Experimental calibration of clumped isotopes in siderite between 8.5 and 62 °C and its application as paleo-thermometer in paleosols
Joep van DijkÁlvaro FernándezJulian-Christopher StorckTimothy S. WhiteMark A. LeverInigo A. MüllerStewart BishopReto F. SeifertSteven G. DrieseА. А. KrylovGregory A. LudvigsonAlexandra V. TurchynChin Yik LinChad WittkopStefano M. Bernasconi
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Previous research assumed deep buried loess as the initial composition of the overlying paleosol and failed to address the long-term continuous pedogenic weathering history in the deep loess-paleosol sequence, which was attributed to little understanding on the difference between loess and paleosol in the long-term deep loess-paleosol sequence. To distinguish between the loess and paleosol, in the long-term deep loess-paleosol sequence in northeast China, the morphology, dust deposition fluxes, geochemical characteristics, magnetic susceptibility, and grain size distributions were investigated. Results showed that the loess layers buried at depth could be differentiated from the paleosol by their poor pedogenic development. The presence of coarser grains in the loess as well as lesser amounts of clay and Fe–Mn coatings compared to paleosol indicated weaker weathering of the loess. Also, optical iron clay films deposited on the surface of the skeleton particles were less visible in the loesses than in the paleosols. From the loess evolution perspective, the pedogenic formation process of the newly formed loess soils should be considered as important as that of the reddish paleosol layer based on criteria of formation time, depth within profile, and morphological characteristics. The formation of the reddish or yellowish paleosol layer was constrained by pedogenic environments associated with climatic change and by the relative rates of deposition and pedogenesis. The terms “loessal paleosol” and “loessal paleosol sequence” are suggested to aid in systematically and consistently addressing the long-term pedogenic weathering evolution recorded in the complex formation of deep loess and paleosol sequences in pedology research. The long-term deep loessal sediments of Northeast China are loessal paleosols, which cannot be simply used as a reference for the overlying paleosol and be deducted from pedogenesis consideration.
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Comparisons of red ratings (RR) with Fed, Fed/Fet, clay content, and magnetic susceptibility (x) of two loess-paleosol sequences at Luochuan and Lingtai on China's Loess Plateau were conducted to study the possible relationship between RR and pedogenic degrees of the two loess-paleosol sequences, and to discuss whether the RR could become new paleo-climatic indicators. Results showed that the RR of the two loess-paleosol sequences had positive, highly significant (P 0.01) correlations with: 1) citrate-bicarbonate-dithionite (CBD) extracted iron (Fed), 2) ratios of CBD extracted iron to total iron (Fed/Fet), 3) clay ( 2 μm), and 4) magnetic susceptibility (x). This suggested that the RR of these loess-paleosol sequences could indicate degrees of loess weathering and pedogenesis and were potential paleo-climatic proxies. The strong correlations of RR to Fed and x also implied that during pedogenic processes, pedogenic hematite in loess and paleosols were closely related to the amount of total secondary iron oxides and pedogenic ferrimagnetic minerals (predominantly maghemite).
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Lithium (Li) and barium (Ba) distributions in the Laoguantai and Liwan loess-paleosol profiles in the south of the Chinese Loess Plateau show distinct variations caused by pedogenesis. Mean Li contents in paleosols are higher than those in the upper or subjacent loess layer. Paleosol units at the two sites, respectively, contain 5% and 18% more Li than that the loess units. Conversely, mean Ba values in paleosol units are noticeably lower than in loess units. Paleosol units contain 33% and 46% less Ba than loess units at the Laoguantai and Liwan sites, respectively. Comparison of Li/Ba ratios shows that Li varied slightly throughout the weathering profiles, but was enriched in the paleosol layers. The Ba distribution showed greater variation throughout the weathering profiles and has a larger impact on the Li/Ba ratios. This indicates that Ba is more actively lost during pedogenesis, whereas Li is relatively stable and accumulates as a pedogenic product. Variations in the Li/Ba ratios in the weathering profiles reflect pedogenic changes after dust deposition. Accordingly, the Li/Ba ratio is a reasonably good proxy for reconstructing the climate change and pedogenesis history contained in the loess-paleosol sequences. The Li/Ba ratios reveal that pedogenic intensity underwent polyphase changes during the Holocene in the eastern Guanzhong Basin of South Central China, likely driven by shifts in the East Asian monsoon.
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