Travertine in western Grand Canyon: paleoclimatically driven aggradation and incision directly dated by U-Series

Quaternary travertines of the Grand Canyon region provide a rich rock record for understanding interactions between canyon incision, geomorphology, paleohydrology, and paleoclimate. Travertine Grotto (TG) displays a long history of travertine deposition recorded by travertine-cemented alluvial terraces and spring mounds derived from a fault-controlled, CO2-rich spring, and exposed along a deeply incised, modern inner gorge carved by TG Creek (TGC). U-Series geochronology has been effective at dating travertine deposition and hence limits the timing of stream aggradation and incision. The oldest dated travertine (354 +/- 15 ka, 2 sigma) is a flowstone remnant on bedrock within a few meters of the present bedrock channel of TGC; this sample indicates that at this location there has been little net bedrock incision in the last 350 ka. The highest and oldest dated terrace remnant (T5- 70 m above TGC) is 243+/- 4 ka . An extensive fill terrace topped by a travertine dam (T4-60-66 m above TGC) yields a 137+/- 10 ka age . The most prominent fill terrace (T3 - 22-25 m) yields ages of 53.9 +/- 1 and 52.8 +/- 1 ka along its base, 31.4 +- 8 ka by optically stimulated luminescence (OSL) to 22.5 +/- 1 ka half way up, and 11.3 +/- 0.2 ka at its top. T3 is interpreted to represent semi continuous alluviation from ca. 50 to 11 ka; incision of the modern TGC inner gorge began after 11.5 ka. OSL dates broadly agree with U-Series dates. Fill terraces T3 and T4 show upward change from cemented alluvium to flowstone, interpreted to reflect climatic-paleohydrologic cycles as follows. Approaching terminations of glacial cycles, colluvial loads delivered to streams apparently decreased, perhaps due to dense vegetation related to high effective moisture levels. Concurrent high spring discharges deposited the relatively pure capping flowstones. Incision ensued during interglacial and early glacial times, possibly characterized by lower effective moisture levels, hence reduced vegetation and enhanced bedloads, and reduced spring discharges. Stable isotope analysis of nine dated travertines involved subsampling of as many as 10 individual laminations, as well as analyses of modern travertine and water. These data indicate that C-13 isotopic variations may reflect depositional environment (i.e., rate of degassing), whereas O-isotopic variation may reflect paleoclimate/paleohydrology. O-isotopic values vary by 1-2 permil across cm-scale laminations reflecting seasonal to decadal changes. Isotopically distinct results are obtained for modern, 11, 47-100, 136 and 354 ka samples.