ORIGIN OF ORGANIC MATTER IN QUATERNARY STROMATOLITES: A MOLECULAR ISOTOPIC STUDY FROM WALKER LAKE, NEVADA

The context of organic matter incorporation and preservation in stromatolites has remained a poorly understood component of stromatolite study. The shoreline of Walker Lake, a closed-basin alkaline lake in western Nevada, hosts numerous Holocene boulder-encrusting stromatolites. Steady lake level retreat since 1882 has exposed some of the stromatolites, while others are currently submerged. The stromatolites reported here likely formed c.a. 2100 yrs B.P. If so, published lake level history would suggest that the stromatolites were subaerially exposed, re-submerged, and re-exposed since they formed. Organic matter extracted from inundated and subaerially-exposed stromatolites was analyzed using GC-IRMS alongside organic matter derived from other shoreline carbonates in an effort to identify and compare their biological sources. Individual n-alkanes extracted from organic matter in stromatolite lamina were found to exhibit δ¹³C values very similar to alkanes of the same chain length found in tufa, carbonate crusts, and macroalgae from Walker Lake. These findings suggests that the organic matter within the stromatolites may represent a mix of sources from the surrounding watershed or water body, rather than organic matter unique to some stromatolite-forming microbial mat community – as is often the tacit assumption when studying stromatolite-associated biotic remains. Stromatolite organic carbon content decreases with increasing elevation above current lake level. Thus, rapid degradation of organic matter occurs upon subaerial exposure. As the stromatolites were subaerially exposed and subsequently re-submerged in the past, it is possible that the bulk of stromatolite-derived organic matter originates partially or completely from modern biological contamination incorporated during their final phase of inundation rather than when the stromatolites initially formed. The incorporation of mixed organic sources, rapid biodegradation of syngenetic organic matter, and the possibility of biological post-depositional contamination are factors that should be taken into account when interpreting the significance of organic isotope and molecular biomarker data in ancient stromatolites.