Author: Hanneke Heida (ICTJA – CSIC, Barcelona)

One of the most challenging aspects of earth sciences is the linking of processes occurring on vastly different scales, from the local to the global, the microscopic to the solar system, the second to millions of years. Sometimes fascinating clues to a large and seemingly ungraspable phenomenon can be found in unexpected places. An example is the development of glacio-isostatic theory, and the characterization of the viscous behavior of the earth’s mantle, from the observation of changing sea level around the Baltic sea over centuries.

But first, a few words about isostasy. In a nutshell, isostasy is the idea that on geological timescales the mantle flows, and as such it allows for the “sinking” of heavy loads on the crust. This explains the formation of crustal roots forming underneath mountain belts, or sedimentary basins deepening due to the loading by sediments1.

In a way, the crust and mantle behave like a waterbed, allowing heavy objects to sink down until the buoyancy of the displaced water (mantle) compensates for the gravitational force. The discovery of this behavior allowed for crucial insights into the way sedimentary basins develop and how mountain ranges are able to form, as well as aided understanding of gravitational anomalies within the earth caused by density differences.

Seal Rock used by Celsius to determine rate of sea level change on Lövgrund Island, with markings starting in the year 1731. Picture courtesy of http://sybelisama.blogspot.com/2019/06/lovgrund-celsius-island-4th-june-2019.html

So how did the Baltic sea aid the discovery of glacio-isostasy?

It turns out that a combination of its restricted connection with the open ocean, its latitude and shape provide the perfect circumstances for studying the isostatic effect of land ice retreat since the last ice age (20.000 years ago). Because this change in load was quite rapid compared to other geological processes, the crust is still adjusting to the melting of the ice, rising slightly every year.

Keen observers around the Baltic sea have noticed changes in sea level for hundreds of years, causing harbors to end up on dry land, and even the forced relocation of entire towns! Celsius, the Swedish scientist famous for the temperature scale that bears his name, used rocks that seals would rest upon until they rose too far above the water to estimate rates of change in sea level with surprising accuracy in the early 18th century. His first estimates of a relative sea level fall of 1,4 cm/year were only slightly above those later determined by more precise methods. Since then a large number of markers have been used to trace the changes in sea level. A fierce scientific debate about the nature of the observed changes ensued, as they could be indicative of either a drop in water level of the sea itself, or a rising of the land! The pattern of sea level change around the Baltic held the key to solving this conundrum, as a uniform change all around would indicate a retreat in sea water, while regional variations could only be explained by uplift of the land itself.

It took great effort tracking sea level at stations all around the Baltic sea over many years to yield observations robust enough to show a clear change in rates of change, with the fastest (highest uplift rate) occurring to the north, where ice thickness would have been greatest. The full history is explored in the book The Changing Level of the Baltic Sea during 300 Years: A Clue to Understanding the Earth by Dr. Martin Ekman.

These systematic investigations eventually led to the wide acceptance of glacio-isostatic theory, which allowed earth scientists to quantify uplift rates, identify the contribution of eustatic sea level changes, and even viscosity of the mantle allowing for the relaxation of the crust. This in turn aided the acceptance of continental drift theory, as it showed the plausibility of convection flow in the mantle.

Isostasy is also a central concept in understanding the development of the Mediterranean during the Messinian Salinity Crisis. The huge amounts of salt deposited in a short timespan would have drastically altered the bathymetry of the basin, and in combination with rapidly changing sea levels by evaporation and flooding, and redistribution of evaporites and sediments by erosion and salt tectonics pose a great challenge towards properly understanding the shape of and the connectivity between various parts of the Mediterranean. Combining our understanding geological processes, lots of data and the simple concept of isostasy has already provided some fascinating insights into the past of our planet, and will continue to do so for many years to come.


Bibliography:

Ekman, M. (2009). The Changing Level of the Baltic Sea during 300 Years: A Clue to Understanding the Earth. In Summer Institute for Historical Geophysics, Åland Islands.

Watts, A. B. (2001). Isostasy and Flexure of the Lithosphere. Cambridge University Press.

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