Description: The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017. This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Description: The vertical distribution of 234Th was measured along the 10°E meridian between 44°S and 53°S in the Antarctic Circumpolar Current (ACC) during the austral summer of 2012. The overarching goal of this work was to estimate particulate organic carbon (POC) export across three fronts: the Sub-Antarctic Front (SAF), the Antarctic Polar Front (APF) and the Southern Polar Front (SPF). Steady state export fluxes of 234Th in the upper 100 m ranged from 1600 to 2600 dpm m−2 d−1, decreasing with increasing latitude. Using large particle (〉53 μm) C/234Th ratios, the 234Th-derived POC fluxes at 100 m ranged from 25 to 41 mmol C m−2 d−1. Observed C/234Th ratios decreased with increasing depth north of the APF while south of the APF, ratios remained similar or even increased with depth. These changes in C/234Th ratios are likely due to differences in the food web. Indeed, satellite images, together with macronutrients and dissolved iron concentrations suggest two different planktonic community structures north and south of the APF. Our results indicate that higher ratios of POC flux at 100 m to primary production occurred in nanophytoplankton dominated surface waters, where primary production rates were lower. Satellite images prior to the expedition suggest that the higher export efficiencies obtained in the northern half of the transect may be the result of the decoupling between production and export (Buesseler 1998). Transfer efficiencies to 400 m, i.e. the fraction of exported POC that reached 400 m, were found to be higher south of the APF, where diatoms were dominant and salps largely abundant. This suggests different remineralization pathways of sinking particles, influencing the transfer efficiency of exported POC to depth.

Description: Global fallout and continuous liquid releases by the two European Nuclear Reprocessing Plants of Sellafield (Great Britain) and La Hague (France) are the major contributors of artificial radionuclides to the Arctic and North Atlantic Oceans. Anthropogenic 236U and the 236U/238U ratio are becoming a new transient tracer in oceanography, which combined to 129I (129I/236U ratio) can be used as a novel oceanographic tool in the Arctic and North Atlantic Oceans. Main strengths of using 129I/236U and 236U/238U atomic ratios are: i) identify sources of artificial radionuclides in water masses (global fallout, reprocessing plants and/or rivers); and ii) evaluating water mass ages. Here we will present results from different GEOTRACES expeditions in the Arctic and North Atlantic Oceans during the years 2011 – 2015, including the two pan-arctic expeditions onboard German RV Polarstern and US RV Healy. Seawater samples and ice cores are analyzed for 129I and 236U, contributing to a better understanding of the general Arctic water circulation and their further transport to the North Atlantic Ocean.

Description: The strength of the Atlantic Meridional Overturning Circulation (AMOC) is a major factor in the global thermohaline circulation system. Because of this central role of AMOC in the global climate system, it is important to have reliable tools to reconstruct the strength of deep water ventilation in general and of AMOC in particular during major phases of past climate variability. The basin-wide Atlantic 231Pa/230Th signature in sediment archives has been identified as a powerful tracer for the intensity of the AMOC in the past. The information on ocean circulation arises from an overall higher export rate of 231Pa over 230Th within NADW from the entire Atlantic basin to the Southern Ocean due to the somewhat lower particle reactivity of 231Pa. A fundamental complication for reliable applicability of this tool arises from the fact that there is a severe lack of data of these isotopes in the critical areas of the modern AMOC. Only very few 230Th and 231Pa profiles in the Labrador Sea, in the NE Atlantic, and in the tropical and South Atlantic have been published [1, 2, 3, 4]. To remove this gap, 14 water column profiles were sampled under trace metal clean conditions during the expedition of RV Pelagia (GEOTRACES Cruise GA02) on a transect following the North Atlantic deep water (NADW). At all stations the deepest sample was collected within the bottom nepheloid layer, providing information on the latest stage of signal development in the water column. Here we will present the first results on 5 profiles of dissolved 230Th and 231Pa along NADW between 15 and 55°N including also the crossover station Bermuda Atlantic Time-Series (BATS). With this study we aim to provide missing information of the factors controlling signal generation in order to answer the questions: What is the 231Pa/230Th isotope composition of the main water masses of the AMOC and how do the 230Th and 231Pa activities in NADW evolve on its way south and east? Can they be explained by ventilation or are there other controlling factors to identify such as the composition of suspended particles? [1] Moran et al. (2002) Earth Planet. Sci. Lett. 203, 999-1014. [2] Moran et al. (1997) Earth Planet. Sci. Lett. 150, 151-160. [3] Moran et al. (1995) Geophys. Res. Lett. 22, 2589-2592. [4] Vogler et al. (1998) Earth Planet. Sci. Lett. 156, 61-74

Description: In this study the first comprehensive dataset of 236U in the North Atlantic Ocean is presented. 236U/238U ratios and 236U concentrations are determined in about 70 seawater samples collected from 9 depth profiles along a transect through the Northwest Atlantic Ocean during the first two legs of the GEOTRACES cruise GA02 in 2010. The cruise track was designed to follow the path of the North Atlantic Deep Water (NADW) from its formation region to the deep parts of the western Atlantic Ocean basins. Our results show that there is a strong North to South and surface to deep ocean gradient spanning a broad range of 236U/238U, from (44±15)x10-12 to (1477±91)x10-12 that correlates well with the distribution of water masses in this region. All measured ratios are above the theoretical pre-anthropogenic levels of 10-13 to 10-14, showing that the whole transect is dominated by anthropogenic 236U. Based on the calculated 236U inventories the 236U inputs of global fallout and direct releases from nuclear reprocessing plants could be constrained to a range from 1000 to 1400 kg and from 115 to 250 kg of 236U, from the two sources, respectively. Our results show that the 236U/238U atomic ratio can be used as a marker of North Atlantic water masses: with high ratios (〉 1000x10-12) characterizing northern water masses (LSW and DSOW); and lowest ratios (50-400 x10-12) corresponding to the increasing influence of AABW and AAIW.

Description: 210Pb and 210Po are naturally occurring radionuclides that are commonly used as a proxy for particle and carbon export. In this study, the distribution of the 210Po/210Pb pair was investigated in the water column of the Barents,Kara and Laptev Seas and the Nansen, Amundsen and Makarov Basins in order to understand the particle dynamics in the Arctic Ocean during the 2007 sea-ice minimum (August–September). Minimum activities of total 210Pb and 210Po were found in the upper and lower haloclines (approx. 60–130 m), which are partly attributed to particle scavenging over the shelves, boundary current transport and subsequent advection of the water with low 210Pb and 210Po activities into the central Arctic. Widespread and substantial (〉 50%) deficits of 210Po with respect to 210Pb were detected from surface waters to 200m on the shelves, but also in the basins. This was particularly important in the Makarov Basin where, despite very low chlorophyll-a levels, estimates of annual new primary production were three times higher than in the Eurasian Basin. In the Nansen, Amundsen and Makarov Basins, estimates of annual new primary production correlated with the deficits of 210Po in the upper 200m of the water column, suggesting that in situ production and subsequent export of biogenic material were the mechanisms that controlled the removal of 210Po in the central Arctic. Unlike 210Po, 234Th deficits measured during the same expedition were found to be very small and not significant below 25m in the basins (Cai et al., 2010), which indicates, given the shorter half-life of 234Th, that particle export fluxes in the central Arctic would have been higher before July–August in 2007 than later in the season.

Description: Natural and anthropogenic radionuclides are used to study a suite of environmental processes. Yet their applications in aquatic systems are hindered by a general lack of knowledge regarding the underlying concepts of radioactivity, the occurrence of radionuclides in ecosystems, and the equations used to describe their decay mechanisms in environmentally applicable ways. The goal of this lecture is to provide upper level undergraduate and graduate students with a basic understanding of how the naturally occurring uranium‐thorium radioactive decay series can be used to address a range of environmentally relevant questions in marine systems. The lecture begins with a brief introduction to uranium‐thorium series decay patterns and their distribution in the marine environment. The remaining lecture focuses on four case studies that cover a range of applications where uranium‐thorium series radionuclides are used and includes: scavenging, air‐sea gas exchange, tracing groundwater, and sedimentation/age dating. This lecture is the second of a four‐part lecture series on radionuclides in the marine environment.

Description: The GEOTRACES section (GN04, TransArc-II) on board the German R/V Polarstern covered a full transect in the Arctic Ocean from the Barents Sea to the Makarov Basin, crossing the Nansen and Amundsen basins. The distributions of 129I concentrations and 236U/238U atomic ratios obtained from more than 300 seawater samples are consistent with different water masses. Atlantic Waters flowing into the Arctic Ocean bring the signal of the two European nuclear reprocessing plants of Sellafield and La Hague, with the highest 129I concentrations (〉 1500 x107 at·kg-1) and 236U/238U ratios (〉 3000 x10-12) in the Norwegian shelf, where the Norwegian Coastal Current penetrates into the Barents Sea. Lowest 129I concentrations (〈 5 x107 at·kg-1) and 236U/238U ratios (〈 10 x10-12) were observed in the deep and bottom waters of the Makarov Basin, proving the long-term isolation of these waters. The combination of 129I/236U and 236U/238U atomic ratios can be used as a dual tracer to understand sources of artificial radionuclides to the Arctic Ocean [1]. This dataset confirms that global fallout and reprocessing plants are the main suppliers of 129I and 236U to the Arctic Ocean, while Siberian rivers would be minor contributors [2]. Different from previous assumptions, results show that the Barents Sea Branch Water (BSBW) has a higher 129I/236U atom ratio than expected from the mixed signal of Sellafield and La Hague. This high 129I/236U ratio suggests that the contribution of La Hague relative to Sellafield is larger than expected in this water mass. Together with the data from other GEOTRACES cruises, a synoptic distribution of 129I and 236U in the Arctic Ocean will be available soon, which will help understanding major water circulation patterns in the Arctic Ocean. [1] M. Christl, N. Casacuberta, C. Vockenhuber, E. C., P. Bailly du Bois, J. Herrmann, H.A. Synal, Reconstruction of the 236U input function for the Northeast Atlantic Ocean: Implications for 129I/236U and 236U/238U-based tracer ages, Journal of Geophysical Research: Oceans, (2015). [2] N. Casacuberta, P. Masque, G. Henderson, M.R. van der Loeff, D. Bauch, C. Vockenhuber, A. Daraoui, C. Walther, H.A. Synal, M. Christl, First 236U data from the Arctic Ocean and use of 236U/238U amd 129I/236U as a new dual tracer, Earth and Planetary Science Letters, 440 (2016) 127-134.