ABSTRACT
Hot and cold spring travertine deposits record integrated histories reflecting changing hydrologic conditions, informing our understanding of the driving forces behind factors impacting local hydrology. We present results from a geologic and geochemical investigation of Cottonwood Travertine, located in Dixie Valley, NV, where it is unclear if the paleospring system that deposited the travertine was driven by deeply sourced hydrothermal fluids, or high fluid flow driven by wetter paleoclimate conditions. The temperature of the spring water that precipitated the Cottonwood Travertine has implications for the relative importance of hydrothermal versus climatic processes
influencing the formation and cessation of this enigmatic deposit. We identified four groups of samples based on geologic setting, sample textures, and stable and clumped isotopic analysis: 1) calcite cemented upper gravels, 2) a mound area at the upper bench of the deposit, 3) samples from the flanks and from vuggy veins and fault zone cements from the base of the deposit, and 4) fibrous sub-travertine veins. The calcite-cemented gravels yielded δ18OC values as low as -18.4‰ (VPDB) and apparent TΔ47 of 52°C. The top mound of the deposit returned calcite δ18OC values between -12.8‰ and -11.7‰
(VPDB) and clumped isotope temperatures (TD47) of 24 – 32°C. Higher d13C and d18Oc values at the mound site are interpreted to reflect off gassing of CO2 and disequilibrium conditions. δ18OC and TD47 values from the slopes and base of the deposit are between -15.9‰ and -14.5‰ (VPDB) and around 20°C, respectively. Structurally, texturally, and isotopically (δ18OC = -29.4‰ (VPDB); TΔ47 = 93°C), the fibrous sub-travertine veins are more consistent with the local Jurassic host rock and probably do not reflect recent hot spring conditions. Our analysis suggests that, despite the impressive volume, Cottonwood Travertine formed from springs that were not particularly hot, and the deposit instead reflects vigorous warm spring activity in a wetter climactic regime rather than fluid flow from an extinct higher temperature hydrothermal system.
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Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA
The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric
CO2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with ‘‘floes”
forming at the atmosphere-water interface, ‘‘snow” of fine particles accumulating at the bottom of pools and terraced constructions
of travertine. Floe material is mainly composed of aragonite needles despite CaCO3 precipitation occurring in
waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters,
in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection.
The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in d13C (9 to 28‰ VPDB) and
d18O (0 to 20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have
previously been attributed to kinetic isotope effects accompanying CO2 hydroxylation but the controls on the d13C-d18O endmembers
and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited
areal extent of the springs (0.5 km2) and the limited range of water sources and temperatures, combined with our sampling
strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations.
We develop an isotopic reaction–diffusion model and an isotopic box model for a CO2-fed solution that tracks the isotopic
composition of each dissolved inorganic carbon (DIC) species and CaCO3. The box model includes four sources or sinks of
DIC (atmospheric CO2, high pH spring water, fresh creek water, and CaCO3 precipitation). Model parameters are informed
by new floe D44Ca data (0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8 107 mol/m2/s) and by previously
published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control
the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric
CO2 and the kinetic isotope fractionation factor (KFF (‰) = (a 1) 1000) accompanying CO2 hydroxylation, estimated
here to be 17.1 ± 0.8‰ (vs. CO2(aq)) for carbon and 7.1 ± 1.1‰ (vs. ‘CO2(aq)+H2O’) for oxygen at 17.4 ± 1.0 C. Combining
our results with revised CO2 hydroxylation KFF values based on previous work suggests consistent KFF values
of 17.0 ± 0.3‰ (vs. CO2(aq)) for carbon and 6.8 ± 0.8‰ for oxygen (vs. ‘CO2(aq)+H2O’) over the 17–28 C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically
equilibrated DIC from creek or surface water (mostly HCO-
3, pH = 7.8–8.7) that occasionally mixes with the high-pH spring
water. The bulk carbonate d13C and d18O values of modern and ancient travertines therefore reflect the proportion of calcium
carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars.
These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common
natural processes, and suggest ancient travertine may be used to retrieve past meteoric water d18O and atmospheric d13C values.
There is evidence that older travertine at The Cedars recorded atmospheric d13C that predates large-scale combustion of
fossil fuels.
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- Award ID(s):
- 1749183
- PAR ID:
- 10280379
- Editor(s):
- Teagle, Damon A
- Date Published:
- Journal Name:
- Geochimica
- Volume:
- 301
- ISSN:
- 0096-3089
- Page Range / eLocation ID:
- 91-115
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi.org/10.1016/j.gca.2021.01.003
