Recordings of Fast Paleomagnetic Reversals in a 1.2 Ma Greigite‐Rich Sediment Archive From Lake Ohrid, Balkans

Abstract

State‐of‐the‐art paleoclimate research strongly depends on the availability of time‐equivalent markers as chronological control to disentangle interrelationships in the climate system from regional to global scale. Geomagnetic reversals are regarded as excellent age constraints because they are global events and independent from climatic conditions. However, spatial variations of timing and internal dynamics of reversals may limit their precision. Our 1.2 Ma high‐resolution (~25 cm/kyr) sediment record from Lake Ohrid is promising to precisely depict the Matuyama‐Brunhes (MB) reversal and the Jaramillo subchron. Two generations of diagenetic ferrimagnetic minerals are present in glacial intervals of the Lake Ohrid record. Early diagenetic greigite acquired a quasi synsedimentary chemical magnetization, while a late diagenetic greigite formation, triggered by the upward diffusion of H2S‐rich waters, obscures the polarity record at the top of the Jaramillo. Interglacial intervals are unaffected by greigite formation, likely due to low iron concentrations. Based on an orbitally tuned age model with tephrostratigraphic markers, the base of Jaramillo can be precisely dated to 1072.4 ka, and the MB reversal to 778.5 ka. Both polarity reversals occurred very rapidly in our record, lasting 2.3 and 1 kyr, respectively. Our results reveal that the dipole component of the Earth's magnetic field fell below the nondipole components only for a short duration in the Mediterranean region. The comparison of the timing of the MB boundary across different archives implies that the onset of the reversal provides a more synchronous age marker compared to often used midpoint ages.

Lake Ohrid’s tephrochronological dataset reveals 1.36 Ma of Mediterranean explosive volcanic activity

“…The results were also validated by a comparison with paleomagnetic age constraints of the succession (base of Jaramillo subchron and Matuyama Brunhes reversal, see also Just, et al . 50 ), suggesting an agreement with all chronologies within errors 2 .…”

Fast Directional Changes during Geomagnetic Transitions: Global Reversals or Local Fluctuations?

“…Nevertheless, upon the careful choice of the input parameters, geodynamo simulations are found to be able to reproduce at least some features of the paleomagnetic observations [30,31]. In particular, most numerical findings [32][33][34] suggest a duration of millennia of polarity reversals, in agreement with paleomagnetic observations and paleomagnetic field models (see for example [21,[35][36][37][38][39] and the references therein). Furthermore, recent geodynamo simulations [26] have been able to reproduce VGP latitudinal changes of up to ∼1 • /y, compatible with the results from [13], and total directional changes of the order of ∼10 • /y.…”

“…Finally, we could not exclude that the observations of [13] can be reproduced by flows capable of global directional change with kinetic energy values available during the M-B reversal. However, given that the subcentennial reversal times that can be derived from these observations are not corroborated by the majority of independent studies (see for example [21,[37][38][39] and the references therein), this scenario seems at odds with our current understanding of geomagnetic field reversals' phenomenology. Taken together, these considerations suggest that it is unlikely that global directional changes are responsible for the rapid directional changes observed in the Sulmona Basin.…”

A High‐Resolution Paleosecular Variation Record for Marine Isotope Stage 6 From Southeastern Black Sea Sediments

“…The age model back to 637 ka is mainly based on tephrochronology and tuning of sedimentological parameters to orbital parameters (Francke et al., 2016). Further age constraints could be derived by some high‐resolution magnetostratigraphic investigation on the Matuyama‐Brunhes reversal and the Jaramillo subchron (Just et al., 2019) with discrete samples taken side by side, while the whole composite profile was generally sampled only every 50 cm. This was decided due to the fact that greigite frequently occurs in the sediments, as confirmed by rock magnetic investigations (Just et al., 2016).…”