HOLOCENE CARBONATES IN LAKE JUNIN PERU: MINERALOGY AND STABLE ISOTOPE RECORDS

Lake Junín (11.0°S, 76.2°W) is an intermontane, high-elevation (4080 m asl) lake in the inner-tropics of the Southern Hemisphere that spans ~300 km². Lake Junin is dammed at its northern and southern ends by coalescing alluvial fans >250 ka that emanate from glacial valleys in both cordillera. An ~20-m-long sediment core obtained in 1996 reveals that sediment deposited during the last glacial cycle (~30-16 ka) is dominated by glacial flour whereas sediment deposited during the last 16 ka consists predominantly of authigenic calcite (marl) and ostracod carapaces. Stable isotope analyses (δ¹⁸O and δ¹³C) of post-glacial carbonates reveals systematic shifts in both isotopic systems that reflect changes in the isotopic composition of lake water and regional climate. However, the potential for contamination of authigenic carbonates by detrital material limits the confidence with which interpretations can be made, especially those of the numerous small (<0.25 ‰) shifts in both records. In order to evaluate the potential effect of detrital contaminants, we measured δ¹⁸O and δ¹³C on 130 bulk samples deposited over the past ~8ka (sample spacing 40-70yr). We then sieved these same 130 samples to separate material >75µm and <75µm, and analyzed those 260 subsamples for both δ¹⁸O and δ¹³C. We sought to determine the isotopic effects (if any) of exposing bulk carbonates to dispersant (dilute sodium metaphosphate) and a strong oxidizing agent (30% H₂O₂) by comparing the δ¹⁸O and δ¹³C of untreated subsamples with those treated with either dispersant, oxidant, or both. Results reveal all three size fractions (bulk, >75µm, <75µm) record nearly identical amplitude variability with the >75µm fraction consistently 0.25-2‰ heavier in both δ¹⁸O and δ¹³C, and the record from bulk sediment plotting between these two curves. Neither size fraction appears to be unduly influenced by regional limestone bedrock (also analyzed for both δ¹⁸O and δ¹³C), suggesting that neither is more or less contaminated by detrital material. Differences between the two size fractions may reflect the presence of ostracod carapaces preferentially in the >75µm fraction, and to a difference between the season of ostracod molting and inorganic carbonate precipitation. Chemical pretreatments each impart small (<0.5‰) reductions in the δ¹⁸O and δ¹³C of bulk sediment.