Freeze-Agglomeration: An Alternative Mechanism for Clay Film Formation
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Abstract:
Oriented clay coatings (argillans, clay cutans, clay films, lamellae) are often interpreted to be caused by illuviation (pervection, lessivage) of fine clay particles. In montane meadow soils (Typic Humaquepts) of the northern Sierra Nevada Range, prominent clay cutans occur on ped faces of a paleosol 3Btgb horizon, which is contemporaneous or younger than 2840 ± 220 yr BP by radiocarbon dating. The unlikelihood of distinct clay cutans forming in such a short period of time via illuviation in a soil with an aquic moisture regime suggests an alternative mechanism. Laboratory studies show that silt- and sand-sized agglomerates can be produced by subjecting colloidal kaolinite (high Fe content), extracted from the montane meadow soil, to repeated freeze–thaw cycles. These agglomerates have a distinct microgranular texture and one to several birefringent domains. Thin section microscopy of the surface Oa horizon reveals that pores and root channels are coated by oriented birefringent zones resembling clay cutans. These clay cutans are similar in optical properties to laboratory-produced freeze-agglomerates. Optical microscopy of clay cutans, scraped from the paleosol 3Btgb horizon, suggests that they too may have formed via freeze-agglomeration. Freeze-agglomeration is proposed as a new mechanism to produce oriented clay films.Keywords:
Agglomerate
Silt
Paleosol
Soil horizon
Illite
Nanocellulose
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The aim of the study. The aim of the study was to find and justify specifics of the paleosol development in the north of West Siberia, revealing soils levels, associated with climate warming during sea isotope stages (SIS), namely MIS-5, MIS-3 and at the end of MIS-2. Location and time of the study. The research was conducted during the last 10 years in the representative area between the Polar Circle and the sub-latitudinal sector of the Ob river all over the area. We carried out paleopedological and paleocryological investigations with fieldwork in the basin of the Vakh River (right tributary of Ob), the Taz and Nadym Rivers, where the soil pits were studied, and paleosols were discovered for the first time. Methods. The methodology has been focused on detailed study of the paleosols discovered by the present authors based on the investigation in respect to the interaction of paleopedogenetic and cryogenetic processes. Also, the encountered paleosols have been, as well compared to the surface Holocene soils that comprises an important element of analysis of the paleopedological records. For age determination we applied radiocarbon dating of organic residues and humus, contained in the paleosols, as well as U/Th-dating in paleosols. Results. Several paleosol levels were identified in the study area. The paleosol of the early substage of MIS-5 (MIS-5e) had signs of forest pedogenesis without permafrost, whereas the soils of late substages of MIS-5 and MIS3 showed evidence of gleying and cryogenesis; the paleosol corresponding to the end of MIS-2 was best represented by gleyic pedosediments in the ice-wedge pseudomorphs. The studied paleosols indicate the permafrost state of the soil-forming sediments in all phases of the Late Pleistocene with the exception of MIS-5e; the finding is important for the paleoenvironmental reconstruction. The article presents the data about wide distribution in the study region of the polygonal structures represented by the ice-wedge pseudomorphs formed in the coldest phases MIS-4 and MIS-2. The pseudomorphs were developed syn-cryogenically when permafrost still existed and were closely related to the formation of cryohydromorphic paleosols. Within the entire sequence of MIS-3 deposits initially ground wedges are common, also indicating syn-cryogenic development, however, under less strong freezing conditions than during MIS-2. Yet at the end of MIS-3 in the upper part of the sequence cryohydromorphic soils appeared again, as a sign of approaching to the MIS-2 cryochron. It was shown that the fills of the ice wedge pseudomorphs contain well preserved redeposited materials of the cryohydromorphic paleosols, including fragments of the humus horizons. It was suggested that the material from such redeposited pedosediments could be used for the radiocarbon dating and for sampling for the paleobotanical investigations. Conclusions. The obtained results cast doubt on the hypothesis of domination of the cold deserts and ice sheets in the study region during the Pleistocene and hence the formation of only extremal soils. There is evidence indicating the existence of well-developed vegetation and soil cover during the cryochrons with sufficient and in some areas excessive soil moisture, due to shallow permafrost table. The interaction of the paleopedogenetic and paleocryogenic processes resulted in rather informative relict objects, and their studies provide highly reliable results about the factors of past pedogenesis.
Paleosol
Marine isotope stage
Pedology
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Paleosols ranging in geological age to 3 billion years are widely used as evidence for ancient surface environments—an enterprise dependent on the enormous literature on factors in soil formation, popularized by Hans Jenny. This kind of inference inverts the logic of Jenny in a manner common to geological sciences—deducing paleoenvironments from observed paleosol features, rather than deducing variation in soil features with observed environmental differences. A paleosol is a single product of many past influences, including alteration after burial, and because of this, some environmental relationships with soil color, clayeyness and organic matter are not useful for interpreting paleosols. One relationship that has proven useful for paleosols is that between depth to calcic horizon and mean annual rainfall. A new compilation of data presented here demonstrates that this relationship holds for aridland soils worldwide. The use of this relationship for interpreting paleoclimate from paleosols is illustrated with an example of the Eocene and Oligocene paleosols of Badlands National Park, South Dakota. Other approaches for the study of paleosols include identifying paleosols within a soil taxonomy, and simulating ancient soil development with mathematical process models. Identification of paleosols leads to broad areas on soil maps unless done with several paleosols. Process models often founder on assumptions, and those of the form δx/δt (where x is a measured soil property) are difficult to apply because time of formation (t) of a paleosol is estimable only to an order of magnitude. Thus the environmental factor approach to the interpretation of paleosols is likely to remain popular for some time to come.
Paleosol
USDA soil taxonomy
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