Chemical weathering intensity and controlling factors in the Changjiang River Basin during the Holocene
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摘要:
大河流域硅酸盐风化可以显著影响海陆物质循环和气候变化。如何通过河流沉积记录解译流域风化历史一直是表生地球系统科学研究的难点。虽然学术界对经典化学风化指标（如CIA、WIP等）的研究认识越来越深入，但碎屑沉积记录的风化信号如何响应气候变化及其时间尺度还存有争议。长江作为发源于青藏高原且流域受季风显著影响的世界大河，是研究风化信号在大河流域间传递和气候-风化-沉积联系的天然实验室。本文选择长江中游、河口和东海内陆架代表性沉积记录，分析全粒级、＜63 μm和＜2 μm（黏土)三种粒级组分的风化指标变化规律。结果显示：细粒级组分（黏土粒级/悬浮物）通常比粗粒级组分（全粒级/河漫滩沉积物）携带了更强的风化信号；经典风化指标反映的是流域累积风化程度，黏土组分的CIA也不能响应千年尺度上的温度变化；在千年尺度上经典风化指标变化主要受控于人类活动/降雨带迁移引起的物源改变，而不是温度变化。该研究揭示了大河流域风化-气候响应与反馈的复杂性，也提出对碎屑沉积记录中风化指标的解释需要谨慎。
Abstract:Chemical weathering of silicates in large river basin has a significant effect on climate change and land-sea material exchange. How to decipher the chemical weathering history from river sedimentary archive has always been a challenge in the study on weathering processes at the Earth’s surface. Although the classical chemical weathering indices (CIA: Chemical Index of Alteration, WIP: Weathering Index of Parker, and so on.) have been well documented, how the weathering proxies in clastic sediments responds to climate change and its time scale remains controversial. As a world-class river originating from the Qinghai-Tibet Plateau and with its basin significantly influenced by the monsoon, the Changjiang (Yangtze) River is a natural laboratory for studying the transmission of weathering proxies among the large basin and the interrelationship of climate, weathering, and sedimentation. Therefore, we examined the changes of different weathering proxies in the river basin since the Holocene by using the sedimentary records from the middle reaches, estuary, and the inner shelf of the East China Sea. The bulk samples, ＜63 μm fractions, and ＜2 μm fractions (clay) show that fine-grained sediments (clay/suspended particulate matter) could carry stronger weathering proxies than coarse-grained ones (bulk samples/floodplain sediments). The classical weathering proxies indicate that the integrated weathering intensity of large basins, and even the CIA in the clays could not reflect the temperature change on millennial scale but the provenance changes that related to rainfall migration and human activities. This study revealed the complexity of weathering-climate responses and feedbacks from large river basins. Therefore, interpretation on weathering proxies in clastic sedimentary archives shall be cautious.
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Keywords:
- silicate chemical weathering /
- climate change /
- Holocene /
- the Changjiang River
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图 1 长江流域季风系统、基性岩分布及收集数据位置示意图
红色箭头：季风系统，绿色圆圈：降水和温度气候记录；ZFCC：浙闽沿岸流，TWC：台湾暖流，KC：黑潮。
Figure 1. Schematic map of monsoon system, basic rock distribution, and location of data collection in the Changjiang River basin
Red arrows: monsoon system; green circle: the sampling locations of precipitation/temperature records; ZFCC: Zhejiang-Fujian Coastal Current; TWC: Taiwan Warm Current; KC: Kuroshio Current
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图 2 中游L02-1孔沉积物组分百分含量、平均粒度（Mz）以及CIA和WIP值变化
空心圆圈代表黏土粒级组分，实心圆圈代表＜63 μm粒级组分；灰色阴影区表示风化指标变化时期。
Figure 2. Downcore variations in percentage composition, mean grain size (Mz), CIA, and WIP for the sediments of Core L02-1
Hollow circle: the clay fraction; solid circle: the ＜63 μm fraction; grey shadow: the variation period of weathering proxies.
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图 3 长江口CM97孔沉积物平均粒度（Mz）、αAlNa、CIA、αAlK和WIP值变化
全粒级组分数据来自文献[21]，黏土粒级组分数据来自文献[22]。空心圆圈代表黏土粒级组分，实心圆圈代表全粒级组分；灰色阴影区表示风化指标变化时期。
Figure 3. Downcore variations in mean grain size (Mz), αAlNa, CIA, αAlK, and WIP for the sediments of Core CM97
Data sources: bulk samples fractions from reference [21], and those of the clay fraction from reference [22]. Hollow circle represents the clay fraction, and solid circle represents bulk samples. Grey shadow area indicates the variation period of weathering proxies.
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图 4 泥质区MD06-3040孔沉积物平均粒度（Mz）、αAlNa、αAlK、CIA和WIP值变化
沉积物数据来自文献[21]。实心圆圈代表全粒级组分，灰色阴影区表示风化指标变化时期。
Figure 4. Downcore variations in mean grain size (Mz), CIA, and WIP for the sediments of Core MD06-3040
Data of sediment are from reference [21]. Solid circle represents bulk samples, Gey shadow area indicates the variation period of weathering proxies.
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图 5 钻孔沉积物中CIA、WIP、αAlNa和αAlK风化参数与平均粒径（Mz）的相关关系
CM97和MD06-3040孔全粒级组分据文献[21]。阴影区为95%置信区间。
Figure 5. The correlation plots of CIA, WIP, αAlNa, and αAlK with mean grain size (Mz) in the core sediments
Data of bulk samples in Cores of CM97 and MD06-3040 are from reference [21]. Shadow area is 95% confidence interval.
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图 6 L02-1、CM97和MD06-3040孔沉积物粗粒级和细粒级沉积物CIA-WIP投点图
长江＜63 μm现代沉积物数据文献[38]，长江＜2 μm现代沉积物据文献[39]。
Figure 6. The WIP vs. CIA diagram of coarse-grained and fine-grained sediments from Cores L02-1, CM97, and MD06-3040
Data sources: ＜63-μm modern sediment of the Changjiang River from reference [38] and＜2 μm ones from [39].
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图 7 长江沉积物A-CN-K和A-CNK-Si图解
a： A–CN–K图解，b：A–CNK–Si图解。CM97孔和MD06-3040全粒级组分据文献[21]，CM97黏土粒级组分据文献[22]，长江悬浮物据文献[8]，平均上陆壳UCC据文献[41]。
Figure 7. The A-CN-K and A-CNK-Si diagram in Changjiang sediments
a: A-CN-K diagram, b: A-CNK-Si diagram. Data sources: bulk samples fractions of Cores of CM97 and MD06-3040 from reference [21], the clay fraction of Core CM97 from reference [22], suspended particulate matter in Changjiang River from reference [8], average upper continental crust UCC from [41].
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图 8 L02-1、CM97和MD06-3040孔化学风化指标的时间变化序列
空心圆圈代表黏土粒级组分，实心圆圈代表全粒级组分。MD06-3040全粒级组分据文献[21]，CM97黏土粒级组分据文献[22]，海平面变化据文献[28]。阴影区代表3个钻孔风化指标发生明显变化。
Figure 8. Time series of the chemical weathering proxies in Cores of L02-1, CM97, and MD06-3040 Hollow circle represents the clay fraction, and solid circle represents bulk samples
Dada sources: bulk samples in Cores MD06-3040 fron reference [21], the clay fraction in Cores CM97 from reference [22], sea level change from reference [28]. Grey shadow area indicates the variation period of weathering proxies.
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图 9 L02-1、CM97和MD06-3040孔εNd与风化指标相关性分析
MD06-3040全粒级组分据文献[21]，CM97黏土粒级组分据文献[22]。阴影区为95%置信区间。
Figure 9. Correlations of the chemical weathering proxies with εNd in Cores of L02-1, CM97, and MD06-3040
Dada sources: bulk samples in Cores MD06-3040 from reference [21], the clay fraction in Cores CM97 from reference [22]. Shadow area indicates 95% confidence interval.
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图 10 L02-1、CM97和MD06-3040孔细粒沉积物风化记录与气候指标对比
A：董哥洞石笋氧同位素[44]，B： 腾冲青海湖沉积物细粒级组分含量[45]，C：青藏高原东南缘天彩湖摇蚊属反演夏季温度（灰线）和5样品滑动平均值（红线）[45]，D： L02-1孔黏土粒级组分CIA值变化，E： 大九湖支链脂肪醇比重建的温度曲线[46]，F：五环三萜烯的平均芳环数[46]，G： CM97孔黏土粒级组分εNd同位素组成[22]，H：CM97孔黏土粒级组分CIA值变化[22]，I： MD06-3040孔全粒级组分CIA值变化[29]和海平面变化（蓝线）[28]。黄色和灰色阴影区分别表示气候控制和人类影响。
Figure 10. Comparisons of fine-grained sediment weathering proxies with climate parameters in Cores of L02-1, CM97, and MD06-3040
A:Oxygen isotopes of stalagmites from Dongge Cave[44], B: Fine-grained component content of Qinghai Lake sediments in Tengchong[45];C: Inversion of summer temperature (gray line) and sliding average of 5 samples (red line) by Chironomid inferred in Tiancai Lake, southeastern margin of the Tibetan Plateau[45], D:The change of CIA value of clay fraction in Core L02-1, E:Temperature reconstructed by branched fatty alcohol ratio in Dajiuhu Lake[46], F: The average number of aromatic rings of pentacyclic triterpenes[46], G: εNd values in the clay fraction of Core CM97[22], H: Change of CIA value of the clay fraction in Core CM97[22], I: Change of CIA value of bulk samples in Core MD06-3040[29] and sea level change(blue line) [28]. Yellow and gray shaded areas indicate climate control and human impact, respectively.
指标名称 表达式 参考文献 化学蚀变指数CIA
（Chemical Index of Alteration）Al2O3/(Al2O3+K2O+CaO*+Na2O)×100 [32] 帕克风化指数WIP
（Weathering Index of Parker）(2Na2O/0.35+MgO/0.9+2K2O/0.25+CaO*/0.7)×100 [33] 化学风化指数CIW
（Chemical Index of Weathering）Al2O3/(Al2O3+CaO*+Na2O)×100 [34] 斜长石蚀变指数PIA
（Plagioclase Index of Alteration）(Al2O3–K2O)/(Al2O3–K2O+CaO*+Na2O)×100 [35] αAlE指数 (Al/活泼元素)样品/(Al/活泼元素)UCC [36] 注：表达式中的CaO*均为硅酸盐岩中的CaO含量。 
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