Insolation and glacial–interglacial control on southwestern African hydroclimate over the past 140 000 years
Introduction
Tropical to sub-tropical southwestern Africa (between about 17°S and 30°S) experiences semi-arid to hyper-arid conditions (Tyson, 1986) due its position in-between the influence of precipitation from low-latitude tropical climate systems in the north and mid-latitude climate systems in the south. It hence provides an important test-region for investigating past changes in the spatial distribution of these climate systems. Unfortunately, due to this aridity, long terrestrial palaeoclimate records are rare and thus our understanding of past changes in precipitation remains incomplete.
There is presently debate regarding the behaviour of hydroclimate in southwestern Africa in response to precessional (19–23 kyr cycle) insolation variations. Increased local summer insolation is expected to increase the land–ocean pressure gradient, bringing more warm, moist air on land and increasing precipitation delivered by the summer monsoon, as has been shown in the northern hemisphere (NH; e.g. Pokras and Mix, 1985, Rossignol-Strick, 1985). In line with this, a 200 ka long sedimentological record from Lake Tswaing in southeastern Africa, suggests that precipitation increased during precessional Southern Hemisphere (SH) summer insolation maxima (Partridge et al., 1997), due to an enhancement of the SH summer East African Monsoon. Similarly, a leaf-wax hydrogen isotope record from the Zambezi River (Schefuß et al., 2011) suggests relatively dry conditions during the mid-Holocene SH summer insolation minimum, relative to the deglacial and late Holocene. In contrast, a hyrax-midden record from the Namib Desert, spanning the last 11.7 ka (one half of the last precessional cycle), suggests a progressive drying from the mid to late Holocene, i.e. wetter rather than drier conditions during the mid-Holocene SH summer insolation minimum (Chase et al., 2009). It was thus suggested that southwestern Africa responds in phase with NH summer insolation (Chase et al., 2009). As such, on precessional timescales it is not yet clear whether the hydrology of southwestern Africa is controlled by NH or SH summer insolation variations.
In addition to precessional insolation control, the effect of glacial versus interglacial boundary conditions on the hydroclimate of southwestern Africa is also under debate. For example, more desert and semi-desert vegetation points to drier conditions during the last glacial period relative to the Holocene and last interglacial period (Shi et al., 2001, Collins et al., 2011). In contrast, a marine grain-size record (Stuut et al., 2002) and a collection of terrestrial records (Chase and Meadows, 2007) suggest wetter conditions in southwestern Africa during the last glacial period. Depleted isotopes in precipitation would also point to wetter glacial conditions (Collins et al., 2013a). Wetter glacial conditions in southwestern Africa have been interpreted to reflect a northward shift of the SH mid-latitude westerly wind belt during the last glacial period (Stuut et al., 2002, Chase and Meadows, 2007, Cockcroft et al., 1987). However, an alternative mechanism is a southward shift of tropical rain-producing systems due to the expanded NH ice sheets, as is sometimes simulated by climate models (e.g. Kageyama et al., 2013). In summary, studies disagree whether southwestern Africa was wetter or drier during the last glacial period and, for those that do agree on the sign of the changes, different mechanisms have been invoked.
We investigate the effect of precessional insolation changes and glacial–interglacial boundary conditions on the climate of southwestern Africa using the hydrogen and carbon isotopic composition of terrestrial plant leaf-wax n-alkanes taken from a marine sediment core. n-Alkanes are straight chain hydrocarbon compounds produced as part of the protective layer on terrestrial plant leaves (Koch and Ensikat, 2008, Eglinton and Hamilton, 1967). The hydrogen isotopic composition () of leaf wax n-alkanes is taken as a recorder of the hydrological history of precipitation (e.g. Sachse et al., 2012). The carbon isotopic composition () of leaf-wax n-alkanes reflects the photosynthetic pathway of the plants i.e. the relative contribution of versus vegetation (e.g. Castañeda et al., 2009). We also assess the contribution of terrestrial inorganic material from XRF-major element analysis. Our sediment core is located off the coast of Namibia at 23°S and receives terrestrial material mainly as dust from southwestern Africa. The sediment core extends back to 140 ka, covering six precessional cycles allowing us to determine whether NH or SH summer insolation controlled southwestern African climate. This time span also allows us test the effect of glacial boundary conditions during the last glacial period (Marine Isotope Stages; MIS 5.4 to MIS 2) on climate by comparison with two interglacial periods; the Holocene (MIS 1; from 10 ka to the present) and the last interglacial period (MIS 5.5; between 130 ka and 116 ka; Kukla et al., 2002). Finally, in order to understand regional-scale shifts in precipitation distribution, we compare our findings with other records from southwestern and tropical central Africa.
Section snippets
Precipitation, moisture sources and controls on of precipitation
Southwestern Africa experiences arid conditions due to the South Atlantic Anticyclone, which is strongest and furthest south during SH winter (e.g. Tyson, 1986). The western part of Namibia experiences the most pronounced aridity due to the cold sea surface temperature of the Benguela upwelling region, which stabilises air and prevents convection (e.g. Eckardt et al., 2013 and references therein). Most precipitation delivered to southwestern Africa is tropical convective precipitation, which is
Sediment core age model
Stratigraphy for the upper part of core MD08-3167 (23.3152°S; 12.3768°E) is based on published radiocarbon ages of planktonic foraminifera (Collins et al., 2013a). For the lower part (below 515 cm), stratigraphy is based on correlation of XRF-scanner Ca/Fe ratio with that of core GeoB1711-4 (23.3150°S, 12.3766°E; Fig. 2a), located at the same coring site as MD08-3167. The age model for core GeoB1711-4 is based on radiocarbon ages of planktonic foraminifera for the upper part and correlation of
Results
Over the last 140 ka, the sedimentation rate of core MD08-3167 (Fig. 3a) displays low values during the last interglacial period (MIS 5.5; ) and the Holocene (MIS 1; ), and an increase during the last glacial period (MIS 5.4 to MIS 2) with maximum values during the Last Glacial Maximum (LGM; MIS 2; ). The terrigenous fraction of the core (Fig. 3b) is lower during most of MIS 5, MIS 3 and MIS 1 and higher during MIS 2 and MIS 4.
CPI values for the sediment core have a
Terrigenous fraction and terrigenous-normalised n-alkane concentrations
The terrigenous fraction of the sediment was generally higher during the last glacial period and lower during the Holocene, the last interglacial period and MIS 3 (Fig. 3b). Changes in the terrigenous fraction depend on changes in either marine productivity or terrestrial input. Sedimentation rate (Fig. 3a) was lowest during the Holocene and last interglacial period, and higher during the last glacial period, particularly at MIS 2, suggesting that the increased terrigenous fraction during the
Conclusions
We have used a sedimentary record of leaf-wax and to reconstruct the hydroclimate of southwestern Africa over the past 140 ka. Leaf-wax and indicate increased SH summer precipitation and greater seasonality during SH summer insolation maxima relative to minima. Similarly, SH summer precipitation and seasonality were increased during the last glacial period versus the Holocene and last interglacial period, with wettest conditions during the LGM. n-Alkane concentrations normalised
Acknowledgments
We are grateful to Britta Beckmann and Ralph Kreutz for assistance in the lab and to Matthias Zabel for performing the EDP-XRF measurements and Marco Klann for performing the opal measurements. We gratefully acknowledge the RETRO consortium for providing samples. The project was supported by the Helmholtz Climate Initiative REKLIM (Regional Climate Change) and the DFG Research Centre/Cluster of Excellence ‘The Ocean in the Earth System’. Comments by E. Niedermeyer and two anonymous reviewers
References (90)
- et al.
Leaf wax n-alkanes and values of CAM plants from arid southwest Africa
Org. Geochem.
(2014) - et al.
Mineralogy and geochemistry of the clay fraction of sediments from the Namibian continental margin and the adjacent hinterland
Mar. Geol.
(1993) Leaf wax n-alkane distributions in arid zone South African flora: environmental controls, chemotaxonomy and palaeoecological implications
Org. Geochem.
(2014)- et al.
Late quaternary dynamics of southern Africa's winter rainfall zone
Earth-Sci. Rev.
(2007) Estimating the hydrogen isotopic composition of past precipitation using leaf-waxes from western Africa
Quat. Sci. Rev.
(2013)The nature of moisture at Gobabeb, in the central Namib Desert
J. Arid Environ.
(2013)- et al.
Controls on the D/H ratios of plant leaf waxes in an arid ecosystem
Geochim. Cosmochim. Acta
(2010) - et al.
A paleohydrologic reinterpretation of the Homeb Silts, Kuiseb River, central Namib Desert (Namibia) and paleoclimatic implications
Catena
(2002) Can sedimentary leaf waxes record D/H ratios of continental precipitation? Field, model, and experimental assessments
Geochim. Cosmochim. Acta
(2008)Leaf water deuterium enrichment shapes leaf wax n-alkane values of angiosperm plants II: Observational evidence and global implications
Geochim. Cosmochim. Acta
(2013)
Leaf water deuterium enrichment shapes leaf wax n-alkane dD values of angiosperm plants I: Experimental evidence and mechanistic insights
Geochim. Cosmochim. Acta
The hydrophobic coatings of plant surfaces: epicuticular wax crystals and their morphologies, crystallinity and molecular selfassembly
Micron
Last interglacial climates
Quat. Res.
How dry was dry? – Late Pleistocene palaeoclimates in the Namib Desert
Quat. Sci. Rev.
Rapid palaeoceanographic changes in the Benguela Upwelling System for the last 160,000 years as indicated by abundances of planktonic foraminifera
Palaeogeogr. Palaeoclimatol. Palaeoecol.
Hydrogen isotope ratios of leaf wax n-alkanes in grasses are insensitive to transpiration
Geochim. Cosmochim. Acta
An automated leaching method for the determination of opal in sediments and particulate matter
Deep-Sea Res., Part I
Spatial distribution of fog in the Namib
J. Arid Environ.
Orbital forcing of climate over South Africa: a 200,000-year rainfall record from the pretoria saltpan
Quat. Sci. Rev.
Eolian evidence for spatial variability of late Quaternary climates in tropical Africa
Quat. Res.
Chemotaxonomic significance of distribution and stable carbon isotopic composition of long-chain alkanes and alkan-1-ols in C4 grass waxes
Org. Geochem.
Mediterranean quaternary sapropels, an immediate response of the African monsoon to variation of insolation
Palaeogeogr. Palaeoclimatol. Palaeoecol.
values of individual n-alkanes from terrestrial plants along a climatic gradient – Implications for the sedimentary biomarker record
Org. Geochem.
The relationship between fog, floods, groundwater and tree growth along the Lower Kuiseb River in the hyperarid Namib
J. Arid Environ.
The classic Marine Isotope Substage 5e
Quat. Res.
Correlation between vegetation in Southwestern Africa and Oceanic Upwelling in the past 21 000 years
Quat. Res.
Southeast trade wind variations during the last 135 kyr: evidence from pollen spectra in eastern South Atlantic sediments
Earth Planet. Sci. Lett.
Influence of physiology and climate on of leaf wax n-alkanes from and grasses
Geochim. Cosmochim. Acta
Tropical sea-surface temperatures during the last glacial period: a view based on alkenones in Indian Ocean sediments
Quat. Sci. Rev.
A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic
Mar. Geol.
A molecular perspective on Late Quaternary climate and vegetation change in the Lake Tanganyika basin, East Africa
Quat. Sci. Rev.
Late quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary
Quat. Sci. Rev.
Anomalous occurrences of Neogloboquadrina pachyderma (left) in a 420-ky upwelling record from Walvis Ridge (SE Atlantic)
Mar. Micropaleontol.
Distribution patterns and stable carbon isotopic composition of alkanes and alkan-1-ols from plant waxes of African rain forest and savanna species
Org. Geochem.
Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records
Quat. Sci. Rev.
Calibration of XRF core scanners for quantitative geochemical logging of sediment cores: theory and application
Earth Planet. Sci. Lett.
An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120–800 ka
Clim. Past
Wet phases in the Sahara/Sahel region and human migration patterns in North Africa
Proc. Natl. Acad. Sci. USA
A record of rapid Holocene climate change preserved in hyrax middens from southwestern Africa
Geology
Rainfall seasonality determines annual/perennial grass balance in vegetation of Mediterranean Iberian
Plant Ecol.
The application of a present-day climatic model to the late quaternary in southern Africa
Clim. Change
Effects of climate and atmospheric CO2 partial pressure on the global distribution of grasses: present, past, and future
Oecologia
Interhemispheric symmetry of the tropical African rainbelt over the past 23 000 years
Nat. Geosci.
Abrupt shifts of the Sahara–Sahel boundary during Heinrich stadials
Clim. Past
Isotopic variations in meteoric waters
Science
Cited by (55)
Did climate change make Homo sapiens innovative, and if yes, how? Debated perspectives on the African Pleistocene record
2024, Quaternary Science AdvancesPrecession and obliquity forcing of the South African monsoon revealed by sub-tropical fires
2023, Quaternary Science ReviewsThe impact of the gut microbiome on toxigenic bacteria
2021, Microbial Pathogenesis
- 1
Present address: Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, D-27568 Bremerhaven, Germany.