Geomagnetic field variations between chrons 33r and 19r (83–41 Ma) from sea-surface magnetic anomaly profiles

Abstract

Sea-surface magnetic profiles exhibit coherent short wavelength "micro-anomalies" (or "tiny wiggles") superimposed to the main anomalies due to reversals. In this study, we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the ∼42 Myr-long period following the Cretaceous Normal Superchron. To this end, we compute stacks of anomaly profiles from different areas in the Indian and Pacific oceans.Using a simple method based on upward continuation, we demonstrate that, the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates. They are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment. A high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates. Modeling the micro-anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr, likely too short to be indeed "true" subchrons. Moreover, the number of detected tiny wiggles clearly depends on the spreading rate. These results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles, as also Preprint submitted to Elsevier Science suggested by recent magnetostratigraphic data. The tiny wiggles are uniformly distributed within chrons, indicating that paleointensity fluctuations are neither inhibited after, nor enhanced before, a reversal beyond a "blind" zone of about 10 km (corresponding to 80 to 250 kyr depending on the spreading rate) for which the anomalies due to reversals prevent the detection of tiny wiggles. Most tiny wiggles probably represent a filtered record of a uniform secular variation regime, as suggested by their uniform spatial distribution over the whole investigated period.

Global Extent of Early Eocene Hyperthermal Events: A New Pacific Benthic Foraminiferal Isotope Record From Shatsky Rise (ODP Site 1209)

“…Plate reorganization could have reduced spreading rates during magnetic polarity Chron C23n leading to less 12 C‐enriched carbon release to the oceanic reservoir due to reduced CO 2 degassing. Spreading rate reconstructions show a one third decrease for the South Pacific and Indian Ocean, an increase for South Atlantic, and no major change in the North Pacific (Bouligand et al, ; Westerhold et al, ), although it is not clear what the global average change is given uncertainties (Bouligand et al, ; Westerhold et al, ). Strontium isotope composition of seawater ( 87 Sr/ 86 Sr) reconstructed for Leg 208 records starts to decrease at 51.5 Ma consistent with a decrease in ocean‐crust production (Hodell et al, ).…”

Magnetic structure of a slow spreading ridge segment: Insights from near‐bottom magnetic measurements on board a submersible

“…Short‐wavelength variations of magnetic anomalies within intervals of constant polarity have been widely observed not only in the Brunhes epoch but also in older polarity epochs, i.e., “tiny wiggles” [e.g., Rea and Blakely , 1975; Cande and LaBrecque , 1974; Cande and Kent , 1992; Bouligand et al , 2006]. As for the origin of tiny wiggles, the following explanations have been proposed: (1) local reduction of magnetization caused by alteration and/or tectonic disruption through extensive faulting or hydrothermal activity [ Tivey and Johnson , 1987; Hussenoeder et al , 1996; Wooldridge et al , 1990, 1992], (2) varying thickness variation of the magnetized layer [ Tivey and Johnson , 1993; Gee and Kent , 1994; Tivey , 1994; Williams et al , 2008], and (3) geomagnetic intensity variation [ Cande and Kent , 1992; Gee et al , 1996, 2000; Schouten et al , 1999; Pouliquen et al , 2001; Bouligand et al , 2006]. In our detailed study area we see no clear relationship between the distributions of magnetization low and faulting zone, and we could therefore not ascribe the variation of magnetization within the Brunhes epoch to tectonic demagnetization.…”

Evolution of the Late Cretaceous crust in the equatorial region of the Northern Indian Ocean and its implication in understanding the plate kinematics

“…The ages are constrained by the synthetic seafloor spreading model generated for profile Circ05ar using the same parameters as in Fig. 4 with additional geomagnetic polarity reversal information of Bouligand et al (2006). The wiggles ‘C’ and ‘B’ represent 76.999 and 75.504 Ma cryptochrons, respectively.…”