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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01703, doi:10.1029/2006GL028155, 2007
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Holocene East Asian monsoon variability: Links to solar and tropical
Pacific forcing
Kandasamy Selvaraj,1 Chen Tung Arthur Chen,1 and Jiann-Yuh Lou2
Received 18 September 2006; revised 20 November 2006; accepted 22 November 2006; published 3 January 2007.
[1] Sedimentary geochemical records from subalpine
Retreat Lake, subtropical Taiwan, document the unstable
East Asian monsoon (EAM) climate over the past
10,300 years, with a weak EAM between 10.3 and
8.6 ky B.P., EAM intensity peaks between 8.6 and
7.7 ky B.P., and then gradually decrease in response to
summer insolation, heat and moisture transport. Our proxy
record reveals several weak monsoon intervals that correlate
to low sea surface temperatures in the western tropical
Pacific and cold events in the North Atlantic, linking the
tropical Pacific, North Atlantic, and Polar climates because
weak EAM events at 8.2, 5.4 and 4.5– 2.1 ky B.P. also
correspond to low values of atmospheric methane and
periods of reduced North Atlantic Deep Water formation.
We therefore suggest that centennial to millennial scale
monsoon variability during the Holocene in the northern
subtropics is globally-mediated via a sun-ocean-monsoonNorth Atlantic linkage. Citation: Selvaraj, K., C. T. A. Chen,
and J.-Y. Lou (2007), Holocene East Asian monsoon variability:
Links to solar and tropical Pacific forcing, Geophys. Res. Lett., 34,
L01703, doi:10.1029/2006GL028155.
1. Introduction
[2] Marine sedimentary records from the North Atlantic
demonstrate that the Holocene was interrupted by a series
of quasi-periodic (1000 – 1500 yr) cold, dry events
[deMenocal et al., 2000a; Bond et al., 2001]. Such climate
instabilities are also apparent in high-resolution marine
[Gupta et al., 2005] and speleothem-based Asian monsoon
(AM) records [Fleitmann et al., 2003; Wang et al., 2005],
suggesting that centennial to millennial scale monsoon
events were caused by solar and glacial boundary forcing.
High-resolution records from the tropical Pacific [Lea et
al., 2000; Stott et al., 2004] show evidence for internal
forcing on decadal to millennial scale monsoon variability
though the importance of internal forcing remains ambiguous [Wang et al., 2005]. This warrants additional data to
evaluate the combined sun-monsoon link and the role of
tropical climate dynamics on a global scale, especially
within the AM system. One ideal location for such study is
the subtropical East Asian island of Taiwan, well known
for its vigorous tropical and extra-tropical land-oceanatmosphere interactions.
[3] Taiwan’s weather and climate are primarily controlled
by the East Asian monsoon (EAM). The EAM is the result
1
Institute of Marine Geology and Chemistry, National Sun Yat-sen
University, Kaohsiung, Taiwan.
2
Department of Marine Science, Naval Academy, Kaohsiung, Taiwan.
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2006GL028155$05.00
of thermal contrast between the Asian landmass and the
tropical Pacific with an additional thermal and dynamic
trigger of the Tibetan Plateau [An, 2000]. The EAM is
extremely important because it regulates global atmospheric
circulation through heat and moisture transport from the
warmest part of the tropical ocean, the West Pacific Warm
Pool (WPWP), to higher latitudes [Wang et al., 2001]. Since
Taiwan has the world’s highest physical weathering rate
(1365 mg/cm2/yr) [Li, 1976], terrestrial vegetation and
weathering are highly dependent on the intensity of the
EAM. Here we report the results of geochemical analyses
(for further details, see auxiliary material1) performed on
sediments from a small (104 m2), hydrologically closed,
subalpine Retreat Lake (24°2903000N, 121°2601500E; 2230 m)
in NE Taiwan. The lake lies directly in the path of the
moisture-laden, southeasterly down-winds of the EAM from
the western tropical Pacific (WTP), and close to the main
axis of the Kuroshio Current, a major boundary current that
transports a large volume (18 – 25 Sverdrup = 106 m3 s1) of
warm, salty seawater from the WTP to the North Pacific.
The modern lake catchments have two different seasons: a
warm, wet season during Northern Hemisphere (NH) summer when the intertropical convergence zone (ITCZ) shifts
northward (Figure 1s) and EAM reaches its maximum and a
cool, dry season during the boreal winter when the Siberian
high establishes a strong anticyclone on the Tibetan Plateau
and ITCZ migrates to a southern position that is associated
with the stronger winter monsoon. The lake is surrounded
by 60 m high mountains comprised of Tertiary metamorphic rocks (slate, phyllite and argillite). Lake level is
primarily controlled by precipitation and evaporation as
there are no in- and out-flow. The combination of mild
mean annual temperature (12°C) and high precipitation
(3000 mm) results in thick vegetation in the catchments
of Retreat Lake.
2. Materials, Methods, and Proxies
[4] Total organic carbon (TOC), total nitrogen and magnetic susceptibility (MS) were measured in 1.7 m long
sediment core collected from the lake that was continuously
sub-sampled at 1 cm interval, which is equivalent to a
temporal resolution of 45 years. Selected major and trace
elements were analyzed from samples of coarse intervals
(2– 4 cm) to provide additional constraints of EAM variability on subtropical weathering. Chronology is based on
8 accelerator mass spectrometry (AMS) and 2 conventional
14
C dates of bulk organic matter (Table S1)1, all uncorrected
dates (see auxiliary material and Figure S2) converted to
1
Auxiliary material data sets are available at ftp://ftp.agu.org/apend/gl/
2006gl028155. Other auxiliary material files are in the HTML.
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Figure 1. Sedimentary geochemical proxy records of Retreat Lake depicting Holocene East Asian monsoon variability in
subtropical Taiwan. (a) Age model and a hiatus between 2.1 and 4.5 ky B.P. (35 – 38 cm), (b) total organic carbon and
d 13C values, (c) carbonorganic: nitrogentotal ratio, (d) magnetic susceptibility, (e) chemical index of alteration, (f) Rb/Sr ratio,
and (g) K/Rb ratio. Cross bars in Figures 1e – 1g represent non-orbital weak monsoon intervals.
calendar years using the calibration [Stuiver and Reimer,
1993]. The linearly interpolated calendar age-depth model
(Figure 1a) indicates that sedimentation was variable with a
mean sedimentation rate of 16.8 cm/ky spanning the last ca.
10.3 calendar kilo years before the present (ky B.P., where
the ‘‘present’’ is defined as the year 1950 A.D.). A hiatus
with extremely slow sedimentation (1 cm/790 years)
between 38 and 35 cm was, however, demarcated by 2
closely-spaced dates (4.5 ± 0.03 and 2.12 ± 0.04 ky B.P.;
Figure 1a) perhaps in large part because of desiccation. To
interpret the Holocene’s EAM variability, we used characteristic sedimentary geochemical proxies namely TOC,
carbonorganic: nitrogentotal (C/N ratio), MS, the chemical
index of alteration (CIA), a proxy for terrestrial weathering
calculated as in work by Selvaraj and Chen [2006], as well
as the Rb/Sr and K/Rb ratios. An increase or decrease in the
proxies is more closely related to the strength of the EAM
than to any other climatic variable in the study area. High
TOC value and C/N ratio, and associated higher negative
carbon isotope (d13C) values, for example, are interpreted to
reflect vascular plant (C3) dominance due to increased
monsoon precipitation during periods of warm/wet climate.
Conversely, high MS and CIA values, and high Rb/Sr but
low K/Rb ratios reflect increased erosion and deposition of
minerogenic sediments due to decreased monsoon precipitation during periods of cool/dry climate.
3. Results and Discussion
[5] The geochemical records for Retreat Lake (Figure 1),
consistent with sediment lithology (auxiliary Figure S2),
show three distinct climatic episodes. First, a cold, dry
period with low precipitation between ca. 10.3 and 8.6 ky
B.P. is indicated by the lowest TOC value and low C/N
ratios. This interpretation is substantiated by high values of
MS, CIA, and high Rb/Sr but low K/Rb ratios indicate
strong erosion in lake catchments due to less dense vegetation cover. Second, a warm and wet interval with strong
EAM precipitation during the ‘‘Holocene optimum’’ between 8.6 and 4.5 ky B.P. is indicated by organic-rich,
peaty sediments with high contents of TOC and a high
C/N ratios. In this interval, negative MS and low CIA values
along with low Rb/Sr and high K/Rb ratios indicate intense
chemical weathering and a drastic reduction in detrital
input into the lake as a result of reduced erosion due to
thick C3 vegetation. Third, a dry climate with weak monsoon precipitation since ca. 4.5 ky B.P. is revealed by low
contents of TOC and low C/N ratios, relative to sediments
deposited in the ‘‘Holocene optimum’’ interval, as well as
higher values of MS and CIA, and the high Rb/Sr but lower
K/Rb ratios. A driest interval at 4.5 to 2.1 ky B.P. is
indicated by acute starvation of organic-inorganic materials
input into the lake, suggesting a significant drop, perhaps
failure, in EAM precipitation which probably brought the
lake into a desiccation stage; an inference very consistent
with an arid period (4.4– 2.1 ky B.P.) shown in speleothem d18O record from central China [Shao et al., 2006].
Typical lacustrine detrital records such as those for Ti, Zr,
and Ti-based detrital percentage, and molar SiO2/Al2O3 - a
grain size proxy, all (auxiliary Figure S3) corroborate the
above inferred EAM fluctuations.
[6] EAM variability is influenced by different factors,
whereas NH summer insolation, the oceanographic characteristics of the WPWP, the strength of the North Equatorial
Current (NEC) in the western Pacific, and the El Niño –
Southern Oscillation (ENSO) in the tropical Pacific are the
most important ones. In subtropical Taiwan, the rapid
increase and peak EAM precipitation between 8.6 and
7.7 ky B.P. (Figures 1 and 2a) indicate a rapid northward
migration of the ITCZ and is in phase with time lag of
2.4 ky to the NH summer insolation anomaly at 11 –
12 ky B.P. [Berger and Loutre, 1991]. During this interval,
a marked change from organic-poor to -rich, peaty sediments and a concomitant negative excursion of d13C values
(from 27% to 30.6%) (Figure 1b) indicating the sudden
expansion of C3 biomass favoring warm, humid conditions.
This increase in EAM intensity in subtropical Taiwan is
consistent with the onset of climatic optimum in the
Loess Plateau [An et al., 2000] and corresponds to the
highest growth rate (as high as 500 mm/yr) between 8.5 and
7.5 ky B.P. in Chinese stalagmite [Dykoski et al., 2005].
The strong, negative relationship between TOC record, an
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Figure 2. East Asian monsoon record from Taiwan
combined with NH insolation, records of low-latitude
monsoons, Pacific SSTs, and North Atlantic hematite
percentages. Time series of (a) Retreat Lake’s TOC and
insolation (dotted line) at 30°N, averaged through June,
July, and August (JJA) [Berger and Loutre, 1991],
(b) Dongge cave d 18O [Wang et al., 2005], (c) Qunf cave
d 18O [Fleitmann et al., 2003], (d) Globigerina bulloides
percentage in Hole 723A [Gupta et al., 2005], (e) Cariaco
Basin Ti [Haug et al., 2001], (f) faunal-based SSTs from
MD81 [Stott et al., 2004], and (g) hematite % from the
North Atlantic, events labeled 0 –5 in work by Bond et al.
[2001]. Grey bars highlight the weak EAM intervals (see
also auxiliary Figure S1 and Table S2).
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ideal proxy for EAM variations, and summer (JJA) insolation evident between 10.3 and 8.6 ky B.P. (Figure 2a)
reveals that the EAM intensity during the early Holocene
was largely controlled by glacial boundary forcing (e.g.,
Tibetan Plateau snow cover and North Atlantic SSTs).
Although decadal to centennial variations in Indian Ocean
monsoon (IOM) precipitation between 10.3 and 8 ky B.P.
indicates an analogous relationship with Greenland temperatures [Fleitmann et al., 2003], the delayed strengthening
of EAM in Taiwan could also be related to the tropical
Pacific SSTs which attained its Holocene average at
8.5 ky B.P. in the WPWP, where EAM prevails [Gagan
et al., 2004]. This, however, contradicts speleothem- [e.g.,
Dykoski et al., 2005] and pollen-based [Liew et al., 2006]
monsoon reconstructions which all show evidence for an
enhanced monsoon during the early Holocene.
[7] Nevertheless, our EAM record shows a strong visible
coincidence with insolation from 8.6 ky B.P. onward
(Figure 2a), indicating that long-term changes in EAM
intensity depend on summer insolation. Importantly, the
gradual long-term decrease in monsoon precipitation in
subtropical Taiwan resembles speleothem-based AM [Wang
et al., 2005], IOM [Fleitmann et al., 2003] records from
China and Oman, faunal-based southwest AM record from
the Arabian Sea [Gupta et al., 2005], and sedimentary bulk
Ti record from the Cariaco Basin, tropical Atlantic [Haug et
al., 2001] (Figure 2). Accordingly, during the ‘‘Holocene
optimum’’, our TOC record, consistent with the Ti record
of the Cariaco Basin, shows increased precipitation and
thus supports a more northerly mean position of the Pacific
ITCZ. Conversely, during the late Holocene, the southward
migration of ITCZ owing to less seasonal NH insolation
would have reduced the strength of summer monsoon in
Taiwan as well as the entire northern subtropics. Equally
important, this low-latitude, wide monsoon similitude indicates that the broad temporal pattern of postglacial to
modern precipitation in the subtropics are controlled, probably on a global scale, by the continuous southward
migration of the mean summer ITCZ and a gradual weakening of the monsoon intensity in response to orbitallyinduced declining summer insolation; an interpretation
consistent with modeling study [Kutzbach and OttoBliesner, 1982].
[8] After 8.6 ky B.P., the long-term monsoon trend is
punctuated by eight discrete intervals of weak EAM (low
TOC values; vertical grey bars in Figure 2) that, within
dating error, can be correlated with other monsoon records
and the low values of faunal Mg/Ca-derived SST record
from the WTP [Stott et al., 2004] (Figure 2f), indicating the
influence of tropical SSTs on the EAM and, in turn, the lowlatitudinal monsoon patterns. Among these intervals, 6 coincide to the timings of millennial-scale events (cold spells
numbered 0 –5; Figure 2g) in the North Atlantic [Bond et
al., 2001] (plotted such that cooling and monsoon weakening are downward), suggesting the influence of variable
solar activity on the EAM. Three weak EAM events of
relatively longer in duration and larger in amplitude at
around 8.3 – 8, 5.7 – 5.1 and 4.5 – 2.1 ky B.P can be
correlated to the low values of atmospheric CH4 recorded
in the GRIP ice core, central Greenland [Blunier et al.,
1995] (Figure 3). Furthermore, these events coincide with
reduced formation of North Atlantic Deep Water (NADW)
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Figure 3. Weak EAM intervals. (a) Retreat Lake’s TOC
record, (b) Holocene methane (CH4) record from GRIP ice
core [Blunier et al., 1995] and benthic foraminifer’s d 13C
record from ODP site 980 [Oppo et al., 2003]. Grey bars
indicate larger amplitude weak EAM intervals.
as demonstrated by d13C values of benthic foraminifera,
Cibicidoides wuellerstorfi, an established proxy for deep
water variability (Figure 3), from the sediments of the
subpolar northeastern Atlantic [Oppo et al., 2003]. These
high-latitude cooling-reduced NADW formation and low
tropical SSTs-weak EAM couples suggest that the reduced
heat and moisture export during these times may reduce
the global wetlands and contributed to decreased atmospheric methane, exactly as noted [Blunier et al., 1995;
Alley et al., 1997]. The event centered at 8.17 ky B.P.
closely correlates with the ‘8.2 ky event’ recorded in
Greenland ice cores [e.g., Alley et al., 1997], probably
related to a catastrophic outburst of freshwater from icemargin lakes to Hudson Bay. A prominent, weak EAM
centered at 5.4 ky B.P. is consistent with the termination timing (5.5 ky B.P.) of the African humid period
[deMenocal et al., 2000b] and slightly leads the onset
(5 ky B.P.) of modern frequencies of the ENSO [Moy et
al., 2002]. A wide interval of weak EAM and consequent
lake desiccation at 4.5 to 2.1 ky B.P. are possibly linked to
cold events 1, 2, and 3 (Figures 2a and 2g). This deterioration is widely represented by lower lake levels in China,
India, and Africa, as well as widespread cultural collapse in
low-latitude regions, including the Neolithic cultural collapse in China [Wang et al., 2005] (NCC in Figure 2b).
[ 9 ] Although orbitally-induced summer insolation
explains the long-term Holocene monsoon patterns in
subtropical Taiwan, abrupt, short-term monsoon variations,
including the weak monsoons at 7, 6.3 and 5.4 ky B.P.
(hatched bars in Figure 1 and auxiliary Figure S3) within the
optimum conditions, seem to have little or no clear external
forcing has drawn the tropical climate dynamics as an
alternate candidate. Ocean-atmospheric interactions, particularly changes in the SST, in the tropical Pacific have
always had a strong influence on the export of heat and
water vapour to high-latitudes and are, therefore, thought
to be an active force for global climate change [e.g., Lea et
al., 2000]. The SSTs were mostly above 29°C before 5 ky
B.P. which is consistent with the strong EAM during the
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‘‘Holocene optimum’’ (Figures 2a and 2f). A remarkable
increase between 9 and 7.3 ky B.P. and relative higher
abundance up to 5 ky B.P. in a deep-dwelling, tropical
planktonic foraminifer, Pulleniatina obliquiloculata, an
indicator species of the Kuroshio path and intensity, in a
number of marine sediment cores in the Okinawa Trough
[e.g., Jian et al., 2000] and the South China Sea [e.g., Wang
et al., 1999] demonstrate the enhanced meridional heat and
moisture transport from the WTP to the North Pacific when
the EAM reached its northernmost limit and the Loess
Plateau was relatively greener than today. Conversely,
reduced heat transport is unambiguous in SST record that
shows less than 29°C with a clear decrease between 5 and
2 ky B.P. (arrow in Figure 2f) and corresponds to a weak
EAM and lake desiccation in subtropical Taiwan. This
inference is evidenced in the abrupt decrease in the
abundance of P. obliquiloculata at 4.6 to 2.7 ky B.P. in
sediment cores from the Okinawa Trough [Jian et al.,
2000], at 4.9 to 2.9 ky B.P. in the South China Sea [Wang
et al., 1999], and in the synchronous drop in SSTs 5 –
3 ky B.P. in the latter area. The decrease in the depth of
the thermocline around 4.3– 2.5 ky B.P. in the Okinawa
Trough further reveals the relaxation of trade winds
throughout the Pacific and thus a weakened NEC and
Kuroshio. Eventually, weak EAM events closely correspond to periods (0.6, 1.7, 3.3, 4.6, 5.9, and 8.1 ky B.P.)
of weakened Kuroshio found in the Okinawa Trough and
periodicities (1500 and 700– 800 yr) of Kuroshio proxies
[Jian et al., 2000] circuitously indicate a climatic forcing
by changes in oceanic thermohaline circulation, including
its subharmonic mode, on EAM variation.
[10] The climate of the WPWP is fundamentally linked to
ENSO variability with relative drought and reduced SSTs,
as seen in Figure 2f, characterizing the El Niño phase
[Tudhope et al., 2001]. Since modern AM failures and
ENSO have a strong correlation [Charles et al., 1997], the
drastic reduction in EAM strength and the lake desiccation
were indeed a response to ENSO in the late Holocene as
indicated by the increased vegetation loss across the Pacific
Basin, Kalimantan, and Australia as well as composite
records of charcoal abundance adjacent to the WPWP,
Papua New Guinea and Indonesia; all show a high frequency of ENSO activities between 5 and 2 ky B.P. (for
further details, see Gagan et al. [2004]). The enhanced runoff variability in the Ti record of the Cariaco Basin between
3.8 and 2.8 ky B.P. indicates a mean southward shift in the
position of the Pacific ITCZ linked to ENSO changes in the
late Holocene [Haug et al., 2001] (Figure 2e). ENSO model
studies [e.g., Liu et al., 2000] and coral records [e.g., Gagan
et al., 2004] display the greatest ENSO variability ca. 3.5–
1.7 ky B.P. This seems to be correlated with the lowest
content of TOC and C/N ratio ca. 2.0 – 1.6 ky B.P. in
sediments of Retreat Lake and highest d18O values of
stalagmites (grey bar 1 in Figure 2), indicative of weak
AM signals. The Oman stalagmite shows a hiatus in
precipitation between 2.7 and 1.4 ky B.P. (Figure 2c),
further supporting the influence of the Indo-Pacific
Warm Pool on variations in the IOM via the Indonesian
Throughflow as well as EAM. Strong supports for the role
of ocean-atmosphere interactions in monsoon variability
come from an analogous pattern of heat transport in the
North Atlantic [e.g., Poore et al., 2003] and numerous
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lacustrine records of late Holocene-ENSO link in the
American-side Pacific [e.g., Moy et al., 2002].
4. Conclusions
[11] Retreat Lake geochemical records invariably show
an abrupt increase of EAM intensity at 8.6 ky B.P. and
since then the long-term EAM variations, similar to other
AM records, were primarily controlled by the mean position
of the ITCZ which affected directly by summer insolation
and indirectly through its effect on the tropical Pacific SSTs,
and related meridional heat and moisture transport. The
centennial to multi-decadal weak EAM intervals and their
link with both the cold events in the North Atlantic and low
SSTs in the tropical Pacific were at least in part ascribed to
changes in solar forcing. An important observation of our
study is that since 5 ky B.P. the EAM, including IOM,
variations seem to be affected by non-linear onset of ENSO
due probably to the enhanced interaction between the
Southern Oscillation and southward Pacific ITCZ. Our study
underscores the importance of additional, high-resolution
climate records in subtropical Taiwan and along the path
of the Kuroshio Current so as to establish more precise
land-sea correlation and to strengthen the monsoon-tropical
Pacific-ENSO linkage.
[12] Acknowledgments. This study was supported by the National
Science Council of Taiwan (NSC-94-2621-Z-110-001 and 94-2611-M110001). Two anonymous reviewers are thanked for their comments on an
earlier draft of this manuscript.
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C. T. A. Chen and K. Selvaraj, Institute of Marine Geology and
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([email protected])
J.-Y. Lou, Department of Marine Science, Naval Academy, P.O. Box
90175, Tsoying, Kaohsiung, Taiwan.
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