WP3: Impact of decadal climatic cycles on sedimentation in the modern Mahakam Delta, Indonesia
Prof. Dr. Salomon B. Kroonenberg
Dr. Jajang Sukarna
Ir. Duddy Ranawijaya
Dr. Simon Troelstra
3. Summary of the project
The project aims to reconstruct changes in sedimentation and geochemical environment in the Mahakam
Delta in the last 200 years, on the basis of (1) biogeochemistry of long-living molluscs, especially
Polymesoda, and (2) granulometry, geochemistry and paleoecology of 210Pb-dated cores from the different
subtidal sedimentary environments.
The geochemical signal preserved in the shells of molluscs is an
exciting new tool to study rapid climate/environmental change. The laser ablation and microdrill technique,
coupled with state of the art geochemical analyses (stable oygen/carbon isotopes, trace elements, ICP-MS,
ICP-AES) opens a detailed record of precipitation/evaporation variability, environmental change, and
run-off patterns which can be directly related to monsoonal, ENSO variability and human impact.
Sampling molluscs along fresh/brackish-water transects in the Mahakam area enables us to link the
effects of specific ENSO-cycles and anthropogenic induced environmental change. Preliminary geochemical
analysis of the shell of the brackish water species Polymesoda showed a detailed climatic/environmental
archive extending ~120 years back. Systematic variations in Sr/Ba ratio suggest important variations in
the influence of marine and terrestrial processes during its growth, possibly caused by ENSO and/or
human interference. Now we propose to carry out a monitor program to calibrate our data, to complete
our analyses on both fossil and recent brackish/freshwater material and to achieve a correlation with
other biota such as corals. ENSO variability over the past centennia will be studied using museum material.
We hypothesize that the changes in Sr/Ba ratio in the molluscs reflect changes in the delicate balance
between fluvial and tidal processes in the delta. We want to test this hypothesis by dating cores
from the different sedimentary environments with 210Pb and other radionuclides, and study granulometry,
geochemistry (including Sr/Ba as well) and paleoecology ( palynology) in those parts of the cores that
according to the radionuclide data have been deposited in the last 200 years. Special attention will
be given to the distribution and thickness of sand-mud couplets thought to represent neap-spring tide cycles.
These data will be compared with data on delta development during the last 5000 years (Project1) and
from the last 50 years from remote sensing imagery (Project 3) and will be used to refine the scenarios
for future development of the delta using the Delft 3D model.
4. Detailed description of the project
a. Scientific Background
In spite of the equable tropical humid climate and the (once) continuous rainforest vegetation of the
Mahakam drainage basin, environmental changes have occurred in the recent past, and more are being
caused by human intervention. The most recent and most dramatic events in the upstream part of the
basin are the forest fires related to ENSO drought but undoubtely sparked by human activity.
From the analysis of charcoal particles in sediments in the Mahakam area human activity is already
discernible in the last 1500 years. There is evidence for forest disturbance near Samarinda around
400 AD (Yulianto et al., 2005). Historical reports indicate also ENSO-related major droughts and
forest fires in 1877-1879, 1902,1914-1915,1940, 1965, 1972, 1982-1983 (Brookfield et al., 1995).
After forest fires the Mahakam river is polluted with fire debris (Boer, 1989), and charcoal
particles in the sediment testify to that. Furthermore logging and mining activities in the
drainage basin have intensified in the last century. All this must have had impact on sedimentation
in the Mahakam delta. The last decade has seen an almost complete clearance of the mangrove
vegetation in the delta for the construction of shrimp ponds. Especially in the tidal channels
the subtraction of large areas for flooding must have changed the sedimentation dynamics in the
This proposal aims to study the impact of these changes on sediment dynamics in the Mahakam
delta during the last 200 years in two ways: (1) detailed microgeochemical analysis of growth
increments in the long-living brackish-water Polymesoda mollusc, which has its habitat in the Nypa
vegetation zone of the Mahakam delta, and has been shown to be a sensitive monitor of environmental
changes in the coastal environment; and (2) detailed sedimentological, geochemical and palynological
analysis of radionuclide-dated sediment cores from the shallow deltaplatform, in order to detect in
the sedimentary record of the last 200 years the same changes that have been recorded by the bivalves.
b. Specific Objective(s)
(1) Mollusc biogeochemistry The geochemical information (stable O/C isotopes, trace elements)
preserved in mollusc shells represents an exciting new source of paleo-environmental information.
Research on both marine, freshwater and terrestrial molluscs has shown that detailed analysis of
the shells yields data that can be directly related to the parameters mentioned above
(Kaandorp et al, 2001, 2002; Troelstra et al, 2001; Verdegaal, 2005). Mollusc distribution is
governed by biotic and abiotic parameters such as current-regime, substrate, temperature, salinity,
oxygen-content, nutrients and predation. Most species have sharply defined ecological niches; small
fluctuations in one of the parameters will result in population modification. It is precisely this
close association with environmental parameters which makes molluscs excellent monitors for ecosystem
variability. Freshwater unionids can reach ages of up to 30-40 years, in cases even exceeding 100 years.
Upon death they leave behind a detailed decennial environmental archive. The recent development of the
microdrill sampling technique enables us the sample the shell in time-series with a resolution of weeks.
We thus have access to a powerful new tool to produce environmental data (temperature, salinity, runoff,
precipitation) from pre-instrumental periods, but which also enables us to detect subtle changes in
the water-chemistry of the recent regime (e.g. human induced pollution).
During the pilot phase of
the E. Kalimantan project a short sampling campaign was carried along two fluvial/coastal
(fresh-brackish-marine) transects: the anthropogenically influenced Mahakam and the relatively pristine
Berau areas. Freshwater unionids were encountered in the Mahakam lakes. This site experiences rapid
environmental change because of rapid sedimentary infill.
The brackish Nypa environment of both
systems contains abundant living Polymesoda, a thick-shelled bivalve. Preliminary analysis shows
that the stable oxygen isotopic signature of this species contains a detailed archive of rapid
climatic change: the El Nino years of 1986/87, 1991/92, 1997/98 and 2003
are clearly reflected in the outer 2 cm of the shell. Subsequent laser-ablation analysis along
the entire margin of the specimen revealed a spectacular seasonal signal of a.o. the elements
Ba and Sr, which serve as proxies recording fluctuations in terrestrial and marine influences
in the habitat. It shows that the lifespan of the mollusc exceeds 100 years while continually
registering the ambient environment.
During the main phase the research will focus on the calibration of parameters by measuring pertinent
data on waterchemistry, temperature and salinity during a two year cycle at a number of carefully
selected monitor stations. Detailed monitoring and analysis of the material will give insight in the
interaction between precipitation, evaporation, run-off, sea-surface temperature and salinity over a
fixed period. In addition the shell chemistry will produce evidence for the degree of human impact
on the ecosystems. The species Polymesoda will be used for our research.
These data will be coupled
with observations on living molluscs (a.o. seasonal growth-rate) and at a later stage with the
geochemical signals stored in the shell during the observational period. In addition detailed
analyses will be carried out on E. Kalimantan material from the period 1700-2000 derived from
museum collections, in order to get insight in ENSO variation and anthropogenic influence over
the last centennia.
(2) Sediment characteristics The principal phase of the research proposed
here is to evaluate how the decadal events recorded in the Polymesoda growth rings and in the
historical data have affected sedimentation in the delta, especially the delicate balance
between fluvial and tidal processes as recorded in the Sr/Ba systematics of the molluscs.
This will be done by detailed studies of shallow cores to be taken in environments of
varying fluvial and tidal impact on the shallow tidal platform.
During the pilot phase it was shown that the overall homogeneous fine-grained sediments from the
Mahakam prodelta are very suitable for 210Pb accumulation rate assessment. Accumulation rates
varied between 0 - 6.5 cm per year (Van den Bergh, 2004, Kneepkens, 2004). These pilot phase
materials will be made available to our project by NIOZ and UU. A sedimentation rate of
0,27-0.42 cm/year was found at Muara Jawa in the southern part of the delta itself
(Darlan et al., 2004). We now will extend these records to the subtidal part of the shallow
delta platform, where fluvial and tidal contributions can still be spatially separated.
However, here, the risk of erosional discontinuities is greater than in the deep prodelta,
which may result in difficulties in interpreting 210Pb data; therefore, we plan to do
additional 137Cs datings and possibly other radionuclides as well. We cannot
sample in the intertidal area where Polymesoda lives, because there the 210Pb signal is
likely to have been destroyed by atmospheric water.
Our own pilot phase coring campaign
in the onshore part of the delta revealed the presence of sand-mud couplets in Holocene
shallow-water delta platform sediments, postulated to represent spring-tide-neap tide cycles
(Storms et al., 2005), a phenomenon also known from the Fly River delta in Papua New
Guinea (..) Also these may serve to assess the completeness of the record on the shallow
delta platform. On the basis of all these data we hope to be able to get a reliable
picture of the amount, nature and distribution of the fluvial and tidal sediments
accumulated in the last 200 years.
The dated sediment cores from the delta platform will
be sampled in great detail (cm-scale) and analysed on grain size (Compton-laser equipment),
geochemistry, petrography and palynology. Fluvial and tidal sediments may be distinguished
from each other in several ways. Tidal sediments are usually finer-grained than fluvial ones
because of the longer pathways (Storms et al., 2005). Fluvial sediments are usually azoic because
of the acid river water (pH 5), while tidal sediments may contain foraminifera, ostracods
and other calcareous microfauna (Carbonel & Moyes, 1987). This might also be evident from
Sr/Ba ratios in the bulk geochemistry of the sediments themselves. Differences in proportions
of upland and mangrove taxa in pollen spectra may also be expected. Fluctuations in the amount
of charcoal particles might give indications of varying influence of forest fires further
Integrating the data from the three sources will lead to a detailed picture of
the sensitivity of the Mahakam delta system for external forcing in fluvial and tidal
processes in the last 200 years. The paleogeographic reconstruction emerging from these data
will be confronted with the models developed on the basis of Delft3D in the first work package.
This will lead to a better prediction of the impact of future changes.
The research will be carried out by two Indonesian researchers, one full-time PhD student
(4 years) with a sedimentology specialisation, and a research assistant (2 years) who will
be in charge of the mollusc work. Additional field assistance from Indonesian and/or Dutch
MSc students is desirable, funding will be sought from StuNed.
(1) Mollusc biogeochemistry:
In the field
Establish two monitor stations, each consisting of
a cage equipped with a thermometer/salinometer for continuous temperature/salinity registration.
In each cage five specimens of Polymesoda will be placed. Water samples for stable oxygen and
carbon isotopic measurements, trace element concentration and nutrients will be taken bi-weekly.
The specimens of Polymesoda will be marked (fluorescence) to indicate the start of the experiment.
This will be repeated every six months to allow accurate correlation with the water dataset.
Furthermore additional Polymesoda specimens will be sampled from their original habitat in the field.
In the laboratory
(2) Sediment characteristics:
Field: In coordination with the other project of the
programme, a sampling cruise with the MGI reseqarch vessel GeoMarin will be set up. Sampling sites will
be selected on the basis of previous work by the ICoMAR partners, Roberts & Sydow (2003) and Storms et
al (2005). Bottom samples using gravity coring will be taken until a maximum depth of 3 metres.
The sediments will be sealed, cut lengthwise and stored in a refrigerator on board of the Geomarin at
6°C. Pictures will be taken of the split cores, and they will be described macroscopically, focusing
on grainsize, colour and organism and plant remains. One half of the split cores will remain in
Cirebon at the MGI facilities, the other half will be transported to the Netherlands for further analysis.
Bulk grain size will be determined at the Free University using Compton analysis
on samples on cm-scale, separating as much as possible the laminae in sand-mud couplets (cf.
Storms et al., 2005). CaCO3 and TOC content will be determined using standard methodologies.
Analyses of the specific 210Pb activity will be carried out at NIOZ. The samples will be freeze-dried
and grinded, spiked with 210Po and leached with 55 ml HNO3 (1.5M) for 7 h. After leaching, 4 ml NH4OH
and 5 ml of 40g/litre ascorbic acid (in 0.5M HCl) will be added. The activity of 210Pb will be measured
via its á-particle emitting granddaughter isotope 210Po with a Passivated Implanted Planar Silicon
(PIPS) detector of Canberra (Boer, in prep.). It will be attempted to carry out the analyses on
samples with homogeneous grain sizes, as grainsize variation within a sample can disturb the results
of the 210Pb-analysis. Pollen analysis will be carried out by Sander van der Kaars at the School of
Geography and Environmental Science, Monash University, Australia. Major and minor element XRF bulk
sediment geochemical analyses and petrographical analyses will be carried out on cores with good 210Pb
dates using the facilities at the Free University of Amsterdam.
Modelling The data will be used to bridge the gap between the revised Delft3D model being developed
in WP1 for millennial time scales to ENSO time scales. DELFT3D is a numerical engineering model
developed at Delft Hydraulics and has proven capabilities of simulating 3D morphology and stratigraphy
on small time scales, max 101 years. In cooperation with the section of hydraulic engineering at
DUT DELFT3D will be upgraded for large time scales by introducing scaling parameters.
d. Scientific Relevance
The unique feature of this project is that it attempts to correlate the high-resolution environmental
record preserved in bivalves with the record preserved in the deltaic sediments, in order to study
the impact of external forcing on seasonal, annual to decadal time scales.
5. Participation in a graduate School ('onderzoeksschool')
CTG (Centre of Technical Geosciences) TUDelft
6. Scientific performance of members of the research group(s)
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