Phoebe J. Lam

The continental margin is a key source of iron to the HNLC North Pacific Ocean

Here we show that labile particulate iron and manganese concentrations in the upper 500m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll (HNLC) region, have prominent subsurface maxima between 100-200 m, reaching 3 nM and 600 pM, respectively. The subsurface concentration maxima in particulate Fe are characterized by a more reduced oxidation state, suggesting a source from primary volcagenic minerals such as from the Kuril/Kamchatka margin. The systematics of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable source of Fe from the upper continental slope. This subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing, and is likely a key source of bioavailable Fe to the HNLC North Pacific.

Authigenic cadmium enrichments in suboxic sediments: Precipitation and postdepositional mobility

The postdepositional mobility of Cd and U has been investigated in North Atlantic turbidites in order to validate their utility as proxies of paleo-organic carbon fluxes. In this region pelagic sediments are interspersed with relatively organic-rich turbidites. Following instantaneous emplacement of turbidite units an oxidation front progressively migrated downwards into the suboxic turbidite, thereby leading to redistribution of authigenic phases according to their affinity to different redox conditions. Authigenic Cd concentrations show a large peak at the redox boundary and are significantly lower in the oxidized portion than in the deeper, unoxidized part of the turbidite. This distribution indicates that Cd has been continuously remobilized by the oxidation front thereby suggesting high sensitivity of the Cd species to the redox state and an efficient immobilization mechanism for Cd under reducing conditions. We propose the precipitation of CdS as a plausible mechanism for removal of Cd in reducing sediments, rather than adsorption onto mineral surfaces. This proposal is consistent with porewater data indicating diffusion of Cd toward, and uptake by, suboxic sediments. Solubility calculations suggest that precipitation of CdS may occur in suboxic sediments in the presence of trace levels of H2S (below routine detection limits). There is no evidence for release of Cd or U back into seawater upon oxidation of reducing sediments. The strong coupling between precipitation of authigenic Cd and U and the early diagenetic degradation of organic carbon offers the potential of reconstructing paleo-productivity using records of Cd and U. However, postdepositional redistribution of authigenic Cd and U may significantly alter records of authigenic Cd and U thereby rendering paleo-productivity reconstructions uncertain. We propose that the(Cd/U)authigenic ratio may provide a qualitative indicator to assess the relative role of steady-state incorporation relative to transient ‘burn-down’ enrichments.

Factors regulating the Great Calcite Belt in the Southern Ocean and its biogeochemical significance

The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the subtropical, Agulhas, and Subantarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO2 (pCO2) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO2 source. There was no calcium carbonate (CaCO3) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234Th-based flux estimates, and remotely sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.

Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.

How well can we quantify dust deposition to the ocean

Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2–5, suggesting that the model may overestimate dust deposition in our study area. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Insights into Particle Cycling from Thorium and Particle Data

Marine particles are a main vector by which the biological carbon pump in the ocean transfers carbon from the atmosphere to the deep ocean. Marine particles exist in a continuous spectrum of sizes, but they can be functionally grouped into a small, suspended class (which constitutes most of the total particle mass) and a large, sinking class (which contributes most of the particle flux). These two classes are connected by aggregation and disaggregation processes. The interplay of processes that create, aggregate, and destroy marine particles determines the strength and transfer efficiency of the biological pump. Measurements of radiocarbon, barium, and organic biomarkers on suspended and sinking particles have provided qualitative insights into particle dynamics, and measurements of thorium isotopes have provided quantitative estimates of rates. Here, we review what has been learned so far about particle dynamics in the ocean from chemical measurements on suspended and sinking particles. We then discuss future directions for this approach.

Coastal ocean and shelf-sea biogeochemical cycling of trace elements and isotopes: lessons learned from GEOTRACES

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Visualising Fe speciation diversity in ocean particulate samples by micro X-ray absorption near-edge spectroscopy

Environmental context. Iron-bearing particles in the ocean have attracted interest due to the role of iron as an essentialnutrientformicroscopicalgae,whichformthebaseofthemarinefoodchain.Moderntechniquesmake it possible to analyse individual particles of iron to determine their composition, but the resulting flood of data can be overwhelming. We show a method of simplifying the data to answer such questions as what groups of minerals are present and whether they are different between ocean basins. Abstract. It is a well known truism that natural materials are inhomogeneous, so analysing them on a point-by-point basis can generate a large volume of data, from which it becomes challenging to extract understanding. In this paper, we show an example in which particles taken from the ocean in two different regions (the Western Subarctic Pacific and the Australian sector of the Southern Ocean, south of Tasmania) are studied by Fe K-edge micro X-ray absorption near- edgespectroscopy(mXANES).Theresultingsetofdataconsistsof209spectrafromtheWesternSubarcticPacificand126 from the Southern Ocean. We show the use of principal components analysis with an interactive projection visualisation tool to reduce the complexity of the data to something manageable. The Western Subarctic Pacific particles were grouped into four main populations, each of which was characterised by spectra consistent with mixtures of 1-3 minerals: (1) Fe 3þ oxyhydroxides þFe 3þ clays þFe 2þ phyllosilicates, (2) Fe 3þ clays, (3) mixed-valence phyllosilicates and (4) magnetite þFe 3þ clays þFe 2þ silicates, listed in order of abundance. The Southern Ocean particles break into three clusters: (1) Fe 3þ -bearing clays þFe 3þ oxyhydroxides, (2) Fe 2þ silicates þFe 3þ oxyhydroxides and (3) Fe 3þ oxides þ Fe 3þ -bearing clays þFe 2þ silicates, in abundance order. Although there was some overlap between the two regions, this analysis shows that the particulate Fe mineral assemblage is distinct between the Western Subarctic Pacific and the Southern Ocean, with potential implications for the bioavailability of particulate Fe in these two iron-limited regions. We then discuss possible advances in the methods, including automatic methods for characterising the structure of the data.

Arctic deep-water ferromanganese-oxide deposits reflect the unique characteristics of the Arctic Ocean

Little is known about marine mineral deposits in the Arctic Ocean, an ocean dominated by continental shelf and basins semi-closed to deep-water circulation. Here, we present data for ferromanganese crusts and nodules collected from the Amerasia Arctic Ocean in 2008, 2009, and 2012 (HLY0805, HLY0905, HLY1202). We determined mineral and chemical compositions of the crusts and nodules and the onset of their formation. Water column samples from the GEOTRACES program were analyzed for dissolved and particulate scandium concentrations, an element uniquely enriched in these deposits. The Arctic crusts and nodules are characterized by unique mineral and chemical compositions with atypically high growth rates, detrital contents, Fe/Mn ratios, and low Si/Al ratios, compared to deposits found elsewhere. High detritus reflects erosion of submarine outcrops and North America and Siberia cratons, transport by rivers and glaciers to the sea, and distribution by sea ice, brines, and currents. Uniquely high Fe/Mn ratios are attributed to expansive continental shelves, where diagenetic cycling releases Fe to bottom waters, and density flows transport shelf bottom water to the open Arctic Ocean. Low Mn contents reflect the lack of a mid-water oxygen minimum zone that would act as a reservoir for dissolved Mn. The potential host phases and sources for elements with uniquely high contents are discussed with an emphasis on scandium. Scandium sorption onto Fe oxyhydroxides and Sc-rich detritus account for atypically high scandium contents. The opening of Fram Strait in the Miocene and ventilation of the deep basins initiated Fe-Mn crust growth ∼15 Myr ago.

Global Spatial and Temporal Variation of Cd:P in Euphotic Zone Particulates

Author(s): Bourne, HL; Bishop, JKB; Lam, PJ; Ohnemus, DC | Abstract: ©2018. American Geophysical Union. All Rights Reserved. Concentrations of Cd and P were determined in particle samples collected using the multiple unit large volume in situ filtration system (MULVFS) from 50 profiles at 34 different locations throughout the Atlantic, Pacific, and Southern Oceans since 1991. Consistent methodology has been used. This data set of Cd:P in size fractionated particles gives insight into the processes that lead to differences in regional Cd:P particle values as well as how the formation and remineralization of these particles lead to dissolved deep water ratios that increase from the North Atlantic to the North Pacific. With large spatial and temporal variation, this data set allows us to study the effects of an El Nino, upwelling, large-scale in situ Fe fertilization, low-oxygen conditions, and seasonal variation on the Cd:P in particles. Overall, Cd:P tends to be higher (~1–2 mmol/mol) in particles gathered in biologically dynamic waters and is much lower (typically ~0.1 mmol/mol) in oligotrophic regions. Using multiple linear regression analysis, we investigate how euphotic zone parameters important to photosynthesis including nitrate, phosphate, silicate, temperature, and euphotic zone depth affect the Cd:P ratio in particles. Using the results of the analysis, we create global seasonal maps of predicted particulate Cd:P distributions. We find that three factors—local dissolved nitrate, silicate concentrations, and euphotic zone depth—can predict 59% of the variation in particulate Cd:P. We verified our projections using in situ filtration samples collected during GEOTRACES expeditions GA03 (North Atlantic) and GP16 (South Pacific).