W. J. Jenkins et al.: A comprehensive global oceanic dataset of helium isotope and tritium measurements 
amount of tritium in the water and the length of time a water 
sample is stored prior to gas extraction, it is generally neces- 
sary to correct the °He/*He results for decay of tritium dur- 
ing storage. For very deep and old samples, e.g., at 3000 m in 
the North Pacific, where tritium concentrations are very low, 
chis correction may be inconsequential. 
2.3 Helium and neon concentration measurements 
To make full use of the helium isotopic ratio measurements, 
one needs to know at least the concentration of helium in 
che samples as well. This allows investigators to calculate 
the actual concentration of both °He and *He in samples 
and, with some assumptions, to estimate the amount of non- 
atmospheric °He — whether it be from tritium decay or hy- 
drothermal input — and *He. The estimate can be further im- 
proved with knowledge of neon concentrations. Since neon 
is similar to helium in terms of its solubility in seawater and 
there are no known significant non-atmospheric sources of 
neon in the ocean, its concentration is a direct tracer of pro- 
cesses like air bubble injection at the sea surface. As an ex- 
ample, we show in Fig. 1 the relationship between the helium 
and neon saturation anomalies! for approximately 2000 near- 
surface samples. The saturation anomalies range from signif- 
icantly below zero to as much as 10 %-15 %. The negative 
values may arise from lower barometric pressures or air-sea 
disequilibrium during cooling. However, there may be sys- 
tematic offsets due to laboratory standardization as well. The 
higher values may also reflect such biases but also may be 
due to air bubble entrapment during sampling at sea. The 
solid line in Fig. 1 represents a type-II linear regression of the 
data and has a slope of near unity, which is expected given the 
similarity of the two gases. However, the slope is greater than 
the precise ratio expected (0.8 to 0.9 depending on tempera- 
ure) based on their solubilities and atmospheric abundances, 
possibly due to differing air-sea eXchange rates. Considera- 
tion must be given to these and other factors when using this 
data (e.g., Fuchs et al., 1987; Roether et al., 1998). 
Helium and neon concentration measurements are typi- 
cally made by mass spectrometric peak height manometry. 
‘hat is, by comparison of major isotope ion currents (*He’ 
and 2°Ne*) between the unknown to helium and neon de- 
ived from an aliquot of marine air. The air aliquot size is 
determined from a knowledge of the barometric pressure, 
‚elative humidity, and temperature at which the previously 
evacuated air standard reservoir was filled and the volume 
of the aliquot. Generally the ion current is assumed to be a 
linear function of the sample size (number of atoms) over 
some narrow range but also can be corrected by construction 
I The saturation anomaly of a gas is the percent deviation of the 
measured concentration relative to the expected concentration of the 
sample in equilibrium with air at one atmosphere, for example, as 
defined by AHe = 100(C/C* — 1), where C* is the solubility equi- 
librium concentration at the temperature and salinity of the sample. 
Aww.earth-syst-sci-data.net/11/441/20 (4 
17 
C 
8 
Slope = 1.04 +- 0.02 
— 
SS 
£ 
; 
A 
„7 
+ 
AHe (%) 
10 
Figure 1. A scatter plot of helium and neon saturation anomalies for 
nearly 2000 near-surface (< 20 m depth) samples from the dataset. 
The solid line is a type-II linear regression of the data. 
of a standard curve using different sized aliquots. Some mea- 
surements (notably the GEOSECS expedition) were made by 
splitting the gas extracted from the water sample and mea- 
suring the helium and neon contents separately using isotope 
dilution (with °He and ??Ne spikes). 
2.4 Data organization 
We have compiled a comprehensive dataset consisting of he- 
lium isotope and tritium measurements in oceanic waters 
made by numerous laboratories over the past 6 decades. The 
dataset includes — 60000 tritium and — 63 000 helium iso- 
tope measurements, — 57000 dissolved helium concentra- 
tions, and — 34.000 dissolved neon concentrations in ocean 
water taken from 1952 to 2015 (for tritium) and from 1967 
to 2015 for helium. In addition to “spot sampling”, there are 
380 cruises, with sampling from > 5400 locations for tri- 
um and — 5600 locations for helium. The helium data are 
rom 8 different laboratories and the tritium data from 15 lab- 
oratories worldwide. In addition to including measurement 
uncertainties, a data quality flag, and data source, each data 
point is accompanied by location (latitude, longitude, depth) 
and time (decimal year) of sampling. When available, water 
temperature, salinity, and dissolved oxygen measurements 
are included. 
A number of the earliest measurements were obtained 
[rom publications. In those cases, the publication source is 
given. If the data were transcribed from tables, the table num- 
ber and page is also given. In the event that the data were 
only available graphically, a computer program to digitize 
the data from plots was used, and in the rare cases in which 
graph quality was sufficiently poor to degrade the precision 
of the data, the uncertainties were commensurately increased 
to reflect it. Where data had been assigned a Digital Object 
Identifier (DON), this is also included. 
Earth Syst. Sci. Data. 11. 441-454, 2019