MAGNETIC PROPERTIES OF LATE HOLOCENE LAMINATED SEDIMENTS FROM CARMEN BASIN, GULF OF CALIFORNIA: THEIR PALEOCLIMATIC AND PALEOCEANOGRAPHIC IMPLICATIONS

We analyze a sedimentary laminated sequence; kasten core K58 (215 cm length), which was recovered in the western part of Carmen Basin at 720 m depth collected aboard of the UNAM R/V "El Puma". Core contains a high resolution record of paleoclimatic and paleoceanographic changes during the late Holocene. Magnetic properties and trace elements are used as proxies of oceanographic and climatic variability. The record provides a tentative scenario for the last 3000 years.

This high resolution laminated sediments (varves) are controlled by two seasonal changes in which we can distinguish two types of sediments: 1) Lithogenic and 2) Biogenic. During summer and autumn occurs transport of sediments from the continent into the basin, producing dark colored laminae, with great amounts of ferromagnetic minerals. Meanwhile, during winter and spring surface waters are affected by upwelling processes, bringing cool nutrient-rich water from subsurface to surface. In consequence biogenic sediments are deposited producing white colored laminae.

For mineral magnetic analyses, low-field magnetic susceptibility was measured in 174 samples at low and high frequencies. Susceptibility displays its higher values at the top of the core, and a characteristic pattern of variations with depth. The remanent magnetization shows normal polarity, with downward inclinations around 60°. The magnetic hysteresis loops and IRM acquisition and back-field IRM demagnetization curves were measured in samples from the dark and light laminae, to investigate any differences in magnetic mineralogy, domain state and relative contents. Plots of hysteresis ratio parameters indicate that all samples fall within the PSD domain field, suggesting a mixture of SD and MD grains. Contribution of fine-grained SP material, which is also suggested from the frequency-dependent susceptibility measurements, is apparent from the data-point distribution above the mixing SD-MD curves.