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The preservation of microbial lipids in saline and acid-saline environments

Claudia Jones1,2, Jill Banfield1 and Jochen Brocks2

1 Department of Earth & Planetary Science, University of California, Berkeley, USA
2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Section of drill core from Site
12 at Lake Tyrrell, ~5m depth.

 

We are currently working on a pair of separate, but related, lipid biomarker analyses on samples from Lake Tyrrell, VIC, Australia, in order to ascertain the timescales and extent of biomarker preservation within a hypersaline lacustrine system.  In the context of the primary project, we have obtained a ~13m drill core (see Fig. 1) from the northern end of the lake during field work in July 2008; from this core we are extracting lipids to construct a temporal sequence. 

Prof S. George and his PhD student, Mr. P.S. Bray, both from Macquarie University, will perform compound specific radiocarbon dating on archaeol extracted from the core and optically stimulated luminescence dating (in collaboration with Dr Kathryn Fitzsimmons, RSES) on quartz lenses within the core to ascertain the timescales over which environmental change occurred. To determine the magnitude and nature of this change, we are examining suites of lipids from each sub-sampled depth to reconstruct the microbial community that was present in the lake at the time the compounds were deposited. The objectives of this work are to 1. Determine the potential for Australian salt lakes to be used as data sources for paleo-environmental reconstruction; 2. Refine estimates of past aridity for the Tyrrell Basin; and 3. Investigate the extent of community re-structuring by micro-organisms in response to salinity changes.

The secondary project examines biomarker preservation at pH 4 acidic seeps found at the southern end of Lake Tyrrell. Anoxic, iron-rich ground waters flowing from these seeps oxidize near the surface and result in ferricrete deposition.  The ferricretes are composed of quartz-rich lake sediments cemented into meter-scale rocks by iron oxides (magnetite) and oxyhydroxides (goethite).  These ferricretes are underlain by typical acid-saline depositional facies, very similar to that encountered at Meridiani Planum on Mars by the rover Opportunity. To determine whether molecular markers for life can coexist with oxidized mineral deposits, we extracted samples of ferricrete and underlying sediments to establish the presence and provenance of free and bound lipid biomarkers.

In order to assess the effects of oxic lithification on redox sensitive lipids, we monitored the concentration of phytanol and its oxidation product, phytanic acid, in ferricretes and surrounding sediment samples.  Results indicate that the jarosite-rich (KFe3+3[(OH)3SO4]2) sediment directly underlying the concretion is a poor matrix for lipid preservation: only small concentrations of phytanol were evident, and phytanic acid was below detection limits. By contrast, both the goethite-rich layer of the concretion and the reduced sulfide-rich sediments surrounding it showed greater concentrations of each compound (20x and 250x, respectively).  Interestingly, the ratio of phytanol to phytanic acid is approximately equal within the oxic concretion and the reduced sediment, indicating that abiotic oxidation is not likely to be a relevant diagenetic pathway for phytanol in this setting.

Differences in compound concentrations between samples demonstrate the differential preservation of lipids within the ferricrete and the underlying sediment. While the concentrations of lipids are ~10 times lower in ferricrete than in sediment, their presence indicates that biomarker molecules may survive the oxidizing conditions of ferricrete formation broadly analogous to those that existed on the Martian surface.