Marius F. Simonsen
A thesis for the degree of Doctor of Philosophy defended August 2018.
The PhD School of Science, Faculty of Science, Physics of Ice Climate and Earth, Niels Bohr Institute, University of Copenhagen
Interglacial ice core dust from Greenland
Atmospheric dust is an active component of the climate system. Paleo dust records from the early Holocene and Eemian, which were both a few degrees warmer than today, are used to constrain models of a future warmer climate. Ice core records provide a strong tie point for paleo dust models, as they have a high temporal resolution and are representative of a large geographical area. However no insoluble dust record from Greenland of the Holocene and Eemian has so far been published. This is partly due to the low dust concentration in the ice, which demands high accuracy from the instruments.
Long ice core dust concentration and size distribution records are often measured by Abakus laser sensor. Abakus measurements deviate from measurements by the more accurate 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 Abakus is due to the non-spherical shape of the particles. The Abakus is strongly inuenced by Mie scattering when measuring standard polystyrene spheres, while the Mie scattering e ects cancel out for real dust particles due to their variable shape, as shown by Amsterdam Discrete Dipole Approximation simulations. Furthermore, the Abakus assigns a larger size to the particles than the Coulter Counter, since the particles are elongated. A Single Particle Extinction and Scattering Instrument (SPES) was used to measure an aspect ratio of 0:390:03 for local east Greenlandic dust and 0:330:03 for remote Asian dust in the east Greenlandic RECAP ice core. The aspect ratio derived from the discrepancy between the Coulter Counter and Abakus agrees with the SPES results, and local and remote dust can be easily discerned. If the aspect ratio is known, the Abakus can be calibrated to the Coulter 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 Greenland coast. Its dust record has been measured by Abakus calibrated using the known particle shape. 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 much higher concentration of small particles than the interglacials. This glacial record is almost identical to the NGRIP dust record, which indicates that the RECAP glacial dust comes from Asia like the NGRIP dust. The 20 m mode and geochemical composition of the interglacial dust shows that it has a local origin, coming from the Scoresby Sund area. The large 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 in large particle concentration at the onset of the glacial was because the Greenland ice sheet and glaciers grew and covered the dust sources. The large particle concentration increased at the same time as the glaciers retreated. The non-zero large particle concentration shows that some ice free areas persisted throughout the glacial.
As opposed to RECAP, which is dominated by local sources, the NEEM Holocene dust record from north central Greenland has a remote source, and can therefore be used as a tie point for global dust models. It has an increasing ux from 10 to 15 mg/m2/year through 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 increase through the Holocene A comparison to the GRIP, NGRIP1 and GISP2 calcium records and the NGRIP2 dust record shows no consistent trend over the Holocene, and there is no geographical variation in ux. Modern global dust models predict up to 20 times more dust at NEEM than measured, and up to 10 times more dust at NEEM than in GRIP and GISP2. This inconsistency shows the need for further development of Greenland atmospheric dust models.