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SUMMARY Palaeomagnetic field intensity measurements, derived from rocks with ages that span geological time, provide a crucial constraint on the evolution of Earth’s deep interior and its magnetic environment. The palaeointensity database PINT has been updated to version v.8.0.0 and includes palaeointensity site-mean records spanning an interval from 50 ka to 4.2 Ga, compiling efforts from the palaeomagnetic community spanning from 1959 to the end of 2019. Nearly all site-mean palaeointensity records have been assessed using the qualitative reliability of palaeointensity (quality of palaeointensity, QPI) framework. This updated database brings together and harmonizes prior QPI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess QPI. The spatio-temporal distribution of the PINT database is analysed, revealing substantial biases towards young records (from the Brunhes chron) in the Northern hemisphere, and intervals with little to no palaeointensity data with a duration of 10s to 100s of millions of years in the Palaeozoic and Precambrian. General QPI compliance is characterized for the PINT database, which shows that the median QPI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing QPI scores in studies published after the year 2000. This illustrates an increasing community awareness of what is required to establish confidence in palaeointensity data and an increasing robustness of the large scale interpretations that can be made with these data. We additionally present a description of the long-term average dipole field strength with descriptive statistics for distinct intervals of Earth history.

Numerical geodynamo simulations capture several features of the spatial and temporal geomagnetic field variability on historical and Holocene timescales. However, a recent analysis questioned the a...

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 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth’s formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ~0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful “age” from a measured “date” or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity.

Deccan Traps flood basalt volcanism affected ecosystems spanning the end-Cretaceous mass extinction, with the most significant environmental effects hypothesized to be a consequence of the largest eruptions. The Rajahmundry Traps are the farthest exposures (~1,000 km) of Deccan basalt from the putative eruptive centers in the Western Ghats and hence represent some of the largest volume Deccan eruptions. Although the three subaerial Rajahmundry lava flows have been geochemically correlated to the Wai Subgroup of the Deccan Traps, poor precision associated with previous radioisotopic age constraints has prevented detailed comparison with potential climate effects. In this study, we use new 40Ar/39Ar dates, paleomagnetic and volcanological analyses, and biostratigraphic constraints for the Rajahmundry lava flows to ascertain the timing and style of their emplacement. We find that the lower and middle flows (65.92 ± 0.25 and 65.67 ± 0.08 Ma, ±1σ systematic uncertainty) were erupted within magnetochron C29r and were a part of the Ambenali Formation of the Deccan Traps. By contrast, the uppermost flow (65.27 ± 0.08 Ma) was erupted in C29n as part of the Mahabaleshwar Formation. Given these age constraints, the Rajahmundry flows were not involved in the end-Cretaceous extinction as previously hypothesized. To determine whether the emplacement of the Rajahmundry flows could have affected global climate, we estimated their eruptive CO2 release and corresponding climate change using scalings from the LOSCAR carbon cycle model. We find that the eruptive gas emissions of these flows were insufficient to directly cause multi-degree warming; hence, a causal relationship with significant climate warming requires additional Earth system feedbacks.

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.

Two timelines for extinction The Cretaceous-Paleogene extinction that wiped out the nonavian dinosaurs 66 million years ago was correlated with two extreme events: The Chicxulub impact occurred at roughly the same time that massive amounts of lava were erupting from the Deccan Traps (see the Perspective by Burgess). Sprain et al. used argon-argon dating of the volcanic ash from the Deccan Traps to argue that a steady eruption of the flood basalts mostly occurred after the Chicxulub impact. Schoene et al. used uranium-lead dating of zircons from ash beds and concluded that four large magmatic pulses occurred during the flood basalt eruption, the first of which preceded the Chicxulub impact. Whatever the correct ordering of events, better constraints on the timing and rates of the eruption will help elucidate how volcanic gas influenced climate. Science , this issue p. 866 , p. 862 ; see also p. 815

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.

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., This study describes how the geomagnetic axial dipole dominance of Earth’s magnetic field remained stable through large parts of the geological time. Since other characteristics of the geomagnetic field have changed substantially on the same timescales, this new observation provides a challenge for future core modeling studies.

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.

The Cretaceous-Paleogene boundary (KPB) in the Hell Creek area of Montana is recognized in several places by an iridium anomaly, which is typically identified at or very near the lithological contact between the Hell Creek Formation and the Tullock Member of the Fort Union Formation. Previous work in the area has argued that organic carbon isotope (δ13Corg) excursions can be used for chemostratigraphic correlation within these continental depositional environments, most importantly for the identification of the KPB where impact evidence is unavailable. However, it is unclear how modern surficial weathering affects δ13Corg values, particularly in terrestrial depositional settings, and whether standard sampling methods are sufficient to obtain unaltered rock material. We tested the fidelity of the terrestrial δ13Corg record with respect to surficial alteration and contamination through investigation of different field sampling techniques, including hand trenches, a backhoe-excavated trench, and sediment coring. We find that δ13Corg values in hand and backhoe trenched sections are more positive than δ13Corg values in cored sections, implying that modern surficial alteration affects δ13Corg values but not overall trends. A negative δ13Corg excursion associated with the KPB is present in most sections we analyzed, but it does not appear to be unique within our sections. The KPB excursion occurs within a coal layer, and we observe similar excursions within other carbon-rich lithologies. Given that we cannot disentangle local lithological effects from global atmospheric changes, we conclude that a negative δ13Corg excursion is not an unequivocal indicator of the KPB in the Hell Creek area.

Long-term temporal variations of the magnetic field (timescales >10 Myr), characterized from paleomagnetic data, have been hypothesized to reflect the evolution of Earth's deep interior and couplings between the core and mantle. By tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations, we have a unique tool for assessing changes in the deep interior back in time. However, numerical simulations are not run in an Earth-like parameter regime and assessing how well they reproduce the geomagnetic field is difficult. Criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and Holocene timescales, but no such criteria exist for longer timescales. Here we present a new set of five criteria (Quality of Paleomagnetic Modeling criteria, Q PM ) that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long-term (∼10 Myr) paleomagnetic field. These criteria measure inclination anomaly, virtual geomagnetic pole dispersion at the equator, latitudinal variation in virtual geomagnetic pole dispersion, normalized width of virtual dipole moment distribution, and dipole field reversals. We have assessed 46 geodynamo simulations using the Q PM criteria. The simulations have each been run for the equivalent of at least ∼300 kyr, span reversing and non-reversing regimes, and include either homogeneous or heterogeneous heat flux boundary conditions. We find that none of our simulations reproduce all salient aspects of the long-term paleomagnetic field behavior for the past 10 Myr. Nevertheless, our simulations bracket Earth values, suggesting that an Earth-like simulation is feasible within the available computationally accessible parameter space. This new set of criteria can inform future simulations that aim to reproduce all aspects of Earth's long-term magnetic field behavior.

The global paleointensity database for 65–200 Ma was analyzed using a modified suite of paleointensity quality criteria (QPI) such that the likely reliability of measurements is assessed objectively and as consistently as possible across the diverse data set. This interval was chosen because of dramatic extremes of geomagnetic polarity reversal frequency ranging from greater than 10 reversals per million years in the Jurassic hyperactivity period (155–171 Ma) to effectively zero during the Cretaceous Normal Superchron (CNS; 84–126 Ma). Various attempts to establish a relationship between the strength of Earth's magnetic field and the reversal frequency have been made by previous studies, but no consensus has yet been reached primarily because of large uncertainties in paleointensity estimates and sensitivity of these estimates to data selection approaches. It is critical to overcome this problem because the evolution of the dipole moment is a first order constraint on the behavior of the geodynamo. Here we show that conventional statistical tests and Bayesian changepoint modeling consistently indicate the strongest median/average virtual dipole moment during the CNS. In addition, the CNS and Jurassic hyperactivity period are characterized by the highest and lowest percentage of virtual dipole moments exceeding the overall median for the 65‐ to 200‐Ma interval, respectively. These observations suggest that the superchron dynamo was able to generate stronger fields than the dynamo operating in the frequently reversing regime. While the precise mechanism remains unclear, our results are compatible with the hypothesis that field strength and reversal rate variation are controlled by changes in core‐mantle boundary thermochemical conditions.

Intermediate composition titanohematite, Fe2-yTiyO3 with 0.5 ≤ y ≤ 0.7, is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature. While largely overlooked in magnetostratigraphic studies, intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of Late Mesozoic and early Cenozoic Laramide clastic deposits throughout the central United States. Intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite, except its unique self-reversing property. Due to these similarities, and with detrital remanent magnetizations masking its self-reversing nature, intermediate titanohematite is often misidentified in sedimentary deposits. Past studies relied upon nonmagnetic techniques including X-ray diffraction and electron microprobe analysis. While these techniques can identify the presence of intermediate titanohematite, they fail to test whether the mineral is the primary recorder. To facilitate the identification of intermediate titanohematite in sedimentary deposits, we characterize this mineral using low-temperature magnetometry and high-temperature susceptibility experiments, and present a new identification technique based on titanohematite's self-reversing property, for sediments that span the Cretaceous-Paleogene boundary (Hell Creek region, Montana). Results from the self-reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field, but that intermediate, self-reversing titanohematite (y = 0.53–0.63) is an important ancillary carrier of remanence. While earlier literature suggests that intermediate titanohematite is rare in nature, reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record, particularly within depositional basins adjacent to calc-alkaline volcanics, than previously thought.

© 2016 Geological Society of America. Paleomagnetism of the North American Midcontinent Rift provides a robust paleogeographic record of Laurentia (cratonic North America) from ca. 1110 to 1070 Ma, revealing rapid equatorward motion of the continent throughout rift magmatism. Existing age and paleomagnetic constraints on the youngest rift volcanic and sedimentary rocks have been interpreted to record a slowdown of this motion as rifting waned. We present new paleomagnetic and geochronologic data from the ca. 1090-1083 Ma "late-stage" rift volcanic rocks exposed as the Lake Shore Traps (Michigan), the Schroeder-Lutsen basalts (Minnesota), and the Michipicoten Island Formation (Ontario). The paleomagnetic data allow for the development of paleomagnetic poles for the Schroeder-Lutsen basalts (187.8°E, 27.1°N; A95= 3.0°, N = 50) and the Michipicoten Island Formation (174.7°E, 17.0°N; A95= 4.4°, N = 23). Temporal constraints on late-stage paleomagnetic poles are provided by high-precision,206Pb-238U zircon dates from a Lake Shore Traps andesite (1085.57 ± 0.25 Ma; 2σ internal errors), a Michipicoten Island Formation tuff (1084.35 ± 0.20 Ma) and rhyolite (1083.52 ± 0.23 Ma), and a Silver Bay aplitic dike from the Beaver Bay Complex (1091.61 ± 0.14 Ma), which is overlain by the Schroeder-Lutsen basalt flows. These Michipicoten Island Formation dates are the youngest yet obtained from Midcontinent Rift volcanic rocks and indicate that rift magmatism was active for at least 25 m.y. The addition of these late-stage paleomagnetic poles to the Laurentian apparent polar wander path suggests that rapid motion of Laurentia continued throughout the entirety of rift volcanism.

We report a palaeomagnetic investigation of the last full geomagnetic field reversal, the Matuyama-Brunhes (M-B) transition, as preserved in a continuous sequence of exposed lacustrine sediments in the Apennines of Central Italy. The palaeomagnetic record provides the most direct evidence for the tempo of transitional field behaviour yet obtained for the M-B transition. 40Ar/39Ar dating of tephra layers bracketing the M-B transition provides high-accuracy age constraints and indicates a mean sediment accumulation rate of about 0.2 mm yr-1 during the transition. Two relative palaeointensity (RPI) minima are present in the M-B transition. During the terminus of the upper RPI minimum, a directional change of about 180 o occurred at an extremely fast rate, estimated to be less than 2 o per year, with no intermediate virtual geomagnetic poles (VGPs) documented during the transit from the southern to northern hemisphere. Thus, the entry into the Brunhes Normal Chron as represented by the palaeomagnetic directions and VGPs developed in a time interval comparable to the duration of an average human life, which is an order of magnitude more rapid than suggested by current models. The reported investigation therefore provides high-resolution integrated palaeomagnetic and radioisotopic data that document the fine details of the anatomy and tempo of the M-B transition in Central Italy that in turn are crucial for a better understanding of Earth's magnetic field, and for the development of more sophisticated models that are able to describe its global structure and behaviour.

Detailed understanding of ecosystem decline and recovery attending the Cretaceous-Paleogene boundary (KPB) mass extinctions is hindered by limited constraints on the pace and tempo of environmental events near the boundary. To mitigate this shortcoming, high-resolution 40Ar/39Ar geochronology was performed on tephras intercalated between fossiliferous terrestrial sediments of the upper Hell Creek and lower Fort Union Formations in the western Williston Basin of northeastern Montana (USA). Tephra samples were collected from 10 stratigraphic sections spanning an area of ∼5000 km2. Several distinctive tephras can be correlated between sections separated spatially by as much as ∼60 km. The tephras are thin distal deposits generally preserved only in lignite beds, which are interbedded with clastic deposits yielding vertebrate faunas of Lancian (late Maastrichtian) to Torrejonian (early Danian) North American Land Mammal Ages. Sanidine from 15 tephra samples was analyzed in 1649 total fusion experiments (1597 on single crystals) and 12 incremental heating analyses of multigrain aliquots. Ages were determined for 13 distinct tephras, ranging from 66.289 ± 0.051 to 64.866 ± 0.023 Ma, including only analytical uncertainties. This level of precision is sufficient to resolve the ages of all of the coal beds that have served as a basis for a regional stratigraphic framework. The data confirm that the Hell Creek–Fort Union formational contact is diachronous, and further support the age of the KPB impact layer at 66.043 ± 0.010 Ma (or ± 0.043 Ma considering systematic uncertainties). Application of the new results to previous magnetostratigraphic data indicates an appreciably compressed time interval between the base of chron C29r and the top of chron C28r, with a maximum duration estimate of 1.421 ± 0.066 Ma. Most notable is the implied brevity of chron C29r, with a maximum estimate of 457 ± 54 ka, and possibly as brief as 345 ± 38 ka, compared to the 710 ka estimate from the Geologic Time Scale 2012 (GTS2012). Further, application of new results to terrestrial biostratigraphy adds higher precision to the timing and tempo of biotic change before and after the KPB. Our results indicate that the timing of pre-KPB ecological decline is constrained to the last ∼200 ka of the Cretaceous, adding further support to the press-pulse extinction hypothesis. Additionally, the duration of the depauperate basal Paleogene Puercan 1 disaster fauna is confined to a 70 ka interval. Faunal recovery in this region, indicated by the appearance of primitive members of the placental mammal radiation and the restoration of taxonomic richness and evenness, occurred within ∼900 ka after the KPB. These results show that biotic recovery after the mass extinction in the terrestrial realm was more rapid than in the marine.

Deccan Traps continental flood basalt eruptions spanned the Cretaceous-Paleogene mass extinction, erupting over a million cubic kilometers of basalt over a total duration of approximately a million years. The environmental consequences of flood basalt eruptions depend on the timing and amount of volatile release; eruption rates are thus needed to evaluate their potential to cause climate change. Radioisotopic dates are not currently sufficient to resolve sub-ten thousand year eruptive tempos, necessary for constraining the effects of short-lifetime volatiles including sulfur dioxide. Recent studies have demonstrated that increases in mercury concentration in sedimentary records correlate with flood basalt eruptions under some circumstances. However, mercury concentrations have primarily been used to show the presence or absence of flood basalt eruptions. We show that this proxy can be used to quantitatively estimate eruptive rates using a mercury geochemical cycle framework. We illustrate this using new mercury chemostratigraphic records from terrestrial Cretaceous-Paleogene boundary sections in eastern Montana, USA, with multiple high-resolution chronologic constraints. We estimate that Deccan eruptions lasted on the order of centuries and released 500–3000 megagrams (Mg) of mercury per year, corresponding to ∼50–250 km3/a of lava. The box model framework highlights the importance of carefully accounting for differences in sedimentation rate and sampling resolution when comparing mercury records from different locations and depositional environments. While there are uncertainties in the box model estimates due to possible variation in flood basalt mercury emissions and sedimentation rates, they provide a useful framework to quantitatively evaluate the global mercury budget change indicated by changing concentration in sedimentary records. Eruptions of the estimated size would have released enough SO2, if it reached the stratosphere, to cause significant cooling for the duration of the eruption. However, given our constraints on the duration of individual eruptions, these colder periods are likely too brief to be clearly visible in most existing paleoclimate records.

Bolide impact and flood volcanism compete as leading candidates for the cause of terminal-Cretaceous mass extinctions. High-precision (40)Ar/(39)Ar data indicate that these two mechanisms may be genetically related, and neither can be considered in isolation. The existing Deccan Traps magmatic system underwent a state shift approximately coincident with the Chicxulub impact and the terminal-Cretaceous mass extinctions, after which ~70% of the Traps' total volume was extruded in more massive and more episodic eruptions. Initiation of this new regime occurred within ~50,000 years of the impact, which is consistent with transient effects of impact-induced seismic energy. Postextinction recovery of marine ecosystems was probably suppressed until after the accelerated volcanism waned.

Made available in DSpace on 2018-11-26T15:28:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-02-01 A recent study of the Matuyama-Brunhes (M-B) geomagnetic field reversal recorded in exposed lacustrine sediments from the Sulmona Basin (Italy) provided a continuous, high-resolution record indicating that the reversal of the field direction at the terminus of the M-B boundary (MBB) occurred in less than a century, about 786 ka ago. In the sediment, thin (4-6 cm) remagnetized horizons were recognized above two distinct tephra layers-SUL2-19 and SUL2-20-that occur similar to 25 and similar to 35 cm below the MBB, respectively. Also, a faint, millimetre-thick tephra (SUL2-18) occurs 2-3 cm above the MBB. With the aim of improving the temporal resolution of the previous Sulmona MBB record and understanding the possible influence of cryptotephra on the M-B record in the Sulmona Basin, we performed more detailed sampling and analyses of overlapping standard and smaller samples from a 50 cm-long block that spans the MBB. The new data indicate that (i) the MBB is even sharper than previously reported and occurs similar to 2.5 cm below tephra SUL2-18, in agreement with the previous study; (ii) the MBB coincides with the rise of an intensity peak of the natural remanent magnetization (NRM) intensity, which extends across SUL2-18; (iii) except for a 2-cm-thick interval just above tephra SUL2-18, the rock magnetic parameters (k, ARM, M-r, M-s, B-c, B-cr) indicate exactly the same magnetic mineralogy throughout the sampled sequence. We conclude that either SUL2-18 resulted in the remagnetization of an interval of about 6 cm (i.e. during the NRM intensity peak spanning similar to 260 +/- 110 yr, according to the estimated local sedimentation rate), and thus the detailed MBB record is lost because it is overprinted, or the MBB is well recorded, occurred abruptly about 2.5 cm below SUL2-18 and lasted less than 13 +/- 6 yr. Both hypotheses challenge our understanding of the geomagnetic field behaviour during a polarity transition and/or of the NRM acquisition process in the Sulmona lacustrine sediment. Ist Nazl Geofis & Vulcanol, I-00143 Rome, Italy CNR, Ist Geol Ambientale & Geoingn, I-00015 Monterotondo, Rome, Italy Columbia Univ, Barnard Coll, New York, NY 10027 USA CEA CNRS UVSQ, IPSL, UMR8212, Lab Sci Climat & Environm, F-91190 Gif Sur Yvette, France Univ Paris Saclay, F-91190 Gif Sur Yvette, France Berkeley Geochronol Ctr, Berkeley, CA 94709 USA Univ Calif Berkeley, Dept Earth & Planetary Sci, Berkeley, CA 94709 USA Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, BR-13506900 Rio Claro, SP, Brazil Univ Estadual Paulista, Inst Geociencias & Ciencias Exatas, BR-13506900 Rio Claro, SP, Brazil

© 2015 Geological Society of America. New constraints on the timing of the Cretaceous- Paleogene mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the "main-stage" eruptions of the Deccan continental flood basalt province suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was under way well before Chicxulub/Cretaceous-Paleogene time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1-1.0 J/m3 throughout the upper ~200 km of Earth's mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume "head." It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai Subgroup) lava flows, which together may account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/Cretaceous- Paleogene time, a huge pulse of mantle plume-derived magma passed through the crust with little interaction and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which inturn might help to determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the Cretaceous-Paleogene extinction.

Paleointensity is one of the least deter­ mined parameters in geomagnetism, yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the Earth’s core and lower mantle. We test two important hy­ potheses that affect our understanding of how paleointensity has changed in Earth’s past: (1) the Mesozoic Dipole Low (MDL) hypoth­ esis and (2) the inverse relationship between geomagnetic polarity reversal rate and paleo­ intensity. We report paleointensity results determined using both the Coe modified Thellier method and the IZZI protocol on the medium­grained hornblende­biotite granite of the Middle Jurassic Shamrock batholith in the southern Singatse Range, western Nevada (USA). Previous zircon U­Pb dating gives an age of batholith emplacement at ca. 165.8 ± 0.4 Ma. This age estimate is coincident with a period of unusually high reversal frequency (~10 reversals/Ma) in the Middle Jurassic. Remanence in the Shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase. The labora tory un­ blocking temperature range for the well­de­ fined thermal remanent magnetization is nar ­ row, between ~540 and 575 °C, and median destructive fields are between 40 and 50 mT, thus we infer that the age of rema nence ac­ quisition is approximated by the U­Pb age estimate for pluton emplacement. Two sets of quality criteria were used to assess the paleo­ intensity results. The first set yielded an 87% success rate while the second set using more stringent criteria resulted in only a 20% suc­ cess rate. Mean paleointensity values for the Shamrock batholith using the “loose” and “strict” quality criteria are 34.0 ± 6.4 µT (N = 13) and 33.6 ± 9.6 µT (N = 3) respectively, with errors reported at one standard deviation. After correction for both remanence anisot­ ropy and cooling rate, the mean values that pass “loose” and “strict” criteria are respec­ tively 17.9 ± 2.7 µT and 17.5 ± 2.6 µT. The uncorrected and corrected paleointensity values for the “strict” estimates yield respec­ tive mean virtual dipole moments (VDMs) of 62.7 ± 18 ZAm 2 (Z = 10 21 ) and 32.7 ± 4.9 ZAm 2 . The uncorrected value is within error of the VDMs reported for plagioclase grains isolated from ca. 160 Ma basalts from Ocean Drilling Program Site 801, however these values have an absolute difference ranging from ~6 to 20 ZAm 2 , while the corrected VDM value is more consistent with the esti­ mated average Jurassic field strength of ~29 ZAm 2 . Using a recent estimate for the long­ term stable field strength of ~42 ZAm 2 , our corrected values add further credence to both the MDL and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses.