signal. We propose that the ratio of IRSL vs. [post-IR] OSL signal provides a measure of the degree of fading of the IRSL signal. It is confirmed that the [post-IR] OSL signal after sufficient IR exposure provides a better performance than that of the IRSL signal in Chinese loess, not only for the Holocene samples, but also for the last glacial samples. A suitable IR exposure time prior to measurement of the [post-IR] OSL signal can be obtained by determination of the plateau region of a plot of the [post-IR] OSL D e value vs. IR exposure duration. The results obtained suggest that both quartz OSL and polymineral [post-IR] OSL signal can produce consistent D e estimates for the loess–paleosol succession. The linear modulation technique was used to explain the good performance of [post-IR] OSL signal in D e determination.

The Plateau Monsoon variation during the past 130 kyr revealed by loess deposit at northeast Qinghai–Tibet (China)

Climate in northeast Qinghai–Tibetan Plateau is characterised by the alternation of summer and winter monsoon circulation, which is generated by thermodynamic and kinetic effects of the immense plateau. The Plateau Monsoon system during the recent geological past has been investigated through a 44-m loess–paleosol section and a 17-m well to obtain the Plateau Monsoon climate changes during the last glacial–interglacial cycle. Thermoluminescence (TL) and optically stimulated luminescence (OSL) methods, as well as field observations and stratigraphic correlation, are used to date the deposit. Magnetic susceptibility (MS), frequency-dependent magnetic susceptibility (FMS), and calcium carbonate content, which are regarded as proxy indicators of strength of the Plateau Summer Monsoon (PSM), are measured. Measurement of grain size distribution, which is regarded as a proxy index of strength of the Plateau Winter Monsoon (PWM), is also carried out. The results show that changes of the unique Plateau Monsoon system during the past 130 kyr are associated with the glacial and interglacial alternations in the Northern Hemisphere. The PSM was unusually strengthened during a period matching the marine oxygen isotope stage 5e (OIS 5e), but it was weak during the OIS 5a and OIS 5c, and was close to that of the OIS 3. The PWM was significantly depressed during OIS 3, and was as weak as that in the Holocene. The PWM and PSM circulations were not always coupled during the last glacial cycle. Changes of the Plateau Monsoon system did not always parallel the SE Asian monsoon, although linkage between them existed to some extent. There are millennial-scale variations in the Plateau Monsoon system, but this is not further discussed in this paper.

The Plateau Monsoon variation during the past 130 kyr revealed by loess deposit at northeast Qinghai–Tibet (China)

Climate in northeast Qinghai–Tibetan Plateau is characterised by the alternation of summer and winter monsoon circulation, which is generated by thermodynamic and kinetic effects of the immense plateau. The Plateau Monsoon system during the recent geological past has been investigated through a 44-m loess–paleosol section and a 17-m well to obtain the Plateau Monsoon climate changes during the last glacial–interglacial cycle. Thermoluminescence (TL) and optically stimulated luminescence (OSL) methods, as well as field observations and stratigraphic correlation, are used to date the deposit. Magnetic susceptibility (MS), frequency-dependent magnetic susceptibility (FMS), and calcium carbonate content, which are regarded as proxy indicators of strength of the Plateau Summer Monsoon (PSM), are measured. Measurement of grain size distribution, which is regarded as a proxy index of strength of the Plateau Winter Monsoon (PWM), is also carried out. The results show that changes of the unique Plateau Monsoon system during the past 130 kyr are associated with the glacial and interglacial alternations in the Northern Hemisphere. The PSM was unusually strengthened during a period matching the marine oxygen isotope stage 5e (OIS 5e), but it was weak during the OIS 5a and OIS 5c, and was close to that of the OIS 3. The PWM was significantly depressed during OIS 3, and was as weak as that in the Holocene. The PWM and PSM circulations were not always coupled during the last glacial cycle. Changes of the Plateau Monsoon system did not always parallel the SE Asian monsoon, although linkage between them existed to some extent. There are millennial-scale variations in the Plateau Monsoon system, but this is not further discussed in this paper.

Spatial and temporal changes of summer monsoon on the Loess Plateau of Central China during the last 130 ka inferred from Rb/Sr ratios

The Rb/Sr ratios for samples from six loess-paleosol sections on the Chinese Loess Plateau during the last glacial-interglacial cycle, and for samples from 18 modern surface soils, have been analyzed. The six sections are along north-south and west-east transects respectively. The results show that the spatial distribution of Rb/Sr is characterized by a remarkable increase from north to south while Rb/Sr ratio changes little from west to east, whose latitude indicates shift of summer monsoon intensity. A regression equation between Rb/Sr ratios and mean annual precipitation (MAP) has been constructed through statistical analysis for the modern surface soils. Paleorainfall during the last 130 ka is estimated for the six sections based on the regression equation. It is evident that precipitation increased remarkably during marine isotope stages 3 and 5, exhibiting the high and similar gradients of variation with latitude. This demonstrates that the environmental effect is more prominent in the south of the Loess Plateau than in the north during the dominance of summer monsoon.

Proxy for Millennial Climate Change during the Last 50,000 Years in the Loess-paleosol Sequence at Shengshan Island in the East China Sea: Estimation by Soil Color Reflectance Indices (L*, a*, and b*)

Based on a multi-proxy record of the loess-paleosol sequences, we studied the millennial climate change and environmental variation in Shengshan Island in the East China Sea for the last 50, 000 years. The proxy color reflectance indices (L*, a*, and b*) of the loess-paleosol sequence in Shengshan show a series of millennial oscillations. The relationship between the standard curve of δ18O in GRIP and sequential changes of L*, a*, and b* indices in the loess-paleosol section of Shengshan during last 50, 000 years showed that L*, a*, and b* are sensitive indicators for climatic change in southeastern China. Patterns of magnetic susceptibility change were very similar to those of L*, a*, and b*. Soil color reflectance indices are evidently very available as a proxy record for identification of loess-paleosol transition, like magnetic susceptibility. Based on soil color proxy, this study identified horizons of Heinrich events (H1 to H5) and interstadial peaks (IS1, 2, 4, 8, and 12) in the loess-paleosol sequence at Shengshan. The Shengshan loess section is a typical loess-paleosol sequence recording climatic oscillations for the last 50, 000 years in the East China Sea. Due to its special location, the section provided new evidence that climatic change indicated by dust sediments in the Loess Plateau is comparable to change in the Korean Peninsula and the Japanese Islands.

An EPR Study on the Chemical form of Mn2+ in the Chinese Loess Samples

Electron paramagnetic resonance (EPR) spectroscopy in combination with thermal methods were used to identify and characterize Mn2+ in the Chinese loess that is a multimineral system. EPR spectra of the loess samples from the classic loess-paleosol section in central China show the presence of trace amounts of Mn2+; whereas paleosol samples present no Mn2+ EPR signal. The spectral changes upon step heating from room temperature to 1000 °C suggest that this EPR signal in the loess arises from Mn substituted into CaCO3. This study provides a direct evidence that the loess-paleosol profiles were formed under the changing redox conditions caused by a past climatic change.

Variations in CaCO3 content and Sr isotopic composition of loess and records of paleoclimatic fluctuations

The carbonate Sr isotopic composition of the Heimugou loess-paleosol section in Luochuan of Shaanxi Province shows large variation, with the87Sr/86Sr ratios ranging from 0.7104 to 0.7120. The carbonate in paleosol has in general higher87Sr/86Sr than the carbonate in loess. In the loess-paleosol section, the Sr isotopic composition vary with an about 100 ka periodicity, corresponding to the change of global glacial cycle and comparable with the variation of Sr isotopic composition of paleosea water. This result indicates that the Sr isotopic composition of carbonate in loess can be an indicator of crustal weathering and paleoclimate change. The fluctuation in Sr isotopic composition of paleoseawater is mainly controlled by the continental riverine flux of Sr and its isotopic composition.

The paleo- and petromagnetic record in the Polish and Ukrainian loess-paleosol sequences

718 samples from five loess-paleosol sections located in the Black Sea region, the western Ukraine and Poland (Lublin Upland) were the subject of a paleomagnetic and petromagnetic study. Strong magnetic enhancement is observed in the cambisols, chernozems and iluvial horizons of the forest (podzolic) and leached brown type (brownerde) soils. Distinct magnetic depletion or dilution occurs in the gley soils and the leached horizons of podsols and brown type soils. Magnetic enhancement in the section from Black Sea region was not simply dependent on paleotemperature. Soils from the interstadial periods could be magnetically enhanced to the same degree as soils which were formed during interglacials. In the Polish and Ukrainian loess-palesol sequences, paleorainfall could be a significant factor controlling the susceptibility signal in addition to paleotemperature. The degree of warming of paleoclimate can be expressed by the amount of secondary maghemite that was formed in the studied paleosols.All sections were deposited after the Brunhes/Matuyama paleomagnetic reversal. No remarkable paleomagnetic event was encountered. However, very distinct directional changes associated most probably with the secular variations were observed in the Polish and western Ukrainian sections. These changes can serve as a stratigraphic correlation tool for comparison with petromagnetic data.

Clay minerals and their paleoenvironmental interpretation in the Baoji loess section, Southern Loess Plateau, China

Clay minerals from the loess-paleosol section at Baoji in north-central China were studied by X-ray Diffraction. Major components, calculated by comparing major peak heights, were estimated as follows: illite (55–75%), kaolinite (10–25%), mixed layer chlorite-vermiculite (10–20%), chlorite (< 7%), and small amounts (< 5%) of smectite and mixed layer illite/smectite. An increasing trend in illite in paleosols and loess from approximately 1.2 Ma to the last glaciation may be indicating an increase in aridity. Likewise an increase in chlorite and chlorite-vermiculite near the base of the Brunhes chron to the present may indicate a decrease in weathering intensity. One of the last warm phases prior to the onset of loess deposition at 2.5 Ma is represented in the Tertiary Red Clay formation by a higher than average kaolinite content (20–25%), a relatively low illite (55–60%) content, and about 15% chlorite/chlorite-vermiculite. The kaolinite content decreases toward the boundary between the Red Clay and the overlying loess-paleosol sediments. This may indicate weaker weathering resulting from climatic deterioration near the Matuyama-Gauss boundary.