A systematic study of floodplains, terraces, and alluvial fans in the Republican River valley of south-central Nebraska provided a well-dated, detailed reconstruction of late Quaternary landscape evolution and resolved outstanding issues related to previously proposed Holocene terrace sequences. Stable carbon isotope (δ13C) values determined on soil organic matter from buried soils in alluvial landforms were used to reconstruct the structure of vegetation communities and provided a means to investigate the relationships between bioclimatic change and fluvial activity for the period of record. Our study serves as a model for geomorphological and geoarcheological investigations in stream valleys throughout the central Great Plains and wherever loess-derived late Quaternary alluvial fans occur, in particular.Holocene alluvial landforms in the river valley include a broad floodplain complex (T-0a, T-0b, and T-0c), a single alluvial terrace (T-1), and alluvial fans that mostly grade to the T-1 (AF-1) and T-0c (AF-0c) surfaces. Remnants of a late Pleistocene terrace (T-2), mantled by Holocene (Bignell) loess, are also preserved, and some Holocene alluvial fans (AF-2) grade to T-2 surfaces.Radiocarbon ages suggest that the T-1 fill and AF-1 fans aggraded between ca. 9000–1000 yr B.P. Hence, nearly all of the Holocene alluvium in the river valley is stored in these landforms. Sedimentation, however, was interrupted by several periods of landscape stability and soil formation. Radiocarbon ages from the upper A horizons of buried soils in the T-1 and AF-1 fills, indicating approximate burial ages, cluster at ca. 6500, 4500, 3500, and 1000 yr B.P. Also, based on the radiocarbon ages, the T-0c fill and AF-0c fans were aggrading between ca. 2000–900 yr B.P. Given that the T-0c fill and upper parts of the T-1 fill were both aggrading after ca. 2000 yr B.P., we suggest that the T-1 surface was abandoned between ca. 4500–3500 yr B.P., but subsequent aggradation of both the T-1 and T-0c fills occurred due to large-magnitude flood events during the late Holocene.The δ13C data indicate a shift from ∼40% C4 biomass at ca. 6000 to ∼85% at ca. 4500 yr B.P. We propose a scenario where (1) a reduction in C3 vegetation after 6000 yr B.P. destabilized the uplands, resulting in an increase in sediment supply and aggradation of the T-1 fill and AF-1 fans, and (2) the establishment of C4 vegetation by ca. 4500 yr B.P. stabilized the uplands, resulting in a reduction in sediment supply and subsequent incision and abandonment of the T-1 and most AF-1 surfaces. The proposed timing and nature of landscape and bioclimatic change are consistent with regional records from the central Great Plains.

中文翻译：

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美国大平原中部晚期第四纪景观演化与生物气候变化

对内布拉斯加州中南部共和党河谷的漫滩，阶地和冲积扇进行的系统研究提供了晚第四纪景观演化的详尽而详尽的重建，并解决了与先前提出的全新世阶地序列有关的突出问题。利用冲积地貌中埋藏的土壤中的有机质确定的稳定碳同位素（δ13C）值可用于重建植被群落的结构，并为研究记录期内生物气候变化与河流活动之间的关系提供一种手段。我们的研究可作为整个大平原中部溪谷以及特别是黄土源的第四纪冲积扇发生的地方的地貌和地质考古研究的模型。河谷中全新世的冲积地貌包括一个宽泛的洪泛区综合体（T-0a，T-0b和T-0c），一个冲积阶地（T-1）和大部分冲积为T-1（AF）的冲积扇。 -1）和T-0c（AF-0c）表面。还保留了由全新世（Bignell）黄土覆盖的晚更新世阶地（T-2）的残余物，并且一些全新世冲积扇（AF-2）评级为T-2表面。和AF-1风扇在大约 9000–1000年BP因此，河谷中几乎所有全新世冲积物都存储在这些地貌中。然而，沉积物被景观稳定和土壤形成的多个时期打断了。在T-1和AF-1填充物中，埋入土壤A层上部的放射性碳年龄，表明大约是埋葬年龄，聚集在ca. 6500、4500、3500和1000年BP 根据放射性碳的年龄，T-0c的填充量和AF-0c的风扇在大约20到20分钟之间逐渐聚集。2000–900年BP鉴于T-0c填充物和T-1填充物的上部在约2,000毫米后都在凝结。2000年BP，我们建议T-1表面在约2年之间被遗弃。4500-3500年BP，但由于全新世晚期发生大水灾事件，T-1和T-0c填充物随后发生了凝结。δ13C数据表明，大约在40％时C4生物量发生了变化。约在6000至〜85％4500年BP我们提出了一种方案：（1）6000年BP破坏了高地后C3植被减少，导致沉积物供应增加，T-1填充物和AF-1风机凝集，（2）约C4植被的建立。4500 BP稳定了高地，导致沉积物供应减少，随后T-1和大多数AF-1表面被切开和废弃。拟议的景观和生物气候变化的时间和性质与大平原中部地区的记录一致。