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

Abstract

S U M M A R YIn 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.

Magnetic mineral diagenesis

“…66 1996; Wilson and Roberts, 1999;Itambi et al, 2010;Nowacyzk, 2011;Chang et al, 2015), and its presence is often attributed to its paramagnetic response to the strong magnetic fields applied during magnetic mineral extraction. It is, therefore, usually ignored in terms of paleomagnetic signal recording.…”

“…Ilmenite and Fe-Cr and Fe-Mn spinels are common relict minerals ( Figure 25) in diagenetically reduced sediments (e.g., Roberts and Turner, 1993;Hounslow et al, 1995;Hounslow, 1996;Wilson and Roberts, 1999;Garming et al, 2005;Itambi et al, 2010;Nowacyzk, 2011) due to their resistance to reductive dissolution under sulphidic conditions (Canfield et al, 1992). Increasing cation substitution can stabilize such minerals against sulphidic dissolution because substitution reduces the concentration of the electron acceptor Fe 3+ .…”

“…Non-steady state magnetite dissolution associated with relatively short-lived enhanced organic carbon burial events can explain cyclicity between relatively strong and weak magnetizations in some instances (e.g., Itambi et al, 2010). Alternatively, enhanced organic carbon supply can result in ferruginous diagenetic conditions that liberate Fe 2+ into sedimentary pore waters, thereby providing the iron needed to enhance magnetite production by magnetotactic bacteria (e.g., Roberts Deposition of organic-rich layers, known as sapropels, in the Eastern Mediterranean Sea provides a clear example of non-steady state magnetic mineral diagenesis in deep marine environments (Dekkers et al, 1994;van Os et al, 1994;van Santvoort et al, 1997;Passier et al, 2001;Passier and Dekkers, 2002;Larrasoaña et al, 2003aLarrasoaña et al, , 2006Liu et al, 2012b).…”

Environmental magnetism: Principles and applications

“…Coastal sediments are mainly characterized by coarse grain sizes, with deposition in proximal locations with discontinuous sedimentation, so they are seldom the focus of environmental magnetic studies. Cosmic particles contribute a much smaller load of 0.002 Tg/yr (Table 1) and generally have negligible significance for environmental magnetic studies [ Itambi at al. , 2010a], although the low terrigenous mineral content of ice means that cosmic flux is measurable in ice [e.g., Lanci et al , 2012].…”

Characterizing magnetic mineral assemblages of surface sediments from major Asian dust sources and implications for the Chinese loess magnetism

“…To better decipher the dust signal preserved in sedimentary archives, it is necessary not only to understand the magnetic properties of terrestrial and marine sediments in depositional sinks to obtain high-resolution records of dust flux from key source regions, but there is also a need to study the magnetic properties of dust material from different sources to their depositional sinks. Previous studies have focused on dust signals recorded by terrestrial and marine sediments, e.g., from the Chinese Loess Plateau (CLP) , the Red Sea (Rohling et al 2008;Roberts et al 2011), the Indian Ocean (Bloemendal and deMenocal 1989), the North Pacific Ocean (Doh et al 1988;Yamazaki and Ioka 1997;Bailey et al 2011), and the North Atlantic Ocean (Bloemendal and deMenocal 1989;Itambi et al 2009Itambi et al , 2010a. Recently, a few studies have been conducted on surface sediments from dust source regions, e.g., the Asian interior (Maher et al 2009;Zan et al 2015) and North Africa (Lyons et al 2010(Lyons et al , 2012Oldfield et al 2014).…”