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

Abstract

Analysis of sedimentary diatom assemblages (10 to 144 ka) form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of Lake Malawi. Lake-level fluctuations on the order of hundreds of meters were inferred from dramatic changes in the fossil and sedimentary archives. Many of the fossil diatom assemblages we observed have no analog in modern Lake Malawi. Cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka, but are not found in significant abundances in the modern diatom communities in Lake Malawi. Salinity- and alkalinity-tolerant plankton has not been reported in the modern lake system, but frequently dominant fossil diatom assemblages prior to 85 ka. Large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka. Similarly, prior to 31 ka, common central-basin aulacoseiroid species are replaced by species found in the shallow, well-mixed southern basin. Surprisingly, tychoplankton and periphyton were not common throughout prolonged lowstands, but tended to increase in relative abundance during periods of inferred deeper-lake environments.

A high-resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wet-sieved fossil and mineralogical residue records. Prior to 70 ka, fossil assemblages suggest that the central basin was periodically a much shallower, more saline and/or alkaline, well-mixed environment. The most significant reconstructed lowstands are ∼ 600 m below the modern lake level and span thousands of years. These conditions contrast starkly with the deep, dilute, dysaerobic environments of the modern central basin. After 70 ka, our reconstruction indicates sustained deeper-water environments were common, marked by a few brief, but significant, lowstands. High amplitude lake-level fluctuations appear related to changes in insolation. Seismic reflection data and additional sediment cores recovered from the northern basin of Lake Malawi provide evidence that supports our reconstruction.

Introduction

Although rare in continental settings, sediment archives spanning more than 100,000 years provide valuable information about long-term climate history. Very deep lakes, which can persist through severe and prolonged periods of aridity (Cohen, 2003), often accumulate sediment continuously over exceptionally long periods. In regions where precipitation and evapotranspiration are nearly equal, climate shifts can produce dramatic fluctuations in lake level, records of which may be preserved in the sediment.

Consequently, the Great Lakes of tropical East Africa, with their vast depths, anoxic bottom waters, high primary productivity, and delicate hydrologic budgets, are ideal candidates for long-term paleoclimatic studies (Nicholson, 1998).

Large-scale fluctuations in lake level often result in substantial changes to lake hydrography, hydrology, nutrient cycling, and biological habitat availability; diatoms are highly sensitive to these types of changes in the water environment, and as such frequently are used to infer paleoclimate change from lake sediments (Fritz, 1996). Although the dominant flora may vary inter-annually, sampling of sediment archives typically averages multiple years. As a result, fossil diatom assemblages that have been extracted from sediments in deep-water regions of large lakes indicate long-term average environmental conditions; often these records are marked by long periods (e.g., millennial in scale) with little change to the fossil assemblage (Kilham, 1990).

Here we present a reconstruction of the Late Pleistocene (10,000 to 144,000 years BP) diatom paleoecology of the central basin of Lake Malawi. Many of the diatom taxa we observed are endemic to Lake Malawi or commonly confined to deep East African lakes. In many cases the fossil assemblages have no modern analog in Lake Malawi; hence, our paleoecological inferences rely on ecological information drawn from the modern distribution of individual taxa in Lake Malawi and other indicators of ancient depth and lake setting. These include the concentration, composition, and taphonomy of the fossil remains of ostracodes (discussed more fully in Park and Cohen (2011-this issue)) and chaoborids, sedimentology and structure of the core, and unambiguous geophysical markers of past shorelines. The fossil diatom data is also used to refine the Cohen et al. (2007) reconstruction of late Pleistocene lake-level change, which was based on biological and sedimentological indicators but did not include diatoms.

Lake Malawi is the southernmost of the Great Lakes of East Africa (9–14°S, Fig. 1); it is one of the largest and deepest lakes in the world, containing approximately 7% of the Earth's total fresh surface water (Patterson and Kachinjika, 1995). The lake and surrounding catchment are highly sensitive to hydrological variations and experience a strongly seasonal climate (Owen et al., 1990). The austral summer (Dec–Apr) is characterized by warm temperatures and gentle, northerly monsoon winds that are accompanied by heavy rainfall associated with the southward migration of the Intertropical Convergence Zone (ITCZ). In the austral winter (May–Aug) the ITCZ shifts toward the equator, producing cool, dry conditions; winters are marked by strong south/south-easterly winds. Typically, the region experiences warm and dry conditions from September to November (Torrance, 1972, Patterson and Kachinjika, 1995).

The annual water budget reflects the highly sensitive hydroclimate of the catchment; approximately 62% of hydrologic inputs are from direct precipitation onto the lake surface (Spigel and Coulter, 1996). There are six major inflows to Lake Malawi (the Ruhuhu, Songwe, South Rukuru, Dwangwa, Bua, and Linthipe), located primarily along the northern and western margins (Patterson and Kachinjika, 1995). A steep north–south gradient exists with respect to annual rainfall (2400–800 mm yr^− 1) and river discharge. Evaporation accounts for about 82% of annual water loss; the remaining water export is via the lake's only outlet, the Shire River, at its southern tip (Eccles, 1974, Spigel and Coulter, 1996). Seasonally, lake levels fluctuate several meters. A drop of 4 m would disconnect Lake Malawi from its outlet, closing the lake basin completely. This may have occurred as recently as the late 1800s (Hecky et al., 1996).

Lake Malawi is divided into three regions based on hydrographic and climatologic gradients (Gasse et al., 2002). The deepest part of the lake (∼ 700 m) is located in the central region; the northern region is generally shallower (maximum depth ∼ 600 m), whereas the southern region is significantly shallower but has a greater surface area (Fig. 1).

The modern lake is meromictic (never mixing completely); stratification is affected by both temperature and dissolved solids, which are both modified by hydrostatic pressure with depth (Wüest et al., 1996). During warm austral summers the density of the surface waters decreases, leading to the development of a well-defined seasonal thermocline; the well-mixed, low-nutrient epilimnetic waters usually are confined to the upper 50–80 m (Eccles, 1974, Patterson and Kachinjika, 1995). Surface water temperatures can rise up to ∼ 28 °C, reaching a maximum near the end of the rainy season. During the austral winter surface water temperatures fall to less than ∼ 24 °C. Together, the cooler waters and strong winter winds mix the epilimnion to a maximum depth of ∼ 125 m (Eccles, 1974, Vollmer et al., 2005). Ineffective transfer of energy with depth, the brevity of the windy season, and a weak chemical gradient keeps the modern lake stratified below ∼ 230 m; temperatures in the hypolimnion remain between 22–23 °C. Although there is some evidence of deeper mixing events in the past (Vollmer et al., 2005), today only about 20% of the metalimnetic water pool mixes with the epilimnion annually, while about 25% of the hypolimnion mixes with the metalimnion (Patterson and Kachinjika, 1995). During the winter season, strong southerly winds stack water in the northern basin, effectively tilting the thermocline. In the southern basin, strong, cold dry winds enhance upwelling and chill surface waters through evaporative cooling.

The epilimnetic water is dilute (∼ 235 μS/cm), with a gradual increase in specific conductivity with depth (Patterson and Kachinjika, 1995, Wüest et al., 1996). The ionic composition of the lake is dominated by calcium, magnesium, sodium, and bicarbonate, which are leached from the metamorphic and igneous rocks of the drainage basin and delivered by inflowing streams (Patterson and Kachinjika, 1995). The epilimnion is characterized by a pH of ∼ 8.6, which declines to circum-neutral conditions with depth. Below the epilimnion, silicic acid and dissolved gases increase with depth and help to maintain meromixis when the water column approaches isothermal conditions (Wüest et al., 1996). Dissolved oxygen declines steadily with depth to barely-oxic conditions at the metalimnion–hypolimnion boundary. Between 200–250 m, concentrations of dissolved oxygen are very low, and below 250 m the water is completely anoxic (Patterson and Kachinjika, 1995). Oxygen levels are mediated by the phytoplankton; in the epilimnion, photosynthesis dominates, leading to elevated pH and dissolved oxygen, while below the photic zone respiration dominates, leading to the anoxic conditions.

The primary sources of biologically-available nutrients (silica, phosphorus, and nitrogen) to Lake Malawi include riverine influx (Si and P), wet and dry atmospheric deposition (P and N), and biological N fixation (Hecky et al., 1996, Johnson, 2002, Bootsma et al., 2003). Nutrient retention is very high; nutrient exports via surface outflow are less than 5% of nutrient imports. Most of the annual nutrient losses occur either through sedimentation or atmospheric degassing. Cycling and vertical distribution of nutrients in Lake Malawi is strongly tied to the structure of the water column; nutrients tend to accumulate in the dark, anoxic hypolimnion where they are inaccessible to phytoplankton, while the epilimnion remains nutrient-poor, particularly during the summer when the seasonal thermocline is shallow and strongly developed (Eccles, 1974). The supply rate of nutrients (particularly Si and P) from rivers is determined by the rates of physical and chemical weathering in the surrounding catchment; however, annual influxes of Si and P are an order of magnitude smaller than the upward transfer of nutrients to the mixed layer through weak eddy diffusion mechanisms during the winter mixing season (Pilskaln and Johnson, 1991). Most of the annual N influx occurs through biological N fixation, but denitrifying bacteria living near the oxycline limit the potential availability of this nutrient in upwelling water (Hecky et al., 1996).

Diatoms are an important component of primary productivity in Lake Malawi. Diatoms often comprise a majority of the total algal biomass during the dry, windy austral winter season, but they tend to be rare throughout the remainder of the year because nutrients (particularly Si and P) are scarce in the well-lit upper waters. Cyanobacteria and green algae dominate the phytoplankton during these periods (Patterson and Kachinjika, 1995). In the open waters of the central basin, primary productivity can increase substantially during the austral winter months as a result of vertical exchange of nutrients (primarily Si) that favor diatom growth (Haberyan and Mhone, 1991, Pilskaln and Johnson, 1991, Pilskaln, 2004). Far removed from diverse littoral habitats and inflowing rivers and characterized by restricted nutrient influx, the open waters of large deep lakes typically feature a low-diversity community dominated by diatom species in direct competition for seasonally-available nutrients (Kilham et al., 1986). Sedimentation of diatoms constitutes the primary sink for silica in the lake system (Johnson, 2002, Bootsma et al., 2003).