Strong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Whitmore, L., Shiller, A., Horner, T., Xiang, Y., Auro, M., Bauch, D., Dehairs, F., Lam, P., Li, J., Maldonado, M., Mears, C., Newton, R., Pasqualini, A., Planquette, H., Rember, R., & Thomas, H. Strong margin influence on the Arctic Ocean Barium Cycle revealed by pan‐Arctic synthesis. Journal of Geophysical Research: Oceans, 127(4), (2022): e2021JC017417, https://doi.org/10.1029/2021jc017417.Early studies revealed relationships between barium (Ba), particulate organic carbon and silicate, suggesting applications for Ba as a paleoproductivity tracer and as a tracer of modern ocean circulation. But, what controls the distribution of barium (Ba) in the oceans? Here, we investigated the Arctic Ocean Ba cycle through a one-of-a-kind data set containing dissolved (dBa), particulate (pBa), and stable isotope Ba ratio (δ138Ba) data from four Arctic GEOTRACES expeditions conducted in 2015. We hypothesized that margins would be a substantial source of Ba to the Arctic Ocean water column. The dBa, pBa, and δ138Ba distributions all suggest significant modification of inflowing Pacific seawater over the shelves, and the dBa mass balance implies that ∼50% of the dBa inventory (upper 500 m of the Arctic water column) was supplied by nonconservative inputs. Calculated areal dBa fluxes are up to 10 μmol m−2 day−1 on the margin, which is comparable to fluxes described in other regions. Applying this approach to dBa data from the 1994 Arctic Ocean Survey yields similar results. The Canadian Arctic Archipelago did not appear to have a similar margin source; rather, the dBa distribution in this section is consistent with mixing of Arctic Ocean-derived waters and Baffin Bay-derived waters. Although we lack enough information to identify the specifics of the shelf sediment Ba source, we suspect that a sedimentary remineralization and terrigenous sources (e.g., submarine groundwater discharge or fluvial particles) are contributors.This research was supported by the National Science Foundation [OCE-1434312 (AMS), OCE-1436666 (RN), OCE-1535854 (PL), OCE-1736949, OCE-2023456 (TJH), and OCE-1829563 (R. Anderson for open access support)], Natural Sciences and Engineering Research Council of Canada (NSERC)-Climate Change and Atmospheric Research (CCAR) Program (MTM), and LEFE-CYBER EXPATE (HP). HT acknowledges support by the Canadian GEOTRACES via NSERC-CCAR and the German Academic Exchange Service (DAAD): MOPGA-GRI (Make Our Planet Great Again—Research Initiative) sponsored by BMBF (Federal German Ministry of Education and Research; Grant No. 57429828)

A call for refining the role of humic-like substances in the oceanic iron cycle

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins

Regulation of the phytoplankton heme b iron pool during the North Atlantic spring bloom

CITATION: Louropoulou, E., et al. 2019. Regulation of the phytoplankton heme b iron pool during the North Atlantic spring bloom. Frontiers in Microbiology, 10:1566, doi:10.3389/fmicb.2019.01566.The original publication is available at https://www.frontiersin.orgHeme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (0.7 μm) from the North Atlantic Ocean (GEOVIDE cruise – GEOTRACES section GA01), which spanned several biogeochemical regimes. We examined the relationship between heme b abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme b concentrations ranged from 0.16 to 5.1 pmol L⁻² (median = 2.0 pmol L⁻², n = 62) in the surface mixed layer (SML) along the cruise track, driven mainly by variability in biomass. However, in the Irminger Basin, the lowest heme b levels (SML: median = 0.53 pmol L⁻², n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L⁻², n = 25; chlorophyll a: median = 2.0 nmol L⁻², n = 23) pointing to regulatory mechanisms of the heme b pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L⁻², n = 11) in the Irminger Basin, but large diatoms (Rhizosolenia sp.) dominated. Hence, heme b depletion and regulation is likely to occur during bloom progression when phytoplankton class-dependent absolute iron requirements exceed the available ambient concentration of DFe. Furthermore, high heme b concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L⁻², n = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme b provides a snapshot of the cellular activity in situ and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.https://www.frontiersin.org/articles/10.3389/fmicb.2019.01566/fullPublisher's versio

SWINGS Cruise Report. MD229. N/O Marion-Dufresne. Jan 11th-March 8th 2021

SWINGS is a multidisciplinary 4-year project dedicated to elucidate trace element sources, transformations and sinks along a section crossing key areas of the Southern Ocean (SO). Major French contribution to the international GEOTRACES program (www.geotraces.org), SWINGS involves ca 80 scientists (21 international laboratories, 7 countries2). As core action of SWINGS, the SWINGS cruise (R/V Marion-Dufresne, MD229, Geotraces section GS02) started from La Reunion on January 11th 2021 and ended at La Reunion 57 days later (March 8th, 2021). This cruise explored a large part of the South Indian Ocean (Figure 0-2) in order to tackle the following objectives: 1) establish the relative importance of sedimentary, atmospheric and hydrothermal sources of TEIs in the Indian sector of the SO 2) investigate the drivers of the internal trace element cycles: biogenic uptake, remineralization, particle fate, and export, and 3) quantify TEI transport by the Antarctic Circumpolar Current and the numerous fronts at the confluence between Indian and Atlantic Oceans

Introduction to the special issue on twenty years of GEOTRACES: An international study of the marine biogeochemical cycles of trace elements and isotopes

This special issue of Oceanography celebrates the transformational findings of the international GEOTRACES program in chemical oceanography, 20 years after drafting of the GEOTRACES Science Plan in 2004 (GEOTRACES Planning Group, 2006). With the section cruise phase of the program ending soon, and a planned pivot toward smaller-​scale process studies, this is an opportune time to look back at the achievements of GEOTRACES during the last two decades and to highlight some of the advances in our understanding of the processes that determine the oceanic distributions of trace elements and isotopes (TEIs)

Intercalibration: A Cornerstone of the Success of the GEOTRACES Program

The international GEOTRACES program was developed to enhance knowledge about the distribution of trace elements and their isotopes (TEIs) in the ocean and to reduce the uncertainty about their sources, sinks, and internal cycling. Recognizing the importance of intercalibration from the outset, GEOTRACES implemented intercalibration efforts early in the program, and consensus materials were generated that included the full range of TEIs dissolved in seawater, in suspended particles, and from aerosols. The GEOTRACES section cruises include “crossover station(s)” that are occupied by two or more sections and whereby all aspects of sample collection, preservation, and processing can be compared and intercalibrated. Once datasets are generated, an international intercalibration committee reviews intercalibration reports and works with the community to address issues and provide intercalibrated data for intermediate data products. This process has resulted in a highly cooperative community that shares advances in protocols to strengthen capacity building and GEOTRACES outcomes, including an unprecedented oceanic atlas of TEIs, with data quality that is state-of-the-art. This article outlines the development and implementation of the successful GEOTRACES intercalibration process

Ironing out Fe residence time in the dynamic upper ocean

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Black, E. E., Kienast, S. S., Lemaitre, N., Lam, P. J., Anderson, R. F., Planquette, H., Planchon, F., & Buesseler, K. O. Ironing out Fe residence time in the dynamic upper ocean. Global Biogeochemical Cycles, 34(9), (2020): e2020GB006592, doi:10.1029/2020GB006592.Although iron availability has been shown to limit ocean productivity and influence marine carbon cycling, the rates of processes driving iron's removal and retention in the upper ocean are poorly constrained. Using 234Th‐ and sediment‐trap data, most of which were collected through international GEOTRACES efforts, we perform an unprecedented observation‐based assessment of iron export from and residence time in the upper ocean. The majority of these new residence time estimates for total iron in the surface ocean (0–250 m) fall between 10 and 100 days. The upper ocean residence time of dissolved iron, on the other hand, varies and cycles on sub‐annual to annual timescales. Collectively, these residence times are shorter than previously thought, and the rates and timescales presented here will contribute to ongoing efforts to integrate iron into global biogeochemical models predicting climate and carbon dioxide sequestration in the ocean in the 21st century and beyond.We would like to thank S. Albani for providing the dust model results (Community Atmosphere Model, C4fn) and the three anonymous reviewers for their constructive comments. The U.S. GEOTRACES work was supported by the National Science Foundation (OCE‐1232669 and OCE‐1518110) and E. Black was also funded by a NASA Earth and Space Science Graduate Fellowship (NNX13AP31H) and the Ocean Frontier Institute. The GEOVIDE work was funded by the Flanders Research Foundation (G071512N), the Vrije Universiteit Brussel (SRP‐2), the French ANR Blanc GEOVIDE (ANR‐13‐BS06‐0014), ANR RPDOC BITMAP (ANR‐12‐PDOC‐0025‐01), IFREMER, CNRS‐INSU (programme LEFE), INSU OPTIMISP, and Labex‐Mer (ANR‐10‐LABX‐19)

Impact of inorganic particles of sedimentary origin on global dissolved iron and phytoplankton distribution

Iron is known to be the limiting nutrient for the phytoplankton growth over ~40% of the global ocean and to impact the structure of marine ecosystems. Dissolved iron (DFe) is assumed to be the only form available to phytoplankton while particulate iron (PFe) has mostly been considered for its role in the biogenic iron remineralization and induced scavenging. Therefore, most studies focused on the nature of DFe external sources to the ocean (i.e. aeolian dust, riverine fluxes, hydrothermal sources and sediment) and their quantification, which still remain uncertain. Among these external sources, the sedimentary sources have been shown to be underestimated. Moreover, the iron supply from sediments has been documented to be often larger in the particle fraction. Here, we test the impacts of an iron sediment source of inorganic particulate iron (PFeInorg) on global DFe and phytoplankton distribution. We use experimentally acquired knowledge to test a parameterization of a PFeInorg pool in a global biogeochemical model and compare with published indirect estimation. Depending on the parameterization of its dissolution and sinking speed, the PFeInorg can noticeably enrich water masses in DFe during its transport from the sediment to the open ocean, notably in regions not usually accessible to external DFe inputs. Indeed, the fact that DFe is prone to scavenging, reduces the impact of equivalent Fe inputs from sediments in the dissolved form in those regions far from the sediment sources. PFeInorg thereby has the potential to fuel the phytoplankton growth in offshore regions impacting the coastal‐offshore chlorophyll gradient

Dissolved trace metals (Ni, Zn, Co, Cd, Pb, Al, and Mn) around the Crozet Islands, Southern Ocean

International audienceA phytoplankton bloom shown to be naturally iron (Fe) induced occurs north of the Crozet Islands (Southern Ocean) every year, providing an ideal opportunity to study dissolved trace metal distributions within an island system located in a high nutrient low chlorophyll (HNLC) region. We present water column profiles of dissolved nickel (Ni), zinc (Zn), cobalt (Co), cadmium (Cd), lead (Pb), aluminium (Al), and manganese (Mn) obtained as part of the NERC CROZEX program during austral summer (2004-2005). Two stations (M3 and M1) were sampled downstream (north) of Crozet in the bloom area and near the islands, along with a control station (M2) in the HNLC zone upstream (south) of the islands. The general range found was for Ni, 4.64-6.31 nM; Zn, 1.59-7.75 nM; Co, 24-49 pM; Cd, 135-673 pM; Pb, 6-22 pM; Al, 0.13-2.15 nM; and Mn, 0.07-0.64 nM. Vertical profiles indicate little island influence to the south with values in the range of other trace metal deprived regions of the Southern Ocean. Significant removal of Ni and Cd was observed in the bloom and Zn was moderately correlated with reactive silicate (Si) indicating diatom control over the internal cycling of this metal. Higher concentrations of Zn and Cd were observed near the islands. Pb, Al, and Mn distributions also suggest small but significant atmospheric dust supply particularly in the northern region