Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Sample Collection

Samples were collected on 21 Hawaii Ocean Time-series (HOT) cruises, onboard the R/V Kilo Moana throughout 2020-2023. All samples were collected at Station ALOHA (22.45°N, 158.0°W) following trace metal clean procedures.

For all cruises except for October and December 2021, trace metal clean particle profile samples were collected using a powder-coated aluminum 12-place 'trace metal' rosette (Seabird) with 8 L external spring sampling bottles (Ocean Test Equipment), deployed using metal-free line (Amsteel). For the October and December 2021 cruises, samples were collected with C-Free bottles (Ocean Test Equipment) that were attached directly to the metal-free line and triggered with Teflon-coated messengers. Prior to sampling, sampling bottles were subjected to a ca. 18 hr soak in seawater collected at a test station near Oahu.

Upon recovery, sampling bottles were taken to a positive-pressure clean van onboard. Particle samples were collected onto acid-cleaned 25 mm diameter 0.2 mm polyethersulfone (Supor) filters using acid-cleaned filter holders and polyethylene tubing. Bottles were pressurized for sampling at 10-15 psi using compressed air and filtrate (~4 L) was collected for gravimetric measurement on land. Filters were stored in acid-cleaned centrifuge tubes and transported back to the lab.

Sample Processing and Measurement

In the lab, filters underwent a two-stage digestion to isolate 'labile' and 'recalcitrant' metal fractions. The 'labile' digestion followed the procedure outlined by Berger et al. (2008). Filters were submerged in 2 mL of a 25% acetic acid, 0.02 M hydroxylamine hydrochloride solution, with 10 ppb Indium added as an internal standard and recovery tracer. The filter and solution were heated in a water bath at 95C for 10 min, and allowed to return to room temperature. After a total contact time of 2 hrs, the filters were removed and the leachate was transferred to acid-cleaned PFA digestion vials (Savillex). To improve recovery, the filter was then rinsed with a few mL of high purity (18.2 M-ohm cm-1) water, which was then added back to the leachate. 100 uL of concentrated nitric acid was added to the leachate, which was then dried overnight at 110C. The sample was redissolved in 0.1 M HNO₃ for ICP-MS analysis.

Following the 'labile' digestion, the filter was then subject to a total extraction following the procedures used by Planquette & Sherrell (2012). 2 mL of a digest solution containing 8 M HNO₃, 2.9 M HF, and 10 ppb Y (also applied as an internal standard and recovery tracer) was added to the filters in a 20 mL PFA vial (Savillex), which was heated for 4 hours at 110°C. The filters were then removed and the sample was evaporated to dryness. 100 ul of concentrated nitric acid was added and the sample was dried again before being redissolved in 0.1 M HNO₃.

Several blank filter and reagent blank analyses (containing no filter) were analyzed to estimate the metal contributions of the filters and reagents.

Digested samples were analyzed using a Thermo Scientific iCAP TQ ICP-MS at the University of Hawaiʻi at Mānoa. All elements were analyzed in triple-quadrupole mode with oxygen as the reactive gas (TQ-O2). A multi-element standard (Inorganic Ventures) was run multiple times throughout a run to monitor changes in instrument sensitivity over the course of a run. 

Accuracy of particulate metals from the digestion protocol was assessed by digestion and measurement of certified reference materials BCR-414, PACS-3. Average recoveries for these standards ranged from 85-92% for Al, 80% for Ti, 79-93% for Mn, 83-93% for Fe, 70-80% for Zn, 122-139% for Cd, 97-113% for Ni, 79-84% for Cu, and 89-98% for Co.

Prior to calibration, ion intensities derived from the ICP-MS were corrected for matrix suppression and/or sample loss by 115-In ('labile' digestions) and 89-Y ('recalcitrant' digestions). Detection of 115-In in the recalcitrant digestions was used to infer and quantify any carry-over of 'labile' metals into the 'recalcitrant' digest, which were subtracted from the recalcitrant total. Metal contents from digestions of blank filters were subtracted from derived concentrations, and subsequent quantity in moles was then re-scaled to a concentration using the volume of seawater filtered (obtained gravimetrically). 

Data were quality controlled by checking for oceanographic consistency across profiles. Samples with concentrations lower than the filter blank were corrected to 0. 

* Sheet 1 of submitted file "HOTpTM_QCflags_nM_v2.xlsx" was imported into the BCO-DMO data system for this dataset.  Table will appear as Data File: 962966_v1_hot-watercolumn-particulate-metals.csv (along with other download format options).

* Data within sheet 2 "Flag Key" appeared redundant with already entered metadata so was not added (see Problems/Issues section for quality flag info)

* ISO DateTime with timezone (UTC) column added in ISO 8601 format.

* After discussion with the data submitter, Cruise ID changed to HOT-### to match the project convention

NSF Award Abstract:
Phytoplankton are the base of marine food webs but their ability to grow in the open ocean by photosynthesis is limited by the scarcity of key nutrients especially iron. To understand how phytoplankton respond to global environmental changes, it is essential to predict how the nutrient content of seawater will change as well. Iron is essential to the light-harvesting machinery of phytoplankton but is an extremely small fraction of seawater (1 part per billion) . Iron is much more abundant in soils and when dust storms blow these soils out to sea, the iron content of seawater increases. It is unknown how long the effects of these iron supply events last, which depends on how well the marine ecosystem can recover and reuse iron before it sinks to the seafloor. It is also unknown if human activities have added to the natural Fe supply. The proposed research will address these questions by conducting a 3 year time-series of iron measurements in the North Pacific Ocean. Here, dust supply from Asia occurs mainly during spring, allowing the loss of iron over the summer and fall months to be documented. Unique chemical signatures will be used to distinguish iron supply from the deposition of desert dust or from human sources. This record of the marine iron cycle will be important for validating ecosystem models that are used to predict how climate change will influence the growth of phytoplankton in the future. The research would make a scientific contribution to the Hawaii Ocean Time-Series, help improve biogeochemical iron models, student training at the graduate and undergraduate level, and support an early career scientist.

A 3 year time-series of iron (Fe) measurements is proposed to constrain the magnitude of external Fe input and Fe recycling in the open ocean. Near-monthly observations will be conducted in the North Pacific Subtropical Gyre onboard Hawaii Ocean Timeseries cruises, which receives regular dust input during springtime and is minimally influenced by deep mixing. Water column profiling of dissolved and particulate Fe concentrations – combined with the flux of Fe recorded in trace-metal-clean sediment traps – will define a residence time of Fe in the upper water column. Iron uptake rates will be quantified through short-term incubations using a novel stable isotope technique and will be used to derive a turnover time with respect to biological uptake. Finally, the isotopic composition of dissolved and particulate Fe in the mixed layer will be measured to evaluate the potential importance of anthropogenic and Hawaiian Fe sources, which are poorly constrained. Together, these measurements will define the tempo and variability of the open ocean Fe cycle and provide a means to validate models that simulate the biogeochemistry of this key micronutrient.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

See more information about the Hawaii Ocean Time Series (HOT) on the related project page:  https://www.bco-dmo.org/project/2101 

Program description text taken from Chapter 1: Introduction from the Global Intercomparability in a Changing Ocean: An International Time-Series Methods Workshop report published following the workshop held November 28-30, 2012 at the Bermuda Institute of Ocean Sciences. The full report is available from the workshop Web site hosted by US OCB: http://www.whoi.edu/website/TS-workshop/home

Decades of research have demonstrated that the ocean varies across a range of time scales, with anthropogenic forcing contributing an added layer of complexity. In a growing effort to distinguish between natural and human-induced earth system variability, sustained ocean time-series measurements have taken on a renewed importance. Shipboard biogeochemical time-series represent one of the most valuable tools scientists have to characterize and quantify ocean carbon fluxes and biogeochemical processes and their links to changing climate (Karl, 2010; Chavez et al., 2011; Church et al., 2013). They provide the oceanographic community with the long, temporally resolved datasets needed to characterize ocean climate, biogeochemistry, and ecosystem change.

The temporal scale of shifts in marine ecosystem variations in response to climate change are on the order of several decades.  The long-term, consistent and comprehensive monitoring programs conducted by time-series sites are essential to understand large-scale atmosphere-ocean interactions that occur on interannual to decadal time scales.  Ocean time-series represent one of the most valuable tools scientists have to characterize and quantify ocean carbon fluxes and biogeochemical processes and their links to changing climate.

Launched in the late 1980s, the US JGOFS (Joint Global Ocean Flux Study; http://usjgofs.whoi.edu) research program initiated two time-series measurement programs at Hawaii and Bermuda (HOT and BATS, respectively) to measure key oceanographic measurements in oligotrophic waters. Begun in 1995 as part of the US JGOFS Synthesis and Modeling Project, the CARIACO Ocean Time-Series (formerly known as the CArbon Retention In A Colored Ocean) Program has studied the relationship between surface primary production, physical forcing variables like the wind, and the settling flux of particulate carbon in the Cariaco Basin.

The objective of these time-series effort is to provide well-sampled seasonal resolution of biogeochemical variability at a limited number of ocean observatories, provide support and background measurements for process-oriented research, as well as test and validate observations for biogeochemical models. Since their creation, the BATS, CARIACO and HOT time-series site data have been available for use by a large community of researchers.
 
Data from those three US funded, ship-based, time-series sites can be accessed at each site directly or by selecting the site name from the Projects section below.