High-Resolution Environmental Magnetic Properties and Relative Geomagnetic Paleointensity of IODP Expedition 339 (Site U1389)

Abstract

The Mediterranean Outflow Expedition, which concluded in early 2012, provides an outstanding opportunity to address paleoceanographic questions about the evolution of the Mediterranean and North Atlantic climate system over the past six million years. The expedition recovered over 6 km of sediments. This provides an excellent archive for the study of paleoclimatic, paleoceanographic, and other paleoenvironmental changes and will also provide chronostratigraphic, plate tectonic, and geomagnetic constraints. Results are presented from the upper part of Site U1389 (36 25.515’N; 7 16.683’W), which is located approximately 90 km west of the Spanish city of Cadiz, in a water depth of 644 mbsl. This site is located in the “channels and ridges” sector of the larger Cádiz Contourite Depositional System (CDS). The sedimentary section extends for 990 m from the early Pliocene to Holocene and is represented by a thick, rapidly accumulated, and very uniform series of contouritic sediment. Paleomagnetic and rock magnetic measurements were carried out at 1-cm resolution on 36 m of U-channel samples from the composite stratigraphic section with the goal of extracting a high-resolution record of the magnetostratigraphy, the relative geomagnetic paleointensity, and the variability of the paleoenvironmental conditions. Step-wise demagnetization of the natural remanent magnetization yielded a well-defined interval over a portion of the Holocene with the deepest part of the section being younger than 60 ka. Sediment ages were determined by using an age model based on proprietary oxygen isotope data from Francisco J. Sierro. Interpolation of the age model shows sedimentation rates averaging near 70 cm/kyr. A relative paleointensity record was extracted by normalizing the NRM by ARM. These results along with the lack of power in the Milankovitch frequencies in the spectral analysis of the NRM/ARM and NRM/k signals support the interpretation that the RPI record is not influenced by environmental factors and accurately represents changes in global-scale paleomagnetic field intensity. Interpretation of the environmental records indicates cyclic warming and cooling, as that correlates well with sapropels, finer grains and warmer climate, and Heinrich Events, coarser grained IRD and cooling climate.

The comparison of IODP U1389 to proven records of GLOPIS reveals a high fidelity with few discrepancies and due to the locality of the site, proves the reliability of the RPI records. All usual criteria for paleointensity studies suggest that the upper portion of Site U1389 yielded a reliable high-resolution record of relative paleointensity, which provides a vital basis for global correlations of future studies.