Question

Deep-sea sediment cores from the North Atlantic provide a high-resolution record of the mid-Pleistocene transition (MPT), a period between 1.2 million and 700,000 years ago when Earth's climate oscillations shifted from a 41,000-year periodicity to 100,000-year cycles. For decades, paleoclimatologists attributed this shift primarily to subtle changes in Earth's orbital eccentricity. However, recent isotopic analyses of benthic foraminifera reveal that global ice volume expanded significantly during the MPT without a corresponding alteration in orbital forcing parameters. This discrepancy has led researchers to investigate internal feedback mechanisms, particularly the drawdown of atmospheric carbon dioxide via enhanced deep-ocean carbon sequestration. Sediment evidence shows marked increases in nutrient utilization in the Subantarctic Ocean contemporaneous with glacial periods post-MPT, suggesting that iron fertilization by windblown dust stimulated phytoplankton blooms, thereby drawing carbon into the deep sea. Critics of the iron hypothesis note that dust flux proxies in Antarctic ice cores demonstrate substantial variability across glacial cycles, arguing that phytoplankton productivity alone cannot account for the magnitude of atmospheric $CO_2$ reduction. Nonetheless, coupled climate-biogeochemical models indicate that even modest variations in ocean stratification, combined with iron inputs, could produce non-linear cooling thresholds capable of stabilizing the 100,000-year glacial cycle without external orbital drivers.

Consider each of the choices separately and select all that apply.

Based on the passage, which of the following can be inferred regarding climate mechanisms during the mid-Pleistocene transition?