Shoreline Tufa and Tufaglomerate from Pleistocene Lake Bonneville, Utah, USA: stable isotopic and mineralogical records of lake conditions, processes, and climate

摘要

Shoreline carbonate deposits of Pleistocene Lake Bonneville record the conditions and processes within the lake, including the evaporative balance as well as vertical and lateral chemical and isotopic gradients. Tufas (swash-zone) and tufaglomerates (cemented, subaqueous colluvium or beachrock) on multiple, well-developed shorelines near the Silver Island Range, Utah, also present an opportunity to examine physicochemical lake processes through time. Three shorelines are represented by carbonate deposits, including the 23-20 ka Stansbury stage, 15-14.5 ka Bonneville stage, and 14.5-14 ka Provo stage.Mean delta(18)O(VSMOW) values of all three shorelines are statistically indistinguishable (similar to27 +/- 1%(0)), when a few Bonneville samples of unusual composition are neglected. However, differences in primary carbonate mineralogy indicate that the correspondence is an artefact of the different fractionation factors between calcite or aragonite and water. Second, in order to sustain a much smaller, shallower lake during the colder Stansbury stage, the climate must have also been relatively dry. Third, delta(18)O values in tufa are higher than tufaglomerate by similar to0.5%(0), consistent with greater evaporative enrichment of lake water in the swash zone. Fourth, mean delta(13)C values for the Provo, Stansbury and Bonneville shorelines (4.4, 5.0 and 5.2%(0), respectively) show that carbon species were dominated by atmospheric exchange, with the variations produced by differences in the oxidation of organic matter.Comparisons of shoreline carbonates with deep-lake marls of the same approximate age indicate that shoreline carbonate was Much higher in delta(13)C and delta(18)O values (both similar to2.5%(0)) during Bonneville time, whereas isotopic differences were minor (both similar to1%(0)) in Stansbury time. In particular, the Bonneville stage may have sustained large vertical or lateral isotopic gradients due to evaporative enrichment effects on delta(18)O values. In contrast, the lake during the Much shallower Stansbury stage may have been well mixed.Differences in the primary mineralogy (Stansbury and Bonneville, aragonite >calcite; Provo, calcite >aragonite) reflect profound differences in lake chemistry in terms of open versus closed-basin lakes. The establishment of a continuous outlet during Provo time probably reduced the Mg2+/Ca2+ ratio of lake water. Curiously, regardless of primary mineralogy, tufaglomerate cements are enriched in Na+ and Cl- and depleted in Mg2+ relative to capping tufa of the same age. This probably reflects vital or kinetic effects in the swash zone (tufa). We suspect that 'abiotic' effects may have been important in the dark pore space of developing tufaglomerate, where the absence of light suppressed photosynthesis. Copyright (C) 2005 John Wiley Sons, Ltd.