Geochemistry, lithology, and bulk density of Cricket Flat paleosol samples

The high-resolution marine isotope climate record indicates pronounced global cooling during the Langhian (16-13.8 Ma), beginning with the warm middle Miocene climatic optimum and ending with significant Antarctic ice sheet expansion and the transition to "icehouse" conditions. Terrestrial paleoclimate data from this interval is sparse and sometimes conflicting. In particular, there are gaps in the terrestrial record in the Pacific Northwest during the late Langhian and early Serravallian between about 14.5 and 12.5 Ma. New terrestrial paleoclimate data from this time and region could reconcile these conflicting records. Paleosols are particularly useful for reconstructing paleoenvironment because the rate and style of pedogenesis is primarily a function of surface environmental conditions; however, complete and well-preserved paleosols are uncommon. Most soils form in erosive environments that are not preserved, or in environments such as floodplains that accumulate in small increments; the resulting cumulic soils are usually thin, weakly developed, and subject to diagenetic overprinting from subsequent soils. The paleosol at Cricket Flat in northeastern Oregon is an unusually complete and well-preserved paleosol from a middle Miocene volcanic sequence in the Powder River Volcanic Field. An olivine basalt flow buried the paleosol at approximately 13.8 ± 0.6 Ma, based on three 40Ar/39Ar dates on the basalt. We described the Cricket Flat paleosol and used its physical and chemical profile and micromorphology to assess pedogenesis. The Cricket Flat paleosol is an Ultisol-like paleosol, chemically consistent with a high degree of weathering. Temperature and rainfall proxies suggest that Cricket Flat received 1120 ± 180 mm precipitation y-1 and experienced a mean annual temperature of 14.5 ± 2.1 °C during the formation of the paleosol, significantly warmer and wetter than today. This suggests slower cooling after the middle Miocene climatic optimum than is seen in the existing paleosol record.

高分辨率海洋同位素气候记录显示，朗格阶（Langhian，16~13.8 Ma）期间存在显著的全球降温过程，该阶段始于温暖的中新世中期气候最适宜期，结束于南极冰盖大规模扩张并向“冰室（icehouse）”气候状态过渡。此时间段内的陆地古气候数据较为匮乏，且部分研究结果存在冲突。尤为突出的是，约14.5~12.5 Ma的朗格阶晚期至塞拉瓦尔阶（Serravallian）早期，美国太平洋西北部地区的陆地古气候记录存在空白。获取该时期该区域的新型陆地古气候数据，或可调和这些相互矛盾的研究结论。古土壤（paleosol）在古环境重建中具有重要应用价值，因为土壤发生学（pedogenesis）中的成壤速率与模式主要受控于地表环境条件；然而完整且保存完好的古土壤较为罕见。多数土壤形成于易受侵蚀的环境中而难以被保存下来，或是形成于如泛滥平原这类以缓慢堆积为主的环境中，由此形成的堆积型土壤通常较薄、发育程度较弱，且易受到后续土壤的成岩叠加改造（diagenetic overprinting）作用。位于俄勒冈州东北部的克里克特平原（Cricket Flat）古土壤，是保存在鲍德河火山岩田（Powder River Volcanic Field）中新世火山序列中的一套异常完整且保存完好的古土壤。基于对该橄榄玄武岩（olivine basalt）流的3次40Ar/39Ar测年结果，其埋藏该古土壤的时间约为13.8 ± 0.6 Ma。本研究对克里克特平原古土壤进行了系统描述，并通过其物理、化学剖面特征及微形态学（micromorphology）分析，对其成壤过程进行了评估。该古土壤属于类老成土（Ultisol），其化学特征与高度风化的特征相符。温度与降水代用指标显示，古土壤形成时期，克里克特平原的年降水量为1120 ± 180 mm·a⁻¹，年平均气温为14.5 ± 2.1 ℃，显著高于现今的气候条件。这一结果表明，中新世中期气候最适宜期之后的降温过程，比现有古土壤记录所揭示的更为缓慢。