Late Miocene paleoecology and paleoclimate in the central High Plains of North America reconstructed from paleopedological, ichnological, and stable isotope analyses of the Ogallala Formation in western Kansas, USA

Brian F. Platt , Jon J. Smith
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Abstract

The late Miocene was a critical time in the development of the North American Great Plains marked by paleoclimate-driven biotic change, including faunal turnovers and the spread of C4 dominated grasslands. The large volume of sediment shed from the Rocky Mountain region during this time preserves a record of these transitions, which can be informed by previously undescribed paleosol and trace fossil properties from the Ogallala Formation of the central High Plains. The purpose of this paper is to reconstruct paleoenvironments, paleoclimate, and paleoecology from outcrops of the Ogallala Formation in western Kansas by integrating paleopedology, ichnology, and stable isotope geochemistry. Eleven lithofacies are recognized in the study area, mostly massive to crudely stratified, fine-to coarse-grained sandstone and pebbly gravel with stratigraphically uncommon fine-grained lithofacies restricted to thinly bedded intervals or lens-shaped geometries within the sand-dominated strata. These host five pedotypes: 1) calcic Vertisols developed on overbank fines, 2) Entisols developed on braid bar gravels, 3) Entisols developed on volcaniclastic sediment lenses, 4) calcic Inceptisols developed on coarse sandy channel fills, and 5) calcareous Mollisols developed on fine sandy bedforms and channel fills. We recognize ten ichnogenera within paleosols, including burrows attributed to ants, bees, beetles or hemipterans, vinegarroon-like arthropods, fossorial reptiles, seed caching mammals, and large carnivorous mammals. Organic carbon stable isotopes indicate that the flora consisted entirely of C3 plants, and paleosols and trace-fossil evidence suggest a tree-limited savanna environment with patches of unvegetated soil. Paleoclimate proxies from paleosol and trace fossil properties indicate mean annual air temperatures between 8 °C and 20 °C with seasonal differences of up to 14 °C between mean monthly temperatures of the warmest and coolest months. Mean annual precipitation was likely between 250 mm and 460 mm with a seasonal difference of up to 250 mm between mean monthly precipitation of the driest and wettest months. While hymenopteran tracemakers were active, soil surface temperatures reached at least 30 °C and moisture content was between 10% and 20%. With timing of deposition constrained biostratigraphically to the Barstovian through Hemphillian, our paleoclimate interpretations are most consistent with the period of relative climate stability after Middle Miocene Climate Transition cooling and before Late Miocene Cooling (∼13.8 Ma–7 Ma). The trace-fossil assemblage reveals previously unknown biodiversity among soil arthropods, as well as important trophic connections between the belowground and aboveground components of the terrestrial food web.

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利用美国堪萨斯州西部奥加拉拉组的古生物学、技术和稳定同位素分析重建北美中部高原区晚中新世古生态和古气候
中新世晚期是北美大平原发展的关键时期,标志着古气候驱动的生物变化,包括动物更替和C4主导的草原的扩张。在此期间,从落基山脉地区流出的大量沉积物保存了这些转变的记录,这可以通过先前描述的古土壤和来自中央高平原奥加拉拉组的痕量化石属性来了解。本文旨在综合运用古地质学、技术和稳定同位素地球化学等方法,对美国堪萨斯州西部奥加拉拉组露头进行古环境、古气候和古生态重建。研究区共识别出11种岩相,多为块状至粗层状、细粒至粗粒砂岩和砾质砾石相,而地层罕见的细粒岩相则局限于砂质地层内的薄层状或透镜状地层。这五种类型的土壤类型分别是:1)在河岸细砂上发育的钙质竖砂,2)在辫状坝砾石上发育的体砂,3)在火山碎屑沉积透镜体上发育的体砂,4)在粗砂质河道填充物上发育的钙质初砂,以及5)在细砂质河床和河道填充物上发育的钙质软泥。我们在古土壤中发现了10个昆虫属,包括蚂蚁、蜜蜂、甲虫或半足类动物的洞穴,醋类节肢动物,化石爬行动物,种子贮藏哺乳动物和大型食肉哺乳动物。有机碳稳定同位素表明,植物区系完全由C3植物组成,古土壤和痕迹化石证据表明,这是一个树木有限的稀树草原环境,土壤中有斑块。古土壤和化石特征的古气候代用物表明,年平均气温在8°C至20°C之间,最暖月和最冷月的月平均气温之间的季节差异高达14°C。年平均降水量可能在250毫米至460毫米之间,最干燥月和最潮湿月的月平均降水量之间的季节差异可达250毫米。当膜翅目跟踪器活跃时,土壤表面温度至少达到30°C,含水量在10%至20%之间。生物地层将沉积时间限制在巴斯托世至亨菲利世,我们的古气候解释与中中新世气候过渡冷却后和晚中新世冷却之前(~ 13.8 Ma - 7 Ma)的相对气候稳定时期最为一致。这一化石组合揭示了土壤节肢动物中未知的生物多样性,以及陆地食物网地下和地上组成部分之间重要的营养联系。
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