C. S. Law

Mixing of a tracer in the pycnocline

A patch of sulfur hexafluoride was released in May 1992 in the eastern North Atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces. The diapycnal eddy diffusivity K estimated for the first 6 months was 0.12±0.02 cm2/s, while for the subsequent 24 months it was 0.17±0.02 cm2/s. The vertical tracer distribution remained very close to Gaussian for the full 30 months, as the root mean square (rms) dispersion grew from 5 to 50 m. Lateral dispersion was measured on several scales. The growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0.07 m2/s at scales of 0.1 to 1 km, larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing. Teasing of the overall patch, initially about 25 km across, into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3±0.5 × 10−7 s−1. The rms width of these streaks, estimated as 3 km and maintained in the face of the streak growth, indicates an isopycnal diffusivity of 2 m2/s at scales of 1 to 10 km, much greater than expected from internal wave shear dispersion. The patch was painted in, albeit streakily, by 12 months, confirming expectations from analytical and numerical models. Homogenization of the patch continued during the subsequent 18 months, while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2/s, acting at scales of 30 to 300 km.

Effect of iron supply on Southern Ocean CO2 uptake and implications for glacial atmospheric CO2

Photosynthesis by marine phytoplankton in the Southern Ocean, and the associated uptake of carbon, is thought to be currently limited by the availability of iron. One implication of this limitation is that a larger iron supply to the region in glacial times could have stimulated algal photosynthesis, leading to lower concentrations of atmospheric CO 2. Similarly, it has been proposed that artificial iron fertilization of the oceans might increase future carbon sequestration. Here we report data from a whole-ecosystem test of the iron-limitation hypothesis in the Southern Ocean, which show that surface uptake of atmospheric CO 2 and uptake ratios of silica to carbon by phytoplankton were strongly influenced by nanomolar increases of iron concentration. We use these results to inform a model of global carbon and ocean nutrients, forced with atmospheric iron fluxes to the region derived from the Vostok ice-core dust record. During glacial periods, predicted magnitudes and timings of atmospheric CO2 changes match ice-core records well. At glacial terminations, the model suggests that forcing of Southern Ocean biota by iron caused the initial ∼40u2009p.p.m. of glacial–interglacial CO2 change, but other mechanisms must have accounted for the remaining 40u2009p.p.m. increase. The experiment also confirms that modest sequestration of atmospheric CO2 by artificial additions of iron to the Southern Ocean is in principle possible, although the period and geographical extent over which sequestration would be effective remain poorly known.

Sulphur hexafluoride as a tracer of biogeochemical and physical processes in an open-ocean iron fertilisation experiment

Abstract The first open-ocean experiment to test the iron hypothesis in the equatorial Pacific was undertaken using the tracer gas sulphur hexafluoride (SF 6 ) to locate and track the fertilised surface water. Continuous surface measurements showed that the SF 6 patch spread rapidly in the first 24xa0h, from an initial release area of ∼64xa0km 2 to a total area of 214xa0km 2 , and remained relatively constant in size for the following three-day period. SF 6 data was mapped in a Lagrangian frame of reference by the use of a drogued GPS buoy released at the centre of the patch. The SF 6 patch remained coherent and exhibited a slow, anti-cyclonic oscillation during the first four days. The buoy was transported downwind of the patch in a northwesterly direction within two days, which has implications for the future use of buoys in surface-water advection studies. Following subduction below a low-salinity front 3–4xa0days after release, the patch centre was relocated by its SF 6 signal at a depth of 25–30xa0m to the east of the residual surface patch. The latter spread rapidly to the southwest during the remainder of the experiment, whilst the subducted patch remained relatively stationary. Density-corrected SF 6 profiles were used to calculate a mean vertical eddy diffusivity of 0.25xa0cm 2 /s across the thermocline following the subduction event. A vertical flux of nitrate of 2.5xa0mmol/m -2 xa0d -1 into the mixed layer was estimated, which implied an f -ratio value of 0.4 on comparison with productivity data. The results demonstrate that SF 6 is a successful tracer of water masses, and emphasise the potential of this technique for the in situ measurement and manipulation of open-ocean processes.

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.

Physical evolution of the IronEx-I open ocean tracer patch

An 8×8 km tracer-enriched patch was successfully formed in an open-ocean mixed layer in the equatorial Pacific during the first uncontained test of the iron hypothesis. To minimize the effects of horizontal advection, the patch formation and subsequent rapid underway sampling of the patch properties were performed in a Lagrangian reference frame centered on a navigated, drogued buoy that tracked the mixed layer movements on O(1 d) timescales. Daily maps of the evolving patch shape, and corrections for the buoy drift relative to the patch center, were based on objectively analyzed surface concentration maps of an inert tracer, SF6, which had been injected into the ocean surface with trace quantities of iron during the patch formation. The Lagrangian reference frame significantly reduced large scale circular and lateral advection errors in maps of the surface patch shape. A strong pycnocline at approximately 35 m depth and very constant 6 m s-1 wind forcing greatly limited turbulent diffusion below the mixed layer. Rapid small scale mixing over the first 24 h was followed by a four day period of slow spreading, primarily in the along-wind direction. Estimates of along wind horizontal diffusivity using a Fickian model were 600±100 m2 s-1, with a mean cross wind value of 200±30 m2 s-1 over a 4 d period. On the fifth day an intruding low salinity surface front effectively capped the patch between a 10–20 m thick fresh surface layer and the pycnocline. This experiment demonstrated that an O(100 km2) open ocean mixed layer tracer tagged patch could be formed, and its evolution mapped the presence of advection over 5–10 d periods.

Low nitrate : phosphate ocean ratios corrected ...

It has become apparent, since publication of our Letter on low nitrate: phosphate ratios in the global ocean (T. Tyrrell and C. S. Law, 387, 793–796; 1997; correction, 393, 396; 1998), that the World Ocean Atlas 1994 (WOA94) database we used contains transcription errors for nutrient measurements. Discrepancies between the original data and the corresponding data sets in WOA94 have been confirmed for the eastern tropical Pacific (identified by A. Longhurst and S. Sutherland), the western North Pacific (M. Aoyama, K. Hirose and K. Ishikawa) and the Agulhas retroflection area south of South Africa (R. Schlitzer and A. Longhurst).