Reconstructing precipitation δ18O from lacustrine carbonates using δ18O, Δ47, and Δ′17O: a modern case study from Junín, Peru with implications for paleoclimate

Abstract:

Paleoclimate studies often aim to reconstruct oxygen isotopes of precipitation (δ¹⁸O_P) because δ¹⁸O_P tracks regional climate change. Lacustrine carbonates are particularly alluring archives of past δ¹⁸O_P, because they allow for the construction of long records, with robust chronologies. However, disentangling the influences of δ¹⁸O_P, formation temperature, and local hydrology on δ¹⁸O of carbonates (δ¹⁸O_C) from lake systems can be challenging, especially if past lake temperatures or water budgets were very different from modern conditions. A recent drilling effort from Lake Junín (Chinchaycocha; 11 °S, 76 °W) in the Peruvian Andes has produced well-dated, high resolution (decadal-centennial) records of δ¹⁸O_C, which have the potential to provide an unprecedented record of δ¹⁸O_P and climate change in the Tropics from the last 700,000 years, if we can control for the influence of water temperature and lake water evaporation. Here, we use a combination of carbonate clumped isotope paleothermometry (∆₄₇) and triple oxygen isotope analyses (Δ′¹⁷O) from lake waters and carbonates from the Junín region today to understand how they constrain temperature and lake water evaporation, respectively. We report data from two fast-turnover and two slow-turnover flow-through lake systems in the Junín Region (residence times of ≤ 1 year and > 1 year, respectively). We find that Δ₄₇ temperatures of actively-forming carbonates agree with measured lake water temperatures. Slow-turnover lake systems have higher average δ¹⁸O values and lower Δ′¹⁷O values than fast-turnover systems, which is consistent with triple oxygen isotope mass balance models for evaporative loss. Finally, we reconstruct unevaporated catchment precipitation δ¹⁸O values (δ¹⁸O_rucp) from modern surface water and calculated carbonate parent-water δ¹⁸O and Δ′¹⁷O values. The average δ¹⁸O_rucp ≅ -15.3 ‰ (1 σ = 1.8 ‰; n = 24; λ_lake = 0.524) is in agreement with local amount weighted mean annual precipitation (δ¹⁸O_P = -15.4 ‰). Our findings suggest that a combination of δ¹⁸O_C, Δ′¹⁷O_C, and Δ₄₇ measurements from Lake Junín carbonates can be used to generate regional δ¹⁸O_rucp records and that this approach can be applied to carbonate from drill cores to build reliable records of δ¹⁸O_P and past hydroclimate in the Peruvian Andes.