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

Age of emplacement and paleolatitude of the Agulhas Plateau – IODP Expedition 392, Site U1582

The Agulhas Plateau (AP), along with Maud Rise (MR) and Northeast Georgia Rise (NEGR), is part of the greater Southeast African Large Igneous Province (LIP) that is hypothesized to have been emplaced above the Bouvet hotspot during breakup of Africa and East Antarctica. Since their emplacement in the Late Cretaceous, these prominent submarine plateaus have been rifted along a triple junction and have controlled connectivity between the South Atlantic, Southern Ocean, and southern Indian Ocean basins. Igneous rocks recovered on a basement high at Site U1582 (located at ~37°S) on the northern Agulhas Plateau hold clues to the age, paleolatitude, nature of LIP basement, and its relation to the mid-ocean ridges that separated the AP from MR and NEGR. At Site U1582, a pillow basalt sequence with intercalated sediments was recovered. Based on initial shipboard biostratigraphy, a Santonian age (~85 Ma) was assigned to this unit. The moderately altered, mildly porphyritic mafic igneous basement rocks recovered at Site U1582 contain abundant veins and carbonate-filled voids. We report in situ U-Pb ages of carbonate vein and void fills (n=20) along with paleomagnetic directions (n=25) from the basaltic basement. The oldest carbonate formed at ~95 Ma (Cenomanian), which we interpret as early diagenetic ages that immediately postdate LIP emplacement during cooling and associated fracturing. Younger vein generations reflect basement carbonation due to seawater circulation, which prevailed until long after LIP emplacement. Basaltic basement rocks at Site U1582 record magnetic field directions reliably. Paleomagnetic analysis yields a negative inclination (normal polarity), which we correlate with Chron C34n (Cretaceous Normal Superchron). We calculate a ~45-50°S preliminary mean paleolatitude that allows us to refine plate kinematic reconstructions and to test the genetic relationship between the AP, MR and NEGR. Our combined age-paleolatitude dataset has implications for Cretaceous paleogeography, ocean circulation, and oceanic crust carbonation timescales

Paleomagnetic and Rock Magnetic Analysis of Sediments and Lavas Obtained on IODP Expedition 392 Agulhas Plateau Cretaceous Climate

IODP Expedition 392 to the Agulhas Plateau (AP) recovered sedimentary and igneous sequences from four sites (Sites 392-U1579, 392-U1580, 392-U1581, and 392-U1582) ranging in age from the Late Cretaceous to the Pleistocene. The primary objectives of this expedition were to examine the nature of the AP basement, the opening of oceanic gateways, and the evolution of the climate system through the Cretaceous hothouse and into the Cenozoic. A key to achieving these objectives is the development of high-quality age models for the sedimentary and igneous sequences recovered from each site. Shipboard age models were developed using a combination of biostratigraphic age constraints, in addition to magnetostratigraphy. To improve upon the age model, shore-based paleomagnetic analysis of discrete samples was performed on intervals where polarity could not be confidently determined from shipboard archive half measurements, specifically focused on intervals where refined age models help achieve the Expedition objectives. Rock and environmental magnetic analysis was also performed on select discrete samples to characterize changes in magnetic mineralogy and grain size throughout the sedimentary sequence captured in each hole. Results from rock magnetic experiments help assess the reliability of measured magnetic signals and further can be used to say something about paleoenvironmental conditions. Magnetic minerals are responsive to many environmental changes including changes in sediment source, redox, weathering, and paleooceanographic conditions and can be utilized as a powerful tool for investigating past environments. Magnetic mineralogic changes will be connected to results from pore water geochemistry and astronomical tuning to help further understand the processes behind the observed changes. Here, we will present on the updated magnetostratigraphy and preliminary rock and environmental magnetic analyses

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

Earliest Palaeocene purgatoriids and the initial radiation of stem primates

Plesiadapiform mammals, as stem primates, are key to understanding the evolutionary and ecological origins of Pan-Primates and Euarchonta. The Purgatoriidae, as the geologically oldest and most primitive known plesiadapiforms and one of the oldest known placental groups, are also central to the evolutionary radiation of placentals and the Cretaceous-Palaeogene biotic recovery on land. Here, we report new dental fossils of Purgatorius from early Palaeocene (early Puercan) age deposits in northeastern Montana that represent the earliest dated occurrences of plesiadapiforms. We constrain the age of these earliest purgatoriids to magnetochron C29R and most likely to within 105–139 thousand years post- K/Pg boundary. Given the occurrence of at least two species, Purgatorius janisae and a new species, at the locality, we provide the strongest support to date that purgatoriids and, by extension, Pan-Primates, Euarchonta and Placentalia probably originated by the Late Cretaceous. Within 1 million years of their arrival in northeastern Montana, plesiadapiforms outstripped archaic ungulates in numerical abundance and dominated the arboreal omnivore–frugivore niche in mammalian local faunas

Expedition 392 summary

During International Ocean Discovery Program Expedition 392, three sites were drilled on the Agulhas Plateau and one site was drilled in the Transkei Basin in the Southwest Indian Ocean. This region was positioned at paleolatitudes of ~53°–61°S during the Late Cretaceous (van Hinsbergen et al., 2015) (100–66 Ma) and within the new and evolving gateway between the South Atlantic, Southern Ocean, and southern Indian Ocean basins. Recovery of basement rocks and sedimentary sequences from the Agulhas Plateau sites and a thick sedimentary sequence in the Transkei Basin provides a wealth of new data to (1) determine the nature, origin, and bathymetric evolution of the Agulhas Plateau; (2) significantly advance the understanding of how Cretaceous temperatures, ocean circulation, and sedimentation patterns evolved as CO2 levels rose and fell and the breakup of Gondwana progressed; (3) document long- and short-term paleoceanographic variability through the Late Cretaceous and Paleogene; and (4) investigate geochemical interactions between igneous rocks, sediments, and pore waters through the life cycle of a large igneous province (LIP). Importantly, postcruise analysis of Expedition 392 drill cores will allow testing of competing hypotheses concerning Agulhas Plateau LIP formation and the role of deep ocean circulation changes through southern gateways in influencing Late Cretaceous–early Paleogene climate evolution

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