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A 23,000-Year Record of Surface Water pH and PCO2 in the Western Equatorial Pacific Ocean

Abstract and Figures

The oceans play a major role in defining atmospheric carbon dioxide (CO2) levels, and although the geographical distribution of CO2 uptake and release in the modern ocean is understood, little is known about past distributions. Boron isotope studies of planktonic foraminifera from the western equatorial Pacific show that this area was a strong source of CO2 to the atmosphere between approximately 13,800 and 15,600 years ago. This observation is most compatible with increased frequency of La Niña conditions during this interval. Hence, increased upwelling in the eastern equatorial Pacific may have played an important role in the rise in atmospheric CO2 during the last deglaciation.
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Mauna Loa volcano, which was not erupting
during the time period 1988 –1998. However,
our analyses relocated many clusters of earth-
quakes parallel to the southeast coast of Ha-
waii at depths of 25 to 50 km (fig. S1),
indicating the existence of several tectonic
fault zones in the mantle throughout this re-
gion. Earthquakes at these fault zones, which
occur seaward of Kilauea and Mauna Loa
volcanoes, may similarly be caused by broad-
er melt movements and the effects of volcano
loading and flexure.
References and Notes
1. J. P. Eaton, K. J. Murata, Science 132, 925 (1960).
2. F. W. Klein, R. Y. Koyanagi, J. S. Nakata, W. R. Tani-
gawa, U.S. Geol. Surv. Prof. Paper 1350, 1019 (1987).
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Res. 89, 5719 (1984).
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503 (1995).
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9. P. M. Shearer, J. Geophys. Res. 102, 8269 (1997).
10. A. M. Rubin, D. Gillard, J.-L. Got, Nature 400, 635
11. F. Waldhauser, W. L. Ellsworth, Bull. Seismol. Soc.
Am. 90, 1353 (2000).
12. R. Y. Koyanagi, B. Chouet, K. Aki, U.S. Geol. Surv. Prof.
Paper 1350, 1221 (1987).
13. B. A. Chouet, Nature 380, 309 (1996).
14. The subjective identification of LP events and tremor
is made at HVO as part of their operations. The
catalog we used only includes located LP events and
tremor and does not include events that could not be
picked and located.
15. Materials and methods are available as supporting
material on Science Online.
16. For the separate analysis of LP events, we used 21.5-s
time windows, bandpass filtered the seismic data 1 to
15 Hz, and took a correlation threshold of 0.4. Note
that of the 632 relocated events above 16-km depth
in Figs. 1A and 2A (15), 297 are high-frequency
earthquakes and 335 are LP events.
17. D. P. Hill, J. J. Zucca, U.S. Geol. Surv. Prof. Paper
1350, 903 (1987).
18. A. M. Dziewonski, G. Ekstrom, M. P. Salganik, Phys.
Earth Planet. Inter. 86, 253 (1994).
19. Materials and methods are available as supporting
material on Science Online.
20. The aftershocks of the 1994 m
5.3 earthquake make
up a large proportion of the seismicity at 30-km
depth between latitudes of 19.2° to 19.3° at –155.3°
longitude, forming a north-south–oriented line of
21. The relocation of earthquakes from 1974 to 2000
using difference pairs from the catalog travel time
data yields images consistent with Figs. 1 and 2.
However, as expected, given the larger errors in
hand-picked arrival times, the patterns are not as
sharply defined as when using arrival time differences
from cross correlation.
22. Seismic swarms are episodes when the rates of earth-
quakes are highly increased without the presence of
a large magnitude mainshock. Although the time
interval and spatial region we examined did not
display swarms, occasional earthquake swarms at
depths of 45 to 65 km were observed from the early
1950s until the fall of 1960 (26).
23. The mechanisms of shallow earthquakes along the
south flank of Hawaii in the Harvard centroid mo-
ment tensor catalog (1976 –2002) (18) generally
show seaward slip on low-angle fault planes.
24. P. T. Delaney, R. S. Fiske, A. Miklius, A. T. Okamura, M.
K. Sako, Science 247, 1311 (1990).
25. S. Owen et al., Science 267, 1328 (1995).
26. J. P. Eaton, D. H. Richter, H. Krivoy, U.S. Geol. Surv.
Prof. Paper 1350, 1307 (1987).
27. This research was supported by NSF under grant
EAR-0106357. We thank J. Battaglia, B. Brooks, G.
Ekstro¨m, J.-L. Got, F. Klein, S. Koyanagi, M. Nettles, G.
Pavlis, A. Rubin, and S. Solomon for thoughtful com-
ments. This is School of Ocean and Earth Science and
Technology (SOEST) contribution number 6219 and
Hawaii Institute of Geophysics and Planetology
(HIGP) contribution number 1272.
Supporting Online Material
Materials and Methods
Figs. S1 to S4
Table S1
References and Notes
9 January 2003; accepted 13 March 2003
A 23,000-Year Record of Surface
Water pH and PCO
in the
Western Equatorial Pacific Ocean
M. R. Palmer
* and P. N. Pearson
The oceans play a major role in defining atmospheric carbon dioxide (CO
levels, and although the geographical distribution of CO
uptake and release in
the modern ocean is understood, little is known about past distributions. Boron
isotope studies of planktonic foraminifera from the western equatorial Pacific
show that this area was a strong source of CO
to the atmosphere between
approximately 13,800 and 15,600 years ago. This observation is most com-
patible with increased frequency of La Nin˜a conditions during this interval.
Hence, increased upwelling in the eastern equatorial Pacific may have played
an important role in the rise in atmospheric CO
during the last deglaciation.
It is generally accepted that the oceans played
a major role in controlling changes in the
content of the atmosphere over glacial-
interglacial time scales (1). Although there is
uncertainty regarding the exact processes (2),
they must have involved changes in the areas
of the surface oceans that are supersaturated
or undersaturated with CO
with respect to
the atmosphere and/or changes in the magni-
tude of the difference between the partial
pressure of CO
) of surface waters and
the atmosphere. Hence, an important con-
straint on the mechanism of glacial-intergla-
cial changes in atmospheric CO
would be
provided by comparing the P
of surface
waters of the glacial ocean with those during
deglaciation and today. Oceanographic stud-
ies have resulted in maps delineating the
in surface waters of the modern ocean
(3), and we show here that boron isotope
B) analyses of planktonic foraminifera
offer the opportunity to provide similar infor-
mation about the past.
Experimental studies have shown that the
planktonic foraminifer Globigerinoides sac-
culifer faithfully records the
B of dis-
solved B(OH)
in the seawater from which
the foraminifer grew its shell (4 ), and that
this is directly related to the pH of the sea-
water. By using the pH of the seawater and
either the alkalinity or the dissolved inorganic
carbon concentrations, the P
of the waters
can be calculated (5).
The equatorial Pacific is the site of the
greatest evasion of CO
(0.8 to 1.0 Pg C
) from the modern oceans (3), and may
thus have played a role in glacial-interglacial
changes in atmospheric P
. In this study,
we determined the
B of 31 handpicked
samples of 50 G. sacculifer (500 to 600
m) from box core ERDC-92 that was raised
from the western equatorial Pacific
(2°13.5S, 156°59.9E; 1598 m) (6). The
samples range from 0.4 to 23.2 thousand
years ago (ka) (7, 8) and cover the period
from the last glacial maximum to almost the
present day.
B values were determined at
Southampton using analytical techniques that
are described in (9) and the data are listed in
Table 1. Carbon and oxygen isotopes were
measured at Cardiff using standard tech-
niques. Because boron isotope fractionation
and the dissociation of boric acid and carbon-
ate species are temperature dependent, the
Mg/Ca ratios of the selected samples were
measured and the temperatures were calculat-
ed using the Mg/Ca-temperature relationship
of Dekens et al.(10) (and by interpolating for
intervening samples).
Figure 1 illustrates the variations in
(Fig. 1A), calculated pH (Fig. 1B), and sur-
face water P
values (Fig. 1C). Also in-
cluded are atmospheric P
levels (from the
Taylor Dome ice core record (11, 12) (Fig.
School of Ocean and Earth Sciences, Southampton
Oceanography Centre, European Way, Southampton
SO14 3ZH, UK.
Department of Earth Sciences,
Cardiff University, Main Building, Post Office Box 914,
Cardiff CF10 3YE, UK.
*To whom correspondence should be addressed. E-
18 APRIL 2003 VOL 300 SCIENCE www.sciencemag.org480
1C) and the difference (PCO
) between the
calculated surface water and atmospheric
values (Fig. 1D). These plots show that
for most of the period covered by this core,
the surface water P
levels were in approx-
imate equilibrium with the atmospheric CO
(within analytical and calculation errors).
However, the interval of 13.8 to 18 ka is
marked by surface water P
values that are
substantially higher than those in the contem-
poraneous atmosphere; the period from 13.8
to 15.7 ka had P
values of 90 parts per
million by volume ( ppmv). The uncertainties
in P
values induced by analytical errors
B values and reasonable uncertainties in
other contributing variables (salinity, temper-
ature, and alkalinity) are about 35 ppmv
during this interval. Hence, the only other
factor that might lead us to question the
validity of this signal is the possibility that
B measured in the foraminifera have
been affected by postdepositional alteration.
Box core ERDC-92 was collected from a
depth of 1598 m (6). This is well above the
calcite lysocline for the past 25,000 years
(13) and well above the depth where other
foraminiferal palaeoceanographic tracers are
affected by partial dissolution (14).
In the modern western equatorial Pacific,
the strongly stratified water column inhibits
upwelling of cold nutrient-rich water into the
euphotic zone, leading to the formation of a
well-mixed warm pool with low levels of
biological productivity and surface water
levels that are in equilibrium with at-
mospheric CO
. To the east, upwelling brings
nutrient- and CO
-rich waters to the surface.
The low iron concentration of these waters
means that the nutrients are not fully used
[i.e., this is a high nutrientlow chlorophyll
(HNLC) area], and as a result, this area of the
ocean is the atmospheres largest natural
source of CO
The longitudinal divide between the warm
pool (from which ERDC-92 was recovered)
and HNLC areas moves to the east during El
Nin˜o events, such that the extent and inten-
sity of the HNLC is reduced and the thermo-
cline shallows in the western equatorial Pa-
cific. Thermocline shallowing can lead to
decreases in sea surface temperature (SST)
and increases in P
(as a result of local
wind stress variations) in the vicinity of
ERDC-92, but these increases (less than 40
ppmv) (15) are smaller than those illustrated
in Fig. 1. In general, surface water P
values vary from close to zero in normal
years to 15 ppmv during El Nin˜o periods at
the site of ERDC-92 (16). In contrast, during
La Nin˜a periods the warm pool retreats to the
west and the area of high P
surface waters
expands (17), such that P
reaches about
80 ppmv at the site of ERDC-92 (16),
although SST does not substantially change
(18). Hence, we hypothesize that the P
record in Fig. 1 would be most easily ex-
plained if there had been more frequent and/
or more intense La Nin˜a conditions between
13.8 and 15.7 ka.
The link between upwelling and produc-
tivity in the Pacific equatorial upwelling zone
Table 1.
BofG. sacculifer from core ERDC-92. Ages are derived from
C dates given by Berger and
Killingley (7) and have been converted to calendar ages according to Stuiver et al.(8).
Sample Depth (cm)
temp. (°C)
1 0–1 0.4 25.8 2.27 –2.00 8.20 0.03 243 22
2 1–2 1.1 25.5 28.6 2.20 –1.79 8.17 0.03 268 24
3 2–3 1.8 25.1 2.31 –1.46 8.13 0.03 304 29
4 3–4 2.5 25.3 28.4 2.46 –1.82 8.15 0.03 284 26
5 4–5 3.2 25.4 2.07 –2.12 8.16 0.03 274 25
6 5–6 3.9 25.5 28.3 2.18 –2.18 8.17 0.03 264 24
7 6–7 4.6 25.1 8.13 0.03 302 29
8 8–9 6.1 24.9 28.2 2.06 –1.24 8.11 0.04 322 31
9 9–10 6.8 25.2 28.1 2.11 –1.49 8.14 0.03 290 27
10 10–11 7.5 25.3 2.30 –1.56 8.15 0.03 280 26
11 11–12 8.2 25.5 28.0 2.28 –1.19 8.18 0.03 261 24
12 12–13 8.9 26.4 28.0 2.08 –1.34 8.27 0.03 195 18
13 13–14 9.6 26.0 2.14 –1.95 8.23 0.03 222 20
14 14–15 10.2 25.9 28.1 2.30 –1.59 8.22 0.03 228 22
15 15–16 10.7 26.0 28.2 2.19 –1.29 8.23 0.03 223 20
16 16–17 11.3 25.4 28.3 2.01 –1.07 8.16 0.03 272 25
17 17–18 11.9 25.4 2.03 –1.10 8.16 0.03 274 25
18 18–19 12.4 25.4 28.4 1.85 –1.65 8.16 0.03 274 25
19 19–20 12.8 25.9 2.12 –1.53 8.21 0.03 233 22
20 20–21 13.3 25.6 28.5 1.98 –1.10 8.18 0.03 258 24
21 21–22 13.8 24.9 8.10 0.03 325 31
22 23–24 14.7 24.8 28.3 1.96 0.82 8.09 0.03 335 32
23 24–25 15.2 24.8 8.10 0.03 333 32
24 25–26 15.7 24.9 28.0 1.63 0.89 8.11 0.03 320 30
25 26–27 16.1 25.4 1.93 0.70 8.17 0.03 267 24
26 27–28 16.9 25.3 27.6 2.12 –1.06 8.16 0.03 275 25
27 28–29 17.9 25.7 2.10 0.48 8.21 0.03 238 21
28 29–30 19.0 26.3 27.0 2.00 0.47 8.27 0.03 195 18
29 30–31 20.0 26.5 26.5 1.99 0.34 8.30 0.03 180 16
30 32–33 22.1 26.1 2.30 0.04 8.26 0.03 204 19
31 33–34 23.2 26.3 26.5 2.06 0.47 8.28 0.03 191 17
Fig. 1. (A)
B values measured in G. sacculifer
from box core ERDC-92. (B) Calculated pH val-
ues. (C) Calculated P
values of surface wa-
ter compared with the ice core record from
Taylor Dome (11, 12). (D) Calculated P
values between surface water and atmospheric
. All carbonate ion calculations were per-
formed according to the numerical routines
described by Zeebe and Wolf-Gladrow (5). To
calculate P
, it is necessary to assume a
value for alkalinity or dissolved inorganic car-
bon. World Ocean Circulation Experiment data
(29) indicate that the modern surface water
alkalinity at this site is 2275 eq kg
the depth range appropriate to G. sacculifer
calcification (10). There is no clear consensus
on the history of alkalinity in the oceans over
the past 25,000 years, so we have simply varied
alkalinity as a function of salinity as deduced
from the sea level curve over this time interval
(30). An uncertainty of 50 eq kg
in alka-
linity yields an average uncertainty of 5.5
ppmv in the calculated P
. Similarly, uncer-
tainties in the temperature estimates of
1.4°C (10) and 1.1 salinity units yield aver-
age uncertainties of 3.5 ppmv and 2.8
ppmv in the calculated P
values. In compar-
ison, the average uncertainty in the P
that results from the measured
B(2 errors
are generally less than 0.3 per mil) is 24.1
is complicated by the fact that this is an
HNLC zone in which upwelled nutrients are
not fully used. Nevertheless, in the western
equatorial Pacific there is an approximate
doubling of primary productivity during La
Nin˜a events relative to El Nin˜o and normal
periods (19). Although marine sedimentary
records of productivity are complicated by
issues such as preservation, sediment focus-
ing, winnowing, and boundary effects (20), it
is noteworthy that a study of variations of
estimated primary productivity (ePP) in a
core from 1°25S, 146°14E showed a peak
in ePP, centered at 14.5 ka (21), which is
similar to the timing of the peak in P
that is identified in this study.
This conclusion is supported by several oth-
er studies in the Indo-Pacific region that have
also suggested that interstadials are character-
ized by La Nin˜a conditions (2224). For exam-
ple, increased wind-driven upwelling along the
Oman margin and productivity in the Cariaco
Basin have both been linked with La Nin˜a
type conditions (22). Hence, there are peaks
in productivity in the Arabian Sea (12.5 to
15.5 ka) (25) and Cariaco Basin (13.1 to 16
ka) (26 ) that are both centered around the
14.6 ka Bølling warming observed in the
Greenland Ice Core Project 2 record (27) and
are coincident with the period of high P
observed in this study.
Finally, the period of high P
served here is approximately coincident with
the deglaciation carbon isotope minimum
event (that reached its greatest intensity at
15.9 0.2 ka) (28) and is also apparent in
our data. Spero and Lea (28) ascribed this
observation to increased upwelling of CO
rich sub-Antarctic Mode Water as a conse-
quence of the reestablishment of circumpolar
deep water that itself resulted from meltback
of Antarctic sea ice. Our data suggest that a
substantial portion of this upwelling occurred
in the equatorial Pacific.
It is increasingly clear that the tropical
oceans had a key part to play in the transition
from glacial to interglacial climate (2326).
Data from our study suggest that there was an
increase in the intensity of upwelling of CO
rich water in the eastern equatorial Pacific
at a time that is coincident with the steepest
rise in atmospheric CO
levels (11, 12) dur-
ing the last deglaciation. Understanding the
extent to which such upwelling and degassing
from the equatorial Pacific contributed to the
rise in atmospheric CO
during this time
requires the generation of other, similar
records from other parts of the ocean relevant
to the physical controls of ocean-atmosphere
exchange [such as temperature and wind
stress (3)]. Nevertheless, our study shows
B studies of planktonic foraminifera
are a powerful tool with which to investi-
gate Pleistocene variations in ocean-atmo-
sphere CO
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Geochim. Cosmochim. Acta 66, 2737 (2002).
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31. This work was funded by the Natural Environment
Research Council and the 6C program of the EC. We
thank W. Berger for supplying us with samples from
core ERDC-92; G. Bianchi for providing the oxygen and
carbon isotope data; M. J. Cooper and R. N. Taylor for
providing assistance with the boron analyses; and E. J.
Rohling, J. G. Shepherd, P. A. Wilson, and two anony-
mous reviewers for their helpful comments.
25 November 2002; accepted 18 March 2003
Published online 27 March 2003;
Include this information when citing this paper.
Role of Polo-like Kinase CDC5 in
Programming Meiosis I
Chromosome Segregation
Brian H. Lee and Angelika Amon*
Meiosis is a specialized cell division in which two chromosome segregation
phases follow a single DNA replication phase. The budding yeast Polo-like kinase
Cdc5 was found to be instrumental in establishing the meiosis I chromosome
segregation program. Cdc5 was required to phosphorylate and remove meiotic
cohesin from chromosomes. Furthermore, in the absence of CDC5 kinetochores
were bioriented during meiosis I, and Mam1, a protein essential for coorien-
tation, failed to associate with kinetochores. Thus, sister-kinetochore coori-
entation and chromosome segregation during meiosis I are coupled through
their dependence on CDC5.
Sexually reproducing diploid organisms
rely on a specialized cell cycle, meiosis, for
gamete formation. During meiosis, DNA
replication is followed by two rounds of
chromosome segregation, in which ho-
mologs segregate during the first and sister
chromatids split in the second. For this
unusual chromosome segregation program
to occur accurately, several meiosis-specif-
ic events must take place (13). First, re-
ciprocal recombination between homologs
generates linkages called chiasmata, which
ensure that homologs are accurately aligned
on the metaphase I spindle. Second, sister
kinetochores face the same spindle pole (co-
orientation) to facilitate sister-chromatid co-
segregation during anaphase I. Before the
second meiotic division, coorientation is lost,
and sister kinetochores attach to microtubules
from opposite spindle poles (biorientation),
which then separate sister chromatids during
anaphase II (4). Lastly, cohesin complexes
that hold sister chromatids together are lost in
a stepwise manner. Loss of arm cohesion
allows for the resolution of chiasmata and
segregation of homologs during meiosis I.
Retention of centromeric cohesion ensures
that sister chromatids properly align on the
meiosis II spindle (5). Sister chromatids sep-
arate at the onset of anaphase II when cen-
tromeric cohesion is lost (4).
To gain insight into how the meiotic seg-
regation pattern is established, we examined
the role of Polo kinase (Cdc5 in budding
Center for Cancer Research, Howard Hughes Medical
Institute, Massachusetts Institute of Technology, E17-
233, 40 Ames Street, Cambridge, MA 02139, USA.
*To whom correspondence should be addressed. E-
18 APRIL 2003 VOL 300 SCIENCE www.sciencemag.org482
... Several studies have proposed that tropical Pacific surface waters acted as a CO 2 source to the atmosphere during the Last Deglaciation (18e9 ka), thus contributing to the deglacial pCO 2(atm) rise, although the proposed mechanisms of carbon release differ. In the western equatorial Pacific, Palmer and Pearson (2003) attributed CO 2 release to an increased frequency of La Niña-like conditions during the Last Deglaciation. In the eastern equatorial Pacific, Martínez-Botí et al. (2015) ascribed positive DpCO 2(sw-atm) to the northward advection and subsequent upwelling of southern-sourced (i.e., from Southern Ocean) intermediate and mode waters with high concentrations of dissolved inorganic carbon (DIC) during the same interval. ...
... Note the westward advection of positive DpCO 2(sw-atm) from the eastern tropical Pacific. Core MD06-3054 (purple circle) and sites of previous studies (cores KR05-15 PC01 (Kubota et al., 2019), ERDC-92 (Palmer and Pearson, 2003) and ODP 1238 (Martinez-Boti et al., 2015), Marquesas site (Douville et al., 2010), and Tahiti sites (Gaillardet and All egre, 1995;Douville et al., 2010;Kubota et al., 2014) discussed in this study are also shown. The signs in the brackets show the state of air-sea CO 2 exchanges in these sites during the Last Deglaciation, with plus, minus and zero signs denoting net CO 2 release and sequestration, and air-sea CO 2 equilibrium, respectively. ...
... In conjunction with previous reconstructions of equatorial Pacific surface-water carbonate chemistry (Fig. 7), our new data provide an opportunity to assess the role of the tropical Pacific in contributing to glacial/interglacial pCO 2(atm) cycles. DpCO 2(sw-atm) records indicate that surface waters of the WPS, western equatorial Pacific (Palmer and Pearson, 2003;Kubota et al., 2019), and central equatorial Pacific (Douville et al., 2010) were in CO 2 equilibrium with the atmosphere during the LGM (Fig. 7bee), but that surface waters of the eastern equatorial Pacific acted as a weak CO 2 source (Martinez-Boti et al., 2015) (Fig. 7f). These results suggest that the tropical Pacific exerted little or no influence on pCO 2(atm) drawdown during the LGM. ...
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The East Asian monsoon (EAM) and El Niño-Southern Oscillation (ENSO) are large-scale oceanic-atmospheric fluctuations that dominate climate variability in the tropical Pacific Ocean. Although the effects of EAM and ENSO on physical and biological processes are increasingly understood, little is known about their influence on seawater carbonate chemistry in the tropical Pacific, especially in the geological past. Here, we present reconstructed variations in surface-water pCO 2 (pCO 2(sw)) and their deviation from atmospheric pCO 2 (DpCO 2(sw-atm)) in the western Philippine Sea (WPS) since 27 ka. Our record displays covariation between DpCO 2(sw-atm) and the intensity of the East Asian summer monsoon (EASM) since the Last Glacial Maximum (LGM), suggesting that EASM-driven upwelling controls long-term changes in surface-water carbonate chemistry and air-sea CO 2 fluxes. Rapid changes in DpCO 2(sw-atm) were linked to the ENSO-like state and, to a lesser extent, the East Asian winter monsoon (EAWM) during the Last Deglaciation, with low values corresponding to La Niña-like phases and strong EAWM during Heinrich Event 1, the Allerød and the Younger Dryas, and high values corresponding to El Niño-like phases and weak EAWM during the Bølling and Pre-Boreal. This interpretation is supported by the relationship of EAM and ENSO to modern surface-water carbonate chemistry in the WPS. Our new record, combined with previously published data, suggests that the tropical Pacific played a minimal role in sequestering CO 2(atm) during the LGM. Tropical Pacific surface waters overall became a pronounced CO 2 source to the atmosphere during the Last Deglaciation, possibly making a substantial contribution to the deglacial pCO 2(atm) rise. We infer that this flux was mainly due to ENSO-related patterns of vertical stratification or lateral advection, perhaps in addition to equatorial upwelling of southern-sourced waters already enriched in dissolved inorganic carbon. Our findings indicate that tropical conditions (i.e., EAM and ENSO-like state) played a crucial role in glacial-interglacial pCO 2(atm) changes, suggesting that this is an important area for future research into the causes of glacial pCO 2(atm) cycles.
... The changes in surface water PCO 2 at the South Iceland Rise is +70 ppmv during the last glacial (21-17.5 ka) to early Holocene (11-10.5 ka) and the subsurface PCO 2 was lower than the contemporary atmospheric CO 2 , although a few data at 11.65, 12.66, and 14.20 ka showed higher PCO 2 values than the contemporary atmospheric CO 2 (Fig. 8c). These changes were similar to those observed in the tropical and subtropical regions over the same time interval (Foster, 2008;Hönisch and Hemming, 2005;Palmer and Pearson, 2003;Sanyal et al., 1997;. The reconstructed subsurface PCO 2 was close to and generally followed the trend of atmospheric CO 2 obtained from Antarctica ice cores (Lemieux-Dudon et al., 2010;Monnin et al., 2001), showing rapid rises during the HS1 and YD, interrupted by a pause during the B/A. ...
... The equatorial Pacific is the site of the greatest evolution of CO 2 (0.8 to 1.0 Pg C year − 1 ) from the modern oceans, and may thus have played a role in glacial-interglacial changes in atmospheric pCO 2 (Palmer and Pearson, 2003). Variations of seawater pH in the western, the central and the eastern equatorial, north and south Pacific Ocean during the last deglaciation were systematically investigated using the boron isotope proxy in planktonic foraminifera and fossil Porites corals at core sites ERDC-92 (2 • 13.5 S, 156 • 59.9 E; 1598 m water depth) by Palmer and Pearson (2003), IODP 310 (17.6 • S, 149.5 • W) by Kubote et al. (2014) and IODP (Fig. 10). ...
... The equatorial Pacific is the site of the greatest evolution of CO 2 (0.8 to 1.0 Pg C year − 1 ) from the modern oceans, and may thus have played a role in glacial-interglacial changes in atmospheric pCO 2 (Palmer and Pearson, 2003). Variations of seawater pH in the western, the central and the eastern equatorial, north and south Pacific Ocean during the last deglaciation were systematically investigated using the boron isotope proxy in planktonic foraminifera and fossil Porites corals at core sites ERDC-92 (2 • 13.5 S, 156 • 59.9 E; 1598 m water depth) by Palmer and Pearson (2003), IODP 310 (17.6 • S, 149.5 • W) by Kubote et al. (2014) and IODP (Fig. 10). ...
Cycling of CO2 between the oceans and the atmosphere has significant impacts on the global climate change. The accurate reconstructions of paleo-pH and atmospheric-oceanic carbon cycling using geochemical tracers (e.g., δ¹¹B, B/Ca) in marine carbonates are reviewed in this work, including the fundamental mechanisms and the remaining challenges in these proxies and the progresses in understanding of evolution of paleo-climate and seawater pH from the late Permian to postindustrial periods. The proxies provide new insight into the evolution of atmospheric CO2 concentrations at time scales from tens of millions to thousands of years, and the direct evidence to the significant ocean acidification during the mass extinction events, and the CO2 cycling in ocean-atmosphere system during the Last Deglaciation and post-industrial periods. On the basis of extensive investigation, it could be concluded that: (i) the carbon dioxide levels and their impacts on Earth surface temperature during the Cenozoic cooling, the Pliocene warmth, and the mid-Pleistocene transition have been evaluated by the combination of multiple proxies; (ii) the benthic/planktonic foraminiferal B/Ca and δ¹¹B data provided consistent implications for global climate variations during the Late Pleistocene, the Late Glacial, Last Glacial Maximum, and the Younger Dryas event; (iii) perturbations of surface ocean pH at the Permo-Triassic (P-T) boundary, the Pliensbachian-Toarcian (Pl-To) boundary, the Cretaceous-Paleogene (K/Pg) boundary and the Palaeocene-Eocene Thermal Maximum (PETM) global warming event were triggered by the large injection of carbon, the short episodic pulses of volcanogenic CO2, the Chicxulub impact, and the volcanism activities of the North Atlantic Igneous Province, respectively; (iv) the ocean acidification in the equatorial and polar Pacific during the Last Deglaciation implied an expanded zone of equatorial upwelling and resultant CO2 emission from higher subsurface dissolved inorganic carbon concentration. The acceleration of modern acidification in post-industrial time was not only driven by anthropogenic CO2 but also varied synchronously with inter-decadal changes in Asian Winter Monsoon Intensity.
... The western tropical Paci c Ocean (WTPO) is a key region for oceanographic research. It has unique ecosystems due to its extraordinary geography and highly complex current system 10,11,12 . Seamounts are generally de ned as isolated and uplifted terrain below the seawater surface with summit depths of at least 1,000 m above the sea oor 13,14,15 . ...
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Zooplankton can affect and regulate the biological carbon pump in the biogeochemical cycles of marine ecosystems through diel vertical migration (DVM) behaviour. The diel vertical distribution and migration of a zooplankton community were studied at a continuous survey station in the Caroline Seamount area of the western tropical Pacific Ocean. Using a MultiNet sampling system, 346 zooplankton species/taxa were collected and identified. The vertical distribution patterns of abundance and composition of the zooplankton community differed between daytime and nighttime. The highest biodiversity index occurred in the 100–200-m ocean depth layer, but some zooplankton species remained in the deep-water layer below 300 m. The DVM patterns of the various dominant species differed, even when the species belonged to the same order or family. Dissolved oxygen and seawater temperature were the main environmental factors affecting the diel vertical distribution of the zooplankton community. The oxygen minimum zone was identified as performing the dual role of “ecological barrier” and “refuge” for zooplankton. The active carbon flux mediated by the zooplankton DVM in the Caroline Seamount area was 14.5 mg C/(m2·d). Our findings suggest that zooplankton DVM can affect and mediate the biological carbon pump in the Caroline Seamount area.
... and hydrothermal process. Pearson and Palmer (2000), Lemarchand et al (2000), Palmer and Pearson (2003). Curry and Lohmann (1983), Ravelo and Anderson (2000), Pagmi et al. (1999) Broecker (1982, Adkins and Boyle (1997) Eroiliani (1955), Shackelton and Opdyke (1973), Duplessyef ai. ...
p>The prime foci of this study have been to investigate two new potential tracers, the Hf and Fe isotopic systems, and their applications to marine environmental studies. My work has focussed on a suite of ferromanganese crusts from the Pacific Ocean (Central Pacific and Izu-Bonin back-arc basin), encompassed different geographic and geologic settings, to obtain new insights into the present and past sources of Hf and Fe in seawater. First, a Hf isotope analytical method has been established for multi-collection Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS, Chu et al , 2002). This refined method has led to the determination of a new set of ytterbium (Yb) isotopic ratios benefiting from improved isobaric interference corrections and offering a promising future application: in situ Laser ablation analyses. The present-day profile of <sup>176</sup>Hf/<sup>177</sup>Hf ratios in the Pacific Ocean has been inferred by analysing surface scrapings of Fe-Mn crusts collected at various water-depths in the Central Pacific. In these samples, Nd isotope and rare earth element distributions correlate well with hydrological properties inferred from WOCE data, demonstrating the applicability of this approach. The Hf isotopic composition does not display any significant variations with depth, throughout the water column, confirming that its oceanic residence time (τ<sub>Hf</sub>) is most probably longer than that of Nd and similar to that of the thermohaline circulation (˜1500 yr). Estimated Hf isotopic compositions for Pacific Intermediate Water and Pacific Deep Water masses are suggested, based upon this vertical distribution. Isotopic depth-profiles for Hf, Nd and Pb drilled into three Fe-Mn crusts have been measured to help decipher the radiogenic isotope budget of the Central Pacific and Izu-Bonin back-arc basin throughout the Late Neogene. Isotopic records for Central Pacific crusts match those from the literature, showing no significant variations over the last 10 Myr. For the Izu-Bonin area, by contrast, Pb-isotope variations suggest mixing between dissolved inputs from aeolian loess and volcanic island arcs. A decoupling of the Hf and Nd isotope records is observed in both Izu-Bonin crusts at ˜4Ma.</p
... Changes in the oceanographic dynamics are expected to affect food availability, light and competition and thereby to increase the likelihood of foraminiferal seasonal preferences along the Papua New Guinea margin relative to the wider WPWP. A Holocene SST record from the Ontong-Java Plateau region 100 , a true open-ocean location in the WPWP, shows monotonic warming across the Holocene, rather than cooling as observed at most marginal sites, thereby providing primary support for these interpretations. ...
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Proxy reconstructions from marine sediment cores indicate peak temperatures in the first half of the last and current interglacial periods (the thermal maxima of the Holocene epoch, 10,000 to 6,000 years ago, and the last interglacial period, 128,000 to 123,000 years ago) that arguably exceed modern warmth1,2,3. By contrast, climate models simulate monotonic warming throughout both periods4,5,6,7. This substantial model–data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change. Here we show that previous global reconstructions of temperature in the Holocene1,2,3 and the last interglacial period⁸ reflect the evolution of seasonal, rather than annual, temperatures and we develop a method of transforming them to mean annual temperatures. We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). However, mean annual temperatures during the last interglacial period were stable and warmer than estimates of temperatures during the Holocene, and we attribute this to the near-constant greenhouse gas levels and the reduced extent of ice sheets. We therefore argue that the climate of the Holocene differed from that of the last interglacial period in two ways: first, larger remnant glacial ice sheets acted to cool the early Holocene, and second, rising greenhouse gas levels in the late Holocene warmed the planet. Furthermore, our reconstructions demonstrate that the modern global temperature has exceeded annual levels over the past 12,000 years and probably approaches the warmth of the last interglacial period (128,000 to 115,000 years ago).
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Identifying the causes and consequences of natural variations in ocean acidification and atmospheric CO 2 due to complex earth processes has been a major challenge for climate scientists in the past few decades. Recent developments in the boron isotope (δ ¹¹ B) based seawater pH and pCO 2 (or pCO 2 sw ) proxy have been pivotal in understanding the various oceanic processes involved in air-sea CO 2 exchange. Here we present the first foraminifera-based δ ¹¹ B record from the north-eastern Arabian Sea (NEAS) covering the mid-late Holocene (~ 8–1 ka). Our record suggests that the region was overall a moderate to strong CO 2 sink during the last 7.7 kyr. The region behaved as a significant CO 2 source during two short intervals around 5.5–4 ka and 2.8–2.5 ka. The decreased pH and increased CO 2 outgassing during those abrupt episodes are associated with the increased upwelling in the area. The upwelled waters may have increased the nutrient content of the surface water through either increased supply or weaker export production. This new dataset from the coastal NEAS suggests that, as a potential result of changes in the strength of the El-Nino Southern Oscillation, the region experienced short episodes of high CO 2 outgassing and pre-industrial ocean acidification comparable to or even greater than that experienced during the last ~ 200 years.
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Chapter 2 assesses observed large-scale changes in climate system drivers, key climate indicators and principal modes of variability. Chapter 3 considers model performance and detection/attribution, and Chapter 4 covers projections for a subset of these same indicators and modes of variability. Collectively, these chapters provide the basis for later chapters, which focus upon processes and regional changes. Within Chapter 2, changes are assessed from in situ and remotely sensed data and products and from indirect evidence of longer-term changes based upon a diverse range of climate proxies. The time-evolving availability of observations and proxy information dictate the periods that can be assessed. Wherever possible, recent changes are assessed for their significance in a longer-term context, including target proxy periods, both in terms of mean state and rates of change.
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Gulev, S. K., P. W. Thorne, J. Ahn, F. J. Dentener, C. M. Domingues, S. Gerland, D. Gong, D. S. Kaufman, H. C. Nnamchi, J. Quaas, J. A. Rivera, S. Sathyendranath, S. L. Smith, B. Trewin, K. von Shuckmann, R. S. Vose, 2021, Changing State of the Climate System. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson- Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.
The use of the boron content and isotopic composition of secondary silicate minerals and siliceous organisms to trace weathering reactions and past ocean pH requires characterizing the fundamental reactions that govern the incorporation and subsequent isotope fractionation of this element in these materials. Toward this goal we have investigated boron adsorption on the surface of amorphous silica (SiO2·0.32H2O) and quantified its isotopic fractionation. Boron adsorption envelopes and corresponding isotope fractionation factors were measured in dilute aqueous solutions (0.01 M) of NaCl and CaCl2. B maximum adsorbed fraction was found to be about 2 times higher in CaCl2 solutions than in NaCl. The modelling of chemical and isotopic data in NaCl solutions allowed to identify the formation of two main B surface species at the SiO2(am)/water interface: a neutral trigonal (B3) inner-sphere complex, >SiOB(OH)2⁰, characterized by a fractionation factor of ∼ -16 ‰ relative to aqueous boric acid, and a negatively charged tetrahedral (B4) inner-sphere complex, >SiOB(OH)3–, with fractionation factors of -5 ‰ with respect to aqueous borate. In CaCl2 solutions the data modelling indicates the presence of B4 inner-sphere complexes with a fractionation factor of -6.5 ‰ relative to aqueous borate, but excludes the presence of trigonal surface species and suggests instead the formation of a Ca-B(OH)4– complex that could partly account for the observed increase of boron adsorption in these solutions. These observations indicate that changes in the surface charge density and interfacial water structure due to different background electrolytes can induce changes in concentration and both chemical and isotopic composition of B adsorbed on silica surfaces. Although this study suggests that the B adsorption reaction on SiO2(am) plays a minor role in the B incorporation and isotopic signature of clay minerals and siliceous cements forming during weathering reactions, the acquired data should allow for an improved knowledge of the biomineralization reactions. The observed B isotopic fractionations between adsorbed and aqueous B species should help determining the relative contribution of various processes, such as cellular transport, biological mediation and silicification reactions, on the amount of B incorporated in siliceous microorganisms and its isotopic signatures. In addition, the presence in NaCl solutions of both trigonal and tetrahedral boron complexes at the silica surface could explain the observed weak pH dependence of the boron isotopic composition of marine diatoms.
Over the last deglaciation there were two transient intervals of pronounced atmospheric CO2 rise; Heinrich Stadial 1 (17.5-15 kyr) and the Younger Dryas (12.9-11.5 kyr). Leading hypotheses accounting for the increased accumulation of CO2 in the atmosphere at these times invoke deep ocean carbon being released from the Southern Ocean and an associated decline in the global efficiency of the biological carbon pump. Here we present new deglacial surface seawater pH and CO2sw records from the Sub-Antarctic regions of the Atlantic and Pacific oceans using boron isotopes measured on the planktic foraminifera Globigerina bulloides. These new data support the hypothesis that upwelling of carbon-rich water in the Sub-Antarctic occurred during Heinrich Stadial 1, and contributed to the initial increase in atmospheric CO2. The increase in CO2sw is coeval with a decline in biological productivity at both the Sub-Antarctic Atlantic and Pacific sites. However, there is no evidence for a significant outgassing of deep ocean carbon from the Sub-Antarctic during the rest of the deglacial, including the second period of atmospheric CO2 rise coeval with the Younger Dryas. This suggests that the second rapid increase in atmospheric CO2 is driven by processes operating elsewhere in the Southern Ocean, or another region.
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A high-resolution ice-core record of atmospheric CO2 concentration over the Holocene epoch shows that the global carbon cycle has not been in steady state during the past 11,000 years. Analysis of the CO2 concentration and carbon stable-isotope records, using a one-dimensional carbon-cycle model,uggests that changes in terrestrial biomass and sea surface temperature were largely responsible for the observed millennial-scale changes of atmospheric CO2 concentrations.
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Dense microearthquake swarms occur in the upper south flank of Kilauea, providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative relocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that generates them. Indeed, relative relocations define a plane dipping about 6 deg northward, although corresponding absolute locations are widely dispersed in the swarm. A composite focal mechanism, built from events providing a correct spatial sampling of the multiplet, also gives a well-constrained northward dip of about 5 deg to the near-horizontal plane. This technique thus collapses the clouds of hypocenters of single-event locations to a plane coinciding with the slip plane revealed by previous focal mechanism studies. We cannot that all south flank earthquakes collapse to a single plane. There may locally be several planes, perhaps with different dips and depths throughout the south flank volume. The 6 deg northward-dipping plane we found is too steep to represent the overall flexure of the oceanic crust under the load of the island of Hawaii. The present work illustates how high pecision relative relocations of similar events in dense swarms, combined with the analysis of geodetic measurements, can help to describe deep fault plane geometry.
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Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Niño conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or −19%) Pg C yr−1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s−1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr−1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr−1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%.
Interpretation of abundant seismic data suggest that Kilauea's primary conduit within the upper mantle is concentrically zoned to about 34-km depth. This zoned structure is inferred to contain a central core region of relatively higher permeability, surrounded by numerous dikes that are in intermittent hydraulic communication with each other and with the central core. During periods of relatively high magma transport, the entire cross section of the conduit is utilized. During periods of relatively low to moderate transport, however, only the central core is active.-from Author
The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (�18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite �18O record into components of temperature and local water �18O reveals a dominant salinity signal that varied in accord with Dansgaard/ Oeschger cycles over Greenland. Salinities were higher at times of highlatitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Nin˜o–Southern Oscillation (ENSO). El Nin˜o conditions correlate with stadials at high latitudes, whereas La Nin˜a conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Nin˜o.
Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittier Narrows earthquake in California (ML=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittier Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short-period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three-dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low-pass filtered, and the P and S wave cross-correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross-correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well-defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a ML=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.
The 3-km-long Greenland Ice Sheet Project 2 (GISP2) ice core presents a 100,000+- year detailed oxygen isotope profile covering almost a full glacial-interglacial cycle. Measurements of isotopic fluctuations in snow, frost, and atmospheric water vapor samples collected during summer field seasons (up to 200/00) are compatible with the large and abrupt 18O/16O changes observed in accumulated firn. Snow pit delta18O profiles from the GISP2 summit area, however, show rapid smoothing of the 18O/16O signal near the surface. Beyond about 2-m depth the smoothed delta18O signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate. The longer climate fluctuations also have regional and often global significance. In the older part of the record, corresponding to marine isotope stages (MIS) 5a to 5d, the effect of orbital climate forcing via the 19- and 23-kyr precession cycles and the 41-kyr obliquity cycle is obvious. From the end of MIS 5a, at about 75,000 years B.P., till the end of the glacial at the Younger Dryas-Preboreal transition, at 11,650 years B.P., the O18O/16O record shows frequent, rapid switches between intermediate interstadial and low stadial values. Fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1.5 kyr, a broad peak at 4.0 kyr, and additional shorter periods. Detailed comparison of the GISP2 18O/16O record with the Vostok, Antarctica, deltaD record; Pacific Ocean foraminiferal 18O/16O; Grande Pile, France, tree pollen; and insolation indicates that a counterpart to many of the rapid 18O/16O fluctuations of GISP2 can be found in the other records, and that the GISP2 isotopic changes clearly are the local expression of climate changes of worldwide extent. Correlation of events on the independent GISP2 and SPECMAP time scales for the interval 10,000-50,000 years B.P. shows excellent chronometric agreement, except possibly for the event labeled 3.1. The glacial to interglacial transition evidently started simultaneously in the Arctic and the Antarctic, but its development and its expression in Greenland isotopes was later suppressed by the influence of meltwater, especially from the Barents Sea ice sheet, on deep water formation and ocean circulation. Meltwaters from different ice sheets bordering the North Atlantic also influenced ocean circulation during the Bølling-Allerød interstadial complex and the Younger Dryas and led to a distinct development of European climate and Greenland 18O/16O values. The Holocene interval with long-term stable mean isotopic values contains several fluctuations with periods from years to millennia. Dominant is a 6.3-year oscillation with amplitude up to 3 to 40/00. Periodicities of 11 and 210 years, also found in the solar-modulated records of the cosmogenic isotopes 10Be and 14C, suggest solar processes as the cause of these cycles. Depression of 18O/16O values (cooling) by volcanic eruptions is observed in stacked GISP2 delta18O records, but the effect is small and not likely to trigger major climate changes.
Carbon-14 determinations on box cores of calcareous ooze from the western and eastern equatorial Pacific suggest that patterns of mixed-layer ages, sedimentation rates, and mixed-layer thicknesses are controlled by gradients of carbonate dissolution and fertility, and by small-scale redeposition processes. Mixed-layer ages range from 3000 to 7000 years, with a mode between 4000 and 5000 years. Sedimentation rates range from 0.8 to 2.4 cm/1000 years. Mixed-layer depths, calculated according to the box model of mixing, range from 7 cm to 16 cm. Observed thicknesses are about one-fourth smaller than calculated ones.
As part of the JGOFS field program, extensive CO2 partial-pressure measurements were made in the atmosphere and in the surface waters of the equatorial Pacific from 1992 to 1999. For the first time, we are able to determine how processes occurring in the western portion of the equatorial Pacific impact the sea–air fluxes of CO2 in the central and eastern regions. These 8 years of data are compared with the decade of the 1980s. Over this period, surface-water pCO2 data indicate significant seasonal and interannual variations. The largest decreases in fluxes were associated with the 1991–94 and 1997–98 El Niño events. The lower sea–air CO2 fluxes during these two El Niño periods were the result of the combined effects of interconnected large-scale and locally forced physical processes: (1) development of a low-salinity surface cap as part of the formation of the warm pool in the western and central equatorial Pacific, (2) deepening of the thermocline by propagating Kelvin waves in the eastern Pacific, and (3) the weakening of the winds in the eastern half of the basin. These processes serve to reduce pCO2 values in the central and eastern equatorial Pacific towards near-equilibrium values at the height of the warm phase of ENSO. In the western equatorial Pacific there is a small but significant increase in seawater pCO2 during strong El Niño events (i.e., 1982–83 and 1997–98) and little or no change during weak El Niño events (1991–94). The net effect of these interannual variations is a lower-than-normal CO2 flux to the atmosphere from the equatorial Pacific during El Niño. The annual average fluxes indicate that during strong El Niños the release to the atmosphere is 0.2–0.4PgCyr−1 compared to 0.8–1.0PgCyr−1 during non-El Niño years.
The Australian contribution to the equatorial JGOFS program consisted of three cruises that focused on a transect from 10°S to 10°N along 155°E. In this paper we report on the last two cruises, in June/July 1992 and November 1993, which coincided with the middle and the end of a prolonged El Nin˜o event, and compare the results with those obtained in October 1990, i.e. after the 1988/1989 La Nin˜a but before the 1991/1992 El Nin˜o. Compared with 1990, the depth of the thermocline was shallower in 1992 and 1993, with the result that the barrier layer was thinner and the deep chlorophyll maximum was always found below the top of the thermocline. Maximum chlorophyll concentrations and depth-integrated concentrations were similar in 1990 and 1992 (0.4 – 0.45 μg 1−1 and 20–30 mg m−2) but were higher in 1993 (0.75 μg 1−1 and 30–40 mg m−2). Modelled productivities varied from 20 to 50 mmol Cm−2 d−1 and were strongly dependent on the depth of the chlorophyll mixed layer and the nutrient supply. In 1992, the surface waters were undersaturated in C02 by about 30 μatm, while in 1993 the waters were closer to equilibrium with atmospheric C02, withpCO2 values largely determined by sea surface temperature. During 1993, large changes in sea surface temperature (1.5°C), salinity (0.4),pCO2 (40 μatm) and near surface currents were observed near the equator over a period of about a week. These changes in the circulation, the physical and chemical structure, and phytoplankton depth distribution and production, appeared to respond to both local meteorological forcing and changes in the large scale equatorial circulation (ENSO). In contrast to sites further east, we always found a low salinity surface layer at 155°E which isolated the surface ocean from changes in carbon pools and fluxes associated with the productive deep chlorophyll maximum. All rights reserved