Estimating palaeowind strength from beach deposits



Abstract The geological record of past wind conditions is well expressed in the coarse gravel, cobble and boulder beach deposits of Quaternary palaeolakes in the Great Basin of the western USA and elsewhere. This paper describes a technique, using the particle-size distribution of beach deposits, to reconstruct palaeowind conditions when the lakes were present. The beach particle technique (BPT) is first developed using coarse beach deposits from the 1986–87 highstand of the Great Salt Lake in Utah, combined with instrumental wind records from the same time period. Next, the BPT is used to test the hypothesis that wind conditions were more severe than at present during the last highstand of Lake Lahontan (≈ 13 ka), which only lasted a decade or two at most. The largest 50 beach clasts were measured at nine beach sites located along the north, west and south sides of Antelope Island in the Great Salt Lake, all of which formed in 1986–87. At these sites, the largest clast sizes range from 10 to 28 cm (b-axis), and fetch lengths range from 25 to 55 km. Nearshore wave height was calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity, which is controlled by wave height. Shoaling transformations are undertaken to estimate deep-water wave heights and, ultimately, wind velocity. Wind estimates for the nine sites, using the BPT, range from 6·5 to 17·4 m s−1, which is in reasonable agreement with the instrumental record from Salt Lake City Airport. The same technique was applied to eight late Pleistocene beaches surrounding the Carson Sink sub-basin of Lake Lahontan, Nevada. Using the BPT, estimated winds for the eight sites range from 9·7 to 27·1 m s−1. The strongest winds were calculated for a cobble/boulder beach with a fetch of 25 km. Instrumental wind records for the 1992–99 period indicate that wind events of 9–12 m s−1 are common and that the strongest significant wind event (≥ 9 m s−1 for ≥ 3 h) reached an average velocity of 15·5 m s−1. Based on this preliminary comparison, it appears that the late Pleistocene western Great Basin was a windier place than at present, at least for a brief time.