RECONCILING EVIDENCE FROM OUTCROPS AND DEEP-LAKE SEDIMENTS: THE CONUNDRUM OF OXYGEN ISOTOPES IN GLACIAL-ERA MONO LAKE, CALIFORNIA

Hydrologically closed lakes, particularly those in arid regions, are exceptional archives of past climatic changes, due to their sensitivity to the balance of precipitation and evaporation. Records of absolute past lake levels, from nearshore facies, berms, wave-cut cliffs and terraces, and high-resolution proxy records from sediment cores have different strengths and weaknesses, but ultimately they must tell the same story about the history of the lake basin. To achieve this goal, indirect proxy records must be examined and interpreted in the context of the absolute changes recorded in stratigraphy and geomorphology.

Mono Lake, California is a highly sensitive closed-basin lake on the eastern (leeward) side of the Sierra Nevada, and has been shown to preserve exceptional paleoclimate records. Ground-breaking study of the Wilson Creek Formation (WCF) facies in outcrops around the basin and their relation to surficial features form the basis of the absolute lake-level curve for the last glacial period, Marine Isotope Stages (MIS) 4-3-2 by K. Lajoie (unpub. UC Berkeley Ph.D. dissertation, 1968). Interpretation of the carbonate content of the correlative deep-lake sediments in the context of Lajoie’s lake-level curve provided a high-resolution record of lake-level for that period (Zimmerman et al. 2011 GSAB v123 p 2320).

Interpretation of oxygen isotopes on carbonates from the same samples in the framework of Lajoie's curve shows the opposite relationship between d¹⁸O and lake level between ~65 and 25 ka than that predicted by simple precipitation-evaporation processes, shifting heavier in times of rising lake level, rather than lighter. One possible explanation for this relationship might be changes in the balance of isotopically heavy precipitation from tropical sources. Recent work on shorelines and nearshore sediments of the last glacial maximum and deglacial period, including the last highstand, suggests the relationship between lake level and oxygen-isotope composition of carbonates likely changes through time, and may be dependent on the relative volume of a change in lake-level, the systematics of carbonate precipitation, and variations in the source of water to the lake, including seasonality of precipitation and meltwater run-off from Sierran glaciers.