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OSL dating provides an opportunity to assess the reliability of other indicators of surface age, such as loess accumulation, pedogenic CaCO3 stage, and the thickness of

pedogenic CaCO3 coats. First, the use of loess deposits to correlate fan surfaces of

similar age throughout the basin proved problematic because of the weak relationship between the thickness of loess deposits and OSL age. A weak relationship would be expected for sites where fan morphology and OSL ages indicate younger incision into older, buried deposits (discussed further below). However, even with these locations removed, as in Fig. 2.17c, the relationship remains weak. Further, we observed a poor correlation between surface age and loess accumulation even on individual fans, with the exception of the Ramshorn fan. These findings may highlight that numerous factors influence the amount of apparent loess accumulation on an individual surface. These

94 factors include vegetation density, the orientation of the surface relative to prevailing wind direction, erosion by water and wind, and proximity to loess source areas. Proximity to loess source areas appears to influence loess accumulation in the LRR, as the thickest deposits are found on the southern fans, Ramshorn and King Canyon, which sit closest to the eastern Snake River Plain where thick loess sequences are common (e.g., Lewis et al., 1975).

We also observed considerable variability in the observed CaCO3 accumulation

stage in alluvial fan soils for deposits of similar age. For example, stage II+

accumulation was observed in soils developed on surfaces with depositional ages ranging from approximately 10-35 ka (Fig 2.17a; sites where fan morphology and OSL ages indicate incision into older, buried deposits have been removed). These results suggest that in this region, the difference in surface ages must approach ~20 kyr before CaCO3

accumulation stage becomes a reliable indicator of age. Kluer (1988) also noted

significant variability in CaCO3 accumulation in the Willow Creek Area, and regarded it

as an unreliable indicator of relative surface age. Some of this variability can likely be accounted for by the subjective nature of determining accumulation stage, but local differences in accumulation rates within the LRR also influence carbonate stage. The rate of pedogenic CaCO3 accumulation within a soil can be influenced by many factors,

including differences in vegetation type and density, amount of bioturbation, amount of loess accumulation, and variability in soil moisture related to elevation or aspect. For example, we noted weaker CaCO3 accumulation both within the northern portion of the

basin and at higher elevations, such as on moraines or glacial outwash surfaces north of the Willow Creek fan. With colder temperatures in the northern part of the basin and at

95 higher elevations, effective moisture is likely greater, reducing CaCO3 accumulation.

These observations suggest that the northern LRR is near the climate boundary for CaCO3 accumulation, and that during past cooler and moister intervals, calcium

carbonate would have been leached through these soils rather than deposited. Finally, CaCO3 accumulation for surfaces may vary from OSL ages because OSL dating

estimates timing of deposition, while carbonate accumulation stage estimates how long a surface has been stable for pedogenesis to occur. The lag time between deposition and soil formation likely varies from site to site, resulting in differences in carbonate accumulation for surfaces with similar depositional timing.

The correspondence of average CaCO3 coat thickness and deposit age proved

much stronger (Fig. 2.17b), providing a more reliable means of estimating surface age as others have previously demonstrated within the basin (Pierce, 1985; Vincent et al., 1994). In addition, the growth rate of 0.4 mm/10 ka (average for the period from 10-60 ka) for pedogenic CaCO3 coats estimated in this study is reasonable when compared to

previously estimated growth rates by Pierce (1985) and Vincent et al. (1994). Pierce (1985) estimated a long-term average growth rate of 0.6 mm/10 ka for the time period between 30-160 ka, while Vincent et al. (1994) estimated a growth rates ranging from 1.1 to 0.4 mm/ 10 ka, with an average of 0.56 mm/10 ka for the Holocene and 0.6-0.7 mm/10 ka for the last 25 ka. While reasonable, the predicted growth rate from this study is less than that of Pierce (1985), and falls into the low end of the range predicted by Vincent et al. (1994). Reasons for this may include (1) differences in methods used to estimate carbonate coat growth rates and (2) locations used to generate the growth-rate estimates. Pierce (1985) generated growth-rate estimates from U-series dating of the coats

96 themselves rather than relying on dating of other material within the same deposit.

Further, the estimated growth rate from Pierce (1985) is based only on carbonate coats collected in the southern portion of the basin along the Arco segment of the Lost River Fault. Vincent et al. (1994) estimated their Holocene growth rates with radiocarbon dating of organic material within deposits, but their growth rate for the last 25 ka relies on assumed ages for LRR moraines. In addition, their study was restricted to the northern portion of the range. The growth rate from this study was generated through OSL dating and measurement of CaCO3 coats throughout the basin, incorporating more

of the potential variability in growth rates driven by the same factors that influence variability in overall accumulation of CaCO3 in soils.

Comparisons between OSL ages for deposits and pedogenic CaCO3 accumulation

and coat thicknesses highlights the ability of OSL dating to detect alluvial strath surfaces. Such surfaces were identified on three of the five LRR alluvial fans, including the surface of site 5 on the Birch Springs fan (Fig. 2.6), the surface of site 18 on the Ramshorn fan (Fig. 2.8) and the surface of site 26 on the King Canyon fan (Fig. 2.9). At the King Canyon fan, the surface of site 26 is interpreted as an alluvial strath because the OSL age estimate here of 21.10 ± 2.00 ka significantly predates the age estimate of ~13-14 ka for the higher and geomorphically older surface of sites 22 and 25. Another OSL sample from this same strath surface (site 23), returns an age of 8.29 ± 0.58; however, we collected the site 23 sample from a sand lens at a depth of only 1 m. This suggests the deeper and older material is ~12.5 ka older than the younger loess cap. Partial bleaching of quartz grains could also produce these anomalously old ages for geomorphically young surfaces, but De distributions for OSL samples from these sites do not display

97 characteristics that suggest partial bleaching (see Chapter 1, Section.1.4.2). While

alluvial strath surfaces make up a small portion of the fans, the distinction between surface formation through incision or deposition is often important in studies investigating links between climate and geomorphic response.