3H/3He Survey

Tritium and helium isotopes in the ocean

WOCE tritium/helium isotope stations. red symbols: shallow tritium/helium isotopes and deep helium isotopes; blue symbols:shallow tritium/helium isotopes only; green symbols: European lines (typically shallow tritium/helium and deep helium isotopes). Stations from the Subduction experiment (North Atlantic; blue symbols are also included). This map does not include a few French lines.

Index:
General background
Contributions to WOCE expected from tritium and helium isotope data
References

General background

Tritium was practically the first transient tracer used extensively in oceanographic studies. Tritium is the radioactive isotope of hydrogen which decays by beta-decay to the noble gas He-3 (half life: 12.43 ys; Unterweger et al., 1980).  Tritium is produced naturally by interaction of cosmic rays with nitrogen and oxygen mainly in the upper atmosphere, and, after oxidation to HTO, takes part in the hydrological cycle. Shortly after the discovery of natural tritium in the environment (von Faltings and Harteck, 1950; Grosse et al., 1951), the potential of tritium as a tracer of water movement in natural water systems, including the ocean was recognized (e.g. Kaufman and Libby, 1954; Begemann and Libby, 1957; Giletti et al., 1958).

Starting in the early 1950's tritium from anthropogenic sources (mainly nuclear weapon tests) was added to the atmosphere in considerable amounts. By the mid 1960's the natural background of tritium in precipitation was masked completely by so-called bomb tritium (e.g. Weiss et al., 1979). The bomb tritium severely limited the use of natural tritium as a tracer due to the fact that only few uncontaminated tritium data from the pre-bomb era are available. However, it offered a new tool for studies of water movement in natural system, i.e. a dye introduced into the environment on a global scale at a relatively well known rate. During the last two decades bomb tritium has developed to a routine parameter in studies of natural water systems (e.g. Muennich and Roether, 1967; Roether et al., 1970; Atakan et al., 1974; Oestlund, 1982; Broecker et al., 1986).

Limitations in using tritium as a water mass tracer arise from its radioactive nature and from dispersion of the bomb signal in the investigated systems, leading to difficulties in interpretation of observed tritium distributions. These problems can be partially overcome by additional measurement of He-3, the decay product of tritium. The combined measurement of tritium and He-3 allows estimates of the elapsed time since a water parcel has been isolated from gas exchange with the atmosphere, i.e. since the tritium/He-3 clock has been set to zero. Jenkins and Clarke (1976) introduced this method to oceanography. Subsequently, the method has been applied to various studies of water movement in surface and ground waters, as well as the ocean (e.g., Torgersen et al., 1977; Jenkins, 1982, 1987; Thiele et al., 1986; Schlosser et al., 1988, 1989 to name just a few examples).

Tritium sources and input functions

Natural tritium concentrations in surface waters are of the order of 10-18 tritium atoms per hydrogen atom. This is the reason that tritium concentrations are reported as TU (Tritium Units) which mean a tritium to hydrogen ratio [T]/[H] of 10-18 (1 TU is equivalent to an activity of 3.2 pCi or 0.12 Bq per liter H2O). The production rate of natural tritium is about 0.5 +- 0.3 atoms tritium per cm2 and second (Craig and Lal, 1961) leading to natural tritium values in mean continental precipitation of about 5 TU (Craig and Lal, 1961; Roether, 1967).

A major part of the bomb tritium was injected into the stratosphere from where it mixed down into the troposphere and participates in the hydrological cycle. Transfer to the surface waters of the oceans which are the largest sink for tritium occurs by water vapor exchange, precipitation and continental run-off (e.g., Weiss et al., 1979; Weiss and Roether, 1980). As most of the nuclear weapons tests were performed in the northern hemisphere, the tritium distribution is strongly asymmetric with relatively high concentrations in the northern hemisphere compared to the southern hemisphere. Fig. 1 shows the tritium concentration in marine precipitation for the northern and the southern hemisphere as a function of time. It is evident from this figure that bomb tritium was released mainly in three pulses during 1954, 1958-1959 and, predominantly, 1963.

Present day tritium concentrations are of the order of 10 TU in continental precipitation. In the ocean the maximum tritium concentrations observed in the northern hemisphere surface water are of the order of 2 TU. Regions with high runoff components like the Arctic Ocean or a location surrounded by continents like the Mediterranean Sea have elevated tritium concentrations.  Present day tritium concentrations in Southern Ocean surface waters are of the order of 0.15 TU.

The delivery of tritium to the oceans ('input function') has been quantified by Roether and Weiss (1980), Dreisigacker and Roether (1979), and Doney et al. (1992). These studies allow us to construct time-dependendent flux boundary conditions, as well as concentration boundary conditions for any given location in the ocean. This capability is important for the application of tritium and tritiogenic He-3 in model calibration studies.

Non-linearity of tritium and the tritium/He-3 age with respect to mixing

The accuracy of the helium-3 measurements allows the detection of tritium-helium ages as short as a few months in the upper thermocline of northern hemisphere waters, nicely complementing the longer time-scales illuminated by the evolution of the large scale tritium fields.  Based solely on measurement issues, the tritium-helium age appears useful on time-scales ranging from a few months (for northern hemisphere surface waters) to the time since the bomb tests (several decades).  However, the tritium-helium age does not respond in a simple, linear fashion to mixing (Jenkins and Clarke, 1976). 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, 1987). 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 0.1 cm/s (e.g., Jenkins, 1998). 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, 1998).

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: it is zero (or very nearly so) at the ocean surface. 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.

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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 3He. 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.g., Jenkins, 1987, 1998). The WOCE-AIMS phase offers a unique opportunity to compare the ventilation rates of several subtropical gyres and oceans. 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.

The long term, quasi-decadal variability of ENSO/El Nino events is neither well understood nor easily predicted. 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 (PIT), introducing 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 14C) is a unique tracer of the interhemispheric, as well as intergyre exchange of waters. The tritium/3He data are being used to quantitatively constrain both the rates of exchange and mixing.

Deep and intermediate water formation

Recently ventilated intermediate and deep waters can be reliably identified through transient tracer measurements.  For example, in the Greenland and Labrador Seas where deep water is formed by deep convection tritium/3He studies are being used to estimate depp water formation rates, as well as their variability (e.g., Peterson and Rooth, 1976; Schlosser et al., 1991; Boenisch and Schlosser, 1995; Boenisch et al., 1997). Deep waters formed in the GIN seas and in the Labrador Sea provide the recently ventilated components of the North

Atlantic Deep Water (NADW) and are found at distinct levels in the NA DWBC corresponding to their respective densities. In the region of the Charlie-Gibbs Fracture Zone, for example, Top et al. (1986) estimated the eastward and southward spreading rates of Labrador Sea Water to be 0.7 cm s-1 and 1.4 cm s-1, respectively. Doney and Jenkins (1994) estimated a similar mean velocity of 1.7 cm s-1 in the DWBC using tritium/3He age gradients. They concluded that the tracer derived velocities (spreading rates) include recirculation and therefore are extremely valuable in considering, for example, the penetration of anthropogenic CO2 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. Additionally, it will allow us to compare the tritium water column inventories determined for the mid 1990s with those obtained from the GEOSECS and TTO data. Such a comparison should reveal features of the large scale circulation of the ocean averaged over periods of roughly one decade, specifically exchange of surface water with deeper levels and interhemispheric exchange (large N/S asymmetry in tritium).

Tritium has also been used in the Southern Ocean to estimate formation rates and pathways of bottom water, specifically Wedell Sea Bottom Water (WSBW) and Antarctic Bottom Water (AABW) (e.g., Michel, 1978; Weiss et al., 1979; Jenkins et al., 1987; Bayer and Schlosser, 1991. Schlosser et al., 1991; Mensch et al., 1996, 1998). The WSBW formation rates estimated on the basis of the tritium data are between 2 and 5 Sv (potential temperature: -0.7 degrees C), in agreement with estimates based on hydrographic data (ref; Muench and Gordon, 1995). These formation rates are much smaller than that derived from a global PO4* and 14C balance (Broecker et al., 1998), posing a major puzzle. 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.

North Pacific Intermediate Water (NPIW) is the widely spread water mass within the 26.7-26.9 sigma theta density range (Talley, 1993). 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, 1993; Talley et al. 1995; Warner et al., 1996). Recent studies however indicate that the northeast Pacific is also a region where ventilation of the intermediate layers takes place. Using available tritium data prior to 1990, van Scoy et al. (1991) concluded that the Alaska Gyre is a site of NPIW ventilation. The fine structure observed in the temperature-salinity data centered at 26.8 sigma theta corroborates this conclusion (Musgrave et al., 1992). WOCE P17N line crosses the Alaska Gyre both latitudinally and longitudinally, and provides tracer data to further examine the ventilation issue. A first look at the P17N CFC data indicates that the Alaska Gyre acts as a reservoir for the North Pacific Current waters and the Alaska Current. 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, 1998). 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 3He 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., 1998; Jean-Baptiste et al., 1990), and, to a lesser extent in the Atlantic Ocean (Roether et al., 1998).  In fact, the average 3He supersaturation (above solubility equilibrium) of the deep waters in the Pacific is about 20 percent.  The fact that this excess 3He is injected at isolated sources results in pronounced gradients in the 3He distribution.  Furthermore, in several areas of the Pacific the hydrothermal activity is of sufficient strength to produce very high 3He 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.g., Craig and Lupton, 1981; Lupton, 1995, 1998).

As an example of helium variations in deep Pacific waters, WOCE WHP line P17 provided helium data from a long meridional section along 135 degrees W in the eastern Pacific (see Figure 1).  This long section shows three distinct deep maxima in 3He, 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 2500 km to the east.  The third maximum at about 42 degrees N (2000 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 3He on depth or density surfaces.  An example is shown in Figure 2, which is a map of 3He at 2500 m depth for the entire Pacific basin. Most of the data for this map have come from the WOCE sections, although some helium values from other expeditions such as GEOSECS and Helios are included.  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 (1997) for 2500 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 3He signatures: the Gulf of Aden (Jean-Baptiste et al., 1990) and the Rodriguez Triple Junction (Jamous et al., 1992). The deep 3He 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, 1982), therefore the 3He field generated is likely a result of the deep circulation.  The theoretical framework provided by the Stommel-Aarons (1960) 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, 1977; 1982). The 3He distribution along a section at 32 S section outlines clearly one such boundary current on the eastern flank of the Madagascar Ridge (at ~45 E) in the depth range between 2 and 3 km (Top, unpublished). On the same 32 S section the deep southward flow centered at about 2.5 km depth is tagged with He-3 from the Gulf of Aden sources, and appears as a core spanning between 70 and 85 degrees E (Top, unpublished). The preliminary results of Indian Ocean WHP lines further identify hydrothermal sources in the eastern Bay of Bengal. The spreading of this signal along with the Pacific throughflow signal (Top et al., 1997) is expected to help resolve the deep circulation in the least explored northeastern sector of the Indian Ocean. One yet-to-be refined issue in the Indian Ocean is the upwelling rate. With deep sources a well defined 3He field could provide answers, at least on a regional scale. The WOCE results for the Indian Ocean are presently being summarized.

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 4He signal in the waters mixing with the glacial meltwatedue to the low solubility of helium in seawater (Schlosser, 1986; Schlosser et al., 1990; Weppernig et al., 1996). 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. The WOCE helium isotope data from WHP sections S4 (Pacific), S4 (Indian Ocean), P17S, P16S, P19S, P12 (SR3), I8S, I9S, as well as WOCE data collected during the German Southern Ocean legs in the Atlantic will help to obtain a better assessment of the flux of freshwater into the surface and deep waters of the Southern Ocean. It will also allow us to contrast the fate of glacial meltwater in different regions around Antarctica. The 4He data will be compared to hydrographic data and measurements of stable isotopes of water. The WOCE data will profit from additional data collected in the framework of CORC (Consortium on the Ocean's Role in Climate). The CORC program is focused on the shelf regions of the Pacific sector (Ross Sea, Amundsen, Bellingshause seas). First results from a combined CORC/WOCE data set have been summarized in Hohmann et al. (2002; in press).

Model calibration

The WOCE transient tracer data sets (mainly CFCs and tritium/3He) provide a unique survey of a dye-type tracer for use in model calibration. The physics of the delivery of CFCs and tritium to the ocean (gas exchange and mainly water vapor exchange, respectively), as 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. Procedures for deriving the input functions on a global scale have been described in the literature (Weiis and Roether, 1980; Dreisigacker et al., 1979; Doney et al., 1992) and model simulations testing the sensitivity of tritium simulations to slightly different input functions were performed (Heinze et al., 1998). 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).

In addition to tritium alone, the combination of tritium and tritiogenic 3He (radioactive mother/daughter pair) offers a tool that extends the capability of tritium alone significantly (e.g., Thiele and Sarmiento, 19xy).

Finally, the use of mantle 3He for calibrating the mid-depth circulation in OGCMs has been explored (Farley et al., 1995). For this purpose, a zero-order input function of mantle 3He into the oceans (linear correlation between spreading rate and 3He release) has been developed. The first results from simulations with the Hamburg LSG model were encouraging in terms of the overall 3He 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. The WOCE data set will eliminate the problems related to the sparseness of the data sets.

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References

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