Summary
In the past 60 years, speleothems have successfully contributed to paleoclimate
reconstruction and provided important insights in climate teleconnections.
Although large-amplitude long-term global variations are well documented,
better insights in the smaller-scaled and shorter-term variations such as during
the Holocene and the last millennium are still needed. To allow such
reconstructions, the behavior of proxy signals measured in speleothems needs to
be understood in more details. In this PhD, speleothems from two contrasting
climate regions, being temperate (Belgium) and semi-arid (Socotra, Yemen)
systems, are studied for multiple proxies at high resolution going up to seasonal
scales to investigate their potential as tool to reconstruct climate variations
during respectively, the most recent 500 years and the Mid- to Late Holocene.
Speleothems from Socotra Island, located in the northern Indian Ocean, indicate
detailed variations of the less-known winter subsystem of the Indian Ocean
Monsoon over the last 6 000 years. Monitoring results of a fast growing (up to 2
mm/y) stalagmite (called ‘Proserpine’) in the Han-sur-Lesse cave in Belgium
provide new insights on how 18O, 13C, growth rate and calcite fabric link to
climate variations at a seasonal scale. Applying these findings on the Proserpine
speleothem allows reconstructing climate variations during the last 500 years up
to seasonal scale in terms of wetter and warmer or colder and dryer winters.
Chapter 1 highlights the important position of speleothems within the broad
climate science area and describes the need for more detailed high-resolution
reconstructions to allow refining the climate models to more regional scales.
Chapter 2 provides an elaborated state-of-the art of the processes and the theory
behind speleothem based climate reconstructions. At the end of this chapter, the
history of speleothem science with the milestone papers are discussed together
with important recent methodological insights, which will further reinforce the
position of speleothems within the field of climate science. Chapter 3 reports the
first part of this PhD research that focuses on using speleothems to reconstruct
monsoon variations in the northern Indian Ocean. Four speleothems from the
eastern side of Socotra Island (Yemen) are studied for their 18O, 13C, Mg/Ca
and Sr/Ca composition to investigate how the Indian Ocean Monsoon (IOM)
evolved in that area over the last 6 000 years. The 4 records provide unique new
insights in the northeast winter IOM subsystem, which is difficultly recorded in
archives since it is often overwritten by the stronger summer IOM variations.
The results show that the northeast winter IOM subsystem evolves differently
than its southwest summer counterpart over the last 6 000 years with no links to
the North Atlantic climate records. Chapter 4 and Chapter 5 report the second
part of this PhD research where the Proserpine stalagmite from the Han-sur-
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Lesse cave is studied to investigate how speleothem proxies record climate
variations at seasonal resolution in temperate climates such as Belgium. First, a
one year (= 2013), biweekly cave monitoring of the Proserpine growth site is
carried out in Chapter 4 to investigate how 18O, 13C, layer thickness and calcite
fabric changes can be used to reconstruct the paleoclimate at seasonal scale. The
most important conclusions of this work are that seasonal climate variations, i.e.
seasonal variations in the climate parameters (in this thesis, effective
precipitation and temperature), are quickly transferred into the cave and that
they are successfully recorded in the speleothem calcite. Seasonal 18O variations
reflect temperature variations, whereas seasonal 13C, layer thickness and
growth rate variations reflect changes in effective precipitation. These acquired
insights are used in Chapter 5 to interpret seasonally resolved 18O, 13C, layer
thickness and calcite fabric changes in the Proserpine stalagmite over the last
500 years to investigate climate variations during the last part of the Little Ice
Age (LIA, ± 1300-1850) and the anthropogenic period (most recent 150 years).
Decadal and centennial changes in the measured proxies reconstruct winter
precipitation intensities and temperatures and indicate the occurrence of
different climatic events that correspond with known European variations.
Seasonal variations are well recorded in the stalagmite and provide new
knowledge on seasonal temperature and effective precipitation over the studied
period in Northern Continental Europe. This work illustrates that, provided a
good knowledge of the cave system combined with a multiproxy approach;
speleothems from temperate climates successfully record lower-amplitude and
seasonal variations.
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