Abstract
Abstract. Glacial climate was characterised by two types of abrupt events. Greenland ice cores record Dansgaard–Oeschger events, marked by abrupt warming in-between cold, stadial phases. Six of these stadials appear related to major Heinrich events (HEs), identified from ice-rafted debris (IRD) and large excursions in carbon- and oxygen-stable isotopic ratios in North Atlantic deep sea sediments, documenting major ice sheet collapse events. This finding has led to the paradigm that glacial cold events are induced by the response of the Atlantic Meridional Overturning Circulation to such massive freshwater inputs, supported by sensitivity studies conducted with climate models of various complexities. These models also simulate synchronous Greenland temperature and lower-latitude hydrological changes. To investigate the sequence of events between climate changes at low latitudes and in Greenland, we provide here the first 17O-excess record from a Greenland ice core during Dansgaard–Oeschger events 7 to 13, encompassing H4 and H5. Combined with other ice core proxy records, our new 17O-excess data set demonstrates that stadials are generally characterised by low 17O-excess levels compared to interstadials. This can be interpreted as synchronous change of high-latitude temperature and lower-latitude hydrological cycle (relative humidity at the oceanic source of evaporation or change in the water mass trajectory/recharge) and/or an influence of local temperature on 17O-excess through kinetic effect at snow formation. As an exception from this general pattern, stadial 9 consists of three phases, characterised first by Greenland cooling during 550 ± 60 years (as shown by markers of Greenland temperature δ18O and δ15N), followed by a specific lower-latitude fingerprint as identified from several proxy records (abrupt decrease in 17O-excess, increase in CO2 and methane mixing ratio, heavier δD-CH4 and δ18Oatm), lasting 740 ± 60 years, itself ending approximately 390 ± 50 years prior to abrupt Greenland warming. We hypothesise that this lower-latitude signal may be the fingerprint of Heinrich event 4 in Greenland ice cores. The proposed decoupling between stable cold Greenland temperature and low-latitude climate variability identified for stadial 9 provides new targets for benchmarking climate model simulations and testing mechanisms associated with millennial variability.
Highlights
Glacial climate is characterised by millennial variability, recorded with specific expressions in different archives and at different latitudes (Voelker, 2002; Clement and Peterson, 2008)
The Tropics exhibit a fingerprint of Dansgaard–Oeschger events (DO) events as suggested e.g. by (i) variations in monsoon strength, (ii) changes in the atmospheric methane concentration with its main source located at low latitudes during glacial periods (e.g. Baumgartner et al, 2012), or (iii) changes in the isotopic composition of atmospheric oxygen δ18Oatm measured in air from ice cores (Landais et al, 2007; Severinghaus et al, 2009), reflecting at this timescale changes in the low-latitude water cycle and global biosphere productivity (Bender et al, 1994b)
The most obvious decoupling between δ18O and 17O-excess is observed during GS-9 (Fig. 4d, green arrows): 17O-excess decreases after the δ18O decrease at the beginning of GS-9 and increases several centuries before the δ18O increase at the beginning of GI-8
Summary
Glacial climate is characterised by millennial variability, recorded with specific expressions in different archives and at different latitudes (Voelker, 2002; Clement and Peterson, 2008). Baumgartner et al, 2012), or (iii) changes in the isotopic composition of atmospheric oxygen δ18Oatm measured in air from ice cores (Landais et al, 2007; Severinghaus et al, 2009), reflecting at this timescale changes in the low-latitude water cycle and global biosphere productivity (Bender et al, 1994b). Greenland ice cores do provide proxy records influenced by high-, mid- and low-latitude climate changes. In the following we will argue that the climatic fingerprint of H4 can be identified in multiple proxy records sensitive to climate and environmental changes at high, mid and low latitudes, archived in the ice and air of Greenland ice cores. The advantage of using the NEEM rather than the Siple Dome δ18Oatm data resides in the smaller NEEM ice–gas synchronisation uncertainty during GS-9 (Appendix A2)
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