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GEOLOGY, September 2010 803
ABSTRACT
Whether eustatic sea-level rise through the Holocene has been
punctuated or continuous has remained controversial for almost
two decades. Resolving this debate has implications for predicting
future responses of remaining ice sheets to climate change and also
for understanding the drivers of human settlement and dispersal pat-
terns through prehistory. Here we present a sea-level curve for the past
8900 yr from Singapore, a tectonically stable location remote from ice-
loading effects. We also present critical and unique sedimentation rate,
organic
δ
13
C, and foraminiferal δ
13
C proxy records of sea-level change
derived from a shallow-marine sediment core from the same area over
the same time interval. The sea-level curve, corroborated by the inde-
pendent proxy records, suggests rapid rise at a rate of 1.8 m/100 yr
until 8100 cal (calibrated) yr B.P., a near cessation in the rate of sea-
level rise between 7800 and 7400 cal yr B.P., followed by a renewed rise
of 4–5 m that was complete by 6500 cal yr B.P. We suggest that this
period of relatively stable sea level during the early to mid-Holocene
enabled modern deltas to advance, providing a highly productive envi-
ronment for the establishment of coastal sedentary agriculture. Peri-
ods of rapid sea-level rise before and after may have catalyzed signifi -
cant postglacial episodes of human dispersal in coastal regions.
INTRODUCTION
Fairbanks (1989) fi rst suggested that the rate of meltwater supply to
the global ocean during deglaciation was not constant, and Blanchon and
Shaw (1995) concluded from coral dating in the Bahamas that a “cata-
strophic rise event” (also called CRE-3, Melt Water Pulse 3, MWP-1d)
raised sea level by 5–6 m over a few hundred years immediately following
7600 cal yr B.P. (calibrated radiocarbon years before 1950) Since that time
considerable additional research effort based on a wide range of coral,
sedimentary, and geophysical records around the world have suggested,
but not proved, the existence of stepped sea-level rise at intervals in the
early Holocene, and the magnitude and timings of periods of slow versus
fast sea-level rise vary widely between individual studies (Blanchon and
Shaw, 1995; Blanchon et al., 2002; Liu et al., 2004; Yu et al., 2007; Bird et
al., 2007; Tamura et al., 2009; Bard et al., 2010). Other studies have con-
cluded that eustatic sea level rose smoothly through the early Holocene
(Bard et al., 1996; Toscano and Macintyre, 2003).
Unambiguously resolving infl ections within any sea-level record is
diffi cult. This is due to the ambiguities associated with determining the
relationship between a dated sample and paleo–sea level, the large iso-
static and/or tectonic corrections that are required at some sites, and large,
often site-specifi c uncertainties in dating that vary with both environment
and dating technique (Edwards, 2006; Stanley, 2001; Woodroffe and Hor-
ton, 2005). These uncertainties are compounded when several records
from dispersed locations are overlain for comparison.
Bird et al. (2007) presented a high-resolution sea-level record from
6500–8500 cal yr B.P. from −16 m to +2.5 m relative to modern mean sea
level, and suggested, but could not conclusively demonstrate, that eustatic
sea-level rise slowed or ceased for a period of a few hundred years follow-
ing 7900 cal yr B.P., rising rapidly by 2–4 m shortly after 7500 cal yr B.P.
This study seeks to further test the inference of the earlier study by refi n-
ing and extending the sea-level curve of Bird et al. (2007) and developing
multiple proxies for sea level using a shallow-marine sediment core from
the same area.
STUDY AREA AND METHODS
A 30 m sediment core was obtained from downtown Singapore
(Geylang district; 1.313713°N; 103.891772°E, +2.65 m relative to mod-
ern mean sea level, rsl; Fig. 1). The 16 m core records the Holocene
marine transgression from 8900 cal yr B.P. in a uniform sequence of fi ne
gray marine clays, overlain by ~1.5 m of transgressive mangrove peat that
prograded over the site in the past 1500 yr as sea level dropped from its
regional mid-Holocene high of +2.5 m.
Key to interpreting the record in the core in terms of sea level is the
location of the core in a low-energy, mangrove-fringed Holocene embay-
ment, remote from any signifi cant fl uvial source of sediment. The Holo-
cene Upper Marine Clay accumulated at the site is largely derived erosion
of the Lower Marine Clay, deposited during the last interglacial period
of high sea level along with sediment eroded from the unlithifi ed, deeply
weathered, kaolinite-rich Old Alluvium of Tertiary age. Both these units
underlie much of the modern Straits of Singapore (Bird et al., 2006).
The upper 1 m of undisturbed sediment (−0.35 to –1.35 m rsl) con-
sists of sandy peat with abundant large organic fragments, and occa-
sional clean sand beds as much as 5 cm thick. The mangrove overlies a
13.65-m-thick sequence of Upper Marine Clay (–1.35 to –15.0 m rsl), a
homogeneous sequence of clays ranging from greenish-gray (10GY 4/1)
to gray (5G 5/1), with diffuse laminations 1–2 mm thick throughout. Both
dry bulk density (0.89 ± 0.07 g/cm
3
; n = 105) and organic carbon content
(0.94 ± 0.18 %C, n = 105) of the sediments are very consistent throughout
(Table DR1 in the GSA Data Repository
1
).
Geology, September 2010; v. 38; no. 9; p. 803–806; doi: 10.1130/G31066.1; 2 fi gures; Data Repository item 2010226.
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
1
GSA Data Repository item 2010226, Tables DR1–DR3, is available online
at www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety
.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Punctuated eustatic sea-level rise in the early mid-Holocene
Michael I. Bird
1,
*, William E.N. Austin
2
, Christopher M. Wurster
1
, L. Keith Fifi eld
3
, Meryem Mojtahid
4
, and Chris Sargeant
2
1
Earth and Environmental Science, James Cook University, PO Box 6811, Cairns, Queensland 4870, Australia
2
Geography and Geosciences, University of St Andrews, Fife KY16 9AL, Scotland, UK
3
Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200, Australia
4
National Oceanography Centre, University of Southampton, Southampton S014 3ZH, UK
Malaysia
Singapore
Straits
KCD
SMU
MAR
NIE
SBU
LCK
GEYLANG CORE (GEY)
STA
SEK
Singapore
Land
Reclaimed land
Holocene Kallang Formation
104°00′E
103°30′E
01°30
′
N
Figure 1. Location of Geylang core and other cores and areas.
804 GEOLOGY, September 2010
The stable isotope composition of organic carbon and carbonate is
widely used as a proxy in paleoenvironmental reconstruction and can be
used to quantify the relative proportions of marine versus terrestrially
derived water and carbon in a range of sample materials (Bouillon et al.,
2008), thus providing information on coastline proximity and hence sea
level. Details for the methodologies used for foram and organic carbon
isotope analysis are provided in Tables DR1 and DR2.
The radiocarbon chronology for the Geylang core was constructed
by dating 13 cleaned, single bivalves or gastropods, and 2 samples of mac-
roscopic mangrove wood (Fig. 2A; Table DR3). The modern elevation
of these samples was measured to ±5 cm, with allowance for as much as
0.5 m of lowering due to autocompaction (Bird et al., 2007). Of the 50
samples used by Bird et al. (2007) in construction of the sea-level curve,
15 were redated for this study at higher precision (Table DR3). In addition,
the total number of index points available for the sea-level curve has been
augmented by 10 dates from 6500 cal yr B.P. to the present (Table DR3).
Discussion of the Singapore sea-level curve and an analysis of the uncer-
tainties associated with the curve were provided in Bird et al. (2007).
Details of calibration to calendar years and determination of local reser-
voir effects are provided in Table DR3.
RESULTS AND DISCUSSION
The extended sea-level curve for Singapore (Fig. 2A) provides
a record for Holocene sea level in Singapore that is consistent with an
infl ection in the rate of sea-level rise between 7800 and 7400 cal yr B.P.
However, dispersion in the data due to uncertainties associated with eleva-
tion, potential minor reworking, and analytical error means that this inter-
pretation cannot be considered conclusive, as previously concluded by
Bird et al. (2007).
The Geylang core provides both physical and geochemical proxy
records that can be used to provide additional independent constraints on
eustatic sea-level rise. As the sea fl ooded the Straits of Singapore, and with
limited opportunity for stabilization by mangrove progradation during
rapid marine transgression, sediment accumulated rapidly at the core site.
The chronology for the core (Fig. 2A) suggests rapid sediment accumula-
tion, at as much as 1 cm/yr, between 8500 and 7800 cal yr B.P., coincident
with a rapid rise in sea level of 1.8 cm/yr. Between 7800 and 7400 cal yr
B.P., coincident with the near cessation inferred from the sea-level curve,
the rate of sediment accumulation at the core site decreased by an order of
magnitude to ~0.1 cm/yr, at a water depth of ~4 m (Fig. 2C). After 7400
cal yr B.P., the rate of sediment accumulation increased progressively to
~0.5 cm/yr, before dropping by an order of magnitude to an average of
0.05 cm/yr upon attainment of the mid-Holocene highstand after 6500
cal yr B.P. The rapid drop in sedimentation rate between 7400 and 7800
cal yr B.P. is consistent with the hypothesis that provision of abundant
sediment accompanying rapid sea-level rise ceased as the result of stabi-
lization of sea level at that time, as was the case when sea level stabilized
again during the mid-Holocene highstand after 6500 cal yr B.P.
Water depth at the core site during the Holocene was controlled
by both the rate of sea-level rise and the rate of sediment accumulation.
Subtraction of the depth to sediment-water interface in the Geylang core
(Fig. 2A) from the inferred maximum and minimum sea-level envelope
derived from the sea-level curve (Fig. 2A) provides a broad estimate of
water depth (Fig. 2B). This suggests that water deepened rapidly at the
core site to a maximum of ~6 m shortly before 8000 cal yr B.P. Water
depth then decreased to ~4 m between 7800 and 7400 cal yr B.P. After
ca. 6000 cal yr B.P., water depth decreased slowly, ultimately leading to
mangrove progradation over the site. While the errors on the estimate of
water depth are comparatively large, it is clear that the location remained
fully subtidal until the late Holocene.
The carbon isotope composition (δ
13
C) of organic matter in the core
refl ects the relative input from terrestrial, dominantly mangrove-derived,
Figure 2. Sea-level curve for Singapore and results from Geylang
core. A: Open circles—relative sea-level curve from 8800 cal (cali-
brated) yr B.P. to present based on new radiocarbon dates of sea-
level index points (Table DR3; see footnote 1) and data of Bird et
al. (2007) (msl—mean sea level). Generalized trend through data
shown by dashed line is within 2σ error of 88% of dates (excluding
three results between 1000 and 3000 cal yr B.P., all of which repre-
sent samples underlain by thick mangrove peat subject to signifi -
cant autocompaction). Filled squares—radiocarbon dated samples
from Geylang core. Age-depth model (solid line) was calculated
using three quadratic functions representing periods of differing
sedimentation rate corresponding to before 7800, 7800–6500 and af-
ter 6500 cal yr B.P. Key intervals of near cessation of sea-level rise
before 7400 cal yr B.P. and renewed rise after 7400 cal yr B.P. are
shown by shaded areas. Other studies are indicated by horizontal
lines: 1—Rapid sea-level rise, Caribbean (Blanchon and Shaw, 1995;
Blanchon et al., 2002). 2—Stillstand followed by rapid fl ooding of the
Mekong delta (Tamura et al., 2009). 3—Rapid sea-level rise, Baltic
Sea (Yu et al., 2007). 4—Yangtze delta rice cultivation (Zong et al.,
2007). 5—Rapid melting of Labrador sector of Laurentide ice sheet
(Carlson et al., 2007, 2008). B: Water depth at Geylang core site is
shown by solid line, dotted lines represent lower and upper limits.
Mean low-water spring tide depth (MLWS, −1.2 m). C: δ
13
C values of
organic carbon in the Geylang core (solid squares), with three-point
running mean (solid line). V-PDB—Vienna Peedee belemnite. D: δ
13
C
(open diamonds) and δ
18
O (solid diamonds) of benthic foraminifera.
Both Ammonia spp. and Elphidium spp. were analyzed in each sam-
ple. The δ
13
C of Elphidium spp. was on average 0.48‰ lower than
Ammonia spp. sample from same interval, due to differing vital ef-
fects. Hence this amount was added to Elphidium δ
13
C results before
calculation of an average ammonia spp equivalent δ
13
C value, with
quoted error being deviation from mean. There was no signifi cant
difference between δ
18
O values of these two species, so results were
averaged, with quoted error being deviation from mean. V-SMOW—
Vienna standard mean ocean water.
GEOLOGY, September 2010 805
carbon (~−27‰) and marine-derived carbon (~−21‰), in turn a measure
of coastline proximity (Bouillon et al., 2008) and hence a measure of sea-
level change. Figure 2C shows that organic δ
13
C values track the inferred
water depth at the site (Fig. 2B), rising from relatively more negative
(dominantly terrestrial) values to a high of ~–25‰ shortly before 8000
cal yr B.P. and rapidly dropping thereafter to 7900 cal yr B.P. This trend
refl ects initial rapid deepening followed by shallowing and rapid mangrove
progradation, made possible by the maintenance of high rates of sediment
supply until the rate of sea-level rise slowed after the 8.2 ka event (Kendall
et al., 2008). δ
13
C values stabilized between 7800 and 7400 cal yr B.P. as
sea level stabilized, but then rose to a second maximum of ~–24‰ shortly
before 6500 cal yr B.P. A rapid drop in δ
13
C values at the onset of Holocene
highstand conditions suggests that mangroves were again able to advance
as the rate of sea-level rise slowed toward the mid-Holocene highstand.
After 6500 cal yr B.P., δ
13
C values stabilized and ultimately decreased to
terrestrial δ
13
C values as mangroves became established on the site.
The averaged δ
13
C and δ
18
O values of two species of benthic fora-
minifera (Ammonia spp. and Elphidium spp.) are shown in Figure 2D.
δ
13
C increased rapidly from −4.5‰ to −2.8‰ until 7800 cal yr B.P. dur-
ing the early period of rapid sea-level rise. Thereafter δ
13
C values stabi-
lized at −2.8‰ until 6900 cal yr B.P., when δ
13
C values rapidly increased
to −1.9‰ before decreasing at 6500 cal yr B.P. The δ
13
C values of these
species are infl uenced by proximity to terrestrially sourced dissolved
inorganic carbon (DIC) from adjacent mangrove areas (Bouillon et al.,
2008). Hence the initial rise in δ
13
C values of these foraminifera is con-
sistent with the inferred rapid increase in water depth at the time, and
subsequent stabilization in δ
13
C values is also consistent with a near ces-
sation in sea-level rise. It is important to note that, for these benthic
species, δ
13
C values will also be partly determined by δ
13
C values of
DIC re mineralized from respired organic matter from the underlying
sediment. As the thickness of the sediments increased, remineraliza-
tion-derived DIC would have increased, partly masking any rapid δ
13
C
response due to sea-level change. Thus while the cessation of sea-level
rise is clearly marked at 7800 cal yr B.P., the response of foraminferal
δ
13
C values to renewed sea-level rise after 7400 cal yr B.P. is delayed and
muted relative to the organic δ
13
C response.
After ca. 6500 cal yr B.P., δ
13
C again decreased in response to sea-
level fall and mangrove progradation, further supporting the link between
foram δ
13
C values and coastline proximity. The δ
18
O values of the forams
(Fig. 2D) show little systematic variability and no anomalies associated
with the changes in sea level implied by the other proxy records, provid-
ing further evidence that the variations in sedimentation rate and carbon
isotope composition observed in the core were not driven by large local
perturbations in circulation or climate.
Other studies have suggested the possibility of a rapid rise in sea level
ca. 7500 cal yr B.P. (Blanchon and Shaw, 1995; Blanchon et al., 2002; Liu
et al., 2004; Yu et al., 2007; Bird et al., 2007; Tamura et al., 2009), but
none have catalyzed general acceptance of the existence of such an event.
This study links a high-resolution sea-level record with three independent
proxy records of sea level (sedimentation rate and organic and foram δ
13
C
values) derived from a shallow-marine core in the same tectonically and
climatically stable continental location, remote from isostatic ice-loading
effects. A fourth proxy, foraminiferal δ
18
O, allows us to eliminate major
changes in climate or circulation as a confounding effect. Synthesis of
these independent records suggests that sea level rose rapidly until shortly
after the 8.2 ka event, slowing suffi ciently after that time to allow man-
grove progradation and slowing dramatically or stopping between 7800
cal yr B.P. and 7400 cal yr B.P. After 7400 cal yr B.P. sea level again rose
by ~4 m by 6500 cal yr B.P., accompanied by further mangrove prograda-
tion. A mid-Holocene highstand of ~+2.5 m was reached after 6500 cal yr
B.P., followed by a lowering, with mangroves prograding over the core
site by ca. 1000 cal yr B.P.
Melting of the Northern Hemisphere ice sheets and Antarctica,
including the catastrophic draining of Lake Agaziz-Ojibway, is thought to
have provided the meltwater fl ux that drove sea-level rise until ca. 8000
cal yr B.P. (e.g., Kendall et al., 2008). Recent work has also indicated that
after a period of relative stability between 8.0 and 7.4 ka (based on
10
Be
exposure dating), the Labrador sector of the Laurentide ice sheet melted
rapidly between 7.4 and 6.8 ka, contributing ~3 m of equivalent sea level
(Carlson et al., 2008). Both the timing and magnitude of this melting event
are consistent with the conclusions of this study, although meltwater con-
tributions from Antarctica after 7400 cal yr B.P. and into the later Holo-
cene are also possible (Conway et al., 1999; Stone et al., 2003).
Horton et al. (2005) modeled sea level at two locations close to Sin-
gapore. Their results suggest rates of sea-level rise of 0.4 to 0.5 cm/yr,
with sea level modeled to be 5–9 m below modern mean sea level at 9000
cal yr B.P., rising to +1 to +3 m above modern mean sea level at 7000
cal yr B.P., with a highstand of +2 to +4 m at 6000 cal yr B.P. at each loca-
tion. The 1000 yr time step used by Horton et al. (2005) does not enable
direct comparison with the centennial-scale variations in the rate of sea-
level rise reported here. However, the results reported here suggest that sea
level rose by 1.8 cm/yr from 8800 cal yr B.P. to 8100 cal yr B.P. (Fig. 2A),
a rate that is signifi cantly higher than suggested by Horton et al. (2005).
This is partly attributable to additional meltwater input associated with the
8.2 ka event (Kendall et al., 2008), but might also provide support for the
suggestion that sea-level rise in the Holocene prior to 8100 cal yr B.P. was
also stepped (Liu et al., 2004).
At other locations similar to Singapore, remote from ice-loading
effects, including the major delta areas of Southeast Asia, a period of rela-
tive sea-level stability would have provided a distinct interval for delta ini-
tiation and progradation. Stanley and Warne (1994) found a modal peak
in global delta initiation between 7800 and 8200 cal yr B.P., at which time
one-third of major deltas were initiated. In Southeast Asia, for example,
in the 400 yr between 7800 and 7400 cal yr B.P., as much as 1400 Gt of
sediment would have been delivered to the coastal zone, based on modern
sediment discharge of major Asian rivers from the Ganges to the Huang
He (Milliman and Meade, 1983). This accumulating sediment may have
provided a base upon which to build the extensive modern delta com-
plexes that now represent the major food-growing areas of Asia, providing
greatly expanded, fertile, resource-rich regions for exploitation by prehis-
toric human populations by the mid-Holocene.
Early rice cultivation from 7700 cal yr B.P. has recently been demon-
strated from the Yangtze delta at Kuahuqiao; the site was abandoned due
to renewed sea-level rise ca. 7550 cal yr B.P. (Zong et al., 2007; Innes et
al., 2009). Other deltas in the region, including the Mekong and Red Riv-
ers, reached maximum fl ooding ca. 8000 cal yr B.P., followed by an inter-
val of stability, with some parts of the deltas fl ooding again after ca. 7500
cal yr B.P. (Tanabe et al., 2006; Tamura et al., 2009). It is possible that this
renewed sea-level rise forced dispersal of populations with a previously
developed ability to exploit delta resources into new coastal areas.
Human migration in prehistory is thought to have long been associ-
ated with coastlines (e.g., Stringer, 2000), and rapid fl ooding of delta areas
after a period of relative stability may have served to focus coastal and/
or maritime human dispersal into a narrow time frame shortly after 7400
cal yr B.P. This possibility is worth further examination in the context
of, for example, controversial postulates concerning early Austronesian
maritime dispersals before 7000 cal yr B.P. from an eastern Indonesian
homeland (Oppenheimer and Richards, 2001).
CONCLUSIONS
The combination of a high-resolution sea-level curve for Singapore
with independently derived indicators of water depth and sea level derived
from a shallow-marine sediment core, also from Singapore, suggests
that there was a period of no, or minimal, eustatic sea-level rise between
806 GEOLOGY, September 2010
7800 and 7400 cal yr B.P., separating intervals of more rapid rise before
and after. Changes in the rate of eustatic sea-level rise prior to the mid-
Holocene may provide part of an explanation for the initiation of deltas in
farfi eld locations and may have served to catalyze and focus episodes of
human migration in prehistory.
ACKNOWLEDGMENTS
This research was funded by the Academic Research Fund of Nanyang Tech-
nological University. We thank G. Cook and J. Cali for the provision of some of the
data supporting this study.
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Manuscript received 27 January 2010
Revised manuscript received 5 April 2010
Manuscript accepted 12 April 2010
Printed in USA
