Scientific drilling in continental East Africa: The dominance of eccentricity-modulated precession and half-precession on continental tropical climate
Abstract
In 2005 the Lake Malawi Drilling Project recovered ~623 m of continuous drill core from seven holes at two sites, including a core to 380 m subbottom in the deep Central Basin (592 m water depth). One of the oldest lakes in the world, Lake Malawi is ~580 km long, spans 5 degrees of latitude, and its sedimentary record integrates signals from across southern hemisphere tropical East Africa. The lake’s great antiquity and stratified water column, with anoxic deep waters below 200 m, provide an ideal location for extracting long, continuous, high-resolution records of Quaternary climate. Initial results from the upper ~90 m of Drill Site 1 demonstrated the influence of eccentricity-modulated precession on the Lake Malawi water balance, and thus the East African climate system (Scholz et al., 2007; Cohen et al., 2007). Rainy season insolation minima at 10 degrees South correlate with several intervals of megadrought over the last ~150 kyr, where drill-core data show evidence for severe low lake stages, dryland vegetation within the catchment, and cooler air temperatures. The entire Site 1 ~380 mblf drill-core record extends more than 0.5 Ma, and key drill-core data sets such as natural gamma ray (NGR), C/N ratios, d13-C of bulk organic matter, split core color (lightness, or L*), and Calcium (Ca) counts from scanning XRF were combined in a principle component analysis. PC(1) from this analysis represents ~64% of the variance, and shows strong power at 8-12 m and 18-25 m in the frequency domain using wavelet spectral analysis, representing half-precessional and precessional climate forcing respectively. Extending the dated age model through the entire length of drilled record suggests a strong correlation between lake level indicators and maximum insolation at 10 degrees South, with evidence for lake lowstands at both precession and half-precession cycles extending well into and beyond oxygen-isotope stage 11. Lowstand intervals are characterized by high density, high NGR, C/N ratios < 5, relative lows in L* values, and high counts of Ca. Stable, high lake stage intervals occur during low eccentricity intervals (since ~60 ka and stage 11) due to a muted precession signal. These results provide additional evidence for the dominant influence of orbital forcing on East African climate at the precession and eccentricity bands.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMPP11B1315L
- Keywords:
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- 4914 PALEOCEANOGRAPHY / Continental climate records;
- 4934 PALEOCEANOGRAPHY / Insolation forcing;
- 4942 PALEOCEANOGRAPHY / Limnology;
- 4946 PALEOCEANOGRAPHY / Milankovitch theory