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Jurassic Magnetic “NOT-” Quiet Zone? –preliminary AUV magnetic survey results from the (very) recent cruise in the western Pacific.


IPGP - Îlot Cuvier


Séminaires Géosciences Marines

Salle 310

Masako Tominaga

Woods Hole Oceanographic Institution

The western Pacific Ocean has the largest extent of Jurassic ocean crust (158-180 My) where the Mesozoic geomagnetic field behavior is recorded. The area encompassed by three magnetic lineation sets (Japanese, Hawaiian, and Phoenix) is marked by low amplitude and relatively indistinct anomalies (“tiny wiggles”) that together are called the Jurassic Quiet Zone (JQZ). The JQZ was originally thought to be a period of few, if any, polarity reversals, similar to the Cretaceous Quiet Zone. However, subsequent near-bottom magnetic surveys of the Japanese lineations in 1992 and 2002/03 reveal that this region is marked by remarkably fast reversals that are lineated and decrease in intensity back in time until M41, prior to which, the magnetic anomalies recover in amplitude. A geomagnetic polarity timescale (GPTS) was constructed from this Japanese anomaly sequence, but the overall global significance of the reversal sequence and systematic field intensity changes require a confirmation of such a GPTS on crust created at different spreading centers. To evaluate the global coherence of geomagnetic field behavior during the Jurassic, we conducted a comprehensive magnetic survey in the Hawaiian Jurassic crust using three different magnetic sensor platforms, including Sentry Autonomous Underwater Vehicle (AUV) for near-bottom data collection, along with a crustal seismic profiling component to survey a spreading corridor in the Hawaiian lineations. Preliminary sea surface and mid-water level magnetic data display a similar anomaly sequence to the Japanese sequence: an overall decrease in anomaly amplitude from M19 to M29, followed by a period of low amplitude, which in turn is preceded by a return to stronger amplitude anomalies. The magnetic anomaly correlations between Hawaiian and Japanese sea-surface level profiles confirm the reversal record back in time, at least, to M38. At the mid-water level and near-bottom AUV level, the magnetic data clearly show the short-wavelength anomaly character of the M29-M38 sequence, indicating that the fast reversals observed in the Japanese lineations are also present in the Hawaiian lineation set. The strong similarity of overall anomaly patterns between Japanese and Hawaiian sequences supports the following preliminary conclusions that: (1) the global coherence of the geomagnetic field behavior during the Jurassic and (2) Earth’s geomagnetic field during the JQZ period was “NOT” so quiet.