Moderate Chemical Weathering of Subtropical Taiwan:Constraints from Solid-Phase Geochemistry of Sediments and Sedimentary Rocks

摘要

The well-known earthquake-and-storm-triggered extremely high physical weathering rate in Taiwan is consistent with present geochemical studies of sediments from different subenvironments (offshore,coastal,river,and lake) and sedimentary rocks of different geological ages,indicating a moderate chemical weathering condition.Major and trace element concentrations normalized to the average upper crust of Yangtze Craton show that the sediments and the average composition of sedimentary rocks of Taiwan are depleted in Ca,Mg,Na,and Sr,enriched in Rb and Zr,and unchanged with respect to K,indicating their moderately altered nature.The mean chemical index of alteration (CIA;71-75) and plagioclase index of alteration (PIA;81-86) values of coastal and offshore sediments reveal the sediments' derivation from sedimentary rocks by moderate silicate chemical weathering processes.The mean CIA value (62) of sedimentary rocks of Taiwan is similar to that for Chinese sediment (61),further confirming the above inference.A-CN-K,(A-K)-C-N,and A-CNK-FM plots (A=A1_2O_3;C=CaO*;N=Na_2O;K=K_2O;F=FeO_T;M=MgO) also confirm that the sediments and sedimentary rocks in Taiwan have undergone moderate silicate weathering,an interpretation consistent with CIA and PIA values.The plots also indicate the presence of illite,chlorite,and a subordinate amount of unaltered feldspars in sediments and sedimentary rocks,which are indicative of the physically weathered and/or moderately chemically altered nature of sediments.The dominance of illite,chlorite,and unaltered feldspars as inferred from geochemical data suggests that the immature nature of sediments and sedimentary rocks is probably a result of low residence times in the source region or river basin and quick removal of materials from the soil profile by steep,mountainous rivers (physical weathering dominates).Elemental ratios such as Rb/Sr,K/Rb,molar K/Na,and Al/Na are close to crustal values.Average shale and river particulates such as those from the Yellow River also indicate moderate chemical weathering conditions for sediments and sedimentary rocks,except for high alpine lake sediments,where the prevailing extreme chemical weathering condition over erosion is clearly differentiated by higher CIA (80-84) and PIA (92-96) values and by their positions on triangular plots.These inferences have also been illustratively corroborated by scatter plots of data such as Rb/Sr versus molar K/Na,and Al/Na versus CIA.Additional evidence from published sources noted here also favors moderate chemical weathering conditions for Taiwan.Geochemical variation of offshore,coastal,and river sediments is mainly controlled by non-steady state weathering dominated by erosion.Steady state weathering,however,seems to produce highly weathered sediments in the alpine region of Taiwan.