Organic matter loss and alteration during black shale weathering

Weathering of organic matter [OM] in ancient sedimentary rocks provides strong controls on carbon cycling in the earth's surface environments and on the O-2 abundance in earth's atmosphere. The rate and nature of OM remineralization is best revealed by examining active weathering environments rich in organic matter. Samples have been collected from several black shale weathering profiles. Analyses reveal a progressive loss of total organic carbon [TOC] upon weathering (from 60% to near 100% in the most weathered samples). Pyrite loss parallels or precedes OM loss in all profiles. Detailed analyses of OM isolated from several depths within each profile indicate little if any systematic change in the structure or composition of residual OM during weathering. These results suggest that for a variety of organic carbon-rich rocks, weathering results in non-selective loss of OM in spite of nearly a 10-fold loss of OM during weathering.