The concept of “sea level” is a curious beast because the oceans aren’t level.
This irregular “sea level” is also continuously moving.
However, after about nineteen years it is possible to calculate the Local Mean Sea Level for a given location provided everything else remains constant.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out.
Nineteen years is preferred because the Earth, moon and sun’s relative positions repeat almost exactly in the Metonic cycle of 19 years, which is long enough to include the 18.613 year lunar nodal tidal constituent.
Unfortunately, not everything remains constant.
The Earth’s surface rises and falls vertically.
The Earth’s crust stretches and compresses horizontally.
The volume of water entering the Earth’s oceans rises and falls.
The volume of water leaving the Earth’s oceans rises and falls.
The resulting variations in Local Mean Sea Level are measured in millimetres per year.
One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes.
Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds.
Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
The resulting variations in the Global Mean Sea Level are called eustatic changes.
“Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Unfortunately, in the long-term geological timescale, this makes it extremely difficult to estimate ancient global sea levels because every variable that can vary has varied.
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the density of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice.
Over much longer geological timescales, changes in the shape of oceanic basins and in land–sea distribution affect sea level.
Additionally, the problems of estimation are compounded by the difficulties in dating ancient geological formations and the incomplete sampling of all the world’s geological formations.
Needless to say, the “science” of geology has overcome these difficulties to determine the global sea level fluctuations for the last 542 million years.
Based upon this reconstruction of “sea level” the mainstream has constructed a narrative.
The most up-to-date chronology of sea level change during the Phanerozoic shows the following long term trends:
● Gradually rising sea level through the Cambrian
● Relatively stable sea level in the Ordovician, with a large drop associated with the end-
● Relative stability at the lower level during the Silurian
● A gradual fall through the Devonian, continuing through the Mississippian to long-term low at the Mississippian/Pennsylvanian boundary
● A gradual rise until the start of the Permian, followed by a gentle decrease lasting until the Mesozoic.
The mainstream narrative is very keen to relate that “ice ages” reduce sea level.
During the most recent ice age (at its maximum about 20,000 years ago) the world’s sea level was about 130 m lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice, mostly in the Laurentide ice sheet.
Most of this had melted by about 10,000 years ago.
Hundreds of similar glacial cycles have occurred throughout the Earth’s history.
Geologists who study the positions of coastal sediment deposits through time have noted dozens of similar basinward shifts of shorelines associated with a later recovery.
This results in sedimentary cycles which in some cases can be correlated around the world with great confidence.
Unfortunately, there are a few problems with this mainstream narrative.
Firstly, the narrative fails to indicate the source of the Earth’s water and its rate of arrival.
Secondly, the narrative ignores the opening up of the oceans around 200 million years ago.
Thirdly, the narrative fails to mention that [by definition] prior to about 200 million years ago all sea level records must relate to the “sea level” of an endorheic basin.
Endorheic basins are closed drainage basins without an outflow to the ocean.
An endorheic basin is a closed drainage basin that retains water and allows no outflow to other external bodies of water, such as rivers or oceans, but converges instead into lakes or swamps, permanent or seasonal, that equilibrate through evaporation. Such basins may also be referred to as closed or terminal basin or as internal drainage systems.
This is a real problem for the mainstream narrative because the current ocean basins only started opening up about 200 million years ago. And to make matters worse the mainstream has no idea when the current ocean basins became connected to establish a Global Sea Level.
The mainstream “sea level” narrative is deeply flawed but the mainstream “sea level” data uncannily [and very informatively] matches the mainstream “seafloor spreading” pattern.
The first logical conclusion [which the mainstream fails to acknowledge] is that rapid “seafloor spreading” has been associated with the rapid “outgassing of water” from the Earth so that the oceans now cover about 71% of the Earth’s surface to an average depth of 3,682 metres.
The second logical conclusion [which the mainstream fails to acknowledge] is that global “sea level” has fallen by about 250 metres during the last 100 million years because the Earth is still [very slowly] inflating and [very slowly] losing water into “space” via the photodissociation of water into oxygen and hydrogen. The heavier oxygen stays in the lower atmosphere whilst the lighter hydrogen is lost into “space” via the exosphere.