This document summarizes an Australian Research Council project to study climate variability in Southeast Asia over the early Holocene using a high-resolution record from crater lakes in Cambodia. The project involves collaboration between researchers from various universities and institutions, and aims to improve understanding of the Asian monsoon as a complex system and climate modeling by developing a direct climate proxy from the region dating back to the early Holocene.
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1.
2. Australian Research Council Discovery
Projects (DP1094367 )
Cambodian Crater Lakes project
collaborators:
D.A. Penny, University of Sydney
B.M. Buckley, Columbia University
Q. Hua, ANSTO
Dr Andy Maxwell (Independent scholar,
USA)
Cambodian Ministry of Environment
ANSTO
AMMRF/ACMM
3. Understanding the Asian monsoon as a complex
system
Responses over mainland Southeast Asia through
the early Holocene ???
High resolution record of climate variability in the
Asian tropics from the early Holocene from a
direct climate proxy
Contribute to improvement of climate modelling /
understanding of the earth systems as a complex system
7. Oman
Indian Summer
Monsoon
Gradual
weakening
Strong summer
monsoon
Tibetan
Plateau
Indian Summer
Monsoon / East
Asian Summer
monsoon
Gradual
weakening
Asynchrono
us start of
weakening
Strong
summer
monsoon
South-
west
China
Indian summer
monsoon / East
Asian Summer
monsoon
Gradual
weakening
Strong
summer
monsoon
South
China
East Asian
Summer monsoon
Gradual
weakening
Strong
summer
monsoon
Central
China
East Asian
Summer monsoon
Gradual
weakening
Strong
summer
monsoon
Region
14C yr 1000 2000 3000 4000 5000 6000 7000 8000 9000
Cal yr BP 930 1940 3200 4400 5700 6800 7800 8800 10000
Strong summer monsoon
Strong summer monsoon
Strong summer monsoon
Strong summer monsoon
Strong summer
monsoon
Gradual weakening
Gradual weakening
Gradual weakening
Gradual weakening
Gradual weakening
Asynchron-
ous start of
weakening
17. y = 8.1951x + 13.463
R² = 0.9911
y = 5.2607x - 11.206
R² = 0.8689
GMWL: y = 8x + 10
-120.00
-100.00
-80.00
-60.00
-40.00
-20.00
0.00
-16.00 -14.00 -12.00 -10.00 -8.00 -6.00 -4.00 -2.00
δ2H(‰)
δ18O (‰)
18O/2H rain water 18O/2H lake surface waters
18.
19.
20.
21. Description EBSD refraction pattern
Name Aragonite
Database Inorganic Crystal Structure Database
Structure
Crystal System Orthorhombic
Shape
LaueGroup 3
Space Group 62
Unit Cell
a 5.74 Å
b 4.96 Å
c 7.97 Å
Alpha 90.00 °
Beta 90.00 °
Gamma 90.00 °
22.
23.
24.
25. Oman
Indian Summer
Monsoon
Gradual
weakening
Strong summer
monsoon
Tibetan
Plateau
Indian Summer
Monsoon / East
Asian Summer
monsoon
Gradual
weakening
Asynchron-
ous start of
weakening
Strong
summer
monsoon
Mainland
Southeast
Asia
Indian summer
monsoon / East
Asian Summer
monsoon
South-
west
China
Indian summer
monsoon / East
Asian Summer
monsoon
Gradual
weakening
Strong
summer
monsoon
South
China
East Asian
Summer monsoon
Gradual
weakening
Strong
summer
monsoon
Central
China
East Asian
Summer monsoon
Gradual
weakening
Strong
summer
monsoon
Region
14C yr 1000 2000 3000 4000 5000 6000 7000 8000 9000
Cal yr BP 930 1940 3200 4400 5700 6800 7800 8800 10000
Asynchron-
ous start of
weakening
Strong summer monsoon
Strong summer monsoon
Strong summer monsoon
Strong summer monsoon
Strong summer
monsoon
Strong summer monsoon
Gradual weakening
Gradual weakening
Gradual weakening
Gradual weakening
Gradual weakening
Gradual
weakening
26. Comparison of broad trends in Donnge cave record (Wang et al., 2005) and Yeak
Kara record of changes in the Holocene Asian monsoon. Both series are displayed
with a three year moving average trend line
1.2 kaBP
Climate is a complex phenomena, and the global climate systems results from interactions between the various earth systems over a range of timescales.
With current changes global weather and climate, it is important to understand patterns of change and mechanisms for these changes over longer periods of time. The instrumental record stretches back just over a hundred years, and we need to look much further back than this.
The Asian monsoon is the world’s largest atmopsheric system, and is associated with the survival and livelihood of more than 2/3 of the world’s population.
The mainland SE Asian region is an important part of the broader Asian monsoon system. Although the physics of monsoon behaviour are uniform through the system, they are mediated by the geography of the regions underlying them, causing spatial changes in monsoon behaviour and a number of sub-monsoon regions or provinces.
The mainland SE Asia region is influenced by two key subsystems, the Indian summer monsoon and the East Asian winter monsoon.
The Indian summer monsoon brings moist parcels of air from the southwest in the northern-hemisphere summer months, depositing monsoonal rainfall over the region.
The East Asian winter monsoon brings dry air from the NE in the northern-hemisphere winter months, , which can act to increase evaporation over the region.
Literature shows that there is variation in the behaviour of the Indian and East Asian monsoons across a number of geographical regions around mainland SE Asia.
In the early Holocene, there was a strong summer monsoon. High-resolution proxy records from a range of sources, including lake sediments and speleothems show a gradual weakening through the mid-Holocene, possibly starting earlier in the Indian monsoon subsystem. There is spatial variation in the timing of the onset of this gradual weakening, and in some regions there is variation in the timings suggested by different proxy records.
This study used carbonate 18O from lake sediment archives as a direct climate proxy to understand the behaviour of the Asian monsoon over the Southeast Asian region. The study sites are based in Cambodia, in a primarily basaltic region.
In the Ratanakiri province, in NE Cambodia, there are a number of maar crater lakes.
These formed as a result of underlying magma coming into contact with groundwater, causing a series of phreatic explosion which resulted in the formation of a number of hollows that have since filled with water and formed lakes. Over time, aeolian-derived, eroded and authigenically produced material fill the crater basins, settling as sediment at the bottoms of these lakes
This study asked a number of sequential questions
The study lakes a relatively round, with steep sides and flat bottom, with no streams running into them.
This is important because the morphology of these lakes results in stratification, with oxygenated, warm surface waters (epilimnion) and anoxic bottom waters (hypolimnion), meaning that there is very little disturbance of sediments at the bottom of these lakes.
Yeak Kara is somewhat different to the other three lakes, as it is very shallow and has a very rapid sediment formation rate. It is hypothesised that, in the past, the morphology of Yeak Kara was very similar to that of the other three deep crater lakes in the present day.
These are very scenic places to work – these are beautiful lakes, and it was great going out on the water to take water and sediment samples. On one of the field trips the inflatable boat didn’t work, and we had to go out on these wooden canoes which was an adventure, but luckily everyone remained on board and no equipment, bar a pair of sunglasses, fell into the water.
Lake waters were tested with depth and over time for limnological, geochemical and isotopic characteristics of 18O and 2H.
It was found that, across the deep stratified lakes, the waters of the epilimnion, from 0-10m, responded to seasonal changes in temperature, pH, conductivity and isotopic ratios, while the hypolimnion waters tended to stay relatively consistent in their limnological and isotopic chemistry.
This plot shows 18O with depth over time for Yeak Oam, and it can be clearly seen that the surface waters respond to the isotopic input from rainfall in the summer months, showing a decrease in D18O as the wet season progresses, and an increase in D18O as the dry season progresses.
The local water line aligns very well with the GMWL, indicating that the meteoric waters that fall in the region where these lakes are is representative of regional climate, that is, the Asian monsoon.
It is also very important to note that the water line for lake surface waters sits off to the side, indicating that these waters are sensitive to evaporation-precipitation dynamics, which confirms that the waters of these lakes respond to the wet-dry seasonal dynamics of the monsoonal climate.
Now that we’ve established that the isotopic ratios of O and H in lake waters are both representative of regional climate and responsive to the seasonal evaporation-precipitation dynamics of the monsoon, it’s time to consider how this is recorded in sediment archives.
Using a modified Livingstone corer, a 13m core was taken from Yeak Kara. These sediments were analysed for sedimentological characteristics and geochemistry.
In some regions of the core, very fine bands of whitish material interspaced with very thin bands of darker mud-like material were found. Geochemical analysis showed that these were high in calcium, and so sediments were examined using a SEM to see if these were calcium carbonate.
Visual analysis showed that there were distinctly-shaped crystals present in the whiter sections, and energy dispersive spectroscopy (EDS) revealed the chemical composition of these to be calcium carbonate.
To determine the species of calcium carbonate present, electron backscatter diffraction was used to determine an orthorhombic structure, characteristic of aragonite.
During analysis, some calcite was also found, and phase mapping was conducted on a number of samples to see the proportions of aragonite (blue) to calcite (red), showing that aragonite was the dominant species of calcium carbonate present.
Now that a link has been stablished between the isotopic composition of the aragonite crystals in the sediment record and the Asian monsoon, let’s consider what this direct climate proxy reveals about the behaviour of the Asian monsoon over mainland SE Asia in the early-mid Holocene.
The behaviour of the Asian monsoon over the SE Asian region has been found to be distinct from other surrounding regions, with a strong summer monsoon, ‘the Holocene Optimum’ ended at ~5850 cal yr BP with an abrupt decrease in the strength of the Indian summer monsoon, followed by further gradual weakening of the monsoon system. The δ18O stable isotope record does not extend beyond 4300 yrBP, however there is scope for future studies to use other proxies to extend this record through the early Holocene to the present, and also further back in time through to the last glaciation and further into the Quaternary.
The Yeak Kara sediment δ18O record was compared with the Donnge speleothem δ18O record (Wang et al 2005) to explore whether these features were visible in other records in the nearby geographical areas where the Asian monsoon is the dominant climate regime (Figure 5.2).
These data, although both representative of regional precipitation dynamics, reflect different monsoon sectors (Indian versus the East Asian moisture sources) and very different pathways for the sequestration of oxygen isotopes.
Despite this, the Donnge cave record shows the same periods of high variability that are recorded in the Yeak Kara record from 7.2-6.9 kaBP and 4.6-4.3 kaBP, although they are not as pronounced. This could reflect the Indian and East Asian sectors of the monsoon responding differently to forcing mechanisms.
The Donnge cave record and the Yeak Kara record show different timing for the end of the Holocene Optimum and transition towards a weaker summer monsoon, with the Donnge record showing this transition occurring at ~7ka, almost 1.2 kaBP earlier than the Yeak Kara record at 5.8kaBP (Figure 5.2). This highlights the need to investigate the behaviour of the Asian monsoon through palaeoclimatic studies in the vast geographical area that its various provinces cover.
There is a distinct response in the Yeak Kara sediment δ18O record to sudden increases in volcanic sulphates (GISP2) which coincides with the recorded abrupt drying at ~7100 yrBP. The anomalous drying with high variability from 7100-6900 yrBP may hence be a response of the Asian summer monsoon to volcanic forcing, where a stronger response was felt in the mainland Southeast Asia region compared to China, since this anomalous dry event is not recorded as strongly in the Donnge record.
The changes in the strength of the intensity of the Asian summer and winter monsoons arises as a result of interactions and teleconnections with other climate systems as well as through responses to climate forcing mechanisms that impact the earth system. It is known that lakes respond to variations in incoming solar radiation, which is one of the key drivers for mixing and biogeochemical changes in composition of waters (Boehrer & Schultze, 2008). These changes result in changes to circulation patterns in lakes directly, for example through impacting the density of water in the epilimnion, and indirectly by waves and currents caused by changes in the weather and climate in surrounding areas and regions. These changes can be stored in variations in the lake sediments as records of palaeoclimate (Brauer, 2004). The sediment δ18O record in Yeak Kara shows the response of the Asian monsoon system over mainland Southeast Asia to insolation changes through the Holocene (Figure 5.3), which shows that the monsoon system gradually decreased in strength in the mid Holocene in correlation with decreasing insolation, which is a feature observed across many studies exploring the Holocene dynamics of the Asian monsoon provinces and their mechanisms
