Marine continental shelf sediments with high deposition rates may provide useful archives of rapid geomagnetic secular variation as long as the primary magnetization is not altered substantially by diagenesis. To quantify the effects of sulfate (SO42-) reduction, which is a dominant early diagenetic processes in such sediments, on paleomagnetic recording, we analyzed four ~6-m long sediment cores from the Mediterranean shelf. Two cores did not reach the methanogenic zone and are characterized by continuous organoclastic sulfate reduction (OSR), while the other two have a distinctive shallow sulfate-methane transition zone (SMTZ). Depth-age models based on 28 radiocarbon ages show that deposition was mostly non-synchronous, suggesting that different parts of the shelf stopped accumulating sediments at different times during the Holocene. The upper sediment column in all cores is dominated by detrital titanomagnetite and biogenic magnetite. OSR-affected sediments record continuous dissolution of the (titano)magnetites, resulting in a steady decrease in magnetic susceptibility and remanent magnetic properties. For cores that reach the methanogenic zone, similar behavior is observed at or above the STMZ, but the magnetic properties stabilize at greater depths. Paleomagnetic directions in these sediments are more coherent, with better agreement with geomagnetic models than sediments affected by OSR. We suggest that methane-rich sediments with a shallow SMTZ and high sedimentation rates can better preserve primary paleomagnetic signals than OSR-dominated sediments due to a lack of dissolved sulfide in the main methanogenic zone, and that a susceptibility decline with depth should be a warning sign for paleomagnetic studies.

Twenty-two sites, subjected to an IZZI-modified Thellier-Thellier experiment and strict selection criteria, recover a paleomagnetic axial dipole moment (PADM) of 62.24$\pm$ 30.6 ZAm$^2$ in Northern Israel over the Pleistocene (0.012 - 2.58 Ma). Pleistocene data from comparable studies from Antarctica, Iceland, and Hawaii, re-analyzed using the same criteria and age range, show that the Northern Israeli data are on average slightly higher than those from Iceland (PADM = 53.8 $\pm$ 23 ZAm$^2$, n = 51 sites) and even higher than the Antarctica average %\cite{asefaw21} (PADM = 40.3 $\pm$ 17.3 ZAm$^2$, n = 42 sites). Also, the data from the Hawaiian drill core, HSDP2, spanning the last half million years (PADM = 76.7 $\pm$ 21.3 ZAm$^2$, n = 59 sites) are higher than those from Northern Israel. These results, when compared to Pleistocene results filtered from the PINT database (www.pintdb.org) suggest that data from the Northern hemisphere mid-latitudes are on average higher than those from the southern hemisphere and than those from latitudes higher than 60$^{\circ}$N. The weaker intensities found at high (northern and southern) latitudes therefore, cannot be attributed to inadequate spatio-temporal sampling of a time-varying dipole moment or low quality data. The high fields in mid-latitude Northern hemisphere could result from long-lived non-axial dipole terms in the geomagnetic field with episodes of high field intensities occurring at different times in different longitudes. This hypothesis is supported by an asymmetry predicted from the Holocene, 100 kyr, and five million year time-averaged geomagnetic field models.

Our understanding of geomagnetic field intensity prior to the era of direct instrumental measurements relies on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases absolute paleointensity datasets are continuous over millennial timescales, in sub-centennial resolution, and directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximal intensity and change rate have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modeling of well-dated archaeomagnetic intensity data from the Levant and Upper Mesopotamia. We report new data from 23 groups of pottery collected from 18 consecutive radiocarbon-dated archaeological strata from Tel Megiddo, Israel. In the Near East, the period of 1700–550 BCE is represented by 84 groups of archaeological artifacts, 55 of which were dated using radiocarbon or a direct link to clear historically-dated events, providing unprecedented sub-century resolution. Moreover, stratigraphic relationships between samples collected from multi-layered sites enable further refinement of the data ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600 BCE, with virtual axial dipole moment (VADM) reaching values of 155–162 ZAm2 – much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ~0.35–0.55 μT/year (~0.7–1.1 ZAm2/year), at least twice the maximum rate inferred from direct observations spanning the past 190 years. The increase from 1750 BCE to 1030 BCE (73 to 161 ZAm2) depicts the Holocene’s largest change in field intensity.