SHORELINE TUFAS OF PLEISTOCENE LAKE BONNEVILLE, UTAH: WHAT CAN BE LEARNED FROM SUCH DEPOSITS?

Tufas (swash-zone) and tufaglomerates (cemented subaqueous colluvium) on well-developed Stansbury (oldest, lowest), Bonneville (highest, intermediate age), and Provo (intermediate elevation, youngest) Lake Bonneville shorelines on the Silver Island Range present an opportunity to examine paleo-chemical gradients between the “open” Great Salt Lake and “restricted” Pilot Valley sub-basins of Lake Bonneville through time and space.

Stable isotope statistics show that outliers may either produce false correlations or mask true correlations. Neglecting outliers, mean d¹⁸O_VSMOW values of all three shorelines are statistically indistinguishable (~27‰). However, deposits from the restricted arm were enriched (~0.3‰) in d¹⁸O, consistent with slightly elevated evaporation in the Pilot Valley sub-basin. d¹⁸O values in tufa were enriched over tufaglomerate by ~0.6‰, consistent with greater evaporative enrichment of lake water in the swash zone.

d¹³C_PDB values show increasing means for the Provo, Stansbury, and Bonneville shorelines (4.4, 5.0, and 5.2‰, respectively), indicating that paleo-productivity was highest during Bonneville time. d¹³C values are also depleted by 0.3‰ in tufas relative to tufaglomerates, suggesting higher productivity below the swash zone, although no discernable difference occurs between sub-basins. ¹⁴C ages of Bonneville and Provo tufas give reliable results, but the apparent age of one Stansbury tufa is too young by thousands of years, although other ages reported for Stansbury tufa appear to be correct (Oviatt et al., 1992). As Stansbury tufa was submerged for ~8 ka following deposition, recrystallization or precipitation of younger cements may overprint primary isotope systematics.

Comparisons with marls of similar age (Oviatt et al., 1994) indicate that shoreline carbonate was much more enriched in d¹³C and d¹⁸O (both ~2.5‰) during Bonneville time, whereas isotopic differences were minor (both ~1‰) in Stansbury time. The lake episodically had an outlet during the Bonneville stage and was closed during the Stansbury stage, suggesting that inflow/outflow conditions or surface area/volume ratios exert an important influence on the isotopic composition of carbonate precipitated in the water column in the middle of a lake versus its shoreline.