Study of deep sea Fe-Mn sediments and the possibility to use these formations as paleoclimatic indicator

Abstract

The iron-bearing compounds in manganese deep sea sediments (nodules and crusts) are studied by Mössbauer spectroscopy at 77, 4.2 and 0.3K. The Mössbauer parameters and the concentration of the two main components α-FeOOH (goethite) and Fe5HO8·4H2O (ferrihydrite), are measured in 1mm sampling steps. The mineral goethite exhibits defect crystal structure. It shows a Mössbauer sextet with broad and asymmetric lines at 77K and at room temperature partially a doublet due to superparamagnetic state. Ferrihydrite has a very low blocking temperature (<20K) and in our measurements at 0.3K, shows a Mössbauer sextet with broad lines. X-ray topography images of Fe-Mn nodules and crusts reveal a specific layer structure related to two long-period variations-1.9 million years (Ma) of the eccentricity e and 1.2Ma of the obliquity ε of Earth's axis, respectively. According to the Milankovitch astronomical theory they can enforce drastic changes of climate. The hydrogenous Fe-Mn nodules and crusts have a very low growth rate of 2-10mm/Ma, while the diagenetic nodules grow faster, 10-50mm/Ma. So time intervals up to 20Ma could be covered. Some global geological events such as the rise of the Isthmus of Panama and the beginning of the modern bottom water circulation (about 6.4Ma before present) have changed the crusts structure and have been discussed. Some specific boundaries in the layer structure of nodules are easily identified. This allows a quick initial self-dating of the nodules without the use of expensive isotopic radiometric methods of dating. © 2012 Elsevier Ltd.