18
Air-Ground Interface: Sediment Tracers
in Tropical Lakes
David Williamson
The climate archives found at low latitudes are mostly sedimentary or geomorphological. Their formation and preservation depend fundamentally on favorable geomorphological
and climatic conditions. Therefore, they differ according to
the environment, the region and the climate and include
moraines left behind by mountain glaciers, ancient shorelines
of old lakes, dunes, flood levels, archeological sites occupied
by animals or humans, continental carbonates (speleothems,
travertine, stromatolites) and sedimentary sequences collected in cores from lakes and bogs. Additional archives
consist of coastal deposits (e.g. corals) and marine hemipelagic sediments with a high deposition rate, especially near
river deltas or coastal upwellings off Mauritania, Benguela
and Somalia. The dating and study of aquifers, as well as of
tree rings complete the range of climate records currently
available and studied, particularly in semi-arid areas.
The physical, chemical and biological controls of the
sedimentary records vary greatly according to the local or
regional specificity of the areas of deposition. For example,
the mixing or overturning of the water column of a lake is
generally recorded by biological (diatoms) or organic
proxies. It may be activated by an earthquake, outgassing
from anoxic hypolimnion or by sediments, but also by the
cooling of surface waters, the melting of ice, by wind
intensity or by a drop in lakewater level. Hence, the climate
or hydrological proxies (e.g. physicochemical properties or
isotopic signatures of water and sediments, terrestrial or
aquatic vegetation markers, erosional or atmospheric inputs)
do not have the same fundamental meaning from one site to
another.
To reduce the uncertainties in timing and interpretation, it
is essential to undertake a detailed reconstruction of the
deposition processes in several sites characteristic of the
region and for a given climate. Among the non-destructive
physical and chemical methods of measuring the variability
of depositional environments, the measurement of magnetic
sedimentary properties (low-magnetic field susceptibility,
induced and remanent magnetizations) is a particularly
appropriate one, because multiple measurements can be
performed. The magnetic signature of sediments—and other
surface materials—depends on changes in the concentration,
grain size and mineralogy of iron minerals (mainly oxides,
sulfides, carbonates). Iron mineral assemblages closely
depend on the iron biogeochemical cycle, in particular on the
weathering of primary minerals, pedogenetic processes,
biomass fires, gravity deposition, redox changes, microbial
activity, authigenesis and early diagenesis. Magnetic methods thus allow for the reconstruction of the dynamics of
surface processes recorded along sedimentary sequences,
and the identification of the main modes of environmental
and climate variability (Thompson and Oldfield 1986).
Intertropical Hydrological Variability in Africa
Africa is characterized by a wide variety of climates, mainly
defined by the amount and distribution of rainfall at the
seasonal, annual, and year-to-year level (from less than
100 mm per year to more than 3000 mm per year) and
temperature (especially at mid-latitudes and at altitude).
From the humid equatorial climate of the Congo basin to the
hyperarid or subtropical climates of the Sahel/Sahara and of
the Kalahari/Namibia, regional climates respond to the
annual and seasonal characteristics of global and tropical
atmospheric circulation (in particular the activity of the
Intertropical Convergence Zone, ITCZ). The temperatures
and surface currents of the neighboring oceans and the
regional orographies (e.g. East-African rift, great escarpment
of Namibia) determine the regional rainfall patterns. Together with the local geomorphology, these regional climates
have a determining influence on the presence, nature and
historical continuity of most of the land-based climate
archives.
D. Williamson (&)
World Agroforestry Centre, Research Institute for Development,
UMR LOCEAN, Université Pierre et Marie Curie, P.O.
Box 30677-00100 Nairobi, Kenya
e-mail: david.williamson@ird.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_18
209
Air-Ground Interface: Sediment Tracers
in Tropical Lakes
David Williamson
The climate archives found at low latitudes are mostly sedimentary or geomorphological. Their formation and preservation depend fundamentally on favorable geomorphological
and climatic conditions. Therefore, they differ according to
the environment, the region and the climate and include
moraines left behind by mountain glaciers, ancient shorelines
of old lakes, dunes, flood levels, archeological sites occupied
by animals or humans, continental carbonates (speleothems,
travertine, stromatolites) and sedimentary sequences collected in cores from lakes and bogs. Additional archives
consist of coastal deposits (e.g. corals) and marine hemipelagic sediments with a high deposition rate, especially near
river deltas or coastal upwellings off Mauritania, Benguela
and Somalia. The dating and study of aquifers, as well as of
tree rings complete the range of climate records currently
available and studied, particularly in semi-arid areas.
The physical, chemical and biological controls of the
sedimentary records vary greatly according to the local or
regional specificity of the areas of deposition. For example,
the mixing or overturning of the water column of a lake is
generally recorded by biological (diatoms) or organic
proxies. It may be activated by an earthquake, outgassing
from anoxic hypolimnion or by sediments, but also by the
cooling of surface waters, the melting of ice, by wind
intensity or by a drop in lakewater level. Hence, the climate
or hydrological proxies (e.g. physicochemical properties or
isotopic signatures of water and sediments, terrestrial or
aquatic vegetation markers, erosional or atmospheric inputs)
do not have the same fundamental meaning from one site to
another.
To reduce the uncertainties in timing and interpretation, it
is essential to undertake a detailed reconstruction of the
deposition processes in several sites characteristic of the
region and for a given climate. Among the non-destructive
physical and chemical methods of measuring the variability
of depositional environments, the measurement of magnetic
sedimentary properties (low-magnetic field susceptibility,
induced and remanent magnetizations) is a particularly
appropriate one, because multiple measurements can be
performed. The magnetic signature of sediments—and other
surface materials—depends on changes in the concentration,
grain size and mineralogy of iron minerals (mainly oxides,
sulfides, carbonates). Iron mineral assemblages closely
depend on the iron biogeochemical cycle, in particular on the
weathering of primary minerals, pedogenetic processes,
biomass fires, gravity deposition, redox changes, microbial
activity, authigenesis and early diagenesis. Magnetic methods thus allow for the reconstruction of the dynamics of
surface processes recorded along sedimentary sequences,
and the identification of the main modes of environmental
and climate variability (Thompson and Oldfield 1986).
Intertropical Hydrological Variability in Africa
Africa is characterized by a wide variety of climates, mainly
defined by the amount and distribution of rainfall at the
seasonal, annual, and year-to-year level (from less than
100 mm per year to more than 3000 mm per year) and
temperature (especially at mid-latitudes and at altitude).
From the humid equatorial climate of the Congo basin to the
hyperarid or subtropical climates of the Sahel/Sahara and of
the Kalahari/Namibia, regional climates respond to the
annual and seasonal characteristics of global and tropical
atmospheric circulation (in particular the activity of the
Intertropical Convergence Zone, ITCZ). The temperatures
and surface currents of the neighboring oceans and the
regional orographies (e.g. East-African rift, great escarpment
of Namibia) determine the regional rainfall patterns. Together with the local geomorphology, these regional climates
have a determining influence on the presence, nature and
historical continuity of most of the land-based climate
archives.
D. Williamson (&)
World Agroforestry Centre, Research Institute for Development,
UMR LOCEAN, Université Pierre et Marie Curie, P.O.
Box 30677-00100 Nairobi, Kenya
e-mail: david.williamson@ird.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_18
209