| 摘要: |
| 古土壤是重建古气候的重要手段，第四纪以前古土壤因受各种地质作用影响不易被识别。本研究以福建西部连城盆地晚白垩世红层——冠豸山剖面为研究对象，通过野外观察、土壤微结构和地球化学元素迁移等方法识别和获得古土壤发育特征，并分析古土壤形成环境。冠豸山剖面共有10个薄层古土壤，主要为新成土和始成土两种类型，土壤发育较弱，没有明显的黏化层和钙结核层。沉积物颗粒和稀土元素分布特征显示剖面中含有一定的风积组分，风积物经短暂流水搬运再次沉积，形成沉积层理。红色碎屑颗粒在沉积之前已经历相当程度的风化，沉积后在干旱环境下成壤较弱，因此，对其古气候或古环境的认识需依据古土壤发育特征，这为研究类似红层提供了新的思路。 |
| 关键词: 古土壤 白垩纪 地球化学 红层 福建 |
| DOI：10.7515/JEE202009 |
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| 基金项目:国家自然科学基金项目（41602184）；福建省自然科学基金项目（201701655）；福建师范大学创新团队项目（IRTL1705） |
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| Late Cretaceous paleosols in Fujian and their paleoenvironmental implication |
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MAO Xuegang, CHEN Jinniu, SHI Yonghui, LIU Xiuming
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1. Institute of Geography, Fujian Normal University, Fuzhou 350007, China
2. State Key Laboratory of Subtropical Mountain Ecology (Funded by Ministry of Science and Technology and Fujian Province), Fujian Normal University, Fuzhou 350007, China
3. School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China
4. Department of Environment and Geography, Macquarie University, Sydney NSW 2109
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| Abstract: |
| Background, aim, and scope Paleosol, the soil formed and preserved in geological periods, is the product of the climate, parent sediment or rocks, time, and vegetation when it formed, of which the climate is the most important driving force. Consequently, paleosol plays an important role in paleoclimate reconstruction, e.g. the successful application of loess-paleosol in Chinese Loess Plateau. Pre-Quaternary paleosols are not easy to be identified due to effects of post diagenesis. Soil horizons are susceptible to be mistaken for sedimentary beddings, which hinders our understanding in their paleoclimatic implication. The present study focuses on Guanzhaishan (GZS) section of Late Cretaceous in western Fujian. It is aimed to identify paleosols in this section combined by field observation, soil micromorphology and geochemistry analysis, and discuss the paleoenvironments when the paleosols formed. Materials and methods GZS section (25.704952°N, 116.766931°E) is located 2 km east of Liancheng County, Longyan City, western Fujian. GZS section is 15.8 m thick, and composed of interbedded red siltstones and sandstones, belonging to the upper part of Shaxian Formation of Late Cretaceous in age. 66 samples (G1—G66) were collected with 10—40 cm increment. A few intact samples were collected for thin sections. Major element concentrations were measured by Epsilon 3 XLE. Rare earth elements (REE) were measured by ICP-MS. Thin sections were made manually with paraffin instead of water to avoid clay expansion. Results Paleosols were identified in field based on root traces, soil horizon and soil structures. Root traces were most companied with animal burrows. Slickensides indicated soil formation due to shrinkage and expansion of clay. 10 paleosols of two different pedotypes (Inceptisol and Entisol) were identified in GZS section. Soil micromorphology and geochemical transport confirmed the paleosols, also indicate that all the paleosols were weakly developed. Discussion The particle grain size, distribution and shape of the red sediment were similar with aeolian dusts. REE distribution was similar with loess-paleosol from Chinese Loess Plateau and subtropical areas, indicating thoroughly mixed. The aeolian dust deposited or transported by temporary water, rather than typical fluvial or lacustrine sediment. Geochemical analysis suggested that the sediment had undergone to an extent chemical weathering (CIA= 80 on average) before deposition. After deposition, the arid climate prevented weathering thus resulted in weakly developed paleosols. Therefore, paleoclimate reconstruction of such red sediment could not rely on sediment geochemistry, but paleosol features. Conclusions 10 paleosol sequences of 2 different pedotypes (Inceptisol and Entisol) were identified, which were weakly developed lacking Bt and Bk horizons and indicating arid condition. The red sediment included aeolian dusts, which were transported and deposited by temporary water, forming sedimentary beddings. The inconsistence between weak paleosol and high weathering intensity suggested that paleoclimate and paleoenvironment reconstruction should rely on paleosol features. Recommendations and perspectives There is long lasting debate on whether red beds indicate paleoclimate and what climate. Paleosols generally occurring in the red beds likely provide an alternative perspective, i.e. paleosol is important archive of paleoclimate. The present study demonstrates a good case to identify paleosols in red beds, and what paleoenvironments were indicated by these paleosols. Paleosols should be emphasized in studying paleoclimate and paleoenvironment. |
| Key words: paleosol Cretaceous geochemistry red beds Fujian |
