Marine Isotope Stage 19: analogue for the present interglacial

点击翻译按钮获取中文摘要

Past interglacials characterized by an orbital geometry similar to the current interglacial (Holocene) provide a basis for assessing the natural evolution of different components of the climate system and the extent of anthropogenic interference during recent millennia. While human modification of vegetation and landscapes has been underway throughout the Holocene, arguably the most significant interference with global environments has been the onset of anthropogenic emissions of greenhouse gases (GHG) over the past two centuries. However, in his 'early anthropogenic hypothesis', Ruddiman (2003) proposed that humans began modifying greenhouse gas concentrations thousands of years before the industrial era, with forest clearance and intensification of rice agriculture leading to increases in atmospheric CO2 and CH4 levels, respectively. More specifically, the concentrations of these gases show early Holocene peaks followed by declines, but the downward trend was reversed after 8 and 5 thousand years ago for CO2 and CH4, respectively. By drawing analogies with natural trends in the previous four interglacials, Ruddiman suggested that the Holocene evolution of GHG concentrations was anomalous, but differences in orbital configurations and uncertainties over their precise synchronization have underlined the limitations of such comparisons. More recently, the period around 780 thousand years ago, known as Marine Isotope Stage (MIS) 19 has been identified as the closest analogue to the current interglacial on the basis of the phasing of orbital parameters and overall climate evolution. As such, it provides a more appropriate testbed for evaluating the Holocene evolution of GHG and vegetation succession. However, despite this potential, MIS 19 remains substantially understudied. The proposed project aims to address this imbalance by generating the most detailed record (~200-yr resolution) of vegetation and hydrological changes during MIS 19 from ODP Site 977 in the Alboran Sea, S Iberian margin and combine it with ongoing analyses of palaeoceanographic proxies from the same samples, currently underway by the Spanish Consortium GRACCIE. In recent years, the western and southern parts of the Iberian margin have emerged as critical areas for linking marine and terrestrial records directly through joint pollen and palaeoceanographic proxy analyses in the same marine archive recovered adjacent to the continent. This allows a direct comparison between ocean and terrestrial changes, bypassing timescale and correlation uncertainties. The results will establish:(1) the duration of the MIS 19 interglacial in sea surface conditions and the precise phasing with vegetation changes;(2) Vegetation trends during MIS 19 vis-à-vis those already observed in the region during the Holocene;(3) the nature of vegetation and hydrological changes during the peak in atmospheric methane concentrations at the end of MIS 19c;(4) the response of vegetation and impact on erosion to short-term climate variability during the intervals before and after the interglacial as well as within it.If the MIS 19 - MIS 1 analogy is correct, these results will have implications for assessing: (i) the natural length of the current interglacial, in the absence of anthropogenic forcing; (ii) the extent to which human impact over the past millennia has overprinted natural vegetation trends in the region; (iii) the underlying causes for the Holocene evolution of methane.