REVISITING A QUATERNARY SOIL CHRONOSEQUENCE ON MARINE TERRACES IN SOUTHERN CALIFORNIA: EVIDENCE FOR MOJAVE DESERT DUST INPUTS TO THE EASTERN PACIFIC OCEAN OVER THE PAST ~700,000 YEARS

Soils on Quaternary marine terraces of remote volcanic islands provide excellent opportunities to investigate the long-term records of far-traveled dust to the world's oceans. Cut into Miocene andesite, San Clemente Island, California has at least 20 uplifted marine terraces, many of which host well-preserved soils. We examined the mineralogy and geochemistry of the fine silt (<38 µm) and clay (<2 µm) components of soils on five of the lowest terraces, with estimated ages of ~100 ka (2b at ~24 m), ~120 ka (2a at ~29 m), ~400 ka (3 at ~50 m), ~550 ka (4 at ~65 m), and ~700 ka (5 at ~85 m). Clay minerals in silt-rich A horizons, studied previously, are dominated by mica (interpreted to be of eolian origin) with minor smectite and kaolinite. Although clay-rich B horizons have smectite contents that increase with depth, mica is also present in all B horizons of all soils, and is not found in the local andesite. Silt fractions of most B horizons contain quartz, also not present in the local andesite. Consistent with the presence of clay-sized mica, concentrations of K, Ba, Rb, and Cs (elements enriched in mica) in the soils are higher than in the local andesite. The presence of both quartz and mica, exotic minerals, leads to the hypothesis that much of the clay and silt in soil B horizons could also be of eolian origin. Immobile trace element geochemistry was determined on the <38 µm fraction of the soils and compared with local island andesite and dust from the Mojave Desert. Sc-Th-La, Cr-Th-Nd, La/Yb vs. Nd/Yb and La/Yb vs. Sm/Yb all show that fine silts and clays have compositions that are intermediate between local andesite and Mojave Desert dust, but are closer to dust. Satellite imagery shows that Mojave Desert dust can reach the eastern Pacific Ocean, including San Clemente Island, under Santa Ana wind conditions, which occur numerous times during the fall, winter, and spring. Santa Ana winds, which flow from northeast to southwest, originate in the interior of western North America, driven by high pressure that settles in the Great Basin after passage of a cold front. The observations presented here suggest that dust inputs to soils on San Clemente Island, and by extension, to deep-sea sediments of the eastern Pacific Ocean, have been an important process for much or all of the middle and late Quaternary, spanning several glacial-interglacial cycles.