5.10 – Geomagnetic Excursions

摘要: Although evidence for geomagnetic polarity reversal can be traced to the work of Brunhes (1906), it was during 1960s that efforts got fully underway resolve time sequence reversals (the so-called timescale or GPTS) from both volcanic rocks and sediments. The importance GPTS geosciences became apparent when realized, in early 1960s, records oceanic magnetic anomalies sedimentary sequences could used as a powerful means correlation and, hence, dating. Early process construction (and its geologic absolute ages), brief excursions began emerge paleomagnetic records. These were often characterized by magnetization directions transitional between two states, did not correlate intervals elsewhere, attributed high amplitude secular variation or, alternatively, artifacts sampling, remagnetization, sedimentological processes. (few kiloyears) duration their recording is fortuitous until recently, no single excursion unequivocally recorded at more than one location. uncertainty has been result poorly constrained ages volcanics As aberrant myriad nongeomagnetic causes, generally treated with skepticism. The change attitude toward about 15 years ago important developments: (1) Improved age control young sediments allowed coeval media correlated. (2) development hydraulic piston corer Deep Sea Drilling Project (DSDP) Ocean Program (ODP), along enhanced penetration other conventional coring devices, such CALYPSO on board Research Vessel Marion Dufresne French Polar Institute (IPEV), led recovery long high-sedimentation-rate deep sea, likelihood excursions. Although subject now considered relatively mature compared situation just ago, are largely restricted last 2 My. We confident existence least seven Chron, perhaps many 11 Matuyama Chron. Prior rare, apart an interval Middle–Late Miocene where nine (or microchrons) location, another four Chrons interval) unlikely intrinsically different chrons; therefore, we may conclude documentation infancy. Progress will require access have sufficiently eruption/sedimentation rates facilitate excursions. The (few) appear less adequately manifest paired intervening define distinct, but brief, opposite relative either side excursion. For best-documented excursions, Laschamp Iceland Basin directional changes globally, there clear locally manifest. excursional without establishment polarity, these represent inadequate incomplete excursion, rather pristine variation. smearing record, detrital remanence (DRM) acquisition process, likely influence all records, severe low accumulation rates. It well established minima paleointensity In future, expect catalog expand considerably, increasingly associated directions. Polarity combined great potential high-resolution stratigraphic correlation. One major challenges study abrupt climate quest appropriate resolution easily achieved using standard marine isotopic methods. is, only relevant understanding field behavior constraining numerical simulation geodynamo, also paleoceanographic paleoclimatic studies precise, millennial-scale,