Geoid Shifting

9 Jul 2021

On the web site of New Concepts in Global Tectonics, you can read all their back issues without charge. For their June 2021 issue go to ... and look for volume 9 number 2. It has a very interesting article for anyone interested in maps and continental shelf systems - and former land surfaces. The article is by Masahiro Shiba of the Museum of Natural and Environmental History in Shizouka, Japan. It is fascinating - and one to make you think.

Many endemic animals from the Pleistocene to the present are found in some islands around the world, stranded and out of reach of the mainland. The fauna of these islands, he suggests, became unique - dwarf forms for example, or very large rodents. His idea is that in the Pleistocene sea levels were much lower and these animals became stranded from their fellows on the mainland as a result of rising seas. In particular, events in the Middle Pleistocene are important, as sea levels at that time were 1000 m lower than at present.

Van der Geer et al [2010] gathered a lot of information on the distribution of fossil mammals and their faunal evolution on islands around the world. They actually showed that these faunal transformations often took place during the Pleistocene - on each island. Possibly not at the same time. However, faunal transformities did coincide in some cases - and many of them occurred in Middle Pleistocene, and later, in Late Pleistocene [the last Ice Age]. Van der Geer supposed that island arcs and continental slopes were formed crustal uplift. Hence, given that as gospel it would require an uplift of 1000 m - since 430,000 years ago. This date roughly corresponds with what in Britain is known as the Anglian Ice Age, by reason that glaciation reached as far south as East Anglia, and possibly even further to the south. It is not a surprise to learn that in that circumstance, a drop of 1000m occurred - more than during the Late Glacial Maximum [which did not penetrate much further than Scotland, some parts of northern England, and the mountainous regions to the western side of Britain]. Whilst one might guess the geoid itself was quite different at both those points in time, to account for such a swing in sea levels, this is not something many geologists would embrace. The physical evidence for this would only be established if it could be proved that while some regions of the globe experienced a drop other regions witnessed the opposite, a drowning of land. The author assumes the drop in sea levels was experienced all around the globe. The main reason for that frame of mind is that the missing ocean water was locked up as ice. One may also suppose that a certain amount of rising, or falling land, was all part of the process, but mainly he foresees a drop in actual sea level as responsible. In that way, he differs from Van der Geer.

Masahiro Shiba begins his article with the Japanese archipelago and shows that it was once joined to Korea and China. Most of the Japanese islands were once a single land mass - but not the Ryukyu Island complex. Most notably, Honshu and Hokkaido. Interestingly, and something I never realised, the Korean Strait and the Tsushima Straits were not dry land during the Late Glacial Maximum. This, in spite of the fact a land bridge, existed on various different occasions from Early Miocene to Middle Pleistocene. Hence, Honshu must have been isolated for a very long time. The straits, it seems, represent a biogeographical boundary.

According to a paleontological study of small mammals in Japan these appear in the fossil record during the Middle Pleistocene. Indeed, during the Pleistocene the Japanese archipelago was connected to the mainland several times - but not after 430,000 years ago, it is claimed. Shiba then goes to the Ryukyu Islands, Okinawa, and Taiwan. At some point the latter was connected to mainland China. In fact, the Ryukyu Islands were connected to China via Taiwan. Next, he turns to the Wallace Line - a hypothetical boundary between the fauna of the regions east and west of this imaginary line in the Indonesia seas. The Huxley Line, on the other hand, roughly coincides with the coastline of SE Asia during the Late Glacial Maximum. The Phillippines, Sulawesi, and Flores are still rich in endemic animal species, as well as fossil endemics such as the water buffalo, wild boars, and deer.  Java, west of the Wallace Line, was affected by volcanism in the Pliocene and during the Pleistocne there were small hippopotamus and deer, small mastodons, and so on. In Middle Pleistocene, a rainforest ecosystem prevailed and the animals of SE Asia migrated into Java - including the early human, Homo erectus. The fauna has strong similarities with that of India - and rainforest continued to be a factor until Late Pleistocene [although it may have come and gone on occasion]. Hence, Java had the Indian elephant, the gibbon and macaque, and the orang utan. Although not mentioned, Sunda Land included Java and existed into the Holocene, when it was drowned around 8000 years ago. After that date, Indonesia was a hotchpotch of islands. Why Sunda Land was overwhelmed is a different story but the idea it involved a tsunami wave following tectonic events in British Columbia [or somewhere in NE America] seems a trifle overloaded. Tsunami waves rise up and then recede. At 8000 years ago the sea level rose suddenly - and remained at that height. Not just in SE Asia but on the continental shelf around Britain and Ireland. This is not addressed by Shiba and conflicts with the notion that sea level rise has been incremental - although it has risen and fallen over the millennia. 

Shiba goes on to look at the Phillippines, islands in the Mediterranean, including Cyprus and Crete, Sardinia, Sicily, and Corsica. Here there were pygmy hippopotamus and elephant - stranded since the Middle Pleistocene, he asserts. Then we off to the Channel Islands of California, the Falklands, and so on. An excellent study and worth reading.