Rob A. Kemp

A record of the Blake Event during the last interglacial paleosol in the western Loess Plateau of China

Abstract A high-resolution record of the Blake Event has been obtained from a 40 m well dug in a loess section at Jiuzhoutai in the City of Lanzhou in the western Loess Plateau of China. The paleomagnetic signal is principally carried by magnetite both in loess and paleosols. The Blake Event is located at the boundary between paleosol S1-c of the last interglacial (equivalent to MIS 5e in deep-sea records) and loess L2-2 (MIS 5d), and is characterized by a triple feature consisting of two short reversed intervals separated by a short normal interval. The directional changes are abrupt, as previously revealed in other records, and suggest geocentric dipole field behavior before, after and during the event, but not necessarily during N R or R N transitions. The age and duration of the event are estimated as between 119.97 and 114.47 kyr BP and 5.5 kyr, respectively, from thermoluminescence dating and astronomically tuned climate stratigraphy based on high-resolution magnetic susceptibility.

Pedogenic modification of loess: significance for palaeoclimatic reconstructions

Abstract This review considers the role of pedogenic processes in modifying wind-blown dust (loess), concentrating particularly on the ways that resulting properties may be interpreted as indicators of past climatic conditions and changes. Emphasis is placed on the sequences of palaeosols developed within loess deposits that are frequently regarded as some of the best terrestrial equivalents of marine-sediment records of long-term global climatic change. A palaeosol is generally interpreted in terms of the broad pedogenic processes and environments assumed to be currently responsible for that type of soil forming at the present surface. Even the very presence of a palaeosol may have palaeoclimatic significance, however, in that it is often taken to indicate a period of relative land surface stability and warmer and/or moister conditions between cold and/or arid phases of loess accumulation. In reality, it may be more useful to consider many loess–palaeosol sequences in terms of changing balances between pedogenesis and loess accumulation over geological time. In most regions, it seems that the balance swings towards pedogenesis during interglacials or interstadials when sediment supply and transport are limited and the climate is warmer and/or wetter. Where accumulation rates are still appreciable during these ‘soil-forming intervals’, however, the soils and palaeosols may be accretionary with surface build-up keeping pace with pedogenesis. Welding may also occur where covering sediments are insufficiently thick to isolate an underlying palaeosol from the effects of pedogenesis active at a new land surface. Further complications occur due to reworking of palaeosols and syndepositional pedogenic alteration of loess units. Generally, such pedocomplexes can only be deciphered if the different pedogenic, geomorphic and sedimentary processes are identified and ordered within a pedosedimentary reconstruction. A recent trend has been to treat some loess–palaeosol sequences as quasi-continuous time series, particularly when comparing depth functions of climatic-proxy properties such as magnetic susceptibility and grain size with marine and ice-core isotope curves.

Isotopic constraints on the source of Argentinian loess – with implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima

We present rare-earth element (REE) and Sr–Nd isotopic data for Argentinian loess (28–38°S) with two aims: (1) to examine the source regions of Argentinian loess and the constraints that these put on palaeo-wind directions; (2) to further investigate the source of Antarctic ice-core dust and to test the hypothesis that some of it could be derived from a region to the north of Patagonia – into which the dry, dusty, westerly dominated Patagonian climate expanded during Quaternary glacial maxima. Sr–Nd isotopic data for Argentinian loess from north of 37°S are distinct from Patagonian loess compositions in that they have more radiogenic Sr (87Sr/86Sr=0.7059–0.7123) and less radiogenic Nd (ϵNd=−0.8 to −6.4). REE patterns and Sr–Nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections. In general, there is a northward change from Patagonia-like REE patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust. The latitudinal Nd isotopic pattern is remarkably similar to that for Andean volcanic rocks and suggests derivation of loess from the Andes by more-or-less direct westerly transport. For loess sections in the north, the data imply a contribution from Palaeozoic gneisses to the northwest in the Chilean Altiplano. Sr–Nd data for extra-Patagonian Argentinian loess north of 37°S do not support a significant role for this source region in supplying dust to Antarctica at the last glacial maximum. This conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts – and in particular the westerlies and the Antarctic Polar Front – during Quaternary glacial maxima. Very systematic relations between the Sr–Nd isotopic composition of loess and their Andean source highlights shifts in the Sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between Patagonian loess and ice-core dust identified by previous workers is due to grain-size differentiation effects.

Micromorphology of loess–paleosol sequences: a record of paleoenvironmental change

Abstract The record of paleoenvironmental change retained within thin sections from loess–paleosol sequences is discussed with reference to case studies from western Europe, China and northwestern USA. The paleoenvironmental significance of individual micromorphological features such as calcitic concentrations, clay coatings and cryogenic structures are first evaluated, and the value of recognising and interpreting microstratigraphic relationships of features in terms of paleoenvironmental change is illustrated. Recent developments have seen a greater emphasis on using micromorphology within a pedosedimentary approach, whereby both pedogenic and sedimentary processes are taken into account when interpreting loess–paleosol sequences. This has led not only to the acquisition of important information on conditions during phases of enhanced loess accretion and periods of climatic transition, but also to an increased awareness of spatial changes in processes and environmental controls during particular time periods. Micromorphologically based reconstructions have enabled the isolation of key stages of pedosedimentary development, although absolute dating of stages is often limited by lack of age controls. A novel approach aimed at addressing this limitation, afforded by presence of datable tephra in microstratigraphic association with key micromorphological features, is discussed.

Variations in loess and palaeosol properties as indicators of palaeoclimatic gradients across the Loess Plateau of North China

Abstract New data are presented on micromorphology, magnetic susceptibility and particle size characteristics of three soil/sediment units at four locations along a WNW-ESE transect across the Loess Plateau of North China. Differences and spatial trends are used to assess the sensitivity of such data as proxies of regional palaeoclimatic gradients. The steepest climatic gradient suggested by the magnetic susceptibility for the S1 palaeosol is at the eastern (humid) end of the traverse. Micromorphology and particle size analysis show that the loess units in the east are richer in clay than the best developed palaeosols in the semi-arid west, and that particle size in the west appears to provide a more sensitive record of climatically-driven changes than magnetic susceptibility. The use of surface and near-surface soils as a datum for inferring palaeoprecipitation values is reviewed in the light of recent evidence of substantial degradation of such soils. Micromorphological and particle size analysis of closely spaced samples from loess-palaeosol sequences is a sensitive addition to magnetic susceptibility stratigraphy and correlation.

Role of micromorphology in paleopedological research

Abstract The current role of micromorphology in paleopedological research is reviewed. Examples are discussed which illustrate its value in the initial identification of paleosols and in their interpretation leading to detailed paleoenvironmental and pedosedimentary reconstructions. Problems concerned with thin-section sampling, description and interpretation are highlighted. These critical issues need to be confronted if the technique is to continue enjoying such high regard amongst specialist and non-specialists alike.

Origin of the magnetic susceptibility signal in Chinese loess

Abstract The quality of the loess magnetic susceptibility (MS) record as a climate proxy has been compared to the ice core and deep-sea records, yet the source and causes of the MS signal remain poorly understood. Four explanations are currently in circulation. The dominant hypothesis, that MS variations largely result from post-depositional weathering and/or formation of magnetic phases by pedogenic processes, has led to the use of MS data as a palaeoprecipitation proxy. We present a different view based on analyses of samples of monthly dust falls over 4 years in North China. These have MS values comparable to those of Pleistocene loess and palaeosols but, unlike the latter, their variations are seasonal and so cannot be explained by weathering differences. Our experiments confirm that MS enhancement in the palaeosols is caused principally by fine-grain magnetic minerals, but, in contrast with previous proposals, the major source of the ultrafine magnetic minerals appears to be decomposed plant litter.

Pedosedimentary and palaeoenvironmental significance of a Holocene alluvial sequence in the southern Pampas, Argentina

A representative section of the Holocene alluvial record of the southern Pampas (Argentina) com prises a sequence of clastic sediments, a diatomite and two palaeosols. Detailed macro- and micromorphological descriptions provide the basis for the reconstruction of its pedosedimentary history. Combining the pedosedi mentary reconstruction with palaeoecological data allows a detailed overview of alluvial landscape development and associated palaeoenvironmental change in the region during the Holocene. The early-Holocene shift from a subhumid dry to humid climate was marked initially by a diminution in clastic sedimentation, the development of the Puesto Callejón Viejo Soil and then re-establishment of fluvial aggradation with a dominance of bioclastic sedimentation. Loess inputs increased during the mid-Holocene as the climate reverted to subhumid dry, and temporary subaerial exposure of the sediments led to the formation of the Puesto Berrondo Soil. More variable environmental conditions followed with natural fluvial aggradation ceasing in the late Holocene, though aeolian sedimentation has continued until the present day. The resultant surface accretionary soil has been modified by recent alluvial inputs from flood events linked to agricultural disturbance in adjacent catchments.

Climate change, loess and palaeosols: proxy measures and resolution in North China

The Loess Plateau of North China is a key region for the study of records of Quaternary climatic change. Various techniques to abstract climatically sensitive data were first applied to the classic loess—palaeosol site at Luochuan, and current views on climatic change in Asia have been strongly moulded by results from this central part of the Loess Plateau. Data including the magnetic susceptibility, granulometry, mineralogy, geochemistry, micromorphology, fossil content, and inorganic and organic carbon content of Asian loess—palaeosol series have been used, singly and in combination, as surrogate measures of palaeoclimates to infer climatic changes with particular reference to changes in the Asian monsoon. Some of this work is reviewed and the status of a number of parameters as climatic proxies is discussed. Some lithostratigraphic sequences from which proxy measures of climate have been derived are incomplete, as shown by erosion surfaces, water-laminated zones and truncated palaeosol profiles, while others provide records of relatively low resolution. Most descriptions of Quaternary climatic changes based on studies of the loess emphasize alternations in the relative dominance of warm and moist v. cool, dry and dust-laden monsoons. Some recent studies of the much higher resolution loess—palaeosol successions on the western margins of the Loess Plateau have suggested that such changes may sometimes be out of phase, with the inference that the current climate model is too simple. Means of testing this hypothesis are proposed.

The pedosedimentary evolution and chronology of Tortugas, a Late Quaternary type-site of the northern Pampa, Argentina

New micromorphological, magnetic susceptibility, geochemical and geochronological data have been collected from Tortugas, a key Late Quaternary type-site of the northern Pampa (Argentina), thus enabling the detailed reconstruction of the pedogenic and sedimentary history of this loess–paleosol sequence. Paleosols developed in the Ceres Formation and the overlying lower member of the Tezanos Pinto Formation retain evidence for leaching, bioturbation and particularly clay translocation processes being dominant. Both paleosols are truncated: evidence for water sorting and sediment reworking, as well as continued bioturbation and redistribution of carbonates, suggests that intervening depositional phases were complex. The previously recorded ‘Hypsithermal Soil’ developed in the upper member of the Tezanos Pinto Formation has similar characteristics to the buried paleosols, yet does not appear to have been truncated at this site and is only covered by a thin layer of sediment (San Guillermo Formation). This latter unit has been pedogenically welded onto the ‘Hypsithermal Soil’, thus creating the present-day surface soil (complex). A series of optically stimulated luminescence dates from throughout the sequence provide a stratigraphically consistent chronological framework (ca. 1–150 ka) for the pedosedimentary reconstruction, yet conflict with the previously reported thermoluminescence-based chronology for the site and region (ca. 1–90 ka).