Comparison between two sediment X-ray Fluorescence records of the Late Holocene from Disko Bugt, West Greenland : paleoclimatic and methodological implications,

Abstract: Popular summary: X-raying the climate archive of Disko Bugt. Disko Bugt is a large bay in West Greenland north of the Arctic Circle. Because of the influence of the relatively warm ocean current coming from the south; the West Greenland Current, it has a mild climate despite its latitude. Over the last 4000 years the climate has varied significantly in the area, which was most likely caused by changes in the strength of the West Greenland Current. By studying the sediments that have accumulated on the sea floor of Disko Bugt over the last millennia, it is possible to acquire more knowledge about the changes in climate, environment, and ocean currents that have occurred in the region. The sediment can be studied in sediment cores. They are obtained by pressing a long cylinder in the sea floor and pulling it out again. Two of these cores were investigated in this study. The almost 10 meter long cores were taken at the same location, in an 855 meter deep submarine trough. One core was split onboard of the ship, and also sampled and analyzed onboard. The other core was taken to Sweden for analysis. Both cores were subjected to a relatively new research method: X-Ray Fluorescence (XRF) scanning. With XRF scanning the composition of chemical elements can be acquired fast and with a high resolution. For either core a different type of XRF scanner was used. In addition to that, various other methods, such as mineral magnetism, were used to study the composition of the sediment. The XRF results showed quite different data for both cores, while the mineral magnetic properties indicated that the composition of the two cores was identical. This would mean that either one or both of the XRF scanners provided erroneous data. From further analysis and comparison it became quite evident that the XRF data from the core scanned onboard was probably wrong. So for the interpretation only the XRF data from Sweden was used. Of the XRF data the most interesting record was provided by the element bromine. The presence of bromine in marine sediment can be related to the production of organic matter by plankton in the surface waters. It appeared from the bromine data that primary productivity by plankton was strongest in the so-called Medieval Warm Period. It is however still debated if it was actually relatively warm or cold in West Greenland. During the earlier Roman Warm Period there were also phases of higher plankton productivity, but they were shorter and periodically alternated by productivity lows. The iron and titanium in the sediment are thought to be of terrestrial origin in the marine sediment, and might be linked to the activity of calving glaciers around Disko Bugt such as Jakobshavn Isbrae. The iron and titanium data both showed a declining trend over the studied period, with cyclic variations superimposed on it, which could imply that glacial activity varied and declined as well.