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CHAPTER 4
Holocene Variability in the intensity
of the Arabian Sea Monsoon
Abstract
Holocene changes in the intensity of the Arabian Sea Summer Monsoon (ASSM)
have been reconstructed using proxies of organic productivity, water column
denitrification and lithogenic input from a Somali margin sediment core. This
reconstruction indicates that the strength of the ASSM has decreased gradually
over the last 10 kyr, following the decrease in precessional forcing. The early
Holocene experienced the strongest monsoon winds, which are reflected by high
productivity (Si/Al, P/Al Ba/Al) values, increased denitrification (δ 15N) and
increased rainfall (Ti/Al). The late Holocene increase in grain size (Zr/Al, %
quartz) at site 905 reflects the consequent reduction in continental moisture as
the monsoon intensity diminished. Though marine proxies of monsoon strength
respond gradually to the decrease in precessional forcing, terrestrial proxies of
continental aridity exhibit a threshold response and indicate that the transition
from the African Humid Period to the dry late Holocene was abrupt and
occurred at approximately 5.5 kyr. Sub-orbital variations in monsoon intensity
are superimposed upon the precessional signal with an evident periodicity of
1830 years. A comparison of the core 905 records with tropical rainfall records
suggests that the strong monsoon of the early Holocene coincides with a northern
position of the ITCZ and that the gradual decline in monsoon strength over the
Holocene has been by accompanied by a progressive shift of the ITCZ to a more
southerly position. By mediating tropical wetland emissions of methane, changes
in the strength of the ASSM and the position of the ITCZ may impact
atmospheric greenhouse gas concentrations. A period of decreased monsoon
intensity has been identified between 8.4-7.8 kyr before present. Though the
onset of this event coincides with the Northern polar 8.2 kyr event, the recovery
of the monsoon occurs 2 kyr later than indicated by the Greenland (GISP2)
records.
Chapter 4. Holocene Variations
Introduction
The seasonal monsoon winds of the Arabian Sea are driven by the temperature
differential between the Indian Ocean and the Asian continent. As the plateau warms
in the boreal summer, the southwesterly winds flowing from the coast of Somalia
over the Arabian Sea to India bring moisture laden winds onshore, resulting in heavy
regional precipitation. These Southwest winds create an anti-cyclonic gyre in the
Arabian Sea inducing intense upwelling off the coast of Oman, which supplies the
surface waters with nutrients and supports high local productivity (Nair et al., 1989).
As the land cools, winter winds become northeasterly, the precipitation over India
diminishes and upwelling ceases in the western Arabian Sea.
There is sound evidence that the intensity of the ASSM has fluctuated
dramatically in the past, varying with precessional changes in the distribution of solar
radiation reaching the earth (Clemens et al., 1991; Kutzbach, 1981; Leuschner et al.,
2003). The Holocene epoch has spanned half of a precessional cycle. During this time
the Northern Hemisphere has changed from having maximum seasonality
approximately 11 thousand years before present (kyr BP), when the summer solstice
was aligned with perihelion, to a minimum today. The tropical seasonal monsoons are
highly influenced by precessional variations in insolation, resulting in a wetter
tropical climate as monsoon intensity increases (Clemens et al., 1991; Kutzbach,
1981; Leuschner et al., 2003). The modern Sahara region of Africa is the largest
desert on Earth. In the early Holocene, between 10 – 6 kyr BP, the same area was a
region of widespread lakes and abundant vegetation (deMenocal et al., 2000; Gasse,
2000). Though the precessional-scale frequency is not detectable in a human life span,
it is possible that non-linear responses to changing insolation can cause dramatic
environment change on a human timescale(deMenocal et al., 2000).
Throughout the last glacial period large amplitude, millennial-scale (suborbital) variation is seen globally. Both the intensity of the ASSM (Altabet et al.,
2002; Leuschner et al., 2000; Schulz et al., 1998) and the interplay between the
summer monsoon and winter monsoon (Porter et al., 1995) fluctuate significantly on
millennial timescales. Recently, millennial-scale climate changes have been
recognised in Holocene records. Although the amplitude of change is less than during
the last glacial period, climate variations over the last 10 kyr are seen in the North
Atlantic (Bond et al., 2001; Bond et al., 1997), throughout the Northern Hemisphere
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Chapter 4. Holocene Variations
(Haug et al., 2001; Hu et al., 2003; Noren et al., 2002; Xiao et al., 2004), the southern
Hemisphere (deMenocal et al., 2000; Gasse, 2000; Russell et al., 2003) and within the
Arabian Sea monsoon system (Fleitmann et al., 2003; Gupta et al., 2003; Jung et al.,
2002a; Jung et al., 2003; Neff et al., 2001). Such widespread indications of climatic
change during the Holocene, a period of limited ice coverage, suggest that ice
discharge and freshwater inputs to the North Atlantic are not the sole force of the
millennial-scale signal. Understanding millennial-scale climate variability is crucial in
determining both the impact of anthropogenic actions and natural climate variability.
Shorter timescale (centennial) specific events, such as the 8.2 event (Alley et
al., 1997; Barber et al., 1999) have been identified in various climate proxies from
terrestrial (Fleitmann et al., 2003; Hu et al., 1999) and marine sources (Haug et al.,
2001) and indicate that discrete occurrences can also have widespread implications.
Though believed to be initiated in the Northern Hemisphere (Barber et al., 1999), the
8.2 kyr event may have influenced change in the tropical hydrological cycle
(Fleitmann et al., 2003; Haug et al., 2001).
In order to determine the orbital and sub-orbital variations in the Arabian Sea
monsoon over the past 10 kyr, a reconstruction of the summer monsoon intensity has
been developed using proxies of productivity, water column denitrification,
continental aridity and wind strength. The use of multiple proxies, all influenced by
the ASSM, allows marine and terrestrial environmental signals to be compared within
the same sediment core. When multiple proxies are presented together, a more
detailed understanding of monsoon variations can be achieved and the discussion can
be focussed on variations in the local, regional and global context.
Setting of core 905
Core 905 was collected from a water depth of 1586 m during the 1993
Netherlands Indian Ocean Project (NIOP). This southwestern Arabian Sea site
(10˚.46’N; 51˚.57’E) is located beneath the core of the ASSM southwesterly winds
flowing from Somalia over the Arabian Sea to India and is therefore distinctly
influenced by summer monsoon induced upwelling (Figure 1)(Van Weering et al.,
1997). Initiated by the same low-pressure cell over Asia, northwesterly winds flow
across the Arabian Peninsula entraining dust. These winds rise above the
southswesterly monsoon winds and deposit sediments over the Arabian Sea. The
northern limb of these northwesterly winds travels down through the Persian Gulf and
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Chapter 4. Holocene Variations
is known as the “Shamal” winds (Clemens et al., 1991; Clemens, 1998; Glennie et
al., 2002b; Membery, 1983). The modern Shamal winds veer to the south and along
the Arabian coast and merge with the southern limb of the northwesterly winds that
travels through the Red Sea region (Figure 1).
NW
Shamal
905
SW
monsoon
Figure 1. Topographic map of the Arabian Peninsula. Site 905, Qunf Cave and the
Omani Coast are identified with red diamonds. The direction of the SW summer
monsoon wind (solid line) and the NW winds (dashed lines) are depicting a scenario
of the Early Holocene.
Today, the northeasterly winter monsoon winds are associated with low
aerosol concentrations (Sirocko et al., 2000; Sirocko et al., 1991) and are not readily
traceable in the 905 lithogenic record. The “inter-monsoon” northwesterly winds have
a much higher dust load but only impact the sediments of the Northeast Arabian Sea
(Sirocko et al., 1991). In the western Arabian Sea, where river runoff is negligible, the
sedimentary records reflect the eolian transport of dust to the sea floor (Clemens,
1998; Clemens et al., 1990; Reichart et al., 1997; Shimmield et al., 1991; Sirocko et
al., 2000; Sirocko et al., 1989). Therefore, although the wind patterns and dust
69
Chapter 4. Holocene Variations
transport of the Arabian Sea area is complex, site 905 is ideally located to record
variations in the ASSM wind intensity.
At site 905 variations in the ASSM intensity can be monitored by changes in
the overhead productivity (Banse, 1987; Honjo et al., 1999; Prell et al., 1981). A
sediment trap study indicates that there is a seasonal frequency in the particle flux of
the western basin, with over 65 % of the organic sedimentation occurring during the
summer monsoon season from June to September (Nair et al., 1989; Conan and
Brummer, 2000). Today both the large flux of organic matter through the water
column and the poorly ventilated intermediate waters (Sarma, 2002) support a stable
basin-wide OMZ between 200-1250m.
Methods (see chapter 1 and appendix A)
Chronology
The Holocene chronology of core 905 is based on 11 AMS 14C dates from a
mixed planktic foraminiferal assemblage and a comparison with the nearby U/Th
dated Q5 speleothem from Oman. (see Chapter 3)
Arabian Sea Monsoon Reconstruction
Measurements of % Organic Carbon (% Corg) indicate that the Somali margin
is a highly productive system where sedimentary % Corg values vary from 1-2.6 %.
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Chapter 4. Holocene Variations
However, the overall structure of the % Corg profile mirrors that of % CaCO3
suggesting that dilution by CaCO3, the dominant constituent of the 905 sediments, is
obscuring the productivity variations of the % Corg profile (Figure 2). The CaCO3
concentrations decrease from 80 % in the early Holocene to 60 % in the youngest
sediments collected (Figure 2).
Figure 3. Determining biogenic silica. A. A comparison of % organic carbon,
biogenic Si (% opal) (provided by E. Koning) and the Si/Al record of core 905. The
arrow indicates the crustal Si/Al value of 3.1. B. A plot of Si vs. Al indicating both
the influence of biogenic Si on the 905 sediments (encircled samples) and two
lithogenic sources (linear relationships). Line “a” describes pre-Holocene samples.
Line “b” describes samples from both the early and late Holocene.
The Holocene record of productivity therefore must be compiled from
addition productivity proxies: Si/Al, Ba/Al and P/Al. The Si/Al record at this site
registers both variations in the biogenic Si (opal) from diatoms and other siliceous
phytoplankton and lithogenic Si (Figure 3a). From the regression plot (Figure 3b), Si
vs. Al, two apparent lithogenic sources to the site and the opal signal can be
identified. The linear Si to Al relationships are related to samples dated between 1211 kyr and 11-10, 2.5-0 kyr. The former has a Si/Al ratio of approximately 3.6, which
is slightly above the crustal average of 3.1 (Wedepohl, 1971) and likely reflects the
input of coarse-grained quartz. The latter, the combined transition and late Holocene
samples, have an elevated Si/Al ratio of approximately 6.4, suggesting that the
lithogenic Si source is highly Si-enriched. Opal, which has no relationship with
lithogenic input, generates a dispersed, non-linear mass in the Si vs. Al plot (Figure
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Chapter 4. Holocene Variations
4), these samples all date from 9.6-2.5 kyr. The comparison of the Holocene Si/Al and
opal records indicates that this intermediate Holocene period is dominated by
variations in opal, which is related to an increase in overhead diatom production.
Therefore throughout the mid-Holocene, from 9.6-2.5 kyr, Si/Al can be used as a
proxy of productivity.
Figure 4. Determining lithogenic and biogenic Ba and P.
In the core 905 sediments, both Ba and P are also elevated above crustal
values and show a non-linear relationship with Al (Figure 4) indicating that biogenic
Ba and P are dominating the sedimentary records off this site. The Ba/Al (x10-4) and
P/Al records vary from 150-600 and 0.038-0.07 respectively. Though the records are
highly variable, spectral analysis indicates a large frequency peak corresponding to a
period of 1830 years (Figure 5). Low Ba/Al and P/Al values of the pre-Holocene
(approximately 12 kyr) are followed by a distinct increase in the early Holocene. The
late Holocene values decrease to a minimum at approximately 2 kyr (Figure 6).
The combined productivity records of core 905, Si/Al, Ba/Al and P/Al,
indicate that the onset of the Holocene is accompanied by an increase in productivity.
Sub-orbital variations in productivity are evident throughout the Holocene. At
approximately 4.5 kyr, the average values of the productivity proxies begin to decline,
reaching a minimum at 2 kyr before present (Figure 6).
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Chapter 4. Holocene Variations
Figure 5. Frequency analysis of the Holocene productivity proxies and the North
Atlantic record of ice rafted debris. A. The frequency spectra of the N. Atlantic record
of hematite stained grains (Bond et al, 1997) identifies a period of 1810 years. B. The
frequency vs. power plot for the 905 P/Al and C. the frequency vs. power plot for the
905 Ba/Al record indicates a period of 1830 years is evident in both of these records.
D. The frequency vs. power plot for 905 Ti/Al record.
2 10 -5
642
Ti/Al power
1828
1 10 -5
0
0.0004
0.0008
0.0012
0.0016
T i/Al frequency
73
0.002
Chapter 4. Holocene Variations
8
!15 N
7.5
7
7.5
6.5
6.5
Si/Al
6
5.5
4.5
0.065
3.5
P/Al
0.055
600
Ba/Al
0.045
400
0.035
200
0
2
4
6
8
10
12
Calendar age (kyr)
Figure 6 from core 905. A. The record of denitrification indicating changes in
monsoon upwelling. B. The P/Al record. C. The Si/Al record. D. The Ba/Al record.
The bold line is a 3 point smooth of the data, which is in plotted in grey. The dashed
lines highlight transitions in the record that are discussed in the text.
Water column denitrification is enhanced through the degradation of organic
matter in the intermediate waters. Upwelling, induced by the summer monsoon winds,
replenishes the surface waters with this residual pool of “heavy” nitrate, which is
incorporated by phytoplankton and subsequently transferred to the sediments with the
flux of organic matter. Therefore, the core 905 denitrification record is used to
support the productivity proxies in the generation of the Holocene Arabian Sea
monsoon reconstruction as a proxy of both intermediate water suboxia and monsoon
induced upwelling. The δ15N values from core 905 decrease from an early Holocene
maximum of 8.1 permil to approximately 6.5 permil in the most recent sediments
(Figure 6). The early Holocene (10.5-8.3 kyr) is characterized by large (0.7 permil),
rapid shifts in the δ15N record over an average of approximately 7.7 permil. Beginning
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Chapter 4. Holocene Variations
at 8.3 kyr, and continuing over the following 500 years, the δ15N values drop more
than 1.0 permil. A partial recovery to 7.3 permil occurs at 7.7 kyr. Between 7.6 and
5.5 kyr there is a gradual decline in the δ15N values. A plateau occurs between 5.5-3
kyr that is followed by a final decrease to late Holocene values of 6.5 permil. As with
the productivity proxies, the dominant feature of the δ15N record is the sharp increase
at 10 kyr, and the gradual decline to minimum values in the late Holocene. The
combined productivity and denitrification records indicate that the ASSM intensity
was enhanced in the Early Holocene and diminished throughout the Holocene to
minimum intensity at approximately 2 kyr.
8
5.5 kyr
7.5
!15 N
0.08
7
6.5
0.075
Ti/Al
6
0.07
36
0.065
Zr/Al
28
6
20
4
% quartz
%dolomite
12
2
0
0
2
4
6
8
10
Figure 7. The lithogenic
records of core 905. A. The
denitrification record is
used as a reference to
determine changes in the
monsoon intensity. B. The
Ti/Al (3 pt smooth)
indicates
changes
in
seasonal precipitation. Tirich soils are produced
during concentrated rainfall
events. C. A 3 pt smooth of
the Zr/Al record indicates
that
coarse-grained
sediments increase in the
late Holocene. The black
lines indicate the average
value before and after the
5.5 kyr transition. D. The
core 905 % quartz (black)
and % dolomite (grey). The
dashed black line highlights
the 5.5 ky transition from
Humid to Arid conditions.
The dashed grey lines
identify changes in the
early Holocene that may be
associated with eth 8.2 kyr
event.
12
Calendar age (kyr)
75
Chapter 4. Holocene Variations
The lithogenic records of core 905 contribute to the monsoon reconstruction
by adding information relating to monsoon induced changes in the continental
environment, terrestrial aridity and wind strength. The Ti/Al record, which closely
resembles the records of denitrification and productivity, reaches an early Holocene
maximum of 0.078 and declines to 0.07 in the most recent sediments, always
remaining above the average shale value of 0.05 (Wedepohl, 1971) (Figure7). The
core 905 Ti/Al values are comparable to values of 0.65-0.08 determined by Sirocko et
al. (2001) and Shimmield et al. (1990) in the northern Arabian Sea. The Ti/Al record
is periodically punctuated by sharp decreases between 12-10.5, 8.6-7.9, 6.6-6.2, 5.55.1 and 2.5-2 kyr (Figure 7). Frequency analysis of the Ti/Al record reveals a
periodicity of 1828, which is similar to that of the productivity proxies, and a second
more dominant periodicity of 642 years (Figure 5). The Zr/Al ratio has a stepwise
transition at 5.5 kyr (Figure 7). Prior to 5.5 kyr the record rapidly fluctuates around a
mean of ~ 20, after 5.5 kyr the frequency of variation diminishes even though larger
amplitude variation occurs. The average value in the late Holocene is approximately
27 (Figure 7).
The differences between the core 905 Ti/Al and Zr/Al records can be used to
gain information about the monsoon variability over the last 10 kyr. The lower values
of Zr/Al in the early Holocene, a period of intense monsoon activity, suggests that
continental moisture is restricting the deflation of large grained sediment at this time.
Zr/Al is negatively impacted by increased continental moisture but positively
impacted by increased wind strength. Consequently, over the duration of the
Holocene, the concentration of Zr/Al in the 905 sediments responds (in a threshold
manner) to the increase in continental aridity as the monsoon intensity diminishes.
However, on sub-orbital scale frequencies, relative peaks in the Zr/Al record occur
during periods of relative increases in wind strength. The 905 % quartz record
resembles the Zr/Al record responding to both changes in winds speed continental
aridity (Figure 7). Quartz is a coarse-grained component of marine sediments
(Shimmield et al., 1991; Shimmield et al., 1990) and therefore supports the use of
Zr/Al as a proxy of sediment grain size. The grain size proxies of core 905 (Zr/Al, %
quartz) indicate that a significant transition occurred in the local terrestrial
76
Chapter 4. Holocene Variations
environment at 5.5 kyr. Prior to 5.5 kyr, monsoon related precipitation and soil
moisture limited the deflation of large grained sediments. After 5.5 kyr, the terrestrial
environment was arid and dusty.
Conversely, the Ti/Al record indicates that higher concentrations of Ti are
present in the 905 sediments during the early Holocene, when the monsoon is strong.
Therefore Ti/Al and Zr/Al are not proxies of the same aspect of the monsoon system.
Titanium is concentrated in tropical soils as a result of the seasonal cycle of intense
rainfall (that leaches away highly reactive elements) and then intense drying
(McFarlane, 1976). The Ti/Al record resembles the δ15N record of upwelling induced
by the monsoon winds (Figure 7) and frequency analysis indicates periodicities of
1828 years and 642 year, the former being shared with the proxies of paleoproductivity. The 905 Ti/Al record is likely reflecting the production of Ti-rich
lateritic soils and is therefore a proxy of intense monsoon rainfall.
Holocene Variations in the Tropical Hydrological Cycle
The seasonal migration of the position of the ITCZ determines the amount of
precipitation over southern Oman, which is recorded in the δ18O record of speleothem
calcite from Qunf Cave (Q5) (17°10 N, 54°18 E) (Figure 8). This record indicates
that changes in precipitation rates, and therefore continental moisture, have occurred
throughout the Holocene. More negative δ18O values result from an increase in
precipitation when the ITCZ is overhead. When the ITCZ moves south, precipitation
decreases and the δ18O values become more positive (Fleitmann et al., 2003). Changes
in the ITCZ position during the Holocene are also recorded in the sedimentary record
of the Cariaco Basin off the coast of Northern Venezuela (Haug et al., 2001) where
increased terrestrial runoff (% Ti and % Fe) occurs during periods of high seasonal
precipitation (Figure 8).
A comparison between the Q5 δ18O record, the Cariaco % Ti and the core
905 δ15N record indicates that changes in the ITCZ position closely resemble changes
in the intensity of the ASSM. The similarity in the overall structure and the detail
between the three records suggests that changes in ITCZ precipitation, both over Asia
and S. America, and monsoon driven upwelling off the Somali coast are responding to
a common external forcing (Figure 8). The dominant Early Holocene maximum and
gradual decrease towards the present day signifies that the precessional signal is
77
Chapter 4. Holocene Variations
underlying the higher frequency changes seen in the tropical records of both ITCZ
position and monsoon intensity.
Variation in the strength of the global monsoon winds is associated with the
precessional cycle (Clemens et al., 1991; Clemens et al., 2003; Kutzbach, 1981;
Leuschner et al., 2003), which determines variations in seasonality. The “Precession
of the Equinox” changes the relationship between the Earth - Sun distance and the
season such that over a period of approximately 21 kyr, summer migrates through
perihelion (least Earth - Sun distance) to aphelion (largest Earth - Sun distance). The
relationship of monsoon intensity to the precessional cycle reflects changes in
seasonal warming and the resulting variations in land-ocean temperature gradient.
Figure 8. Holocene variations
in the Arabian Sea Monsoon
intensity and the position of
the Intertropical Convergence
Zone (ITCZ). A.The downcore
sedimentary Ti concentrations
from the Cariaco Basin
(coastal Venezuela) indicating
more
terrestrial
runoff
(increased % Ti) associated
with increased precipitation as
the ITCZ moves to a southerly
position. Decreases in % Ti
and lower rainfall indicate a
southerly position of the ITCZ.
(Haug et al., 2001) B. The
rainfall reconstruction from
Qunf Cave (Oman). More
negative δ18O values indicate
increased rainfall (Fleitmann
et al., 2003). C. The 905 δ15N
record. Larger values reflect
increased denitrification. D.
The upwelling record of G.
bulloides from coastal Oman
(Gupta et al., 2003). The
shaded band highlights the 8.2
kyr event. Orange lines are for
visual comparison of the
structure of these three
records. The red dots are AMS
14
C dated samples from the
905 record.
78
Chapter 4. Holocene Variations
The independent age models generated for these three sites substantiate the
coherent variation in ITCZ movement and ASSM intensity throughout the Holocene.
The G. bulloides upwelling record of from a coastal Omani site presented by Gupta et
al. (2003) further confirms the Holocene variability of ASSM intensity (Figure 8).
The 905 δ15N record provides evidence of Holocene changes in wind driven
upwelling, primary productivity and water column denitrification, all variables
directly related to the strength of the ASSM. The Q5 δ18O record and Cariaco % Ti
show a global variation in the ITCZ position that relates to ASSM intensity, where a
northern position of the ITCZ coincides with increased ASSM intensity and a
southern position of the ITCZ coincides with decreased ASSM intensity. The paleorecords presented here indicate that significant change in the tropical hydrological
cycle has occurred throughout the Holocene with the underlying precessional forcing
being the strongest signal evident.
Changes in the tropical hydrological cycles have global consequences, not
only through changes in atmospheric water vapour, the most abundant greenhouse
gas, but through the mediation of additional greenhouse gas emissions from tropical
wetland areas. Methane (CH4), the third most abundant greenhouse gas after water
vapour and carbon dioxide, has multiple natural sources as well as anthropogenic
sources, the latter which have now become dominant (Wuebbles et al., 2002). A
conservative estimate of the natural sources of methane indicates that tropical
wetlands release approximately 100 Tg CH4/yr to the atmosphere, accounting for over
50% of the natural annual emissions (IPCC, 2001). Boreal wetlands account for
approximately 21 % of naturally produced CH4, oceanic gas hydrates, termites and
wild ruminants make up the balance of 28 % (Wuebbles et al., 2002). Methane is
optimally produced during the anaeobic breakdown of organic matter in water-logged
soils when temperatures reach 37 °C (Wuebbles et al., 2002). Therefore changes in
the hydrological cycle are likely to have impacted CH4 production in tropical
wetlands through changes in precipitation patterns and regional waters table levels.
A comparison of the 905 δ15N record and CH4 recorded in the EPICA Dome C
Antarctic ice core (Flückiger et al., 2002) and the Greenland GRIP ice core (Blunier et
al., 1995) indicate that there is a close precessional-scale relationship between tropical
monsoon intensity and atmospheric CH4 concentrations (Figure 9). Although methane
is mixed in the atmosphere quickly, an offset between the two hemispheres exists as a
79
Chapter 4. Holocene Variations
result of the higher CH4 production rate in the north (Chappellaz et al., 1993). This
explains the consistently lower CH4 values in the Dome C record compared to the two
GRIP records that are duplicate measurements of the same core done in two different
laboratories (Flückiger et al., 2002) (Figure 9). The chronology of the Dome C and
GRIP records are independent from one another and not synchronised with core 905
therefore the differences in the Early Holocene profiles of the CH4 records and the
905 δ15N are likely related to chronology discrepancies. Following the Early Holocene
precessional maximum, a 20 % reduction in CH4 corresponds to the gradual decrease
in monsoon strength over the last 10 kyr. After 3.2 kyr, the monsoon records and CH4
records diverge: the CH4 records increase without an identifiable monsoon increase.
As atmospheric CH4 concentrations are increased through rice cultivation and other
human activities, the divergence between the CH4 and monsoon records at 3.2 kyr
may mark the onset of anthropogenic CH4 emissions (Ruddiman, 2003).
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Chapter 4. Holocene Variations
Though over the past 90 kyr atmospheric CH4 has varied in common with the
changes in monsoon intensity (see Chapter 1) and has responded to changing
temperatures over the last 400 kyr (Petit et al., 1999), anthropogenic emissions of CH4
have disrupted this balance. The modern day concentration of atmospheric CH4 is
approximately 1700 ppm, more than double the concentrations seen over the last four
glacial cycles (400 kyr) which ranged from 350-800 ppm. The divergence of the
Holocene 905 δ15N records from the atmospheric CH4 records at 3.2 kyr supports the
hypothesis of Ruddiman (2003) that anthropogenic activities prior to the industrial
revolution
altered
greenhouse
gas
production
and
atmospheric
methane
concentrations.
Nitrous oxide, also an important greenhouse gas, is produced within highly
productive oceanic upwelling zones and terrestrial wetland areas as a by-product
during both the nitrification and denitrification processes. Having both a marine and
terrestrial source causes differences in the behaviour of N2O during rapid climate
changes with respect to CH4 (Sowers et al., 2003). The Holocene N2O record shows
very small magnitude variations that do not closely resemble either the ice core CH4
records (Flückiger et al., 2002) or the 905 reconstruction of ASSM intensity
indicating that Holocene fluctuations in the multiple sources of N2O are small or are
in contradiction with one another.
Linear and threshold responses to Precessional forcing
The Holocene Epoch, the last 10 kyr, spans half of a precessional cycle. The
Early Holocene “African Humid Period”, from 10.5-6 kyr BP, has been described
extensively in the literature (deMenocal et al., 2000; Gasse, 2000) (and references
therein) and is once again confirmed by the 905 δ15N records (Figure 8). This period
of intense summer monsoon convection is followed by a general decline in wind
intensity that continues until approximately 6 kyr when a plateau or slight recovery
occurs and persists until 3 kyr. At that time the hiatus in the Q5 δ18O record and the
lowest values in both the 905 δ15N and the G. bulloides upwelling records (Figure 8)
suggest a minimum in monsoon wind strength. This progression from maximum
monsoon intensity to minimum monsoon intensity over the past 10 kyr corresponds to
the precession of the equinox where June occurred at perihelion 10 kyr ago and is
occurring at aphelion today.
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Chapter 4. Holocene Variations
Our understanding of how “abrupt” the transition was between the African
Humid Period and the late Holocene arid environment depends on the proxy used in
the reconstruction. Proxies of monsoon intensity such as the 905 δ15N and the Q5
precipitation records suggest that wind strength, upwelling and precipitation
responded linearly to the decrease in insolation (Fleitmann et al., 2003). However a
Holocene record of terrestrial input from the African continent to the Northern
Equatorial Atlantic Ocean (deMenocal et al., 2000) suggests that an abrupt transition
from moist continental conditions to arid continental conditions occurred in less than
four centuries. Water level and paleo-chemistry records from Lake Abiyata (Ethiopia)
indicate that a stepwise transition from freshwater to saline water occurred at 5.4 kyr
(Chalie et al., 2002). The lithogenic records from core 905 confirm a stepwise
transition at 5.5 kyr and indicate that the west African continental response to the
decrease in the African monsoon seen by deMenocal et al. (2000) is duplicated by the
east African continental response to changes in the ASSM. At 5.5 kyr the Zr/Al and %
quartz records from core 905 show an increase in the average concentration compared
to the Early Holocene. Rather than indicating an increase in wind strength, this mid
Holocene transition appears to reflect an increase in continental aridity that allowed
the deflation of larger grained sediments to be transported as dust from the Arabian
Peninsula and to the Somali margin (Figure 7). The stepwise nature of the terrestrial
response is likely enhanced by vegetation changes as rainfall decreased (Gasse, 2000).
The contrasting scenarios depicted by the marine and terrestrial proxies
highlight the need for multi-proxy reconstructions in order to understand fully how
different aspects of this complex environment have responded to changing climatic
conditions. Abrupt transitions in terrestrial ecosystem dynamics are likely to have
significant and unpredicted impacts on human populations. The collapse of the
Akkadian civilisation, living in ancient Mesopotamia (modern Syria and Iraq),
occurred approximately 4 kyr before present, concurrently with a period of decreased
monsoon activity and increased aridity (Cullen et al., 2000). Present day variations in
monsoon activity are still associated with increased rates of human mortality whether
because of drought or torrential flooding. As human induced climate change
supersedes the natural variation, our understanding of ecosystem thresholds is more
important today than ever.
82
Chapter 4. Holocene Variations
Sub-orbital variation: Millennial-scale
The productivity proxies from core 905 indicate that superimposed over the
first order precessional signal (the early Holocene maximum to late Holocene
minimum), is a higher frequency variation. Frequency analysis of the 905 productivity
proxies indicates power corresponding to a period of 1830 kyr (Figure 5). The Ti/Al
record has a period of 1828, but a sharper more dominant peak at 642 (Figure 5).
Though Bond et al. (1997) reported a 1500 ± 500 year cycle, frequency analysis of
their Holocene North Atlantic record of hematite stained grains indicates a dominant
period of 1800 years (Figure 5).
Keeling et al., (2000) proposed that variations in the physical relationship
between the Earth and Moon produce a cycle of variable tidal mixing with a period of
1823 and calculated that the Earth-Moon distance was at a minimum 525, 2348, 4293,
5921, 7744, and 9640 years before present (Figure 10). At these times, the strength of
tidal mixing is thought to have increased (Keeling et al., 2000). Tidal mixing provides
more than half the total power for vertical mixing in the surface waters (Keeling et al.,
2000). Consequently, increases in vertical mixing would incorporate deeper waters
into the mixed layer and cool the surface waters. The common periodicities of 1830
from the core 905 productivity records and 1823 from the tidal cycle suggest a
possible relationship between monsoon-induced upwelling and the tidal mixing of the
surface ocean. Decreased values of the core 905 productivity proxies occur during the
predicted times of maximum tidal mixing (Figure 10) suggesting that mixing induced
changes in the Arabian Sea surface water temperature might be affecting the intensity
of the ASSM.
The intensity of the ASSM would likely be affected by a slight cooling in the
surface waters of the Western Equatorial Pacific (WEP), a possible result of increased
tidal mixing. When the easterly equatorial trade winds are strong, warm surface
waters are concentrated in the WEP and are referred to as the “warm pool”. The
presence of the warm pool reinforces the low pressure area over Asia that initiates the
ASSM (Joseph et al., 1994; Ju et al., 1995; Krishnamurthy et al., 2003). Cooling of
the WEP warm pool though enhanced tidal mixing may have decreased the intensity
of the Asian low-pressure reinforcement and therefore diminished the strength of the
ASSM. Liu et al. (2003) performed model simulations of the evolution of the six
major monsoon systems (Asian, N. African, S. African, Australian, N. American and
83
Chapter 4. Holocene Variations
S. American) during the Holocene and found that the Asian monsoon (the East Asian
and Arabian Sea monsoon combined) was influenced by ocean dynamics, which
complicated the insolation induced monsoon forcing.
12
16
GISP2
20
35
Be
10
24
Zr/Al
30
28
25
20
0.075
P/Al
15
0.065
10
0.055
8
!15 N
7.5
0.045
7
0.035
6.5
6
0
2
4
6
8
10
12
Calendar age (kyr)
Figure 10. Tidal (lunar) forcing vs. solar forcing of the Arabian Sea Monsoon. A. The
GISP2 10Be record indicating solar activity (Finkel and Nishiizuni, 1997). Note that
the scale is inverted. Increased The dashed blue arrows indicate times of maximum
tidal mixing as predicted by Keeling and Whorf, 2000. The dashed grey lines
highlight the 8.2 kyr event.
Stott et al. (2004) presents a reconstruction of Holocene variations in the sea
surface temperature (SST) of the WEP from two sites, 6° N and 7° S using Mg/Ca
measurements on foraminiferal CaCO3 (Figure 11). The SST decrease of 0.5 °C over
the Holocene appears consistent with the decrease in precessional forcing seen in 905
δ15N record (Figure 11). As well, some detail seen in the Northern Hemisphere record
84
Chapter 4. Holocene Variations
(Figure 11a) is comparable to sub-orbital variations in the ASSM. The Southern
Hemisphere record however shows no obvious coherence with the intensity of the
ASSM. During the late Holocene (after 5 kyr), cold SST’s in both sites coincide with
periods of maximum tidal mixing. This relationship does not appear to exist in the
Early Holocene. It is likely that the influence of tidal mixing is secondary to the
precessional influence that dominates the early Holocene. Therefore the coherence
between the cold WEP temperatures and maximum tidal mixing may only be evident
during periods of reduced precessional forcing and tidal mixing does not explain the
sub-orbital variations of the early Holocene.
Figure 11. A comparison of the Western Equatorial Pacific Warm Pool sea surface
temperatures and the intensity of the Arabian Sea summer Monsoon. A. SST
determined from Mg/Ca measurements from a site 7 ºN in the warm pool and the 905
δ15N record (data is from Stott et al, 2004) B. SST determined from Mg/Ca
measurements from a site 6 ºS in the warm pool and the 905 δ15N record (data is from
Stott et al, 2004). Dashed lines indicate times of maximum tidal mixing. The arrows
indicate error associated with the temperature records.
Contrary to a lunar mechanism forcing the millennial-scale variation, a solar
mechanism is more frequently suggested. Solar activity has also been employed as the
mechanism generating both high latitude (Bond et al., 2001; Hu et al., 2003) and low
latitude (Fleitmann et al., 2003; Jung et al., 2002b; Jung et al., 2004; Neff et al., 2001)
Holocene climatic variability. Bond et al (2001) indicate that variations in solar
activity, as expressed in the cosmogenic production of
14
C and
10
Be, is strongly
correlated with millennial-scale variation in records of North Atlantic ice rafted
debris. Higher production of 14C and 10Be occurs when solar winds decrease and solar
radiation is reduced, allowing variations in solar activity to be tracked in ice core
records (14C and 10Be) and tree rings (14C).
85
Chapter 4. Holocene Variations
A comparison between the core 905 paleoproductivity records and the GISP2
10
Be record shows periods of increased
10
Be production (decreased solar activity)
correspond to decreases in productivity identified by the P/Al record (Figure 10). A
coherent relationship between monsoon intensity, determined from the core 905 δ18O
record, and solar activity has also been reported by Jung et al, 2004. The record of
continental aridity, Zr/Al, also decreases with enhanced 10Be production between 8-3
kyr (Figure 10) suggesting a less monsoon related precipitation at these times. As
well, though detail is absent in the low-resolution % dolomite and % quartz records,
decreases in these mineral concentrations at site 905 generally occur during periods of
decreased solar activity. Overall, sub-orbital periods of diminished solar activity
correspond with indications of reduced ASSM intensity: a decrease in productivity
(P/Al), diminished wind strength (Zr/Al) and less intense seasonal rainfall (Ti/Al).
It is not possible here to discern whether the ultimate forcing of millennialscale monsoon variability is lunar or solar in origin. The tropical monsoon climate is
very sensitive to changes in sea surface temperature and insolation, for both affect the
location and total area of tropical convection zones (Pierrehumbert, 1999). It is
therefore understandable that more than one forcing mechanism will influence proxies
of monsoon variability. The 905 records of marine paleoproductivity appear to show
the distinct 1828 year periodicity of the tidal forcing with low productivity occurring
during periods of increased tidal mixing. As well the influence of variable solar
activity is evident in the productivity records. The 905 Ti/Al record has a periodicity
of 1823 years but a more dominant peak with a period of 642 years and therefore does
not correspond as closely with changes in tidal mixing but does resemble the higher
frequency (though not regular) variation in solar activity.
The differences between the land-derived proxies of continental aridity and
marine upwelling and productivity signals highlight the sensitivities in the land-based
and sea-based systems and suggest multiple forcings may be influencing the system.
There are different implications with respect to future change, between a cyclical
mechanism such as a lunar forcing and the more erratic variations in solar activity.
For prediction of future monsoon variability the degree of forcing attributed to these
two possibilities needs to be determined.
86
Chapter 4. Holocene Variations
The 8.2 kyr event
A distinct event which lasted from 8.4-8 kyr ago is recorded in the GISP2 ice
core indicating a period of reduced ice accumulation, reduced atmospheric methane
(Figure 9), increased fire activity, and increased dustiness disrupted the relatively
stable Early Holocene period (Alley et al., 1997). This event is believed to be a result
of the draining of glacial Lakes Agassi and Ojibway into Hudson Bay as the
Laurentide ice sheet retreated (Barber et al., 1999). The nature of the ice core proxies,
which record local, regional and global events, suggest that this event had widespread
climate impacts. This is becoming more apparent as evidence of the “8.2 kyr” event is
now available from North America (Dean et al., 2002; Hu et al., 1999; Schuman et al.,
2002), Europe (Tinner et al., 2001) and the tropics (Fleitmann et al., 2003; Gasse,
2000; Haug et al., 2001).
A centennial-scale event occurring at approximately 8.2 kyr is characterized in
the Q5 δ18O and Cariaco % Ti records as a period of decreased tropical precipitation
(Fleitmann et al., 2003; Gasse, 2000; Haug et al., 2001). However, these tropical
records indicate that the event terminated around 7.8 kyr, making the duration of the
tropical 8.2 kyr event 2 kyr longer than the polar event initiated at approximately the
same time (Figure 8). Between 8.4-7.8 kyr, the decreased precipitation recorded from
the tropical sites has been interpreted as a southerly shift in the position of the ITCZ
(Fleitmann et al., 2003; Gasse, 2000; Haug et al., 2001). Corresponding to this shift in
the ITCZ, a stepwise decrease in monsoon intensity is seen in the 905 δ15N record.
The first decrease is initiated at 8.4 kyr and the second occurs at 8.2 kyr (Figure 9).
The extended duration of this tropical event suggests that either that the tropical
hydrological cycle, once perturbed by the fresh water input in the North Atlantic, is
relatively slow to recover, or that the 8.2 kyr event identified in the Northern polar
region is not directly associated with the change in the tropical hydrological cycle that
occurred between 8.4-7.8 kyr before present.
The 905 Ba/Al paleoproductivity record indicates a decrease in monsoon
intensity and productivity from 8.5-7.8 kyr during which low 905 Ba/Al (Figure 6)
values correspond to the drop in % G. bulloides seen in records from the Oman coast
(Gupta et al., 2003) (Figure 8). The core 905 P/Al and Si/Al records do not show as
distinct a decrease during the 8.2 kyr event. However, the Si/Al record is complicated
by changes in the aluminosilicate dust input (see discussion above) and the broad
87
Chapter 4. Holocene Variations
local minimum in the % opal record beginning at 8.4 kyr (Figure 3) supports a
decrease in productivity during the 8.2 kyr event.
A sharp increase in the core 905 Zr/Al record occurs between 8.4-8.0 kyr
indicating that large grain sediments are being transported to site 905 even though the
ASSM winds diminished. This is followed by a distinct low in the Zr/Al record
between 8.0-7.8 kyr. As discussed above, changes in continental moisture appear to
determine the 5.5 kyr transition from low to high Zr/Al values (Figure 7). The
increase in Zr/Al at 8.2 kyr may also be a result of a decrease in soil moisture
indicating a higher rate of sediment deflation from the nearby continent, or it might
reflect a change in the wind direction.
NW winds
“straighten”
SW monsoon
weakens
Figure 12. Changes in atmospheric winds patterns during the 8.2 kyr Event. The
diminished summer monsoon winds move offshore from the Arabian Peninsula. The
Shamal winds extend eastward, no longer impacting site 905. The southern limb of
the northwesterly winds also extends, now directly influencing site 905.
The orientation of sand dunes on the Arabian Peninsula suggests that during
the last glacial period when the summer monsoon winds were weak and located
88
Chapter 4. Holocene Variations
offshore, the Shamal winds were strong and extended beyond the coastline of the
Arabian Peninsula out to the sea (Glennie et al., 2002b) (Figure 12). In contrast during
the early Holocene period, when the intensity of the summer monsoon increased, the
Shamal winds retreated and the Arabian coastline was influenced by the summer
monsoon winds (Glennie et al., 2002a). The increase in Zr/Al and decrease in
dolomite at site 905 (Figure 7) during the 8.2 kyr event may be reflecting a relative
increase in the strength of the southern limb of the northwesterly winds that flow
through the Red Sea region. The dolomite-rich Shamal winds, increasing in kind,
would have less impact on the sediments of site 905 as they would extend eastward
rather than southward (Figure 12).
Conclusions
Variations in the intensity of the Arabian Sea summer monsoon have occurred
throughout the Holocene. These changes are evident on a variety of time scales from
precessional (20 kyr) to centennial (100 years). Precessional cycles change
seasonality, causing a slow transition from hot summers and cold winters to cool
summers and cool winters. The resultant change in the land–sea temperature gradient
drives the variations in monsoon strength. Past changes in monsoon wind strength,
marine upwelling and upwelling-induced productivity are linearly related to
precessional forcing, as are the atmospheric greenhouse gas (CH4, N2O)
concentrations. However, the terrestrial response to the linear change in precessional
forcing is abrupt, exhibiting threshold behaviour between an arid, dusty, desert
environment and a humid environment. A distinct transition in continental moisture
occurred at 5.5 kyr indicating the end of the Early Holocene humid period in Eastern
Africa and the onset of the arid conditions apparent today.
It is evident that superimposed over the precessional mid-Holocene transition
out of African Humid Period is a higher frequency variation in the strength of the
Arabian Sea monsoon. Changes in the monsoon intensity, as registered by the marine
productivity proxies of core 905, occur with a cyclicity of approximately 1830 years,
a period also seen in the Holocene sedimentary records of the N. Atlantic (Bond et al.,
2001; Bond et al., 1997). This periodicity is similar to the 1823 period of tidal mixing
driven by the changes in the earth- moon relationship (Keeling et al., 2000). Changes
in solar activity also correlate with the paleo-productivity and lithogenic records from
site 905.
89
Chapter 4. Holocene Variations
An event occurring between 8.4-7.8 kyr is apparent in the 905 records of
Arabian Sea monsoon variability. During this period, the intensity of the Arabian
monsoon decreased. Though the onset of this tropical event corresponds to the 8.2 kyr
event associated with a fresh water flux into the North Atlantic Ocean (Barber et al.,
1999), the tropical event appears to endure 2 kyrs longer than the “8.2 event”
identified in the Greenland ice core (GISP2) (Alley et al., 1997). The relationship
between these two events is therefore uncertain.
Understanding the natural Holocene variation in the strength of the ASSM or
at least the variation that has occurred over the last 10 kyr, allows the prediction of
future climate change to be better and more informed. As well it furthers our
understanding of the mechanism driving millennial-scale changes in climate. It is well
documented that a change in the tropical convection patterns are felt globally through
changes in the hydrological cycle. Variations in the ASSM intensity on a millennialscale are also likely to have global repercussions, both climatically and economically.
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