The formation of biogeochemical laminations in Lake Bosumtwi, Ghana, and their usefulness as indicators of past environmental changes

Timothy M. Shanahan, Jonathan T. Overpeck, J. Warren Beck, C. Winston Wheeler, John A. Peck, John W. King, Christopher A. Scholz

Research output: Contribution to journalArticlepeer-review

33 Scopus citations

Abstract

The sediments from Lake Bosumtwi, Ghana contain a unique record of fine-scale (mm to sub-mm) laminations, which will provide a valuable annual chronometer for reconstructing paleoenvironmental changes in West Africa covering much of the last 1 Ma. Comparisons of laminae counts to independent 210Pb dates and the rise in anthropogenic "bomb" radiocarbon support the interpretation of the laminations in the uppermost sediments as registering annual events. Radiocarbon dates on in-situ fish-bone collagen are in agreement with varve counts, further supporting the annual nature of our varve chronology. Over the instrumental period (1925-1999), dark-varve thickness measurements are correlated with local rainfall (r = 0.54) and appear able to resolve decadal-scale changes in precipitation. The relationship between varve thickness and rainfall provides support for our interpretation that dark-colored varve thickness records catchment runoff during the rainy season rather than dust flux during the dry season. Dark laminae alternate with organic and carbonate-rich light laminae formed during the fall period of enhanced productivity. Downcore, varves undergo significant microstratigraphic and geochemical variations, but retain the same pattern of alternating clastic and organic-rich laminae, providing support that the laminae may represent annual time markers for reconstructions of the deeper part of the record.

Original languageEnglish (US)
Pages (from-to)339-355
Number of pages17
JournalJournal of Paleolimnology
Volume40
Issue number1
DOIs
StatePublished - Jul 1 2008

Keywords

  • Africa
  • Climate
  • Crater lake
  • Monsoon
  • Stratigraphy
  • Varve

ASJC Scopus subject areas

  • Aquatic Science
  • Earth-Surface Processes

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