PhD defence by Ernesto Kettner – Niels Bohr Institute - University of Copenhagen

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PhD defence by Ernesto Kettner

Title of thesis:
Highly resolved paleoclimatic aerosol records: continuous flow analysis and the potential of flow cytometry for the characterisation of aeolian dust in polar ice cores

Abstract - in Danish below
In ice cores a plethora of proxies for paleoclimatic conditions is archived. Air trapped in
the ice during firnification allows for direct measurements of the concentrations and isotope ratios of paleoatmospheric gases while, the isotopic composition of the ice matrix itself is related to paleotemperatures. Impurities in the matrix are comprised of particulate and soluble aerosols, each carrying information on its source’s activitiy and|or proximity.

Opposed to gases and water isotopes, the seasonality of many aerosols is not smoothed out in the firn column so that large concentration gradients with frequently changing signs are preserved. Therefore, these aerosol records can be used for dating by annual layer counting. However, with increasing depth the annual layer thicknesses decreases due to pressure and ice flow and accurate dating is possible only as long as the rapid variations can be resolved experimentally.

Over the last decades Continuous Flow Analysis (CFA) has become a well-established
technique for aerosol quantification. In CFA, a piece of core is melted continuously and
the melt water is analysed for an array of chemical impurities. When designing a CFA
system, a trilemma between high sample throughput, high resolution and a large number of detected species is encountered.

In this experimental work, subsequent to contributing to the construction of a CFA
system optimised for high resolution, possibilities for the improvement of the insoluble
aerosol records’ quality were investigated. Whereas soluble aerosols can be analysed for concentration changes only, insoluble aeolian dust can reveal additional information on its atmospheric residence time via changes in the mean grain sizes.

Volumes of particulate matter in ice cores are most reliably determined with Coulter
counters, but since a Coulter counter performs measurements on discrete samples, it cannot be connected to a CFA system. Attenuation sensors, on the other hand, can be integrated into a CFA set-up, but are known to yield poor dust size records.

The dilemma between high quality sizing and high depth resolution was found impossible to circumvent by employing a third detection technique - laser scattering. Reliable information on size changes, even relative ones, cannot be obtained using optical methods. It is therefore proposed to focus further efforts on electrical measurements, making use of the advancements made over the past decade in impedance cytometry.

Resumé
Et væld af proxydata for paleoklimatiske forhold bliver arkiveret i iskerner. Luftbobler
fanget i isen under firnifikationsprocessen giver mulighed for direkte at måle koncentrationen og isotopsammensætningen af paleoatmosfæriske gasser, mens den isotopiske sammensætning af selve isen er relateret til paleotemperaturer. Urenheder i ismatricen består af både partikulære og opløselige aerosoler, som alle indeholder oplysninger om aktiviteten og|eller nærheden af hver sin kilde.

I modsætning til gasser og vandisotoperne bliver sæsonudsving i mange aerosoler ikke
udglattet i firnen, hvilket bevirker at store koncentrationsgradienter med skiftende fortegn bevares. Derfor kan disse aerosoldata benyttes til datering af iskernen ved optælning af årlag. Med dybden mindskes årlagstykkelserne dog på grund af øget tryk og isens flydning, og en præcis datering er kun mulig så længe de hurtige variationer kan opløses eksperimentelt.

Gennem de seneste årtier er Continuous Flow Analysis (CFA) blevet en veletableret
teknik til at kvantificere de varierende aerosolkoncentrationer. I CFA bliver et stykke iskerne smeltet kontinuerligt, og smeltevandet analyseres for en række kemiske urenheder. Når man designer et CFA system støder man på et trilemma mellem at balancere høj produktivitet, høj opløsning og detektion af et stort antal forskellige slags urenheder.

I dette eksperimentelle arbejde, efter at bidrage til opbygningen af et CFA system
optimeret til høj dataopløsning, blev mulighederne undersøgt for at forbedre kvaliteten af målingerne af uopløselige aerosoler.

Den mest pålidelige måde at bestemme en iskernes partikelindhold er ved hjælp af Coulter counterer, men eftersom en Coulter counter udfører målinger på diskrete prøver, kan dette instrument ikke tilsluttes et CFA system. Dæmpningssensorer kan derimod integreres i et CFA system, men disse er kendt for at give dårlige data for støvkornsstørrelser.

Dilemmaet mellem høj kvalitet af støvkornsstørrelse data og høj dybdeopløsning blev
fundet umuligt at omgå ved at benytte en tredje detektionsteknik – laserspredning. Pålidelig information om ændringerne i støvkornsstørrelse, selv relative, kan ikke opnås ved hjælp af optiske metoder. Det foreslås derfor at fokusere en yderligere indsats på elektriske målinger, der gør brug af de fremskridt, der er gjort i løbet af det seneste årti indenfor impedanscytometri.

Supervisor
Anders Svensson, Ass. prof. at Centre for Ice and Climate, NBI, University of Copenhagen

Assessment committee:
Christine Hvidberg, Ass. prof. Centre for Ice and Climate, NBI, University of Copenhagen
Jean-Robert Petit, Prof., CNRS, Université Joseph Fourier, Grenoble
Aloys Bory, Ass. prof. University of Lille-I

If interested please contact Ernesto Kettner for a copy of the thesis. Contact Ernesto Kettner