Rafter
et al
.,
Sci. Adv.
8
, eabq5434 (2022) 16 November 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
1 of 9
OCEANOGRAPHY
Global reorganization of
deep-sea circulation
and
carbon storage after the
last ice age
Patrick A.
Rafter
1
*, William R.
Gray
2
, Sophia K.V.
Hines
3
, Andrea
Burke
4
, Kassandra M.
Costa
3
,
Julia Gottschalk
5
, Mathis P.
Hain
6
, James W.B.
Rae
4
, John R.
Southon
1
, Maureen H.
Walczak
7
,
Jimin Yu
8,9
, Jess F.
Adkins
10
, Timothy
DeVries
11
Using new and published marine fossil radiocarbon (
14
C/C) measurements, a tracer uniquely sensitive to circulation
and air-sea gas exchange, we establish several benchmarks for Atlantic, Southern, and Pacific deep-sea circula-
tion and ventilation since the last ice age. We find the most
14
C-depleted water in glacial Pacific bottom depths,
rather than the mid-depths as they are today, which is best explained by a slowdown in glacial deep-sea overturning
in addition to a “flipped” glacial Pacific overturning configuration. These observations cannot be produced by
changes in air-sea gas exchange alone, and they underscore the major role for changes in the overturning circulation
for glacial deep-sea carbon storage in the vast Pacific abyss and the concomitant drawdown of atmospheric CO
2
.
INTRODUCTION
The ocean’s ability to store and release carbon via changes in biology,
chemistry, and physics (
1
) makes it a prime candidate for driving
changes in glacial-interglacial atmospheric carbon dioxide (CO
2
)
and the global ice ages of the late Pleistocene. Physical changes in
deep-sea ventilation—the combined influence of air-sea gas ex-
change and circulation-driven transfer of gases, including CO
2
—
are especially important because they can alter the carbon storage
capacity of the ocean. Several studies indicate reduced deep-sea
ventilation during glacial periods (
2
–
6
), which has been linked to a
slower ocean overturning rate (
7
). However, other studies suggest
that the ventilation proxy data are indistinguishable from modern
circulation (
8
,
9
), instead suggesting that the reduced glacial deep-
sea ventilation could solely derive from reduced air-sea gas ex-
change in the subpolar Southern Ocean (
10
–
12
), although these
studies have typically used datasets that poorly constrain the Pacific
Ocean. Differentiating between these two drivers of glacial deep-sea
ventilation is valuable for determining the climate and carbon cycle
dynamics underpinning glacial-interglacial variations in CO
2
. If
glacial ocean circulation was no different from today, the apparent
glacial reduction in deep-sea ventilation and amplification of ocean
carbon sequestration would derive from relatively small-scale, high-
latitude surface ocean conditions. This contrasts with reduced gla-
cial deep-sea ventilation and deep ocean carbon storage that is
compelled by basin- or even global-scale change in overturning cir-
culation. Here, using a compilation of new and published marine
fossil radiocarbon (
14
C) measurements (Fig. 1A), we reconstruct
glacial-interglacial marine
14
C/C along the density surfaces of
modern ocean overturning in all major ocean basins over the past
25,000 years (25 ka). When compared to new stable carbon isotope
measurements and numerical model results, our deep-sea
14
C/C data-
set suggests that glacial ventilation of the Pacific Ocean, the largest
ocean basin on Earth and therefore a key carbon reservoir, was pro-
foundly different from today and cannot be solely explained by a
change in air-sea gas exchange.
The radiocarbon content of seawater (
14
C/C, commonly ex-
pressed as a
14
C age or as
14
C; see the Supplementary Materials for
more information) has long been recognized as a “most useful tracer”
(
13
) of ocean circulation and ventilation. This is because
14
C is in-
troduced to the global ocean via the air-sea exchange of CO
2
, and as
surface waters descend into the ocean interior, seawater
14
C/C is
progressively lowered only by radioactive decay or mixing with
water masses with lower
14
C/C levels. Therefore, outside of rare in-
stances of significant geologic carbon addition from the seafloor
(
14
,
15
), the
14
C/C of seawater reflects the same processes affecting
the partitioning of CO
2
between the ocean and the atmosphere, i.e.,
changes in ocean biology, chemistry, and physics [see (
1
)]. There
are various methods for interpreting marine fossil
14
C/C [see (
16
–
19
)],
and here, we use the conventional “marine fossil–minus–the con-
temporaneous atmosphere” method because it is the simplest ap-
proach that is applicable to all observations, including both benthic
foraminifera and deep-sea corals. As per convention, this “
14
C ven-
tilation age” is used throughout our study. All new and published
planktic foraminifera
14
C age models (those requiring calibration to
atmospheric
14
C/C) were updated to the most recent IntCal20 at-
mospheric
14
C/C (
20
), with regionally appropriate reservoir ages
(see the Supplementary Materials). This multisubstrate archive allows
us to provide a comprehensive view of
14
C/C in the deep Atlantic,
Southern, and Pacific Oceans. A paucity of glacial-interglacial
14
C/C
observations from the deep Indian Ocean (Fig. 1A) does not allow
us to estimate basin-scale values, and this region will not be further
discussed in our study.
MATERIALS AND METHODS
New radiocarbon analyses were made using sediment cores from
the subarctic North Pacific, central North Pacific, and Subantarctic
sites (see diamonds in Fig. 1A and fig. S4). Each sample was washed
using deionized water in a 63-
m sieve, and mixed benthic fora-
minifera species (notably without
Pyrgo
spp.) were selected from
1
University of California, Irvine, Irvine, CA, USA.
2
Laboratoire des Science du Climat et
de l’Environnement (LSCE/IPSL), Université-Paris-Saclay, Gif-sur-Yvette, France.
3
Woods Hole Oceanographic Institution, Woods Hole, MA, USA.
4
University of
St. Andrews, St. Andrews, Scotland, UK.
5
Institute of Geosciences, Kiel University,
Kiel, Germany.
6
University of California, Santa Cruz, Santa Cruz, CA, USA.
7
Oregon
State University, Corvallis, OR, USA.
8
Pilot National Laboratory for Marine Science
and Technology (Qingdao), Qingdao 266237, China.
9
Australia National University,
Canberra, Australia.
10
California Institute of Technology, Pasadena, CA, USA.
11
De-
partment of Geography and Earth Research Institute, University of California, Santa
Barbara, CA, USA.
*Corresponding author. Email: prafter@uci.edu
Copyright © 2022
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
License 4.0 (CC BY).
Rafter
et al
.,
Sci. Adv.
8
, eabq5434 (2022) 16 November 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
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the >250-
m fraction. Samples were graphitized using standard Keck
Carbon Cycle Accelerator Mass Spectrometry Laboratory methods
at University of California, Irvine, including a 10% leach of each
sample to remove potential secondary calcite. More details on the
pretreatment, graphitization, and assigning the calendar ages for
sample depths are detailed in the Supplementary Materials.
RESULTS
On the basis of this ensemble of measurements (Fig. 1A), we find
that marine fossil
14
C/C recorded since 4 ka before the present (BP)
[as in (
21
)] from all depths compares well with pre-1950 seawater
(pretreated by removing
14
C influenced by 20th-century thermonu-
clear weapons testing; see the Supplementary Materials) (Fig. 1B).
The seawater-proxy
14
C/C comparison in Fig. 1B illustrates the clas-
sic global ocean “conveyor belt” (
22
), where the Atlantic (green) has
the highest and the Pacific (brown) has the lowest seawater
14
C/C
(low
14
C or old
14
C ventilation age), providing a first-order valida-
tion of the
14
C ventilation proxy (Fig. 1A). Additional sensitivity
analyses (fig. S1) indicate a robust, linear relationship between ma-
rine fossil
14
C/C and preindustrial seawater
14
C/C, with slopes vary-
ing from 0.96 ± 0.1 to 1.03 ± 0.1 and
R
2
value from 0.60 to 0.82 (all
with
P
<< 0.001). The age models used by each study are shown as
symbols in Fig. 1B (and fig. S1) and indicate that age model assump-
tions do not impose a systematic bias on our results.
Modern deep-sea
14
C ventilation ages and
global
overturning circulation
The relatively young Atlantic and old Pacific
14
C ventilation ages in
Fig. 1B arise from the overturning of upper and lower circulation
cells that intertwine like the loops of a “figure 8” (
23
). We show this
circulation and modern Atlantic, Southern, and Pacific
14
C ventila-
tion ages as a “boomerang” plot in Fig. 2A to emphasize both the
Southern Ocean’s central role within the interlocking loops of the
figure 8 and the vastly different ocean volumes (the Southern Ocean
is simplified to average values between density surfaces in all plots
of Fig. 2). Following the arrows in Fig. 2A, the formation of the
North Atlantic Deep Water (NADW) introduces well-equilibrated
surface waters (and therefore young
14
C ventilation ages) to the
deep sea [here defined as greater than or equal to the 27.5 kg m
−3
neutral density surface (
n
)]. These well-equilibrated Atlantic waters
move south, joining older
14
C age Indian and Pacific Deep Water in
the deep Southern Ocean (
23
). These mixed Southern Ocean waters
(
24
) upwell along isopycnals to the Southern Ocean surface, and the
least dense waters (here,
n
< 27.5 kg m
−3
) move equatorward (
25
),
eventually reconnecting with the North Atlantic surface (see dots in
Fig. 2A). The densest of these upwelled Southern Ocean waters
spend a relatively brief time at the surface such that the
14
C ventila-
tion age is not reset to the contemporaneous atmosphere (
26
,
27
),
making this relatively small subpolar region an important zone for
deep-sea ventilation (
10
–
12
). The densification of these subpolar
surface waters on the periphery of the Antarctic continent forms
Antarctic Bottom Water (AABW) (
28
,
29
), here defined as
n
>
28 kg m
−3
(see the Supplementary Materials). Because they have not
fully equilibrated with the atmosphere, AABW introduces a large
“preformed”
14
C ventilation age to global bottom waters (
26
). The
closure of this modern deep-sea overturning (i.e., the reconnection
of the upper and lower loops of the figure 8) occurs between the
Pacific bottom and mid-depth waters via diapycnal mixing (
23
)
(dashed blue arrow in Fig. 2A). Since there is no deep-water forma-
tion in the modern North Pacific, this diapycnal mixing is one of the
only mechanisms for delivering
14
C/C to the mid-depth Pacific,
placing it downstream of the abyssal Pacific. The result of this mod-
ern ocean overturning circulation is that the modern mid-depth
Pacific Ocean—not Pacific bottom water—has the oldest
14
C venti-
lation ages of any water mass in the modern ocean.
DISCUSSION
Reduced deep-sea ventilation during the
LGM
Our compilation of new and published marine fossil
14
C/C mea-
surements illustrate that the
14
C ventilation age of the deep Atlantic,
Southern, and Pacific Oceans were collectively much older during
the Last Glacial Maximum (LGM; 23 to 18 ka BP) than today
(Fig. 2D and fig. S2). Following the LGM, our data show a general
decrease in
14
C ventilation age for most deep-sea basins in time slices
during the deglaciation (Fig. 2, B and C). To quantify the basin-scale
changes in
14
C ventilation, we apply a simple depth-bias correction
AB
Fig. 1. An ensemble of marine fossil
14
C/C and a first-order test of fidelity.
(
A
) Locations for all deep-sea proxy seawater
14
C/C measurements used in this study, all of
which are below the neutral density surface (
n
) = 27.5 kg m
−3
(below the depth of intermediate water masses). Open symbols are mid-depth sites above or on the neutral
density surface
n
= 28.0 kg m
−3
. Closed symbols are
n
> 28.0 kg m
−3
“bottom water” sites. Diamonds are sites with new data provided in this study. The basin colors in (A)
identify the location of measurements in (
B
), which compares preindustrial seawater dissolved inorganic carbon (DIC) from (
30
) and all compiled proxy
14
C/C (differenced
from the contemporaneous atmosphere) over the past 4 ka (see fig. S1 for additional age ranges). The dashed line in (B) is 1:1, the solid line is the slope, and the red en-
velope is 68 and 95% error range.
Rafter
et al
.,
Sci. Adv.
8
, eabq5434 (2022) 16 November 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
3 of 9
as well as locally weighted/estimated scatter plot smooting (LOESS)
smoothing of our proxy data over the past 25 ka (tables S1 and S2;
details in the Supplementary Materials). LOESS provides a conserva
-
tive estimate of the deep-sea
14
C ventilation age because it applies
different weights to values according to their distance from the binned
mean value while also considering the overall trend of the time series
when calculating the best fit. We propagate calendar age and analytical
uncertainties through to our final LOESS uncertainties (see the Sup
-
plementary Materials for more detail and for the error propagation
method). The data density is high enough that we also parse the
14
C/C
using the characteristic
n
ranges of the upper and lower loops of the
modern figure 8 configuration of ocean overturning: “mid-depth”
A
B
C
D
Fig. 2. Deep-sea
14
C ventilation ages of the LGM and early deglaciation were much older than today.
This plot shows the
14
C ventilation age of Atlantic, Southern,
and Pacific Ocean seawater and is composed of five panels, from left to right: meridional-average North Atlantic (20°N to 60°N), zonal-average Atlantic (60°N to 40°S),
Southern Ocean average within chosen density surfaces, zonal-average Pacific (60°N to 40°S), and meridional-average North Pacific (20°N to 60°N). Note that the size of
each ocean basin’s end piece—the tips of the boomerang—and the width of the Southern Ocean are scaled to the volume of each ocean (the Pacific being >2 times the
volume of the Atlantic). Symbols are site depths (color-coded for Southern Ocean density averages: Atlantic, red; Indian, black; Pacific, blue). The modern ocean
14
C ven-
tilation age (
A
) [data from (
15
,
57
)] is considerably younger than the (
D
) LGM (23 to 18 ka BP) and (
C
) the first stage of deglacial global warming (HS1; 18 to 14.7 ka BP).
However, by the (
B
) BA (14.7 to 12.8 ka BP), global
14
C ventilation ages approach modern values. Intermediate-depth
14
C ventilation ages are not shown because these
are not directly reflecting the overturning of the upper and lower cells and have known influences from the input of geologic carbon (
21
). The black lines represent neutral
density (
n
) surfaces of 27.5 and 28
kg m
−3
and provide a rough approximation for the upper and lower cells of the modern figure 8 circulation [arrows in (A)].