Line-scan imaging and XRF scanning

10.4 Line-scan imaging and XRF scanning

Matthias Forwick¹, ¹UoTr Henrriette Kolling^{2 2}AWI 10.4.1 Line-scan imaging

Objectives

Line-scan imaging was performed to document e.g. sediment colour, structures and lithological boundaries of split gravity cores, as well as kastenlot cores and box cores. The results shall be used for stratigraphic correlation (based on colour), as well as correlation with other proxies, e.g. micropalaeontology, element geochemistry or physical properties.

Work at sea

Line-scan images were acquired with a Jai CV L107 camera with RGB (red-green-blue) channels at 630 nm, 535 nm and 450 nm, respectively, mounted to an Avaatech XRF core scanner. The camera contains three CCD sensors and a beam splitter to separate the RGB signal. Images were acquired with a down-core resolution of approx. 70 µm. They were stored in *.bmp and *.jpg formats. The images have a fixed width of 15 cm and their lengths were adjusted depending on the lengths of the sections to be photographed. A total of 203 core sections was imaged, equaling approx. 180 meters. Section lengths varied between 12 and 101 cm. In addition to the virtual colour images, RGB, CIE-L*, a* and b* are available as *.txt files. However, these informations are obtained from the entire image width, i.e. they include also areas that were not covered with sediment core (e.g. ruler and core holders).

The light source was switched on at least 30 minutes prior to image acquisition to allow the light bulbs to warm up and stabilize. Furthermore, a white calibration, using a white tile, was carried out prior to every series of image acquisition. All images were acquired with aperture 11 and 10 ms exposure time. A ruler with mm-scale was placed along the imaged sections.

Whenever possible, measurements were performed during transit in open water or during station work in the ice to minimize the negative effect of vibrations on image quality.

The sediment surfaces were cleaned and smoothened with a plastic card (credit card) across-core prior to image acquisition. The corners of the plastic cards were smoothened as thoroughly as possible. However, it could occasionally not be avoided that small incisions in the cards remained, resulting in thin visible stripes across the image, particularly in very fine grained and water-rich intervals (see example on Fig. 10.4.1C). Such stripes can easily be misinterpreted as lamination and caution should be given when analyzing images of such intervals.

Preliminary results

The quality of the images is generally very good. The majority of the imaged sediment is brown (see example on Fig. 10.4.1A). Beige, grey and green are sub-ordinate colours. The surfaces of intervals with high clast contents appear more irregular than intervals where clast numbers are low or absent (Fig. 10.4.1B).

In case sub-mm analyses of the images are planned, it should be noted that the vibrations of the ship led to quasi-continuous distortions of 3-5 pixels, i.e. up to 350 µm (Fig. 10.4.1D).

Furthermore, it should be noted that the width of the gravity-core liners did not allow the numbers of the ruler to be entirely visible on the image. However, the upper parts of the numbers are visible and the distances between these are 10 mm (see example of ruler on Fig. 10.4.1D).

10. Marine Geology

Fig. 10.4.1: A) Example of line-scan image from core PS87/056-1-KAL-OG-0102-0200.5 cm. B) Irregular sediment surface due to coarser sediment composition. C) Thin stripes resulting from irregularities in plastic card used for cleaning of surface. D) Irregularities caused by vibrations of the

vessel.

Data management

The *.bmp, *.jpg and *.txt files will be made available in PANGAEA Data Publisher for Earth &

Environmental Science .

10.4.2 XRF core scanning Objectives

X-ray fluorescence (XRF) core scanning was performed to obtain an overview of the qualitative element-geochemical composition of sediment cores recovered during the expedition. The results shall contribute to multi-proxy reconstructions of palaeo-environmental conditions, as well as to identify sediment input from various circum-Arctic sediment sources. In geographically restricted areas, the results should provide a useful tool to establish chemo-stratigraphies that can be used for core correlation.

Work at sea

XRF core scanning was performed with an Avaatech XRF core scanner installed in a 20-foot container that was placed in the container storage room below deck in the front of Polarstern.

Primary radiation was emitted from an Oxford 100 Watt water cooled X-ray source with a Rhodium anode and a 125 µm Beryllium window. The secondary radiation was detected with a digital Canberra X-PIPS detector with an electrically cooled Peltier system allowing an energy resolution down to 190 eV at 5.9 keV. Measurements were carried out through a

helium-10.4 Line-scan imaging and XRF scanning

flushed triangle system landing on the sediment surface to avoid measurement through air, the latter exposing much higher friction on the radiation than He, thus, leading to a reduction of the detected signal.

All measurements were carried out with a down-core resolution of 10 mm and cross-core resolution of 12 mm. The step sizes between the measurements were 10 mm, thus, providing continuous records. Measurements including core PS87/080-4-SL were carried out in three runs to excite light, medium and heavy elements, using the following settings: 10 kV, 1000 µA, 10 seconds count time, no filter (1. run); 30 kV, 2000 µA, 10 seconds count time, Pd-thick filter (2. run); 50 kV, 2000 µA, 10 seconds count time, Cu filter (3. run; Tab. 10.4.1). However, due to time constraints, the measurement settings for the cores PS87/086-3-SL to PS87/096-1-SL had to be modified. For those cores, two runs with reduced count times were carried out to exclusively excite light and medium elements: 10 kV, 1000 µA, 5 seconds, no filter (1. run); 30 kV, 2000 µA, 5 seconds, Pd-thick filter (2. run; Tab. 10.4.1).

The X-ray source was turned on at least 30 minutes before the start of to the measurements to provide sufficient time for heat-up. Prior to and after each measurement series three pressed pellets were measured to control the stability of the measuring setup, i.e. the stability of the X-ray source, the absence of leakage from the He-chamber and the function of the detector.

The sediment cores were stored in the container at least one day prior to the measurements allowing the material to adjust to room temperature. The surfaces were cleaned with a plastic card and covered with a 4 µm thick ultralene foil to avoid contamination of the measurement triangle affecting further measurement.

A total of 203 core sections were scanned, resulting in approx. 47,800 point measurements.

Section lengths varied between 12 and 101 cm.

After the measurements, the raw data were processed using WinAxil. The following models were applied:

PS87-10kV.Afm (including Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Rb, Rh) PS87-30kV.Afm (including Cu, Zn, Ga, Br, Rb, Sr, Y, Zr, Nb, Mo, Ir, Pb, Bi) PS87-50kV.Afm (including Ag, Cd, Sn, Te, Ba)

Preliminary results

The XRF core scanner performed very well, despite of the constant vibrations of the vessel, as well as repeatedly strong shaking during ice breaking. Only during very strong collisions with ice floes, the X-ray source turned off automatically (Tab. 10.4.1). The intensity of the X-ray source was only marginally reduced during the expedition. Communication errors between Scanner PC and the instrument occurred rarely (Tab. 10.4.1). Measurements were resumed from the depth where the X-ray source turned off or where communication errors occurred.

10. Marine Geology Tab. 10.4.1: Summary of PS87 cores that have been scanned onboard Polarstern. Different measurement settings are listed.

10.4 Line-scan imaging and XRF scanning

Count rates varied during the measurements, particularly at section boundaries (Fig. 10.4.2B).

However, decreases in count rates occurred also in relatively porous/coarse intervals or where the sediment surface was irregular so that the contact between the landing triangle and the sediment surface was reduced forcing the primary and/or secondary radiation to travel through air. To minimize the negative influence of such matrix effects, ratios or log-ratios of elements should be applied for future analyses (see Tjallingii et al. 2007; Weljte & Tjallingii 2008).

Fig. 10.4.2: A) Line-scan image of core PS87-070-1-KAL-OG. B) Sum of the 12 most abundant elements. C) Fe/Ca-ratio. D) Mn/Fe-ratio.

Certain correlation of sediment colour and changes in element ratios can be observed. Whereas the Mn/Fe ratio increases in dark brown intervals, the Fe/Ca ratio increases in light grey/

beige intervals (see example from core PS87/070-1-KAL, Fig. 10.4.2C, 2D). However more extensive post-cruise work is needed to properly address the objectives mentioned above.

Data management

Raw data, processed data and models applied for data processing will be made available in PANGAEA Data Publisher for Earth & Environmental Science.

10. Marine Geology

References

Tjallingii R, Röhl U, Kölling M, Bickert T (2007) Influence of the water content on X-ray fluorescence core-scanning measurements in soft marine sediments. Geochemistry, Geophysics, Geosystems, 8, doi:10.1029/2006GC001393.

Weltje GJ, Tjallingii R (2008) Calibration of XRF core scanners for quantitative geochemical logging of sediment cores: Theory and application. Earth and Planetary Science Letters, 274, 423-438.