Figure 5: DRM acquisition experiments.
From: Microbially assisted recording of the Earth’s magnetic field in sediment
(a) DRM acquisition in a 60-μT field with 50° inclination for the sediment groups A, C, D and E, as well as subsequent decay in a null field. Zero-field decay curves have been modelled using equations (2 and 3) (solid lines). The median rotational diffusion coefficients deduced from these models were used to fit the DRM acquisition curves, taking into consideration that the initial DRM (that is, DRMi) is progressively replaced by a PDRM acquired in the applied field. Because of initial sediment compaction and stabilization, it was assumed that the equilibrium PDRM intensities (Meq) decrease exponentially from an initial state, characterized by DRM=PDRM at the beginning of deposition (that is, Meq/DRMi=1, see inset), to a final value of Meq/DRMi matching the measurements shown in b. The exponential decrease of Meq needed to model the observed DRM changes in time (inset) is faster for sediments containing more bacteria (that is, groups A and C). (b) Dependence of DRMi and Meq on field intensity. Meq was calculated from the PDRM acquisition curves as shown in Fig. 3. Solid lines are least-squares fits with M=M0S(B/B0), where 
is a suitable approximation of equation (1), M0 is the magnetization corresponding to full alignment of the magnetic moments (dashed lines), and B0=15.5 μT and 26.7 μT for DRMi and Meq, respectively. (c) Inclination of DRMi and PDRM. Lines are averages of all experiments from the groups A–E. DRMi inclinations are slightly shallower, especially in sediments containing less bacteria, while no systematic shallowing is observed for the PDRMs. (d) Inclination of DRMi in 60-mT fields with 0°, 20°, 50° and 80° inclinations (circles). Solid lines are plots of the inclination shallowing law tan I=f tan IB, where I and IB are the inclinations of DRMi and the applied field, respectively, and f is an empirical flattening factor29. These results confirm that DRMi has typical properties of a DRM, as seen in the traditional redeposition experiments.
