Studies of iron in deep-sea sediments by Mossbauer spectroscopy

The distribution of the forms of Fe in the solid phases in core samples of sediments from the Peru Basin has been investigated by Mossbauer spectrosco py with special attention to the cause of the sharp color transition betwee n an upper green colored and a lower tan colored part. An important part of sample handling includes strict exclusion of oxygen during preparation of absorbers and measurements at cryogenic temperatures, The measurement strat egy includes measurements between 77 K and 300 mK in zero external magnetic field, supplemented by measurements in external magnetic fields at 4.2 and 300 mK (up to 6.2 and 1 T, respectively). The temperature scans allow dete ction, identification and quantification of superparamagnetic iron oxides ( goethite and hematite). The oxides are only present in samples from the upp er tan-colored part of the core. The major part of the Fe(II) and Fe(III) ( >80%) is present in a magnetic structure similar to that of layer silicates . The relative Fe(II) content of the layer silicates is practically identic al to that determined from the paramagnetic components measured at liquid n itrogen temperature. This shows that the color transition in the sediment c oincides with a change in the relative Fe(II) content in layer silicates fr om 11 to 37%. The color change can thus be explained by an increase in occu rrence of Fe(II)-Fe(III) pairs exhibiting absorption bands due to intervale nce electron transfer.