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

During 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 high-resolution 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 (Fe3S4). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 degrees 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 similar to 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.