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| Titel |
Source to sink element geochemistry and clay mineralogy in Lake Towuti,
Indonesia: understanding climate-induced controls on sediment composition
during the past 60 kyr BP |
| VerfasserIn |
Marina Morlock, Hendrik Vogel, Valentin Nigg, Ascelina Hasberg, Martin Melles, James M. Russell, Satria Bijaksana |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130532
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| Publikation (Nr.) |
 EGU/EGU2016-10801.pdf |
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| Zusammenfassung |
| Lake Towuti is a large (560 km2 surface area; 198 m max. water depth) ultraoligotrophic lake
hosted in the East Sulawesi ophiolite, characterised by high iron and very low sulphur
contents. The lake is surrounded by several 10s of metres thick deeply weathered laterite soils
and closed-canopy rainforest. In May-July 2015, we recovered more than 1000 m of sediment
core capturing the entire sediment infill to bedrock in the course of the ICDP Towuti Drilling
Project.
In the tropics very little is known about the influence of climatic changes on weathering
and erosion on glacial-interglacial time-scales. It is expected that varying hydroclimatic
conditions will lead to changes in the weathering and erosion rates and greatly influence
terrestrial elemental cycling. The direction of change and more quantitative estimates of the
rates of changes are, however, unknown.
In order to characterise modern erosional processes and element cycling in the
lake and its catchment, we collected catchment-characteristic bedrock samples and
profiles of their overlying laterites, riverine sediments, and 85 samples of surface
sediments from the lake. All samples were analysed for their geochemical and
clay-mineralogical (<2 μm) composition in order to define the composition of erodible
substrates, trace source-to-sink changes in sediment composition, and assess the
spatial variability in Lake Towuti. The relationships found in the modern system
were then applied to two sediment cores, dating back 30,000 and 60,000 years BP,
respectively.
The laterite soils in the catchment show a characteristic zonation with high concentrations
of Al, Ti, Fe, and Cr in the uppermost horizon, while Mg is enriched in the saprolite zone
directly above bedrock. Weathering intensity increases from bedrock (least weathered) across
river bedload of the 15 inlets to the sediments in the deepest basin of the lake (most
weathered). The largest inlet to Lake Towuti, the Mahalona River, supplies sediments with
low Al and high Mg concentrations and exerts a dominant control on the present-day
sediment composition of Towuti’s northern basin. This indicates that the Mahalona River and
its tributaries cut deep into the laterite soils, transporting relatively unweathered material to
the lake.
In the past 60,000 years, the Al/Mg ratio is lowest between 35,000 and 15,000
years BP, and kaolinite is the dominant clay mineral during this period. During
most of the Holocene and >35,000 years BP, Al/Mg is comparable to today and
smectites (Holocene) and illites (MIS 3) are the most abundant clay minerals. The clay
mineralogy suggests deeper soil erosion during wet interglacials and more surficial
erosion during dry glacial climate conditions. These findings imply that erosion
and element cycling are mainly driven by changes in precipitation amount and
terrestrial runoff in Towuti’s catchment. The Al/Mg ratio on the other hand points
to a stronger (lesser) contribution of relatively unweathered sediments sourced
from the Mahalona River catchment during dry (wet) phases, likely as a result of
lake-level changes and associated changes in shoreline proximity to our coring sites. |
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