Abstract
Atmospheric dust is an active component of the climate system. Paleo dust records fromthe early Holocene and Eemian, which were both a few degrees warmer than today, are usedto constrain models of a future warmer climate. Ice core records provide a strong tie pointfor paleo dust models, as they have a high temporal resolution and are representative of alarge geographical area. However no insoluble dust record from Greenland of the Holoceneand Eemian has so far been published. This is partly due to the low dust concentrationin the ice, which demands high accuracy from the instruments.Long ice core dust concentration and size distribution records are often measuredby Abakus laser sensor. Abakus measurements deviate from measurements by the moreaccurate Coulter Counter, and the discrepancy can not be solved by a simple calibration.In this thesis it is shown that the discrepancy between the Coulter Counter and the Abakusis due to the non-spherical shape of the particles. The Abakus is strongly inuenced by Miescattering when measuring standard polystyrene spheres, while the Mie scattering eectscancel out for real dust particles due to their variable shape, as shown by AmsterdamDiscrete Dipole Approximation simulations. Furthermore, the Abakus assigns a largersize to the particles than the Coulter Counter, since the particles are elongated. A SingleParticle Extinction and Scattering Instrument (SPES) was used to measure an aspect ratioof 0:390:03 for local east Greenlandic dust and 0:330:03 for remote Asian dust in theeast Greenlandic RECAP ice core. The aspect ratio derived from the discrepancy betweenthe Coulter Counter and Abakus agrees with the SPES results, and local and remote dustcan be easily discerned. If the aspect ratio is known, the Abakus can be calibrated to theCoulter Counter, so it gives accurate concentrations and size distributions.The RECAP ice core was drilled in 2015 on the Renland ice cap by the East Greenlandcoast. Its dust record has been measured by Abakus calibrated using the known particleshape. It has a high concentration of large particles during the Holocene and Eemian,but low concentrations during the glacial. On the other hand, the glacial has a muchhigher concentration of small particles than the interglacials. This glacial record is almostidentical to the NGRIP dust record, which indicates that the RECAP glacial dust comesfrom Asia like the NGRIP dust. The 20 m mode and geochemical composition of theinterglacial dust shows that it has a local origin, coming from the Scoresby Sund area. Thelarge particle concentration fell by more than 90% from 116:6 0:7 to 111:1 0:5 ka b2k(before year 2000 CE) and rose again from 12:10:1 to 9:00:1 ka b2k. The decrease inlarge particle concentration at the onset of the glacial was because the Greenland ice sheetand glaciers grew and covered the dust sources. The large particle concentration increasedat the same time as the glaciers retreated. The non-zero large particle concentration showsthat some ice free areas persisted throughout the glacial.As opposed to RECAP, which is dominated by local sources, the NEEM Holocene dustrecord from north central Greenland has a remote source, and can therefore be used asa tie point for global dust models. It has an increasing ux from 10 to 15 mg/m2/yearthrough the Holocene, and the same concentration during the Eemian and early Holocene.The NEEM calcium ux, a proxy for dust, is around 1.5 mg/m2/year with no increasethrough the Holocene A comparison to the GRIP, NGRIP1 and GISP2 calcium recordsand the NGRIP2 dust record shows no consistent trend over the Holocene, and there isno geographical variation in ux. Modern global dust models predict up to 20 times moredust at NEEM than measured, and up to 10 times more dust at NEEM than in GRIPand GISP2. This inconsistency shows the need for further development of Greenlandatmospheric dust models.
| Originalsprog | Engelsk |
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| Forlag | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
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| Status | Udgivet - 2018 |
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