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W. J. Jenkins et al.: A comprehensive global oceanic dataset of helium isotope and tritium measurements
Tritium sampling locations
tributions is also valuable as they penetrate the subtropi-
cal thermocline and also intermediate and deep waters. Fig-
ure 5 shows four snapshots of tritium distributions on a sec-
tion along approximately 52° W in the North Atlantic be-
tween the South American and North American coasts (see
map inset). Tritium concentrations are decay-corrected to
a common midpoint in time (1997) for comparison. While
the last three occupations are conveniently along the same
cruise track (courtesy of WOCE, CLIVAR, and GO-SHIP),
the first is a composite from several cruises of opportunity
taken over an approximately 1-year period at roughly the
same longitude. One can readily see the downward propaga-
tion and ultimate dispersion of the bomb tritium pulse within
the main thermocline (the upper 1000 m), and the progressive
ingrowth of tritium in the intermediate layer (1500-2500 m
depth) and in the bottom layers (- 4000 m) is striking. One
can also see the repartitioning of tritium inventories in the up-
per waters, both due to vertical exchange and ventilation but
also due to the continued accumulation of this bomb fallout
isotope at subtropical latitudes (similar to bomb radiocarbon,
as observed by Broecker et al., 1995) from more tritium-rich
waters to the north.
Equally important is evidence of the southward propaga-
ion of the transient tracer along the deep western boundary
current system from Nordic and Labrador seas (Doney and
Jenkins, 1994). This first appears in 1982 at the northern end
of the section between 3000 and 4000 m depth near the bot-
(om of the continental slope. As time progresses, it gr0ows
in at intermediate (— 1500 m) and deeper (3500-4000 m)
depths at the southern end. This clearly marks the net advec-
tive timescales for the deep- and intermediate-depth western
houndary currents.
Figure 6 shows the corresponding helium isotope anomaly
distribution for the tritium sections. Interpretation of the he-
lium isotope ratio anomaly is a little more complicated than
the invasion of bomb tritium, but the buildup of tritiugenic
*He within the main thermocline is an important diagnos-
tic of vertical transport for the subtropical main thermo-
cline. Its retention and back-flux to the ocean surface is a
uniquely valuable transient tracer observation, one that par-
allels the buildup and reflux of inorganic nutrients in the ther-
mocline (e.g., nitrate and phosphate) but in a quantifiably de-
fined manner. Observations of surface water °He excesses
(not shown here) have been used as flux gauges to quan-
tify/constrain regional-scale new production rates (Jenkins,
1988; Jenkins and Doney, 2003; Stanley et al., 2015) as well
as upwelling rates (Rhein et al., 2010).
We also include a time series (plotting only the upper
2000 dbar in Fig. 7) for stations within 200km of Bermuda
(32.3° N, 64.7° W) in the subtropical North Atlantic, which
highlights some high temporal resolution features in the pen-
etration of tritium into and ingrowth of °He within the main
thermocline and intermediate waters at one location.
Perhaps the most notable features of the oceanic distri-
bution of the helium isotope ratio anomaly are the large
ar
EN
A nd A
A YR *
| take, ar Fa
; Ve DET LE
F A HE CN PS ._.
SA I Sc Ft ra Ar
SA NT
= Hase Mt
va.
DA
Helium sampling locations
A
Sa
| 5 P N a
Ä
1A
SE Sn
{A 2
5 ul a}
AR
3
sa
Figure 3. Tritium and helium sample locations.
It is well known that the delivery of bomb tritium to
he ocean was a reflection of the distribution of the atmo-
spheric tests and occurred in two principle modes: a domi-
nant pulse-like injection in the Northern Hemisphere and a
much smaller more “diffuse” input into the Southern Hemi-
sphere (Doney et al., 1992). The former mode was dominated
by the early 1960s American and USSR tests that occurred
just prior to the 1963 Partial Test Ban Treaty, while the south-
ern hemispheric tests were carried out predominantly by the
UK and France. Weiss and Roether (1980) estimated the de-
livery of approximately 1500 GCi to the Northern Hemi-
sphere and 480 GCi to the Southern Hemisphere by the end
of 1972. The latitudinal trends can be seen in Fig. 4, which is
a plot of near-surface (< 50m depth) water tritium concen-
trations vs. time for a number of latitude bands. Most strik-
ing is the northward increase in the concentration (y-axis)
ranges. The time-latitude trends in Fig. 4 reflect this glob-
ally asymmetric delivery, but much of the structure caused
by regional variations in atmospheric input and ocean circu-
lation may be masked due to conflating major ocean basins
in the groupings.
As a transient tracer, tritium offers an opportunity to vi-
sualize the ventilation of deep waters on decade to century
timescales. Benchmark observations of water column tritium
concentrations during the GEOSECS Atlantic Expedition re-
veal North Atlantic deep water formation in a graphic man-
ner (Östlund et al.. 1974). The evolution of the tritium dis-
zarth Syst. Sci. Data. 11. 441—454. 20 ii.
www.earth-svst-sci-data.net/11/441/2019:
