11.2: Sea Level Terminology
When talking about “sea level”, it's important to make sure that we define some important terms:
Water Depth
Water depth is the distance between the sediment surfaces and the water surface. This is an instantaneous measurement that is location specific.
Eustatic Sea Level
Eustatic sea level is the distance between the center of the Earth and the sea surface. Given that the center of the Earth is a universal datum, this allows this value to be measured or calculated globally. You can change global sea level by changing the amount of water in the oceans or changing the amount of water the ocean basins can hold. Drivers of eustatic change include:
- Thermohaline changes – changes in the volume of water in the ocean because of temperature changes. Over a scale of decades, changes in the temperature of the shallow ocean can change eustatic sea level by centimeters to tens of centimeters. Over centuries to millennia, circulation can warm the deep ocean as well and cause several meters of eustatic change.
- Glacial ice volume – when large amounts of glacial ice accumulate on land, sea level falls. When glacial ice on land melts and runs back to the oceans, eustatic sea level rises. Because it's largely contained in the ocean already, the amount of floating sea ice has much less of an effect. Changes in the volume of glacial ice have the potential to cause very large changes in eustatic sea level over relatively short periods of time (many tens of meters over a few millennia).
- Volume of terrestrial aquifers – this is the amount of groundwater stored on land. It has a relatively minor and gradual contribution, something on the order of a few meters over tens of millennia.
- Volume of the ocean basins – Long term tectonic changes can change the amount of water held in the ocean basins. Although it is potentially a very large amount of water, these changes take a very long time (hundreds of meters over tens of millions of years).
Relative Sea Level (RSL)
Relative sea level is the distance between a local datum and the sea surface. It’s a function of tectonism (subsidence or uplift) and eustasy and allows us to track the difference between them through time. You increase relative sea level when the distance between the two increases; you decrease relative sea level when the two get closer together.
Relative sea level (RSL) is a useful concept to consider with regards to transgressions because it determines “ accommodation (A)”, which is the room available for sediment to accumulate. An increase in RSL increases accommodation and gives us the potential to preserve sediment. An overall decrease in RSL gives us less (or removes) accommodation because there is less space for sediment to accumulate. In terms of what causes transgressions and regressions, accommodation is a useful concept, but it alone is not enough to control transgression versus regression. We must also consider sediment supply (S) which is the amount of sediment that is delivered to an area (which is a function of climate, tectonism, etc.).
Given all of that, we can revisit the definitions of transgression and regression that we talked about at the beginning of this section to think about how the balance between sediment supply and accommodation controls transgressions and regressions. In cases where the the sediment supply (S) is greater than accommodation (A), a regression occurs and the shoreline is pushed in a seaward direction (overall shallowing). In cases where the sediment supply is less than accommodation, a transgression occurs and the shoreline migrates in a landward direction (progressive deepening).