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.