388
Chemical Oceanography, 4th Edition
The concentrations of nitrate, phosphate, and silicate were 10.8 ± 0.4 μM, 0.92 ± 0.02 μM,
and 3.9 ± 0.1 μM, respectively, and the chlorophyll concentrations were 0.24 ± 0.02 μg L –1 ,
typical of the equatorial HNLC region. The low chlorophyll conditions were substantiated by overflights of the NASA airborne optical laboratory prior to the experiment. The
primary production (15 to 25 μg C L –1 day –1 ) for surface waters is typical for this region.
An area of approximately 64 km 2 (8 × 8 km) was enriched by adding 455 kg (7800 moles)
of iron as FeSO 4 (0.5 M) in acidic seawater (pH = 2) to the prop wash of the ship. Along with
the iron, sulfur hexafluoride, SF 6 , was added as an inert tracer. The SF 6 solution (Ledwell
et al., 1993) was prepared by dissolving 0.35 moles of SF 6 in seawater in a 2500-L steel tank.
The SF 6 was pumped (1.4 min –1 ) together with the iron solution (12 L min –1 ) into the prop
wash as the ship steamed at about 9 km h –1 . The goal was to increase the iron concentration
to 4 nM (a level, in bottle experiments, that was sufficient to cause large increases in chlorophyll and complete depletion of the available major nutrients within 5 to 7 days). This
enriched patch was tracked for 10 days and monitored for changes in biological, chemical,
and physical parameters. The iron hypothesis required that we detect increased rates of
growth in the patch relative to water outside the patch. The increased growth in the patch
should reduce major nutrient concentrations and decrease the fugacity of carbon dioxide.
Three tracking strategies were employed to maintain contact with the iron- enriched area.
The iron deployment and initial sampling were performed about a central Lagrangian reference point located by a buoy equipped with a global positioning system (GPS) receiver
interfaced to a VHF (very- high- frequency) packet radio transmitter and receiver. The
GPS buoy was interrogated every 5 min to determine the location of the ship relative to
this central point. In addition, four World Ocean Circulation Experiment (WOCE) drifter
buoys equipped with ARGO floats buoy positions could be received. All of these buoys
had a holey sock drogue set at 10 m to reduce slippage relative to the enriched patch.
The inert chemical tracer SF 6 also added to the patch was detected continuously by electron capture gas chromatography (GC) at detection limits of less than 10 –16 M. A flowthrough pumping system with the intake set at 3 m on the ship’s bow was used to sample
surface seawater. SF 6 measurements were used to determine the position of the enriched
iron patch relative to the GPS buoy. The concentration of iron in the patch was also determined every 8 min by both colorimetric and chemiluminescent analyses based on flow
injection techniques. Surface seawater was supplied for iron analyses through the Teflon
pumping system with the intake on the ship’s bow. Samples were acidified to pH = 3 before
analyses, resulting in the detection of dissolved iron, freshly precipitated colloidal iron,
and much of the aged colloidal iron (total dissolvable iron).
The patch was sampled for 3 days in a star- shaped pattern using the surface pumping
systems while the ship steamed at approximately 9 km h –1 . Each day, one major hydrographic station was occupied within the patch and one outside the patch at a time near local
noon. Samples were collected at these stations for primary productivity, species composition, chlorophyll, pigments, nutrients, POC (particulate organic carbon), PON (particulate
organic nitrogen), DOC (dissolved organic carbon), DMS, dimethylsulfonium propionate
(DMSP), dissolved and particulate trace metals, halocarbons, and CO 2 system parameters.
The concentrations of the SF 6 tracer, together with the ship’s position relative to the GPS
buoy, were used to distinguish inside from outside stations. Overflights of the NASA P-3
Orion optical laboratory were performed on four additional occasions during the experiment to assess the large- scale effects of iron on surface water chlorophyll. The aircraft flew
legs of approximately 180 km in length in a star pattern centered on the GPS buoy.
The core of the patch was subducted beneath a low- salinity front that passed through
the area from east to west late on the fourth day after the iron addition, to a depth of 30 to
Chemical Oceanography, 4th Edition
The concentrations of nitrate, phosphate, and silicate were 10.8 ± 0.4 μM, 0.92 ± 0.02 μM,
and 3.9 ± 0.1 μM, respectively, and the chlorophyll concentrations were 0.24 ± 0.02 μg L –1 ,
typical of the equatorial HNLC region. The low chlorophyll conditions were substantiated by overflights of the NASA airborne optical laboratory prior to the experiment. The
primary production (15 to 25 μg C L –1 day –1 ) for surface waters is typical for this region.
An area of approximately 64 km 2 (8 × 8 km) was enriched by adding 455 kg (7800 moles)
of iron as FeSO 4 (0.5 M) in acidic seawater (pH = 2) to the prop wash of the ship. Along with
the iron, sulfur hexafluoride, SF 6 , was added as an inert tracer. The SF 6 solution (Ledwell
et al., 1993) was prepared by dissolving 0.35 moles of SF 6 in seawater in a 2500-L steel tank.
The SF 6 was pumped (1.4 min –1 ) together with the iron solution (12 L min –1 ) into the prop
wash as the ship steamed at about 9 km h –1 . The goal was to increase the iron concentration
to 4 nM (a level, in bottle experiments, that was sufficient to cause large increases in chlorophyll and complete depletion of the available major nutrients within 5 to 7 days). This
enriched patch was tracked for 10 days and monitored for changes in biological, chemical,
and physical parameters. The iron hypothesis required that we detect increased rates of
growth in the patch relative to water outside the patch. The increased growth in the patch
should reduce major nutrient concentrations and decrease the fugacity of carbon dioxide.
Three tracking strategies were employed to maintain contact with the iron- enriched area.
The iron deployment and initial sampling were performed about a central Lagrangian reference point located by a buoy equipped with a global positioning system (GPS) receiver
interfaced to a VHF (very- high- frequency) packet radio transmitter and receiver. The
GPS buoy was interrogated every 5 min to determine the location of the ship relative to
this central point. In addition, four World Ocean Circulation Experiment (WOCE) drifter
buoys equipped with ARGO floats buoy positions could be received. All of these buoys
had a holey sock drogue set at 10 m to reduce slippage relative to the enriched patch.
The inert chemical tracer SF 6 also added to the patch was detected continuously by electron capture gas chromatography (GC) at detection limits of less than 10 –16 M. A flowthrough pumping system with the intake set at 3 m on the ship’s bow was used to sample
surface seawater. SF 6 measurements were used to determine the position of the enriched
iron patch relative to the GPS buoy. The concentration of iron in the patch was also determined every 8 min by both colorimetric and chemiluminescent analyses based on flow
injection techniques. Surface seawater was supplied for iron analyses through the Teflon
pumping system with the intake on the ship’s bow. Samples were acidified to pH = 3 before
analyses, resulting in the detection of dissolved iron, freshly precipitated colloidal iron,
and much of the aged colloidal iron (total dissolvable iron).
The patch was sampled for 3 days in a star- shaped pattern using the surface pumping
systems while the ship steamed at approximately 9 km h –1 . Each day, one major hydrographic station was occupied within the patch and one outside the patch at a time near local
noon. Samples were collected at these stations for primary productivity, species composition, chlorophyll, pigments, nutrients, POC (particulate organic carbon), PON (particulate
organic nitrogen), DOC (dissolved organic carbon), DMS, dimethylsulfonium propionate
(DMSP), dissolved and particulate trace metals, halocarbons, and CO 2 system parameters.
The concentrations of the SF 6 tracer, together with the ship’s position relative to the GPS
buoy, were used to distinguish inside from outside stations. Overflights of the NASA P-3
Orion optical laboratory were performed on four additional occasions during the experiment to assess the large- scale effects of iron on surface water chlorophyll. The aircraft flew
legs of approximately 180 km in length in a star pattern centered on the GPS buoy.
The core of the patch was subducted beneath a low- salinity front that passed through
the area from east to west late on the fourth day after the iron addition, to a depth of 30 to