Stratigraphy and paleoenvironments of the early to middle Holocene Chipalamawamba Beds (Malawi Basin, Africa)

Abstract

We describe the Chipalamawamba Beds, early to middle Holocene deposits at the southern margin of long-lived Lake Malawi. The beds are exposed because of downcutting of the upper Shire River. The Chipalamawamba sediments are medium to coarse, yellow to brown sands deposited in lenses varying in horizontal extent from a few meters to several hundreds of meters. Four units are recognized; the first three mainly contain lacustrine sediments deposited during lake high-stands about 10.6–9.7 cal ka BP (Unit 1), 7.6–6.5 cal ka BP (Unit 2) and 5.9–5.3 cal ka BP. Sediments of Unit 4 top units 1 to 3, are coarser and display regular foresets and oblique-bedding, suggesting deposition in riverine environments after installation of the Shire River (~5.0 ka BP). Freshwater mollusk assemblages and bioturbations regularly occur in the lacustrine sediments, but are largely absent from Unit 4. Diverse and often contradicting hypotheses on the lake levels of Lake Malawi have been proposed for the early and middle Holocene. The Chipalamawamba Beds allow straight-forward recognition of water levels and provide strong evidence for oscillating lake levels during this period, rather than continuous high or low levels. Sedimentation rates have been high and individual shell beds have typically been deposited during a few decades. Because the Chipalamawamba Beds contain a sequence of mollusk assemblages with intervals between subsequent shell beds ranging from a century to a few millennia, they enable paleontological analysis of the fauna with unusually high temporal resolution. That some mollusk lineages inhabiting Lake Malawi are in the early stages of diversification and radiation increases the paleobiological relevance of these beds

Morphological stasis in an ongoing gastropod radiation from Lake Malawi

“…No taphonomic biases exist that could explain the absence of multiple shallow-water Bellamya species in the fossil assemblages if they were existing locally at the time of deposition, and except for Bellamya, all other currently diversified mollusk genera show diversities in the Chipalamawamba Beds similar to modern levels (15) (Table S2). These observations are based on the identification of more than 40,000 fossil specimens from diverse shallow-water habitats and exclude inadequate sampling as explanation (SI Text and Tables S1-S3).…”

“…Radiocarbon dating indicates minimal time averaging; a single bed spans a few decades maximum (15). The modern material was predominantly collected from a shoreline stretch of 50 km along the southwestern shores of the southeastern arm of Lake Malawi to sample the same diversity of shallow-water habitats in close proximity to the geographically confined fossil sites (SI Text).…”

“…In addition to its extant diversity, this Bellamya clade left abundant, well-preserved fossil shells in a high-resolution, radiocarbon-dated sediment record that spans much of the Holocene (15). The study system presents a particularly promising opportunity to study radiations because some of the life-history traits that are thought to have led to the divergence of the modern descendants affect shell morphology and, hence, can be traced in fossils.…”

Recurrent camouflaged invasions and dispersal of an Asian freshwater gastropod in tropical Africa

“…The nominal, endemic Melanoides species of Lake Malawi have been aggregated by some authors [ 22 , 54 ] into a ‘ M. polymorpha complex’, even though distinct morphs can be discerned in the modern [ 31 , 55 ] and fossil faunas of Lake Malawi [ 56 ]. We have not altered previous assignments of specimens in GenBank, but consider the traditional species valid until a formal taxonomic revision demonstrates otherwise.…”

A progressively wetter climate in southern East Africa over the past 1.3 million years

“…Holocene climate shifts linked to precessional forcing are reported throughout northern Africa and much of tropical eastern Africa (including the socalled "African Humid Period" 19 ), but the antiphased response in the Southern Hemisphere (that is, an early Holocene arid period) appears to have been subdued and intermittent (for example, an early Holocene drop of about 120 m in the level of Lake Malawi 20 interspersed with highstands 21 ; brief, early Holocene highstands of Lake Makgadikgadi at about 20° S (ref. 22) and Lake Chilwa at about 15° S (ref.…”