<p>The Middle Pleistocene Transition (MPT) has been characterized as the transition in temperature and sea level from low-amplitude, 41-kyr variability to high-amplitude, quasi-100-kyr variability in the absence of any orbital forcing between 1.2 and 0.7 Ma. The regolith hypothesis is one of a class of hypotheses developed to explain the MPT in sea level, which has been largely inferred from d<sup>18</sup>O<sub>benthic</sub> records. Here we use a global array of 130 sea-surface temperature (SST) records based on Mg/Ca, alkenone, and faunal proxies to reconstruct global and regional SST change over the last 4.5 Myr. Average global temperature cooled by ~6.5<sup>o</sup>C since ~3.5 Ma, with the MPT represented by a significant increase in the rate of cooling between ~1.4 and 0.8 Ma, and a change from dominant 41-kyr to dominant quasi-100-kyr frequencies at ~1.2 Ma that are well correlated with CO<sub>2</sub> over the last 800 ka (r<sup>2</sup>=0.6). Temperature terminations after 1.2 Ma correspond to skipped obliquity beats and, for the last 800 ka, large increases in CO<sub>2</sub>. We use our global SST reconstruction to remove the temperature signal from the Ahn17 d<sup>18</sup>O<sub>benthic</sub> stack to derive d<sup>18</sup>O<sub>seawater</sub>. Accounting for the influence of changing temperature on the isotopic composition of ice sheets, we use the d<sup>18</sup>O<sub>seawater</sub> record to reconstruct global sea level for the last 4.5 Myr. These results suggest sea-level minima equivalent to or lower than the LGM sea-level low stand (130 m) throughout the Pleistocene. Since inception of Northern Hemisphere glaciation ~3 Ma, sea level varied linearly with obliquity until ~1.2 Ma, when sea-level began to vary nonlinearly with obliquity, with the largest terminations occurring at the same time as temperature terminations that correspond to increasing obliquity and CO<sub>2</sub>. These results suggest that the MPT is largely a temperature phenomenon likely associated with CO<sub>2</sub>. The regolith hypothesis other hypotheses developed to explain a transition from low- to high-amplitude sea level variability during the MPT are no longer required, with the MPT change in sea-level response to obliquity likely due to modulation by CO<sub>2</sub>.</p>
The role of orbital forcing in the Early Middle Pleistocene Transition
