CARBON ISOTOPE SIGNATURES IN PERENNIALLY ICE-COVERED LAKE FRYXELL, MCMURDO DRY VALLEYS, ANTARCTICA

In this study, we assess microbial mat carbon isotope signatures with depth and through time in Lake Fryxell in the McMurdo Dry Valleys, Antarctica. Lake Fryxell is covered with 3 - 6 m of ice year-round, has a steep oxycline at 8 - 9 m depth, and is host to cyanobacterial benthic microbial mats. Previous work has documented distinct δ¹³C values for particulate organic matter, benthic organic matter above and below the oxycline, and mat material from seasonal melt water moats that form around the lake margin. The distinct carbon isotope values of these carbon pools provide a potential signature of changing lake conditions through time. We tracked the content and isotope signatures of organic carbon from the active mats on the lake bottom (sampled in 2023 at 4.3, 6.1, and 8.0 m depth) into the recent (Holocene) sedimentary record (32 cm long core sampled in 2012 at 11.2 m depth) and compared these values to those of a previously published study (Lawson et al., 2004 at various depths 4 - 19 m). From the shallow sediment core (0 - 3.60 cm depth in core), we interpret alternating laminated organic matter (2 - 5 mm thick) and sand (~2 mm thick) as a record of benthic microbial mat growth interrupted by episodes of greater sand deposition through the perennial ice cover. Further downcore (0 - 30.5 cm depth in core), we observe organic-poor graded sands and consolidated muds with no distinct laminated organic matter. The mean δ¹³C value of organic matter in sediment layers is -27.8‰ (VPDB) and of microbial mat-associated laminae is -30.4‰. For a 2.5 cm-thick layer of graded sand interpreted as rapidly deposited sediment dump from the ice cover, there was insufficient carbon for analysis. δ¹³C values of organic carbon down core are consistent with values reported (~-30.0 - -14.0‰) for benthic organic matter in 2004. More carbon isotope analyses should be done to constrain modern carbon isotope signatures in Lake Fryxell, particularly with regard to differences in isotopic composition of benthic microbial mats. Variables such as ice cover thickness, sand deposition, and available photosynthetically active radiation may contribute to preserved δ¹³C values. Determining the effect of these variables on δ¹³C values will allow for interpretable recent climate records in this unique environment.