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Past warmer climate periods at the Antarctic margin detected from proxies and measurements of biogenic opal in the ABD-1B core: the XRF spectral silver (Ag) peak used as a new tool for biogenic opal quantification

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Kuhn, G. , Helling, D. , von Eynatten, H. , Niessen, F. and Magens, D. (2008): Past warmer climate periods at the Antarctic margin detected from proxies and measurements of biogenic opal in the ABD-1B core: the XRF spectral silver (Ag) peak used as a new tool for biogenic opal quantification , AGU Fall Meeting, December 14-19, San Francisco, USA. .
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Abstract:

Quantification of biogenic opal in marine sediments is a time consuming job, but the results could indicate periods of higher bioproductivity and warmer conditions than today at the Antarctic margin. Within the international Antarctic Geological Drilling Program (ANDRILL), core AND-1B was drilled and recovered a 1285 m sequence from a flexural moat basin filled with glacimarine, terrigenous, volcanic and biogenic sediments below the McMurdo Ice Shelf. Our main goal is to study the variability and the stability of the Ross Ice Shelf from Miocene to Recent. The melting and collapse of large Antarctic ice shelves may cause a significant sea level rise because of accelerated inland ice glacier surges into the ocean. Biogenic opal content in sediments can be deduced indirectly from grain density measurements on single samples, or faster and more continuous by gamma ray attenuation measurements on the core, with subsequent wet bulk and grain density calculations. Spectral colour reflectance (b* value) measurements on the split core surface can also be a fast tool for opal content quantification. Of course, they all have disadvantages in comparison to direct measurement on samples using X-ray diffraction or geochemical leaching methods. Some major and minor chemical elements were measured directly on split core surfaces with a non- destructive X-Ray Fluorescence Core Scanner method (XRF-CS, Avaatech) in the field. Quantitative geochemical analyses like determination of total inorganic and organic carbon (TOC), biogenic opal as well as major and minor elements were done on core samples. We found a strong positive correlation between the counts per second of the XRF-CS Ag peak area and the biogenic opal content of the samples (r=0.81) not only in the AND-1B core but in others as well from the Antarctic margin. In literature, it is noted that diatoms could accumulate Ag in sediments, so at first we were pleased to find this Ag enrichment with our tool. But further geochemical analyses revealed that measuring these low Ag concentrations and their variability (< 2ppm) is not possible or at least problematic with the XRF-CS. The detector of the XRF-CS has an Ag collimator, possibly acting as an amplifier on perhaps higher induced X-ray emissions in opal rich sediments within the Ag energy spectrum range, which might have nothing to do with Ag itself. However, we are still studying the physics behind this measurement phenomenon. Nevertheless, this Ag peak can be used as a proxy for biogenic opal concentrations. It is negatively correlated to Fe and Ti and variability downcore has a high signal to noise ratio. Combining the opal calculations from fast measurements of the Ag peak (opal-Ag), the grain density (opal-GD), and the b* value (opal-b*) we yielded a new multi-parameter proxy (opal-MP) for a high-resolution record of biogenic opal concentration in the upper 600m of the core (spacing: about 2cm or 300y). This opal-MP proxy correlates very well with measured opal leaching data (r=0.88, n=481). The biogenic opal concentrations in combination with other high-resolution data will be used as a cyclostratigraphic approach to understand paleoenvironmental and climate changes. Periods with much higher accumulation of biogenic opal than today were detected in the core that indicate a retreat and perhaps a total decay of the Ross Ice Shelf

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