Records of the Laschamps geomagnetic polarity excursion from Black Sea sediments: magnetite versus greigite, discrete sample versus U-channel data

Norbert R Nowaczyk,Helge W Arz,Jiabo Liu

doi:10.1093/gji/ggaa506

Abstract

SUMMARY Magnetostratigraphic investigation of sediment cores from two different water depths in the SE Black Sea based on discrete samples, and parallel U-channels in one of the cores, yielded high-resolution records of geomagnetic field variations from the past about 68 ka. Age constrains are provided by three tephra layers of known age, accelerator mass spectrometry 14C dating, and by tuning element ratios obtained from X-ray fluorescence scanning to the oxygen isotope record from Greenland ice cores. Sedimentation rates vary from a minimum of ∼5 cm ka−1 in the Holocene to a maximum of ∼50 cm ka−1 in glacial marine isotope stage 4. Completely reversed inclinations and declinations as well as pronounced lows in relative palaeointensity around 41 ka provide evidence for the Laschamps geomagnetic polarity excursion. In one of the investigated cores also a fragmentary record of the Mono Lake excursion at 34.5 ka could be revealed. However, the palaeomagnetic records are more or less affected by greigite, a diagenetically formed magnetic iron sulphide. By definition of an exclusion criterion based on the ratio of saturation magnetization over volume susceptibility, greigite-bearing samples were removed from the palaeomagnetic data. Thus, only 25–55 per cent of the samples were left in the palaeomagnetic records obtained from sediments from the shallower coring site. The palaeomagnetic record from the deeper site, based on both discrete samples and U-channels, is much less affected by greigite. The comparison of palaeomagnetic data shows that the major features of the Laschamps polarity excursion were similarly recovered by both sampling techniques. However, several intervals had to be removed from the U-channel record due to the presence of greigite, carrying anomalous directions. By comparison to discrete sample data, also some directional artefacts in the U-channel record, caused by low-pass filtering of the broad magnetometer response functions, averaging across fast directional and large amplitude changes, can be observed. Therefore, high-resolution sampling with discrete samples should be the preferred technique when fast geomagnetic field variations, such as reversals and excursions, shall be studied from sedimentary records in the very detail.

Highlights

Geomagnetic field variations are an expression of short- to longterm geodynamo processes inside the Earth’s liquid outer core (e.g. Merrill & McFadden 1999; Roberts 2008)
For dating of investigated Holocene and late-glacial sediments from the SE Black Sea, accelerator mass spectrometry (AMS) 14C ages from two cores recovered from the western Black Sea during RV METEOR cruise M51/4 in 2001 were adopted (Fig. 1, Table 1)
Individual virtual geomagnetic pole (VGP) positions from the four cores from site M72/5–22 are shown as coloured closed diamonds (Fig. 10a) while the corresponding stack is shown as a black bold dotted line in all three projections of Fig. 10

Summary

Introduction

Geomagnetic field variations are an expression of short- to longterm geodynamo processes inside the Earth’s liquid outer core (e.g. Merrill & McFadden 1999; Roberts 2008). Geomagnetic field variations are an expression of short- to longterm geodynamo processes inside the Earth’s liquid outer core Merrill & McFadden 1999; Roberts 2008). Lacustrine and marine sediments, as well as archaeological artefacts can serve as recorders of these field variations, comprising palaeosecular variation, excursions, reversals, chrons and superchrons. The first geomagnetic excursion reported to have occurred in the Brunhes Chron is the Laschamps excursion, named after its type-locality close to the village ‘Laschamps’ in the Massif Central, France

Methods

Results

Discussion

Conclusion