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Tracking Rare Earth Element Distribution in San Francisco Coastal Sediment

Session: Geochemical Studies of Sediments (Posters)

Presenting Author:

Hessa Patwa

Authors:

Patwa, Hessa¹, Malkowski, Matthew², Sickmann, Zachary³, Visser, Jason⁴

(1) University of Texas at Austin, Austin, TX, USA, (2) University of Texas at Austin, Austin, TX, USA, (3) University of Texas at Dallas, Richardson, TX, USA, (4) University of Texas at Austin, Austin, Texas, USA,

Abstract:

Rare Earth Elements (REEs) are essential for many technologies, such as smartphones, batteries, superconductors, and wind turbines. As demand for REEs increases, recognizing the processes that concentrate REEs in sedimentary deposits is vital in identifying potential sources, especially in accessible coastal zones. Light REE-bearing minerals, such as monazite, and heavy REE-bearing minerals, such as xenotime and zircon, contain many of these crucial REEs. Beaches and tidal ebb-deltas may produce placers, where heavy REEs accumulate due to hydrodynamic fractionation and by wave and tidal action winnowing lighter particles. To better understand how depositional environments and surface processes affect REE distribution, we combine new and existing geochemical analyses of sediment across regions of the San Francisco (SF) Bay and outer coast. The dataset includes grab samples, dredges, and piston cores of SF Bay area beaches, ebb and flood delta deposits, and offshore marine samples of sand and silt. New samples were powdered and pressed into homogeneous discs and analyzed by wavelength dispersive X-ray fluorescence (WD-XRF) spectrometry to determine the concentration of major and trace elements, including select REEs. We evaluated REE distributions as total REE (REEt) concentration, which is the sum of concentrations of select REEs, and as chondrite-normalized Lanthanum to Ytterbium (La/Yb) ratios, which quantifies the relative fractionation between light and heavy REEs. Bulk sediment REEt concentrations were highest in the Outer Coast (82.3 ppm) and South Ocean Beach (71.2 ppm) and lowest in Stinson Beach (24.8 ppm) and Rodeo Beach (30.6 ppm). The lowest La/Yb ratios were found in Stinson Beach (1.6) and Rodeo Beach (2.3) (indicating a greater presence of xenotime and zircon), whereas the highest ratios were found in the Outer Coast (9.6) and Crissy Field (7.6). Variations in REE concentrations within the same region may be attributed to differences in grain size and mineralogy, specifically evident in the Rodeo Beach samples. Excluding the Rodeo Beach sample, we found that zirconium (Zr) is strongly correlated with REEt (linear fit R²=0.959), but not with La/Yb ratios (linear fit R²=0.457), demonstrating that Zr, which is most abundant in zircon, may be a good proxy for concentration, but not for fractionation.