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Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer insolation. Conversely, short-term precipitation changes in the northern African tropics have been linked to North Atlantic sea surface temperature anomalies, affecting the northward extension of the Intertropical Convergence Zone and its associated rainbelt. Our knowledge of large-scale hydrological changes in equatorial Africa and their forcing factors is, however, limited. Here we analyse the isotopic composition of terrigenous plant lipids, extracted from a marine sediment core close to the Congo River mouth, in order to reconstruct past central African rainfall variations and compare this record to sea surface temperature changes in the South Atlantic Ocean. We find that central African precipitation during the past 20,000 years was mainly controlled by the difference in sea surface temperatures between the tropics and subtropics of the South Atlantic Ocean, whereas we find no evidence that changes in the position of the Intertropical Convergence Zone had a significant influence on the overall moisture availability in central Africa. We conclude that changes in ocean circulation, and hence sea surface temperature patterns, were important in modulating atmospheric moisture transport onto the central African continent. [ABSTRACT FROM AUTHOR]

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants-adapted to warm, dry conditions and low atmospheric CO2 concentrations-have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature and CO2 concentration are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content-driven by tropical sea surface temperature changes and the strength of the African monsoon-controlled aridity on the African continent, and hence large-scale vegetation changes. [ABSTRACT FROM AUTHOR]

The presence and abundance of anaerobic ammonium-oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300-3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q-PCR) analysis of anammox-specific 16S rRNA genes and anammox-specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g-1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]-ladderane monoether-PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox-specific 16S rRNA gene copy numbers and the concentration of the C20 [3]-ladderane monoether-PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 x 108 copies g-1 sediment and 30 pg g-1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification. [ABSTRACT FROM AUTHOR]

We analyzed the distribution of branched tetraether membrane lipids derived from soil bacteria in a marine sediment record that was recovered close to the Congo River outflow, and the results enabled us to reconstruct large-scale continental temperature changes in tropical Africa that span the past 25,000 years. Tropical African temperatures gradually increased from ~21° to 25°C over the last deglaciation, which is a larger warming than estimated for the tropical Atlantic Ocean. A direct comparison with sea-surface temperature estimates from the same core revealed that the land-sea temperature difference was, through the thermal pressure gradient, an important control on central African precipitation patterns. [ABSTRACT FROM AUTHOR]

Abstract: The debate of climate versus CO2 in controlling the long-term dynamics of tropical African vegetation has focused on events at the upper tree-line, since the relevant paleodata tend to be from mid-elevation sites (~2000–3000m). Less well known is the relative importance of CO2 in regulating the dynamics of tropical lowland (<1500m) vegetation, particularly that of the dry open woodlands, bush- and grasslands covering much of eastern equatorial Africa. Here we examine the stable carbon isotopic composition of n-alkanes in the sediment record of Lake Challa, a lowland crater lake near Mt. Kilimanjaro, covering the last 25,000years. The distributions of the n-alkanes, with dominance of the long-chain odd-carbon-numbered components, and their isotopic composition reveal a mixed origin. The C23 and C25 n-alkanes are depleted in 13C, with δ13C values between −30 and −50‰. In shallow lakes these n-alkanes are thought to derive from non-emergent aquatic plants, but this is unlikely in this steep-sided crater lake as it lacks a significant littoral habitat. The C27+ n-alkanes are predominantly derived from leaf wax lipids of terrestrial plants, brought into the lake predominantly by local soil run-off. Their δ13C values, in particular that of the n-C31 alkane, reveal a marked transition in local lowland vegetation from being dominated (~70–100%) by C4 plants during the glacial period until 16.5cal kyr BP, to a more mixed C3/C4 composition (~30–60% C4) during the Holocene. The start of the late-glacial trend towards a greater proportion of C3 plants coincided with the start of increasing monsoon rainfall, ~1500years after the onset of the rise in atmospheric CO2 and ~3500years after the onset of post-glacial warming. The transition was interrupted during the dry Younger Dryas period (13.0–11.7cal kyr BP), when C4 plants again became much more prevalent, almost reaching their glacial-period abundance. Notably, the principal trend in leaf-wax δ13C values infers C4 dominance during both wet and dry phases of the glacial period, and a mixed C3/C4 vegetation during both wet and dry phases of the Holocene. Our results indicate that long-term variation in pCO2 exerted important control on the composition of drought-adapted savanna vegetation in this currently semi-arid lowland region of equatorial East Africa. [Copyright &y& Elsevier]

Abstract: High-resolution records of organic and inorganic geochemical proxies document environmental changes during Oceanic Anoxic Event 1b for the Albian upwelling system at DSDP Site 545 Mazagan Plateau off NW Africa. Sea surface temperature estimates based on TEX86 analyses show an abrupt rise by ~3 °C concurrent with a more than 2-fold increase in accumulation rates for organic carbon and siliciclastic sediment components. Geochemical grain-size indicators (Si/Al and Zr/Al) indicate the influx of overall finer grained sediment during the event. Our data suggest that the warming phase associated with OAE 1b resulted in an attenuation of the NE trade wind system and a weakening of local upwelling conditions. Higher amounts of precipitation over NW Africa and an associated higher continental run off may have supplied nutrients to the eastern North Atlantic and fostered the production of organic matter and black shale deposition. [Copyright &y& Elsevier]

Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer insolation. Conversely, short-term precipitation changes in the northern African tropics have been linked to North Atlantic sea surface temperature anomalies, affecting the northward extension of the Intertropical Convergence Zone and its associated rainbelt. Our knowledge of large-scale hydrological changes in equatorial Africa and their forcing factors is, however, limited. Here we analyse the isotopic composition of terrigenous plant lipids, extracted from a marine sediment core close to the Congo River mouth, in order to reconstruct past central African rainfall variations and compare this record to sea surface temperature changes in the South Atlantic Ocean. We find that central African precipitation during the past 20,000 years was mainly controlled by the difference in sea surface temperatures between the tropics and subtropics of the South Atlantic Ocean, whereas we find no evidence that changes in the position of the Intertropical Convergence Zone had a significant influence on the overall moisture availability in central Africa. We conclude that changes in ocean circulation, and hence sea surface temperature patterns, were important in modulating atmospheric moisture transport onto the central African continent.

The dominant forcing factors for past large-scale changes in vegetation are widely debated. Changes in the distribution of C4 plants--adapted to warm, dry conditions and low atmospheric CO2 concentrations--have been attributed to marked changes in environmental conditions, but the relative impacts of changes in aridity, temperature and CO2 concentration are not well understood. Here, we present a record of African C4 plant abundance between 1.2 and 0.45 million years ago, derived from compound-specific carbon isotope analyses of wind-transported terrigenous plant waxes. We find that large-scale changes in African vegetation are linked closely to sea surface temperatures in the tropical Atlantic Ocean. We conclude that, in the mid-Pleistocene, changes in atmospheric moisture content--driven by tropical sea surface temperature changes and the strength of the African monsoon--controlled aridity on the African continent, and hence large-scale vegetation changes.