Phosphorus geochemistry in the Luochuan loess section, North China and its paleoclimatic implications

摘要

Total P (P_t) on a carbonate-free basis in an entire loess-paleosol sequence and P_t, organic P (P_o) and inorganic P (P_i) in the S_0-L_1-S_1 sequence were investigated in detail with different resolutions for the Luochuan loess section from northern China. P_t content varies between 393 and 786 ppm throughout the loess-paleosol sequence, and is generally higher in the loess than in interstratified paleosols, showing fluctuation cycles of 100ka in correspondence to loess-paleosol alternations. P_t variations on a carbonate-free basis in the loess-paleosol sequence could indicate variations in atmosphere precipitation resulting in different leaching loss of P from paleosols. P_i has an average value of 499 ppm with a range of 324-560 ppm, accounting for more than 70% of P_t in the S_0-L_1-S_1 sequence, where the minimum of P_i in the Malan loess is higher than the maximum of P_i in S_1. P_o ranges between 59 and 233ppm with an average of 132ppm in the S_0-L_1-S_1 sequence. Phosphorus (P) was initially delivered to the Luochuan loess section via influx of aeolian dust from the northern desert and Gobi areas by the East Asian winter monsoon, and then was modified by pedogenesis associated with the East Asian summer monsoon during the last 130 ka. "Preserved P_t" in the loess L_1 is tightly correlated with grain size without leaching loss of P due to enrichment of P in fine-grained fractions, as well as "initial P_t". "Leaching P_t" data show that paleosol S_1 had lost 15-40% of its "initial P_t", and that there was much more precipitation in S_1 than in L_1. P_i subject to slightly weak pedogenesis was completely transformed into P_o without leaching loss of P in loess L_1. By contrast, much P_i disappeared from paleosol S_1 due to strong pedogenesis, partly through leaching and partly through conversion to organic forms during P cycling processes. P_o variation is similar to those of MS and the < 7.8 μm fraction in L_1, but contains more information on the East Asian winter monsoon due to weak pedogenesis without leaching of P. P_o in S_1 lower than L_1SS_1 as a consequence of strong decomposition of the organic matter kept constantly in the middle of S_1 where P_i kept at the lowest of 423 ppm, suggesting that there existed a very warm and humid climate related to the enhanced summer monsoon during that period. The mean organic P/ inorganic P ratio (P_o/P_i) is lower in the L_1LL_1 and L_1LL_2 than in the S_0, S_1 and L_1SS_1, indicating that low P_o/P_i ratios coincide with weak weathering-pedogenesis, and higher P_o/P_i ratios correspond to strong weathering-pedogenesis. P_o/P_i ratio can eliminate the effect of grain size on aeolian dust because of chemical uniformity of aeolian dust and enrichment of P_o and P_i in the fine-grained fractions. Thus, P_o/P_i ratio is solely linked to pedogenesis of the Luochuan loess section. Variation in P_o/P_i ratio is similar to those of MS and the Marine Oxygen Isotope composition, indicating the summer monsoon evolution during the last 130 ka and providing the biogeochemical evidence for further understanding the genetic links between the East Asian monsoon and global climate change.