Characterizing magnetofossils from first‐order reversal curve (FORC) central ridge signatures

Abstract

The central ridge structure of a first-order reversal curve (FORC) distribution is indicative of uniaxial noninteracting single domain magnetic particles, which provides the opportunity to identify and characterize biogenic magnetic mineral remains (magnetofossils) in sediments. Recent studies have shown that magnetofossils are widespread in the geological record and that they carry useful environmental information and contribute to paleomagnetic recording, which makes it essential to quantify how these biogenic components contribute to the magnetic properties of sediments. We present results from six sedimentary sequences whose magnetic mineral assemblages contain a significant magnetofossil contribution. Using principal component analysis, we find that the central ridge properties exhibit both intra-sequnce and intersequence variability that may be ascribed to external environmental factors. While samples from individual sediment sequences tend to cluster together, there is a continuum of inter-sequence behavior that appears to be related to a variety of magnetofossil properties. We demonstrate the complexity of biogenic magnetic components in sedimentary environments, but also the power and potential of FORC central ridges for understanding magnetic mixtures and unraveling environmental information.

Magnetic mineral diagenesis

“…This should be within a few kyr of deposition if, in the worst-case scenario, the bacterium lived within the sediment. As indicated by Kopp and Kirschvink (2008), reports of putative greigite magnetofossils are still relatively rare (Pósfai et al, 2001;Vasiliev et al, 2008); however, a recent report of ancient greigite magnetofossils (Chang et al, 2014) suggests that it is possible to magnetically detect such signals, which should simplify paleomagnetic interpretation of greigite-bearing sediments.…”

“…Egli (2004a, b, c) recognised this difficulty and, therefore, grouped particles produced by dissimilatory iron-reducers with detrital and pedogenic magnetite because of their similar magnetic coercivity distributions. A magnetically non-interacting low-coercivity stable SD component is being identified increasingly in modern and ancient sediments; it has been attributed to extracellular magnetite, although definitive evidence is lacking (Egli, 2004a, b, c;Ludwig et al, 2013;Heslop et al, 2014). Much more work is needed to understand the contribution of extracellular magnetite to the magnetic record of sediments.…”

“…Greigite has also been observed to form rapidly within various sediment types (e.g., Pye, 1981;Pye et al, 1990;Reynolds et al, 1999;Blanchet et al, 2009;Nowaczyk et al, 2012), so that any paleomagnetic signal would be equivalent to that carried by detrital particles. Studies of ancient marine sediments have also provided evidence that greigite provides a paleomagnetic signal that was acquired sufficiently early to carry a reliable record of geomagnetic field variations (Vasiliev et al, 2007;Hüsing et al, 2009;Chang et al, 2014). In other cases, claims of geomagnetically reliable early remanence acquisition by greigite (Tric et al, 1991) are difficult to reconcile with petrographic observations of multiple generations of greigite formation .…”

“…Such considerations are also relevant to marginal seas, such as the Baltic Sea, Black Sea or Caspian Sea, that have been isolated from the open ocean for significant parts of their history. Transitions to brackish conditions particularly favour greigite formation and preservation (e.g., Sohlenius, 1996;Jelinowska et al, 1998;Strechie et al, 2002;Jørgensen et al, 2004;Neretin et al, 2014;Chang et al, 2014;Holmkvist et al, 2014).…”

“…If the bacteria lived within sediment, particularly below the surface mixed layer, it will be necessary to understand whether the associated biogeochemical remanent magnetization produced by such biogenic particles (Tarduno and Wilkison, 1996;Tarduno et al, 1998) is widely responsible for paleomagnetic signals in the geological record (Roberts et al, 2013a;Larrasoaña et al, 2014). It is also expected that, in the coming years, greigite magnetofossils will prove to be much more commonly preserved in sulphidic sediments (Chang et al, 2014).…”