Simulating Prehistoric Mid-Latitude Oxygen Isotopes in Precipitation using IsoGSM
Wednesday, 17 December 2014
The oxygen isotope ratio (δ18Op) of precipitation at mid-latitudes varies in response to multiple weather and climate influences and is difficult to interpret without constraining each potential influence. This is particularly true in pre-historic records when climate boundary conditions and forcings were different. As an example Middle East (ME) Holocene records have been interpreted in multiple, conflicting ways. We investigate the previous proxy interpretations with an isotope-enabled climate model in order to assess how each atmospheric influence affected the δ18Op record. Two model simulations were conducted with the Isotope-incorporated Global Spectral Modal (IsoGSM): 1) with present-day conditions and 2) with mid-Holocene (MH) conditions. For the MH simulation, changes were made to surface forcing, orbital parameters and greenhouse gas concentrations. The model results show the annual averaged δ18Op 6 kya was lower than modern by 0.3-1‰ within the ME, consistent with published proxy records. Today the δ18Op exhibits lower δ18Op during the winter and higher δ18Op in both fall and spring. Our results show that the primary driver of mean annual δ18Op at mid-latitudes during the MH was variations in the strength and duration of the seasonal precipitation cycle, specifically atmospheric conditions in late spring. Focusing on the late spring, we find that the lower MH δ18Op signal can be explained by a strengthened springtime Cyprus low, resulting in additional moisture transport from the western Mediterranean. This reduces springtime temperatures by bringing colder air from the European continent, and this additional moisture increases seasonal precipitation. Finally, there is lower δ18O of vapor at the upstream moisture source during the MH. Results from IsoGSM suggest that the fractional change in the annual mean δ18Op 6000 years was 60% due to upstream δ18O of vapor changes, 30% to temperature change (-0.7 C), and 10% to precipitation amount change (+10 mm) during the spring. Therefore, δ18O proxy records in the ME are found to be highly sensitive to multiple springtime influences, both local and upstream, and must be carefully evaluated to ensure they are not misinterpreted.