Magnetic mineral inventory of equatorial Atlantic Ocean marine sediments off Senegal-glacial and interglacial contrast

Abstract

In complex sedimentary environments with different sources of the magnetic particles, it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data. Palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources, and diagenesis may alter the primary magnetic mineral assemblage. Rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages. In this study of core-retrieved sediments from the last 150 kyr off the coast of Senegal (NW Africa), the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant. In glacial stages, ferrimagnetic iron and iron-titanium oxides magnetically dominate with few signs of a diagenetic overprint. Low- and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low-temperature AC susceptibility measurements and SEM observations. In contrast, the interglacial samples appeared to have been severely affected by diagenesis, with pyrite making up over 50 per cent of the iron-bearing phases. The Verwey transition observed at 110 K confirms the presence of magnetite. The glacial and interglacial contrast reveals the close link between northwest African climate and the magnetic mineralogy of the sediments. © 2010 The Authors Journal International © 2010 RAS.