M.I. Liddicoat

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers

Measurements of air-sea gas exchange rates are reported from two deliberate tracer experiments in the southern North Sea during February 1992 and 1993. A conservative tracer, spores of the bacterium Bacillus globigii var. Niger, was used for the first time in an in situ air-sea gas exchange experiment. This nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period. The first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported. This allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium/sulphur hexafluoride ratio to be assessed. Data from earlier dual tracer experiments are reinterpreted, and findings suggest that results from all dual tracer experiments are mutually consistent. The complete data set is used to test published parameterizations of gas transfer with wind speed. A gas ex- change relationship that shows a dependence on wind speed intermediate between those ofLiss and Merlivat [1986] and Wanninkhof [1992] is found to be optimal. The dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb-derived radiocarbon. The degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates.

Vertical eddy diffusion and nutrient supply to the surface mixed layer of the Antarctic Circumpolar Current

[1] Dispersion of the tracer sulphur hexafluoride (SF6) during the Southern Ocean Iron Enrichment Experiment (SOIREE) provided an estimate of vertical exchange at the base of the surface mixed layer (60 m) at 61� S 140� E. Budget analysis confirmed that the SF6 patch was well constrained by surface mapping, with the decline in total SF6 showing good agreement with that predicted from wind speed parameterizations. Two approaches were used to calculate the mean effective vertical diffusivity Kz from the diapycnal transfer of SF6, with complementary error function and second-moment fits to the SF6 depth profiles indicating that Kz was less than 0.3 � 10 � 4 m 2 s � 1 . This result was examined using a three-dimensional diffusion model that incorporated lateral dispersion and air-sea exchange losses, which confirmed that vertical shear and subpycnocline dispersion did not influence the Kz estimate. Current shear at the base of the mixed layer was generated by wind-driven inertial oscillation, with a decrease in wind speed and increasing stratification in the latter half of the experiment reducing diapycnal transfer of

Automated vacuum analysis of sulphur hexafluoride in seawater: derivation of the atmospheric trend (1970–1993) and potential as a transient tracer

Abstract A fully-automated analysis system for the tracer gas sulphur hexafluoride (SF 6 ) is described. The system could be readily adapted to analyse seawater samples of varying volume and a concentration range exceeding 7 orders of magnitude. Automation facilitated a high precision despite continual analysis for periods of 30 days, and the performance of the instrument over a two year period of intensive use is discussed. Background SF 6 profiles from different oceans are compared and the surface concentrations utilised to obtain atmospheric concentrations. These are incorporated with other published data to derive a history for atmospheric SF 6 concentration. The data suggest an initial exponential rise in atmospheric SF 6 to 1979, followed by a linear increase to late 1993 at a rate of 5.5% p.a. With the sensitive analytical system and documented atmospheric history described in this paper, SF 6 has potential application as a transient tracer of recently ventilated waters, as demonstrated by comparison with the analytical parameters of the chlorofluorocarbons.

A Lagrangian SF6 tracer study of an anticyclonic eddy in the North Atlantic: patch evolution vertical mixing and nutrient supply to the mixed layer

Biological and biogeochemical change in the surface mixed layer of an anticyclonic eddy at 60°N in the North Atlantic were monitored within a Lagrangian time-series study using the tracer sulphur hexafluoride (SF6). Four ARGOS buoys initially released at the patch centre remained closely associated with the SF6 patch over a 10-day period, with the near-circular eddy streamlines contributing to the stability and coherence of the patch. Progressive deepening of the surface mixed layer was temporarily interrupted by a storm, which increased mixed-layer nitrate and accelerated the transfer of SF6 to the atmosphere. Diapycnal exchange of SF6 was relatively rapid due to the shallow pycnocline gradient, and a vertical eddy diffusivity (Kz) of 1.95 cm2 s?1 at the base of the mixed layer was estimated from vertical SF6 profiles at the patch centre. Application of Kz to the nutrient gradients inferred vertical nitrate and phosphate fluxes of 1.8 and 1.25 mmol m?2 d?1, respectively, for the pre-storm period, which accounted for 33% and 20% of the reported in vivo uptake rates. Integration of the vertical nitrate flux and decline in surface layer nitrate suggest a total loss of 0.54 mmol N m?3 d?1 during the 5-day pre-storm period, of which in vivo nitrate consumption only accounted for 49%. Vertical transport of ammonium regenerated in the pycnocline accounted for up to 25% of in vivo phytoplankton uptake. The results suggest that the contribution of vertical turbulence to the mixed-layer nutrient pool was less important than that recorded in other regions of the open ocean, inferring that advective processes are more significant in an eddy. This study also emphasises the potential of SF6 for oceanic Lagrangian time series studies, particularly in dynamic regions, and in constraining estimates of new production.

The flow of Antarctic bottom water to the southwest Indian Ocean estimated using CFCs

Observations of chlorofluorocarbons (CFCs), carbon tetrachloride, temperature, and salinity from five sections following the outflow path of Antarctic Bottom Water (AABW) into the southwest Indian Ocean are reported. The transient tracer data clearly show the plume of recently ventilated water whose hydrographic properties are progressively altered by mixing with the overlying waters. We use the CFC measurements to estimate the mean speeds (or transit times) and mixing rates (or dilutions) of the abyssal flow at each section using simple kinematic circulation models. Given our assumptions, the CFC ventilation age equals the transit time. The results suggest a transit time of 23±5 years (outflow speed of 1.2±0.3 cm s−1) to the Crozet-Kerguelen Gap with a dilution of 8–15 from the surface waters of the Weddell Sea. The estimated horizontal diffusivity is 30–70 m2 s−1, and the vertical diffusivity is 3–7×10−4 m2 s−1 Combined with the estimate of R. R. Dickson (unpublished data, 1998) for the AABW transport at this point, we conclude that a volume flux of 0.8–1.6 Sv (106 m3 s−1) is leaving the continental shelves of the Weddell Sea to eventually enter the abyssal Indian Ocean past Crozet Island.

Horizontal dispersion within an anticyclonic mesoscale eddy

Abstract Ship-mounted ADCP and buoy data are used to fit an azimuthal velocity profile to a N.E. Atlantic mesoscale eddy chosen as a site for a Lagrangian biogeochemical survey, the cruise forming part of the UK Plankton Reactivity in the Marine Environment (PRIME) initiative. Together with the buoy-derived locus of the eddy centre, the ADCP-derived velocity field allows observations of a sulphur hexafluoride (SF6) tracer release within the eddy to be put into a non-translating, non-rotating frame. Analysis of the transformed data suggests that the patch of tracer, though spreading, remained coherent throughout the nine-day survey. A linear regression on the square of the radial patch width versus time gives an estimate for the effective horizontal diffusion coefficient of 22±10 m 2 s −1 . This is consistent with previous estimates of diffusion rates at the O(10 km) lengthscale of the patch. Theory predicts a corresponding along-streamline spreading time of ∼14 days. This implies that the tracer patch mixed little with the surrounding waters during the first five days of the survey, suggesting that biogeochemical processes were little affected by lateral mixing during this period. The theory is inapplicable at later times because the area was struck by a storm at the start of the sixth day, which resulted in the halving of SF6 concentrations. Using a model of a circular eddy with the calculated velocity profile, the dispersion of the initial patch of tracer is simulated for different diffusivities, the results confirming the estimate of along-streamline mixing time and demonstrating the enhancement of azimuthal diffusion by the radial shear of the flow.

PHYSICAL EXCHANGES AT THE AIR-SEA INTERFACE UK-SOLAS Field Measurements

As part of the U.K. contribution to the international Surface Ocean–Lower Atmosphere Study, a series of three related projects—DOGEE, SEASAW, and HiWASE—undertook experimental studies of the processes controlling the physical exchange of gases and sea spray aerosol at the sea surface. The studies share a common goal: to reduce the high degree of uncertainty in current parameterization schemes. The wide variety of measurements made during the studies, which incorporated tracer and surfactant release experiments, included direct eddy correlation fluxes, detailed wave spectra, wind history, photographic retrievals of whitecap fraction, aerosol-size spectra and composition, surfactant concentration, and bubble populations in the ocean mixed layer. Measurements were made during three cruises in the northeast Atlantic on the RRS Discovery during 2006 and 2007; a fourth campaign has been making continuous measurements on the Norwegian weather ship Polarfront since September 2006. This paper provides an overview of the three projects and some of the highlights of the measurement campaigns.

Sulphur hexafluoride and helium-3 as sea-water tracers: deployment techniques and continuous underway analysis for sulphur hexafluoride

Abstract Methods are described for the preparation and release of sulphur hexafluoride (SF 6 ) and 3 He to surface sea water for studies of gas exchange and as oceanographic tracers. Known amounts of the tracers (0.28 mol SF 6 and 0.05 mol 3 He, measured by gas chromatography with thermal conductivity detection) were dissolved in 1000 l of sea water in a gas-tight steel tank and later released at a depth of 10 m without modification of the initial 3 He: SF 6 ratio, by excluding all air from the tank. Techniques for deploying the tracers as a discrete patch in shallow, tidally active waters (30 ± 2 m, S.E. North Sea) and the labelling of the patch with drifter buoys are also given, Concentrations of SF 6 in the patch were mapped from a survey vessel using an analysis system that determined concentrations in surface water by purge and trap followed by gas chromatography with electron-capture detection, in a fully automated, continuous sequence of analyses with a 3-min repeat time. Data were computer-matched with the ships navigational data and used to construct a near real-time plot of the tracer patch in a reference frame which moved with the water mass. The plot was used to aid further navigation of the ship around the patch. The results from a typical mapping exercise are presented.

Marked seasonality in the concentrations and sea‐to‐air flux of volatile iodocarbon compounds in the western English Channel

[1] In the first seasonal study of volatile iodinated organic compounds (VICs) in the open sea, concentrations of five VICs were measured approximately weekly at four depths, over 20 months from July 2002 to April 2004, in the western English Channel. Seawater concentrations varied seasonally by an order of magnitude for all five compounds, with winter minima and, generally, late summer/autumn maxima. The average contribution to the dissolved VIC pool was chloroiodomethane (39%), diiodomethane (33%), iodomethane (22%), iodoethane (6%), and bromoiodomethane (4%). Total sea-to-air flux of iodine atoms carried by the VICs (15.5 mmol I m � 2 yr � 1 ) was approximately fourfold higher than that for iodomethane alone. This contrasts with previous studies that indicated that iodomethane was the main vector of iodine away from macroalgal beds. The estimated sea-to-air flux takes into account the significant airside control of the gas exchange of certain VICs, particularly diiodomethane; for which a 30% reduction in average daily flux was observed when an additional airside transfer velocity was included in the calculations. Because of their high reactivity, chloroiodomethane and diiodomethane are likely to drive the atmospheric organic iodine chemistry over these shelf seas, rather than the monohalogenated VICs.