Geochemical constraints on the sources of beach sand, southern Sendai Bay, northeast Japan

Unraveling the precise sources of sand and their quantitative contributions to a beach is beneficial for effective coastal management. To demonstrate a method for provenance analysis serving that practical purpose, we conducted a case study of the southern coast of Sendai Bay, a storm-wave dominated coast facing the NW Pacific Ocean. To eliminate the effect of hydraulic sorting, which changes sediment composition independently of the sediment source, we analyzed only medium-sand-sized light minerals (density < 2.65 g cm(-3)), which consisted mainly of quartz and feldspars. Concentrations of Al, Na, K, Ca, Mg, Fe, Rb, Sr, Ba, and Pb in the fraction were measured. In addition to that, the Sr isotope ratio, Sr-87/Sr-86, was considered to compensate for the weakened compositional contrast due to the limited mineralogy. We first compared the chemical compositions, which were transformed to centered log-ratio in order to get rid of the constant-sum constraints, and Sr isotope ratio between beach samples and possible sources. Second, we estimated the contributions of the presumed sources by comparing the chemical and isotopic composition of beach sand samples with an ideal composition calculated by assuming simple mixing of source sediments. Finally, under the assumption that the contributions derived by analyzing medium-sand-sized light minerals are also applicable to bulk beach sand, we calculated the total amount contributed by the sources to the beach in the past few decades as the sum of the products of the contributions of each source and the change of the sediment volume at each site along the coast. Our measurements of 19 samples of beach sand and 12 of riverbed sediment showed that the main sources of beach sand were the Abukuma River, which enters the ocean near the center of the study area, and coastal cliffs composed of Mio-Pliocene sedimentary rocks at the southern end of the beach. Contributions from other rivers were negligible. The contribution of the Abukuma River gradually increased northward from about 10% at a point 20 km south of the river's mouth to more than 80% at a point 20 km north of the mouth. We inferred that this alongshore change was due to the dominant northward direction of longshore currents and the locations of the two main sources. Thus, sediments from both sources migrate preferentially northward, transported by the longshore currents, and the amount of sediment derived from the Abukuma River, relative to the amount from the coastal cliffs, increases northward. We assumed that the sources of the medium-sand-sized light-minerals are the same as those of the bulk sediments and estimated the annual contribution of the Abukuma River and the coastal cliffs to the whole beach system to be 3.8 x 10(4) m(3) and 4.1 x 10(4) m(3), respectively. The annual contribution of the river to the beach sediment is about 34% of its estimated annual sediment discharge. The rest of the discharged sediment is presumably retained on the seafloor.