Radiocarbon Geochemistry of Modern and Ancient Arctic Lake Systems, Baffin Island, Canada

prior to being transported into the lake system. The combinaThe accuracy of Arctic lake chronologies has been assessed by tion of these factors commonly leads to radiocarbon ages measuring the C activities of modern carbon sources and that are apparently too old. Discrete macrofossils are rarely applying these isotopic mass balances to dating fossil lake materifound in sufficient quantities for AMS C dates. Even when als. Small (õ1 km) shallow (õ25 m) Arctic lakes with watersheds plant macrofossils are present, their geologic ages are susõ12 km have soil and peat stratigraphic sections with C activipect because macrofossils and sedimentary carbon can be ties ranging from 98 to 51% Modern. The C activity of particulate reworked from preexisting sediments and soils, and the meorganic carbon, dissolved organic carbon, and dissolved inorganic tabolism of ancient carbon can generate a significant resercarbon from lake and stream waters ranges from 121 to 95% voir effect for living aquatic plants (Nelson et al., 1988). Modern. The sediment–water interface of the studied lakes shows We examined modern sedimentary and geochemical analogs consistent C ages of Ç1000 C yr, although the C activity of to improve our understanding of the chronology of sediment living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the sequences. Our objective is to describe all carbon sources enterÇ1000 C yr age of the sediment–water interface results from ing a series of Arctic lakes whose biological productivity and deposition of C-depleted organic matter derived from the waterbedrock geology are representative of lakes commonly used shed. q 1996 University of Washington for paleoclimatic reconstructions. The three principle components in evaluating the C inventory of the lake basins are: (1) measure the contemporary C activity of the dissolved INTRODUCTION inorganic carbon (DIC), dissolved organic carbon (DOC), and particulate organic carbon (POC); (2) measure the C age– depth relationship of soil and peat profiles; and (3) extract The Arctic Region is fundamental to understanding and specific organic chemical fractions from lake sediments to imdescribing late-Quaternary global climatic changes because prove the C chronologies by dating similar organic comit is considered to be sensitive to even small climatic perturpounds. By first measuring the radiocarbon activity in modern bations (Dansgaard et al., 1989; IPCC Scientific Assessment, carbon sources, we can assess the accuracy of C ages from 1990; Douglas et al., 1994). Depending upon environmental lacustrine sediments and define specific subsets of carbon in conditions, the Arctic also has the potential to be an enorbulk lake sediments that are optimal for C dating. mous reservoir or source for carbon which is stored as sediWe tried dating numerous chemical fractions to assess the mentary organic carbon or released as gaseous or solid caraccuracy of C dates when terrestrial plant macrofossils bon compounds (cf. Oechel et al., 1993). The timing of past were not present in sufficient quantities for an AMS C climatic changes in the Arctic is fundamental to modeling measurement. These include dating bulk sediment, fulvic regional climatic changes. Although Arctic lakes are comacid, humic acid, humin compounds, and aquatic plant macmonly used as sources of climatic proxy data, accurate radiorofossils. Which of these materials yields the most accurate carbon ages have been difficult to obtain. Radiocarbon dating geologic age depends on each lake system’s bedrock geolof Arctic lake sediments is frequently inaccurate because ogy, soil and peat development, water source(s), and the these systems have low rates of productivity and sedimentaabsolute organic carbon content in sediments. We measured tion resulting in low organic matter concentrations in the the radiocarbon activity of modern organic compounds for sediments. Furthermore, low rates of decomposition allow terrestrial organic matter to be stored for prolonged periods each lake basin to help evaluate the dates on fossil carbon.