Climatic influences on stable water isotope variability in palaeo-precipitation

Abstract

Stable water isotopes are useful tracers in the hydrological cycle that preserve integrated histories of water transport. Water isotope archives such as speleothems (calcium carbonate cave deposits) and ice cores provide evidence of past changes in the atmospheric water cycle. Climatic interpretations from past isotopic variability in proxy records often consider changes as an index of a single climate parameter, such as temperature or precipitation amount. The isotopic composition of precipitation, however, more accurately integrates multiple variations in atmospheric circulation from source to site.

In this study, the impact of abrupt and orbital- scale climatic changes on the tropics and poles is examined using geochemical and modelling techniques. The dominant climatic influences on variability in water isotopes in precipitation are investigated for a variety of different types of climatic change, and the climatic interpretations applied to speleothem and ice core records are evaluated using general circulation model simulations of isotope and climate variability.Results for comparison with proxy records are obtained from a suite of simulations of past and present climates using the coupled atmosphere-ocean Goddard Institute for Space Studies (GISS) ModeIE-R. Water isotope tracers are incorporated into the model and are tracked through all stages of the hydrological cycle, where appropriate fractionations are applied at each phase change. In addition, simulations utilise a novel suite of generalised vapour source distribution (VSD) tracers, whereby the sources of water vapour to a location can be traced back through any cloud process to the site of original surface evaporation.

Firstly, geochemical analyses from a speleothem from southern Indonesia are combined with model results to constrain the spatial and temporal evolution of abrupt climate change in the low latitudes. This demonstrates that Heinrich stadial hydrological changes in the southern tropics are more complex than inter-hemispheric anti-phasing and are dependent on the magnitude of the perturbation and its ability to propagate southwards. Secondly, using this hosing simulation, modelled oxygen isotopic ({u03B4}{u2091}{u2078}O) [or delta-O-18 ] changes are compared with reconstructed values from numerous low-latitude proxy records covering Heinrich events and sites are classified according to the dominant modelled influence on {u03B4}{u2091}{u2078}O variability. The prevailing amount effect interpretation is found to be most useful for coastal or tropical sites under direct Intertropical Convergence Zone influence. Lastly, the skill of deuterium excess as a tracer of oceanic source conditions in the high latitudes is evaluated using a suite of VSD-enabled model experiments and deuterium excess is found to record complex changes in the atmospheric water cycle in both the northern and southern polar regions.

Water isotope variability integrates a complete air mass history from the evaporative source to the site of rain-out. Isotope-climate relationships are generally complex and dynamic; site-specific, multi-variable interpretations are often warranted. This study demonstrates the utility of generalised VSD-tracers as a circulation diagnostic and of combining multiple approaches to improve site-specific interpretations from water isotope archives. Overall, palaeo-simulations using isotope-enabled models allow the climatic influences on isotopic variability to be evaluated and the signals recorded in archives to be more precisely understood.