Sedimentological and stable-isotope evolution of lakes in the Vestfold Hills, Antarctica

Abstract

The sedimentological, chemical and isotopic characteristics of sediment cores from three slightly saline to hypersaline lakes (Highway, Ace and Organic Lakes) and two marine inlets (Ellis Fjord and Taynaya Bay) in the Vestfold Hills, Antarctica, have been examined. Sections of the cores deposited in marine environments are characterized by uniform, regularly laminated, fine-grained, organic-rich sediments, with uniform organic δ 13C values (−18.0 to −19.4% vs. PDB) and sulfur contents. In contrast, sediments deposited in lacustrine environments are extremely heterogeneous, varying from finely laminated mat-like sequences to poorly sorted clastic-rich sediments. Authigenic monohydrocalcite and aragonite occur in some lake sediments. The δ 13C values of organic matter in the lacustrine sediments exhibit an extremely wide range (−10.5 to −25.3‰) that can be related to variations in physico-chemical conditions in the lake waters. Strongly negative organic- δ 13C values coupled with high sulfur contents are indicative of an anoxic zone in the overlying lake waters, whereas less negative organic- δ 13C values coupled with low sulfur contents are indicative of well-mixed oxic conditions. Particularly high organic- δ 13C values result during high levels of microbial activity in the lakes, due to high rates of photosynthetic CO 2 fixation. The large shifts in organic- δ 13C are not necessarily accompanied by any change in macroscopic sedimentological characteristics, illustrating the utility of isotopic investigations in these environments. The δ 13C composition of authigenic carbonate in hypersaline Organic Lake sediments provides a record of changes in palaeoproductivity, while the δ 18O of the carbonate provides information on rates of meltwater input and evaporation in the lake. 14C-dating suggests that Highway Lake was isolated from the sea by isostatic uplift at least 4600 yr before present (B.P.) whereas Organic Lake was isolated at approximately 2700 yr B.P. Apparent emergence rates calculated from the 14C ages range from 1.0 to 2.1 mm yr −1. The ‘reservoir effect’ in the lacustrine and marine environments is variable, but probably does not exceed ∼ 1000 yr in any of the lakes examined.