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 bipolar glaciation
from ~3.4 Ma. We compare different sea-level and
deep-water temperature reconstructions to build a resource for
validating long-term numerical model-based approaches. We present: (a) a
new template synthesis of ice-volume and deep-sea temperature variations
for the past 5.3 million years; (b) an extended template 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 present that require
attention in further research. These include offsets between
calibration-sensitive versus 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; 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 uniform
manner. Uncertainties in proxy-based reconstructions hinder further
distinction of “reality” among reconstructions. It seems more
promising to further narrow this using three-dimensional ice-sheet
models with realistic ice-climate-ocean-topography-lithosphere coupling,
as computational capacities improve.