Early diagenetic greigite as a recorder of the palaeomagnetic signal in Miocene-Pliocene sedimentary rocks of the Carpathian foredeep (Romania)

Abstract

S U M M A R YDuring the Miocene-Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe 3 S 4 ). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 • C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have 'rectangular' shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of B c = 35-45 mT and B cr = 52-67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with B c values of 45-90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ∼ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.

A Greigite-Based Magnetostratigraphic Time Frame for the Late Miocene to Recent DSDP Leg 42B Cores from the Black Sea

“…The FORC diagrams indicate the presence of a highly interacting (pseudo) single domain with an B c peak at ∼60 mT but with a spread up to ∼85 mT ( Figure 2P). This FORC diagram shows many similarities to diagrams obtained from other greigite-bearing rocks of the Paratethys (Vasiliev et al, 2007;Chang et al, 2014). A more pronounced central ridge and less pronounced GRM are observed in comparison to type 2 ( Figure 2Q).…”

“…Type 4 IRM acquisition curves appear to be saturated or nearly saturated at 700 mT, making a fit with a detrital magnetite component similar to types 1-3 unlikely. Therefore, we propose a fit with two IRM components of approximately equal B 1/2 at ∼60-65 mT (Figures 3B,C) implying two overlapping greigite components (see also Vasiliev et al, 2007). FORC diagrams are more noisy for Type 4, which agrees with the lower amounts of greigite ( Figure 3D).…”

“…We preferred to thermally demagnetize the specimens rather than use alternating field demagnetization, because greigite-bearing samples are prone to acquire a gyro-remanent magnetization (GRM); the demagnetization trajectory is biased by this GRM acquisition from 30-40 mT upward (e.g., Vasiliev et al, 2007Vasiliev et al, , 2008Roberts et al, 2011). GRM can become stronger than the starting NRM.…”

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

“…On the other hand, the primary magnetic mineral assemblage of detrital origin is often overprinted by post-depositional alteration (Hounslow and Maher, 1999;Roberts et al, 1996). The latter results from changing redox conditions at the lake/sea floor and in subsurface sediments leading to dissolution of iron oxides and formation of ferrimagnetic iron sulfides, such as greigite and pyrrhotite (Demory et al, 2005;Froelich et al, 1979;Karlin and Levi, 1983;Rowan et al, 2009;Sagnotti, 2007;Vasiliev et al, 2007) or paramagnetic minerals, such as pyrite, siderite, and vivianite (Dong et al, 2000;Karlin and Levi, 1983).…”

The Blake Event recorded near the Eemian type locality – A diachronic onset of the Eemian in Europe

“…This type of FORC distributions is typical of diagenetic SD greigitebearing sediments (e.g. Roberts et al, 2011Roberts et al, , 2006Roberts et al, , 2000Rowan and Roberts, 2006;Sagnotti et al, 2010;Vasiliev et al, 2007). Many FORC diagrams have a variable mixture of these two FORC distributions: the central ridge representing one end member and the vertically spread concentric contours the other (Fig.…”