Greig A. Paterson

On improving the selection of Thellier‐type paleointensity data

The selection of paleointensity data is a challenging, but essential step for establishing data reliability. There is, however, no consensus as to how best to quantify paleointensity data and which data selection processes are most effective. To address these issues, we begin to lay the foundations for a more unified and theoretically justified approach to the selection of paleointensity data. We present a new compilation of standard definitions for paleointensity statistics to help remove ambiguities in their calculation. We also compile the largest-to-date data set of raw paleointensity data from historical locations and laboratory control experiments with which to test the effectiveness of commonly used sets of selection criteria. Although most currently used criteria are capable of increasing the proportion of accurate results accepted, criteria that are better at excluding inaccurate results tend to perform poorly at including accurate results and vice versa. In the extreme case, one widely used set of criteria, which is used by default in the ThellierTool software (v4.22), excludes so many accurate results that it is often statistically indistinguishable from randomly selecting data. We demonstrate that, when modified according to recent single domain paleointensity predictions, criteria sets that are no better than a random selector can produce statistically significant increases in the acceptance of accurate results and represent effective selection criteria. The use of such theoretically derived modifications places the selection of paleointensity data on a more justifiable theoretical foundation and we encourage the use of the modified criteria over their original forms.

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

A new mechanism for the magnetic enhancement of hematite during heating: the role of clay minerals

The magnetic properties of loess, lake, and ocean sediments are often used as indicators for paleoclimatic/paleoenvironmental changes. Thermomagnetic analysis is a conventional approach for identifying magnetic phases and thermal alteration of samples. Magnetic concentration parameters are often enhanced after thermal treatment. In this study, the role of clay minerals in magnetic enhancement at elevated temperatures is systematically investigated. The results indicate that the clay minerals (saponite, Ca-montmorillonite, kaolinite, and chlorite) are dominated by paramagnetic behaviour and that the magnetic properties remain relatively stable after heating to 700°C in argon. In contrast, mixtures of hematite and chlorite have a high degree of magnetic enhancement after heating in argon, which indicates that clay minerals play important role in magnetic enhancement. These results improve our understanding of the processes involved in complicated mineral transformations, which is important for retrieving paleoclimatic/paleoenvironemntal signals from magnetic proxies.

A new set of qualitative reliability criteria to aid inferences on palaeomagnetic dipole moment variations through geological time

Records of reversal frequency support forcing of the geodynamo over geological timescales but obtaining these for earlier times (e.g. the Precambrian) is a major challenge. Changes in the measured virtual (axial) dipole moment of the Earth, averaged over several millions of years or longer, also have the potential to constrain core and mantle evolution through deep time. There have been a wealth of recent innovations in palaeointensity methods, but there is, as yet, no comprehensive means for assessing the reliability of new and existing dipole moment data. Here we present a new set of largely qualitative reliability criteria for palaeointensity results at the site mean level, which we term QPI in reference to the long-standing Q criteria used for assessing palaeomagnetic poles. These represent the first attempt to capture the range of biasing agents applicable to palaeointensity measurements and to recognise the various approaches employed to obviate them. A total of 8 criteria are proposed and applied to 312 dipole moment estimates recently incorporated into the PINT global database. The number of these criteria fulfilled by a single dipole moment estimate (the QPI value) varies between 1 and 6 in the examined dataset and has a median of 3. Success rates for each of the criteria are highly variable, but each criterion was met by at least a few results. The new criteria will be useful for future studies as a means of gauging the reliability of new and published dipole moment estimates.

New methods for unmixing sediment grain size data

Grain size distribution (GSD) data are widely used in Earth sciences and although large data sets are regularly generated, detailed numerical analyses are not routine. Unmixing GSDs into components can help understand sediment provenance and depositional regimes/processes. End-member analysis (EMA), which fits one set of end-members to a given data set, is a powerful way to unmix GSDs into geologically meaningful parts. EMA estimates end-members based on covariability within a data set and can be considered as a nonparametric approach. Available EMA algorithms, however, either produce suboptimal solutions or are time consuming. We introduce unmixing algorithms inspired by hyperspectral image analysis that can be applied to GSD data and which provide an improvement over current techniques. Nonparametric EMA is often unable to identify unimodal grain size subpopulations that correspond to single sediment sources. An alternative approach is single-specimen unmixing (SSU), which unmixes individual GSDs into unimodal parametric distributions (e.g., lognormal). We demonstrate that the inherent nonuniqueness of SSU solutions renders this approach unviable for estimating underlying mixing processes. To overcome this, we develop a new algorithm to perform parametric EMA, whereby an entire data set can be unmixed into unimodal parametric end-members (e.g., Weibull distributions). This makes it easier to identify individual grain size subpopulations in highly mixed data sets. To aid investigators in applying these methods, all of the new algorithms are available in AnalySize, which is GUI software for processing and unmixing grain size data.

Insolation driven biomagnetic response to the Holocene Warm Period in semi-arid East Asia

The Holocene Warm Period (HWP) provides valuable insights into the climate system and biotic responses to environmental variability and thus serves as an excellent analogue for future global climate changes. Here we document, for the first time, that warm and wet HWP conditions were highly favourable for magnetofossil proliferation in the semi-arid Asian interior. The pronounced increase of magnetofossil concentrations at ~9.8 ka and decrease at ~5.9 ka in Dali Lake coincided respectively with the onset and termination of the HWP, and are respectively linked to increased nutrient supply due to postglacial warming and poor nutrition due to drying at ~6 ka in the Asian interior. The two-stage transition at ~7.7 ka correlates well with increased organic carbon in middle HWP and suggests that improved climate conditions, leading to high quality nutrient influx, fostered magnetofossil proliferation. Our findings represent an excellent lake record in which magnetofossil abundance is, through nutrient availability, controlled by insolation driven climate changes.

Magnetic domain state diagnosis using hysteresis reversal curves

We present results for a series of hysteresis measurements that provide information about remanent, induced, transient-free, and transient magnetization components. These measurements, and differences between measurement types, enable production of 6 types of first-order reversal curve (FORC)-like diagrams with only double the number of measurements involved in a conventional FORC measurement. These diagrams can be used to distinguish magnetic signatures associated with each domain state. When analyzing samples with complex magnetic mineral mixtures, the contrasting domain state signatures are mixed together in a traditional FORC diagram, but these signatures can be identified individually when using the various FORC diagrams discussed here. The ability to make different FORC measurements and to identify separately each magnetic component by investigating different magnetization types can provide much-improved understanding of the information provided by FORC diagrams. In particular, the transient hysteresis FORC diagram provides a method to measure the nucleation field of magnetic vortices and domain walls. We provide a simple explanation for FORC results from natural multi-domain samples that are not explained by conventional domain wall pinning models. We also provide software for processing the different types of FORC data.

Origin of microbial biomineralization and magnetotaxis during the Archean

Significance A wide range of organisms sense Earth’s magnetic field for navigation. For some organisms, like magnetotactic bacteria, magnetic particles form inside cells and act like a compass. However, the origin of magnetotactic behavior remains a mystery. We report that magnetotaxis evolved in bacteria during the Archean, before or near the divergence between the Nitrospirae and Proteobacteria phyla, suggesting that magnetotactic bacteria are one of the earliest magnetic-sensing and biomineralizing organisms on Earth. The early origin for magnetotaxis would have provided evolutionary advantages in coping with environmental challenges faced by microorganisms on early Earth. The persistence of magnetotaxis in separate lineages implies the temporal continuity of geomagnetic field, and this biological evidence provides a constraint on the evolution of the geodynamo. Microbes that synthesize minerals, a process known as microbial biomineralization, contributed substantially to the evolution of current planetary environments through numerous important geochemical processes. Despite its geological significance, the origin and evolution of microbial biomineralization remain poorly understood. Through combined metagenomic and phylogenetic analyses of deep-branching magnetotactic bacteria from the Nitrospirae phylum, and using a Bayesian molecular clock-dating method, we show here that the gene cluster responsible for biomineralization of magnetosomes, and the arrangement of magnetosome chain(s) within cells, both originated before or near the Archean divergence between the Nitrospirae and Proteobacteria. This phylogenetic divergence occurred well before the Great Oxygenation Event. Magnetotaxis likely evolved due to environmental pressures conferring an evolutionary advantage to navigation via the geomagnetic field. Earth’s dynamo must therefore have been sufficiently strong to sustain microbial magnetotaxis in the Archean, suggesting that magnetotaxis coevolved with the geodynamo over geological time.

Asian monsoon modulation of nonsteady state diagenesis in hemipelagic marine sediments offshore of Japan

We have identifiedmillennial-scale variations inmagneticmineral diagenesis from Pacific Ocean sediments offshore of Japan that we correlate with changes in organic carbon burial that were likely driven by Asian monsoon fluctuations. The correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred frommagnetic and geochemical analyses. Episodes of intense monsoon activity and attendant sedimentmagneticmineral diagenesis also appear to correlate with Heinrich events, which supports the existence of climatic telecommunications between Asia and the North Atlantic region. Several lines of evidence support our conclusions: (1) fluctuations in down-coremagnetic properties and diagenetic pyrite precipitation are approximately coeval; (2) localized stratigraphic intervals with relatively strongermagneticmineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to nonsteady state diagenesis; (3) down-core variations in elemental S content provide a proxy for nonsteady state diagenesis that correlate with key records of Asianmonsoon variations; and (4) relict titanomagnetite that is preserved as inclusions within silicate particles, rather than secondary authigenic phases (e.g., greigite), dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution.We suggest that such millennial-scale environmentalmodulation of nonsteady state diagenesis (that creates a temporal diagenetic filter and relict magneticmineral signatures) is likely to be common in organic-rich hemipelagic sedimentary settingswith rapidly varying depositional conditions. Ourwork also demonstrates the usefulness of magnetic mineral inclusions for recording important environmentalmagnetic signals.

The effects of secondary mineral formation on Coe‐type paleointensity determinations: Theory and simulation

Thellier-type experiments are the most widely applied approaches for determining the absolute paleointensities of Earths magnetic field. One major problem, however, is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments. Linear Arai plots with acceptable partial thermal remanent magnetization (pTRM) checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high-quality paleointensity estimates. However, by simulating the Coe variant of the Thellier method on assemblages of single domain (SD) magnetite particles, it is demonstrated that new magnetic minerals, which form during thermal treatments, can result in linear, concave-up, or concave-down Arai plots depending on the magnetic properties of both the primary and secondary magnetic phases. Among this range of behavior, pseudoideal Arai plots, which are linear with acceptable pTRM check statistics, would lead to paleointensity underestimates. It is further demonstrated that pTRM checks are proportional to the degree of underestimate with a magnetic granulometry dependency for SD particles. Due to the complexity of this dependency, pTRM check statistics are only comparable when specimens have similar magnetic properties. This suggests that a universal threshold for pTRM check statistics is not likely to be effective. Since the criteria of linearity and low pTRM check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature-dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration.