The mineralogical compositions of 119 samples collected from throughout the San Francisco Bay coastal system, including bayfloor and seafloor, area beaches, cliff outcrops, and major drainages, were determined using X-ray diffraction (XRD). Comparison of the mineral concentrations and application of statistical cluster analysis of XRD spectra allowed for the determination of provenances and transport pathways. The use of XRD mineral identifications provides semi-quantitative compositions needed for comparisons of beach and offshore sands with potential cliff and river sources, but the innovative cluster analysis of XRD diffraction spectra provides a unique visualization of how groups of samples within the San Francisco Bay coastal system are related so that sand-sized sediment transport pathways can be inferred.The main vector for sediment transport as defined by the XRD analysis is from San Francisco Bay to the outer coast, where the sand then accumulates on the ebb tidal delta and also moves alongshore. This mineralogical link defines a critical pathway because large volumes of sediment have been removed from the Bay over the last century via channel dredging, aggregate mining, and borrow pit mining, with comparable volumes of erosion from the ebb tidal delta over the same period, in addition to high rates of shoreline retreat along the adjacent, open-coast beaches. Therefore, while previously only a temporal relationship was established, the transport pathway defined by mineralogical and geochemical tracers support the link between anthropogenic activities in the Bay and widespread erosion outside the Bay. The XRD results also establish the regional and local importance of sediment derived from cliff erosion, as well as both proximal and distal fluvial sources. This research is an important contribution to a broader provenance study aimed at identifying the driving forces for widespread geomorphic change in a heavily urbanized coastal-estuarine system.
The mineralogical compositions of the sand-sized fraction of 119 total beach, offshore grab, cliff outcrop, and river/streambed samples throughout the San Francisco Bay coastal system were determined using X-ray diffraction (XRD). All sediment sample analyses were done on a consistent size fraction, 0.15 - 0.5 mm (mean D10-D90 range of beach samples), after shell was removed by weak hydrochloric acid leach. Potential source rocks were analyzed in bulk. All samples were ground in a McCrone micronizing mill with 4 ml of methanol for 5 min. The ground mixture was then dried at 80° C overnight, and the resulting powder was ground with a mortar and pestle to fit through a 106 mm sieve and back-packed into an aluminum sample holder to ensure randomization. Samples were analyzed using a Philips XRD with graphite monochromator at 40 kv and 45 mA. Step scans were run from 5° to 65° 2-theta with 0.02° steps, using CuKa radiation and a count time of 2 s/step. XRD digital scan data were analyzed using Philips X'Pert High Score search and match function to identify peaks and mineral composition. Semi-quantitative mineral percentages were determined by multiplying unique peak intensities for each mineral in a sample by relative intensity factors as described by Cook et al. (1975) and the products for all minerals in a sample were then summed to 100%. Intensity factors were estimated for minerals not found in Cook et al. (1975).
Supplement to: Hein, James R; Mizell, Kira; Barnard, Patrick L (2013): Sand sources and transport pathways for the San Francisco Bay coastal system, based on X-ray diffraction mineralogy. Marine Geology, 345, 154-169
