Analysis of the Distribution of Land and Oceans

Post date: May 01, 2011 9:13:0 PM

Here we present a quick analysis of the distribution of landmasses from the data generated as part of our Visible Paleo-Earth project. Previous post (here and here) showed the analysis done to a few periods. Now we have all 30 periods to compare (all individual analysis attached as a single ZIP file at the end). We plotted the distribution of land and ocean areas from 750 Mya to today, including North to South, and East to West ratios (Figure 1). The analysis shows an increasing small trend in the land to ocean ratio during this period. Suggesting that our planet has been steadily loosing oceanic water in the last 750 million years, among other explanations. There are associated variation due to movement of landmasses and glacial periods, but this trend seems specially true after 500 Mya.

Water can escape from our planet in various ways. Photolysis in the upper atmosphere dissociates water into hydrogen and oxygen, the hydrogen escapes to space and the oxygen usually combines with the crust creating oxides or forming carbon dioxide. Water also escapes by thermal processes. Some water is replenished from cometary impacts or from geological activity. Also impacts contribute to atmospheric and oceanic erosion (Chyba, 1990). The protection of Earth's magnetic field also complicate matters. All these factors combine to a net loss of water to space at a very small rate. This rate changed through time due to variations in solar luminosity, terrestrial heat flow, impact rates, crust size, and carbon reservoirs.

We fitted a straight line to the land to ocean ratio trend to get a sense of this potential oceanic loss (Figure 2). Assuming the same trend before 750 Mya, the fit suggest that landmasses started to appear about 1,900 Mya. However, we know that continents grew rapidly between 3,000 and 2,700 Mya (Condie, 1986), and there is even evidence of continental crust as old as 4,400 Mya (Wilde et al., 2001). We can explain this by assuming that the loss rate was slower in the past, probably due to the lower solar luminosity. The sea-leve curve (Hallam, 1984) was included in a plot of the land to ocean ratio for comparison (Figure 3).

If we assume the same loss rate in the future, our planet will have the same land and ocean area by 3,000 million years from today (now 29% land, 71% ocean). However, solar luminosity will increase due to helium fusion in the Sun and it is expected that Earth will loss its oceans much earlier, between 1,000 to 2,000 million years from now (Sackmann et al., 1993).

Figure 1. Percent of land and ocean areas of Earth in the last 750 million years. Blue color for the ocean, dark brown for northern land areas, and light brown for southern land areas. The black line shows the distribution of land areas along the East to West direction, showing that the supercontinents Rodinia (750Mya) and Pangea (240Mya) were almost occupying one side of the planet, during their existence. There were 10 to 30% of land areas during all the periods. 

Figure 2. Ratio of land and ocean areas in the last 750 million years. There is a small increasing trend in the land to ocean ratio probably associated with the natural loss of water to space by various factors. This is about 1% increase in the ratio every 20 million years. At 220 Mya occurred a transition from a south dominated land to a north dominated land planet.

Figure 3. Comparison of the land to ocean ratio with sea-level. As expected, there appears to be an anti-correlation between both, as see level decreases more land area is exposed. Sea-level data from Hallam, 1984.

References

• Chyba, C. F. (1990). Impact delivery and erosion of planetary oceans in the early inner solar system. Nature 343, 129-133.

• Condie, K. C. (1986). Origin and early growth rate of continents. Precambrian Research, 32, 4, 261--278.

• Hallam, A. (1984). Pre-Quaternary sea-level changes. Annual Review of Earth and Planetary Sciences, 12, 205.

• Sackmann, I. J., Boothroyd, A. I., and Kraemer, K. E. (1993). Our sun. III. Present and future. The Astrophysical Journal, 418, 457.

• Wilde, S. A., Valley, J. W., Peck, W. H., and Graham, C. M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature, 409, 6817, 175-178.