Termination 1 Millennial
-
scale Rainfall Events over the Sunda Shelf
F. Buckingham
1
, S.A. Carolin
1,2
, J.W. Partin
3
, J.F. Adkins
4
, K.M. Cobb
5
, C.C. Day
1
, Q.
Ding
6
, C. He
7
, Z. Liu
7
, B. Otto
-
Bliesner
8
, W.H.G. Roberts
9
, S. Lejau
10
, J. Malang
10
1
Department of Earth Sciences, University of Oxford, Oxford, UK
2
Department of Earth Sciences, University of Camb
ridge, Cambridge, UK
3
Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin,
TX, USA
4
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena,
CA, USA
5
Department of Earth and Atm
ospheric Sciences, Georgia Institute of Technology, Atlanta, GA,
USA
6
Department of Geography and Earth Research Institute, University of California Santa Barbara,
Santa Barbara, USA
7
Department of Geography, The Ohio State University, Columbus, OH, USA
8
C
limate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder,
CO, USA
9
Geography and Environmental Sciences, Northumbria University, Newcastle
-
Upon
-
Tyne, UK
10
Gunung Mulu National Park, Sarawak, Malaysia
Correspond
ing
author:
St
acy Carolin
(
sac219@cam.ac.uk
)
Key Points:
A new stalagmite record reveals for the first time distinct Bølling
-
Allerød and Younger
Dryas oxygen isotope variations in northern Borneo
The stalagmite oxygen isotope pattern strongly resembles the recent iCESM
transient
annual rainfall simulation from 20
-
11 ka
Borneo drying during Heinrich 1 and the Younger Dryas
may be
attributed to an
anomalous boreal winter
W
estern
N
orth Pacific anticyclone
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. Please cite this article as
doi: 10.1029/2021GL096937
.
This article is protected by copyright. All rights reserved.
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Abstract
Recent paleoclimate reconstructions have suggested millen
nial
-
scale variability in the Indo
-
Pacific Warm Pool region coincident with events of the last deglaciation. Here we present a new
stalagmite oxygen isotope record from northern Borneo, which today is located near the center of
the region’s mean annual int
er
-
tropical convergence zone. The record spans the full deglaciation,
and reveals for the first time distinct oxygen isotope variations at this location
connected
with the
Bølling
-
Allerød
onset
and the Younger Dryas event. The full deglaciation
in the
Born
eo
stalagmite proxy reconstruction appears remarkably similar to
a
20
-
11 ka
transient simulation of
rainfall
over the area
produced using the isotope
-
enabled Community Earth System Model.
In
this
model
, periods of weakened Atlantic Ocean meridional overtur
ning circulation are
associated with
an
anomalous
W
estern
N
orth Pacific
anticyclone
, which is produced in boreal
autumn and shifts south over Borneo during boreal winter, causing dry conditions
.
Plain Language Summary
Here
we
aim
to
resolve
conflicting
ev
idence
of
how
tropical
convection
in
Borneo
in
the
I
ndo
-
Pacific
oceanic
region
,
a
critical
region
associated
with
El
Niño
events
and
the Asian and
Australian
-
Indonesian monsoon
system
s,
was
affected
by
past
changes
in
the
strength
of
Atlantic
Ocean
circulation.
Analyzing
a
20,000+
year
old
Borneo
cave
calcite
formation
along
its
growth
axis
,
we
find
large
oxygen
isotope
shifts
coincident
with
two
prominent
millennial
-
scale
periods
of
reduced
Atlantic
Ocean
overturning
circulation
strength.
The
isotop
e
record
is
similar
to
a
neighboring
south
west
ern
Philippine
stalagmite
record
,
and
w
e
interpret
the
signal
as
drier
mean
annual
conditions
over
the
region
during
these
events
compared
to
the
background
state;
an
interpretation
supported
by
simulations
in
three
independent
state
-
of
-
the
-
art
climate
model
s
.
One
model
suggests
that
these
conditions
in
Borneo
were
driven
by
the
southwards
shift
of
an
anomalous
anticyclone
in
the
boreal
winter
season.
By
combining
geochemical
and
model
evidence,
this
study
has
s
hown
that
rainfall
decreased
for
millennial
periods
during
the
end
of
the
ice
age,
when
the
Atlantic
Ocean
overturning
circulation
weakened.
In
doing
so,
we
have
elucidated
further
how
this
region
responds
to
major
changes
in
global
climate
and
ocean
condi
tions.
1 Introduction
The Indo
-
Pacific Warm Pool (IPWP) is a critical region of the global climate system,
associated with the Pacific and Indian Walker cells and the Asian and Australian
-
Indonesian
monsoon systems. Partin et al. (2007) presented the firs
t stalagmite δ
18
O record used to
reconstruct convection changes over the IPWP spanning the region’s transition from the Last
Glacial Maximum to the Holocene. The δ
18
O record was constructed from three individual
stalagmites, collected from Gunung Buda Nati
onal Park cave systems in northern Borneo (4° N,
115° E). The strongest millennial
-
scale δ
18
O signal evident in all three records was a gradual
shift through Heinrich Stadial 1 (HS1) to the
highest
δ
18
O values of the record, interpreted as a
signal of
weakening convection over the western tropical Pacific through this period. The record
also demonstrated the warm pool’s relatively smooth variability on both orbital and millennial
timescales, in contrast to the abrupt shifts seen in mid
-
latitude Chinese
stalagmite δ
18
O records
(e.g. Wang et al., 2001).
A significant feature of the northern Borneo stalagmite record was the lack of a δ
18
O
signal coincident with the Younger Dryas (YD) event, despite a clear HS1 signal (Partin et al.,
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2007). This appeared to
suggest convection in the IPWP responded differently to the HS1 versus
YD North Atlantic events (Partin et al., 2007), both of which are associated with a weakening of
the Atlantic Meridional Overturning Circulation
(AMOC)
(e.g. McManus et al., 2004; Böhm
et
al., 2014; Ng et al., 2018).
T
he YD conclusion at the time was based on a single stalagmite
record, SCH02,
as vertical growth in the
other two
Borneo
stalagmites
stopped prior to the YD
event, and resumed after the event
. A more recent study identified
U
-
Th dating disparities in
stalagmite SCH02 during Heinrich Stadial 4 (Carolin et al., 2013), warranting additional study of
the Borneo rainfall response to millennial events of the last deglaciation. Indeed,
other stalagmite
records found in the IPWP in
Sumatra, Indonesia to the west (0° S, 100° E) (Wurtzel et al.,
2018), Palawan, Philippines to the northeast (10° N, 119° E) (Partin et al., 2015), and Flores,
Indonesia to the south (8.5° S, 120° E) (Griffiths et al., 2009; Ayliffe et al., 2013) all reveal
distinct δ
18
O shifts coincident with the YD event.
Tropical climate perturbations associated with a weakened AMOC are
proposed to
result
from processes such as a southward shift of the inter
-
tropical convergence zone (ITCZ) over the
Pacific (Zhang and Del
worth, 2005; 2007) or cross
-
basin interactions between the Atlantic and
the El Niño
-
Southern Oscillation
(ENSO)
or Pacific Decadal Oscillation (Timmerman et al.,
2007; Wang, 2019).
To better understand underlying dynamics linking the AMOC with the local
ci
rculation change over the IPWP, a
thorough analysis of
the various proposed processe
s
is
needed.
Improved climate proxy information in observation records
that can be used in
model
-
data comparison contributes to this aim
.
Despite the increasing number of s
talagmite sites and proxy records, it remains
challenging to interpret IPWP stalagmite δ
18
O records as changes in past rainfall amount and
perturbations to large
-
scale atmospheric systems over the last deglaciation. This is largely due to
the complexity of
the maritime continent region.
Under modern conditions, individual sites
around the IPWP have unique rainfall patterns contributing to the mean annual rainfall δ
18
O
value, as a result of the seasonal ITCZ migration, the intra
-
annual Madden
-
Julian Oscillat
ion,
and the Pacific and/or Indian Ocean Walker circulation cells’ inter
-
annual variability (e.g.
Moerman et al., 2013; Belgaman et al., 2017; Wurtzel et al., 2018
; Konecky et al., 2019
). During
the last deglaciation, the Sunda Shelf was widely exposed (e.
g. Hanebuth et al., 2000),
placing
the paleo
-
coastline
further from
some cave sites
than modern,
such as those in northern Borneo
,
and likely reorganizing regional circulation patterns
(
e.g.
DiNezio et al., 2016;
Windler et al.,
2020)
. Fully
-
coupled
general circulation model simulations with enabled water isotopes is a tool
that can be utilized to assist with IPWP stalagmite δ
18
O interpretations
(e.g. Windler et al., 2020;
Du et al., 2021)
.
Here we present a new northern Borneo stalagmite δ
18
O record,
the first from the area
with continuous growth through the last deglaciation, to
test for climate responses in the IPWP to
changes in AMOC and associated climate events in the North Atlantic.
The stalagmite contains
clean calcite that is ideal for U
-
Th da
ting, and has previously demonstrated its ability to capture
millennial scale events through the last glacial period (Carolin et al., 2013). We use a recent
transient simulation of the evolution of global climate and water isotopes over the last
deglaciati
on (He et al., 2021a) to investigate if the new Borneo stalagmite δ
18
O response and
other stalagmite δ
18
O responses around the IPWP are present in a simulated climate with water
isotopes, and if so what atmospheric mechanism causes rainfall variability ove
r the Sunda Shelf
in the model simulation. We compare the recent isotope
-
equipped transient simulation’s Heinrich
Stadial event response with two other state
-
of
-
the
-
art
climate
model simulations to review model
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consistency. We aim to reconcile evidence lea
rned from the real world and model world to gain
new physical insights on how IPWP convection responded to
orbital forcing as well as
significant changes to the AMOC
over the North Atlantic.
2 Methods
2.1
Site Location
The stalagmite presented in this stu
dy, SC02, was collected from Secret Cave in
Gunung Mulu National Park, which is southwest
-
adjacent to Gunung Buda National Park
in northern Borneo (
S
upporting
I
nformation
Text S1
, Figure S1). The site sits in the
center of the tropical convection zone, tod
ay receiving ~5000 mm rainfall annually, with
little seasonal variability in either temperature or rainfall amount. Over a decade of daily
rainwater and biweekly drip water monitoring shows a small deviation to
higher
rainwater δ
18
O values in the February
-
March
-
April months (Figure 1). The linear
regression correlation coefficient between annually averaged δ
18
O and rainfall amount at
the study site is
-
0.67, suggesting ~ 45% of the variance in annual rainfall δ
18
O can be
explained by variability in local in
terannual rainfall amount (Ellis et al., 2020). Dry local
conditions associated with El Niño events are responsible for the largest interannual
rainfall δ
18
O excursions at this site under modern conditions
(Moerman et al., 2013; Ellis
et al., 2020)
.
2.2 Ge
ochemical methods
The U
-
Th age model for this study was constructed on the upper portion of
stalagmite SC02
from 18 U
-
Th samples
. Carolin et al. (2013; 2016) reported U
-
Th ages
measured in the lower portion of stalagmite SC02, 105
-
31 ka (thousand years bef
ore
1950 C.E.).
In the upper portion, f
our
of the 18
U
-
Th samples were analyzed in the
Division of Geological and Planetary Sciences at the California Institute of Technology,
and 14 samples were analyzed in the Department of Earth Sciences at the University of
Oxford, following the methods of Carolin et al
. (2016; 2019). A detailed description of
the U
-
Th age sampling, multicollector inductively
-
coupled
-
plasma mass spectrometer
(MC
-
ICP
-
MS) measurements, and sample age error analysis is found in
Supporting
Information Text S2
.
Previous studies have found a
wide range of initial detrital
230
Th/
232
Th values in
Buda and Mulu stalagmites
, which are used to correct for any detrital
230
Th captured
within the calcite crystals during formation
. Individual isochrons suggest values from 45
to 200 ppm as atom ratios (o
r 8 to 37 as activity ratios) (Partin et al., 2007; Carolin et al.,
2013; Carolin et al., 2016; Chen et al., 2016), several times larger than the commonly
used bulk surface silicate
230
Th/
232
Th value of ~5 ppm. The initial detrital
230
Th/
232
Th
value of 111
± 41 ppm was previously calculated for SC02 based on two isochrons
measured in separate stalagmites from Secret Cave in Mulu (
methods and calculations in
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Carolin et al.,
2013
Supplementary Materials
)
, and is used here to correct the U
-
Th ages
in the upper
portion of SC02 for initial detrital Th contamination
.
Stable isotope samples were milled along the growth axis of SC02 in the
Department of Earth Sciences at the University of Oxford using a New Wave Micromill.
Samples were milled in a continuous trench
style at 0.2 mm intervals around the YD
calcite section and at 1 mm intervals in the other sections (
S
upporting
I
nformation
Text
S
3
). Samples were analyzed on a Delta V Advantage isotope ratio mass spectrometer
coupled to either a Kiel IV carbonate device
or Gas Bench II introductory system at the
University of Oxford. Oxygen and carbon isotope results are given in parts per mill (‰)
relative to the Vienna Pee Dee Belemnite Standard (VPDB).
Average m
easurement error
is
less than 0.0
5
‰ (1σ) for δ
13
C and 0.
1
0
‰ (1σ) for δ
18
O. Interpolated ages at all stable
isotope sample
locations
between individual U
-
Th age sample trenches were computed
using the Poisson
-
process deposition model feature in OxCal(v4.4) (Bronk Ramsey,
2008, 2009; Ramsey & Lee, 2013) (
S
upportin
g
I
nformation
Text S
4
).
2.3 Climate simulation
For this study we compare stalagmite δ
18
O records with iTRACE (He et al.,
2021a), a 20 ka
-
11 ka transient simulation of global climate using the
Community Earth
System Model version 1.3 with fully coupled water isotope modules (iCESM) (Brady et
al., 2019). The iTRACE
simulation
well reproduce
s
the deglacial climate and water
isotope evolution in the pan
-
Asian monsoon regions and Greenland (He et al. 2021a, b).
The iTRACE experiments generally follow the strategy of the t
ransient TraCE
-
21ka
simulations
(Liu
et
al.,
2009;
He
et
al.,
2011),
with meltwater flux
designed to be largely
consistent with reconstructed sea level change
(Lambeck et al., 2014)
, continental ice
sheets modified at the beginning of each 1000 year interval
based on the ICE
-
6G
reconstruction (Peltier et al., 2015)
, and gre
enhouse gases prescribed throughout
following reconstructions
(Petit et al., 1999; Lüthi et al., 2008; Schilt et al., 2010)
(Fig. 2a
in He et al., 2021a). At 14 ka, the bathymetry of the Sunda Shelf coast line shifts to the
-
75 m pre
-
industrial isobath, an
d at 12 ka, sea level rises further around the Sunda Shelf,
according to ICE
-
6G (Peltier et al., 2015). As such, any abrupt shifts in the simulated
climate at precisely 14 ka and 12 ka are discounted, as these are artifacts of the imposed
abrupt coastline
changes in the model at those times (He et al., 2021a). A full description
of the model and its experiments can be found in He et al. (2021a) Supplementary
Materials.
3
Results
3.1
SC02 δ
18
O record
Growth within the upper portion of SC02 spans the full deg
laciation (Figure 2
,
Figure S2 and S3
). The U
-
Th ages do not include any age reversals, with all ages within
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the sequential order of growth within error (
Figure S3, Data Set
S1). Vertical extension
rate is slow but relatively constant, between 5
-
15 μm/yr (
Figure S3, Data Set S2
).
Over orbital time scales (~ 5 kyr), changes in δ
18
O in SC02 match those in the
original Buda δ
18
O record (Partin et al., 2007), however millennial
-
scale δ
18
O shifts are
most prominent in this study’s SC02 record (Figure 2). The SC
02 δ
18
O record
’s
mean
value is also the lowest of the group of four Mulu and Buda stalagmite records. Larger
amplitude δ
18
O variability and lower mean δ
18
O values suggest this study’s Mulu Secret
Chamber
stalagmite was least affected by mixing (Moerman et
al., 2014; Ellis et al.,
2020) and kinetic effects (Hendy,
19
71).
The SC02 δ
18
O record looks similar to the original Buda
Snail Shell Cave
SCH02
δ
18
O record (Partin et al., 2007) on depth scale, however the 95% CI of the timing of
onset and ending of δ
18
O
events are not in alignment between the Mulu SC02 and
original Buda SCH02 records (Figure S
4
). The disagreement between the Mulu SC02 and
Buda SCH02 event timing is not caused by the initial detrital
230
Th/
232
Th correction
(
Figure S
4
). We conclude that
something inherent to the original Buda SCH02 sample
geochemistry caused inaccuracies in calculated U
-
Th ages on millennial timescales,
possibly related to the greater amount of detrital material, larger number of growth
hiatuses, slow growth portions
,
ura
nium loss due to open
-
system behavior,
or unresolved,
short hiatuses found in this sample during certain time intervals. The inaccurate timing of
certain millennial scale events in SCH02 could only be recognized through comparison
with other same
-
site samp
les after the original publication.
The new SC02 δ
18
O record reveals for the first time evidence of
abrupt northern
Borneo
stalagmite
δ
18
O changes
which
overlap within
age
error
, and appear similar to,
the North Atlantic Bølling
-
Allerød
onset
(~14.7 ka, e.
g. Buizert et al., 2014), Younger
Dryas onset (12.85 +/
-
0.06 ka, Cheng et al., 2020), and Younger Dryas ending (11.70
-
11.61 +/
-
0.04 ka, Cheng et al., 2020) (Figure S
4
).
3.2
iTRACE IPWP model
-
data comparison
Fig. 3 shows the full iTRACE simulated timeser
ies of rainfall and calculated
calcite δ
18
O, a function of rainfall δ
18
O and surface temperature (
Supporting Information
Text S5
), compared with stalagmite δ
18
O from northern Borneo (
stalagmite
SC02) and
five other sites around the IPWP (see Fig. 1 map), o
n a 50
-
yr moving mean of the annual
average. The shape of the simulated rainfall variability around the northern Borneo site is
remarkably similar to the SC02 stalagmite δ
18
O record (inverse relationship between Fig.
3b left and middle columns) through the
whole deglaciation, and there is also general
agreement between the derived model calcite δ
18
O and the SC02 stalagmite δ
18
O record
(Fig. 3b middle and right columns). The directional shift in δ
18
O in both northern and
southern IPWP stalagmites is inversel
y related to each site’s respective simulated rainfall
shift during the deglaciation millennial events, changes consistent with the amount effect
(e.g. Rozanski et al., 1993).
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4
Discussion
The regional coherence of multiple stalagmite δ
18
O deglaciation si
gnals spaced
throughout the maritime continent of the IPWP (Fig. 3) suggests broad
-
scale atmospheric
dynamics, not local site
-
specific climate changes, drove the rainwater and stalagmite δ
18
O
pattern. This interpretation is supported by the mean annual rai
nfall response simulated in
iTRACE, in which the broad area of the Sunda Shelf north of the equator is drier, while the
Indian Ocean south of the Sunda Shelf is wetter, in response to meltwater forcing and AMOC
weakening simulated in the model (Fig. 3,
Fig
. S
7
).
The iTRACE IPWP mean annual rainfall response to North Atlantic freshwater forcing
and a weakened AMOC is generally consistent with two other fully coupled general circulation
climate model simulations: TraCE
-
21ka
(Liu
et
al.,
2009;
He
et
al.,
2011)
and
HadCM3 (Roberts
and Hopcroft, 2020) (Fig. S
8
c
).
TraCE
-
21ka is a 21 ka
–
modern transient simulation of global
climate using the
Community Climate System Model version 3 (CCSM3) (Collins et al., 2006).
HadCM3 (Gordon
et al., 2000; Pope et al., 2000) has been extensively used for climate studies
since the late 1990s, and performs reasonably well with respect to mean climate (Valdes et al.,
2017), particularly in the IPWP region under glacial conditions (DiNezio and Tier
ney, 2013).
While the spatial rainfall anomaly pattern is consistent between all three simulations, the
amplitude of the change in mean annual rainfall during the Heinrich event is much greater in the
CESM1 (iTRACE) versus the CCSM3 simulation (Fig. S
8
c
).
The CESM1 mean annual rainfall
response is similar in amplitude to the HadCM3 0.25 Sv forcing simulation (Fig. S
8
c
). DiNezio
et al. (2016) previously noted the agreement of IPWP paleoclimate proxies with both the CESM1
and HadCM3 simulated steady
-
state LGM
climate. This study extends this agreement between
proxy and model to include these two models’ mean annual response to a North Atlantic
freshwater forcing event.
The iTRACE simulated drying over Borneo during freshwater forcing events is largely
caused b
y an anomalous local anticyclone
in this model
which shifts equatorward from the
Philippine Sea over Borneo during the boreal winter months when AMOC is weakened (Fig. 4).
This
anticyclone
was recently recognized
in the iTRACE simulation
in He et al. (2021
c). The
authors suggested that the
anticyclone
was yielded and persisted in boreal autumn by
anomalously low moist static air from the midlatitudes, which was advected by mean
northeasterly winds. The
low moist static air
result
s
from a
n enhanced
meridiona
l sea surface
temperature gradient in the
W
estern
N
orth Pacific which is associated with a weakened AMOC
state. During the winter months, these mean northeasterly winds strengthen, pushing the
anticyclone southward over Borneo (Fig. 4 in He et al., 2021c),
affecting both northern Borneo
and nearby Palawan stalagmite locations in the model world
(Fig. 4)
.
The western North Pacific anticyclone is also known to be sensitive to SST variab
i
lity
over the ENSO region and tropical Indian Ocean on interannual times
cales (e.g. Wang et al.,
2000; Xie et al., 2009; Li et al., 2017), events which have been observed to similarly cause
significant
rainfall anomalies
over Borneo under modern conditions
(
e.g.
Ropelewski and
Halpert, 1987; Lau and Nath, 2003; Cobb et al., 20
07)
.
The simulated meridional SST change
along the coast of East Asia, rather than the zonal SST in the ENSO region, coupled with the
northeasterly wind flow
, is shown
in iTRACE
simulations, as well as TraCE
-
21ka simulations, to
also be able to produce
such an
anticyclone response (He et al., 2021c).
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The modeled boreal winter drying anomaly and rainwater δ
18
O anomaly over Borneo is
stronger during HS1 versus the YD event (Fig. 4), and thus this season appears to have driven
the stronger mean annual HS
1 versus YD drying over Borneo in the model (Fig. S
7
). A larger
deviation in northern Borneo stalagmite SC02 δ
18
O is also observed during HS1 versus the YD
event, in support of the model results. Notably, while iTRACE simulates a weak drying over the
Sunda
Shelf north of the equator through boreal summer and an increase in austral summer
rainfall over the Indian Ocean south of the Sunda Shelf during both HS1 and the YD, there is
very little change in modeled rainwater δ
18
O over the Sunda Shelf during the bo
real summer
(Fig. S
9
). Future
improvements in model isotope modules and
further
analysis using isotope
tracers
may help
to better determine what factors are influencing rainwater δ
18
O over the Sunda
Shelf during boreal summers
as a result of meltwater forc
ed events in the model world
.
5 Conclusions
The new SC02 δ
18
O record provides better chronological constraints and continuous
temporal coverage, as compared to previously published results, and reveals for the first time
evidence of abrupt δ
18
O changes in
northern Borneo coincident with the North Atlantic Bølling
–
Allerød
onset
and the Younger Dryas millennial event. The record affirms the regional
consistency amongst multiple stalagmite δ
18
O deglaciation records throughout the maritime
continent, suggestin
g broad
-
scale atmospheric dynamics drove the records’ δ
18
O pattern. A
comparison of the spatial and temporal pattern of the stalagmite δ
18
O records with a recent
isotope
-
equipped transient simulation shows general mean annual model
-
data agreement at most
s
talagmite sites in the IPWP. The anomalous drying over Borneo during HS1 and the YD is
attributed to an anomalous
W
estern North Pacific anticyclone found in the iTRACE
and TraCE
-
21ka
simulation
s
when AMOC is in a weakened state, which shifts south over Bor
neo during
winter months. The distinct spatial and temporal IPWP δ
18
O signature over the last deglaciation
captured in the IPWP stalagmite δ
18
O records presents a robust regional target for future isotope
-
enabled climate model experiments.
Figure Captions
Figure 1. Location of study site (circled) and other stalagmite paleoclimate sites in the text:
Palawan, 10° N (Partin et al., 2015); Borneo, 4° N (this study); Sumatra, 0° S (Wurtzel et al.,
2018); Sulawesi, 5° S (Krause et al., 2019); Flores, 8.5° S (Gr
iffiths et al., 2009; Ayliffe et al.,
2013); Western Australia, 17° S (Denniston et al., 2013). Triangle symbols indicate cave site is
predominantly boreal (up) or austral (down) summer rainfall. Circle symbol sites have little
rainfall seasonality. Contou
rs show mean annual sea surface temperatures (SST)
(
Hersbach et al.
,
2019)
. Black filled shapes show modern landmasses, with the gre
y contours at 60 meters below
sea level modern bathymetry, indicating the estimated Younger Dryas event coastlines. Long
term average (2006
-
2018) monthly rainfall δ
18
O with 1σ error bars at this study site are shown
on the right (Moerman et al., 2013; Elli
s et al., 2020). Note the y
-
axis is inverted.
Figure 2. (a) SC02 (this study; black) and BA04, SSC01, and SCH02 (Partin et al., 2007; green,
red, and blue, respectively) δ
18
O records, plotted on a single y
-
axis. (b) Individual SC02 (this
study), and BA04,
SSC01, and SCH02 (Partin et al., 2007) δ
18
O records. For both (a) and (b), the
BA04, SSC01, and SCH02 records are plotted on their originally published age models. U
-
Th
Accepted
Article
This article is protected by copyright. All rights reserved.
ages of all samples were corrected using the mean initial detrital
230
Th/
232
Th listed.
Solid black
vertical lines in (b) are at 14.7, 12.87, and 11.7 ka.
Figure 3. iTRACE and IPWP region stalagmite δ
18
O records. (i) iTRACE annual precipitation at
50
-
yr moving mean (He et al., 2021a) averaged from grid cells near respective stalagmite sites
(
Figure S5). (ii) Stalagmite δ
18
O at sampling resolution published: (a) Partin et al., 2015, (b) this
study, (c) Wurtzel et al., 2018, (d) Krause et al., 2019, (e) Griffiths et al., 2009, Ayliffe et al.,
2013, (f) Denniston et al., 2013. (iii) Light blue: i
TRACE precipitation
-
weighted annual
precipitation δ
18
O at 50
-
yr moving mean (He et al., 2021a) averaged from grid cells near
respective stalagmite sites (Figure S5). Dark blue: Forward
-
modeled calcite δ
18
O, a function of
iTRACE
-
simulated rainfall δ
18
O and
surface temperature (Supporting Information Text S5). The
plotted size of y
-
axis tick intervals is the same between stalagmite and modeled δ
18
O for each
respective site. Grey bars indicate timing of HS1 (18
-
14.7 ka) and the YD (12.87
-
11.7 ka).
Figure 4. iT
RACE modeled change in seasonal rainfall in response to YD and HS1 forcing in the
IPWP. (a) “Heinrich event” 15.5
-
15 ka 500
-
yr monthly mean minus “background” 20
-
19 ka
1000
-
yr monthly mean and (b) “YD” 12.5
-
12 ka 500
-
yr monthly mean minus “background”
13.5
-
13 ka 500
-
yr monthly mean rainfall for the indicated 3
-
month period. Black outline is
modern bathymetry at (a) 120 meters below sea level today and (b) 60 meters below sea level
today, indicating estimated Heinrich 1 and Younger Dryas event coastlines, re
spectively. Arrows
indicate low level wind anomalies (850 hPa). Markers indicate cave site locations, same as Fig.
1.
Acknowledgments
We wish to thank all of the staff
, freelance guides, and locals
at Gunung Mulu National Park
for
their assistance in field
work and sample collection over the past 15+ years.
Thank you to David
Battisti for his
efforts in assisting select
authors of this article
with computation and model
analysis basics
in preparation for
this project.
Thank you to Alan Hsieh and Andrew
Mason at
the University of Oxford, and Guillaume Paris
and Sophie Hines when
at Caltech for their
assistance with U
-
Th analysis.
Thank you to Gideon Henderson for his support and
encouragement
, and to Nele Meckler, Yves Krüger, and Marit
Løland for helpful
discussion
.
Thank you to the reviewers for their time and comments.
Permits for this work were granted by
the Malaysian Economic Planning Unit, the Sarawak State Planning Unit, and the Sarawak
Forestry Department. This work was supported by a Geological S
ociety 2017 Research Grant
(UK) and a Royal Society International Exchanges Award (UK).
Open Research
All data are publicly available on NOAA NCDC
(
https://www.ncei.noaa.gov/access
/paleo
-
search/study/35433
)
and Zenodo (
https://doi.org/
10.5281/zenodo.6026615
)
. The authors
acknowledge no conflicts of interest.
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