Figure 3: Sr compositions and dust contributions.

Seasonal record of strontium (Sr) isotopic composition and relative proportion of Central Valley dust to the total flux incident at the four sites, plotted as a function of time since last precipitation in the Central Valley (data from Friant Dam). Symbols for the four sites match those in Fig. 1: circles are Shorthair (elevation ∼2,700 m); pentagons are Providence (elevation ∼2,000 m); diamonds are Soaproot (elevation ∼1,100 m); and squares are SJER (elevation ∼400 m). Colours reflect passage of time during the sampling interval: yellow (July), orange (August) and red (September). (a) Sr data from multiple collectors in July indicate the variability in dust composition at a single site (s.d. of the mean of each sample measurement is <0.0015). For sites with multiple months of data, Sr compositions trend towards less radiogenic compositions as the dry season progresses, towards the composition of the Central Valley (Fig. 2). (b) Relative proportions of Central Valley versus Asian dust calculated using the Sr compositions and two-component isotope mixing model. The low-elevation site is dominated by Central Valley dust (∼80%) throughout the dry season, whereas the higher elevation sites decline in the relative contribution of Asian sources from as much as 40% early in the sampling interval to 20% at the end. (c) Sr isotopic composition as a function of elevation, indicating the general trend of more radiogenic compositions with higher elevation. (d) Relative proportions of Central Valley versus Asian dust as a function of elevation. The lowest elevation site has the largest fraction of Central Valley dust, while the highest elevation site has the largest fraction of Asian dust.