Radiocarbon distribution and the effect of legacy in lakes of the McMurdo Dry Valleys, Antarctica
Limnol. Oceanogr., 59(3), 2014, 811-826 | DOI: 10.4319/lo.2014.59.3.0811
ABSTRACT: The water of the ice-covered lakes of the McMurdo Dry Valleys is derived primarily from glacial melt streams and to a lesser extent permafrost seeps and subglacial outflow. The result is a mixture of radiocarbon ages that reflect both the end-member water source and the biogeochemical processing of waters as they migrate to the lake-water column. Samples were collected from various locations within perennially ice-covered Antarctic lakes and the streams that feed them, and they were analyzed for radiocarbon abundance of organic and inorganic carbon. Stream gradient and length were shown to affect the degree of equilibration of water with the modern atmosphere prior to entering the lakes. Stream microbial mats assimilate inorganic carbon flowing over them. Seasonal ice-free ‘moat’ water dissolved inorganic carbon (DIC) is largely dependent on the amount of meltwater input from streams (modern) vs. that from direct glaciers input (old). Under the ice cover, 14C ages of lake-water DIC and organic matter are dependent on lake history, composition, and quantity of particulate matter fallout. Bottom waters of the west lobe of Lake Bonney have a DIC age of ∼ 27,000 14C yr before present, which we believe are the most radiocarbon-deficient lake waters on Earth. Comparison of the radiocarbon profiles in the two lobes of Lake Bonney, along with previously published geochemical data, provides a new chronology of the evolution of these two waterbodies and shows that currently deep saline water is being displaced over the sill separating them.