HOT SPRINGS AND POND LIPS: UNDERSTANDING CONTROLS OF PRECIPITATION RATES ON CARBONATE TERRACED ARCHITECTURE AT SPRING AT-3, ANGEL TERRACE, MAMMOTH HOT SPRINGS, YELLOWSTONE NATIONAL PARK, WYOMING, USA

Travertine precipitation rates, morphology, distribution, and chemistry vary between facies at Spring AT-3. Understanding these parameters is essential to interpret physical, chemical, and biological controls on carbonate terraced architecture. The central hypothesis of this project is to determine if living microbes/microbial communities directly influence crystal growth rates, morphology, distribution and chemistry. This study compares travertine precipitation rates, morphology, and chemistry; water chemistry between five defined facies and transition zones (Fouke et al., 2000); and physical controls on terrane morphology. Temperatures at each facies are as follows: Vent (68-72°C); Apron and channel (45-71°C); Pond (35-61°C); Proximal Slope (28-54°C); and Distal Slope (28-30°C). The transition zones between the facies range from razor sharp boundaries to several centimeters in width and may extend laterally from the flow for several meters.

In order to quantify the morphology and rate of travertine precipitation at Spring AT-3, 750 2.5cm-diameter stainless steel washers were placed on the spring substrate at 24 locations along the outflow within and between facies. Analyses of washers at specific time intervals (4, 12, and 48 hours) for 4 days yielded growth measurement ranges of: 0-0.2 g/hr; 0-0.56 g/hr; and 0-1.7 g/hr, respectively. Initial results indicate faster precipitation rates at transition zones and near the system flow, with the fastest growth at the pond lip.

Preliminary studies identified arcuate aragonite structures growing into the direction of flow at the pond lips. This increase in precipitation rate and morphology at pond lips is further suggested by increases in pH and decreases in temperature and total dissolved solids (TDS), as compared to other areas. Additional in situ diurnal measurements of TDS, temperature and pH (2,104 total measurements), along with analyses of selected samples by ESEM, cathodoluminescence (CL) and standard plane light (PL) microscopy, δ¹³C and δ¹⁸O ratios, and dissolved inorganic carbon (DIC), provides insight on factors that control crystal growth rate and morphology. Furthermore, analyses of samples collected from the pond to proximal slope transition area determine composition, architecture, genesis and growth history of the pond lips.