Novel isotope geochemical tracing of Greenland ice core dust

Many signs of contemporary climate change are being observed on our planet. This affects or will affect large-scale patterns of atmospheric circulation on the global scale, influencing and controlling both temperatures and precipitation. Atmospheric simulations may predict these processes, but it is imperative to test the performance of such models under climatic conditions that were profoundly different from those of the present-day. Remarkable climatic/environmental changes (interglacial warming/glacial cooling) happened some thousands of years ago in the geologic past, which altered atmospheric circulation and changed dust emissions significantly from time to time due to hydrological and vegetation changes. Mineral dust emitted from the continents loaded the atmosphere and subsequently accumulated on the Greenland ice sheet after long-range (several thousands of km) transport. Impurity concentration variations measured in central-Greenland ice cores provide a record of past climatic changes and activities of dust emission centers, but it is equally important to identify which sources contributed dust to the ice sheet and how their role may have changed. We attempt at identifying these dust sources in the present research using novel approaches (Hf isotopic fingerprinting in combination with Sr and Nd isotopic fingerprinting). Constraining the origin of dust helps to better understand the effects of climatic changes on land (precipitation/vegetation) and to test the aforementioned atmospheric circulation models. Improved model simulations may provide in turn better forecasts on future atmospheric circulation changes. 1

Greenland ice cores are archives of palaeoclimatic change, extending as far back as the last interglacial (~130-115 kiloyears) with similar climate to present-day. Better understanding of past climate change and its effects is crucial to better predict what will happen in the future, when atmospheric greenhouse gas concentrations will reach high levels. Climate change affects large-scale patterns of atmospheric circulation on a global scale, influencing and controlling both temperatures and precipitation patterns over the continents. Atmospheric model simulations may predict these processes, but it is imperative to test the performance of such models under climatic conditions some thousands of years ago in the geological past, when these conditions were profoundly different from those of the present day. Aerosols/dust particles deposited during the cold glacial period on the Greenland ice sheet originate from the continents of the northern hemisphere. Constraining the dust origin and examining the impurity content of ice helps to better understand the effects of climatic changes on land, precipitation and vegetation, dust source activity and to validate atmospheric circulation models under cold glacial climate conditions. In this Lise Meitner project we developed methods to measure the hafnium, strontium and neodymium (Hf-Nd-Sr) isotope ratios of low volume aerosol/dust samples (3-6 mg), which are not just cheaper than previous methods, but allow high sample throughput without compromising accuracy and reproducibility. Also, in cooperation with the University of Lausanne, the hydrogen isotope compositions of water in clay mineral crystal structures were analyzed to use these isotopic data as "fingerprints" of dust sources, which is also a novel approach to tracing dust sources. Measurements of the isotopic compositions of last glacial dust material from the NorthGRIP ice core and potential source are samples revealed that the dust was very likely to be sourced from the Taklimakan desert (NW China), but its origin in Southeast Europe cannot be excluded. This is a crucial finding, which must be considered in future model simulations of the global dust cycle and the Earth system in general. Computer simulations for the Last Glacial Maximum (LGM) in a smaller model domain, specifically done for this project in cooperation with the Karlsruhe Institute of Technology using the Max Planck Institute for Meteorology Earth System Model demonstrated that dust material from Europe could reach Greenland over the LGM. This has not been considered a plausible scenario in previous research of dust in central Greenland ice cores, simply because of the fact that Europe is downwind to Greenland. This Lise Meitner project, therefore, contributed a lot to our general understanding of potential sources of central Greenland glacial aerosols and demonstrated that the Hf-Nd-Sr isotopic composition of dust and hydrogen isotopes of structurally bound water are powerful tracers in aerosol provenance studies.