Ancient Forests and Climate in Greenland
Revealed by Inter-Disciplinary Studies between Geosciences and Biosciences
Accelerated melting of glaciers along the margin of the Greenland ice sheet, has made scientists ponder how Greenland would look without ice. One way of answering this question is to look at remnants from the ecosystem that existed before the ice sheet was formed. The problem is that the Greenland ice sheet now covers most locations where plant and animal remains might be found. Fortunately, at Kap København, a remote location on the northeastern-most coast of Greenland, well-preserved specimens can be found in abundance from the time before the ice sheet. Very few other locations in Greenland provide ancient biological evidence and inter-disciplinary efforts have, therefore, been crucial in making the most of existing material, providing a peek into Greenland’s past.
In 2012, a small expedition team led by Danish geologist Svend Funder set out to gather more material from the Kap København formation. The new samples were studied and compared to basal ice material from the bottom of the ice sheet retrieved by deep ice core drilling operations in south and northwest Greenland.
Greenland with Kap København, north of the current ice sheet, and the drill sites Camp Century and DYE-3 that were studied for ancient DNA, as well as the GRIP site that holds the oldest dated ice. View of present-day study sites: Top: Greenland ice sheet. Bottom: Kap København. Photos by A. Schmidt.
A forested north Greenland
The Kap København formation is a unique locality in terms of geological information with a remarkably high diversity in old flora and fauna material. Plant and animal remains have been preserved in layers of sediment that were transported by rivers from the inland and later deposited in an estuary close to the northern coastline. The composition of biological
material deposited during the warm period, indicates that the landscape most likely contained boreal forest with rivers and mountains in the background. A similar park tundra environ-ment can be found in present-day Siberia and Labrador. Following this warm climate a decrease in temperature occurred so suddenly that the deposited material was locked into the permafrost. This frozen condition has remained until today and is also the reason why a small amount of ancient plant DNA could still be obtained from the permafrost.
An example of how the landscape in north Greenland could have been at the time before the Greenland ice sheet settled. Photo is from present day Alaska (source: Pixabay).
A tooth’s importance for dating of the northern forest
Different ages have been proposed for the site, since the discovery of the location by Svend Funder in the 1970s. The youngest proposed age is around 1 million years and the oldest more than 2 million years. New and advanced dating methods were applied to the samples from the 2012 expedition, in order to determine the magnetic orientation in the sediment layers and to attempt a dating based on an extinct rabbit’s tooth, found by Danish geologist, Ole Bennike, in the 1980s.
The magnetic orientation of the minerals in the layers reveals the magnetic orientation on Earth during the period when the sediment layer was deposited. As the material has remained frozen since deposition, the magnetic orientation has also been preserved. Currently, we are in a period of so-called normal magnetic orientation but at the time of deposition the orientation was reversed. By comparing this information to other reference curves that go back 2.5 million years, there are only a few candidates for periods that would fit both the climate conditions and the reversed magnetic orientation.
The rabbit tooth helps narrow down the time period, as the species became extinct 1.5 – 2 million years ago in North America. This particular species was poorly adapted to a cool climate and since the cooling happened earlier in north Greenland than in more southern latitudes in North America, it is thought that the rabbit died out in Greenland first. Therefore, the finding of this rabbit tooth in the fossil-rich layers suggests at least 2 million years as the age of the site.
The Kap København formation also contains information about even older climate events. Before the warm period with the boreal landscape it appears that a significant sea-level rise occurred. The melting of continental ice sheets likely caused this and evidence of the sea-level rise is seen from a layer rich in marine deposits. This period is interpreted to have occurred 2 – 2.5 million years ago.
Age reference curves based on: top, d18O profile middle, solar insulation and bottom, magnetostratigraphy with black in normal magnetic orientation and white in reversed. From Melles et al. (2012).
DNA provides a new means to study more locations
In order to obtain more knowledge about past climate in Greenland, ancient environmental DNA was studied thereby revealing the species composition of ancient ecosystems. This DNA comes from dead organisms that lived in the region at the time prior to the ice sheet formation. The study of ancient DNA is based on the fact that all organisms leave behind dead organic material that becomes degraded over time and adsorbed to the soil particles. Thus, the DNA can provide information of what kind of organisms lived in a certain ecosystem. As soil particles in permafrost and basal ice have remained frozen at low temperatures the chance of finding preserved but degraded DNA is possible but not easy.
Basal ice sample from Camp Century (top) and a permafrost core sample (bottom). Photos: A. Schmidt & Eske Willerslev.
Aside from the unique Kap København formation, insights into a Greenland without ice cover can be obtained from only very few places, such as the deep ice core drilling sites. Here scientists have drilled through thousands of meters of ice to reconstruct past climate and gain access to bedrock at the bottom of the ice. Not all of these deep drillings returned basal ice material with soil or debris entrained but two of the sites provided material comparable to the Kap København findings.
Forest in both northern and southern parts
The latest results from the studies of the ancient DNA in basal ice from Camp Century and DYE-3 in northwest and south Greenland show traces of boreal forest. The vegetation at the two sites differs somewhat but both ecosystems had species that required a climate 5-10°C warmer than today. This is quite similar to the results from Kap København with its boreal forest and tundra vegetation.
Evidence that the three different areas in Greenland were all forested brings up the exciting question of whether the forest was present during the same period at these locations.
A range of dating methods was used to determine the age of the basal ice material from DYE-3 and Camp Century. Some of the methods were targeting the age of the ice, another the time since the sediment became incorporated into the ice matrix, and finally one method estimated the approximate age of the DNA sequences. Ultimately, a minimum age of the biological remains was obtained but not a maximum age. Therefore, the consensus is that the two locations were definitely ice covered during the last interglacial, between 123,000 and 116,000 years ago and most likely also during the older super-interglacial 420,000 – 360,000 years ago.
Did the forest come back several times or did it overlap in time?
The challenge of finding a maximum age is ongoing and researchers are working on the puzzle of distinguishing which inter-glacial periods supported the boreal forest in the three regions. Ice sheet models are used to simulate past dynamics of the ice sheet, thereby helping to solve this puzzle. So far we are pushing the limits of when the ice sheet most likely started to form.
Previous studies estimated a minimum age of the oldest basal ice from the GRIP drill site to be 900,000 years old. Therefore, we know central Greenland has been covered by ice for at least 1 million years. The question still remains whether this is also the minimum age of the Camp Century and DYE-3 ecosystems or if these sites perhaps co-existed with the Kap København formation?
Map of Greenland, illustrating how forest could have been distributed at the time of the Kap København Formation. Dark green = coniferous forest, light green = deciduous forest, brown = tundra and white = local ice caps. From Funder (1996).
Melles, M., J. Brigham-Grette, P. S. Minyuk, N. R. Nowaczyk, V. Wennrich, R. M. DeConto, P. M. Anderson, A. A. Andreev, A. Coletti, T. L. Cook, E. Haltia-Hovi, M. Kukkonen, A. V. Lozhkin, P. Rosen, P. Tarasov, H. Vogel, B. Wagner. (2012). 2.8 Million Years of Arctic Climate Change from Lake El’gygytgyn, NE Russia. Science 337, 315-320; published online EpubJul (doi:10.1126/science.1222135).
Funder, S. (1996). Et Anderledes Klima.Varv, 4:18. (in Danish).