Download Ocean Depth through Deep Time

Comparative Tectonics and Geodynamics (2015)
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Ocean Depth through Deep Time
S. Sim1 , D.R. Stegman1 and N. Coltice2
1. Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla,
California 92093, USA
2. Laboratoire de Gologie de Lyon, Universit Lyon 1, Ecole Normale Suprieure de Lyon, Universit de Lyon, 69007
Lyon, France
The Earth’s oceans have played an important role in the evolution of life and tectonics on Earth, and yet our
understanding of basic connections between these remains limited. One of the central, and still unanswered
questions, is whether Earth’s oceans have been present over all of Earth’s history, and how deep were any
oceans that may have existed. Global tectonics provides a large influence on the long term fluctuations in
sea level through varying the volume of ocean basins. The volume of ocean basins over time can be estimated
from the seafloor age distribution as observed in plate reconstructions, which gives the proportion of younger,
elevated seafloor to older, subsided seafloor.
Mller et al. (2008) reported that 200m of sea level variation over the past 140 Myr may be associated with
changes in global tectonics and associated seafloor age distributions, while also accounting for other major
contributions such as the volume of ice sheets, the presence of large igneous provinces on the seafloor and
thickness of sediments on the seafloor. Plate reconstructions show periods during the Cretaceous with a
larger number mid-ocean ridges and correspondingly larger areal extent of younger, elevated seafloor that
produce shallower ocean basins and an associated rise in sea level (Mller et al., 2008). We reproduce their
results by constructing the plate age-area histograms (areal extent of seafloor with a particular age vs age)
for oceanic crust from the dataset for each million year. From the plate age-area distributions, we calculate
the depth-area distribution of oceanic crust using plate cooling models and thus the volume of water that
can be accommodated by the ocean basins. Assuming that the oceanic volume of water has not changed,
we can calculate the sea level from the assumed total volume of oceanic water and the amount of water that
can be accommodated by oceanic basins. As a result, we establish a relationship between sea level and the
age-area distribution of oceanic plates.
We then extend this methodology back into earlier times during Earth’s history by using synthetic plate
reconstructions derived from numerical models of mantle convection in 3D spherical geometry. To approximate conditions for earlier in Earth’s history, we consider that both the Rayleigh number would have been
higher in the past, resulting in faster surface velocities and, on-average, younger seafloor. Thus, we vary
the surface velocity from the modern day value of 4cm/yr to what is predicted for early Earth conditions
of 80cm/yr (corresponding to Rayleigh number of 108 to 1010 , respectively). Coltice et al. (2014) showed
that the shape of seafloor age distribution is influenced by the growth of continental area over time, with
an increasingly younger-aged, triangular shaped distribution favored for increasing continental surface. We
also vary the amount of continents on Earth between 0, 10%, and 30% continents.
These models are based upon those previously done by Coltice et al. (2012, 2014). Further details about the
model are provided in supplementary materials of Coltice et al (2012). The difference between the model in
this study and the ones done in previous studies is that these include 30% basal heating, a viscosity jump
of 30 between upper and lower mantle, and more than 6 orders of magnitude of viscosity variations with
temperature. In order to generate enough statistics to indicate the natural variability of the system, we
perform 5 billion year long time-integrations generating hundreds of age-area distributions for each of the
scenarios we consider. Using those, we can calculate the sea level associated using similar principals.
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