Comparison and synthesis of sea-level and deep-sea temperature variations over the past 40 million years

Abstract

Global ice volume (sea level) and deep-sea temperature are key measures of Earth's climatic state. We synthesize evidence for multi-centennial to millennial ice-volume and deep-sea temperature variations over the past 40 million years, which encompass the early glaciation of Antarctica at ˜34 million years ago (Ma), the end of the Middle Miocene Climate Optimum, and the descent into the bipolar glaciation state from ˜3.4 Ma. We compare different sea-level and deep-water temperature reconstructions that are grounded in data to build a resource for validation of long-term numerical model-based approaches. We present: (a) a new ice-volume and deep-sea temperature synthesis for the past 5.3 million years; (b) a single template reconstruction of ice-volume and deep-sea temperature for the interval between 5.3 and 40 Ma; and (c) a discussion of uncertainties and limitations. We highlight key issues associated with glacial state changes in the geological record from 40 Ma to the present that require specific attention in further research. These include offsets between calibration-sensitive versus more thermodynamically guided deep-sea paleothermometry proxy measurements; a conundrum related to the magnitudes of sea-level and deep-sea temperature change at the Eocene-Oligocene transition at 34 Ma; a discrepancy in deep-sea temperature levels during the Middle Miocene between proxy reconstructions and model-based deconvolutions of deep-sea oxygen isotope data; and a hitherto unquantified non-linear reduction of glacial deep-sea temperatures through the past 3.4 million years toward a near-freezing deep-sea temperature asymptote, while sea level stepped down in a more linear manner.