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Ocean Circulation and Shelf Processes in the Arctic Mediterranean Traced by Radiogenic Neodymium Isotopes, Rare Earth Elements and Stable Oxygen Isotopes

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Ocean Circulation and Shelf Processes in the Arctic Mediterranean Traced by Radiogenic Neodymium Isotopes, Rare Earth Elements and Stable Oxygen Isotopes Ocean circulation and shelf processes in the Arctic Mediterranean traced by radiogenic neodymium isotopes, rare earth elements and stable oxygen isotopes Georgi Laukert Dissertation Kiel, 2017 Ocean circulation and shelf processes in the Arctic Mediterranean traced by radiogenic neodymium isotopes, rare earth elements and stable oxygen isotopes Dissertation zur Erlangung des Doktorgrades Dr. rer. nat. der Mathematisch-Naturwissenschaftlichen Fakultät der Christian-Albrechts-Universität zu Kiel vorgelegt von Georgi Laukert Kiel, 2017 Gutachter 1. Gutachter und Betreuer: Prof. Dr. Martin Frank 2. Gutachter: Prof. Dr. Don Porcelli Eingereicht am: 26. Januar 2017 Tag der Disputation: 20. Februar 2017 Zum Druck genehmigt: 20. Februar 2017 Gez. Prof. Dr. Natascha Oppelt, Dekanin Gutachter - i - Erklärung Erklärung Hiermit erkläre ich an Eides statt, dass ich die vorliegende Abhandlung, abgesehen von der Beratung durch meinen Betreuer, nach Inhalt und Form selbstständig erarbeitet habe und keine anderen, als die von mir aufgeführten Quellen und Hilfsmittel, verwendet wurden. Diese Arbeit ist unter Einhaltung der Regeln guter wissenschaftlicher Praxis der Deutschen Forschungsgemeinschaft entstanden und wurde weder in Auszügen noch in ganzer Form an einer anderen Stelle im Rahmen eines Prüfungsverfahrens eingereicht. Teile dieser Arbeit sind bereits in einer Fachzeitschrift veröffentlicht, wurden zur Veröffentlichung eingereicht oder sind in Vorbereitung eingereicht zu werden. Kiel, den 26. Januar 2017 Georgi Laukert - ii - CONTENTS CONTENTS ABSTRACT ....................................................................................................................1 KURZFASSUNG ........................................................................................................... 3 1. INTRODUCTION...................................................................................................... 6 1.1 The changing Arctic Mediterranean ................................................................................ 6 1.2 Bathymetry, hydrography and ocean circulation in the Arctic Mediterranean..............7 1.3 Annual hydrographic variability of the Siberian shelf seas............................................ 10 1.4 Principles and application of neodymium isotopes as water mass tracer ..................... 11 1.5 Principles and application of REEs in seawater ............................................................. 14 1.6 REE sources in the Arctic Mediterranean and their εNd signatures............................... 17 1.7 Stable oxygen isotope systematics and source-defined components ............................ 21 1.8 Objectives and outline of the thesis and contributions to the chapters ...................... 22 2. METHODS ............................................................................................................. 25 2.1 Sample collection and pre-concentration ..................................................................... 25 2.2 Chemical procedures including chromatographic purification .................................. 26 2.3 Neodymium isotope and ID concentration measurements via MC-ICP-MS .............. 28 2.4 Rare earth element concentration measurements ....................................................... 28 2.5 Oxygen isotope, nutrient and sample salinity measurements ..................................... 29 2.6 Binary and ternary mixing based on salinity, Nd isotopes and [Nd]........................... 29 3. PUBLICATION AND MANUSCRIPTS ..................................................................... 31 CHAPTER I: Ocean circulation and freshwater pathways in the Arctic Mediterranean based on a combined Nd isotope, REE and oxygen isotope section across Fram Strait .... 31 CHAPTER II: Transport and transformation of riverine Nd isotope and rare earth element signatures in high latitude estuaries: A case study from the Laptev Sea ........... 63 CHAPTER III: Propagation of Greenland freshwater in the western Fram Strait - Evidence from dissolved Nd isotopes and REEs ............................................................ 89 4. SUMMARY, CONCLUSIONS AND OUTLOOK .................................................... 100 4.1 Summary and Conclusions ...........................................................................................100 4.2 Outlook ......................................................................................................................... 103 DANKSAGUNG .........................................................................................................105 REFERENCES ........................................................................................................... 106 DATA TABLES ........................................................................................................... 127 - iii - ABSTRACT ABSTRACT The warming of the Arctic region in the recent past has proceeded at rates double that of the global average and has been accompanied by rapid sea ice retreat and increased heat and freshwater fluxes to the Arctic Mediterranean (i.e. the Arctic Ocean and the Nordic Seas, AM). Further warming will have strong impacts on ocean circulation, freshwater pathways, and marine ecosystems in the AM. Disentangling the sources, distribution and mixing of water masses involved in the transport and transfer of heat and freshwater is therefore critical for the understanding of present and future hydrological changes in the high-latitude and polar regions and their consequences. This study refines the knowledge of water mass circulation and mixing in the AM and provides new insights into the processes occurring on the Arctic shelves and in high-latitude estuaries. A multi-proxy approach is used combining dissolved radiogenic Nd isotopes (εNd), rare earth elements (REEs) and stable oxygen isotopes (δ18O) together with standard hydrographic tracers. The sources, distribution and mixing of water masses that circulate in the AM and pass the Fram Strait are assessed through evaluation of dissolved εNd and REE, and δ18O data obtained from samples recovered in 2012 along a full water depth section extending between Svalbard and Greenland at ~79 °N, and through a compilation and reassessment of literature Nd isotope and concentration data previously reported for other sites within the AM. The Nd isotope and REE distribution in the central Fram Strait and the open AM primarily reflects the lateral advection of water masses and their mixing, whereas seawater-particle interactions exert important control only above the shelf regions. For example, on the NE Greenland Shelf, remineralization of biogenic and/or release from detrital particles is recorded in bottom waters. Advection of warm Atlantic Water (AW) in the upper water column of the eastern and central Fram Strait is clearly reflected by an εNd signature of -11.7 and a Nd concentration ([Nd]) of 16 pmol/kg. Freshening and cooling of the AW on its way through the AM are accompanied by a continuous change towards more radiogenic Nd isotope compositions (e.g. -10.4 of dense Arctic Atlantic Water). This change results from mixing with intermediate waters but also mirrors the admixture of dense Kara Sea waters and Pacific-derived waters. Exchange with basaltic formations of Iceland and southeastern Greenland is suggested to impart the intermediate and deep waters of the AM with more radiogenic εNd signatures, which reach -9.5 in the Fram Strait. Significant inputs of Nd from Svalbard are not observed and surface waters and Nd on the western Svalbard Shelf originate in the Barents Sea. Shallow (< 200 m) waters of Arctic origin form the core of the East Greenland Current above the Greenland slope and have relatively radiogenic εNd (reaching -8.8) and elevated [Nd] (21-29 pmol/kg), which together with δ18O and standard hydrographic tracers are used to determine the proportions of Pacific-derived (< 30 % based on Nd isotopes) and Atlantic-derived seawater, as well as of river waters (< 8 %). A change in the Nd isotope compositions to less radiogenic values (-12.4) and an increase in [Nd] (38 pmol/kg) are observed at water depths above 100 m near the Greenland coast documenting addition of Greenland-sourced freshwater (GFW). The - 1 - ABSTRACT amount of GFW contained in the upper water column on the NE Greenland Shelf reached 6 % in 2012. Data obtained for the years 2014 and 2015 for the northern and southern NE Greenland Shelf suggest similar fractions of GFW for shallow waters in the Norske Trough and east of Ob Bank, indicating southward and northward propagation of GFW along the Greenland coast assuming that the NE Greenland Ice Stream is the freshwater source. The Nd isotope compositions of Arctic-derived waters (εNd ~ -9) and other water masses were essentially constant over the time period 2012-2015, which provides a solid basis for quantitative estimates of GFW admixture. The GFW distribution suggests that future increased GFW supply forced by global warming will lead to additional freshening of shallow Arctic waters, which, once these waters have traversed the Nordic Seas, may ultimately affect overturning strength in the northern Labrador Sea. Overall, the results obtained from the Fram Strait demonstrate that the pronounced gradients in εNd and REE contents in the upper water column provide a reliable basis for assessments of short-term shallow hydrological changes within the AM. New insights into the processes occurring in high latitude estuaries are provided by dissolved Nd isotope and
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