Data for Synthesis Mechanisms Producing Isotopically Enriched Acetic Acid in Deep Groundwater Systems from the Canadian Shield

Fracture waters collected from the Kidd Creek Observatory (KC) in Timmins, Ontario, Canada are hosted by 2.7 Ga Canadian Shield rocks with minimal surface water exchange over geologic timescales resulting in residence times of more than 1Ga. The dissolved organic carbon compound (DOC) inventory of these waters is predominantly composed of acetate and formate. Acetate is notably 13C enriched (-4.9 to -7.3‰) compared to typical photosynthetically derived organic matter and is present at among the highest concentrations ever measured in natural groundwaters. Acetate and formate present in these fracture waters are hypothesized to been abiotically formed via radiolytic water-rock interactions over millions of years.
To better understand the importance of radiolytic processes among the variety of abiotic organic synthesis reactions in the deep subsurface, we utilized the capabilities of a modified gas chromatography – Orbitrap – mass spectrometry (GC Orbitrap-MS) system to measure the intramolecular carbon isotope signature of acetate collected from Kidd Creek Observatory. These results were compared to acetate from another groundwater system at Birchtree, Manitoba, where fracture waters have a well-established microbial community and evidence of significant paleo-meteoric water recharge and mixing. The carboxyl (C1) carbon of acetate from KC was enriched by 15.4‰ on average relative to the methyl (C2) carbon, whereas both positions were isotopically similar for the Birchtree sample. The contrasting intramolecular isotope patterns observed in acetate collected from KC vs. Birchtree fracture waters indicate the activity of two separate acetate synthesis pathways; one abiotic (likely radiolysis; KC) and the other biotic (metabolisms that consume or produce acetate; Birchtree). This approach demonstrates the utility of intramolecular isotope analysis for distinguishing abiotic and biotic sources of acetate in environments on Earth and elsewhere.