Organic geochemical proxies of paleoceanographic, paleolimnologic, and paleoclimatic processes

http://ruby.fgcu.edu/courses/twimberley/envirophilo/wanner.pdf

Mid- to Late Holocene climate change: an overview

https://www.ldeo.columbia.edu/~peter/site/Papers_files/deMenocal.etal.2000.pdf

Abrupt onset and termination of the African Humid Period:rapid climate responses to gradual insolation forcing

http://www.ldeo.columbia.edu/~peter/site/Papers_files/deMenocal.2004.pdf

African climate change and faunal evolution during the Pliocene-Pleistocene

Marine records of African climate variability document a shift toward more arid conditions after 2.8 million years ago (Ma), evidently resulting from remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles. African climate before 2.8 Ma was regulated by low-latitude insolation forcing of monsoonal climate due to Earth orbital precession. Major steps in the evolution of African hominids and other vertebrates are coincident with shifts to more arid, open conditions near 2.8 Ma, 1.7 Ma, and 1.0 Ma, suggesting that some Pliocene (Plio)-Pleistocene speciation events may have been climatically mediated.

https://science.sciencemag.org/content/sci/270/5233/53.full.pdf

Plio-Pleistocene African Climate

The Croll–Milankovitch astronomical theory of climate1–3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession, although nonlinearity had to be assumed for eccentricity4,5. Moreover, interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6,000–5,000 yr is seen between an astronomical climate index and terrestrial climate indicators, dominated by the northern ice-sheet dynamics. Climate proxy data of low latitudes are less directly dependent on the ice sheets. Here it is shown that during the past 464,000 yr, African monsoons signalled by the East Mediterranean sapropels were heaviest always and only when a northern summer monsoon index, computed from the orbital variation of insolation, reached maximum values. This occurred during all the interglacials, but also twice during glacial periods. Thus tropical aridity is not the single climatic pattern of glacial periods. An immediate terrestrial climate response to orbital variations of insolation is established.

African monsoons, an immediate climate response to orbital insolation

Mediterranean Quaternary sapropels, an immediate response of the african monsoon to variation of insolation

In an East Mediterranean marine core, the upper sapropel begins soon after the start of a global event—a very heavy precipitation which occurred in the equatorial latitudes during the late Glacial–early Holocene. This heavy precipitation in Africa, channelled by the Nile River across 35° of latitude, produced a low-salinity surface layer in the East Mediterranean. In this confined basin, with high bottom salinity, the steep salinity gradient stratified the water column. The stagnant bottom waters triggered the sapropel formation. Cretaceous sapropels in the tropical oceans may result from the same chain of events in warm, humid climates, with contrasting wet-and-dry seasonal rhythm.

After the deluge: Mediterranean stagnation and sapropel formation

Fragments of the Orgueil meteorite were macerated in mineral acids (HNO3-HF-HNO3) to dissolve the mineral matrix and separate the acid-resistant organic residues; a routine procedure in the extraction of pollen and spores from terrestrial sediments. Numerous spherical hollow objects were found, optically resembling the brown amorphous residual organic matrix of the meteorite. Their morphology, size-distribution, and chemical composition, revealed by electron microprobe with reference to carbon and phosphorus, are described, and evaluated in connection with criteria of biogenicity. The intrinsic criteria are satisfactorily met, but the extrinsic requirement of a sedimentary environment is not met. A review of the literature concerning the meteoritic environment suggests an explanation of these spheres based on the environment of their formation. It is proposed that they are organic coatings on olivine microchondrules, magnetite and glass globules, the mineral component of which has been dissolved by the acid maceration. They could have initially resulted from the polymerization of dispersed small organic molecules condensing on the surface of the microchrondrules. The latter were injected from a volcanic ‘nuee ardente’ into the dispersed cold primordial cosmic dust of hydrated silicates and organic molecules, around the meteorite parent-body. This presumably occurred before the cosmic dust accreted as the carbonaceous chondritic outer layer of the parent-body. Upsurging reducing hot gases from the ‘nuee ardente’ would polymerize part of the dispersed organic matter as the insoluble brown amorphous matrix, possibly the ‘sticking’ agent when the cosmic dust accreted. The spiraled form of several of the organic structures described here are suggestive of atmospheric heat microturbulences. Organic membranes and comet-form tails of spherical coatings suggest polymerization in the wake of injected microchondrules. These diverse organic structures would result in our view from the abiogenic thermal organization of organic matter in an extraterrestrial gas-solid system.

Martine Rossignol-Strick

Papers

African monsoons, an immediate climate response to orbital insolation

Mediterranean Quaternary sapropels, an immediate response of the african monsoon to variation of insolation

After the deluge: Mediterranean stagnation and sapropel formation

Extra terrestrial abiogenic organization of organic matter: The hollow spheres of the Orgueil meteorite