Wednesday, December 16, 2015

Congo's Permo-carboniferous Paleoclimate had a Long Term Warming Trend With Five/Six Frigid Snaps

Permo-carboniferous paleoclimate of the Congo Basin: Evidence from lithostratigraphy, clay mineralogy, and stable isotope geochemistry


Milleson et al


Approximately 1000 m of strata in the Upper Paleozoic Lukuga Formation in the Dekese core in the central Congo Basin provide lithostratigraphic, mineralogical, and isotopic evidence for substantial climatic variation within a long-lived lacustrine basin. Lithostratigraphic indicators of cold climate include polymictic strata (dropstone deposits) and coupled laminations of fine clay-size material and coarse silt (glacial varves). Dropstones are concentrated in three stratigraphic zones in the lower ~ 425 m of the Lukuga Formation, and varved strata occur in two broad stratigraphic zones in the lower ~ 700 m of the formation. These sedimentological indicators suggest that the lower ~ 2/3 of the Lukuga Formation was strongly influenced by frigid conditions and glacial-like processes.

The clay-size fraction of 97 samples is dominated by detrital minerals, including quartz, feldspar, chlorite, illite, and poorly ordered expansible 2:1 phyllosilicates. Based on variation in the mineralogy of these samples, the Lukuga Formation is divisible into three Clay Mineral Zones (CMZs), numbered in ascending stratigraphic order. CMZ 1 and CMZ 3 include several horizons of expansible 2:1 phyllosilicates that represent warmer/wetter intervals. Intervening CMZ 2 is a long (~ 500 m) zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold/frigid interval.

There are numerous calcite-cemented layers, including spar-filled veins that cross depositional bedding and represent postburial alteration, radiaxial fibrous cements that displace detrital grains, and micrite that crystallized near the time of deposition. Eighty-two stable isotope analyses of micrite yield δ13C values that range from –44.6‰ to –4.1‰ and δ18O values that range from –20.0‰ to 5.0‰ (VPDB). The carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions. In contrast, stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes.

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