Sediment Magnetic Acquisition

As we seek ever more detailed information from paleomagnetic records we run up against our inadequate understanding of the sediment magnetic acquisition process. In the simplest case, the natural remanent magnetization (NRM) of sediment is aligned with and a function of the intensity and direction of the geomagnetic field at the time of deposition. However, bioturbation and diagenesis influence the magnetic acquisition process in incompletely understood ways. Observational studies support a post-depositional remanent magnetization (pDRM) process (Fig. 1) where the magnetization is locked-in at depth. In the pDRM model, the depositional remanence is acquired by locking in different grains over a range of depths (acquisition zone) below the mixed layers. Leading to smoothing and an offset between the sediment/water interface and the lock-in zone. This implies that sediments act as a time lagged, low-pass filter leading to a paleomagnetic record that is temporally offset and smoothed relative to the original geomagnetic input signal (Fig. 2). In contrast, recent theoretical and empirical studies suggest that sediments acquire their magnetization at a shallow depth over a few centimeters. Suggesting that deep-sea sediment preserve a relatively complete geomagnetic record with little smoothing or time offset. Therefore, how one interprets the paleomagnetic record, is governed by the magnetic acquisition process they believe to be correct.

 

Comparing Paleomagnetic Output with Geomagnetic Input

The development of a firm understanding of the magnetic acquisition process(es) has been hindered by a lack of fundamental observations from natural settings.  We are now working to derive a new set of Holocene observations from two distinct locations:

Northern North Atlantic Sediments

The recent development of high quality, well-dated directional paleomagnetic secular variation (PSV) records from ultra-high accumulation rate northern North Atlantic sediments provide a previously missing geomagnetic template for the region. Our study aims to use this as a geomagnetic input function to be compared with paleomagnetic output measured on well-constrained cores from a variety of environments and accumulation rates across the North Atlantic.

Observations from multicores from the Southeast Alaska Margin

By comparing paleomagnetic output, measured from well-constrained multicores that recover the upper 50 cm from a range of southeastern Alaskan margin environments, with the historical geomagnetic input, we seek to constrain some of the influences of environment on the magnetic acquisition process.

 

Figure 1.  Cartoon illustrating the pDRM sediment magnetic acquisition process (after Tauxe, 1993)

Figure 2.  Post-depositional remanent magnetization model (after Channell & Guyodo, 2004)