Terrestrial responses to millennial and sub-millennial climate variability in the Early Glacial and implications for the hydrological cycle

While much work has focused on glacial terminations, less is known about the transitions from warmer to cooler climate states. As such, the transition from the Last Interglacial Complex to the Early Pleniglacial, 67-85 thousand years ago (ka), represents an important case study for gaining insights into the interaction of orbital and short-term climate variability and ice sheet growth. This interval is characterized by a series of millennial-scale climate oscillations, while sea level fell by about 50m, reflecting the expansion of ice sheets in northern high latitudes. For ice sheets to expand a supply of warm, moist air to cold regions is required, so that ice accumulation exceeds ablation. Reconstructed Greenland temperatures show a series of warm events (interstadials) during this period, but corresponding changes in atmospheric methane concentration are subdued, pointing to reduced emissions and a weakening of the hydrological cycle. In parallel, new evidence from ice cores has revealed the presence of centennial oscillations in Greenland temperatures during this interval. These are accompanied by variations in atmospheric methane concentrations, underlining the large geographical extent of these climate changes. This project aims to deliver better constraints on changes in terrestrial environments and the hydrological cycle and on links between key parameters in the climate system during the interval 67-85 ka. This will be achieved by generating a high-resolution pollen record, linked with sedimentological and palaeoceanographic proxies within the same reference marine sequence from the S Portuguese margin. In recent years, the S. Portuguese margin has emerged as a critical area for tracing millennial-scale variability and linking marine and terrestrial records. A key aspect is the geographical setting of the area where the combined effects of the Tagus River and a narrow continental shelf lead to the rapid delivery of terrestrial material, including pollen, to the deep-sea environment. Joint pollen, sediment and palaeoceanographic proxy analysis within the same sequence allows a direct comparison between ocean and terrestrial changes, bypassing timescale and correlation uncertainties. More specifically, the project aims to establish: (1) the strength of the hydrological cycle at mid-to-low latitudes during interstadials: given that changes in vegetation in southern Europe are mainly controlled by variations in the continental hydrological balance, a pollen record for this interval will provide an independent index of changes in the strength of the hydrological cycle, which can be compared with that inferred from the atmospheric methane record. (2) the response of vegetation to sub-millennial events and impact on erosion: while work on the Portuguese margin has established the immediate response of vegetation to millennial-scale variability, sub-millennial variations have not been previously documented. A new ultra high-resolution record of changes in sediment composition in the reference marine core has now revealed the presence of sub-millennial events, coeval with variations in ice core records. By increasing the resolution of the pollen record during these events, the project will investigate whether the oscillations in sediment composition were accompanied by vegetation changes. This, in turn, will test the hypothesis that the rapid changes in sediment composition reflect variations in the terrigenous supply from the Tagus River, which are ultimately controlled by the interplay of the hydrological balance and extent of vegetation cover. The results will have implications for understanding moisture sources for ice sheet growth during transitions into full glacial conditions and the response of terrestrial environments to short-term climate variability.