Paleo- and Environmental Magnetic Record of U1396: a 4.5Myr Record from the Caribbean Sea
Tuesday, 16 December 2014
Long continuous paleomagnetic records provide unique insights into the evolution of the geomagnetic field. Sediments recovered from IODP Site U1396 (16.51°N, 62.45°W) contain a 140 m sequence of hemiplegic sediment and intercalated tephra related to the evolution of the Lesser Antilles volcanic arc. Shipboard measurements of the NRM demagnetized at 20mT documented 17 polarity reversals dating back ~4.5Myrs with magnetic susceptibility (MS) efficient at identifying the visible tephra. These initial results suggested Site U1396 presents a rare, low-latitude, sedimentary magnetic record stretching back into the Gilbert chron. After developing a composite depth scale for the site, we sampled the continuous splice with u-channels to improve the 2.5cm shipboard resolution. Centimeter scale u-channel measurements of MS, NRM AF demagnetized in 12 steps between 20-80mT, and ARM were accompanied by hysteresis measurements of discrete samples to assess lithologic variability, establish the magnetic carriers and quality of the geomagnetic signal, refine the shipboard magneto-stratigraphic reversal record, and derive an estimate for relative paleointensity (RPI). Magnetic mineralogy is dominated by PSD-MD size ferrimagnetic minerals and MS efficiently discriminates sub-cm scale tephra that can drive lithologic variances in the NRM from the carbonate dominated hemipelagic sediment. Maximum angular deviation (MAD) values range from < 1° to > 40° in volcanogenic deposits, though 5-10° are more representative of hemipelagic intervals whose average values are lower in the Pliocene than the Pleistocene. Inclination varies around GAD predictions for both polarities which alongside azimuthally orientated declination establishes the refined reversal based age model. Pliocene sedimentation rates (~4 cm ky-1) are twice that of the Pleistocene and MS reveals a greater Pliocene eruption frequency potentially identifying a behavioral change in the volcanic history of the arc. The NRM was normalized by ARM and MS to generate estimations of RPI. Comparison to ODP Site 983 shows preservation of similar directional and intensity features through the B/M boundary while lower Pliocene MAD values and the accompaniment of older reversals with RPI lows reinforces our geomagnetic interpretation back into the Gilbert.