Return to Palaeo-sea-level research Topics
Ellipsoid is a hypothetical smoothed version of the geoid which, in turn, approximates to mean sea level. It assumes that the planet is completely homogeneous with none of the gravitational variations that cause undulations in the geoid. The ellipsoid is a datum for GPS elevation readings (ellipsoidal height). Many GPS devices can, however, calculate and output orthometric height measurements i.e. traditional heights relative to the geoid.
Geoid is an artificial representation of mean sea level (collective local means sea levels, (LMSLs), which extends across the globe, even under the continents where local tide-gauge data is not available. It is an undulating surface generated by mathematical models which take into account the Earth’s gravitational irregularities. It serves as a reference surface, or datum, against which precise topographic elevations can be measured and expressed as orthometric heights. The geoid is not fixed. It varies in response to long-term changes in volume, as well as to redistribution, of ocean water.
Global mean sea level (GMSL) is the average height of the global sea surface. More specifically it is the area-weighted mean of all global sea surface heights measured by satellite altimetry. GMSL is a manifestation of the volume of water in the ocean. It is a smoothed surface which excludes the gravitational and circulatory effects on sea level. Unlike the ellipsoid it is not a fixed datum. It is equivalent to eustatic sea level and is time-dependent. GMSL rises and falls in response to changes in ocean water volume. (NOTE: Past GMSLs should not be quoted relative to present LMSL at a particular location because LMSL varies from place to place whereas GMSL is represented by a smooth surface. Ideally past GMSL should explicitly be quoted “relative to present GMSL”)
Local mean sea level (LMSL), or simply Mean sea level (MSL), is the mean or average position of sea level at mid-tide in a given region or a country. It is generally calculated from a continuous recording of water levels at a tide gauge situated in a sheltered harbour. It has traditionally been adopted as a local datum for meausing topographic heights. Due to irregularities in the Earth’s gravitational field, mean sea level is not a smooth surface. Its distance from the center of the Earth varies from place to place and it is thus has only local applicability. It is also time dependent because long-term sea level and land elevations are constantly changing. Unless otherwise stated, elevation measurements quoted on this website represent the elevation of a feature relative to LMSL.
Local relative sea level (LRSL), or simply relative sea level (RSL), is a palaeo-sea-level height expressed relative to a local mean sea level (LMSL) datum established on the land. This is equivalent to the height above the geoid, known as the orthometric height. LRSL is unique to a given location, and thus strictly speaking is “local”. The vertical position of LRSL is interpreted from a marine imprint known as a sea level indicator (SLI). LRSL includes two components: 1). eustatic sea level at the time the SLI was created and 2). subsequent vertical land motion (VLM). LRSL includes global eustatic effects , GIA effects and tectonic effects.
Sea level indicator (SLI) is a biogenic, sedimentary, erosional or chemical marine feature preserved on the Earth’s topographic surface. By studying a modern analog of an ancient sea level indicator, the approximate relative position of contemporaneous sea level can be ascertained and a palaeo relative sea level (LRSL) can be estimated. The elevation of an SLI is measured in the field using traditional surveying techniques or a GPS receiver. It is expressed relative to a local mean sea level (LMSL) datum as an orthometric height.
Glacio-isostatic adjustment (GIA) is isostatic vertical land motion (VLM) associated with the planet’s deformational, gravitational and rotational response to climate-driven changes in ice-water loads. The pattern of GIA for a particular interglacial will be a function of the earth and ocean response to ice loads applied before, during and after the interglacial. It can be expected to exhibit considerable spatial variability that depends on the location of a site relative to ice margins of both the immediately preceding glacial maximum and of the previous glacial maximum. (Lambeck et al., 2012).
Bermuda Geology 