INTRODUCTION
Ice core records provide a rich record of climate change across much of Antarctica, but a similar record has not been thoroughly constructed for the ice-free regions of the continent. In ice-free areas like the McMurdo Dry Valleys (MDV) in West Antarctica, modern and ancient lacustrine carbonate deposits provide a means to develop such a record. Lacustrine carbonate deposits are useful sedimentary archives, often preserving rich multi-proxy records of changes in local climate, hydrology, and biogeochemistry. The physical and geochemical information contained in lacustrine carbonates can aid in detailed reconstruction of past local environmental conditions. The rich lacustrine carbonate record has been most thoroughly recognized and explored in low- and mid-latitude lakes, including both open- and closed-basin lakes (e.g. DeMott et al. 2019, Hudson et al. 2017, Petryshyn et al. 2015, Talbot 1990). In Antarctic closed-basin lakes, where processes of sedimentation differ considerably from those at lower latitudes, the carbonate record has not yet been thoroughly characterized. Sedimentation in Antarctic lakes is impacted by factors such as transport of sediment through permanent ice covers on lakes, restricted liquid water input due to sub-freezing temperatures, low weathering rates, prevalence of freeze concentration over evaporative concentration, and limitation of primary productivity by the seasonal light-dark cycle and low nutrient availability (Craig et al. 1992, Green et al. 1988, Rivera-Hernandez et al. 2019, Vincent et al. 2008 and references therein). Thus, it is necessary to explore modern carbonate deposition in order to more accurately interpret drivers of carbonate precipitation in paleolake deposits (e.g. Doran et al. 1994) and their implications for terrestrial Antarctic climate records.
At present, the MDV contain a number of perennially ice-covered lakes which contain liquid water year-round. Many of these lakes contain abundant benthic carbonate deposits (e.g. Lawrence and Hendy 1985, Mackey et al. 2018); the fraction of the total sediment represented by these carbonates varies among the lakes (Lawrence and Hendy 1985, Mackey et al. 2015, Sutherland and Hawes 2008). Modern benthic carbonates in Lake Fryxell (Taylor Valley, MDV) comprise a record of modern climate, hydrology, and biogeochemistry in the lake and its surroundings. Lake Fryxell carbonates are found in close spatial association with metabolically active microbial mats on the lake floor (Wharton et al. 1982), and sediment cores indicate that carbonate precipitation occurred through changes in regional climate and hydrology over tens of thousands of years (Lawrence and Hendy 1985; 1989). This study assesses the distribution, texture, and composition of carbonates in modern microbial mats of Lake Fryxell to better interpret the roles of climate, hydrology, and biology in carbonate precipitation in this perennially ice-covered lake. Such terrestrial carbonates present a promising sedimentary archive that can enhance scientific understanding of terrestrial Antarctic environmental change.