Reconstruction of Western Mediterranean precipitation δ18O through the Last Glacial: methodology using speleothem from monitored caves in Gibraltar
Abstract
We describe a methodology for reconstructing the δ18O of rainfall through the Last Glacial period using well-dated speleothem records and information from cave monitoring. This study is based on new record from flowstone and stalagmite from Ragged Staff Cave, Gibraltar spanning 30 - 110 ka and constrained by 52 U-Th ages. The decadally resolved δ18O record resembles N-GRIP and δ18O variations between 40 and 62 ka align closely with NGRIP's GICC05modelext timescale. The relationships between calcite deposition, δ18O and rainfall are well-constrained from an eight-year cave monitoring campaign which shows that the water isotope signal that Gibraltar speleothems capture is averaged over a period of around one year, and that modern speleothem calcite closely tracks the δ18O of winter rainfall.
Three factors determine the δ18O of speleothem calcite: the δ18O of winter recharge (as seepage water), the cave temperature (that determines equilibrium fractionation between water and calcite), plus non-equilibrium fractionation caused by CO2degassing and/or surface reactions. To constrain temperature we used Uk'37 estimates of annual average sea surface temperatures (SSTs) in core ODP-977A in the Alboran Sea, after converting its original timescale to that of NGRIP. We assume that in coastal Gibraltar Mean Annual Air Temperatures (MATs) equalled SSTs. Cave temperatures lag MATs due to the thermal capacity of the bedrock and we use a 1-D heat flow model to set up two bounding scenarios for the evolution of cave temperatures enabling time-series of 'equilibrium' and 'disequilibrium' values for δ18O in cave water to be reconstructed. Cave monitoring shows that average seepage δ18O is currently 0.6 to 0.75 ‰less than rainfall, because of complete evaporation of summer rains. In the cooler climates of the Last Glacial evaporation would have been less than today and the isotopic composition of seepage may have been closer to the annual average of rainfall. A reconstructed envelope for the average δ18O of palaeo-precipitation in the western Mediterranean indicates 1) that the isotopic composition of precipitation in Gibraltar between 40 and 62 ka was within 1‰ of modern rainfall and 2) in the speleothem archive the main expression of Dansgaard-Oescher events is through temperature, not the δ18O of rainfall.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP21A..07M
- Keywords:
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- 3344 Paleoclimatology;
- ATMOSPHERIC PROCESSES;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1655 Water cycles;
- GLOBAL CHANGE;
- 1833 Hydroclimatology;
- HYDROLOGY