ALBERT

Description: Radium isotopes are produced by decay of an isotope of thorium, a highly particle reactive element. Radium is relatively soluble in seawater such that once a thorium isotope in suspended or bottom sediments decays, a fraction of the produced radium isotope may be released to solution. 224Ra (3.7-day half-life) and 223Ra (11.4-day half-life) are used extensively as tracers for interaction on the shelf and for submarine groundwater discharge (SGD). 228Ra is strongly enriched in shelf waters and in the Transpolar Drift (TPD) and its half-life (5.8 years) is well suited to study the fate of this shelf-derived water in the central Arctic. This dataset collected in the framework of GEOTRACES during PS94 in 2015 shows that the input of 228Ra in the TPD has increased since previous studies in 2007 and 2011. The distribution of 228Ra-daughter 228Th (1.9 y half-life) and the 228Th/228Ra ratio are measured to trace particle fluxes. The accumulation of 226Ra (1620 y half-life) in the deep Makarov Basin can be used as a measure of basin ventilation time.

Description: Radium isotopes are produced by decay of an isotope of thorium, a highly particle reactive element. Radium is relatively soluble in seawater such that once a thorium isotope in suspended or bottom sediments decays, a fraction of the produced radium isotope may be released to solution. 224Ra (3.7-day half-life) and 223Ra (11.4-day half-life) are used extensively as tracers for interaction on the shelf and for submarine groundwater discharge (SGD). 228Ra is strongly enriched in shelf waters and in the Transpolar Drift (TPD) and its half-life (5.8 years) is well suited to study the fate of this shelf-derived water in the central Arctic. This dataset collected in the framework of GEOTRACES during PS94 in 2015 shows that the input of 228Ra in the TPD has increased since previous studies in 2007 and 2011. The distribution of 228Ra-daughter 228Th (1.9 y half-life) and the 228Th/228Ra ratio are measured to trace particle fluxes. The accumulation of 226Ra (1620 y half-life) in the deep Makarov Basin can be used as a measure of basin ventilation time.

Description: Radium isotopes are produced by decay of an isotope of thorium, a highly particle reactive element. Radium is relatively soluble in seawater such that once a thorium isotope in suspended or bottom sediments decays, a fraction of the produced radium isotope may be released to solution. 228Ra is strongly enriched in shelf waters and in the Transpolar Drift (TPD) and its half-life (5.8 years) is well suited to study the fate of this shelf-derived water in the central Arctic. The 228Th/228Ra ratio in the subsurface water column can be used to trace particle fluxes. In previous studies up to PS94 (2015) we had identified the strong and increasing 228Ra signal in the Transpolar Drift. During PS100 we studied the distribution of 228Ra in Fram Strait to investigate how this 228Ra signal in the TPD is changed by dilution and decay on its way to the East Greenland Current. Water from in situ pumps was filtered and passed over MnO2-coated cartridges to absorb the Ra and Th. On board, short-lived Radium isotopes and 228Th were analysed using RaDeCC counting. In the lab, the cartridges were leached in refluxing HCl and long-lived Radium isotopes were measured by gamma spectroscopy. At depths where in situ pumps were deployed with two cartridges in series, the Ra adsorption efficiency was determined from the ratio of the Ra activities in the two cartridges. At depths where only one cartridge was mounted, we used the average collection efficiency of all other cartridge pairs, corrected after a mass spectrometric analysis of 226Ra activities in discrete samples (Vieira et al., 2021, doi:10.1002/lom3.10428) (82 ± 8%).

Description: Radium isotopes are produced by decay of an isotope of thorium, a highly particle reactive element. Radium is relatively soluble in seawater such that once a thorium isotope in suspended or bottom sediments decays, a fraction of the produced radium isotope may be released to solution. 228Ra is strongly enriched in shelf waters and in the Transpolar Drift (TPD) and its half-life (5.8 years) is well suited to study the fate of this shelf-derived water in the central Arctic. The 228Th/228Ra ratio in the subsurface water column can be used to trace particle fluxes. In previous studies up to PS94 (2015) we had identified the strong and increasing 228Ra signal in the Transpolar Drift. During PS100 we studied the distribution of 228Ra in Fram Strait to investigate how this 228Ra signal in the TPD is changed by dilution and decay on its way to the East Greenland Current. Water from the ship's seawater intake was filtered and passed over MnO2-coated cartridges to absorb the Ra and Th. On board, short-lived Radium isotopes and 228Th were analysed using RaDeCC counting. In the lab, the cartridges were leached in refluxing HCl and long-lived Radium isotopes were measured by gamma spectroscopy.

Description: To better understand the inputs of riverine REEs and Hf to the ocean and their impact on the open ocean we collected filtered seawater samples from 16 full water column profiles along the Congo River plume at the coast of Congo and Gabon and along an offshore section at 3°S, complemented by three freshwater samples collected directly from the Congo River. The water samples from the Congo River plume were collected during GEOTRACES cruise GA08 with German RV Meteor (M121) in November and December 2015 with a stainless steel CTD rosette and a towed fish for surface waters. River water end-member samples from the Congo River at zero salinity were taken upstream by boat in May, July and October 2017 near the center of the river at ~6°S and ~12.5°E (stations 12.1–12.3). We determine for the first time REE and Hf concentrations and isotope compositions within the Congo River plume and quantify Nd and Hf fluxes. Based on these data we evaluate how far the Congo River plume is traceable by these geochemical parameters, how it contributes to their budgets in the South East Atlantic Ocean, and to which degree the Nd and Hf isotope and REE distributions can be used as water mass tracers in areas of high continental inputs.

Description: To better understand the association between barium (Ba) and silicon (Si) at ocean margins, we collected filtered seawater samples along the Congo River plume at the coast of Congo and Gabon and along an off-shelf section at 3°S, complemented by three freshwater samples collected directly from the Congo River. The water samples from the Congo margin were collected during GEOTRACES cruise GA08 with German RV Meteor (M121) in November-December 2015 with a stainless steel CTD rosette and a towed fish for surface waters. Congo River freshwater samples were taken upstream by boat in May, July and October 2017 near the center of the river at ~6°S and ~12.5°E (stations 12.1-12.3). We determine for the first time Ba and Si stable isotopes at the Congo River-dominated Southeast Atlantic margin. Based on these data we evaluate the contribution of Congo River plume to Ba and Si budgets in the South East Atlantic Ocean, and the association between Ba and Si at the Congo margin.

Description: Meteor Cruise M121 was dedicated to the investigation of the distribution of dissolved and particulate trace metals and their isotopic compositions (TEIs) in the full water column of the Angola Basin and the northernmost Cape Basin. A key aim was to determine the driving factors for the observed distributions, which includes the main external inputs, as well as internal cycling and ocean circulation. The research program of the cruise is official part of the international GEOTRACES program (www.geotraces.org) and cruise M121 corresponds to GEOTRACES cruise GA11. Subject of the cruise was the trace metal clean and contamination-free sampling of waters and particulates for subsequent analyses of the TEIs in the home laboratories of the national and international participants. Besides a standard rosette for the less contaminant prone metals, trace metal clean sampling was realized by using for the first time a new dedicated, coated trace metal clean rosette equipped with Teflon-coated GO-FLO bottles operated via a plastic coated cable from a mobile winch of GEOMAR Kiel. The particulate samples were collected under trace metal clean conditions using established in-situ pump systems operated from Meteor’s Aramid line. The cruise track led from Walvis Bay northwards along the West African margin until 3°S, then turned west until the Zero Meridian, which was followed southwards until 30°S. Then the cruise track turned east again until the Namibian margin was reached and then completed the near shore track northwards until Walvis Bay. The track crossed areas of major external inputs including dust from the Namib Desert and exchange with the west African continental margin and with the oxygen depleted shelf sediments of the Benguela upwelling, as well as with the plume of the Congo outflow, that was followed from its mouth northwards. Our investigations of internal cycling included the extremely high productivity associated with the Benguela Upwelling and the elevated productivity of the Congo plume contrasting with the extremely oligotrophic waters of the southeastern Atlantic Gyre. The links between TEI biogeochemistry and the nitrogen cycle forms an important aspect of our study. The major water masses contributing the Atlantic Meridional Overturning Circulation were sampled in order to investigate if particular TEI signatures are suitable as water mass tracers, in particular near the ocean margin and in the restricted deep Angola Basin. A total of 51 full water column stations were sampled for the different dissolved TEIs, which were in most cases accompanied by sampling for particulates and radium isotopes using the in-situ pumps. In addition, surface waters were continuously sampled under trace metal clean conditions using a towed fish and aerosol and rain samples were continuously collected.

Description: Highlights • Frequent sediment resuspension may have buffered D-Fe released from shelf sediments. • 228Ra was used to estimate trace element fluxes from the Chukchi shelf sediments. • The estimated sediment 228Ra flux ranks among the highest reported globally. • About 10–25% of the Chukchi shelf sediment Fe flux is exported to the Arctic Ocean. The Chukchi Sea is a primary site for shelf-ocean exchange in the Arctic region and modifies Pacific-sourced water masses as they transit via the Bering Strait into the Arctic Ocean. The aim of this study was to use radium and trace metal distributions to improve our understanding of biogeochemical cycles in the Bering and Chukchi Seas, and evaluate their potential response to future changes in the Arctic. We investigated the distributions of dissolved and total dissolvable trace metals (Cd, Fe, Ni, Cu, Zn, Mn, Co, and Pb) in the Bering and Chukchi Seas during spring. In addition, the long-lived radium isotopes (226Ra and 228Ra) were measured as tracers of benthic trace metal inputs. Trace metal concentrations, especially Fe and Mn, were highly elevated in Chukchi shelf waters compared with the open Arctic Ocean and Bering Strait. Trace metal, nutrient, and Ra patterns suggested that Fe, Mn, and Co concentrations were predominantly controlled by reductive benthic inputs, whereas the other trace metals were influenced by biological uptake and release processes. We propose that Fe, Mn, and Co in the Chukchi Sea are supplied from shelf sediments during winter overturning, and we combine the 228Ra fluxes with the distributions of Fe, Mn, and Co to provide a first estimate of their benthic fluxes in the region. The average benthic flux of 228Ra was 1.49 × 108 atoms m−2 d−1, which is among the highest rates reported globally. Estimated dissolved Fe (D-Fe) flux from the sediments was 2.5 μmol m−2 d−1, whereas D-Mn and D-Co fluxes were 8.0 μmol m−2 d−1 and 0.2 μmol m−2 d−1, respectively. The off-shelf transport of D-Fe to the Arctic Ocean is estimated to be about 10–25% of the benthic Fe flux, with the remainder retained on the shelf due to scavenging and/or phytoplankton uptake. Our results highlight the importance of the Chukchi Sea as a major source of the micro-nutrients to the Arctic Ocean, thereby supporting primary production. Long-term changes in factors that affect cross-shelf mixing, such as the observed reduction in ice cover, may therefore enhance shelf nutrient inputs and primary productivity in the Arctic.

Description: Trace elements (TEs) play crucial roles in regulating ocean processes including marine biogeochemical cycles, and are therefore vital to support marine life. Understanding the biogeochemical cycling of TEs requires knowledge of their sources/ sinks and transport in the oceans. Radium isotopes are suitable tools to study inputs of elements from the continental margins, as they are produced by the decay of thorium isotopes in sediments, and are soluble in seawater. Therefore, in this Ph.D. thesis, I used Ra isotopes to determine boundary TE fluxes from two diverse environments that constrain the major TE sources, including shelf sediments in an Arctic shelf region and in an eastern boundary system off the western African coast, as well as rivers such as the River Congo. First, the distributions of the dissolved and total dissolvable TEs (Cd, Fe, Ni, Cu, Zn, Pb, Mn, and Co) were investigated in the Chukchi Sea and 228Ra isotope was applied as tracer of benthic TE inputs. The results show that elevated benthic TE inputs on the Chukchi shelf provided suitable conditions for phytoplankton blooms. The 228Ra activities in spring appear to be 2-fold higher than in summer, which was a surprising observation and requires further investigation. Next, 228Ra was used to investigate the influence of the Congo River on surface ocean TE concentrations in the South Atlantic Ocean. The results show that the Congo River plume constitutes a large and unexpected input of TEs (Fe, Mn, and Co) into the South Atlantic. Finally, Ra isotopes were used to investigate a number of key ocean boundary processes in the Southeast Atlantic Ocean. Upwelling in the Benguela region is visible in the Ra distributions, and elevated Ra isotopes, Fe (II) and Si concentrations were observed in the Benguela oxygen minimum zone, possibly due to inputs from the reducing shelf sediments, or submarine groundwater discharge along the Namibian shelf.