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.

Abstract

Two parallel sediment cores were retrieved from the Inner Egedesminde Dyb marine trench at Disko
Bugt, West Greenland. Disko Bugt is the most northern point along the Western Greenland coast where
relatively warm Atlantic water penetrates through the West Greenland Current. Both cores were subjected to Xray
fluorescence spectrometry (XRF) scanning to obtain the elemental composition of the sediment. The first
core, labeled 343310-5-1, was scanned on board the research vessel Maria S. Merian using an Avaatech XRF
scanner. The second, labeled 343310-6-1, was scanned at Stockholm University with an Cox Analytical Itrax
core scanner. In addition magnetic susceptibility and mineral magnetism were measured on both cores.
Subsamples from core 343310-6-1 were also subjected to thermogravimetric analysis, so that the weight loss on
ignition could be determined. In order to establish an age model, two samples from 343310-5-1 where
radiocarbon dated, and paleomagnetism was measured on subsamples from core 343310-6-1.The datings
resulted in an age model, which indicated that the cores covered at least the last 3000 years.
The downcore mineral magnetic profiles of both cores are practically identical, but the XRF data showed some
significant differences between the cores. It was assumed that one or both of the used scanners might have
produced erroneous measurements. Not many opportunities to independently verify the outcomes of the XRF
data were available, but the good correlation between the bromine record from core 6-1 and the loss on ignition
at 550 degrees (LOI550) from the same core suggested that the XRF analysis by the Itrax core scanner was
probably not to be blamed.
It was hypothesized that there might be a correlation between the various magnetic properties and certain
elements, but no significant correlations were found. Overall the mineral magnetic data proved hard to interpret,
but it was useful for correlating the cores, as the mineral magnetic signals of the two parallel cores were close to
identical. Of the XRF data the most interesting record was provided by bromine from core 6-1. Bromine has
been suggested as a proxy for paleoproductivity, and its correlation with LOI550, which represents the percentage
of organic carbon in the sediment, seemed to confirm this. The bromine record showed variations that could be
correlated with results from other studies, and could be interpreted as evidence for changing oceanographic
conditions in Disko Bugt resulting in enhanced and reduced primary productivity in the surface waters. It
appeared that primary productivity was strongest in the so-called Medieval Warm Period, of which it is still
debated if it was actually relatively warm or cold in West Greenland. During the Roman Warm Period there were
also phases of enhanced productivity, but they were periodically alternated by productivity lows. The iron and
titanium records both showed a declining trend over the studied period, with cyclic variations superimposed on
it. Both elements 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.

Abstract (Swedish)

I det här arbetet har två parallella marina sedimentkärnar från Inre Egedesminde Dyb i Diskobukten, västra Grönland, undersökts. Diskobukten är den nordligaste delen längs den Grönländska västkusten var det förhållandevis varma vattnet från Atlanten transporteras med den västra Grönlandströmmen. Båda kärnorna scannades med hjälp av fluorescencespectrometry (XRF) för att erhålla den elementära sammansättningen av sedimenten. Den första kärnan, märkt 343310-5-1, scannades ombord forskningsfartyget Maria S. Merian med en Avaatech XRF scanner. Den andra, märkt 343310-6-1, scannades med en Cox Analytical Itrax core scanner vid Stockholm universitet. Även magnetisk susceptibilitet och mineralmagnetiska analyser utfördes på de båda kärnorna. Prov från kärnan 343310-6-1 utsattes även för termogravimetrisk analys, så att viktförlusten vid förbränning kunde bestämmas. För upprättandet av en ålderskurva daterades två prov från 343310-5-1 med kol-14-metoden och paleomagnetiska analyser utfördes på kärna 343310-6-1. Dateringarna resulterade i en ålderskurva, som indikerade att kärnorna åtminstone spänner över de senaste 3000 åren.

De mineralmagnetiska profilerar från båda kärnorna var praktiskt taget identiska, medan XRF-datan visade på signifikanta skillnader mellan kärnorna. Det antogs att en eller båda av de använda scanningsutrustningarna gav felaktiga mätningsresultat. Det fanns små möjligheter att göra en oberoende verifiering av resultaten från XRF-scanningarna, men den goda korrelationen mellan bromprofilerna från kärna 6-1 och kolförlusten vid förbränningen på 550 grader (LOI550) från samma kärna visade på att XRF-analysen utförd på Itraxen inte var felaktig.

Det antogs att det kunde finnas en korrelation mellan de magnetiska egenskaperna och vissa element, men inga signifikanta korrelationer påvisades. Det visade sig att resultaten från de mineralmagnetiska mätningarna var svåra att tolka, men de var användbara för att korrelera kärnorna, eftersom de mineralmagnetiska signalerarna från de två parallella kärnorna nästan var identiska. Den mest intressanta XRF-datan var bromprofilen från kärna 6-1. Brom anses vara ett mått för paleoproduktivitet och dess korrelation med LOI550, som visar procentsatsen av organiskt kol i sedimenten, verkar bekräfta detta. Bromprofilen visade variationer som kunde korreleras med resultat från andra studier och kunde tolkas som bevis för ändrade oceanografiska förhållanden i Diskobukten, som resulterat i förhöjd och förminskad primärproduktion i ytvattnet. Det verkade som om den primära produktionen var som störst under den medeltida värmeperioden, vilket det fortfarande debatteras om det faktiskt var förhållandevis varmt eller kallt i västra Grönland. Under den romerska värmeperioden var det också delvis förhöjd produktivitet, men även växelvisa perioder med låg primär produktion. Järn- och titanprofilerna visade båda en neråtgående trend över den studerade perioden, med cykliska variationer ovanpå denna trend. Båda elementen är troligen av terrestriskt ursprung och kan knytas till aktiviteten hos de kalvande glaciärerna runt om Diskobukten ,så som, Jakobshavn Isbrae.

• author: Jäckel, Oskar
• supervisor: Ian Snowball; Per Sandgren
• organization: Department of Geology
• alternative title: En jämförelse mellan två röntgenfluorescensprofiler av sen-holocena sediment från Diskobukten, västra Grönland : implikationer för paleoklimat och metodik
• year: 2009
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Disko Bugt, X-ray fluorescence, mineral magnetism, magnetic susceptibility, paleomagnetism, bromine, productivity, Medieval Warm Period
• publication/series: Dissertations in Geology at Lund University
• report number: 248
• language: English
• id: 2302080
• date added to LUP: 2012-01-25 14:11:52
• date last changed: 2012-01-25 14:11:52