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.