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W. J. Jenkins et al.: A comprehensive global oceanic dataset of helium isotope and tritium measurements
{ Introduction
radioactive counting of tritium decay rate, and the daugh-
ter product (He) ingrowth method. The first method (AMS)
has not been used for the measurement of environmental
tritium levels but is better suited to measuring high tri-
tium concentrations in small samples, largely for biomedical
tracer research (Brown et al., 2005; Chiarappa-Zucca et al.,
2002; Glagola et al., 1984; Roberts et al., 2000). The second
method usually involves isotopic enrichment of hydrogen
From the water samples, either by electrolysis (e.g., Östlund
and Werner, 1962) or thermal diffusion (Israel, 1962) fol-
lowed by low-level radioactive counting, either by liquid
scintillation (Momoshima et al., 1983) or gas proportional
counting (Bainbridge et al., 1961). Measurements are made
relative to prepared standards (Unterweger et al., 1980), and
accuracy appears to be limited by the reproducibility of the
enrichment process to 3 %-10 % (Cameron, 1967).
The third method, He ingrowth, is a three-step method.
First, it involves degassing of a quantity (— 1 to 1000 mL) of
water to remove all dissolved helium. Second, the degassed
water is stored in a helium leak-tight container (usually low-
He-permeability aluminosilicate glass or metal) for a period
of several weeks to a year or more. Experience indicates
that it is necessary to shelter the stored samples from cos-
mic rays since there is a latitude-dependent cosmogenic He
production rate that masquerades as “tritium signal” (Lott
and Jenkins, 1998). Finally, the ingrown He is extracted
from the water sample and mass spectrometrically analyzed
(Clarke et al., 1976; Ludin et al., 1997). The last method, al-
though it involves a potentially lengthy incubation period, is
chemically simpler and does not involve isotopic enrichment
steps. As such, it offers intrinsically greater accuracy (limited
by standardization of the mass spectrometer, typically better
than 1 %) and a lower ultimate detection limit (Jenkins et al..
1983; Lott and Jenkins, 1998).
The global oceanic distributions of tritium (°H, a radioactive
ısotope of hydrogen with a half-life of 12.3 years), its daugh-
:er product He, and helium isotopes in general arise from
the complicated interplay of ocean ventilation, circulation,
and mixing, with the hydrologic cycle, air-sea exchange,
and geological volatile input. Observations of the delivery
of tritium to the ocean and its redistribution are a useful tool
for diagnosing gyre- and basin-scale ventilation and circula-
:jon (Doney et al., 1992; Doney and Jenkins, 1994; Dorsey
and Peterson, 1976; Dreisigacker and Roether, 1978; Fine
and Östlund, 1977; Fine et al., 1987, 1981; Jenkins et al.,
1983; Jenkins and Rhines, 1980; Michel and Suess, 1975;
Miyake et al., 1975; Östlund, 1982; Sarmiento, 1983; Weiss
and Roether, 1980; Weiss et al., 1979).
In shallow waters, away from seafloor hydrothermal vents,
:he combination of tritium and °He may be used to deter-
mine the time elapsed since a water parcel was at the sea
surface, making it a useful tool for diagnosing ventilation
and circulation on seasonal through decade timescales (Jenk-
ins, 1987, 1998, 1977). The ingrowth and evasion of tri-
tiugenic °He from the thermocline is also useful as a flux
gauge for constraining the rate of nutrient return to the ocean
surface (Jenkins, 1988; Jenkins and Doney, 2003; Stanley
et al., 2015) as well as upwelling in coastal regions (Rhein
et al., 2010). Finally, the distribution of helium isotopes in
the deep sea provides important quantitative constraints on
‘he impact of submarine hydrothermal venting on many el-
ements because the global hydrothermal helium flux is well
known (Bianchi et al., 2010; Holzer et al., 2017; Schlitzer,
2016). This makes °He useful as a flux gauge (German et al.,
2016; Jenkins et al., 1978, 2018a; Lupton and Jenkins, 2017;
Resing et al., 2015; Roshan et al., 2016). Consequently, there
have been numerous measurements of these properties over
the years, particularly under the aegis of major observational
programs like GEOSECS (Bainbridge et al., 1987), TTO
(Jenkins and Smethie, 1996), WOCE (http://www.nodc.noaa.
zov/woce/wdiu/diu_summaries/whp/index.htm, last access:
1 April 2019), CLIVAR (http://www.clivar.org, last ac-
cess: 1 April 2019), GO-SHIP (http://www.go-ship.org,
last access: 1 April 2019), and GEOTRACES (http://www.
geotraces.org, last access: 1 Aprıl 2019). It seems valuable to
assemble all existing data, including those measured prior to
and outside of these programs, along with appropriate meta-
data, in one place to facilitate further use and analysis. This
1s a report of these efforts.
2.2 Helium isotope measurement methodology
Water samples are usually drawn from Niskin bottles into
a helium leak-tight container either for shipboard (Lott and
Jenkins, 1998; Roether et al., 2013) or shore-based gas ex-
traction. The latter involves either clamped (Weiss, 1968)
or crimped copper tubing (Young and Lupton, 1983). The
extracted gases are subsequently purified and concentrated,
usually eryogenically (Lott, 2001; Lott and Jenkins, 1984;
Ludin et al., 1997), and expanded into a mass spectrometer
for isotopic analysis. While time-of-flight mass spectrome-
try has been used (Mamyrin, 2001; Mamyrin et al., 1970),
most oceanic helium isotope measurements have been made
using multi-collector magnetic sector instruments (whereby
ions are electrostatically accelerated and deflected by a mag-
netic field, e.g., Bayer et al., 1989; Clarke et al., 1969; Lott
and Jenkins, 1998, 1984; Ludin et al., 1997). Measurements
are typically standardized to marine air and corrected for
any sample-size-dependent ratio effects determined by mea-
surement of different-sized air aliquots. Depending on the
2 Methods
2.1 Tritium measurement methodology
There are at present three distinct methods for the determi-
nation of tritium in water samples: direct measurement of
tritium abundance by accelerator mass spectrometry (AMS),
carth Syst. Sci. Data. 11. 441-454. 201=
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