Recycling of dissolved iron in the North Pacific Subtropical Gyre

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Hawco, N. J., Yang, S.-C., Pinedo-Gonzalez, P., Black, E. E., Kenyon, J., Ferron, S., Bian, X., & John, S. G. Recycling of dissolved iron in the North Pacific Subtropical Gyre. Limnology and Oceanography, 67(11), (2022): 2448-2465, https://doi.org/10.1002/lno.12212.The importance of iron as a limiting nutrient in the open ocean is widely recognized, but there is substantial uncertainty about the rate that it cycles in the marine environment. Here, we combine measurements from the water column, sediment traps, and incubations to constrain Fe turnover during summer at Station ALOHA in the North Pacific Subtropical Gyre. Using low levels of 57Fe–58Fe double spike, measured with high precision by multi-collector inductively coupled plasma mass spectrometry, we find Fe uptake rates of 30–60 pM d−1 throughout the euphotic zone. Dissolved Fe turnover times are estimated at 10–15 d in the mixed layer and 1–3 d near the deep chlorophyll maximum. Aerosol Fe supply inferred from a thorium isotope mass balance indicates that the dissolved Fe residence time is approximately 6 months in the upper euphotic zone (0–75 m), relative to external sources, and 2 months in the lower euphotic zone (75–150 m). To reconcile these observations, the average Fe atom must be recycled over 25 times at Station ALOHA in both the upper and lower euphotic zones (an “Fe ratio” equal to 0.04 and 0.03, respectively), a level of conservation that has only been documented in Fe-limited regions thus far. At steady state, this scenario requires an aerosol Fe solubility of 4.5%, which is similar to dissolution experiments from Pacific Ocean aerosols. Our results suggest that the oligotrophic ocean is capable of recycling iron efficiently even when these ecosystems are not demonstrably iron-limited.This work was also supported by the Simons Foundation (602538 and 823167 to N.J.H., 329108 to S.G.J) and National Science Foundation grants 2022969 to N.J.H. and 1911990 to S.F

Dissolved REE concentrations from the US GEOTRACES Arctic cruise (GN01, HLY1502) from August to October 2015

Dataset: GN01 Dissolved REEs LDEODissolved REE concentrations from the US GEOTRACES Arctic cruise (GN01, HLY1502) from August to October 2015. These samples were analyzed at Lamont-Doherty Earth Observatory (LDEO). For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/835533NSF Division of Ocean Sciences (NSF OCE) OCE-1459513, NSF Division of Ocean Sciences (NSF OCE) OCE-1458936, NSF Division of Ocean Sciences (NSF OCE) OCE-145971

Isopycnal Transport and Scavenging of 230

The Southern Ocean hosts complex connections between ocean physics, chemistry, and biology. Changes in these connections are hypothesized to be responsible for significant alterations of ocean biogeochemistry and carbon storage both on glacial-interglacial timescales and in the future due to anthropogenic forcing. Isotopes of thorium (230Th and 232Th) and protactinium (231Pa) have been widely applied as tools to study paleoceanographic conditions in the Southern Ocean. However, understanding of the chemical behavior of these isotopes in the modern Southern Ocean has been limited by a paucity of high-resolution observations. In this study, we present measurements of dissolved 230Th, 231Pa, and 232Th on a meridional transect along 170°W from 67°S to 54°S in the Pacific sector of the Southern Ocean, with high vertical and meridional sampling resolution. We find Th/Pa fractionation factors below 1, highlighting the preferential removal of Pa relative to Th in a region with low lithogenic inputs where the particle flux is dominated by biogenic opal. We also find steep gradients in all three of these isotopes along neutral density surfaces from north to south, demonstrating the importance of isopycnal mixing in transporting these nuclides to the Southern Ocean. Our results suggest that 231Pa and 230Th in the Southern Ocean are highly sensitive tracers of physical transport that may find use in studies of Southern Ocean biogeochemical-physical connections in the past, present, and future

A new purification method for Ni and Cu stable isotopes in seawater provides evidence for widespread Ni isotope fractionation by phytoplankton in the North Pacific

Nickel and copper are cofactors in key phytoplankton enzymes and the stable isotope composition of Ni and Cu (δ60Ni and δ65Cu) in seawater have the potential to identify major processes that influence their biogeochemistry. However, accurate analysis of δ60Ni and δ65Cu is challenging because of the difficulties in separating these metals from interfering elements in the seawater matrix. Here we report a fast and simple method for purification of Ni and Cu from seawater samples that is able to completely remove interfering elements Mn, Ti, Cr, and Fe. This method was verified by analyzing four reference materials that contain significant levels of interfering elements (powdered plankton, natural soils, and two marine sediments). Using this technique, we generated a dataset of 49 seawater δ60Ni and δ65Cu measurements from the upper water column of the North Pacific Ocean, which show preferential uptake of light Ni isotopes by phytoplankton (αbio-sw = 0.9997 ± 1) but no net fractionation of Cu isotopes. This new method simplifies treatment of seawater samples for Ni and Cu isotope analysis, enabling high-throughput investigations of δ60Ni and δ65Cu throughout the global ocean

Diel Changes in Trace Metal Concentration and Distribution in Coastal Waters: Catalina Island As a Study Case

Understanding biogeochemical cycling of trace metals in the ocean requires information about variability in metal concentrations and distribution over short, e.g., diel, time scales. Such variability and the factors that influence it are poorly characterized. To address this shortcoming, we measured trace metal concentrations in the total dissolved, colloidal, and soluble fractions every 3–4 h for several consecutive days and nights in surface waters from a coastal station. Our results show that both the concentration and the size partitioning of some biologically essential (Fe, Cu, Co, and Cd) and anthropogenic (Pb) metals are subjected to diel variations that may be related to both inorganic and biological processes (e.g., photolysis of high-molecular-weight dissolved organic matter, photoinduced reduction/oxidation of metal­(hydrous)­oxides, uptake by growing phytoplankton, degradation of organic matter, lysis, and grazing). The largest fluctuations were observed in the soluble and colloidal pools. Soluble Fe varied during the day-night cycle by a factor of 40, and the contribution of colloidal Pb to the total dissolved fraction increased from 6 ± 3% during the day to as much as 70–80% during the night. Our results suggest that changes occurring over time scales of hours need to be considered when collecting and interpreting trace metal data from the surface ocean

Metal data collected during the Tara Pacific Expedition 2016-2018

The Tara Pacific expedition (2016-2018) sampled coral ecosystems around 32 islands in the Pacific Ocean, and sampled the surface of oceanic waters at 249 locations, resulting in the collection of nearly 58,000 samples. The expedition was designed to systematically study corals, fish, plankton, and seawater, and included the collection of samples for advanced biogeochemical, molecular, and imaging analysis. Here we provide the total dissolvable (i.e. acidified unfiltered whole seawater) Fe, Zn, Mn, Ni, Cd, Co, Cu, and Pb concentrations for 242 surface seawater samples. Trace metal analyses were performed with the goals of characterizing the surface seawater trace metal distribution across the open ocean and coastal regions in both the Atlantic and Pacific, and exploring metal-dependent ecosystem structure and metabolism. Some of the findings include high concentrations of iron (Fe) and manganese (Mn) in several regions, such as the North Atlantic Ocean and near the South Pacific islands, possibly due to Saharan dust and hydrothermal vent input, respectively. Elevated lead (Pb) was found in the North Pacific near southeast Asia, where anthropogenic sources may contribute. We also observe interbasin differences in concentrations for most of the metals, such as cobalt (Co), which is relatively high in the North Atlantic in comparison to the Pacific, perhaps due to dust deposition or continental weathering. There are also intrabasin differences in metal concentrations between oligotrophic and upwelling regions, exemplified by the higher cadmium (Cd) concentrations near the Peruvian coast, likely due to upwelling. Overall we captured high-resolution trace metal data that depicts the nuances in the metal distribution of the global ocean

Latitudinal constraints on the abundance and activity of the cyanobacterium UCYN‐A and other marine diazotrophs in the North Pacific

The number of marine environments known to harbor dinitrogen (N2)-fixing (diazotrophic) microorganisms is increasing, prompting a reassessment of the biogeography of marine diazotrophs and N2 fixation rates (NFRs). Here, we investigate the diversity, abundance, and activity of diazotrophic microorganisms in the North Pacific Subtropical Gyre (NPSG), a diazotrophic habitat, and the North Pacific Transition Zone (NPTZ), a region characterized by strong physical, chemical, and biological gradients. Samples were collected on two springtime meridional cruises during 2016 and 2017, spanning from 23.5°N to 41.4°N along 158°W. We observed an abrupt decrease in diazotrophic abundances near the southern edge of the NPTZ, which coincided with a salinity front and with a ∼10-fold increase in Synechococcus abundance, but without a concomitant change in phosphate or nitrate concentrations. In NPSG waters south of this diazotrophic boundary, nifH genes and NFRs were consistently detected and diazotrophic communities were dominated by UCYN-A, an uncultivated, symbiotic cyanobacterium (2.8 × 103 to 1.0 × 106 nifH gene copies L−1). There was a significant positive relationship between quantitative polymerase chain reaction-derived UCYN-A nifH gene abundances and community NFRs in the NPSG, suggesting a large contribution of UCYN-A to community NFRs. In the NPTZ waters to the north, NFRs were low or undetected and nifH genes were rare, with the few detected sequences represented by UCYN-A and noncyanobacterial diazotrophs. The patterns we observed in UCYN-A abundance in the context of local biogeochemistry suggest that the environmental controls of this organism may differ from those of cultivated marine cyanobacterial diazotrophs

Latitudinal constraints on the abundance and activity of the cyanobacterium UCYN‐A and other marine diazotrophs in the North Pacific

The number of marine environments known to harbor dinitrogen (N2)-fixing (diazotrophic) microorganisms is increasing, prompting a reassessment of the biogeography of marine diazotrophs and N2 fixation rates (NFRs). Here, we investigate the diversity, abundance, and activity of diazotrophic microorganisms in the North Pacific Subtropical Gyre (NPSG), a diazotrophic habitat, and the North Pacific Transition Zone (NPTZ), a region characterized by strong physical, chemical, and biological gradients. Samples were collected on two springtime meridional cruises during 2016 and 2017, spanning from 23.5°N to 41.4°N along 158°W. We observed an abrupt decrease in diazotrophic abundances near the southern edge of the NPTZ, which coincided with a salinity front and with a ∼10-fold increase in Synechococcus abundance, but without a concomitant change in phosphate or nitrate concentrations. In NPSG waters south of this diazotrophic boundary, nifH genes and NFRs were consistently detected and diazotrophic communities were dominated by UCYN-A, an uncultivated, symbiotic cyanobacterium (2.8 × 103 to 1.0 × 106 nifH gene copies L−1). There was a significant positive relationship between quantitative polymerase chain reaction-derived UCYN-A nifH gene abundances and community NFRs in the NPSG, suggesting a large contribution of UCYN-A to community NFRs. In the NPTZ waters to the north, NFRs were low or undetected and nifH genes were rare, with the few detected sequences represented by UCYN-A and noncyanobacterial diazotrophs. The patterns we observed in UCYN-A abundance in the context of local biogeochemistry suggest that the environmental controls of this organism may differ from those of cultivated marine cyanobacterial diazotrophs