CIO researchers contribute to a new study; the largest to date of ancient Europeans’ DNA

CIO researchers contribute with C-14 dating to a new study; the largest to date of ancient Europeans’ DNA

The North Sea, the ice age and isotopes

Article by em. professor Hans J. van der Plicht

We all know about the North Sea. We also all know about sea level rise in the near future. What is generally less known is sea level rise in the past. In short, during ice ages there was so much water trapped in polar ice, that the sea level was lowered by more than 100 m. The present depth of the southern part of the North Sea is significantly less, about 30 m. Hence during the last ice age (about 120-12 thousand years ago) the region was dry land. It was inhabited by plants, animals and humans, and is known as Doggerland. The British isles were connected with continental Europe, the major rivers like Rhine/Meuse and Thames came together and flowed south into what is now the Channel. See the map (figure 1) which shows a reconstruction of the geography at various times in the last 18 thousand years. Global warming at the end of the ice age caused a dramatic sea level rise, drowning Doggerland, isolating Britain (yes there are brexit jokes about this) and eventually changing the geography to the situation of the present day.

Hence, the present North Sea is a drowned landscape. It contains remains of the past, for animals and humans in the form of fossil bones. These are found in fishing nets, and the beach and on artificially reclaimed lands like Maasvlakte and Zandmotor, constructed using sands from deposits far from the coastline. See for example figure 2; instead of fish, the catch of these fishermen was a mammoth skull. 

One of the prime research questions is: how old are the fossils? This is impossible to establish by direct observations such as archaeological or geological stratigraphy. There is no clear context, the sea erased traces and possibly relocated many samples during the past millennia. 

Radiocarbon dating is the only way to answer this question. During the past decades, large series of fossil bones from the North Sea have been analyzed at the CIO. The datelist has recently been published. In total, it contains 218 animal bone dates: 144 for the ice age period (Late Pleistocene), and 74 for the Holocene, the present warm period. The published human bone dataset contains 123 dates. The vast majority of the latter are (sub)recent and are not of interest for Doggerland studies; these are submitted by NFI, the forensic institute when it is of importance for missing persons or "cold cases". The old Doggerland dataset for humans is limited in size. The datelist is almost completely based on work done in Groningen; a limited number of samples has been dated elsewhere.

The tables form a valuable "legacy collection" for present and future research. A few observations / conclusions are highlighted here. 

The fauna contains extinct animals, such as the woolly mammoth (the ice age icon), woolly rhinoceros, cave lion and sabre-toothed cat. Doggerland was part of what is known as the "mammoth steppe fauna". The mammoth is the most dated species; it contains of course a sampling bias because of the mere size of the animal, and the demise of the mammoth is still somewhat mysterious. Fragile bones from small animals are easily missed, or have disappeared by degradation. 

The youngest mammoth dated is 36-41 thousand years old (2 sigma uncertainty range). The upper limit for the 14C method is roughly 50.000 years. Many mammoth bones show ages older than this limit; their 14C radioactivity can not be distinguished from the background. The dates of whales and other sea animals obviously indicate a marine location at that time (provided the bones are in situ finds).

In terms of climate, the Last Glacial Maximum (LGM, defined as the time most water was present in the form of ice) is assumed to be around 20 thousand years ago. It is of significance that no fossil bones from the North Sea are dated to this time; there is an obvious gap of more than 10 millennia during the LGM period. In addition, also 14C dated peat deposits from Twente (a large series of 90 dates covering the whole 14C dating range) show this hiatus on the continent. 

Concerning human bones with ice age dates from the North Sea, the following results are of significance. The oldest modern human bone date is a Late Glacial skull part found in the North Sea, 14C dated to 13 thousand years. There is also an indirect human date, from the same location (known as the Brown Bank): a decorated bison bone, also 13 thousand years old, representing the oldest art from the region. See figure 3.

But there are older dates, albeit indirect: in particular a Neandertal tar-hafted flint tool, the tar made from birch bark, is 14C dated to 50 thousand years ago. A very rare and significant find. Note there is only one Neandertal skull part (Krijn) known from the North Sea, but this bone was degraded and not datable.

Most human prehistoric bones from Doggerland date to the middle stone age (Mesolithic, roughly 10-7 thousand years ago). Finds of human remains from this era are rare, also on the continent. Other dated sample materials from the North Sea testifying human presence in Doggerland are artefacts, made by wood or bone (usually antlers).

Apart from using the radioactive isotope 14C, the stable isotopes 13C and 15N of the bone collagen (which is the datable fraction) also provide information of the past. The ratios 13C/12C and 15N/14N are indicators for the environment the dated fossil lived in, and for its diet. They are indicators for trophic level (herbivore, carnivore, omnivore) and aquatic food resources.

For example, changing 13C contents in herbivore bone through time - from Pleistocene to Holocene - indicate different plants as food source. Mammoths bones show in general atypically high 15N contents, in the carnivore regime. These numbers become more "normal" in some cases for the latest mammoths (about 15 thousand years ago) which we do not have in the present North Sea database.  For a more detailed treatise of the stable isotope analysis see the PhD thesis by Margot Kuitems

The stable isotope ratios for the Doggerland humans reveals information on their diet. The15N/14N ratio indicates they consumed primarily aquatic food, most likely fish. The 13C/12C ratios show that it was freshwater fish from rivers; thus, not marine fish. 

The Nederlandse Geologische Vereniging (NGV) dedicated a special issue of their journal Grondboor en Hamer to the Northsea: the past geological setting, paleobotany, paleontology and archaeology. The issue is published in English. The CIO contributed with two chapters, one on 14C dating and one on the stable isotopes 13C and 15N of the fossil bones. A more technical article has been published in the journal Radiocarbon. This NorthSea Special issue of the NGV can be purchased. The PDF's of the two CIO articles are available from the author. Other articles published on the North Sea with a CIO contribution are listed below, most are available via the RUG library.

Most actual news: in the journal Nature a large and trendsetting article on ancient DNA in human bones from the ice age was published in early march. Many bones are dated by 14C, by the CIO and several other laboratories. Also the North Sea human dated 13.000 years ago mentioned above is analyzed for its DNA.

The isotope work on the North Sea is continuing with a new NWO funded project led by the Groningen Institute of Archaeology (GIA), with participation of the CIO for the Radiocarbon dating

Additional literature is availble by em. professor Hans J. van der Plicht.