Introduction

In our quest to understand the geomagnetic field of past millennia, we rely on two primary sources of data. The first source is archaeomagnetic data, which provides us with valuable information about the geomagnetic field based on ancient artifacts. However, one major challenge we encounter is the uneven distribution of this data in both space and time. It’s like trying to put together a historical jigsaw puzzle without all the pieces.

But don’t worry, there is another data source that helps overcome these limitations and provides a more comprehensive understanding of the past geomagnetic field. Sedimentary records, a treasure trove of information spanning vast time periods and offering improved spatial coverage. Imagine these records as growing bars, each representing a time series of the geomagnetic field at a specific location. In contrast, archaeomagnetic data appears as dots, providing snapshots of the geomagnetic field at specific locations. As we venture further back in time, sediment data becomes increasingly essential since archaeomagnetic data becomes sparser.

Now let’s unravel the different magnetization mysteries within these sediments. Archaeomagnetic data captures the geomagnetic field through a fascinating process known as thermoremanent magnetization (TRM). Picture this: ancient artifacts, like pottery and kiln structures but also lava flows are heated and then left to cool down. But here’s the magical part - during this process, they become magnetized, preserving a snapshot of the geomagnetic field at that exact moment. It’s like capturing a piece of geomagnetic field’s history in a magnetic time capsule.

The magnetization process in sediments is known as detrital remanent magnetization (DRM). During the sedimentation process, magnetic particles settle in such a way that their magnetic moments tend to point in the direction of the geomagnetic field. It’s like they have an ancient compass within them, pointing in the direction of the geomagnetic forces. With the accumulation of additional sediment material, the magnetic particles become mechanically fixed within the sediment structure, preserving their magnetic orientations. It’s as if they have been frozen in time, capturing the magnetic field’s influence at the lock-in moment.

The magnetization in sediments is influenced by several factors, such as the interaction between the magnetic particles and the substrate at the sediment-water interface, as well as the dewatering process of the sediment. It’s worth mentioning that there are different terminologies and classifications to describe these effects. For our discussion, we’ll refer to the general remanent magnetization found in sediments as detrital remanent magnetization (DRM). We can further categorize the magnetization acquired through the interaction of particles with the substrate at the sediment-water interface as depositional DRM (dDRM). On the other hand, post-depositional DRM (pDRM) refers to magnetization acquired after the particles have settled on the sediment-water interface, occurring over longer timescales.

Within depositional DRM, various effects come into play. For instance, there is the inclination error, which arises when non-spherical particles settle flat on the sediment-water interface. This leads to a distortion of the inclination, resulting in smaller inclination values than expected. Another form of inclination distortion occurs when aligned particles roll into the nearest depression on the sediment-water interface. These effects contribute to the complexity of interpreting sediment magnetization data.

In our investigation, we’re particularly interested in unraveling the secrets of post-depositional DRM. Initially, only the larger sediment particles become mechanically fixed shortly after deposition. Smaller particles, on the other hand, enjoy a freer journey, moving within water-filled voids and pore spaces for a longer duration. However, as the sediment consolidates and dries out, these smaller particles slowly become locked in too. It’s a mesmerizing process, like witnessing magnetic particles tell stories of the ever-changing geomagnetic field.

Check out the figure below that illustrates the journey of magnetic particles during the lock-in or pDRM process.

A The lock-in adventure begins when the particles settle on the sediment-water interface. Sediments are composed of a mix of magnetic and non-magnetic particles, creating a vibrant playground. During the early stages of the lock-in process, particles rotate freely and align with the geomagnetic field. It’s like a magnetic ballroom dance conducted by the geomagnetic field forces.

B As time passes and sedimentation continues, the surrounding material consolidates. Larger magnetic particles begin to lose their mobility and get locked in. They find their forever spots, holding onto the memories of the geomagnetic field at that time. But what about the smaller particles? They’re still lively and free, closely following the twists and turns of the geomagnetic field.

C After ample sedimentation and consolidation, the lock-in process reaches its grand finale. Each particle becomes a storyteller, carrying a piece of the geomagnetic field’s history within it. The sediment layer becomes a mosaic of magnetic moments, depicting diverse states of the geomagnetic field throughout the entire lock-in period. It’s like a magnetic symphony composed of the melodies of the geomagnetic field.

The magnetic moment of a whole sediment layer represents a weighted average of the geomagnetic field over the lock-in period. A weighted average is a way to calculate a more accurate average by giving more importance to certain values over others. It involves assigning weights to different values based on their significance, so that the final average reflects their contributions accordingly. When it comes to the lock-in process and the recorded magnetic moment of a sediment layer, we need to consider the varying contributions of the geomagnetic field over the lock-in period. This is where the concept of a lock-in function comes into play. The lock-in function assigns weights to the different geomagnetic field values, reflecting their significance during the lock-in process.

The investigation of the lock-in process and the development of a modeling concept to estimate the lock-in functions for individual core samples is the primary goal of our studies. Our research outcomes, outlined in our papers (https://doi.org/10.1029/2023JB027373), provide in-depth details, methodologies, and findings that shed light on the fascinating world of sediment records and post-depositional DRM. With our results we make sediment data a more reliable data source for modeling the geomagnetic field. It’s time to unlock the secrets of the geomagnetic past!