Lithostratigraphy, physical properties and organic matter variability in Lake Malawi Drillcore sediments over the past 145,000 years

https://doi.org/10.1016/j.palaeo.2010.10.028Get rights and content

Abstract

Scientific drill cores recovered from Lake Malawi exhibit a remarkable down-core lithologic variability, and are indicative of radically changing environmental conditions forced by large-amplitude lake-level shifts over the past 150 kyr. Here we present detailed lithologic and sedimentary fabric descriptions of the key sedimentary units, along with down-core physical properties data, down-core organic matter geochemistry (TOC, C/N, and δ13C data sets), and images and descriptions from core sections and from sediment smear slide microscopy. These data reveal a fundamental change in Lake Malawi's limnology and regional climate at ca. 60–70 ka. Prior to this time the lake was characterized by large-amplitude variations in lake level and water chemistry, but after 60 ka the lake remained comparatively high, and the central basin drill site accumulated mainly organic-rich, laminated sediments. Organic matter sources changed dramatically during the different lake stages. During major lake high stands, a mixed assemblage of algal (diatom-dominated), woodland and aquatic macrophyte (C4-pathway), and grassland (C3-pathway) organic matter was deposited, whereas during extreme low lake stages (water depths < 200 m), when saline, alkaline lakes persisted in the basin, sediments with minimal amounts of algal-dominated organic matter accumulated and were preserved.

Introduction

The tropical latitudes contribute significantly to the global climate system by receiving most of the solar energy that strikes the earth, and then redistributing energy to the high latitudes via atmospheric and oceanic circulation. The long-term natural variability of tropical climate on the continents is best known from lake deposits and speleothems, and new lake records are now extending our understanding of key regions around the continental tropics (e.g. Hodell et al., 2008, Peck et al., 2007, Scholz et al., 2006). Lake Malawi is situated between 9° and 14° S within the western branch of the east African rift (Fig. 1), and the sediments within this basin provide an opportunity for a continuous long-term record of natural climate variability in the continental tropics of southern hemisphere East Africa.

The climate of tropical East Africa is dominated by seasonal variability in rainfall and winds, with relatively minor seasonal temperature variability. Tropical convection associated with Intertropical Convergence Zone (ITCZ) gives rise to a single rainy season lasting from ~ December to March (Fig. 2). Moisture is derived from both the Atlantic and Indian Oceans in monsoonal circulation associated with summer heating of the continental interior.

Section snippets

Background

Lake Malawi is one of world's largest and deepest lakes extending more than 560 km from end to end, with a maximum water depth of about 700 m (Fig. 1). The lake is permanently stratified and anoxic at depth, with a chemocline depth that averages about 250 m (Patterson and Kachinjika, 1995). The strong seasonal pattern of a short, intense rainy season and then a strong windy season during the austral winter (Fig. 2) results in a bimodal sedimentation pattern of terrigenous inputs and then

Drilling and field program efforts

Lake Malawi is landlocked and located ~ 400 km from the Indian Ocean, with only overland connections to ports on the Indian Ocean coast. The project used the lake barge Viphya, as the drilling platform (Scholz et al., 2006, Scholz et al., 2011-this issue). Prior to this effort site survey studies carried out on small local vessels were used to help develop a stratigraphic framework for the basin, and to help identify preferred drill sites (Lyons et al., 2011-this issue; Fig. 1, Fig. 3).

At drill

Geochronology summary

Age dating of sediment drill cores recovered from Site 1C in Lake Malawi was completed using several different dating methods. For the interval from 0 to 52 kyr BP, 14C dating of bulk organic matter generated internally consistent results and a high-resolution chronology (Fig. 4, Fig. 5). The deeper, older parts of the core below 22 m were dated using Optically Stimulated Luminescence (OSL), and paleomagnetic inclination and paleointensity methods. Paleointensity was determined using the ratio of

Section 1: 10 ka–61 ka

The upper part of core 1C, from the top of the core at 6.51 mblf to 28 mblf is characterized by moderate to high organic carbon values, H2O-saturated, low-density diatomaceous mud in which Pediastrum may be prominent, and comprises a lithology (LI) comparable to that which has been accumulating in the lake over the last few thousand years (e.g. Barry, 2001, Johnson et al., 2002). Accordingly, this lithology is interpreted to be reflective of limnological conditions not unlike the modern lake,

Discussion and conclusions

  • 1.

    Lithology, organic matter, and physical properties vary markedly at Site 1 in Lake Malawi, a drill site set in 593 m of water in a hemipelagic setting in the central basin of the lake. The last ~ 150 kyr of record is observed in the upper 90 m of section in core 1C. Down-core variability in sediment lithology, physical properties, and organic matter character is pronounced on millennial and orbital timescales, especially for the period prior to 60 kyr BP.

  • 2.

    Over the past 60,000 years the water depths at

References (50)

  • C.A. Scholz et al.

    Scientific drilling in the Great Rift Valley: The 2005 Lake Malawi Scientific Drilling Project — An overview of the past 145,000 years of climate variability in Southern Hemisphere East Africa

    Palaeogeogr. Palaeoclimatol. Palaeoecol.

    (2011)
  • T.D. Specht et al.

    Architecture of the Lake Malawi Rift, East Africa

    Journal of African Earth Sciences

    (1989)
  • J.C. Stager et al.

    A high-resolution 11, 400-yr. diatom record from Lake Victoria, East Africa

    Quaternary Research

    (1997)
  • J.R. Stone et al.

    Late Pleistocene paleohydrography and diatom paleoecology of the central basin of Lake Malawi, Africa

    Palaeogeogr. Palaeoclimatol. Palaeoecol.

    (2011)
  • F.A. Street-Perrott et al.

    Late Quaternary changes in ecosystem and carbon cycling on Mt. Kenya, East Africa: a landscape-ecological perspective based on multi-proxy lake-sediment influxes

    Quaternary Science Reviews

    (2007)
  • M.R. Talbot et al.

    A high resolution palaeoclimatic record for the last 27, 500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter

    Earth and Planetary Science Letters

    (1992)
  • M.R. Talbot et al.

    Hydrogen Index and carbon isotopes of lacustrine organic matter as lake level indicators

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (1989)
  • M.J. Wooller et al.

    Late Quaternary fires and palaeoecology of Mount Kenya, East Africa: evidence from charred grass cuticles in lake sediments

    Palaeogeography, Palaeoclimatology, Palaeoecology

    (2000)
  • Barry, S., 2001. Stratigraphic correlation and geochronology of varved sediments from Lake Malawi, East Africa. M.Sc....
  • K.R.M. Beuning et al.

    Sensitivity of carbon isotopic proxies to paleoclimatic forcing: a case study from Lake Bosumtwi, Ghana, over the last 32,000 years

    Global Biogeochemical Cycles

    (2003)
  • E.T. Brown et al.

    Abrupt change in tropical African climate linked to the bipolar seesaw over the past 55,000 years

    Geophysical Research Letters

    (2007)
  • A.S. Cohen et al.

    Ecological consequences of Early Late-Pleistocene megadroughts in Tropical Africa

    Proceedings of the National Academy of Sciences

    (2007)
  • I.W. Croudace et al.

    ITRAX: Description and Evaluation of a New Multi-Function X-Ray Core Scanner

  • B.P. Finney et al.

    Late Quaternary lake-level changes of Lake Malawi

  • D. Harris et al.

    Acid fumigation of soils to remove carbonates prior to total organic carbon or carbon-13 isotopic analysis

    Soil Science Society of America Journal

    (2001)
  • Cited by (0)

    View full text