Expansion of C4 grasses in the Late Miocene of Northern Pakistan: evidence from stable isotopes in paleosols
Expansion of C4 grasses in the Late Miocene of Northern Pakistan: evidence from stable isotopes in paleosols
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DOI：
10.1016/0031-0182(94)00108-k
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发表时间：
1995-05
期刊：
影响因子：
--
通讯作者：
J. Quade;T. Cerling
中科院分区：
文献类型：
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作者：
J. Quade;T. Cerling
对巴基斯坦北部新近纪 Siwalik 剖面的古土壤进行了稳定同位素、粘土矿物学和大量化学分析，以重建过去约 17 Ma 的洪泛区环境。代表 17− ∼ 7.3 Ma 的古土壤中土壤碳酸盐的稳定碳同位素组成（平均 δ13C (PDB) = -10.2%）和相关有机质（平均 δ13C (PDB) = -24.1%）表明，洪泛区植被以 C3 植物为主。 7.3 Ma 时，碳同位素值开始转向正值，这标志着 C4 草逐渐扩展到洪泛区。从6 Ma至今，古土壤碳酸盐（平均δ13C（PDB）=+0.6%）和有机质（平均δ13C（PDB）=-14.4%）的碳同位素值均匀富集于13C，表明存在近乎纯的C4草地。大多数古土壤中高岭石的稀缺性和蒙脱石和成土碳酸盐的丰富性表明，在整个 17 Ma 的记录中，该地区的降雨量保持在 1.0-1.25 m/年或更少。该剖面下部的古土壤缺乏有机 A 层位，但具有淡红色 B 层位，通常含有次生氧化铁结核。这些较古老的古土壤中土壤碳酸盐的淋滤深度通常大于该剖面的上更新世部分，其中有机 A 层很常见，而 B 层明显更黄。综合证据表明，记录的 7.3 Ma 之前部分的成熟古土壤主要是钙质 Alfisols 或 Mollisols，它们曾经位于几乎纯 C3 植被（可能是树木和灌木）的下面，而 C4 草地下面的钙质 Mollisols 主导了记录的上部。巴基斯坦古土壤记录中的碳和氧同位素趋势在哺乳动物化石的饮食和尼泊尔的古土壤中也很明显，从而表明洪泛区植被的这些古环境变化可能是整个大陆的。局部影响，例如青藏高原抬升驱动的亚洲季风的发展或加强，可能导致了 C4 草的扩张。然而，如果 C4 草的扩张被证明是全球同步的，那么更大范围的原因，例如 ϱCO2 的显着减少，可能是驱动机制。
Stable-isotopic, clay-mineralogic, and bulk-chemical analyses were conducted on paleosols of the Neogene Siwalik sections in northern Pakistan in order to reconstruct floodplain environments over the past ∼ 17 Ma. The stable carbon isotopic composition of soil carbonate (mean δ13C (PDB) = -10.2%) and associated organic matter (mean δ13C (PDB) = −24.1%) in paleosols representing 17− ∼ 7.3 Ma reveal that floodplain vegetation was dominated by C3plants. At 7.3 Ma, a shift toward more positive carbon isotopic values began, signaling the gradual expansion of C4grasses onto the floodplain. From 6 Ma to present, carbon isotopic values for paleosol carbonate (mean δ13C (PDB) = +0.6%) and organic matter (mean δ13C (PDB) = −14.4%) are uniformly enriched in13C, indicating the presence of nearly pure C4grassland. The scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1.0–1.25 m/yr or less for the entire 17 Ma of record. Paleosols in the lower portion of the section lack organic A horizons but have reddish B horizons often containing secondary iron-oxide nodules. Leaching depths of soil carbonate in these older paleosols are typically greater than those in the Plio-Pleistocene part of the section, where organic A horizons are common, and B horizons are markedly more yellow. The combined evidence suggests that the mature paleosols in the pre-7.3 Ma part of the record are dominantly calcareous Alfisols or Mollisols that once underlay nearly pure C3vegetation, perhaps trees and shrubs, while calcareous Mollisols underlying C4grassland dominate the upper part of the record. The carbon- and oxygen-isotopic trends in the paleosol record in Pakistan are also evident in the diet of fossil mammals, and in paleosols from Nepal, thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent-wide. Local effects, such as the development or intensification of the Asian Monsoon driven by uplift of the Tibetan Plateau, may have led to the expansion of C4grasses. If, however, the expansion of C4grasses proves globally synchronous, then a larger scale cause, such as a marked decrease in ϱCO2, may be the driving mechanism.