H. Göte Östlund

The penetration of tritium into the tropical Pacific

Abstract The persistence of subsurface tritium maxima coincident with the Equatorial Currents is used to show that advection along isopycnals by the mean wind-driven circulation is the dominant process in the at most 14-year time scale for the penetration of high northern latitude water to the equator (above 26.2 sigma-theta). Ventilation of the equatorial Pacific thermocline from the north contrasts sharply with the equatorial Atlantic thermocline which is ventilated from the south. The most striking manifestation of the North Pacific circulation is evidenced by a tritium maximum and salinity minimum at the equator between 145° and 125°W located above 25.6 sigma-theta. It shows that regardless of time of sampling the eastern&sol/central equator has received the moat high latitude water, probably as a consequence of recirculation by the Equatorial Currents. Between the same meridians there is a tritium maximum on and north of the equator at the surface, which is interpreted as an expression of upwelling. ...

Circulation of Tritium in the Pacific Ocean

Abstract The input of bomb tritium into the high-latitude Northern Hemisphere waters has demonstrated the spread of a tracer in three dimensions in the North Pacific Ocean. Subsurface tritium maxima in middle and low latitudes clearly show the importance of lateral mixing (along isopycnals) in the upper waters. The tritium pattern as mapped on isopycnal surfaces puts definite time bounds on the exchange between the subtropical anticyclonic gyre of the North Pacific and both the subarctic cyclonic gyre and the system of zonal flows in the equatorial region. The penetration of bomb tritium to depths below 1000 m in the western North Pacific Ocean shows that these waters have been ventilated at least partially in the past 17 years of the post-bomb era. From the tritium pattern the upper waters of the North Pacific can be divided into three regions: a mixed layer that exchanges rapidly with the atmosphere, a laterally ventilated intermediate region (between the mixed layer and at most the winter-outcrop isopy...

Two decades of mixing tritium into the North Pacific Ocean

Abstract Tritium data collected in the North Pacific Ocean mostly from Long Lines cruises (1983–1985) are compared with GEOSECS data (1973–1974) to characterize the dominant circulation processes in the second decade after the major bomb injection. The total decaycorrected inventory for the North Pacific Ocean remained nearly constant during the decade between GEOSECS and Long Lines, although the spatial distribution changed. Tritium was lost from the subpolar region and was gained by the tropics. Exchange between the subpolar and subtropical regions appears to be occurring at both the extreme eastern and western boundaries. On isopycnals that are not thought to outcrop ≥26.80 σ θ ), the tritium inventory is about 30% higher in the Long Lines data. The GEOSECS tritium data show that the ventilation time scale of the thermocline is rapid; less than 10 years. The GEOSECS tritium distribution is characterized by north-south gradients, which are strongest in the east. The Long Lines tritium data show that the eastern portion of the subtropical gyre is well mixed and has similar values to the subpolar region. We attribute this difference in the data sets to the sharp decrease in the tritium source function after the mid-1960s. Without the addition of tritium at northern outcrops, gradients cannot be maintained in the presence of mixing. Higher tritium values in the eastern, as compared with the western, portion of the basin are consistent with Subtropical Mode Water and shallow salinity minimum water formation. Furthermore, differences between the eastern and western portions of the basin suggest that mixing and/or recirculation may be more important in the west. On non-outcropping isopycnals there is poleward and westward contraction of the subtropical gyre in both data sets. Cross i~opycnal mixing in the subpolar region ventilates isopycnals (as evidenced by the presence of bomb tritium) which do not outcrop. The Long Lines data suggest that after formation in the northwest, intermediate waters may be further ventilated within the Alaskan Gyre. The residence time in the subpolar region is estimated to be less than 14 years for North Pacific Intermediate Water and greater than 20 years for deeper waters. In the Long Lines data tritium penetration reaches at least to the oxygen minimum (27.40 σ θ ) in the subpolar and most of the SUbtropical region. The average increase in vertical tritium penetration in the Long Lines data is approximately 100 m. This is consistent with the small increase (from 12 to 14%) in the volume of the North Pacific Ocean exposed to bomb tritium. Fourteen per cent is a lower bound estimate of the ventilated volume of the North Pacific Ocean

Detection, Structure, and Origin of Extreme Anomalies in a Western Atlantic Oceanographic Section

Abstract Ten anomalous water parcels were detected in the water column of a western Atlantic oceanographic section (0–5550 m; 70°W; 23–33°N). The parcels had extreme properties lying either two standard deviations from historical mean values, or estimated origins lying 2000 km from the oceanographic section. Detection, structure, and origin of the parcels were determined from analyses on isobaric and isopycnal surfaces using eight kinds of measurements temperature, salinity, oxygen, light scattering, silicate, phosphate, nitrate, and tritium. The parcels originated from seven of the major water masses comprising the North Atlantic Ocean. As observed along the oceanographic section the parcels had the following average (range) characteristics vertical pressure extent = 650 db (100–150 db); horizontal extent = 130 km (30–260 km); and distance to origin = 2300 km (400–5200 km). A population density equaling approximately one parcel per 100 km was obtained as the number of parcels (10) divided by the length o...

Temporal variations of bomb radiocarbon inventory in the Pacific Ocean

The natural and anthropogenic components of the radiocarbon measurements from seawater samples can be successfully separated by an improved method, which is based on a very well-defined relationship between natural radiocarbon and dissolved silica observed mainly during the GEOSECS survey for waters beneath 1000 m depth. This relationship is further reconfirmed by the l4 C measurements from large volume samples taken in the deep waters in the Pacific Ocean during the recent WOCE survey program. Analysis of upper ocean 14 C measurements made along 152°W, and north of 20°N, in the northeastern Pacific Ocean during the NOAAs CGC91 cruises which is a part of the WOCE survey program, indicates that the bomb l4 C inventory in this part of the ocean has increased by 22% since the GEOSECS measurements made in 1974. This increase is consistent with the model prediction of 25% for the northern hemisphere ocean. Change of the surface water bomb Δ 14 C values during this period is insignificant. This feature is also consistent with the model simulation. Results of this new analysis will provide useful information of the temporal variations of bomb l4 C inventory in the ocean, in addition to the spatial distribution, which can be used as powerful constraints in calibrating the global ocean carbon cycle models, especially those based on three-dimensional ocean general circulation models, for estimating the uptake of CO 2 by the ocean.

Penetration of bomb radiocarbon in the tropical Indian Ocean measured by means of accelerator mass spectrometry

Abstract Radiocarbon measurements performed on seawater samples by means of accelerator mass spectrometry (AMS) enable to reduce by a factor of 2000 the water sample size needed for the 14 C measurements. Therefore no chemical treatment on board the oceanographic vessel is required. Seventy-four AMS 14 C determinations on samples collected in the tropical-equatorial Indian Ocean during the second leg of the INDIGO program (1986) are presented and compared with the β-counting results obtained during the same campaign and the GEOSECS program (1978). A pronounced reduction of the equatorial 14 C deficit suggests that substantial amounts of bomb- 14 C are associated with the westward flowing Pacific water which enters the Indian Ocean via passages through the Indonesia archipelago and/or to meridional mixing with 14 C-rich water of the southern subtropical gyre.

Time dependence of organic matter decay and mixing processes in Framvaren, a permanently anoxic fjord in South Norway

Three different layers have been identified in Framvaren, which has a maximum water depth of 184 m. One oxic layer above the redoxcline at 18–20 m. One anoxic layer from 20 to 100 m which is occasionally ventilated by a flow over the sill (which has a depth of 2.5 m), and finally a stagnant layer below 100 m. Using the release rate of silica from the bottom and measurements of the concentration of HTO it is possible to make some calculations on the annual volume of interleaving in the layers 25–50 m, 50–75 m, and 75–100 m together with the advective flows. Reliable values of the sulfide concentration were obtained by precipitating and weighing HgS together with careful protection of all anoxic water samples with argon. The light yellow color of the precipitate in the depth range 25 to 80 m indicates that the occasional ventilation will cause such reactions as 0.502 + H2S S(colloidal) + H2O. The elemental sulfur, being stabilized with HS−, is set free upon the precipitation of HgS. The new data for the concentration of sulfide give an acceptable stoichiometry for the decay reaction of organic matter. This is not the case with the data of Yao and Millero. The mean values for the concentrations of ammonium and phosphate agree with the new data of Yao and Millero. The mol/mol C/N ratio of 10.1 found in trapped material by Naess and coworkers (1988) agrees with the stoichiometry of the dissolved constituents, i.e. C/N = 9.92 ± 0.45. A denitrification reaction is suggested to explain the high values of C/N. The vertical diffusion coefficient at 100 m calculated from the depth profile of silica was 0.92 × 10−6 m2 s−1 which lies in the range of values given by Fröyland. Finally, the 14C age of the total dissolved inorganic carbon (Ct) in the water below 90 m was about 1600 years indicating a bioproduction in the period 8000 years B.P. to A.D. 1853 when a channel was opened between the fjord outside (Helvikfjord) and Framvaren.

Exchange times in the Pacific Equatorial Current system

Abstract Tritium data from the GEOSECS eastern and western track stations sampled in the thermocline of the North Equatorial Counter Current (NECC) showed high values, which are indicative of the contribution of the North Equatorial Current (NEC) to the equatorial region. A tritium front at 6°N is evidence of the penetration of northern latitude water into the NECC. The results of a two-☐ model show no net flux between NEC and NECC and a lower bound estimate of five years for an exchange time scale between the NEC and NECC in the thermocline; this is consistent with a horizontal diffusivity of 5 × 10 7 cm 2 /s.

A Sampling System for Atmospheric HT and HTO

A system is briefly described, which is designed for the sampling of tritium gas (HT and T2) and tritiated water vapor in the air at levels far below MPC and down to present-day atmospheric background levels. The tritium gas is extracted from the air by catalytic combustion on Pd at ambient temperature. The resultant water is absorbed on molecular sieve carrying the Pd. The system is well suited for field work. Present-day world-wide HT levels are about 2.3 pCi/kg air, and the HTO levels about the same in the troposphere.

Radiocarbon in dissolved oceanic CO2

Abstract The measurement criteria for the use of 14 C as an oceanic water-mass tracer are reviewed, including the particular conditions of ship-board water sampling. A status report is then given on the effort of the US government to meet the special requirements of an AMS (Accelerator Mass Spectrometer) facility for oceanography.