Table of Contents

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

Uptake Incubations:
Fe uptake incubations were performed on 12 Hawaii Ocean Time-series (HOT) cruises, onboard the R/V Kilo Moana throughout 2021-2023. All samples were collected at Station ALOHA (22°45' N, 158°0' W) following trace metal clean procedures. Incubations were performed and harvested following the method described in Hawco et al. (2022), with minor modifications. See the attached supplemental file for a listing of specific cruise IDs.

For all cruises except October 2021, trace metal clean seawater was collected using a powder-coated aluminum framed 12-place 'trace metal' rosette (Seabird) with 8-liter (L) external spring sampling bottles (Ocean Test Equipment), deployed using metal-free line (Amsteel). For the October 2021 cruise, 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 collection, sampling bottles were subjected to a ca. 18 hour soak in seawater collected at a test station near Oahu. Prior to each cruise, all materials were soaked in Citranox for one day, soaked in 10% HCl for 7 days, and rinsed multiple times with 18.2 M-Ohm Milli-Q water (Millipore).

To condition the Fe spike at low nanomolar (nM) concentrations in seawater, seawater was collected at 25 meters (m) to "pre-incubate" the isotope spike with natural iron binding ligands present in seawater. Upon recovery, sampling bottles were taken to a positive pressure clean van onboard. For each "pre-incubation", 100 milliliters (mL) of seawater was filtered through a 0.2 micrometer (um) Acropak filter into an acid-cleaned 150 mL PC bottle. The seawater was amended with 100 microliters (uL) of a ~1 micromolar (uM) Fe double spike (Fe_DS) containing 57Fe and 58Fe and left in the dark at room temperature for approximately 24 hours.

Seawater for the incubations was collected after sunset from the mixed layer (25 m). The incubations were set up in the clean van under red light to minimize light shock. The 100 mL of pre-equilibrated solution was added to a 2 L acid-cleaned polycarbonate bottle, pre-rinsed with filtered seawater, which was then filled with seawater. The incubations were performed in triplicate, alongside a control incubation that was filtered at 0.2 um. Incubation bottles were placed in an on-deck flow-through incubator, shaded to light levels equivalent to a depth of 25 m and temperatures matching the surface mixed layer. At 24 hours, the incubations were removed and harvested via vacuum filtration onto acid-cleaned 0.2 millimeter (mm) polyethersulfone filters (47 mm), with the filtrate collected in 1 L bottles.

Sample Purification and Measurement:
Filtrate from the incubations was acidified to pH 1.8 using Optima grade hydrochloric acid and left to equilibrate for a minimum of 2 months.

Laboratory processing of samples followed methods described in Hawco et al. (2022). Briefly, filters were digested overnight in 50% nitric acid (optima grade) in perfluoroalkoxyl vials (Savillex) at 95 degrees Celsius. Dissolved iron in acidified filtrates was extracted using Nobias PA-1 resin after adjusting sample pH to ~6, rinsed with water and eluted in 1M nitric acid. Both sample types were then evaporated to dryness, heated in a 1:1 HNO₃:HCl solution for 2 hours, dried down again, and then resuspended in 10 M HCl for column chemistry purification of Fe with AG-MP1 resin. Sample was loaded at 10 M HCl, rinsed at 5M HCl, and then Fe was eluted at 1 M HCl. Following resin extraction, samples were again dried down and resuspended in 0.1 M HNO₃ for MC-ICP-MS analysis.

Measurements were performed using a Thermo Scientific Neptune MC-ICP-MS at the University of Southern California, using the IRMM-014 standard to define the natural abundance ratio of Fe.

Data were processed following Hawco et al. (2022) with minor modifications. The mean particulate Fe_DS of all control samples (1 pM, n = 12) was subtracted from the pFe_DS for each sample.

Dissolved iron turnover times (in days) were calculated as:

Fe_DS turnover time = ((pFe_DS + dFe_DS)/pFe_DS) x incubation duration

based on the measured concentrations of Fe double spike in the particulate (pFe_DS) and dissolved (dFe_DS) phases.

The Fe uptake rate (pM/d) was calculated as:

Fe uptake rate = [dFe] / Fe_DS turnover time

where [dFe] reflects the dFe concentration at 25 m measured by isotope dilution (reported in BCO-DMO dataset "Water Column Dissolved and Total Dissolvable Metal Concentrations, Hawaii Ocean Timeseries 2020-2023" (DOI: 10.26008/1912/bco-dmo.962986.2)).

- Loaded original file "HOTFeuptake.xlsx" (sheet 1) into BCO-DMO processing system; used row 1 as headers.
- Renamed the following fields: "Time_start" to "DateTime_start_HST"; "Time_end" to "DateTime_end_HST".
- Converted "DateTime_start_HST" from HST to UTC in ISO 8601 format: parsed as python datetime with format %Y-%m-%dT%H:%M:%S; wrote to "Start_ISO_DateTime_UTC" formatted as %Y-%m-%dT%H:%M:%SZ.
- Converted "DateTime_end_HST" from HST to UTC in ISO 8601 format: parsed as python datetime with format %Y-%m-%dT%H:%M:%S; wrote to "End_ISO_DateTime_UTC" formatted as %Y-%m-%dT%H:%M:%SZ.
- Reordered columns.
- Set field types/formats.
- Saved the final file as "994200_v1_hot_fe_uptake.csv".

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.