Paleosol Horizons Research Articles

Early pleistocene lake deposits and Lower Paleolithic finds in Nahal (wadi) Zihor, Southern Negev desert, Israel

An Early Pleistocene fluvio-lacustrine sequence from the extremely arid southern Negev desert, Israel, indicates that climatic conditions during that period were humid enough to support the formation of a lake (Lake Zihor) there. The lacustrine sediments were deposited in a tectonic valley that developed along the Zihor tectonic line after the deposition of the Pliocene Arava Formation. They intercalate with fluvial sediments that contain several horizons of reddish calcic paleosols. The lacustrine sequence comprises three sedimentary cycles. Each starts with a dark clay layer overlain by a white limestone unit, both of which contain abundant freshwater fauna, and terminates with a green detrital limestone that contains only one type of brackish-water ostracod. The top of each green limestone unit is characterized by pedogenic features and microrelief, indicating desiccation of the lake and soil development. Isotope data support the hypothesis that the white limestone was deposited in an open, freshwater lake. The incision of the present channel of Nahal (wadi) Zihor in the lacustrine sediments is manifested by a series of rock-cut and fluvial terraces (Q1–Q4) capped by gypsic-salic soils, which reflect the onset of the present, extremely arid climate. Over 100 find-spots and larger occurrences of prehistoric artifacts assigned to the Lower Paleolithic were discovered near Lake Zihor. On the basis of techno-typological and stratigraphic considerations, these assemblages are divided into two groups, the first of which may be contemporaneous with the lake, while the second is found mainly on the younger Q1 and Q2 terraces. It is estimated that the lake existed for more than 100,000 years. The climate during this period was probably semiarid, but water budget calculations suggest that, in addition to surface runoff, seepage of groundwater along the Zihor Line contributed a considerable amount of water to the lake.

Paleoclimatic significance of mineral magnetic properties of loess sediments in northeastern Qinghai-Tibetan Plateau

Loess deposit in the northeastern Qinghai-Tibetan Plateau offers an excellent record of climate and environment changes in this region. We have conducted multiparameter mineral magnetic analyses of samples of loess deposits from the Dongchuan, Lalakou and Panzishan sections. The methods used include magnetic susceptibility, frequency-dependent susceptibility, temperature-dependent susceptibility, isothermal remanent magnetization acquisition, and magnetic hysteresis loops. The results reveal that magnetite is the dominant contributor to magnetic susceptibility. Maghemite and hematite is also present in the loess layers and paleosol horizons. The higher concentration of maghemite in paleosols suggests that the formation of maghemite occurred during in situ pedogenesis, which plays an important role in the enhancement of the magnetic susceptibility. Similar to that in the Chinese Loess Plateau lying to the east of the Liupan Mountains, magnetic granulometry in the studied loess and paleosols is predominantly pseudo-single-domain (PSD). However, the mean grain-size of the ferrimagnetic minerals in loess is evidently coarser (larger PSD and multidomain-like) than in paleosols (mainly PSD). The magnetic susceptibility of loess and paleosols is positively correlated with the content of ultrafine magnetite/maghemite grains, and hence with the intensity of in situ pedogenesis. Therefore, the enhanced magnetic susceptibility in the studied loess-paleosol sequences can be interpreted as being due to climatically induced in situ pedogenesis.

Vertebrate extinction across Permian–Triassic boundary in Karoo Basin, South Africa

Distinct assemblages of paleosols above and below the Permian–Triassic boundary in the Karoo Basin of South Africa are evidence for reorganization of ecosystems following this greatest of all mass extinctions. The Permian–Triassic boundary is recognized from the last appearance of Dicynodon and from a series of negative excursions in the isotopic composition of carbon within therapsid tusks, pedogenic carbonate nodules, and organic matter. The boundary is also marked by laminated beds with very weakly developed paleosols, a change from purple (10R) to brownish red (2.5YR) paleosols, and a thin (10-cm) claystone breccia of reworked soil clasts. Paleoclimatic changes include a shift from arid and highly seasonal paleoclimate inferred from diffuse and shallow calcareous nodules in Permian paleosols to semiarid and less seasonal paleoclimate inferred from deep and well-focused calcic horizons in Triassic paleosols. An earliest Triassic shift to warmer and wetter paleoclimate is also indicated by increased chemical weathering, abundance of lycopsids, and diversity of labyrinthodonts. Permian paleosols have root traces comparable to those of open shrubland and riparian woodland, whereas Triassic paleosols have root traces and profile forms like soils of open woodland. This is a significant paleoenvironmental change, but not as dramatic a change as would be expected from the devastating extinctions of 88% of fossil vertebrate genera. Latest Permian therapsid reptiles were diverse and ecologically specialized. In contrast, the principal earliest Triassic therapsid, Lystrosaurus , was a burrower with no specific habitat preference. Its short internal nares, barrel chest, and high neural spines would have given it greater aerobic scope than preexisting therapsids and may have been an advantage under conditions of hypercapnia and hypoxia. These adaptations and associated ecosystem changes are compatible with widespread vertebrate mortality by acidosis and pulmonary edema in a post-apocalyptic greenhouse created by the voluminous release of methane from shallow marine and permafrost clathrates.

Chemical weathering history of the southern Tajikistan loess and paleoclimate implications

Major chemical elements and Rb, Sr, Ba abundances were measured for loess samples from Chashmanigar loess-soil sequence in southern Tajikistan. And the degree of element loss and intensity of weathering of the samples were calculated. From these calculations we found that the paleosol horizons of the Chashmanigar section were chemically weathered to some extent and the weathering intensity of the soils is stronger than that of loess horizons, indicating that paleosols in southern Tajikistan were deposited in relatively warm and humid interglacial ages. In addition, the southern Tajikistan loess deposited before 0.9Ma BP was more intensely weathered than that deposited after 0.9Ma BP, which may imply the intensification of aridity in this area since 0.9Ma BP.

Correlation of loess–paleosol sequences in East and Central Asia with SE Central Europe: towards a continental Quaternary pedostratigraphy and paleoclimatic history

As recent small climatic fluctuations on a 10 2–10 3 year time scale can be correlated worldwide, and represent a decline of mean annual temperature of only ⩽1°C, major climatic changes on at least a 10 5 year scale (glacial-interglacial cycles) and probably a 10 4 year scale (the approximate length of an interglacial) must be of similar ages throughout the temperate climatic belt of the Northern Hemisphere. This concept allows continental pedostratigraphical correlations. Detailed knowledge of the genesis of paleosols is needed to establish loess–paleosol stratigraphies that can be used for paleoclimatic reconstruction. Most paleosols, however, are truncated and largely recalcified from overlying loess. Micromorphological studies allow primary and secondary carbonates to be distinguished and provide unequivocal evidence of clay illuviation. This enables the recognition of typical loess, weathered loess and the recognition of different genetic soil horizons, such as CB, BC, Ah, Bw and Bt horizons. For the Brunhes epoch, the sequence at Karamaydan, Tadjikistan, is more detailed than the corresponding section in Luochuan, China, and even more than in SE Central Europe except for the last glaciation. The very good correlation with the deep-sea oxygen isotope record, which includes the development of an accurate astronomical time scale, allows a detailed chronostratigraphical subdivision of the loess–paleosol sequence in Karamaydan, which therefore should be regarded as a key sequence for reconstructing the climatic history of the Brunhes epoch. This implies that the loess–paleosol sequences in SE Central Europe are less complete than thought earlier. Pedocomplexes in Karamaydan correspond with single paleosols in SE Central Europe, e.g. the F6 paleosol in Stari Slankamen correlates with pedocomplexes PK VI and V at Karamaydan, which were formed over a period of about 140 ka , although pedogenesis was interrupted several times by loess deposition. The F6 soil is therefore an example of a welded paleosol, as are the F5, the F4 and the F2 paleosols in SE Central Europe. For most of the Matuyama epoch, the central and lower parts of the sequence at Chashmanigar show more pronounced paleosols than the equivalent parts at Luochuan; in SE Central Europe only at Stari Slankamen are three strongly developed though truncated paleosols (F9–F11) above Neogene sediments. Mineralogical investigations of the silt and clay subfractions show that there is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols of the Brunhes epoch at Karamaydan and of the Matuyama epoch at Chashmanigar. This suggests that the interglacial climates represented by the B or Bt horizons of the buried paleosols of late, middle, and early Pleistocene age were similar to that of the Holocene.

Paleosols and paleoenvironments of the middle Miocene, Maboko Formation, Kenya.

The middle Miocene (15 Ma) Maboko Formation of Maboko Island and Majiwa Bluffs, southwestern Kenya, has yielded abundant fossils of the earliest known cercopithecoid monkey (Victoriapithecus macinnesi), and of a kenyapithecine hominoid (Kenyapithecus africanus), as well as rare proconsuline (Simiolus leakeyorum, cf. Limnopithecus evansi) and oreopithecine apes (Mabokopithecus clarki, M. pickfordi), and galagids (Komba winamensis). Specific habitat preferences can be interpreted from large collections of primate fossils in different kinds of paleosols (pedotypes). Fossiliferous drab-colored paleosols with iron-manganese nodules (Yom pedotype) are like modern soils of seasonally waterlogged depressions (dambo). Their crumb structure and abundant fine root-traces, as well as scattered large calcareous rhizoconcretions indicate former vegetation of seasonally wet, wooded grassland. Other fossiliferous paleosols are evidence of nyika bushland (Ratong), and early-successional riparian woodland (Dhero). No fossils were found in Mogo paleosols interpreted as saline scrub soils. Very shallow calcic horizons (in Yom, Ratong, and Mogo paleosols) and Na-montmorillonite (in Mogo) are evidence of dry paleoclimate (300-500 mm MAP=mean annual precipitation). This is the driest paleoclimate and most open vegetation yet inferred as a habitat for any Kenyan Miocene apes or monkeys. Victoriapithecus was abundant in dambo wooded grassland (Yom) and riparian woodland (Dhero), a distribution like that of modern vervet monkeys. Kenyapithecus ranged through all these paleosols, but was the most common primate in nyika bushland paleosols (Ratong), comparable to baboons and macaques today. Mabokopithecus was virtually restricted to riparian woodland paleosols (Dhero), and Simiolus had a similar, but marginally wider, distribution. Habitat preferences of Mabokopithecus and Simiolus were like those of modern colobus monkeys and mangabeys. A single specimen of Komba was found in dambo wooded grassland paleosol (Yom), a habitat more like that of the living Senegal bushbaby than of rainforest galagids. A shift to non-forest habitats may explain the terrestrial adaptations of Victoriapithecus, basal to the cercopithecid radiation, and of Kenyapithecus, basal to the hominoid radiation. Both taxa are distinct from earlier Miocene arboreal proconsulines, oreopithecines and galagids.

Paleowind directions from the magnetic fabric of loess profiles in central Alaska

Two parallel profiles of alternating loess and paleosol horizons from the Halfway House site west of Fairbanks, in central Alaska, were evaluated in terms of their potential for preserving paleowind directions. Combined, the profiles span SPECMAP marine oxygen isotope stages 1–6, approximately the last 150 ka. Our analytical tools consisted of measuring the sample’s anisotropy of magnetic susceptibility and characterizing the magnetic mineralogy by multiple approaches. Both primary eolian and secondary reworked magnetic fabrics are recognized along certain intervals of both profiles. Primary eolian fabrics display a horizontal magnetic foliation and a well defined magnetic lineation, corresponding to the paleodirection of sediment transport and deposition. A systematic change in transport direction is observed where long-term prevailing winds appear to shift from a NW–SE to N–S direction during glacial and interglacial periods, respectively.

Isotopic and geomorphic evidence for Holocene Climate, Southwestern Kansas

The Cimarron Bend of southwestern Kansas is characterized by loess, dunes, and sand sheets that reflect a complex eolian–alluvial history. Stratigraphic evidence indicates that at least two major dune and sand sheet reactivation episodes occurred. These follow intervals of alluviation, stability, and soil development in the Holocene. Radiocarbon ages from paleosol horizons bracket these periods and their δ13C values provide evidence of a shift from a cooler, wetter climate in the late Pleistocene and early Holocene dominated by C3 plants, to a warmer, drier middle Holocene period characterized by an increase in C4 plants. Stable isotopic results for late Holocene soils may reflect local rather than regional climatic conditions. Our results compare favorably with studies in the central and southern Great Plains. Together, these results imply that a regional change toward a warmer, drier climate occurred from the early to the middle Holocene on the southern Great Plains.

Morphogenesis of Antarctic Paleosols: Martian Analogue

Samples of horizons in paleosols from the Quartermain Mountains of the Antarctic Dry Valleys (Aztec and New Mountain areas) were analyzed for their physical characteristics, mineralogy, chemical composition, and microbiology to determine the accumulation and movement of salts and other soluble constituents and the presence/absence of microbial populations. Salt concentrations are of special interest because they are considered to be a function of age, derived over time, in part from nearby oceanic and high-altitude atmospheric sources. The chemical composition of ancient Miocene-age paleosols in these areas is the direct result of the deposition and weathering of airborne-influxed salts and other materials, as well as the weathering of till derived principally from local dolerite and sandstone outcrops. Paleosols nearer the coast have greater contents of Cl, whereas near the inland ice sheet, nitrogen tends to increase on a relative basis. The accumulation and vertical distribution of salts and other soluble chemical elements indicate relative amounts of movement in the profile over long periods of time, in the order of several million years. Four of the six selected subsamples from paleosol horizons in two ancient soil profiles contained nil concentrations of bacteria and fungi. However, two horizons at depths of between 3 and 8 cm, in two profiles, yielded several colonies of the fungi Beauveria bassiana and Penicillium brevicompactum, indicating very minor input of organic carbon. Beauveria bassiana is often reported in association with insects and is used commercially for the biological control of some insect pests. Penicillium species are commonly isolated from Arctic, temperate, and tropical soils and are known to utilize a wide variety of organic carbon and nitrogen compounds. The cold, dry soils of the Antarctic bear a close resemblance to various present and past martian environments where similar weathering could occur and possible microbial populations may exist.

Paleoenvironmental changes at the Messinian–Pliocene boundary in the eastern Mediterranean (southern Cyprus basins): significance of the Messinian Lago-Mare

The study of the late Messinian–Early Pliocene sediments in south Cyprus (Pissouri and Polemi basins) and the comparison with the offshore coeval deposits drilled in the ODP Leg 160 boreholes, provide new data illustrating the environmental changes which occurred in the eastern Mediterranean at the end of the salinity crisis. The Messinian–Pliocene transition is marked by a very rapid environmental change indicated by sharp variations in lithology, sedimentology, microfaunal assemblages and stable isotope composition. The latest Messinian interval is dominated by the ‘Lago-Mare’ oligo-mesohaline environments. The most striking feature in the Pissouri Basin is the intercalation between the uppermost gypsum bed and the base of the Zanclean deposits of four main horizons of paleosols interbedded with carbonates and conglomerates. Paleosols indicate periods of subaerial exposure long enough to permit pedogenesis to develop. Conglomerates indicate intense erosion of the Troodos Massif and its sedimentary cover as a result of the water level drop, tectonic activity and more humid climatic conditions. Thus, several subaerial exposure phases occurred in the marginal areas like the Pissouri Basin whereas shallow oligohaline conditions persisted in the deeper parts of the basin, as indicated at ODP Site 968. Both in the Pissouri Basin and on the flank of the Erathostenes Seamount the late Messinian paleosols are directly overlain by the lowermost Pliocene sediments deposited in well-oxygenated deep marine conditions. This drastic change of water depth shows that the water level of the late Messinian saline to brackish lakes dropped far below that of the world ocean, at least more than several hundred meters. Both these base level fluctuations and the oligohaline conditions caused intensive erosion–karstification of the upper gypsum beds as also reported from many other Mediterranean regions. Moreover, the study of the Polemi sections shows that oligohaline conditions had already started during the deposition of the upper gypsum sub-unit. Typical brackish and fresh water assemblages are present in the last two gypsum layers and intergypsum beds. This indicates that freshwater dilution, i.e. the Lago-Mare conditions, resulted from a progressive change of the hydrological budget which started during deposition of the upper evaporites and climaxed after the deposition of the uppermost gypsum layer (classical Lago-Mare facies). During this time, periods of increased evaporation over precipitation led to deposition of the uppermost gypsum layers. This evolution is interpreted as being due both to the severance from the open ocean and to the increasing contribution of the runoff–precipitation versus marine inputs in the hydrological budget of the Mediterranean.

Seven million-year iron geochemistry record from a thick eolian red clay-loess sequence in Chinese Loess Plateau and the implications for paleomonsoon evolution

Recently, the Tertiary red clay sequence underlying Pleistocene loess has attracted much attention. The remarkable progress is the recognition that the Tertiary red clay sequence is also wind-blown in origin, thus providing a good opportunity to reconstruct long-term monsoon changes. In contrast to the loess deposits, the magnetic susceptibility and the pedogenic characteristics are almost independent of each other in the red clay, thus challenging the validity of the readily measurable magnetic susceptibility in describing the monsoon history recorded by the red clay. With the objective to address the long-term East Asia summer monsoon evolution, the free to total Fe2O3 ratios were calculated in a continuous eolian red clay-loess sequence at Lingtai, the Chinese Loess Plateau, which has a thickness of 305 m and a basal age of 7.0 Ma. The seven million-year Fe2O3 ratio record from the red clay-loess sequence indicates that (i) variations of the free to total Fe2O3 ratios of the loess can correlate generally with the alternations of the loess and paleosol horizons; (ii) the East Asia summer monsoon was stronger as a whole in Neogene than that in Quaternary; (iii) the strongest East Asia summer monsoon may occur between 4.0 and 4.8 Ma. The relatively small ice volume and high global temperature may be responsible for the strong summer monsoon during the early Pliocene.

The distribution and discrimination of shallow, authigenic carbonate in the Pliocene-Pleistocene Palomas Basin, southern Rio Grande rift

Nine types of authigenic carbonate are present in the Pliocene–Pleistocene alluvial-fan and axial-fluvial sediment of the Palomas half graben in the southern Rio Grande rift. Pedogenic and other vadose carbonate includes (1) pedogenic carbonate of stage II and stage III morphology underlying Bw and Bt horizons, (2) mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone, (3) pedogenic calcic nodules and tubules in eolian sand, and (4) gully-bed cement of proximal-fan conglomerates formed by infiltration and evaporation of surface runoff. Shallow groundwater carbonate exists as (5) thin (30–50 cm), massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe, (6) thick (1.5–3 m), massive beds deposited by lateral flow of groundwater or at springs, and (7) thin (30 cm), calcified root mats associated with near-surface, water-saturated sediment or springs. Phreatic spar cements (8) occupy the interstices of conglomerates and sandstones and locally exist as (9) oriented concretions. Groundwater carbonates are best developed near the toes of the large, hanging-wall–derived alluvial fans, whereas phreatic cement preferentially exists in footwall-derived, alluvial-fan conglomerates. Pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons, a predominantly vertical arrangement of root traces, peds, desiccation cracks, and calcic tubules, and by gradational contacts. The δ 13 C and δ 18 O values are similar among pedogenic and shallow groundwater carbonate, although locally nodules in eolian sand and gully-bed cement have higher δ 18 O values, perhaps due to the effects of evaporation. Some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of δ 13 C and δ 18 O. Authigenic carbonate in footwall-derived, alluvial-fan sediment has consistently higher values of δ 13 C than that in hanging-wall–derived sediment, which may reflect differences in vegetative type and/or density on either side of the basin.

00/00681 Study of structure analysis of oil shale and fuel manufacturing technology. 9

Lower Paleozoic rocks are exposed in various regions of Egypt (south central Sinai, north Eastern Desert and southwest Western Desert), in addition to occurring in the subsurface such as north Western Desert and the Gulf of Suez. The Lower Paleozoic rocks in Egypt include surface and subsurface rock units of formational status. The surface rock units are the Taba, Araba and Naqus formations. The subsurface rock units include the Shifa, Kohla and Basur formations.The Infracambrian Taba Formation has been discovered recently in the outcrops of the southeast central Sinai in the Taba-Ras El-Naqab area. It is missing and/or not recognized in the subsurface. The Taba Formation consists mainly of reddish brown, unfossiliferous gravelly fine- to medium-grained kaolinitic sandstones and subordinate horizons of paleosols. The Cambrian Araba Formation and its subsurface equivalent (the Shifa Formation) are essentially composed of reddish brown, fine-grained laminated sandstone and siltstone with abundant Skolithos and Cruziana sp. In contrast, the Ordovician–Silurian Naqus Formation and its subsurface equivalents (the Kohla Formation and Basur Formation) are mainly composed of white, unfossiliferous, cross-bedded, medium- to coarse-grained sandstones with haphazardly distributed pebbles and cobbles. Sedimentological analysis indicates that the Araba Formation and its subsurface equivalent were deposited in a marginal-marine environment, whereas the Naqus Formation and its subsurface equivalents were laid down in a fluvio-glacial environment.Integrated stratigraphic and sedimentological studies of the Lower Paleozoic rocks permit reconstruction of the paleogeography of Egypt at that time. Egypt has been largely controlled since the Cambrian by the pre-existing structural framework of the pre-Phanerozoic basement rocks inherited from the Late Proterozoic Pan-African event. Additionally, sedimentation processes were controlled during Cambro-Ordovician times by tectonic movements, whereas glacio-eustatic control predominated during the Late Ordovician–Silurian Period. These studies suggest that most areas of Egypt were exposed lands with episodically transgression by epi-continental seas related to the paleo-Tethys. These lands formed a part of a stable subsiding shelf at the northern Gondwana margin.

Stable carbon isotope stratigraphy of the Paleogene pedogenic series of southern France as a basis for continental-marine correlation

The Upper Cretaceous–lower Paleogene formations in the Aix-en-Provence basin are characterized by interfingering lacustrine carbonates and flood-plain alluvium. The deposits from both environments display numerous horizons of carbonate-rich paleosols. By using a combined magnetostratigraphy and carbon isotope stratigraphy for the pedogenic carbonate glaebules, we constructed a detailed integrated stratigraphy for the entire lower Paleogene. This method provides a way to correlate marine and terrestrial sequences when C3 plants dominated the environment. Both long-term and short-term δ 13 C variations were identified in the Provence series: (1) the uniform carbon isotope record established through the end of the Maastrichtian followed by the abruptly negative carbon isotope excursion in the lowermost Paleocene provides a marker for the precise location of the Cretaceous-Tertiary boundary in this basin, 5 m below the Calcaire de Vitrolles Formation; (2) a gradual δ 13 C increase through the middle upper Paleocene similar in amplitude to that of the marine realm; (3) a short-term negative δ 13 C excursion in the latest Paleocene, as seen within the Calcaire de St. Marc Formation; and (4) a slow decrease during the late Paleocene to early Eocene, with a probable hiatus in the sedimentary record. The Provence mammal site of Palette is shown to be stratigraphically younger than the late Paleocene negative δ 13 C excursion.

Illitization in a Paleozoic, peat-forming environment as evidence for biogenic potassium accumulation

Potassium enrichment is common in surface horizons of many paleosols and has been attributed to the formation of authigenic illite from smectites during burial diagenesis. In such cases, the presence of primary smectite is key to the illitization process, which then results in potassium enrichment as a byproduct. Potassium enrichment in paleosols where no primary smectite was present, however, requires a different process of accumulation. A Lower Pennsylvanian, tropical Ultisol from the Southern Anthracite Field in Pennsylvania is enriched in potassium in the surface horizon and formed in an environment unfavorable for smectite accumulation. Kaolinite dominates clay mineral fractions of soils that form in such highly weathering environments. In the absence of primary smectite to scavenge potassium during illitization, there should be no potassium enrichment during burial diagenesis. However, in Lykens Valley paleosol, the pattern of potassium concentration, organic matter distribution and authigenic illite growth suggests that organic matter may have provided an important source of potassium for illite formation. Illitization of kaolinite in the organic-rich surface horizon is complete but, illite coexists with kaolinite in the B horizon. Within the paleosol matrix, illitization is more complete in areas of high organic concentration where organic fragments often have illite haloes with illite whorls growing from their edges. We propose that the potassium enrichment both in the soil profile and in the overlying coal is the result of the luxury uptake of potassium by peat-forming plants (nutrient intake at rates exceeding metabolic requirements). Further, potassium immobilized by adsorption on the peat provided interlayer cations for illite authigenesis to begin. If, as we suggest, luxury nutrient uptake during peat formation sequestered sufficient potassium to promote later illite formation, the pattern and extent of illitization within the paleosol profile reflects early biogenic nutrient accumulation. Thus, illite, in some paleosols, may represent fossil evidence for a biological process: the luxury uptake of potassium.

Pedogenic calcite as evidence for an early Holocene dry period in the San Francisco Bay area, California

Rainfall at the site of Union City, California, during early Holocene time appears to have been about half that of today, 470 mm/yr. We base this conclusion on detailed descriptions and particle-size analyses of 12 soil profiles and 1:20 scale logs of the fluvial stratigraphy in two 100-m-long, 5-m-deep excavations dug perpendicular to the axis of an alluvial fan along the Hayward fault. Subsidence and right-lateral movement along the fault allowed an offset stream to produce a nearly continuous alluvial record documented by 35 14 C ages on detrital charcoal. Bk (calcitic) horizons in paleosols developed in the fan suggest that a relatively dry climatic period occurred from 10 to 7 ka (calendar-corrected ages). The pedogenic calcite exists primarily as vertically oriented filaments and fine, cavernous nodules formed at ped intersections. Soils and paleosols formed before 10 ka or since 7 ka did not have Bk horizons. Bk horizons that were buried suddenly at 7 ka were overlain by leached zones averaging 41 ± 3 cm thick—about half the current depth of leaching.

Weathering and clay mineral formation in two Holocene soils and in buried paleosols in Tadjikistan: towards a Quaternary paleoclimatic record in Central Asia

The upper part of the Karamaydan section, Tadjikistan, shows the most detailed loess–paleosol sequence yet known for the Brunhes chron, and the central and lower parts of the Chashmanigar section provide similar detail for most of the Matuyama chron. To enable paleoclimates to be deduced, the primary and secondary minerals in the silt and clay fractions must be determined separately to evaluate the type and intensity of mineral weathering and clay mineral formation. To distinguish between inherited and pedogenetically formed clay minerals, the original petrographic homogeneity of the parent material from which a soil developed must be established. The main sources of pedogenic clay minerals are phyllosilicates in the silt fractions. Illites and vermiculites are the dominant pedogenetically formed clay minerals in the B or Bt horizons of the Holocene climaphytomorphic soils and in all paleosols (S) and pedocomplexes (PK) in Karamaydan and Chashmanigar, except S XVII in which large amounts of smectites were formed. There is little difference in the type and amount of pedogenic clay mineral formation between the Holocene soils and the paleosols in the Brunhes epoch at Karamaydan as well as during most of the Matuyama epoch at Chashmanigar. These results indicate that the climates of the interglacials represented by the B or Bt horizons of the buried paleosols of young, mid and old Pleistocene age were similar to that of the Holocene.

MAJOR CONSTRAINTS ON THE USE OF RADIOCARBON DATING FOR TEPHROCHRONOLOGY

It is well recognised that radiocarbon dating is a powerful tool capable of providing a detailed chronostratigraphy for areas that have been subjected to recurrent volcanic activity during Late Pleistocene and Holocene time. In such geological contexts carbon-bearing materials suitable for 14 C dating are, for the most part, refractory components of pedosphere (e.g. paleosol humic matter) and organic detritus (e.g. charred/uncharred remains of higher plants) which occur within the tephra suites. Since such materials derive initially from atmospheric carbon dioxide, as well as record significant lapses in volcanic activity, it is then often assumed that these conditions comply ideally with the fundamental requirements of the radiocarbon dating theory. However, two recent 14 C dating programmes based on materials from the volcanic districts of the Phlegrean Fields and the island of Procida (Campania, southern Italy) yielded evidence to the contrary and, in turn, gave rise to concern over the validity of the 14 C dating method when applied in volcanic regions. In particular, it has been found that the conventional radiocarbon ages recorded by modern tree-leaves from the caldera of Solfatara volcano range from ‘modern’ to ca. 5000 yr BP. Furthermore, the magnitude of the age anomaly is determined by the pattern of localised dilution of the mean atmospheric 14 C concentration by juvenile ( 14 C-free and 13 C-enriched) carbon dioxide issuing from nearby volcanic vents. In the case of paleosols, a series of 14 C measurements on different chemical fractions isolated from the humic matter in a suite of four paleosol horizons entrained in tephra deposits at Procida island yielded, for each paleosol horizon, a systematic ca. 2000 yr scatter of 14 C ages. It is shown that such intra-soil age differences, which reflect the origin and history of humic constituents, can be used to afford a better definition of the true paleosol age.

Magnetic properties of modern soils and Quaternary loessic paleosols: paleoclimatic implications

The magnetic properties of paleosols developed in Quaternary sequences of loess have been used for: stratigraphic definition; correlation with other terrestrial and deep-sea sequences; and paleoclimatic (paleorainfall) reconstruction. In some loess/paleosol sequences, including those of the Chinese Loess Plateau, Tajikistan and the Czech Republic, maxima in magnetic susceptibility values correspond with the paleosol horizons, and minima with the least-weathered loess layers. In other loess/soil sequences, including those of Siberia, Alaska and Argentina, the relationship is completely opposite, with susceptibility minima associated with the most developed paleosols. To account for these opposite relationships, the respective roles of: (1) magnetic enhancement and (2) magnetic depletion and/or dilution in determining soil magnetic properties are investigated for a range of modern soil types. Most magnetic enhancement is seen in the upper horizons of well drained cambisols. Absence or loss of magnetic iron oxides is apparent in acid, podsol profiles and waterlogged soils. For the cambisol profiles, significant correlation is found between susceptibility and organic carbon, cation exchange capacity and clay content. The mineralogy, morphology and grain size of soil magnetic carriers, extracted from three modern enhanced soils and a paleosol from the Chinese Loess Plateau, are also identified by independent petrographic means (microscopy and X-ray diffraction of magnetic extracts). Magnetite and maghemite of ultrafine grain size (from ∼0.4 to < ∼0.001 μm) are the major contributors to the magnetically enhanced soils. Weathering can concentrate detrital magnetic grains, especially in the fine silt size fractions of soils. The magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site-specific basis, taking into account the possibilities of pedogenic enhancement, pedogenic dilution or depletion, and allochthonous inputs of magnetic minerals. Excessively arid, wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets. Well drained, intermittently wet/dry soils, with reasonable buffering capacity and a substrate source of Fe, show most magnetic enhancement. For those soils which favour magnetic enhancement processes, correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall. The almost unique pedogenic system in the Chinese loess plateau, where variation in soil-forming factors other than climate is reduced to a global minimum, allows use of the paleo-susceptibility values as proxy paleorainfall values. To identify the mechanism of any link between magnetic properties, for example, susceptibility, and climate change, the mineralogy and grain size of soil magnetic carriers, requires detailed investigation. Given the prospect of: (1) quantitative paleorainfall records from continental loess records; (2) predictions of present and future climate change; and (3) the value of paleoclimate data for testing of numerical climate models; such magnetic investigations can contribute significantly to our understanding of past and future environmental change.

Meteoric sphaerosiderite lines and their use for paleohydrology and paleoclimatology

Sphaerosiderite, a morphologically distinct millimeter-scale spherulitic siderite (FeCO 3 ), forms predominantly in wetland soils and sediments, and is common in the geologic record. Ancient sphaerosiderites are found in paleosol horizons within coal-bearing stratigraphic intervals and, like their modern counterparts, are interpreted as having formed in water-saturated environments. Here we report on sphaerosiderites from four different stratigraphic units, each of which has highly variable 13 C and relatively stable 18 O compositions. The unique isotopic trends are analogous to well-documented meteoric calcite lines, which we define here as meteoric sphaerosiderite lines. Meteoric sphaerosiderite lines provide a new means of constraining ground-water δ 18 O and thus allow evaluation of paleohydrology and paleoclimate in humid