Geochemical constraints on the regolith hypothesis for the middle Pleistocene transition

摘要

The transition from 41- to 100-kyr glacial cycles and concomitant increase in global ice volume similar to1 Ma remain an enigmatic feature of late Cenozoic climate. Here, we examine the petrology, mineralogy, and geochemistry of the silicate fraction of tills spanning the past 2 Ma from the north-central United States to evaluate the hypothesis that this so-called middle Pleistocene transition (MPT) occurred by erosion of regolith and subsequent exposure of underlying Canadian Shield bedrock by the Laurentide ice sheet. These data indicate that late Pliocene tills are depleted in crystalline lithologies, unstable minerals, and major-element oxides derived from plagioclase and ferromagnesians and are enriched in kaolinite, quartz, iron oxides, TiO2-bearing resistates, and meteoric Be-10. In contrast, early and middle Pleistocene tills show enrichment in crystalline lithologies, stable minerals, and major oxides derived from plagioclase and ferromagnesians and depletion in meteoric Be-10, whereas late Pleistocene tills show major-element concentrations that are most similar to that of fresh shield bedrock. Marine isotope records of Sr, Os, and Hf show significant changes around the MPT that are consistent with the removal of a regolith and the exhumation of fresh silicate bedrock. These results indicate that ice sheets initially expanded on highly weathered bedrock and progressively exhumed a fresher rock source, thereby supporting the hypothesis that a change in the composition of the substrate underlying ice sheets best explains the origin of the MPT. (C) 2004 Elsevier B.V. All rights reserved.