Sample collection and preparation
Benthic observations and sampling were carried out by divers in November 2012, following a transect established in 2006 (see Dillon et al. 2020, Jungblut et al. 2016). Video imagery was collected from 8.5 to 10.5 m depth in 2012 and from below the ice cover to 7.1 m depth in 2006. All water depths referenced in this study are based on the lake level in 2012. Divers measured the depth to carbonate from the mat-water interface by probing mats with an incrementally-marked metal wire until a rigid surface was encountered. Selected push cores and dredge samples were dissected in the field, dried, and shipped to the University of California, Davis. Remaining samples were frozen during the field season for shipment and subsequently stored below -20 °C.
Carbonates in sediment cores were located by halving the frozen cores. Acrylic core tubes were cut lengthwise using a hand-held rotary cutting tool with a circular diamond-edged blade, then cores were halved along the same line using a water-cooled Hi-Tech Diamond 6” trim saw (Westmont, IL, USA) to expose the interior of the core. Carbonate pieces and surrounding sediments were excised from cores using the trim saw, then lyophilized in order to separate carbonates from sediments. Dried carbonates were shipped to Spectrum Petrographics (Vancouver, WA, USA) for preparation as thin sections; carbonate pieces which appeared particularly fragile were first embedded in Epo-Tek 301-2 clear epoxy (Billerica, MA, USA) according to manufacturer instructions to prevent damage during shipping. Samples were prepared as thin sections 30 or 80 μm thick.