Tracing water masses and continental weathering by neodymium and hafnium isotopes in the Atlantic sector of the Southern Ocean

Radiogenic isotopes, such as the ones of hafnium (Hf) and neodymium (Nd), have been used to investigate present and past ocean circulation patterns, hydrothermal inputs and continental weathering regimes. This thesis presents a detailed study of the dissolved Hf and Nd isotope composition and concentration of seawater in the Atlantic sector of the Southern Ocean. The goal is to better understand their controlling factors, mainly weathering inputs, biogeochemical cycling, and water mass mixing. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during expedition ANTXXIV/3 with R/V Polarstern in 2008 in the frame of the International Polar Year (IPY) and the GEOTRACES program. Volumes of seawater between 60 L (deeper than 200 m) and 130 L (surface) were collected for Hf, and a volume of 20 L for Nd isotopes following GEOTRACES protocols. For isotopic analysis, the samples were chemically and ion chromatographically purified, for which existing methods had to be significantly modified, as described in detail in Chapter 2. Hafnium and Nd concentrations were determined on aliquots of the same samples by isotope dilution. The isotope compositions and concentrations were measured by Thermal Ionisation Mass Spectrometry (TIMS) or Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS). The distribution of Hf and Nd concentrations in the surface water (Chapter 3) is generally similar with essentially constant values south of the Polar Front (~0.3 pmol/kg and ~18 pmol/kg, respectively). Minimum Hf (~0.12 pmol/kg) and Nd (~7 pmol/kg) concentrations are observed between the Subtropical Front and the Polar Front, most probably due to the limited terrigenous flux in this area, but may also result from scavenging by biogenic opal. In contrast, at the northernmost station, 200 km southwest of Cape Town, a pronounced increase of the Nd concentration is observed, while the Hf concentration is at its minimum. This indicates a lower amount of Hf than of Nd released by weathering of the Archean cratonic rocks of South Africa. In the vicinity of landmasses the Hf and Nd isotope composition is clearly affected by terrigenous inputs, whereas most of the open surface waters are characterized by the same isotope composition as the deep waters (presented in Chapter 4). Only in the area of the Agulhas retroflection low Nd isotope values (ε Nd = -18.9) are Abstract observed implying that unradiogenic inputs originating from South Africa are supplied via the Agulhas Current. The Nd isotope compositions are more radiogenic (ε Nd ~ -8) towards the Subtropical Front and within the Antarctic Circumpolar Current and slightly decrease to ε Nd ~ -8.5 in the Weddell Gyre (WG). Near the volcanic King George Island (KGI) the isotopic data show significant increases to ε Hf = 6.1 and ε Nd = - 4.0. The Hf and Nd concentrations show peak values (0.38 pmol/kg and 22.6 pmol/kg, respectively), suggesting an enhanced release from mafic rocks of the Antarctic Peninsula. The overall Hf isotope composition varies within a small range (between ε Hf = 6.1 at KGI and ε Hf = 2.8 in the WG). Near the Antarctic continent the isotope composition drops from ε Hf ~ 5 to ε Hf ~ 3, indicating a release of unradiogenic Hf by glacial grinding and destruction of weathering resistant minerals such as zircons. The Hf isotope composition and concentration data show that it is a sensitive tracer for physical weathering conditions on the adjacent continental landmass. The range of Nd isotopes is a factor of five larger than of Hf isotopes, which confirms that Nd isotopes are a highly sensitive tracer for the provenance of weathering inputs to marine surface waters. In Chapter 4 the first combined deepwater profiles of dissolved Hf and Nd concentrations and isotope compositions from the Atlantic sector of the Southern Ocean are presented. Eight full-depth profiles were analyzed for both Hf and Nd, whereas four additional profiles were analyzed for Nd only. In the upper few hundred meters Hf concentrations are low (0.2 pmol/kg to 0.4 pmol/kg) and increase to relatively constant values (0.6 pmol/kg) in the deeper water column. North of the Polar Front, Nd concentrations increase linearly towards the bottom indicating uptake and release by biogenic opal, whereas in the WG the Nd concentrations are essentially constant (25 pmol/kg) at depths greater than ~ 1000 m. Hafnium shows homogenous isotope compositions (average value εHf = 4.6), whereas Nd isotopes mark distinct differences between water masses, such as modified North Atlantic Deep Water (εNd = -11 to εNd =-10) and Antarctic Bottom Water (εNd = -8.6 to εNd =-9.6). Waters locally advected via the Agulhas Current can also be identified by their unradiogenic Nd isotope compositions. Mixing calculations suggest that a small fraction of Nd is removed by particle scavenging during mixing of water masses north of the Polar Front. The calculation of the mixture of a North Pacific and a North Atlantic end-member shows that Nd isotope and concentration patterns in the Lower Circumpolar Deep Water can be explained by ~ 30:70 contributions of these end-members. In conclusion, Hf and Nd isotopes and their concentration patterns provide valuable insights into the weathering inputs and their provenance in the Atlantic sector of the Southern Ocean. The Hf isotopes largely reflect local imprints in the surface layer, whereas the deeper parts are isotopically invariant. The enhanced release of Hf from volcanic rocks implies that those rock types are an important source for radiogenic Hf into the ocean. Both elements readily adsorb onto siliceous frustules of diatoms, whereas scavenged Nd is released easier during reminerlization than Hf. Mixing calculation reveals that Nd is removed by 10 % to 20 % during mixing between northern and southern sourced waters. However the entire Nd budget in the Atlantic sector of the Southern Ocean is governed by Pacific (~ 30 %) and Atlantic (~ 70%) contributions, implying that local contributions from Antarctica are inferior. The Nd isotopes reflect the modification of CDW during AABW formation. This information can be used to evaluate paleo-AABW production and distribution

Marine Ice: A sleeping iron giant in the Southern Ocean?

The Polar Southern Ocean (PSO) provides an excess amount of macro-nutrients but productivity is largely limited by the availability of essential micro-nutrients, namely iron, manganese, zinc and others. Seasonal patches of increased productivity off major ice shelfs around Antarctica suggest that local sources of these deficient micro-nutrients must be present. With this session contribution we present a new study on marine ice from the Filchner-Ronne Ice Shelf (FRIS) as a potential source of iron and other limiting micro-nutrients for the Atlantic sector of the PSO. Marine ice is formed via partial melting of meteoric shelf ice near the grounding line of large ice shelves (e.g. FRIS). During this process small refrozen ice platelets accumulate in a layer of over 100 m thickness underneath the ice shelf to form marine ice containing high amounts of particulate material. In a project funded by the German Research Foundation (DFG) within the priority program SPP1158, we analyse 2 marine ice cores (B13: 62m, B15: 167m of marine ice) recovered in the 1990’s from the FRIS on their geochemical compositions. The coring location of B13 was about 40 km away from the shelf ice edge and B15 was drilled another 136 km further inland along the reconstructed flow line of B13. Due to shelf ice migration over the last 30 years, their locations have shifted about 30 km towards the shelf ice edge. First results show dissolved Fe (dFe) and Mn (dMn) concentrations ranging between 30 and 300 nMol and particulate Fe (pFe) of 20 to 120 µMol (0.2 to 1.4 µMol for pMn). These concentrations are orders of magnitude higher than the ones currently found in the PSO for those elements. Basal melting and ice-berg calving of marine ice with the accompanied release of these essential trace metals could therefore fuel local productivity in regions with large extent of shelf ice. With our study we aim to evaluate marine ice as potentially overlooked source for limiting micro-nutrients that could explain high productivity areas within an otherwise relatively low productive PSO

Neodymium in the oceans: a global database, a regional comparison, and implications for palaeoceanographic research

The neodymium (Nd) isotopic composition of seawater has been used extensively to reconstruct ocean circulation on a variety of time scales. However, dissolved neodymium concentrations and isotopes do not always behave conservatively, and quantitative deconvolution of this non-conservative component can be used to detect trace metal inputs and isotopic exchange at ocean–sediment interfaces. In order to facilitate such comparisons for historical datasets, we here provide an extended global database for Nd isotopes and concentrations in the context of hydrography and nutrients. Since 2010, combined datasets for a large range of trace elements and isotopes are collected on international GEOTRACES section cruises, alongside classical nutrient and hydrography measurements. Here, we take a first step towards exploiting these datasets by comparing high-resolution Nd sections for the western and eastern North Atlantic in the context of hydrography, nutrients and aluminium (Al) concentrations. Evaluating those data in tracer–tracer space reveals that North Atlantic seawater Nd isotopes and concentrations generally follow the patterns of advection, as do Al concentrations. Deviations from water mass mixing are observed locally, associated with the addition or removal of trace metals in benthic nepheloid layers, exchange with ocean margins (i.e. boundary exchange) and/or exchange with particulate phases (i.e. reversible scavenging). We emphasize that the complexity of some of the new datasets cautions against a quantitative interpretation of individual palaeo Nd isotope records, and indicates the importance of spatial reconstructions for a more balanced approach to deciphering past ocean changes

Particle-seawater interaction of neodymium in the North Atlantic

Dissolved neodymium (Nd) isotopes (expressed as εNd) have been widely used as a water mass tracer in paleoceanography. However, one aspect of the modern biogeochemical cycle of Nd that has been sparsely investigated is the interplay between dissolved and particulate phases in seawater. We here present the first regional data set on particulate Nd isotope compositions (εNdp) and concentrations ([Nd]p) from five stations in the western North Atlantic Ocean along the GEOTRACES GA02 transect, in conjunction with previously published dissolved Nd isotope compositions (εNdd) and concentrations ([Nd]d)1. Key observations and interpretations from our new particulate data set include the following: (1) A low fractional contributions of [Nd]p to the total Nd inventory per volume unit of seawater (~5%), with significant increases of up to 45% in benthic boundary layers. (2) Increasing Nd concentrations in suspended particulate matter ([Nd]SPM) and fractions of lithogenic material with water depth, suggesting the removal of Nd poor phases. (3) Different provenances of particulates in the subpolar and subtropical gyres as evidenced by their Nd isotope fingerprints reaching from εNdp ≈ -20 near the Labrador Basin (old continental crust), over εNdp ≈ -4 between Iceland and Greenland (young mafic provenance), to values of εNdp ≈-13 in the subtropics (similar to African dust signal). (4) Vertical heterogeneity of εNdp, as well as large deviations from ambient seawater values in the subpolar gyre, indicate advection of lithogenic particles in this area. (5) Vertically homogenous εNdp values in the subtropical gyre, indistinguishable from εNdd values, are indicative of predominance of vertical particulate supply. The process of reversible scavenging only seems to influence particulate signatures below 3 km. Overall, we do not find evidence on enhanced particle dissolution, often invoked to explain the observed increase in dissolved Nd in the North Atlantic

Online preconcentration ICP-MS analysis of rare earth elements in seawater

The rare earth elements (REEs) with their systematically varying properties are powerful tracers of continental inputs, particle scavenging intensity and the oxidation state of seawater. However, their generally low (∼pmol/kg) concentrations in seawater and fractionation potential during chemical treatment makes them difficult to measure. Here we report a technique using an automated preconcentration system, which efficiently separates seawater matrix elements and elutes the preconcentrated sample directly into the spray chamber of an ICP-MS instrument. The commercially available “seaFAST” system (Elemental Scientific Inc.) makes use of a resin with ethylenediaminetriacetic acid and iminodiacetic acid functional groups to preconcentrate REEs and other metals while anions and alkali and alkaline earth cations are washed out. Repeated measurements of seawater from 2000 m water depth in the Southern Ocean allows the external precision (2σ) of the technique to be estimated at <23% for all REEs and <15% for most. Comparison of Nd concentrations with isotope dilution measurements for 69 samples demonstrates that the two techniques generally agree within 15%. Accuracy was found to be good for all REEs by using a five point standard addition analysis of one sample and comparing measurements of mine water reference materials diluted with a NaCl matrix with recommended values in the literature. This makes the online preconcentration ICP-MS technique advantageous for the minimal sample preparation required and the relatively small sample volume consumed (7 mL) thus enabling large data sets for the REEs in seawater to be rapidly acquired

Spatio-temporal evolution of the West African monsoon during the last deglaciation

On the basis of a multi-proxy data set from the Gulf of Guinea (eastern equatorial Atlantic) we reconstruct the spatio-temporal evolution of the West African monsoon (WAM) and present evidence for a decoupling between latitudinal shifts of the rain belt and WAM intensification. The onset of deglacial monsoon invigoration at ∼16,600 years before present lagged northward migration of a weak rainfall zone by ∼2800 years. Conversely, during the Younger Dryas (YD) time interval, WAM precipitation was severely reduced but we find no evidence for a large-scale retreat of the rainfall front. This observation is not in agreement with the hypothesis of a large-scale shift of the intertropical convergence zone south of the tropical WAM region during the YD. Our results can be better reconciled with the newly emerging concept of a strong influence of Tropical Easterly and African Easterly Jets on modern WAM

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes

Rare Earth Elements input from proglacial sediments into Kongsfjorden, Svalbard

Rare earth elements (REE), such as the lanthanides, yttrium and scandium together with neodymium isotopes (εNd) are useful tracers in geochemistry to characterise sediment provenances. Furthermore, they can help to comprehend other elemental fluxes from land to sea. However, input mechanisms of REE into seawater are still being investigated to better understand REE cycling. Particularly, shelves in glacial areas are subject to environmental transformations due to modern climate change. Retreating glaciers expose relatively reactive sediments which are ideal to study aquatic geochemical processes governing REE distribution. We have collected filtered (0.45 µm) seawater from the Kongsfjord (Svalbard) and meltwater samples from the glaciers draining into the fjord for REE and εNd. Samples were analysed for REE on a ThermoFisher® Element2 after an offline pre-concentration using a seaFAST®. The REE were quantified with a known amount of thulium (Tm) as an internal standard prior to pre-concentration. With a neglectable fractionation amongst REE on the seaFAST column, this method allows an efficient and accurate quantification of such elements. Neodymium isotopes will be analysed at a later stage to better understand the provenance of the meltwater distributaries. The distributions of REE in the meltwater show clear enrichment of MREE and low HREE/LREE, in particular close to one land terminating glacier front on the Brogger-peninsula, typical for freshwater. Patterns from the fjord show a seawater distribution with high HREE/LREE and low Ce/Ce* in the deep outer fjord. Lower salinity surface waters are enriched in REE and show lower HREE/LREE than deep waters. Surface samples in the outer fjord show highest REE concentrations, suggesting that at this location REE inputs from the large tidewater glacier Kronebreen at the head of Kongsfjorden are subordinated to the inputs by smaller glaciers draining meltwater over proglacial sediments from the Brogger-peninsula