Impact of Grassland Radiation on the Nonmarine Silica Cycle and Miocene Diatomite
Impact of Grassland Radiation on the Nonmarine Silica Cycle and Miocene Diatomite
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DOI：
10.2110/palo.2003.p03-108
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发表时间：
2005-04
期刊：
影响因子：
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通讯作者：
D. L. Kidder;E. Gierlowski‐Kordesch
中科院分区：
文献类型：
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作者：
D. L. Kidder;E. Gierlowski‐Kordesch
摘要早中新世草占主导地位的生态系统的崛起是一个合理的触发器急剧增加中新世非海洋硅藻沉积物的积累，以及多样化的非海洋硅藻。这种草原辐射引入了一种生物地球化学机制，以增强可用二氧化硅和其他营养物质的广泛和持续动员。火山作用可能是负责从晚白垩世到渐新世已知最古老的非海洋硅藻的出现，情节性的非海洋硅藻沉积物。虽然多产的中新世火山活动无疑在许多火山岩的发展中仍然很重要，但火山土壤的草原殖民化的反馈可能解释了为什么在中新世之前的记录更稀疏之后，白垩纪沉积在中新世激增。草为主的生态系统，增加非海洋硅藻积累，早中新世的演化辐射的非海洋硅藻类群的初步崛起，至少近似同时代。虽然已知最早的草是古新世，但多条证据，包括软表土层古土壤，高齿有蹄类食草动物的化石和植硅石化石，表明以草为主的生态系统直到中新世早期才显着扩张。在欧亚大陆、非洲和澳大利亚的部分地区，草原辐射明显延迟到中新世中期。如果这一延迟是真实的，那么这些地区的硅藻/硅藻记录应该与之相吻合。中新世硅藻积累增加的开始日期尚未准确确定，但本文预测与草占主导地位的生态系统的兴起相吻合。早中新世辐环藻和海链藻的多样性与早中新世草原扩张的时间一致。随后在二氧化硅循环中的调整也可能归因于草原。晚中新世和上新世的非海洋硅藻辐射与急剧的回归相吻合，可能释放了中新世土壤和古土壤中储存的营养物质和可溶性植硅体蛋白石，以及土壤孔隙沃茨中溶解的二氧化硅。植硅体的海退侵蚀脉冲为更新世冰期海洋硅藻低Ge/ Si比值提供了新的解释。非海洋硅藻从回归间隔应记录较低的Ge/Si比比之前和之后的回归，因为植硅体的贡献与低Ge/Si比。中新世晚期C4和潮湿高草生态系统的辐射可能比短、干燥气候的早中新世草动员了更多的二氧化硅。即使在草原扩张之后，火山活动的程度也可能随着火山活动程度的变化而波动，但其水平要比这种新的陆地生态系统出现之前高得多。
Abstract The Early Miocene rise of the grass-dominated ecosystem is a plausible trigger for a sharp Miocene increase in accumulation of nonmarine diatomaceous sediment as well as diversification of nonmarine diatoms. This grassland radiation introduced a biogeochemical mechanism for enhancing widespread and sustained mobilization of usable silica and other nutrients. Volcanism was probably responsible for episodic nonmarine diatomaceous sediments from the advent of the oldest known nonmarine diatoms in the Late Cretaceous through the Oligocene. Although prolific Miocene volcanism was undoubtedly still important in the development of many diatomites, feedback from grassland colonization of volcanic soils may explain why diatomaceous sedimentation surged in the Miocene following a more sparse pre-Miocene record. The initial rise of the grass-dominated ecosystem, increased nonmarine diatomite accumulation, and Early Miocene evolutionary radiations of nonmarine diatom taxa are at least approximately coeval. Although the earliest known grass is Paleocene, multiple lines of evidence, including mollic-epipedon paleosols, fossil occurrences of hypsodontic ungulate grazers, and fossil phytoliths, suggest that the grass-dominated ecosystem did not expand significantly until Early Miocene. The grassland radiation apparently was delayed until Middle Miocene in parts of Eurasia, Africa, and Australia. If that delay is real, the diatom/diatomite record in those regions should coincide with it. The onset of increased Miocene diatomite accumulation is as yet imprecisely dated, but coincidence with the rise of the grass-dominated ecosystem is predicted herein. Early Miocene diversifications of Actinocyclus and Thalassiosira diatoms are consistent temporally with grassland expansion where it is Early Miocene. Subsequent adjustments in the silica cycle also may be attributed to grasslands. Nonmarine diatom radiations in the Late Miocene and Pliocene coincide with sharp regressions that may have released nutrients and soluble phytolith opal stored in Miocene soils and paleosols as well as dissolved silica in soil pore waters. Regressional erosive pulses of phytoliths provide a new explanation for low Ge/ Si ratios in marine diatoms during Pleistocene glacial intervals. Nonmarine diatoms from regressive intervals should record lower Ge/Si ratios than before and after those regressions because of phytolith contributions with low Ge/Si ratios. Late Miocene radiations of C4 and moist tall-grass ecosystems may have mobilized even more silica than the short, dry-climate Early Miocene grasses. Abundance of diatomite may have fluctuated in concert with changes in degree of volcanism, even after grassland expansion, but at substantially higher levels than before this new terrestrial ecosystem arose.