Insolation and glacial–interglacial control on southwestern African hydroclimate over the past 140 000 years

https://doi.org/10.1016/j.epsl.2014.04.034Get rights and content

Highlights

  • Southwestern Africa was wetter during southern hemisphere summer insolation maxima.

  • Southwestern Africa was wetter during glacial versus interglacial conditions.

  • The glacial precipitation increase was tropical rather than westerly-wind derived.

Abstract

The past climate evolution of southwestern Africa is poorly understood and interpretations of past hydrological changes are sometimes contradictory. Here we present a record of leaf-wax δD and δC13 taken from a marine sediment core at 23°S off the coast of Namibia to reconstruct the hydrology and C3 versus C4 vegetation of southwestern Africa over the last 140 000 years (140 ka). We find lower leaf-wax δD and higher δC13 (more C4 grasses), which we interpret to indicate wetter Southern Hemisphere (SH) summer conditions and increased seasonality, during SH insolation maxima relative to minima and during the last glacial period relative to the Holocene and the last interglacial period. Nonetheless, the dominance of C4 grasses throughout the record indicates that the wet season remained brief and that this region has remained semi-arid. Our data suggest that past precipitation increases were derived from the tropics rather than from the winter westerlies. Comparison with a record from the Congo Basin indicates that hydroclimate in southwestern Africa has evolved in antiphase with that of central Africa over the last 140 ka.

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 (δD) 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 (δC13) of leaf-wax n-alkanes reflects the photosynthetic pathway of the plants i.e. the relative contribution of C3 versus C4 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 δD 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; 5 cmkyr1) and the Holocene (MIS 1; 4 cmkyr1), 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; 27 cmkyr1). 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 δD and δC13 to reconstruct the hydroclimate of southwestern Africa over the past 140 ka. Leaf-wax δD and δC13 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

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    Present address: Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, D-27568 Bremerhaven, Germany.

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