Cyclostratigraphic analyses were performed on magnetic susceptibility (MS), and elemental Ti and Fe series along the upper Aptian-lower Albian interval of the Poggio le Guaine (PLG) core, a Cretaceous pelagic succession in the Umbria-Marche Basin (central Italy). This interval represents one the most detailed and complete sedimentary archives and records oceanic perturbations associated with Oceanic Anoxic Event (OAE) 1b. The MS, Ti and Fe orbital control indicates a timespan of 2.68 Myr for OAE 1b event (114.10 to 111.34 Ma) and short eccentricity cycles played a key role, in controlling the amount of detrital input from weathering during monsoonal periods. Our chronostratigraphic study also provides age of 114.09 Ma for 113/Jacob, 113.25 Ma for Kilian, 112.67 Ma as a central age of the Monte Nerone cluster, 111.70 Ma for Urbino and 111.37 Ma for Leenhardt subevents, and a timespan of ~20 kyr for 113/Jacob, 70 kyr for Kilian, 670 kyr for Monte Nerone cluster, 60 kyr for Urbino and 60 kyr for Leenhardt levels. This study provides compelling evidence of the enormous potential C-isotope stratigraphy as tie points for cyclostratigraphic studies and as a valuable way to evaluate diachronism of bioevents. The organic-rich levels encompassing OAE 1b event has particular characteristics resulting from the combination of warm climate triggered by volcanic CO2 input, heavy precipitation, intense weathering and rapid marine transgressions, which leads the oceanic-atmospheric perturbations, acting as amplifiers of orbital forcings paleoclimate changes, resulting in deoxygenation and carbon burial during OAE 1b.

Quantifying the contributions of distinct mineral populations in bulk magnetic experiments greatly enhances the analysis of environmental and rock magnetism studies. Here we develop a new method of parametric unmixing of susceptibility components in hysteresis loops. Our approach is based on a modified Gamma-Cauchy exponential model, that accounts for variable skewness and kurtosis. The robustness of the model is tested with synthetic curves that examine the effects of noise, sampling, and proximity of susceptibility components. We provide a Python-based script, the Hist-unmix package, which allows the user to adjust a direct model of up to three ferromagnetic components as well as a dia/paramagnetic contribution. Optimization of all the parameters is achieved through least squares fit (Levenberg-Marquardt method), with uncertainties of each inverted parameter calculated through a Monte Carlo error propagation approach. For each ferromagnetic component, it is possible to estimate the magnetization saturation (Ms), magnetization saturation of remanence (Mrs) and the mean coercivity (Bc). Finally, Hist-unmix was applied to a set of weakly magnetic carbonate rocks from Brazil, which typically show distorted hysteresis cycles (wasp-waisted and potbellied loops). For these samples, we resolved two components with distinct coercivities. These results are corroborated by previous experimental data, showing that the lower branch of magnetic hysteresis can be modeled by the presented approach and might offer important mineralogical information for rock magnetic and paleomagnetic studies.

Carbonate rocks frequently undergo remagnetisation events, which can partially/completely erase their primary detrital remanence and introduce a secondary component through thermoviscous and/or chemical processes. Despite belonging to different basins hundreds of kilometres apart, the Neoproterozoic carbonate rocks of South America (over the Amazon and São Francisco cratons) exhibit a statistically indistinguishable single-polarity characteristic direction carried by monoclinic pyrrhotite and magnetite, with paleomagnetic poles far from an expected detrital remanence. We use a combination of classical rock magnetic properties and micro-to-nanoscale imaging/chemical analysis using synchrotron radiation to examine thin sections of these remagnetised carbonate rocks. Magnetic data shows that most of our samples failed to present anomalous hysteresis properties, usually referred to as part of the “fingerprints” of carbonate remagnetisation. Combining scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS), highly sensitive X-ray fluorescence (XRF), and X-ray absorption spectroscopy (XAS) revealed the presence of subhedral/anhedral magnetite, or spherical grains with a core-shell structure of magnetite surrounded by maghemite. These grains are within the pseudo-single domain size range (as well as most of the iron sulphides) and spatially associated with potassium-bearing aluminium silicates. Although fluid percolation and organic matter maturation might play an important role, smectite-illitisation seems a crucial factor controlling the growth of these phases. X-ray diffraction analysis identifies these silicates as predominantly highly crystalline illite, suggesting exposure to epizone temperatures. Therefore, we suggest that the remanence of these rocks should have been thermally reset during the final Gondwana assembly, and locked in a successive cooling event during the Early-Middle Ordovician.

Paleoclimate studies on speleothems commonly use oxygen isotopes as a record of precipitation variability and carbon isotopes to document soil, vegetation, and atmospheric processes. Magnetic minerals in speleothems record complementary paleoclimate information but need to be interpreted within the context of the particular geographic and geologic setting in which a karst environment occurs. This study surveys 23 caves in South America (7°N to 25°S latitude). The present-day climate is dominated by a monsoon regime, with variable precipitation between 50 to 800 mm/month covering different biomes, therefore making South America a good candidate to explore the properties of magnetic minerals at the tropical/subtropical climate. We share a database of magnetic properties from 23 stalagmites samples (90 specimens), 4 soil samples (34 specimens) and 2 limestone samples (15 specimens). Measured rock magnetic parameters include magnetic susceptibility, natural, anhysteretic, and isothermal remanent magnetization (NRM, ARM, IRM), as well as low-temperature magnetometry and first-order reversal curves. These data help constrain the types and granulometry of the magnetic mineralogy that commonly occur in South American speleothems, their host carbonates, and their overlying soils. We show that concentration-dependent parameters in soils overlying the caves are two to three orders of magnitude higher than those in stalagmite and limestones. Despite these differences, unmixed coercivities between soil (median value of 19 mT) and stalagmites (median value 20 mT) and substantially different from those of host limestones (median 39 mT). Our results suggest that much of the magnetite in South American speleothems is pedogenic in origin, and may allow magnetic measurements to capture changing soil and vegetation dynamics in the epikarst through time.

Tracing the field in the past at the centennial and millennial timescale (i.e., the archeomagnetic timescale) is important in improving geomagnetic field models and also for field forecast analyses. The distribution of archeomagnetic data across the globe is very inhomogeneous with almost all data coming from the northern hemisphere, particularly from Europe and Asia. Yet, the southern hemisphere is the one presenting the highest variability for the past millennia and, it is also the region comprising the South Atlantic Anomaly (SAA). The SAA is characterized by the lowest total field intensity and it is presently located in Southern Brazil. Some studies suggest that SAA is a persistent field feature in South Atlantic for periods longer than historical times, but the beginning of the influence of strong non-dipole fields at the centennial-scale evolution in South Atlantic region, could give important information about the recurrence of SAA at longer timescales. Here, we report six new high-quality archeointensity results for the Pelotas city region, in South Brazil with ages ranging from 1790 to 1943 CE. Archeointensity measurements were performed with the double heating technique, including partial termoremanent magnetization (pTRM) checks and pTRM tail-checks. Measurements were corrected from anisotropy of TRM and cooling-rate. Archeointensity results vary from 36.4±0.6 μT to 27.8±2.0 μT. The new data complement previous results obtained in South and Southeast Brazil, Argentina and Chile and provide additional evidence for the rapid decay of the field in the region. In addition, it demonstrates the gradual increase in the contribution of non-dipolar components in the geomagnetic field of South America since 1800 CE, therefore tracking the arrival of the South Atlantic Anomaly to these times.