stable Isotopes and depositional environment

The isotopic compositions of primary lacustrine carbonates is dependent on the composition of the lake waters and the temperature of mineral precipitation. (Stuiver, 1970; Talbot, 1990; Gasse and Van Campo, 1994). The isotopic composition of lake waters will vary depending on the isotopic composition of the rainfall of the catchment area, its amount and seasonality, the temperature, the rate of evaporation, the relative humidity and biological

between isotopic enrichment of dissolved inorganic carbon (DIC) by photosynthetic activity and isotopic depletion by oxidation of organic matter (Stuiver, 1975; McKenzie, 1981; Rosen et al., 1995; Benson et al., 1996, Wachniew and Rozanski, 1997). Variations in all

of these factors over periods of months to thousands of years will result in highly variable

lacustrine carbonate δ18O and δ13_C.

In a closed system total dissolved inorganic carbonate (TDIC) is mainly controlled by

isotopic exchanges with the atmosphere (Fontes and Gasse, 1990), and 12_{C is removed}

preferentially by organisms (Casanova and Hillaire-Marcel, 1993), hence 13_{C will become}

enriched in brines. As δ18O and δ13C vary in a systematic way, both increasing with increasing

concentration of the brine, they should be correlated, and studies have identified covariance

between δ18O and δ13C in lacustrine carbonates (e.g., Janaway and Parnell, 1989; Casanova

and Hillaire-Marcel, 1993; Camoin et al., 1997). Talbot (1990) established that covariant trends typify closed basins. Typically the covariance trend should be positive but Camoin

et al., (1997) also found coexisting positive and negative correlations of increasing δ18_O

and decreasing δ13C, from carbonates from perennial and ephemeral carbonate lakes in the

central palaeo-Andean Basin of Bolivia. They attributed the co-existing trends to changes between perennial (negative correlation) and ephemeral (positive correlation) lacustrine

conditions. In the case of decreasing δ13C, 12_{C is supplied from oxidation of organic matter}

in deeper waters (Camoin et al., 1997; Wachniew and Rozanski, 1997).

Therefore, in an open lacustrine setting, the δ18O and δ13_{C of carbonates will be highly}

variable as the balances between the various inputs change. In contrast, in a closed lacustrine setting, there will be a correlation between δ18O and δ13_{C. If either or both of these conditions}

are found to occur in the Curdimurka Subgroup carbonates, this will provide supporting

evidence for a lacustrine depositional environment. 5.1.5 the aims of this study.

In Chapter 3 sedimentological analysis of the Curdimurka Subgroup proved ambiguous in

determining its depositional environment. The Dome Sandstone was deposited in a fluvial environment but from the Rook Tuff to the Boorloo Siltstone, the depositional environment varied from mud-flat to sub-wavebase, without conclusive evidence for either a marine or lacustrine setting. Although a marine origin is favoured, further evidence is sought to provide

support for the conclusions of the sedimentological analysis. Stable isotopes can provide a method of determining between marine and non-marine depositional environments in

two ways. Firstly, δ13C of carbonate rocks may be compared with global δ13C curves, and

if the data does not match the curve, the carbonates were likely deposited in a lacustrine

environment. Secondly, depositional factors can influence the δ13C of carbonate rocks, and

by comparing δ13C results against those expected from lacustrine and marine environments,

it may be possible to identify the depositional enivronment.

5.2

M

Samples were collected from throughout the Curdimurka Subgroup, although most were collected from the two carbonate-dominant levels of stratigraphy; the Dunns Mine Limestone

and the Boorloo Dolomite (Figures 5.4). About half of the samples were collected from diamond drill core and half from outcrop. Several samples from the Burra Group were

also collected from diamond drill core for comparison. One sample of a weathered rind to a carbonate vein from the Dunns Mine Limestone was sampled to determine the effects

of weathering on the isotopic values. A sample of pedogenic carbonate (aragonite?) from above a black shale in the Cooranna Formation was also collected. Appendix 2 has the

sample details.

A dental drill was used to grind material from a fresh surface. Samples from outcrop were first sliced to expose a fresh surface and the sample for analysis taken from that surface. Selective samples (from the matrix and clasts of breccias, veins, and individual beds) were

drilled out but otherwise the samples were taken to be representative of the sample as

a whole. The standard procedure of measuring C and O isotopes of carbonate rocks to determine marine isotopic composition insist on micro-sampling early diagenetic (non- luminescent) cement (e.g., Kaufman et al., 1991; Bartley et al., 2001) however, the samples collected for this study are too fine-grained to allow this. Several studies have demonstrated

that whole-rock δ13_C

carb analyses can differ insignificantly from the non-luminescent phases

(e.g., Aharon, 1987; Fairchild and Spiro, 1987; Kaufman et al., 1991; Melezhik and Fallick, 2003). Hence the sampling method here is considered to be valid.

CO₂ gas for analysis was extracted at the University of Tasmania Central Science Laboratory

using a modification of the method of McRea (1950). Samples were reacted in sealed,

evacuated pyrex glass tubes with H₃PO₄ at a controlled temperature of 50o_{C in a water bath}

for 24 hours. The CO₂ generated was separated from any traces of water vapour by passing

it through a trap at the freezing point of acetone (-95oC), before collection under liquid

nitrogen. The CO₂ was analysed on a Micromass Optima Stable Isotope mass spectrometer

using a reference gas calibrated against NBS-19. Results are expressed relative to V-PDB (for oxygen) and V-PDB (for carbon). Analyses of the ANU-M1 calcite, used as a working

standard for several years in this laboratory, show standard errors of +/-0.06 permil for δ13_C

and +/-0.1 permil for δ18_O.

5.3

r