The prevailing consensus among paleoclimatologists traditionally attributed Pleistocene glacial-interglacial transitions exclusively to orbital forcing—cyclical variations in Earth's orbit known as Milankovitch cycles. However, recent high-resolution isotopic analysis of benthic foraminifera indicates that orbital forcing alone is insufficient to explain the magnitude and characteristics of the Mid-Pleistocene Transition (MPT). During the MPT, the dominant periodicity of glacial cycles shifted from 41,000 years to approximately 100,000 years. Some researchers propose that long-term endogenous climate feedbacks, specifically involving subduction-zone carbon sequestration, amplified the external orbital signal. Yet, this carbon-feedback hypothesis struggles to account for the abruptness of the MPT.
An alternative model posits that the progressive removal of regolith—weathered, unconsolidated bedrock—by repeated early Pleistocene glaciations ultimately altered ice-sheet dynamics. According to this model, the exposure of underlying high-friction crystalline bedrock increased basal friction, which allowed for the formation of thicker, more stable ice sheets. These massive ice sheets took significantly longer to accumulate and subsequently melt, thereby shifting the glacial periodicity to 100,000 years without necessitating any concurrent change in external orbital parameters or atmospheric forcing.
Based on the passage, which of the following statements can be logically inferred? Select all that apply.
- The transition to a 100,000-year glacial cycle during the MPT did not inherently require a corresponding shift in the cyclical variations of Earth's orbit.Answer
- BHigh-resolution isotopic analysis of benthic foraminifera demonstrates that Milankovitch cycles had virtually no measurable effect on glacial periodicity during the MPT.
- Prior to the MPT, Pleistocene ice sheets likely rested on terrestrial surfaces that provided less resistance to ice flow than did the surfaces underlying post-MPT ice sheets.Answer
- DThe 100,000-year glacial cycles observed after the MPT correspond to variations in the eccentricity of Earth's orbit, whereas the earlier 41,000-year cycles correspond to variations in Earth's axial tilt.
- EThe regolith-removal model is currently accepted by a majority of paleoclimatologists as a more comprehensive explanation for the MPT than the endogenous carbon-feedback hypothesis.