Georg H. Grathoff

Department of Geology, Portland State University, Portland, Oregon USA GrathoffG@pdx.edu;

Minerals of varying crystallinity were identified in dunal soils, suggesting redistribution of Fe, Al, and Si within the last 20 to 70 ky. These soils are located near Newport, Oregon, USA, receive 1720mm of precipitation annually, and were emplaced during the late Pleistocene, 20ky to 70ky ago. Authigenic minerals identified using XRD include: crystalline gibbsite and vermiculite as well as poorly crystalline allophane, imogolite, and ferrihydrite. By SEM we identified Fe-Si-Al phases in varying proportions coating and cementing the sand grains in the B and C horizons. we identified as poorly crystalline goethite and allophane with XRD. Vermiculite is commonly found in the <2mm size fraction of the B horizon. In the C horizon we find allophane and imogolite replacing roots and woody material, as well as nodules of crystalline gibbsite precipitating in the pore spaces of coarser sands and cavities. Overall the poorly crystalline Fe-Si-Al cement imparts significant strength to the sands.

In addition, we find active ferrihydrite precipitation together with Fe oxidizing bacteria on the surface of ephemeral pools at the sea cliff beach interface, suggesting active Fe leaching from the dune deposits. These pools stain the beach sands red during the summer and fall. The water that drains these sand dunes is weakly acidic with a pH close to 6, slightly reducing, and a TDS of up to 600ppm. The amorphous Fe is removed from these pools with high tide events and may serve as an important source of Fe to near shore fisheries.