Rock magnetic evidence for a middle Holocene transition in marine sediments from La Paz basin, southern Gulf of California

Abstract

Results of a rock magnetic study of a marine sediment core from La Paz Basin are used to investigate the Holocene paleoclimate in the Gulf of California. Chronological control based on AMS radiocarbon dating shows that the sequence covers the past ~7650 cal yr BP. The coercivity, magnetization intensity, susceptibility and magnetic hysteresis loops point to dominant fine-grained magnetite and titanomagnetites as main magnetic carriers. In the magnetic hysteresis ratio plot, magnetization and coercivity ratios fall in the pseudo-single domain field. High magnetic susceptibilities and magnetization intensities characterize the upper core section, with lower values downcore on a low amplitude oscillation trend. The late Holocene is marked by low susceptibilities, frequency-dependent, paramagnetic slope factors and variable hysteresis parameters with low Mr/Ms magnetization ratios, low Bc and Bcr coercivities and high Bcr/Bc ratios. The lower core section in the ~7650-5000 cal yr BP interval shows increased susceptibilities as compared to the upper sections with lower values. The magnetic hysteresis logs for the interval around 5000-4000 cal yr BP show some samples with distinct lower magnetization (Mr/Ms) and higher coercivity (Bcr/Bc) ratios, which characterize the interval from the downcore variation trend. The pattern in the magnetic properties is interpreted in terms of a change in precipitation from high to low humidity from the middle to the late Holocene. The sediment section records a middle Holocene climate transition in the southern Gulf ~5000-4000 cal yr BP, supporting a northward migration of climate patterns along the western North America margin, from the southern Gulf of California to Alaska.