Abstract
AbstractState‐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.
Highlights
Geomagnetic reversals are indispensable as chronological markers in sedimentary archives and volcanic rocks, giving age constraints over time intervals of millions of years
Greigite formed in shallow sediment depth and acquired a quasi‐synsedimentary chemical remanent magnetization
This can be explained by the formation of early diagenetic greigite within the complete glacial layer in the subsurface
Summary
Geomagnetic reversals are indispensable as chronological markers in sedimentary archives and volcanic rocks, giving age constraints over time intervals of millions of years. For the global benthic oxygen isotope stack LR04 (Lisiecki & Raymo, 2005) and for the Geomagnetic Polarity Time Scale (GPTS; Ogg, 2012) an age of 782 ka (±4 kyr) is suggested for the MB transition, stratigraphically positioned in Marine Isotope Stage (MIS) 19. The midpoint ages of the MB in different sedimentary archives integrate over 782 to 770 ka (Figure 1). This age range covers a stratigraphic interval from the peak interstadial MIS 19c to the onset of stage MIS 19a, which is a significant difference from a paleoclimatic perspective with implications for stratigraphic correlation. The most rapid postulated reversal was evidenced at sites in Italy (Macrì et al, 2018; Sagnotti et al, 2014; Sagnotti et al, 2016) and high‐resolution sites from
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