Comparison of Thermal and Microwave Paleointensity Estimates in Specimens Displaying Non‐Ideal Behavior in Thellier‐Style Paleointensity Experiments

Determining the strength of the ancient geomagnetic field is vital to our understanding of the core and geodynamo but obtaining reliable measurements of the paleointensity is fraught with difficulties. Over a quarter of magnetic field strength estimates within the global paleointensity database from 0‐5 Ma come from Hawaiʻi. Two previous studies on the SOH1 drill core gave inconsistent, apparently method‐dependent paleointensity estimates, with an average difference of 30%. The paleointensity methods employed in the two studies differed both in demagnetization mechanism (thermal or microwave radiation) and Thellier‐style protocol (perpendicular and Original Thellier protocols) – both variables that could cause the strong differences in the estimates obtained. Paleointensity experiments have therefore been conducted on 79 specimens using the previously untested combinations of Thermal‐Perpendicular and Microwave‐Original Thellier methods to analyze the effects of demagnetization mechanism and protocol in isolation. We find that, individually, neither demagnetization mechanism nor protocol entirely explains the differences in paleointensity estimates. Specifically, we found that non‐ideal multi‐domain‐like effects are enhanced using the Original Thellier protocol (independent of demagnetization mechanism), often resulting in paleointensity overestimation. However, we also find evidence, supporting recent findings from the 1960 Kilauea lava flow, that Microwave‐Perpendicular experiments performed without pTRM checks can produce underestimates of the paleointensity due to unaccounted‐for sample alteration at higher microwave powers. Together, these findings support that the true paleointensities fall between the estimates previously published and emphasize the need for future studies (thermal or microwave) to use protocols with both pTRM checks and a means of detecting non‐ideal grain effects

Intensity of the Earth's magnetic field: Evidence for a Mid-Paleozoic dipole low

The Mesozoic Dipole Low (MDL) is a period, covering at least ∼80 My, of low dipole moment that ended at the start of the Cretaceous Normal Superchron. Recent studies of Devonian age Siberian localities identified similarly low field values a few tens of million years prior to the Permo-Carboniferous Reverse Superchron (PCRS). To constrain the length and timing of this potential dipole low, this study presents paleointensity estimates from Strathmore (∼411 to 416 Ma) and Kinghorn (∼332 Ma) lava flows, United Kingdom. Both localities have been studied for paleomagnetic poles (Q values of 6 to 7), and the sites were assessed for their suitability for paleointensity from paleodirections, rock magnetic analysis, and microscopy. Thermal and microwave experiments were used to determine site mean paleointensity estimates of ∼3 to 51 μT (6 to 98 ZAm2) and 4 to 11 μT (9 to 27 ZAm2) from the Strathmore and Kinghorn localities, respectively. These, and all the sites from 200 to 500 Ma from the (updated) Paleointensity database (PINT15), were assessed using the Qualitative Paleointensity criteria (QPI). The procurement of reliable (QPI ≥ 5) weak paleointensity estimates from this and other studies indicates a period of low dipole moment (median field strength of 17 ZAm2) from 332 to 416 Ma. This “Mid-Paleozoic Dipole Low (MPDL)” bears a number of similarities to the MDL, including the substantial increase in field strength near the onset of the PCRS. The MPDL also adds support to the inverse relationship between reversal frequency and field strength and a possible ∼200-My cycle in paleomagnetic behavior relating to mantle convection.</jats:p

Chemical and Pb Isotope Composition of Phenocrysts from Bentonites Constrains the Chronostratigraphy around the Cretaceous-Paleogene Boundary in the Hell Creek Region, Montana

An excellent record of environmental and paleobiological change around the CretaceousPaleogene boundary is preserved in the Hell Creek and Fort Union Formations in the western Williston Basin of northeastern Montana. These records are present in fluvial deposits whose lateral discontinuity hampers long-distance correlation. Geochronology has been focused on bentonite beds that are often present in lignites. To better identify unique bentonites for correlation across the region, the chemical and Pb isotopic composition of feldspar and titanite has been measured on 46 samples. Many of these samples have been dated by 40Ar/39Ar. The combination of chemical and isotopic compositions of phenocrysts has enabled the identification of several unique bentonite beds. In particular, three horizons located at and above the Cretaceous-Paleogene boundary can now be traced—based on their unique compositions—across the region, clarifying previously ambiguous stratigraphic relationships. Other bentonites show unusual features, such as Pb isotope variations consistent with magma mixing or assimilation, that will make them easy to recognize in future studies. This technique is limited in some cases by more than one bentonite having compositions that cannot be distinguished, or bentonites with abundant xenocrysts. The Pb isotopes are consistent with a derivation from the Bitterroot Batholith, whose age range overlaps that of the tephra. These data provide an improved stratigraphic framework for the Hell Creek region and provide a basis for more focused tephrostratigraphic work, and more generally demonstrate that the combination of mineral chemistry and Pb isotope compositions is an effective technique for tephra correlation

Characterization of Magnetic Mineral Assemblages in Clinkers: Potential Tools for Full Vector Paleomagnetic Studies

Abstract High‐quality paleointensity data are essential for improving our understanding of the geomagnetic field; however, it is challenging to find materials that reliably record full vector magnetization going back in time. Here, we examine a new candidate material for paleointensity studies: clinkers, which are rocks that have been baked, metamorphosed, or melted by underlying coal seam fires. Previous studies conducted on clinkers suggest that they may be high‐fidelity magnetic field recorders. However, due to the inhomogeneity of clinker deposits and limited scope of previous studies, it is unknown under what conditions these conclusions hold true. To better assess this, we quantified the variation of magnetic properties within clinker deposits collected from the Powder River Basin, Montana, as a function of lithology, oxidation state, distance from the coal seam, and location. Our results indicate that the clinker products contain three main magnetic minerals: magnetite, hematite, and the rare ε‐Fe2O3. Clinker lithology was found to be the primary control on magnetic mineralogy, where strongly baked sediment and porcellanite are dominated by varying proportions of hematite, ε‐Fe2O3, and magnetite, and paralavas are dominated by low‐Ti magnetite. All clinker materials are thermally stable and likely experienced temperatures in excess of the magnetite Curie temperature. Grain size analysis indicates that the magnetic particles in all clinker materials are amenable to high‐quality paleointensity study. In total, our study confirms that clinkers should be reliable full vector paleomagnetic recorders

Deccan volcanism at K-Pg time

The last major mass extinctions in Earth history (e.g., end-Guadalupian, end-Permian, end-Triassic, and end-Cretaceous) are all correlated closely in time with the main-phase eruptions of major flood basalt provinces (Emeishan, Siberian, Central Atlantic Magmatic Province, and Deccan Traps, respectively). The causal relationship between flood volcanism and mass extinction is not clear, but likely involves the climate effects of outgassed volatile species such as CO2, SO2, Cl, F, etc., from some combination of magma and country rocks. In a surprising “coincidence,” the end-Cretaceous (K-Pg boundary) micro-faunal extinction also corresponds precisely in time to what may have been the largest meteor impact of the past billion years of Earth history, the Chicxulub crater at 66.05 Ma. The Deccan Traps eruptions were under way well before K-Pg/Chicxulub time and are most likely the result of the mantle plume “head” that initiated the presently active Reunion hotspot track-thus the Deccan Traps were clearly not generated, fundamentally, by the impact. However, recent high-precision 40Ar/39Ar geochronology indicates that conspicuous changes in basalt geochemistry, lava flow morphology, emplacement mode, and a possible 50% increase in eruption rate at the Lonavala/Wai subgroup transition in the Deccan Traps lava group corresponded, within radioisotopic age precision, to the K-Pg boundary and the Chicxulub impact. This has led to the testable hypothesis that the Mw ~11 seismic disturbance of the Chicxulub impact may have affected the Deccan eruptions. Here we review a broad landscape of evidence regarding Deccan volcanism and its relation to the K-Pg boundary and attempt to define what we see as the most important questions than can and should be answered by further research to better understand both the onshore and largely unknown offshore components of Deccan-related volcanism, and what their climate and environmental impacts at K-Pg time may have been

The eruptive tempo of Deccan volcanism in relation to the Cretaceous-Paleogene boundary.

Late Cretaceous records of environmental change suggest that Deccan Traps (DT) volcanism contributed to the Cretaceous-Paleogene boundary (KPB) ecosystem crisis. However, testing this hypothesis requires identification of the KPB in the DT. We constrain the location of the KPB with high-precision argon-40/argon-39 data to be coincident with changes in the magmatic plumbing system. We also found that the DT did not erupt in three discrete large pulses and that &gt;90% of DT volume erupted in &lt;1 million years, with ~75% emplaced post-KPB. Late Cretaceous records of climate change coincide temporally with the eruption of the smallest DT phases, suggesting that either the release of climate-modifying gases is not directly related to eruptive volume or DT volcanism was not the source of Late Cretaceous climate change

A Hyperactive Geomagnetic Field in the Late Visean (Early Carboniferous) From the Late Asbian Stratotype Section in Northwest England, UK

Plain Language Summary: Nearly synchronous global changes in geomagnetic polarity give both a detailed irregular pacing to geological time and provide a glimpse into heat transfer processes across the core—mantle boundary which drives the Earth's geodynamo. Although the Late Carboniferous is characterized by some well‐studied reversals, details of the tempo of polarity changes in the Early Carboniferous are unknown. This work addresses this by providing a detailed record of polarity changes over a ∼2 million year interval at around 334.5–332.5 million years ago‐from the Trowbarrow Quarry section in NW England. We demonstrate that these limestones likely preserve magnetization from close to their time of formation and record at least 31 polarity reversals. These observations support the idea that the Earth's dynamo was in a hyperactive reversing state similar to those sustained for tens of Myr in the Late Jurassic, parts of the Cambrian and the Late Ediacaran. It further corroborates a ∼200 Myr cyclicity in paleomagnetic field behavior since the Precambrian, potentially linked to variable core heat flow forced by mantle convection

Quantitative estimates of average geomagnetic axial dipole dominance in deep geological time

A defining characteristic of the recent geomagnetic field is its dominant axial dipole which provides its navigational utility and dictates the shape of the magnetosphere. Going back through time, much less is known about the degree of axial dipole dominance. Here we use a substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth’s surface. Applying this relation to published estimates of equatorial angular dispersion implies that geomagnetic axial dipole dominance averaged over 107–109 years has remained moderately high and stable through large parts of geological time. This provides an observational constraint to future studies of the geodynamo and palaeomagnetosphere. It also provides some reassurance as to the reliability of palaeogeographical reconstructions provided by palaeomagnetism