Hydrological factors in the interpretation of stable isotopic proxy data present and past : a European perspective

In the context of this paper, stable isotopic proxies are those preserving directly, or via known or calculable fractionation factors, a record of 18O/16O and/or 2H/1H in the water which contributed to their growth. Suitable proxies include amongst others tree-ring cellulose, bone phosphate, lake sediment minerals and speleothem fluid inclusions. A principal aim of studying isotopic proxies is to gain more information about past climates through a knowledge of how rainfall isotope compositions have changed over time. However, converting back from proxy to rainfall is not necessarily straightforward, because however well the isotopic fractionation due to proxy growth or formation is understood, a variety of modifying processes may have intervened between this and the actual rainfall events. Three main reservoirs of environmental water may contribute to the isotopic compositions of terrestrial proxy indicators: surface waters, soil moisture, and groundwater. Each of these can in principle be isotopically modified compared to rainfall in the locality. This review examines these effects and how they may change under different European climate types at the present day. It is concluded that while groundwater isotopic compositions typically represent closely the bulk composition of rainfall under a variety of climatic regimes, soil and particularly lake waters require detailed study if their isotope systematics are to be adequately understood. The review also finds that to a large extent “the present is the key to the past” in terms of recharge processes during the late Pleistocene, though glaciation may have had profound isotopic consequences in certain areas, while a more general aridity led to the apparent absence of recharge in most areas for much of the late Devensian