News Release 

A new record of deglaciations in last million years shows persistent role of obliquity pacing

American Association for the Advancement of Science

Over the last million years, small variations in Earth's orbit continued to trigger and terminate global glaciations, throughout and after the Mid-Pleistocene Transition, according to a new study, which presents a novel high-resolution record of the last 11 deglaciations. Beyond what was possible using existing less well-dated environmental records, the new precisely dated chronology reveals the persistent influence of obliquity and insolation in pacing Earth's glacial-interglacial cycle. The Quaternary - the current period of Earth's history that began a little more than 2.5 million years ago (Ma) - is often characterized by a series of glacial and interglacial periods, which repeatedly set continent-sized ice sheets to ebb and flow like a frozen tide across much of the Northern Hemisphere. Before the Mid-Pleistocene Transition (MPT, 1.25 to 0.7 Ma), Quaternary global glacial cycles repeated roughly every 40 thousand years (ka). However, during the MPT, the pattern of Quaternary glaciation fundamentally shifted and expanded to approximately 100 ka intervals. While it is widely agreed that the 40 ka pre-MPT cycles were driven by the cyclical variation in Earth's orbital tilt or obliquity, orbital forcing theories fail to explain the longer post-MPT glacial-interglacial periods adequately. A central challenge in evaluating orbital theories of Earth's ice age cycles is the inherent chronological uncertainty of the deep-ocean sediment records often used to identify them. Recent studies using precisely dated speleothem records to anchor the ages of glacial terminations recorded in marine sediments as far back about 640 ka, however, suggest that post-MPT intervals are instead collections of shorter, orbitally driven cycles. Using a uranium-lead radiometrically dated Italian speleothem from Italy to constrain the chronology of new high-resolution marine sediment records, Petra Bajo and colleagues expand this record to cover the eleven glaciation-deglaciation events of the last million years. The authors show that the first two post-MPT deglaciation events are separated by two obliquity cycles and conclude that Earth's obliquity remained the main driver of glacial-interglacial cycles throughout the Quaternary - across the MPT and beyond.

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