MICROBIALLY MEDIATED DISSOLUTION FEATURES ON FOSSIL SILICEOUS SPONGE SPICULES IN A WEST ANTARCTIC ICE SHEET SUBGLACIAL LAKE

The NSF funded Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project obtained sediment cores from Subglacial Lake Whillans (SLW), a subglacial lake in the lower ice plane of the Whillans Ice Stream, West Antarctica. The lake and broader subglacial environment hosts a thriving microbial community of extremophile bacteria (Christner et al, 2014), as well as microfossil evidence of past marine deposition in the West Antarctic interior (Scherer et al., 1998).

We are examining fossil sponge spicules from two multi-cores from SLW to evaluate their potential as a nutrient source for the subglacial microbial ecosystem. Sponge spicules in the lake sediments are found to exhibit pitting and flaking structures on the exterior and mineral precipitants within some pits. Here we report initial analyses of these pitting structures to evaluate whether they result from direct microbial colonization.

More than 200 sponge spicules from the upper 26 cm of the lake sediment have been analyzed and measured with light microscopy. We document the exterior dimensions of the sponge spicule, the axial canal width, etch pitting structures, opaque mineral precipitants, and organic material. Using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) and elemental mapping we have, to date, analyzed 17 samples from the upper 20 cm of sediment in the lake. We document pitting, elemental data, and direct observation of collapsed bacterial cells on spicules.

We hypothesize that subglacial microbes are specifically targeting proteinaceous material preserved within the biosiliceous structures. Preliminary results indicate higher concentrations of pitting and flaking structures occur in the upper 10 cm in the core, where microbial activity is highest (Christner et al., 2014). Results also indicate fossil sponge spicules are likely an important nutrient source supporting subglacial communities. Furthermore, the concentration of pitting structures in the active biologic zone indicates that microbially mediated dissolution is occurring rapidly in the lake. There is little evidence of relict dissolution features from past microbiological activity, as indicated by generally good spicule preservation in the till below the upper ~10cm.