Mid-Pleistocene paleoceanography during a low-eccentricity climate cycle

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Our climate has always been paced by complex but mathematically determinable changes in Earth's orbit around the Sun. These changes explain a geological record of glacial-interglacial oscillations during the past 2.7 million years, each representing a slightly different orbital configuration. The oscillations are labelled marine isotope stages: 28 are recognized in the past 1 million years. Marine Isotope Stage (MIS) 19, occurring 770,000 years ago, is significant for two reasons. Firstly, the orbital configuration of its warmest interval was almost identical to that of today's interglacial -- both feature low eccentricity orbits. The study of MIS 19 therefore provides a baseline to understand how humans have affected our modern climate and to predict its natural duration. Secondly, the most significant reversal of Earth's magnetic field in the past 2.6 million years occurred within MIS 19. This reversal is the primary guide to the Early-Middle Pleistocene boundary, and both the reversal and the climatic signals associated with MIS 19 allow precise global recognition of this boundary. Five sites have been selected to elucidate MIS 19, two in Japan, and one in the northern North Atlantic, eastern North Atlantic, and Italy. Each site is highly time-resolved and contains numerous paleoclimatic signals. Moreover, the Japanese site is the global reference section for the Early-Middle Pleistocene boundary and the Italian site is potentially its auxiliary stratotype. However, none of the proposed sections has been studied for dinoflagellate cysts even though this largely planktonic microfossil group is used commonly for quantitative paleoenvironmental reconstructions across a wide range of past oceanographic conditions. MIS 19 has a distinctive global climate signature beginning with an interruption of deglaciation, an early rise to full interglacial conditions, the inception of glaciation, and finally three or four warm and cold oscillations marked by rapid shifts in the climate state. Subtle oscillations during full interglacial conditions were driven by low-latitude orbital harmonics. Glacial inception represents a threshold crossed as global ice volume increased, and the subsequent high-amplitude oscillations reflect disruptions to the Atlantic meridional overturning circulation by meltwater at high latitudes, thereby activating the thermal bipolar seesaw. The coherence of signals between Pacific and Atlantic-Mediterranean sites suggests the influence of the Intertropical Convergence Zone, perhaps amplified regionally by low-latitude monsoon dynamics. A refined understanding of MIS 19 and the mechanisms then operating, facilitated by the application of dinoflagellate cysts as a powerful climate proxy, will help assess generally which features of the climate system are sensitive to low-eccentricity orbital configurations. This research will also sharpen MIS 19 as an instrument for locating the Early-Middle Pleistocene boundary globally.