Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

Dissolved and particulate trace elements collected on cruise RR1604 (GO-SHIP transect IO9N) in the Eastern Indian Ocean from March to April 2016.

Samples were collected from 5 L Teflon-coated Niskin-X bottles hung on non-metal line, and clean techniques were followed throughout (Bruland et al., 1979). Niskin-X bottles were transferred to a clean lab immediately after retrieval. At most stations a single sample was collected from 20 m. Four-depth profiles extending to 200 m were collected at 6 stations along the transect.

For dissolved metal analysis, 300 mL of seawater was filtered through acid-washed 47 mm 0.4 μm polycarbonate filters into 250 mL LDPE bottles which were acid-cleaned following GEOTRACES protocols (Cutter et al., 2010). A vacuum filtration apparatus was used, with all parts in contact with the sample composed of acid-washed PFA. The assembly was rinsed with 3 aliquots of sample prior to sample collection. Samples were acidified with Optima HCl to 0.024 M, double bagged, and stored for analysis on shore.

Dissolved samples were analyzed using an ESI seaFAST SP2 coupled to a Perkin Elmer Nexion 350D ICP-MS. Sample aliquots were transferred to 30 mL LDPE bottles. Approximately 8 mL of sample was taken up to rinse and fill a 6 mL preconcentration loop. Samples were buffered in-line with resin-cleaned 4 M ammonium acetate buffer-adjusted to pH = 6.0 ± 0.1 and passed through an ESI pre-concentration column with a 200 μL resin bed volume. The final concentration of the buffer in the samples was 0.88 M. Metals were eluted off the column with 200 μL of 1.6 M Optima-grade HNO₃. Eluted metals passed through a PFA nebulizer, quartz spray chamber, and nickel cones on their way to an ammonia-filled dynamic reaction cell (0.55 mL/min flow).

Quantification was achieved by standard additions automated by the seaFAST system. A standard addition solution of 6.9 nM Fe, 46 nM Mn, 4.5 nM Zn, and 1.1 nM Pb was made in 0.024 M HCl. Concentrations in this solution were quantified using an external curve. Additions were introduced as the sample filled the preconcentration loop. The volume introduced by standard additions was kept constant, with 0.024 M Optima HCl making up the difference for samples with no additions. Each sample was spiked with 2 additions averaging roughly 100% and 200% of the sample concentration.

Approximately 4L of water from Niskin bottles was filtered for particulate metals. The bottles were pressurized to approximately 5 psi with 0.2 μm filtered air and seawater passed through 25 mm 0.4 μm Supor filter membranes. The membranes were housed in Swinnex polypropylene filter holders and attached to the Niskin-X bottles with Teflon tubing and polycarbonate fittings. The membrane was transferred to an acid washed petri-slide, double bagged, and kept frozen at -20°C for analysis on shore.

Labile and total particulate concentrations were analyzed through sequential digestion of the Supor membranes following protocol of Rauschenberg and Twining (2015). Membranes were first leached using an acetic acid-hydroxylamine solution (Berger et al., 2008) then transferred to a PFA vial and digested with a 4M HCl/4M HNO3/4M HF mixture (Ohnemus et al., 2014). Reference materials and process blanks – filters that had 2 L of 0.2 μm filtered seawater passed through them –were sequentially digested alongside the samples.

Digests were analyzed using a Thermo Element2 HR-ICP-MS equipped with a quartz nebulizer, cyclonic spray chamber, and nickel cones. Cd was analyzed in low resolution while all other elements were analyzed in medium resolution. In-115 was used as a drift monitor, and quantification was performed using an external curve.

Dissolved metal samples were instrument blank corrected and quantified using standard additions. Multiple 'process blanks' (0.024 M HCl spiked with 0.1% seawater) were run daily, and daily detection limits were calculated based on 3x the standard deviation of the process blanks.

All particulate ICP-MS data were normalized to an internal standard (In-115) and quantified using external calibration curves. Concentrations of each element per filter were calculated and the contribution of the process blank (measured as the elements contained in an acid-washed filter through which 0.2-µm filtered water was passed during the cruise) was then subtracted. Separate process blanks were calculated for the labile and total digestions. Element concentrations (per liter of water filtered) were calculated by dividing the per filter concentration by the volume of water passed through each filter.

Data quality: 
Data are flagged with the following:
1 = Good data, passed QC;
3 = Questionable or suspect data, used when a data point was oceanographically inconsistent;
4 = Bad;
6 = Below detection limit.

BCO-DMO Processing: modified parameter names (removed units, replaced hyphens and spaces with underscores, added "_flag" to flag columns, replaced blank cells with "nd")

Description from NSF award abstract:
The Indian Ocean accounts for nearly a fifth of global ocean photosynthesis and is likely a key component in global ocean nutrient and carbon cycles. However, the Indian Ocean may be the least studied major marine body on the planet. Our limited understanding suggests extensive variations in physical and chemical environmental conditions, but how this variation influences biodiversity, nutrient stress, and more broadly regional differences in the functioning of phytoplankton is unknown. To help address these gaps, the investigators will conduct a study by joining an already-funded major research cruise to this region. It will cover a northern region with some of the highest temperatures recorded in open ocean waters, an area around 10°S of predicted (but not tested in situ) iron stress, and a southern subtropical gyre with unique nitrogen to phosphorous(or N:P) ratios. The focus of this project is to quantify and synthesize the interconnectedness of environmental conditions, phytoplankton diversity and genome content, and nutrient biogeochemistry, with the goal of understanding how these may lead to unique biogeochemical regions in Indian Ocean. The research will have broader impacts on many levels. First, it will increase public awareness of the role of phytoplankton on ocean functioning, climate, and people's lives through a new partnership with the Aquarium of the Pacific (AOP), which is the fourth most-attended aquarium in the nation. Secondly, the project will train a postdoctoral scholar as well as a graduate and undergraduate students. Third, the research will dramatically increase our basic knowledge ocean biogeochemistry and in many cases will be the first measurements of their kind made in the Indian Ocean.

This project will address two major questions: How do environmental conditions, phytoplankton diversity, phytoplankton physiology, and biogeochemistry vary across the central Indian Ocean? Are there distinct biogeochemical regimes in the central IO? The researchers hypothesize that environmental conditions, including the relative availability of nitrogen (N) and iron (Fe), lead to three distinct phytoplankton communities and biogeochemical regimes. They will employ a series of advanced analytical tools including high sensitivity measurements of dissolved and particulate nutrients (nitrogen, phosphorus, and iron), genomics, bioassays to test for nutrient stress, and cell-sorting of specific taxa followed by measures of nutrient content and uptake. A focus of this project is to quantify and synthesize the interconnectedness of environmental conditions, phytoplankton diversity and genome content, and nutrient biogeochemistry, and how these lead to unique biogeochemical regions in Indian Ocean. This extensive set of observations can ultimately be linked to ocean models and satellite data to provide a comprehensive view of regional differences in chemistry, biodiversity and phytoplankton biogeochemical functioning in the Indian Ocean.