Abstract Archaeointensity and rock-magnetic studies were undertaken on 49 baked clay artefacts from four archaeological sites (Ter, Junnar, Nalasopara and Kanheri) in Maharashtra, India. Rock-magnetic properties, including bulk magnetic susceptibility, magnetic remanence and thermomagnetic analysis, indicate the presence of a low-coercivity magnetite in fine (superparamagnetic, single domain) grain-sizes. The ratio of anhysteretic remanent magnetization to saturation isothermal remanent magnetization, the reversible high-temperature susceptibility curves and the 3-axes isothermal remanent magnetization tests also indicate that the artefacts dominantly possess fine-grained magnetic particles, carrying a stable thermoremanent magnetization (TRM). Archaeointensity was estimated using Coe's modified Thellier method corresponding to the linear behaviour of natural remanent magnetization loss and TRM gained plots, which were evaluated with ThellierTool4.0 software. Cooling rate and anisotropy of the TRM corrections were applied and the corrected intensities were used to calculate a mean archaeointensity value for each one of the four sites. The new archaeointensity values were plotted along the existing Indian archaeointensity values derived only from archaeological artefacts, and were compared with the SHA.DIF.14k and ARCH10k.1 global models’ predictions. The present study aims to improve the overall understanding of Indian geomagnetic field variation in the past by providing new high-quality archaeointensity results. However, still more archaeointensity values are required to develop a reliable secular variation curve for India.

The Early Cretaceous represents a time interval in the greenhouse world that was characterized by dramatic changes in the paleogeography, paleoceanography, and paleoclimate of the Earth system. Furthermore, a striking, prominent feature of the geomagnetic polarity time scale is the ~34 m.y. period of the normal polarity field (Cretaceous Normal Polarity Superchron). Although marine anomalies and paleomagnetic data from deep-sea cores and land sections indicate that a reversed polarity (M0r) with a duration of ~0.4 m.y. occurred before the superchron, incomplete exposure, coupled with gaps in sampling due to the presence of marl layers, has limited the identification of M0r in a number of sections. An integrated multidisciplinary investigation of lower Cretaceous sediments at the base of the Poggio le Guaine (PLG) core (Northern Apennines, central Italy) was carried out to identify the Barremian-Aptian contact, which is defined by the M0r lower boundary. Rock magnetic measurements of the studied interval of the PLG core reveal magnetite as the main magnetic carrier. Paleomagnetic results indicate a short interval characterized by reverse polarity. This interval is in the uppermost part of the Hedbergella excelsa planktonic foraminiferal zone and in the upper part of the Chiastozygus litterarius (NC6) calcareous nannofossil zone. Stable carbon (δ 13 C) and oxygen (δ 18 O) isotopes indicate a chemostratigraphy of the PLG core with the signature of oceanic anoxic event 1a (OAE 1a). The lithological expression of OAE 1a is the organic-rich black shale unit known as the Selli Level. The comparison of our magnetostratigraphic, biostratigraphic, and chemostratigraphic records throughout the Barremian-Aptian boundary with those available from previously investigated oceanic and land-based sites allows recognition of the magnetochron M0r and OAE 1a at PLG for the first time.

A detailed total intensity magnetic survey of a local negative magnetic anomaly located in the southern sector of the inner ring in the Ries impact structure was carried out in 2006–2007. As the suevite of the Ries crater is known to have an often strong reverse remanent magnetization causing negative magnetic anomalies, a suevite body lying below shallow lake sediments upon the crystalline basement rocks of the inner ring was suspected to be the cause of the anomaly. A drilling program conducted by the Geological Service of Bavaria offered the opportunity to drill a 100-m-deep core hole into this anomaly in 2006. The core stratigraphy involves from 0 to 4.5 m fluviatile Quaternary lake sediments, from 4.5 to 21 m Neogene clays of the Ries crater lake, and from 21 to 100 m suevite and impact melt rock. The suevite and the impact melt rock have a strong reverse remanent magnetization and very high Koenigsberger ratios. Thermomagnetic and coercivity analyses indicate that magnetite is the dominant carrier of the magnetization. The borehole unfortunately did not penetrate the crystalline basement rocks of the inner ring, but modeling of the magnetic source body indicates that the bottom of the hole could not be far from the contact. A macroscopic survey shows suevite from 21 to 87 m, highly diverse in terms of suevite types, and a gradational transition to massive impact melt rock constituting the lowermost 13 m of the drill core. A detailed macroscopic description and first results of microscopic observations reveal that suevite groundmass is substantially altered to secondary phyllosilicates (mostly smectite, minor chlorite) and locally extensive development of calcite. Crystalline basement–derived lithic clasts and minerals dominate the clast population, and only traces of clastic material derived from the upper sediment parts of the target could be recorded. Macroscopically and microscopically, melt fragments have mostly irregular shapes, which lead to the tentative conclusion that only part of the melt—and by implication suevite—mass is derived from fallout of the ejecta curtain. On the other hand, most melt fragments and larger lithic clasts are seemingly oriented subperpendicular to the core axis. This could be interpreted as being due alternatively to settling through air or lateral movement within the actual crater. The gradational zone between proper suevite and massive impact melt rock is characterized by increasing enrichment of melt component and concomitant reduction of suevitic groundmass, until in the uppermost impact melt rock, only millimeter-wide stringers of groundmass remain between densely packed centimeter- to decimeter-size melt fragments.