Why are the δ 13 C org values in Phanerozoic black shales more negative than in modern marine organic matter?

The δ 13 C org values of Phanerozoic black shales average −27‰, whereas those of modern marine organic matter average −20‰. The black shale isotopic values mimic those of continental organic matter, yet their organic geochemical properties mandate that they contain predominantly marine organic matter. Hypotheses that proposed to explain the low δ 13 C values of black shales include diagenetic losses of isotopically heavier organic matter components, releases of isotopically light carbon from methane clathrates or extensive magmatic events, greater photosynthetic discrimination against 13 C during times of higher atmospheric p CO 2 , and greenhouse climate stratification of the surface ocean that magnified photic zone recycling of isotopically light organic matter. Although the last possibility seems contrary to the vertical mixing that leads to the high productivity of modern oceanic upwelling systems, it is consistent with the strongly stratified conditions that accompanied deposition of the organic carbon‐rich Pliocene‐Pleistocene sapropels of the Mediterranean Sea. Because most Phanerozoic black shales contain evidence of photic zone anoxia similar to the sapropels, well‐developed surface stratification of the oceans was likely involved in their formation. Existence of isotopically light land plant organic matter during several episodes of extensive magmatism that accompanied black shale deposition implies massive release of mantle CO 2 that added to the greenhouse conditions that favored oceanic stratification. The 13 C depletion common to most Phanerozoic black shales apparently resulted from a greenhouse climate associated with elevated atmospheric p CO 2 that led to a strongly stratified ocean and photic zone recycling of organic matter in, augmented by magmatic CO 2 releases. Key Points Photic zone recycling of organic carbon is responsible for their low δ 13 C values Black shales deposited during periods of strong surface ocean stratification Periods of greenhouse climate established conditions for black shale depositionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108327/1/ggge20506.pd