The effects of continental growth on global sea level

Abstract:

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.

First we establish a relationship between sea level and the age-area distribution of oceanic crust using reconstructed oceanic plate age for recent 140 Myr from Müller et al. (2008), 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.

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 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 4 cm/yr to what is predicted for early Earth conditions of 80 cm/yr (corresponding to Rayleigh number of 10^8 to 10^10, 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 vary the amount of continents on Earth from 0, 10%, to 30% of surface area of the Earth. These newer models 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. Using those, we can calculate the mid-oceanic ridge depth associated using similar principals as for the 140 Myr plate reconstruction models.