Pier van der Merwe

Modern sampling and analytical methods for the determination of trace elements in marine particulate material using magnetic sector inductively coupled plasma-mass spectrometry

Trace elements often limit phytoplankton growth in the ocean, and the quantification of particulate forms is essential to fully understand their biogeochemical cycling. There is presently a lack of reliable measurements on the trace elemental content of marine particles, in part due to the inadequacies of the sampling and analytical methods employed. Here we report on the development of a series of state-of-the-art trace metal clean methods to collect and process oceanic particulate material in open-ocean and sea ice environments, including sampling, size-fractionated filtration, particle digestions and analysis by magnetic sector inductively coupled plasma-mass spectrometry (ICP-MS). Particular attention was paid to the analysis of certified reference materials (CRMs) and field blanks, which are typically the limiting factor for the accurate analysis of low concentrations of trace metals in marine particulate samples. Theoretical detection limits (3 s of the blank) were low for all 17 elements considered, and varied according to filter material and porosity (sub-microg L(-1) for polycarbonate filters and 1-2 microg L(-1) for quartz and polyester filters). Analytical accuracy was verified using fresh water CRMs, with excellent recoveries noted (93-103%). Digestion efficiencies for various acid combinations were assessed using sediment and plankton CRMs. Using nitric acid only, good recoveries (79-90%) were achieved for Mo, Cd, Ba, Pb, Mn, Fe, Co, Ni, Cu, Zn and Ga. The addition of HF was necessary for the quantitative recovery of the more refractory trace elements such as U, Al, V and Cr. Bioactive elements such as P can also be analysed and used as a biomass normaliser. Our developed sampling and analytical methods proved reliable when applied during two major field programs in both the open Southern Ocean and Antarctic sea ice environments during the International Polar Year in 2007. Trace elemental data are presented for particulate samples collected in both suspended and sinking marine material, and also within sea ice cores.

Advances in the offline trace metal extraction of Mn, Co, Ni, Cu, Cd, and Pb from open ocean seawater samples with determination by sector field ICP-MS analysis

Trace metals are fundamental components of various biochemical reactions for phytoplankton. They serve as micronutrients and therefore play a key role in marine biogeochemical cycles. International programs such as GEOTRACES require fast, sensitive and reliable methods for the simultaneous analysis of multiple trace elements in seawater. This paper reports the development of a simplified, automated, low cost, portable, off-line extraction method with high sample throughput. The extraction uses the chelating resin Nobias-chelate PA1 offering an extraction factor of 18 from 27 mL of seawater. This solid phase extraction has been coupled with Sector Field-Inductively Coupled Plasma-Mass Spectrometry (SF-ICP-MS) for analysing dissolved manganese (dMn), cobalt (dCo), nickel (dNi), copper (dCu), cadmium (dCd) and lead (dPb). An optimum pH of 6.2 was selected allowing quantitative recovery of most elements of interest, offering stable Cu and minimum molybdenum (Mo) recoveries, limiting interferences of Cd determination. Picomolar or subpicomolar trace metal blank concentrations and detection limits were obtained suitable for open ocean sample measurements. Regular analysis of reference seawater samples (SAFe, GEOTRACES and in-house seawater) showed excellent short-term and medium-term precision (1–8% RSD) and accuracy of the method. Twenty four samples, 3 blanks, 6 standard addition calibration samples, 3 replicates of in-house seawater and 2 reference seawater samples were extracted daily. The method has been successfully applied to the analysis of seawater samples from the Southern and Pacific Oceans.

Dissolved iron in the North Atlantic Ocean and Labrador Sea along the GEOVIDE section (GEOTRACES section GA01)

Dissolved Fe (DFe) samples from the GEOVIDE voyage (GEOTRACES GA01, May–June 2014) in the North Atlantic Ocean were analyzed using a seaFAST-picoTM coupled to an Element XR sector field inductively coupled plasma mass spectrometer (SF-ICP-MS) and provided interesting insights into the Fe sources in this area. Overall, DFe concentrations ranged from 0.09± 0.01 to 7.8± 0.5 nmol L−1. Elevated DFe concentrations were observed above the Iberian, Greenland, and Newfoundland margins likely due to riverine inputs from the Tagus River, meteoric water inputs, and sedimentary inputs. Deep winter convection occurring the previous winter provided iron-to-nitrate ratios sufficient to sustain phytoplankton growth and lead to relatively elevated DFe concentrations within subsurface waters of the Irminger Sea. Increasing DFe concentrations along the flow path of the Labrador Sea Water were attributed to sedimentary inputs from the Newfoundland Margin. Bottom waters from the Irminger Sea displayed high DFe concentrations likely due to the dissolution of Fe-rich particles in the Denmark Strait Overflow Water and the Polar Intermediate Water. Finally, the nepheloid layers located in the different basins and at the Iberian Margin were found to act as either a source or a sink of DFe depending on the nature of particles, Published by Copernicus Publications on behalf of the European Geosciences Union. 918 M. Tonnard et al.: Dissolved iron in the North Atlantic Ocean with organic particles likely releasing DFe and Mn particle scavenging DFe.

Insights Into the Biogeochemical Cycling of Iron, Nitrate, and Phosphate Across a 5,300 km South Pacific Zonal Section (153°E–150°W)

Iron, phosphate and nitrate are essential nutrients for phytoplankton growth and hence their supply into the surface ocean controls oceanic primary production. Here, we present a GEOTRACES zonal section (GP13; 30-33oS, 153oE-150oW) extending eastwards from Australia to the oligotrophic South Pacific Ocean gyre outlining the concentrations of these key nutrients. Surface dissolved iron concentrations are elevated at >0.4 nmol L-1 near continental Australia (west of 165°E) and decreased eastward to ≤0.2 nmol L-1 (170oW-150oW). The supply of dissolved iron into the upper ocean (<100m) from the atmosphere and vertical diffusivity averaged 11 ±10 nmol m-2 d-1. In the remote South Pacific Ocean (170oW-150oW) atmospherically sourced iron is a significant contributor to the surface dissolved iron pool with average supply contribution of 23 ± 17% (range 3% to 55%). Surface-water nitrate concentrations averaged 5 ±4 nmol L-1 between 170oW and 150oW whilst surface-water phosphate concentrations averaged 58 ±30 nmol L-1. The supply of nitrogen into the upper ocean is primarily from deeper waters (24-1647 μmol m-2 d-1) with atmospheric deposition and nitrogen fixation contributing <1% to the overall flux, in remote South Pacific waters. The deep water N:P ratio averaged 16 ±3 but declined to <1 above the deep chlorophyll maximum (DCM) indicating a high N:P assimilation ratio by phytoplankton leading to almost quantitative removal of nitrate. The supply stoichiometry for iron and nitrogen relative to phosphate at and above the DCM declines eastward leading to two biogeographical provinces: one with diazotroph production and the other without diazotroph production.

Sustained Upwelling of Subsurface Iron Supplies Seasonally Persistent Phytoplankton Blooms Around the Southern Kerguelen Plateau, Southern Ocean

Although the supply of iron generally limits phytoplankton productivity in the Southern Ocean, substantial seasonal blooms are observed over and downstream of the Kerguelen plateau in the Indian sector of the Southern Ocean. Surprisingly, of the oceanic blooms, those associated with the deeper southern plateau last much longer (~3 months) than the northern bloom (~1‐month downstream of northern plateau). In this study, iron supply mechanisms around the southern plateau were investigated, obtaining profiles of dissolved iron (<0.2 μm, dFe) to 2,000‐m deep at 25 stations during austral summer 2016. The dFe concentrations in surface waters (≤100‐m depth) ranged from below the detection limit (DL, median of 0.026 nmol/kg) to 0.34 nmol/kg near the Antarctic shelf, with almost half the data points below detection. These low and—with few exceptions—largely spatially invariant concentrations, presumably driven by seasonal drawdown of this essential micronutrient by phytoplankton, could not explain observed patterns in chlorophyll a. In contrast, dFe concentrations (0.05–1.27 nmol/kg) in subsurface waters (100–800 m) showed strong spatial variations that can explain bloom patterns around the southern Kerguelen plateau when considered in the context of frontal locations and associated frontal processes, including upwelling, that may increase the upward supply of dFe in the region. This sustained vertical dFe supply distinguishes the southern blooms from the bloom downstream of the northern Kerguelen plateau and explains their persistence through the season.