3H/3He Survey | Lamont-Doherty Earth Observatory
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However, the tritium-helium age does not respond in a simple, linear fashion to mixing Jenkins and Clarke, For example, a mixture between two water masses with differing tritium concentrations produces an "average" age which is weighted toward the water mass with more tritium. The degree to which the tritium-helium age is affected by mixing depends on the strength of mixing and the contrast in tritium concentrations.
This was quantified in terms of an advection-diffusion relationship for the tritium-helium age Jenkins, The equation resembles that for an "ideal age tracer" but includes an undsteady term due to the transient nature of the tritium distribution and an additional, non-linear mixing term. In typical upper ocean conditions the magnitude of the non-linear effects are negligible on timescales less than a decade, and become more significant only for longer timescales.
These effects can be accounted for, and the age distribution has been used to calculate absolute velocities in the North Atlantic subtropical main thermocline to an accuracy of order 0. In the deeper part of the thermocline, the temporal evolution of the age distribution becomes significant, and can be used as a diagnostic of mixing rates Robbins and Jenkins, Using the tritium-helium age, rather than the evolution of the tritium and helium-3 distributions, thus is a more complicated exercise, as the equations are not linear.
However, the use of the age offers two advantages that suggest that it is a useful, parallel avenue to explore in analyzing the tracer data. First, the boundary condition for the age is well known: The tritium-helium clock is set by gas exchange with the atmosphere, and except in cases of very deep, rapid convection, this exchange is sufficiently rapid to zero the clock.
Second, the temporal evolution of the age distribution, particularly many decades after the bomb tests, is small, making one time mapping of the age distribution useful and more easily in interpretation than non-synoptic sampling of a highly evolving field. Contributions to WOCE expected from tritium and helium isotope data Thermocline ventilation, shallow water circulation and intergyre exchange There are a number of processes and features of ocean circulation that will be studied using tritium and 3 He.
Thermocline ventilation is an important aspect of any coupled ocean-atmosphere model, primarily because ventilation occurs and can vary on the crucial annual to decadal time scales. Transient tracer distributions yield valuable insights into the processes responsible, and have lead to surprises.
For example, Sarmiento deduced from tritium distributions that the North Atlantic main thermocline ventilates at a rate far faster than implied by Ekman pumping alone. This was confirmed and quantified in detail by later tracer measurements e. Differences in wind-stress patterns and thermohaline stratification combined with the tracer observations will lead to valuable insight into the mechanisms of subduction and ventilation these regions.
Another important byproduct of this work will be the determination of oxygen utilization rates within the upper thermocline. Vertically integrating these rates will lead to estimates biological export production on a regional basis, and the vertical scale height of the oxygen utilization rate is an important unknown in carbon system modeling.
Recent work has implicated the Hadley Cell exchange between the subtropics and the tropics. In this cell, subtropical thermocline waters move into the tropics to replace upwelling water. It has been postulated that the inward advection of thermal anomalies serves to modify the tropical thermocline structure and the character of waters upwelling at the equator.
Moreover in the Pacific, numerical models indicate that the relative contributions from northern versus southern hemispheric subtropical thermocline waters is modulated by the intensity of the Pacific Indonesian Throughflow PITintroducing interesting possibilities for teleconnections to the ENSO events. The trajectories and time-scale of this advection, and the role of mixing along its path clearly play a role in this process.
Tritium, with its strong interhemispheric concentration gradient compared to the weaker contrasts in CFCs and 14 C is a unique tracer of the interhemispheric, as well as intergyre exchange of waters.
Deep and intermediate water formation Recently ventilated intermediate and deep waters can be reliably identified through transient tracer measurements. Doney and Jenkins estimated a similar mean velocity of 1.
They concluded that the tracer derived velocities spreading rates include recirculation and therefore are extremely valuable in considering, for example, the penetration of anthropogenic CO 2 or other surface perturbations into the deep Atlantic.
The WOCE data will allow us to develop a much better visualization of the penetration of this surface dye into the interior ocean. The WSBW formation rates estimated on the basis of the tritium data are between 2 and 5 Sv potential temperature: The WOCE data will allow us to improve our estimates and to contribute to a better understanding of the deep and bottom water formation in the Southern Ocean. Its ventilation is generally accepted to occur in the subpolar western Pacific, specifically in the Okhotsk Sea, where outcropping of NPIW densities, as well as deep convection has been observed Talley, ; Talley et al.
Recent studies however indicate that the northeast Pacific is also a region where ventilation of the intermediate layers takes place.
mkaplan | Lamont-Doherty Earth Observatory
Using available tritium data prior tovan Scoy et al. The fine structure observed in the temperature-salinity data centered at WOCE P17N line crosses the Alaska Gyre both latitudinally and longitudinally, and provides tracer data to further examine the ventilation issue. In the gyre the lateral components are further modified by mixing with near surface waters over a period of approximately 2 years Aydin et al, The calculations are likely to be refined with the inclusion of the tritium-helium data which in general indicate similar trends.
Circulation of intermediate waters The volcanic activity and associated hydrothermal circulation along the global mid-ocean ridge system introduces a 3 He rich signal into the deep ocean basins, which can be used to trace patterns of ocean circulation at depth intervals which are largely untagged by transient tracers.
This is especially true for the Pacific Ocean, where the spreading rate of the ridges is the greatest, resulting in a correspondingly high rate of mantle helium injection, but there are also clear mid-depth signals in the Indian Ocean GEOSECS; Schlosser et al.
In fact, the average 3 He supersaturation above solubility equilibrium of the deep waters in the Pacific is about 20 percent. The fact that this excess 3 He is injected at isolated sources results in pronounced gradients in the 3 He distribution. Furthermore, in several areas of the Pacific the hydrothermal activity is of sufficient strength to produce very high 3 He enrichments of up to 50 percent above solubility equilibrium.
It has been demonstrated that in these areas the helium plumes provide valuable insight into the regional circulation patterns e. This long section shows three distinct deep maxima in 3 He, each corresponding to a section through a distinct hydrothermal plume. The strong maxima at 14 degrees S and 8 degrees N, divided by a minimum on the equator, are plumes originating on the East Pacific Rise EPR axis some km to the east.
The third maximum at about 42 degrees N m depth is helium from the Juan de Fuca Ridge in the far northeastern Pacific. The sampling density for deep helium in the WOCE Hydrographic Program is high enough that it is now possible to construct maps of 3 He on depth or density surfaces.
An example is shown in Figure 2, which is a map of 3 He at m depth for the entire Pacific basin. The figure shows quite clearly the strong helium plumes which are being transported westward from the EPR axis at 14 S and 8 N, as well as the plume from the Juan de Fuca Ridge which has a trend to spread southwest toward Hawaii. Comparison with the steric height maps of Reid for m depth shows that the flows indicated by the helium plumes agree reasonably well with flows based on dynamic height calculations.
One of the goals of the WOCE synthesis is to complete the merging of the helium data and to compare the integrated field with the hydrographic data which will allow us to produce similar maps of the helium field on various density surfaces.
In the Indian Ocean two regions of hydrothermal activity can be distinguished on the basis of the 3 He signatures: The deep 3 He signal provides a unique tool for tracing the deep circulation. The complex topography of the Indian Ocean is likely to provide competing flows to a heat-driven plume over the vents Stommel,therefore the 3 He field generated is likely a result of the deep circulation. The theoretical framework provided by the Stommel-Aarons model of deep circulation predicts boundary currents in the complex multiple basins of the Indian Ocean.
In most of these basins boundary currents carrying water northward were detected Warren, ; On the same 32 S section the deep southward flow centered at about 2.
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The spreading of this signal along with the Pacific throughflow signal Top et al. One yet-to-be refined issue in the Indian Ocean is the upwelling rate. With deep sources a well defined 3 He field could provide answers, at least on a regional scale. Freshwater addition to the Southern Ocean It has been shown previously that air bubbles trapped in glacial ice dissolve during the melting of ice shelves and produce a measurable 4 He signal in the waters mixing with the glacial meltwatedue to the low solubility of helium in seawater Schlosser, ; Schlosser et al.
This signal can be used to tracer the pathways of so-called Ice Shelf Water formed in contact with glacial ice, as well as for estimates of melting rates of glacial ice and the addition of freshwater to the shelf waters around Antarctica related to this source.
It will also allow us to contrast the fate of glacial meltwater in different regions around Antarctica. The 4 He data will be compared to hydrographic data and measurements of stable isotopes of water.
The physics of the delivery of CFCs and tritium to the ocean gas exchange and mainly water vapor exchange, respectivelyas well as the delivery rates as a function of time, often called 'input functions' is very different. Therefore, the combination of the two transient tracers in model runs allows us to constrain the simulation of oceanographic features much firmer.
Some basic work in preparation of incorporation of tritium into OGCMs has already been done.
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Procedures for deriving the input functions on a global scale have been described in the literature Weiis and Roether, ; Dreisigacker et al.
The main contribution of this proposal to this topic will be the preparation of global tritium fields which can be used by interested modeling groups for calibration of their OGCMs and other regional models. For this purpose, a zero-order input function of mantle 3 He into the oceans linear correlation between spreading rate and 3 He release has been developed.
The first results from simulations with the Hamburg LSG model were encouraging in terms of the overall 3 He budget. The main problem with the simulations were related to the overall sparse data sets available for model-data comparison and specifically the lack of reliable data for the Atlantic where the signal is relatively small.
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