Extrinsic Controls and Sea Level

Longer-term extrinsic, but periodic (repeating) processes that play a role in sea level change

Geologists have been investigating sea level change and its impact on the distribution of sedimentary rocks on continents for some time. Scientists, including Dr. J. Tuzo Wilson, explained the relationship between the tectonic opening and closing of ocean basins and periods of high sea level relative to periods of lower sea level (more info) . The cycle of opening and closing of ocean basins is referred to today as the "Wilson Cycle."

Dr. Larry Sloss, in his investigations of rock units of North America in the 1940's to 1960's, began to group rock strata into sedimentary sequences which are now referred to as "Sloss Sequences." Each Sloss Sequence (of which there were 6 original sequences recognized since the Cambrian Period) is recognized on the basis of an unconformities produced by sea level regression and accompanying erosion. During the ensuing transgression, packages of sedimentary rocks are deposited as sea level rises back across the formerly exposed and eroded landscapes.

Since the pioneering work of Wilson and Sloss and many other geologists, global sea level fluctuations have been a significant area of research. Geoscientists who focus on investigating the pattern of sediment deposition relative to sea level change are called sequence stratigraphers and these individuals are often employed by the private sector to investigate, locate, and extract natural resources.

In Figure 4.23 below (graphic produced by Robert A Rohde), you can see two different reconstructed sea level curves for the last 542 million years (since the Cambrian). The figure was produced to highlight two separate research groups and their sea level rise reconstructions based on geologic evidence from the last 542 million years (commonly referred to as the Phanerozoic Eon by geologists). Although the two curves vary in the level of detail, the fact that modern sea level is used as the datum on the graph demonstrates that sea levels of the past 542 million years were rarely at modern levels. In fact, based on these datasets, sea levels were routinely higher than modern sea level and rarely much lower than modern sea level except for during a few geologic periods. During times when sea level has been at or lower than at present, geologists typically refer to Earth's climates as having been in "Ice House" conditions, and when sea levels were above or significantly above modern levels, these episodes are considered to be "Green House" conditions.

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Figure 4.23: Global Sea Level Fluctuations from the last 542 million years. Two different curves are shown and include different methods for assigning sea level. The datum is modern sea level, which is assigned a 0 value on the vertical axis. These datasets show that sea level has been both higher and lower than modern throughout different time intervals. One thing is for certain, sea level is never static and is always changing – either increasing or decreasing.
Credit: https://en.wikipedia.org/wiki/Past_sea_level#/media/File:Phanerozoic_Sea_Level.png

The very fact that these sea level oscillations are so pronounced and even for some time periods fairly regular suggests an additional mechanism may yet be at play when it comes to understanding the driving forces behind sea level change in geologic time.

• Milankovitch Cyclicity & Earth Orbit Variations
• 542 Million Years of Sea Level Change: Exxon's Sea Level Reconstruction
• Sea Level Change During the Last 5 Million Years
• Case Study: Miocene Tectonics, the Gulf Stream, Runaway Global Cooling and Sea Level Change
• Sea Level Rebound After Cooling?
• Earth Orbit, Solar Insolation Variability & Sea Level Change
• Climate Proxy Datasets: How Do We Know How Sea Levels Have Changed?
• Learning Check Point
• Solar Luminosity Cycles
• Case Study: 11,000 Years of Sea Level Change
• Holocene Sea Level Curves: A Closer Look