2,000 Cores to decode the Superchron

Magnetic seafloor anomalies documented several reversals of the Earth’s magnetic field: the magnetic north pole and south pole did flip places. The last of these flips occurred about 780 000 years ago. But the length of a so called chron is not constant: there also existed periods with more frequent reverals or even with millions of years without any reversal at all.

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Abrupt vs. gradual geomagnetic transitions?

The Cretaceous Normal Superchron (CNS) is one of those long intervals of stable polarity, taking place between 84 to 121 million years ago. What exactly happens at a transition between a superchron and a reversing period is not understood. Hypotheses range from gradual changes to abrupt changes of the magnetic field. A gradual change would suggest a link of the geodynamo to core-mantle boundary conditions, an abrupt change is more likely related to the sudden reorganization of the chaotic geodynamo in the outer core.

Anang Sahroni’s doctoral project explores this debate by focusing on the transition towards the end of the CNS. He examines the paleosecular variation of the Earth’s magnetic field, which represents the spatial and temporal fluctuations of the geomagnetic field. Rocks such as lava flows record the prevailing geomagnetic field at the time of their formation. Anang is studying a sequence of lava flows that were emplaced round the time of the transition at the end of the superchron on the ocean floor and were pushed onto the continental crust through plate collisions, so-called ophiolites.

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Paleomagnetism of the East Sulawesi Ophiolite

The East Sulawesi Ophiolite (ESO) is one of the largest ophiolites in the world: its upper part consists of ~3 km-thick basaltic lava flows, preserving excellent recorder of the ancient geomagnetic field. Through paleomagnetic and paleosecular variation (PSV) analysis of these flows, the behavior of the geomagnetic field across the transition can be studied. Over 200 lava flows consisting of over 2000 drill cores were sampled between 2022-2024 during extensive 5-weeks field trips. During fieldwork for paleosecular variations, care must be taken to accurately document the orientation of the drill cores. The magnetization of the lava flows can deflect a magnetic compass in this process. The orientation of the cores was measured using the sun compass to avoid these magnetic anomalies.

Back in the laboratory Anang obtained the paleomagnetic directions through standard thermal and alternating field demagnetization. Finally, to remove the potential oversampling contributed by lava flows that were emplaced in a short period and recorded the same geomagnetic field, correlation test was done by comparing the directions between successive flows, resulting in over 90 independent records of paleomagnetic directions.

Assuming a dipolar field the position of the pole of the Earth’s magnetic field during the time of the deposition of the lava flow is calculated, known as the virtual geomagnetic pole (VGP).

Time evolution of the virtual geomagnetic poles scatter

The paleosecular variation, the fluctuation of the past Earth’s magnetic field, is now quantified using the VGP scatter as a proxy: after filtering transitional directions the angular standard deviation is calculated

Because VGP scatter varies with latitude Anang will compare his results with well-studied lava sequences emplaced at similar latitudes (25–35°), including the Deccan Traps in India and the Mojave–Sonora Desert in the USA, to examine how it evolved through time. From this temporal evolution, he aims to determine whether the CNS was characterized by a distinct regime compared to the period afterward, offering insights into how the Earth’s magnetic field recovers following long periods of stability.

Abstract

Geomagnetic secular variation from the East Sulawesi Ophiolite (Banggai Regency, Indonesia)

Anang Sahroni, Michelle Nadya Putri, Florian Lhuillier, Mochamad Nukman, Wiwit Suryanto

The East Sulawesi Ophiolite (ESO), thought to be emplaced in the Late Cretaceous, is one of the three largest ophiolites in the world. Its upper section consists of an ~1 km-thick sequence of basaltic flows that preserve palaeomagnetic records, providing insights into its palaeogeography and the behaviour of the Earth’s magnetic field towards the end of the Cretaceous Normal Superchron (CNS; 121–84 Ma), an enigmatic 40-Myr period during which the Earth’s magnetic field stopped reversing its polarity.

We collected palaeomagnetic samples from 230 lava flows in the Balantak (Batumandi, Teku, Pulo Dua), Toili, and Siuna districts. For each flow, we drilled 8–10 cores and oriented them using a sun compass to avoid misorientation errors due to magnetic anomalies. Systematic alternating field demagnetisation of the samples was conducted using the automated “sushibar” system with an embedded high-precision cryogenic magnetometer, supplemented by thermal demagnetisation of three sister samples per flow to ensure the robustness of the palaeodirectional data.

Based on the analysis of the distribution of palaeodirections derived from the 230 lava flows, we intend to provide a new estimate of the dispersion of the virtual geomagnetic poles at the end of CNS. This value will be compared with other available estimates over the past 120 Myr in order to better characterise the geomagnetic secular variation and the state of the geodynamo. In particular, we will address the controversial issue of the dependency between secular variation and reversal rate over the past 120 Myr.

A03p-021 Geomagnetic secular variation from the East Sulawesi Ophiolite (Banggai Regency, Indonesia) Anang Sahroni, Michelle Nadya Putri, Florian Lhuillier, Mochamad Nukman, Wiwit Suryanto. IAGA / IASPEI Joint Scientific Meeting, 31 August - 5 September 2025, Lisbon, Portugal.