3-4 Reconstructing Lake Paleo-DIC Concentrations using Dual C-14 Age Data, with Examples from Paulina Lake and East Lake, Newberry Volcano, OR

Session: Lakes of the World Through Space and Time: Archives of Climate, Paleoenvironments, Ecosystems, Geohazards, and Economic Resources

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Abstract:

Lake organic carbon and carbonate ¹⁴C age dates in lake deposits commonly suffer from “reservoir effects”, whereas ¹⁴C dates on terrestrial organic matter usually provide more realistic ages. The reservoir effects create ‘apparent ages’ that are too old, caused by dis-equilibrium of atmospheric ¹⁴C with lake water DIC. The ¹⁴CO₂(air) to ¹⁴CO₂(aq) equilibration is relatively fast, but the ¹⁴CO₂(aq) to ¹⁴HCO₃^- and ¹⁴CO₃⁼ equilibration is slow. For example, the time scale for full isotopic equilibrium in the marine surface layer is estimated at a decade! The degree of isotopic disequilibrium is influenced by the pH-dependent ratio DIC/CO₂aq and by physical lake processes such as mixing regime, thermal stratification, and water/carbon residence times. High-DIC lakes are commonly less equilibrated than low-DIC lakes, which suggests that the age mismatch between apparent and real ages may be phrased in terms of paleo-DIC.  We derived a set of expressions from the ¹⁴C decay equation that relate the age mismatch to the DIC. We then normalize that expression on the age mismatch in the DIC of the modern lake, with its measured DIC concentration. This normalization on the modern lake is key, largely eliminating the influence of physical lake processes. We derived the equation DIC(t^x) = DIC(modern) EXP(ΔΔt/8033), where DIC(t^x) is the lake DIC at time t=x, DIC(modern) is the measured DIC today, and ΔΔt is the difference between the age differences (apparent minus real) in the t^x and t(modern) cases. We applied this principle to sediment cores and waters from Paulina Lake and East Lake at Newberry Volcano, OR, using sets of dual ages and the modern cases, providing DIC values in Paulina Lake of 400 ppm HCO₃ today, 240 ppm at ~4000 BP and 700 ppm at ~7000BP. Calcite was not stable in the more dilute version of Paulina Lake at ~4000BP (PHREEQ simulations), which agrees with the absence of ostracod valves in core segments of that age. The 700 ppm time interval coincides with local volcanic activity that created the volcanic ridge between Paulina Lake and East Lake, which presumably increased the hydrothermal carbonate lake inputs. These preliminary results suggest that this novel, dual-age approach holds promise for the reconstruction of paleo-DIC and possibly paleo pH in various lake systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9975

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