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Paleomagnatism and Dendrochronology DatingTechniques • What are magnetic fields? Regions around magnetic material or a moving electric charge within which the force of magnetism acts. Magnetostratigraphy: uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells and to date the sediment. (GPTS) geomagnetic polarity time scale Paleomagnetism: is the study of the record of the Earth's magnetic field in rocks, sediment, or archeological materials. Curie point: the critical temperature of about 500°C - 600°C (for magnetite) where iron-bearing minerals become magnetized in alignment with the Earth’s magnetic field. When cooled further, the magnetite molecules will retain this orientation unless subsequently heated past Curie point. This semipermanent alignment is referred to as thermal remnant magnetism. Detrital remnant magnetism: When the preferred orientation of magnetic minerals in sedimentary rocks imparts bulk magnetic properties to the rocks. How? During deposition of sediments, small magnetic mineral grains are able to rotate in the loose unconsolidated sediment of the depositional surface and thus align themselves mechanically with Earth’s magnetic field. In a lab, rock samples are tested with a magnetometer. Black Stripes represent normal polarity while the White Stripes represent reversed polarity. In order to tie the patterned measured in an individual succession to be able to estimate the polarity of the stratigraphy it is essential to have a tie point to the geologic time scale. This may be provided by absolute dating of the unit. Examples: Lava, and biostratigraphy Individual normal and reverse polarity intervals ("Chrons") typically range from ~10 thousand to 10 million years in duration. One problem with this time of dating is that microscopic iron particles in some sediments undergo chemical changes after they have settled through the water column. This causes the iron particle to realign with the Earth’s magnetic field at the time of the chemical change. Global magnetic reversal patterns from Spreading ridges. Dedrochronology: The word comes from 1. Dendros – using trees 2. Chronos – time 3. Ology – Study of Lab Processes: Two Processes 1. The Fundamental Technique - Match ring-growth samples. 2. Skelton Process - Marking trees ring width variation. Problems: Why not Just count rings? Because of locally absent rings and false bands What do tree rings tell us? 1. To put the present into proper historical context. 2. To better understand current environment process and conditioning. 3. To improve understanding of possible future issues.