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

Palaeomagnetic field intensity measurements from the 2.6 Ga Yandinilling dyke swarm (Western Australia)

SUMMARY Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N = 103) and 10–500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today.</jats:p

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

Reconstructing fluvial incision rates based upon palaeo‐water tables in Chalk karst networks along the Seine valley (Normandy, France)

Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high‐level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo‐water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest‐level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka−1, dropping to ~0.08 m ka−1 between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka−1 from MIS 5 to the present time

Recurring magnetic field anomalies in the South Atlantic and the first palaeointensities from Saint Helena

&amp;lt;p&amp;gt;A long-lived hypothesis is that, if averaged over sufficient time (ca 10 million years), the Earth&amp;amp;#8217;s magnetic field approximates a geocentric axial dipole (GAD). Despite this common assumption, the question of how significant the non-GAD features are in the time-averaged field is an important and unresolved one. In the present-day field, the South Atlantic Anomaly (SAA) is the biggest irregularity in the field. We know that this anomaly has not always been a part of the field, but in Engbers et al., 2020, it was shown that the magnetic field shows irregular behaviour in this region on a million-year timescale. The irregular behaviour was demonstrated through a substantially high VGP dispersion (21.9&amp;amp;#186;) for lava flows from Saint Helena that are between 8 and 11 million years old. The island of Saint Helena is located at the margin of the present-day SAA and has declination -16.6&amp;amp;#186;, inclination -57.5&amp;amp;#186; relative to expected GAD values of 0.0&amp;amp;#186;/-7.8&amp;amp;#186; (Dec/Inc). We have now commenced the measurements of absolute palaeointensity data from this location. So far, we have performed thermal and microwave IZZI-Thellier experiments on 2 localities from Saint Helena. The site mean results show variable but generally very low field intensities, although further work is required to make these sufficiently robust. Our low field estimates suggest a field in the South Atlantic that is not only unstable, but mainly weaker than expected. This could mean that recurring reversed flux patches (RFP) are responsible for the irregularities and weaknesses in the field in this region, stretching back up to 11 million years ago.&amp;lt;/p&amp;gt;</jats:p

Comparison of thermal and microwave paleointensity estimates in specimens that violate Thellier’s laws

Deep in the Earth, thermodynamic behavior drives the geodynamo and creates the Earth’s magnetic field. Determining how the strength of the field, its paleointensity (PI), varies with time, is vital to our understanding of Earth’s evolution. Thellier-style paleointensity experiments assume the presence of non-interacting, single domain (SD) magnetic particles, which follow Thellier’s laws. Most natural rocks however, contain larger, multi-domain (MD) or interacting single domain (ISD) particles that often violate these laws and cause experiments to fail. Even for samples that pass reliability criteria designed to minimize the impact of MD or ISD grains, different PI techniques can give systematically different estimates, implying violation of Thellier’s laws. Our goal is to identify any disparities in PI results that may be explainable by protocol-specific MD and ISD behavior and determine optimum methods to maximize accuracy. Volcanic samples from the Hawai’ian SOH1 borehole previously produced method-dependent PI estimates. Previous studies showed consistently lower PI values when using a microwave (MW) system and the perpendicular method than using the original thermal Thellier-Thellier (OT) technique. However, the data were ambiguous regarding the cause of the discrepancy. The diverging estimates appeared to be either the result of using OT instead of the perpendicular method or the result of using MW protocols instead of thermal protocols. Comparison experiments were conducted using the thermal perpendicular method and microwave OT technique to bridge the gap. Preliminary data generally show that the perpendicular method gives lower estimates than OT for comparable Hlab values. MW estimates are also generally lower than thermal estimates using the same protocol