Earth and Planetary Science Letters 296 (2010) 115–123
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Constant Holocene Southern-Ocean
14
C reservoir ages and ice-shelf flow rates
Brenda L. Hall a, Gideon M. Henderson b,⁎, Carlo Baroni c, Thomas B. Kellogg a
a
b
c
Department of Earth Sciences and the Climate Change Institute, University of Maine, Orono, ME 04469 USA
Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK
Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
a r t i c l e
i n f o
Article history:
Received 6 January 2010
Received in revised form 23 April 2010
Accepted 30 April 2010
Available online 1 June 2010
Editor: M.L. Delaney
Keywords:
marine reservoir effect
Southern Ocean
radiocarbon
U/Th dating
coral
McMurdo Ice Shelf
a b s t r a c t
Southern Ocean radiocarbon reservoir ages (i.e. non-zero radiocarbon ages in seawater) are the highest in
the world's surface ocean. Constraining these reservoir ages at present and in the past is important not only
because unknown reservoir ages limit the interpretation of Antarctic radiocarbon chronologies, but also
because reservoir ages provide information about ocean circulation (as a recorder of past circulation and as
an end member for major deep-water masses in today's ocean). In this study, we use paired U/Th and 14C
ages of an unusual set of solitary coral samples trapped by fringing ice shelves in the Ross Sea to provide the
first detailed study of Holocene reservoir ages for the Southern Ocean. Our results indicate a relatively
constant marine radiocarbon reservoir age of 1144 ± 120 years for the past 6000 years. These results are
consistent with extrapolation of the relationship between 14C and alkalinity seen elsewhere, supporting the
use of this empirical relationship in high latitudes. The results also suggest constant deep-ocean circulation
and air–sea exchange during the Holocene and provide a good target for tuning ocean models of modern
circulation. Combining the new ages for corals with their distance from the modern-day ice-shelf grounding
line provides some of the first long-term records of ice-shelf velocities for any region and indicates constant
flow of the McMurdo Ice Shelf during the Holocene, at a rate similar to that observed today.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Because surface seawater contains carbon that has not equilibrated
fully with that in the atmosphere it has a non-zero 14C age known as
the reservoir age. This reservoir age complicates dating of marine
materials which form with a non-zero age and can therefore appear
too old. The reservoir age also encodes information about ocean
circulation and air–sea gas exchange. Constraining the reservoir-age
history for key sites is therefore an important goal, particularly for
regions where it is large and/or variable (Bondevik et al., 2006). The
Southern Ocean is one such setting (we define the Southern Ocean to
mean ocean water south of the Antarctic Convergence). In this region,
upwelling of old deep-waters and the relatively short residence time
of surface waters, lead to the highest surface-water reservoir ages
anywhere on Earth (Key, 2004). Uncertainty in the precise value and
possible temporal variability of the pre-bomb (particularly the prehistorical) reservoir age in the region limits the precision and possibly
the accuracy of 14C chronology in the Southern Ocean and on the coast
of Antarctica (Berkman and Forman, 1996). Because terrestrial
organic material is scarce, many radiocarbon dates from Antarctica
are from marine organisms, all of which are subject to this reservoir
effect. This presents a severe limitation when comparing the timing of
⁎ Corresponding author. Tel.: + 44 1865 282123.
E-mail address: [email protected] (G.M. Henderson).
0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2010.04.054
past changes in circulation, climate, and ice-sheet dynamics in the
Antarctic and Southern Ocean with those in other regions.
Knowledge of Southern Ocean reservoir ages also has special
significance because of the important role of the region in the global
ocean. This is a major site of deep-water formation, so models which
tune their deep-ocean circulation to observed 14C distributions
require accurate knowledge of the 14C content of the waters in this
source region. Similarly, the use of 14C as a tracer for past patterns and
rates of deep-ocean circulation (e.g., Robinson et al., 2005a) requires
knowledge of the history of source-water compositions. The possibility of changes in 14C of this southern source, independent of any
change in water flux to the deep ocean, has been suggested
(Schmittner, 2005) and would confound the simple use of Δ14C as a
tracer of deep-water flow. A history of Southern Ocean reservoir age
also provides information about past circulation in its own right,
because waters upwelling in the region reflect the pattern and rate of
deep-water flow into the Southern Ocean.
Unfortunately, 14C released by weapons testing in the mid-20th
century prevents direct measurement of the natural Southern Ocean
reservoir age in the modern era. Attempts to deconvolve natural
from bomb 14C signatures have used the relationship observed in the
deep ocean between 14C and either silica (Broecker et al., 1995) or
alkalinity (Rubin and Key, 2002), and the latter suggests a spatially
uniform natural Southern Ocean reservoir age (Key, 2004)
of ≈ 1100 years [Δ14C ≈ − 128‰, where Δ14C is the deviation from
atmospheric 14C/12C in parts per thousand (a full discussion of 14C
116
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
nomenclature is in the Supplementary material — SM); Fig. 1]. A
more direct assessment is possible using material of known age
collected prior to weapons testing. Such data from locations around
Antarctica indicate a reservoir age of 1131 years (± 125 years 1s.d.;
n = 12) (Berkman and Forman, 1996; Björck et al., 1991; Mabin,
1985; Stuiver et al., 1981; Whitehouse et al., 1989) for the historical
era (∼ 1840–1940) in good agreement with that derived from
seawater relationships. Neither the alkalinity approach, nor the
historical samples, indicate any significant spatial variation within
the Southern Ocean, reflecting the rapid circumpolar mixing of water
masses and the relatively short residence time of surface waters in
this region. Since these features of the Southern Ocean are unlikely to
change during the Holocene, characterizing the reservoir age at any
one site should provide a history of the whole region.
Neither seawater measurements nor historical museum collections allow an assessment of variations in reservoir age prior to A.D.
1840, which must come instead from a natural archive that can be
dated both by 14C and by an independent chronometer. The only
previous attempt to assess prehistoric 14C reservoir ages in the
Southern Ocean used 226Ra dating of marine barites to derive five
Holocene reservoir ages (van Beek et al., 2002). That work was
conducted at 53°S, 5°W, significantly north of Antarctica and sites of
bottom water formation, and far enough north that seawater
deconvolution suggests a reservoir age lower than that in the south
(e.g. Fig. 1 — upper panel).
A more complete assessment of past reservoir ages in the Southern
Ocean would allow reconstruction of the timing and rates of past
climate and environmental processes. One such process, which can be
assessed directly with the data presented in this study, is flow of ice
away from Antarctica in the floating ice shelves that border the
continent. Of particular relevance here, the McMurdo Ice Shelf is
marginal to the large Ross Ice Shelf that transports continental ice
from both the West and East Antarctic Ice Sheets to the Southern
Ocean. Assessment of the Holocene history of flow of this, or any other
Antarctic ice shelf, has not been readily possible and is a secondary
goal of the study we present here.
2. Study site, samples, and methods
Our objective was to quantify past reservoir variations in the
Southern Ocean. Archives dateable with two chronometers are scarce in
the Southern Ocean but, in this study, we used a set of solitary coral
samples found in a unique setting on the surface of ice shelves in the
Ross Sea region. Unlike other ice shelves which accumulate snow on the
upper surface, parts of the McMurdo and Hell's Gate Ice Shelves in the
western Ross Sea (Fig. 2) have surface ablation zones due to active
Fig. 1. Δ14C in the Southern Ocean (Pacific sector). Colour panels show gridded and contoured Δ14C from the GLODAP dataset (Key, 2004). The lower panel is based on direct
observations, while the upper panel is corrected for bomb 14C assuming the relationship with alkalinity (Rubin and Key, 2002). The study site is marked with the arrow in the top left.
Note that expected pre-bomb surface waters in this Ross Sea region are similar to those found northwards to ≈ 62°S such that the Ross Sea characterises the whole Southern Ocean.
The black line shows the location of the nearest full water-column profile to the Ross Sea measured in the 1970s and demonstrating deep-ocean values of ≈−156‰ with less
negative surface water due to air-sea exchange and addition of bomb radiocarbon. The red bar represents the range of surface values for the pre-bomb Holocene derived from this
study (− 133 ± 13‰; 1s.d.).
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
117
Fig. 2. Location map of the McMurdo Ice Shelf samples, giving their U/Th age and position. Samples are keyed to Table 1. Data for two samples not appearing in Table 2, because they
lack radiocarbon ages (K76-50B18 and K76-23B7) can be found in Table 1.
sublimation and low snowfall (Debenham, 1919; Gow and Epstein,
1972; Kellogg et al., 1990; Stuiver et al., 1981). Freezing adds ice to the
base of the shelf and results in parts of these ice shelves being of marine
origin (Kellogg et al., 1990; Souchez et al., 1991). In shallow areas near
the grounding line or adjacent to islands, this basal freezing captures
sea-floor sediment, including biota such as solitary coral. The sediment
is transported upwards through the ice because of the continued surface
sublimation and basal-freezing and emerges on the ice-shelf surface
(this process is shown schematically in Fig. 3). The McMurdo and Hells
Gate Ice Shelves are ∼10–110 m thick (Souchez et al., 1991; Swithinbank, 1970) and residence times of water under the ice shelves are short
(on the order of months). Thus, the biota frozen onto the basal ice record
the properties of the open Ross Sea surface water, including its reservoir
age. The process of incorporation of biota into the ice at the grounding
line and subsequent movement also provides the potential for
calculating ice-shelf flow rates.
To resolve the Holocene reservoir effect in the Ross Sea (and hence
the Southern Ocean), we dated 43 solitary corals (Gardineria
antarctica; Fig. 4) from debris bands on the McMurdo and Hell's
Gate Ice Shelves by both the 14C and U–Th disequilibrium methods (a
further three samples were dated by U/Th but were too old for the 14C
technique). Solitary corals similar to these are found at all depths and
all latitudes in the global ocean and have been used in recent studies
to derive information about past ventilation ages (e.g. Robinson et al.,
2005b), but not previously to assess surface-ocean reservoir ages.
Typical samples for this study ranged from 2 to 3 cm in length and
showed little evidence of alteration.
Prior to uranium–thorium chemistry, we followed cleaning
procedures specific to solitary corals (Lomitschka and Mangini,
1999) designed to remove Th-rich coatings adhering to the coral
surfaces. Test samples showed that this procedure reduced the 232Th/
230
Th ratio by 1–2 orders of magnitude, thus reducing the need for
large corrections for initial 230Th and improving the precision and
quality of our analyses greatly. After cleaning, the samples were
dissolved, spiked with 236U–229Th solution, and then processed
broadly following the approach of Robinson et al. (2002), using
multicollector ICP mass spectrometry. Final ages were calculated with
ISOPLOT (Ludwig, 1991), using half lives of Cheng et al. (2000).
Following convention, all U/Th ages are given with 2-sigma error and
are quoted relative to 1950. Radiocarbon samples from each coral
(taken close to the U/Th subsamples to avoid problems with longlived specimens) were cleaned and sent to the NOSAMS and NERC
radiocarbon laboratories. Samples identified as SUERC were prepared
to graphite at the NERC Radiocarbon Facility and analysed at the
SUERC AMS in East Kilbride (UK). Others (labelled ‘OS’) were
prepared and analysed at the NOSAMS laboratory in Woods Hole,
Massachusetts (US). There is no discernible difference between
results from the two laboratories. Radiocarbon dates are listed with
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B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
Fig. 3. A schematic representing the way in which debris, including solitary corals, is incorporated at the grounding line and then moves through the ice shelf and towards the calving
front. The photograph shows debris bands on the surface of Hell's Gate Ice Shelf resulting from this process.
1-sigma error — again following convention for that chronometer. All
dates and other pertinent information are in Tables 1 and 2.
We note that more than one approach has been taken for the
calculation of reservoir ages in previous publications, with the various
approaches leading to significantly different values for the Southern
Ocean. Here, (see SM for full details) we calculate Δ14C values (and
corresponding reservoir ages) based on the difference in radiocarbon
content of the surface ocean and co-existing atmosphere at the given
calendar age (from the U/Th age). Atmospheric values are taken from
IntCal04 (Reimer et al., 2004). This is an identical approach to that
used in the 14C-CHRONO marine reservoir database (http://intcal.qub.
ac.uk/marine/).
As a secondary focus to the study, the unique transport history of
the corals and their relationship to the grounding line of the McMurdo
Ice Shelf allow us to calculate average ice-shelf flow rates over time.
Because the corals dated here were first incorporated into the ice at
the grounding line and then moved away by ice flow, their age,
coupled to their modern distance from the grounding line, provides an
average velocity of the ice since the sample became incorporated. A
suite of 11 of our dated corals form a transect along the Black Island
medial moraine, parallel to ice flow, with increasing distance from the
present-day grounding line. These data provide an average flow rate
for each sample to enable one of the first reconstructions of the longterm changes in ice-shelf flow rates for any region.
3. Results
U/Th ages range from modern to ∼ 6000 years B.P. (Table 1) All but
one yield an initial (234U/238U) activity ratio between 1.142 and 1.151
and therefore close to modern seawater (=1.146; Robinson et al.,
2004) suggesting that the ages are robust. We reject the one sample
with a high 234U/238U (1.175) because of potential contamination or
open-system issues.
Pre-bomb reservoir ages reconstructed from these samples, with
respect to the globally averaged atmosphere, show no significant
long-term trend for the last 6000 years and have a mean of 1144 ±
120 years (1s.d.) (corresponding to a Δ14C of −133 ± 13; Fig. 5;
Table 2). Small deviations from this mean may be real and reflect
minor variations in the reservoir ages but cannot be resolved with our
current dataset.
Fig. 5 also shows the calculated ice-shelf flow rates along the Black
Island medial moraine. Those samples located farthest from the
grounding-line have the greatest average velocity (as much as ∼ 10 m/
year); flow rates decrease towards the present-day grounding line.
4. Southern Ocean reservoir ages in the Holocene
Fig. 4. Examples of solitary corals collected from the McMurdo Ice Shelf. Scale is in
centimetres.
Our reservoir age of 1144 years agrees well with that obtained
from pre-bomb historical samples (1131 ± 125 years; Berkman and
Forman, 1996; Mabin, 1985; Stuiver et al., 1981 — see Fig. 5) when
these are calculated in an identical way to that used here. Our data
therefore extend knowledge of the Southern Ocean reservoir age
beyond the historical and indicate that it has not varied significantly in
at least the last 6000 cal year B.P. The use of a constant Holocene
reservoir correction for Southern Ocean/Antarctic marine radiocarbon
chronologies, as previous studies have had to assume (i.e., Baroni and
Hall, 2004; Hall and Denton, 1999; Hall et al., 2004; Licht et al., 1996)
is therefore reasonable. The magnitude of the Holocene reservoir
value is, however, smaller than the ∼ 1300 years commonly applied
(i.e., Berkman and Forman, 1996; Hall et al., 2004), which will result in
a small correction to most existing chronologies. Moreover, methods
of calculation and application of reservoir corrections to Antarctic
samples, particularly when converting to calendar years, have been
inconsistent and sometimes incorrect (see SM), which will result in
further adjustments.
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
119
Table 1
U and Th concentrations and isotope ratios with calculated ages for corals from the McMurdo Ice Shelf and Terra Nova Bay. All errors are 2σ and incorporate analytical measurement
uncertainty, together with weighing error, spike uncertainty and an assumed blank uncertainty. Half lives used are those of Cheng et al. (2000). Ages are quoted relative to 1950. Raw
ages are corrected for the small amounts of initial 230Th using an assumed 232Th/230Th of 50,000. Uncertainty on the corrected ages is the quadratic sum of the raw age error and the
full size of the correction required for each age. The 2nd and 3rd digit of the sample names refers to the year of collection, with following numbers referring to the sample site, and the
final letter/number, the sample identifier at that site. Sample K76-13 is rejected based on its high δ234U value. Three complete replicates (i.e. separate pieces of the same coral sample
taken through chemistry and mass spectrometry independently) were performed for K76-50A and demonstrate reasonable agreement.
238
δ234U
232
McMurdo ice shelf
K76-67A
K78-67
K78-66
K78-66A
K81-1C
K78-64
K76-24
K76-24A
K81-66
K76-23A
K76-25
K81-2A
K76-23
K76-23B
K76-22
K81-2
K81-51A
K76-55C
K7655
K8147
K76-20
K76-55A
K76-17A
K8151
K76 13
K76-10C
K76-50B
K81-45
K76-10B
K76-10D
K77-9
K76-10
K76-8A
K76-50A (rep. 1)
K76-50A (rep. 2)
K76-50A (rep 3)
K76-50A (mean)
K76-9
K76-50
K76 10-2
K76-51
K76-7
K76-5
K76-58
K81-24
K76-65
3.288 ± 0.006
3.776 ± 0.005
4.221 ± 0.007
5.905 ± 0.007
3.573 ± 0.006
5.773 ± 0.014
5.508 ± 0.007
5.376 ± 0.008
6.625 ± 0.009
6.724 ± 0.009
5.799 ± 0.008
5.822 ± 0.007
5.325 ± 0.010
6.022 ± 0.008
7.380 ± 0.009
6.451 ± 0.009
6.999 ± 0.011
5.449 ± 0.007
6.100 ± 0.008
5.132 ± 0.008
6.610 ± 0.008
5.889 ± 0.007
6.664 ± 0.013
4.976 ± 0.007
7.697 ± 0.010
7.012 ± 0.015
6.390 ± 0.008
5.442 ± 0.007
7.485 ± 0.010
6.943 ± 0.009
6.934 ± 0.010
5.954 ± 0.007
6.280 ± 0.008
7.234 ± 0.009
6.956 ± 0.009
6.598 ± 0.008
6.929 ± 0.009
7.890 ± 0.010
6.925 ± 0.009
5.574 ± 0.007
7.230 ± 0.009
4.707 ± 0.007
5.542 ± 0.007
6.526 ± 0.015
6.704 ± 0.008
5.451 ± 0.007
146±2
144 ± 2
146 ± 2
149 ± 2
149 ± 2
144 ± 2
147±2
147 ± 2
147 ± 2
144 ± 2
145 ± 2
151 ± 2
146 ± 2
147 ± 2
144 ± 2
145±2
144±2
145 ± 2
148 ± 2
150 ± 2
145 ± 2
145 ± 2
145 ± 2
146 ± 2
175 ± 2
147 ± 2
143 ± 2
144 ± 2
144 ± 2
142 ± 2
144 ± 2
149 ± 2
146 ± 2
143 ± 2
143 ± 2
149 ± 2
145 ± 2
145 ± 2
145 ± 2
149 ± 2
144 ± 2
145 ± 2
146 ± 2
122 ± 2
77 ± 2
76 ± 2
0.496 ± 0.002
1.888 ± 0.008
0.350 ± 0.017
0.192 ± 0.005
0.029 ± 0.001
0.089 ± 0.002
3.207 ± 0.027
0.380 ± 0.010
0.829 ± 0.007
0.365 ± 0.003
0.138 ± 0.000
0.165 ± 0.001
1.117 ± 0.007
0.037 ± 0.001
0.694 ± 0.004
1.259 ± 0.010
0.609 ± 0.002
26.470 ± 0.094
1.956 ± 0.014
0.412 ± 0.003
1.020 ± 0.008
0.640 ± 0.004
0.599 ± 0.003
4.552 ± 0.104
2.426 ± 0.008
2.150 ± 0.012
1.059 ± 0.004
0.619 ± 0.003
4.395 ± 0.012
3.314 ± 0.014
0.997 ± 0.005
2.438 ± 0.012
0.268 ± 0.002
0.092 ± 0.001
0.833 ± 0.005
0.137 ± 0.001
0.354 ± 0.002
0.392 ± 0.002
0.279 ± 0.001
2.905 ± 0.012
0.131 ± 0.000
2.550 ± 0.010
0.422 ± 0.002
1.211 ± 0.002
0.384 ± 0.001
1.051 ± 0.001
Terra Nova Bay
TNB-1
TNB-2
TNB-3
TNB-5
TNB-6
TNB-9
5.758 ± 0.007
4.932 ± 0.006
6.286 ± 0.008
4.224 ± 0.006
5.478 ± 0.007
6.167 ± 0.008
146 ± 2
145 ± 2
144 ± 2
143 ± 2
145 ± 2
147 ± 2
1.042 ± 0.005
5.010 ± 0.014
5.091 ± 0.014
0.851 ± 0.003
2.899 + 0.007
4.001 ± 0.011
Sample
U conc.
(ppm)
Th conc.
(ppb)
230
(232Th/238U)
(×106)
(230Th/238U)
Raw age
(kyr)
Cor. age
(kyr)
0.001 ± 0.001
0.044 ± 0.011
0.066 ± 0.032
0.109 ± 0.016
0.088 ± 0.034
0.169 ± 0.038
1.083 ± 0.030
1.100 ± 0.078
1.901 ± 0.038
1.985 ± 0.035
1.778 ± 0.027
1.849 ± 0.029
1.709 ± 0.020
1.919 ± 0.138
2.455 ± 0.026
2.242 ± 0.046
2.824 ± 0.054
2.844 ± 0.033
3.001 ± 0.036
2.576 ± 0.036
3.322 ± 0.040
2.969 ± 0.038
4.570 ± 0.037
3.586 ± 0.095
6.373 ± 0.026
6.221 ± 0.044
5.648 ± 0.039
4.848 ± 0.051
6.735 ± 0.031
6.232 ± 0.044
6.230 ± 0.048
5.430 ± 0.038
5.684 ± 0.090
6.628 ± 0.120
6.190 ± 0.052
6.125 ± 0.094
6.315 ± 0.088
7.264 ± 0.068
6.376 ± 0.050
5.217 ± 0.038
6.757 ± 0.077
4.687 ± 0.059
5.534 ± 0.045
82.742 ± 0.364
106.932 ± 1.100
89.393 ± 0.224
49.4 ± 0.2
163.7 ± 0.7
27.2 ± 1.3
10.7 ± 0.3
2.7 ± 0.1
5.1 ± 0.1
190.6 ± 1.6
23.1 ± 0.6
41.0 ± 0.4
17.8 ± 0.1
7.8 ± 0.0
9.3 ± 0.0
68.7 ± 0.4
2.0 ± 0.1
30.8 ± 0.2
63.9 ± 0.5
28.5 ± 0.1
1590.5 ± 6.0
105.0 ± 0.8
26.3 ± 0.2
50.5 ± 0.4
35.6 ± 0.2
29.4 ± 0.1
299.6 ± 6.9
103.2 ± 0.4
100.4 ± 0.6
54.2 ± 0.2
37.2 ± 0.2
192.3 ± 0.6
156.3 ± 0.7
47.1 ± 0.2
134.1 ± 0.7
14.0 ± 0.1
4.2 ± 0.0
39.2 ± 0.2
6.8 ± 0.0
16.7 ± 0.1
16.3 ± 0.1
13.2 ± 0.1
170.6 ± 0.7
5.9 ± 0.0
177.4 ± 0.8
24.9 ± 0.1
61.1 ± 0.2
18.8 ± 0.1
63.1 ± 0.1
0.0000 ± 0.0000
0.0007 ± 0.0002
0.0010 ± 0.0005
0.0011 ± 0.0002
0.0015 ± 0.0006
0.0018 ± 0.0004
0.0120 ± 0.0003
0.0125 ± 0.0009
0.0175 ± 0.0004
0.0180 ± 0.0003
0.0187 ± 0.0003
0.0194 ± 0.0003
0.0196 ± 0.0002
0.0195 ± 0.0014
0.0203 ± 0.0002
0.0212 ± 0.0004
0.0247 ± 0.0005
0.0319 ± 0.0004
0.0301 ± 0.0004
0.0307 ± 0.0004
0.0307 ± 0.0004
0.0308 ± 0.0004
0.0419 ± 0.0004
0.0440 ± 0.0012
0.0506 ± 0.0002
0.0542 ± 0.0004
0.0540 ± 0.0004
0.0544 ± 0.0006
0.0550 ± 0.0003
0.0548 ± 0.0004
0.0549 ± 0.0004
0.0557 ± 0.0004
0.0553 ± 0.0009
0.0560 ± 0.0010
0.0544 ± 0.0005
0.0567 ± 0.0009
0.0557 ± 0.0008
0.0562 ± 0.0005
0.0563 ± 0.0004
0.0572 ± 0.0004
0.0571 ± 0.0007
0.0609 ± 0.0008
0.0610 ± 0.0005
0.7787 ± 0.0040
0.9745 ± 0.0101
1.0021 ± 0.0028
− 0.05 ± 0.00
0.01 ± 0.02
0.04 ± 0.04
0.05 ± 0.02
0.09 ± 0.06
0.12 ± 0.04
1.10 ± 0.03
1.14 ± 0.09
1.63 ± 0.03
1.68 ± 0.03
1.75 ± 0.03
1.80 ± 0.03
1.83 ± 0.02
1.82 ± 0.14
1.90 ± 0.02
1.99 ± 0.04
2.32 ± 0.05
3.03 ± 0.04
2.84 ± 0.04
2.90 ± 0.04
2.91 ± 0.04
2.92 ± 0.04
4.01±0.04
4.22 ± 0.12
4.75 ± 0.02
5.23 ± 0.04
5.23 ± 0.04
5.26 ± 0.06
5.32 ± 0.03
5.31 ± 0.04
5.31 ± 0.04
5.37 ± 0.04
5.34 ± 0.09
5.42 ± 0.10
5.26 ± 0.05
5.47 ± 0.09
5.39 ± 0.08
5.44 ± 0.05
5.44 ± 0.05
5.51 ± 0.04
5.53 ± 0.07
5.90 ± 0.08
5.91 ± 0.05
126.26 ± 1.33
239.11 ± 9.16
265.48 ± 4.14
− 0.08 ± 0.09
− 0.04 ± 0.11
0.03 ± 0.08
0.05 ± 0.06
0.09 ± 0.08
0.11 ± 0.07
1.03 ± 0.13
1.13 ± 0.11
1.61 ± 0.08
1.67 ± 0.07
1.74 ± 0.06
1.80 ± 0.06
1.81 ± 0.08
1.81 ± 0.15
1.89 ± 0.07
1.97 ± 0.09
2.31 ± 0.08
2.45 ± 0.63
2.80 ± 0.10
2.89 ± 0.08
2.89 ± 0.08
2.91 ± 0.08
4.00 ± 0.07
4.11 ± 0.20
4.71 ± 0.09
5.19 ± 0.10
5.21 ± 0.08
5.25 ± 0.09
5.25 ± 0.13
5.25 ± 0.12
5.29 ± 0.08
5.32 ± 0.11
5.34 ± 0.11
5.42 ± 0.12
5.25 ± 0.08
5.47 ± 0.10
5.38 ± 0.10
5.44 ± 0.08
5.44 ± 0.07
5.45 ± 0.12
5.53 ± 0.09
5.83 ± 0.14
5.90 ± 0.08
126.20 ± 1.34
239.05 ± 9.16
265.23 ± 4.15
0.176 ± 0.020
0.180 ± 0.012
1.254 ± 0.060
0.528 ± 0.008
1.124 ± 0.009
1.617 ± 0.018
59.2 ± 0.3
332.6 ± 1.0
265.2 ± 0.8
66.0 ± 0.2
173.3 ± 0.5
212.4 ± 0.6
0.0019 ± 0.0001
0.0022 ± 0.0001
0.0122 ± 0.0001
0.0263 ± 0.0004
0.0125 ± 0.0001
0.0160 ± 0.0002
0.13 ± 0.01
0.16 ± 0.01
1.12 ± 0.01
0.68 ± 0.01
1.15 ± 0.01
1.49 ± 0.02
0.10 ± 0.07
0.04 ± 0.17
1.02 ± 0.15
0.65 ± 0.08
1.09 ± 0.12
1.41 ± 0.13
Th conc.
(ppt)
The new results also can be used in conjunction with the
commonly used CALIB programme for conversion of 14C ages to
calendar ages (http://calib.qub.ac.uk/calib/). For marine ages, that
programme calls for a delta-R value, where this value describes the
regional deviation in reservoir age from the global average value. The
results presented here indicate that, for Holocene Southern Ocean
samples, a delta-R value of 791 ± 121 should be applied. In detail,
because mean ocean reservoir ages change slightly through the
Holocene, a more precise method for correcting marine samples
would be to apply a time-dependent delta-R, ensuring that the
Southern Ocean reservoir age remains constant (see SM for further
details).
5. Implications for the global ocean and ocean circulation
The reconstructed natural Δ14C of − 133 ± 13‰ agrees with
modern seawater measurements corrected for bomb radiocarbon
using the relationship between 14C and alkalinity (Key, 2004). Earlier
techniques for the separation of bomb and natural 14C made use of the
relationship with silica (Broecker et al., 1995) but were found to be
120
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
Table 2
U/Th and 14C ages for Ross Sea solitary corals, with calculated 14C parameters. U/Th errors are 2σ, 14C are 1σ (following convention in the literature for the two chronometers). Full
details of the calculation of 14C parameters are given in the SM. Briefly: “Raw 14C age” is that provided directly by analysis at the radiocarbon lab; “Calc. 14C age” is the U/Th age
converted to an atmospheric 14C value at the time the coral lived using INTCAL04; “Res. Age” is the difference between 14C age of the atmosphere and the surface water at that time
(except for the first two samples and K78-64, which are from the post-bomb period and have reservoir ages calculated relative to the 1950 atmosphere); “R′(t)” is the Res. Age
expressed against the Southern Hemisphere calibration curve and is the value that should be applied to 14C dates for a local reservoir correction; ΔR is the deviation from the mean
ocean value and should be used with the CALIB program; and “Δ14C” is Res. Age expressed as the 14C content of the surface waters relative to the atmosphere in parts per thousand.
“Dist” is the distance from Black Island for those samples along or close to the Black Island debris band. Superscripts after the sample number refer to locations on Fig. 2. All samples
represent single analyses, except for K-76-50A, for which three U/Th analyses were averaged. Sample shown in grey text was rejected because of an anomalous δ234U value.
14
C age (years) Calc.
14
C age (years) Res. age (years) R′(t) (years) ΔR (years) Δ14C (‰)
Dist. (km)
Sample with map key U/Th Age (years) Lab number
Raw
McMurdo ice shelf
K78-671
K78-662
370 ± 25
modern
130 ± 7
− 354 ± 23
44 ± 21
1119 ± 21
117 ± 7
1002 ± 22
962
644
− 117.3 ± 2.4
1280 ± 30
modern
127 ± 8
130 ± 8
1153 ± 31
82 ± 24
1113
769
− 133.7 ± 3.4
10.2 ± 3.0
2470 ± 35
2284 ± 21
1115 ± 13
1203 ± 14
1355 ± 37
1081 ± 25
1315
1041
1044
699
− 155.2 ± 3.9
− 125.9 ± 2.8
2715 ± 24
1709 ± 15
1006 ± 28
966
677
− 117.7 ± 3.1
2803 ± 24
1759 ± 15
1044 ± 28
1004
722
− 121.9 ± 3.1
2971 ± 24
1808 ± 15
1163 ± 28
1123
828
− 134.8 ± 3.1
2967 ± 25
1855 ± 15
1112 ± 29
1072
774
− 129.3 ± 3.2
3060 ± 30
3010 ± 45
2997 ± 21
1864 ± 15
1946 ± 14
2019 ± 13
1196 ± 34
1064 ± 47
978 ± 25
1156
1024
938
847
725
640
− 138.3 ± 3.6
− 124.1 ± 5.2
− 114.6 ± 2.7
3349 ± 25
2275 ± 14
1074 ± 29
1034
744
− 125.1 ± 3.1
3594 ± 25
2408 ± 14
1186 ± 29
1146
862
− 137.3 ± 3.1
4040 ± 55
3722 ± 22
2697 ± 14
2789 ± 15
1343 ± 57
933 ± 27
1303
893
1011
610
− 154.0 ± 6.0
− 109.7 ± 3.0
3850 ± 35
3847 ± 25
2796 ± 15
2807 ± 15
1054 ± 38
1040 ± 29
1014
1000
731
716
− 123.0 ± 4.2
− 121.4 ± 3.2
4630 ± 35
4642 ± 22
3674 ± 13
3750 ± 13
956 ± 37
892 ± 26
916
852
594
517
− 112.2 ± 4.1 13.0
− 105.1 ± 2.9
5930 ± 55
5874 ± 23
4148 ± 13
4497 ± 12
1782 ± 57
1377 ± 26
1742
1337
1415
1009
− 199.0 ± 6.0 16.9
− 157.5 ± 2.7 22.4
5580 ± 40
5742 ± 26
4526 ± 11
4547 ± 12
1054 ± 41
1195 ± 29
1014
1155
683
838
− 123.0 ± 4.5
− 138.2 ± 3.1
5844 ± 24
4550 ± 12
1294 ± 27
1254
911
− 148.8 ± 2.8
5782 ± 26
4570 ± 12
1212 ± 29
1172
840
− 140.0 ± 3.1
5810 ± 35
5806 ± 26
4591 ± 12
4608 ± 12
1219 ± 37
1198 ± 29
1179
1158
843
809
− 140.8 ± 4.0
− 138.5 ± 3.1 25.8
5851 ± 23
4656 ± 12
1195 ± 26
1155
819
− 138.2 ± 2.8
5844 ± 26
4704 ± 12
1140 ± 29
1100
762
− 132.3 ± 3.1 23.7
5836 ± 21
4702 ± 12
1134 ± 24
1094
754
− 131.7 ± 2.6
6020 ± 55
5910 ± 40
6180 ± 45
6239 ± 27
4712 ± 12
4775 ± 12
5085 ± 13
5128 ± 14
1308 ± 56
1135 ± 42
1095 ± 47
1111 ± 30
1268
1095
1055
1071
939
768
711
711
− 150.3 ± 6.0
− 131.8 ± 4.5 21.9
− 127.4 ± 5.1 26.8
− 129.2 ± 3.3 28.7
1406 ± 39
1356 ± 42
2303 ± 48
1929 ± 38
2453 ± 54
2590 ± 38
128 ± 8
99 ± 7
1099 ± 13
692 ± 12
1173 ± 14
1512 ± 14
1278 ± 40
1257 ± 43
1204 ± 50
1237 ± 40
1280 ± 56
1078 ± 40
1238
1217
1164
1197
1240
1038
941
905
827
904
966
758
− 147.1 ± 4.2
− 144.9 ± 4.5
− 139.2 ± 5.3
− 142.7 ± 4.3
− 147.3 ± 5.9
− 125.6 ± 4.4
K78-66A2
3
− 45 ± 113
28 ± 77
49 ± 60
88 ± 78
115 ± 68
K81-1C
K78-644
K76-245
K76-24A5
1028 ± 126
1133 ± 106
K81-666
1613 ± 77
K76-23A7
1675 ± 68
K76-258
1743 ± 63
9
1800 ± 65
7
K76-23
K76-2210
K81-29
1806 ± 82
1891 ± 68
1966 ± 88
K81-51A11
2313 ± 79
K81-2A
K76-55C
12
2454 ± 630
K76-5512
K81-4713
2805 ± 99
2887 ± 77
K76-2014
K76-55A12
2895 ± 81
2908 ± 77
K76-17A15
K81-5111
4002 ± 74
4112 ± 200
K76-1316
K76-10C17
4710 ± 94
5191 ± 100
K81-4519
K76-10B17
5249 ± 89
5249 ± 128
K76-10D17
5255 ± 119
K77-920
5291 ± 84
17
K76-10
K76-8A21
5318 ± 111
5339 ± 106
K76-50A18
5379 ± 101
K76-922
5437 ± 81
K76-50
18
K76 10-2
K76-5123
K76-724
K76-525
5438 ± 75
17
Terra Nova Bay
TNB-1
TNB-2
TNB-3
TNB-5
TNB-6
TNB-9
5450 ± 125
5531 ± 87
5835 ± 143
5904 ± 82
103 ± 75
41 ± 172
1020 ± 149
655 ± 78
1086 ± 116
1406 ± 131
OS-42499
SUERC3943
SUERC5901
OS-42497
SUERC3942
OS-42560
SUERC5890
SUERC3951
SUERC5889
SUERC5892
SUERC5904
OS-42559
OS-42558
SUERC3945
SUERC5907
SUERC5897
OS-42562
SUERC3947
OS-42557
SUERC5894
OS-42556
SUERC3948
OS-42555
SUERC5884
OS-42498
SUERC3935
SUERC5887
SUERC5900
OS-42501
SUERC3838
SUERC5893
SUERC3941
SUERC3937
OS-42554
OS-42561
OS-42500
SUERC3936
AA-56092
AA-56091
AA-56088
AA-56094
AA-56090
AA-56093
− 45.0 ± 3.0
5.5 ± 2.6
4.0
5.1
6.0
8.2
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
121
Fig. 5. Top: reservoir age and Δ14C for coral samples from McMurdo Ice Shelf (MCM) and Terra Nova Bay (TNB), as well as data from historical samples (Berkman and Forman, 1996;
Björck et al., 1991; Mabin, 1985; Stuiver et al., 1981; Whitehouse et al., 1989). Grey band shows average reservoir value with 1-sigma range. Note the constancy of reservoir age and
the recent invasion of bomb 14C; Bottom: distance versus age for samples from the Black Island medial moraine, showing range of average ice velocities.
unreliable at high latitude and in surface waters (Rubin and Key,
2002). The alkalinity method, though more successful, is by the
admission of the original authors, “based on circumstantial evidence”.
Given the importance of this technique for establishing whole-ocean
bomb radiocarbon inventories and, of particular relevance here, the
radiocarbon composition of Antarctic Bottom Water (AABW), it is
reassuring that the alkalinity technique generates the same Δ14C as
our new direct assessment for high-latitude surface ocean waters.
Models have struggled to replicate reservoir ages as great as
1100 years in the surface Southern Ocean, but these new observational constraints suggest that such a value should be a tuning target
for ocean carbon models. Butzin et al. (2005), for instance, derived
model Southern Ocean surface waters of ≈600 years (≈−75‰ Δ14C)
and only generated values close to those observed here with
significant sea-ice cover in the Southern Ocean and reduced North
Atlantic Deep Water (NADW) flow. Similarly, Muller et al. (2008), in
an explicit investigation of the role of air–sea exchange in setting
ocean 14C distributions, failed to replicate the reservoir ages seen here
even with quite dramatic changes to gas exchange parameters. In both
cases these models are of intermediate complexity and may suffer
from too much lateral mixing in the Southern Ocean. In such cases
where reservoir ages in the formation region for AABW are incorrect,
122
B.L. Hall et al. / Earth and Planetary Science Letters 296 (2010) 115–123
care must be taken that models are not getting a good match to
observed distribution of 14C values in the deep-ocean for the wrong
reasons.
The constancy of Δ14C in the Southern Ocean during the last
6000 years places constraints on changes in the global pattern of
ocean circulation. Southern Ocean deep-waters (Δ14C ≈ − 150‰) can
be considered as a mixture of Atlantic waters (Δ14C = −90‰) and
Pacific waters (Δ14C = − 200‰) (Matsumoto and Key, 2004). Air–sea
exchange at the surface lowers Δ14C from −150‰ to −133 ‰ in
surface waters. The lack of change in this surface value during the last
6000 years is most readily explained by approximate constancy in
both air–sea exchange (e.g. similar winds and sea–ice cover) and in
the relative mixture of Atlantic and Pacific waters to the Southern
Ocean. Although it is possible that complementary changes in air–seaexchange and ocean circulation might lead to a constant Southern
Ocean reservoir age, it would be very unlikely for these changes to
exactly cancel one another in magnitude and timing, so constancy of
conditions appears more likely. Models suggest sensitivities of
Southern Ocean surface reservoir ages of as much as 500 years due
to changes in sea ice and NADW strength (Butzin et al., 2005). Such
models typically have focused on the large changes associated with
last ice age (Butzin et al., 2005) or the Younger Dryas (Ritz et al.,
2008). For the Holocene period studied here, a simple mass balance
can be used to interpret the changes in relative NADW strength
through time. To keep surface ocean Δ14C within 13‰ of − 133‰,
while assuming a constant rate of air–sea exchange and Δ14C values
for the Atlantic and Pacific as above, mass balance suggests that
changes in the relative fraction of Atlantic input to the Southern Ocean
cannot have been larger than 25% about the modern value. Such a
change of flow is broadly consistent with previous estimates of NADW
flow from proxies such as 231 Pa/230Th (McManus et al., 2004) or Nd
isotopes (Piotrowski et al., 2005).
6. Ice-shelf flow rates
The chronology of these coral samples also provides information
about the flow rate of the McMurdo Ice Shelf during the last
6000 years. Ice shelves are an important feature of the Antarctic
environment and are thought to buttress the continental ice sheets
that feed them, thereby influencing ice volume and sea level. Satellite
and field observations provide data for the modern flow-rate of ice
shelves but, although sedimentary archives can provide information
about the presence or absence of ice shelves though time, there are no
existing archives that provide quantitative information about past
flow rates. Kellogg et al. (1990) first suggested that materials captured
by the ice shelves could be used to calculate ice velocity, but could not
perform this calculation accurately at the time due to poor marine
reservoir constraints and lack of U/Th chronology. For the 11 samples
from the Black Island medial moraine found as much as ∼29 km from
the present-day grounding zone (Fig. 5), the age-distance relationship
can be explained most simply by relatively constant ice flow across
the grounding line with gradual acceleration towards the calving front
as a result of ice-shelf conservation of mass and momentum (e.g.,
Schoof, 2007). Such flow-velocity patterns are observed on many
Antarctic ice shelves today (e.g. Thomas and MacAyeal, 1982). The
only observations of modern ice-shelf flow rates from the Black Island
medial moraine are from still closer to the calving front and are
consistent with our data and interpretation (Glasser et al., 2006).
Large changes in ice-shelf flow rates observed recently on the
Antarctic Peninsula have been linked to warming climate and
expansion of surface melt (e.g. Cook and Vaughan, 2009; Scambos
et al., 2000; Vieli et al., 2007). The fact that such changes in flow rates
apparently have not occurred on the McMurdo Ice Shelf, despite
evidence for warmer-than-present ocean conditions ∼ 6000–
1000 years ago, inferred from nearby marine mammal remains (Hall
et al., 2006), suggests that air temperatures were not sufficiently high
to induce widespread surface melt zones on the ice shelf.
7. Conclusions
We document the Southern Ocean marine reservoir effect over
the past 6000 years and find that it has not varied significantly from
1144 ± 120 years (1 s.d.) (corresponding to a Δ14C of −133 ± 13‰).
This implies that use of a constant Southern Ocean reservoir correction for the mid-to-late Holocene is valid, although there needs to
be consistency in the method by which this correction is applied. The
relatively constant Δ14C values over this time also have implications
for ocean circulation, and suggest that changes in Atlantic input to the
Southern Ocean cannot have been larger than 25% of the modern flow
to maintain Δ14C within 13‰ of −133 ‰ as these new Ross Sea
results indicate. The data also provide a first Holocene history of iceshelf flow rates and indicate a constant rate, despite variable climate
in the region.
Contributions
BLH initiated the project to determine the past reservoir effect in
the Southern Ocean by dating solitary corals. The corals were from
collections made by CB and TBK. BLH and GMH analysed the corals
and made the reservoir calculations. GMH made the Δ14C calculations
and ocean circulation interpretations. BLH and GMH wrote the paper
with contributions from all authors.
Acknowledgements
This work was supported by the Office of Polar Programs of the
National Science Foundation, the Italian Antarctic Research Program,
and the National Environmental Research Council (NERC). Radiocarbon ages were provided by the NOSAMS facility at Woods Hole
Oceanographic Institution and by the NERC Radiocarbon facility at
East Kilbride. We especially would like to thank Dr. Paula Reimer, who
answered numerous questions concerning the calculation of marine
radiocarbon reservoir effects. Two reviewers and Editor P. Delaney
provided very helpful comments.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.epsl.2010.04.054.
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