Preliminary Estimation of Paleoproductivity via TOC and Habitat Types: Which Method Is More Reliable? —A Case Study on the Ordovician–Silurian Transitional Black Shales of the Upper Yangtze Platform, South China

Abstract

New total organic carbon (TOC) data from the two Ordovician–Silurian transitional graptolite-bearing black shale intervals, the Wufeng Formation and the Longmaxi Formation in Central Guizhou and West Hubei, respectively, as well as previously reported TOC data from the same intervals in other places on the Yangtze platform of South China, have been used to produce an initial estimate of the primary paleoproductivity via a conventional inverse method (i.e., R_pp-inverse). The values of the R_pp-inverse are estimated to be 32 (43–21) gC/(m²·a) (Wufeng Formation) and 21 (27–16) gC/(m²·a) (Longmaxi Formation). Also, simultaneously, the habitat types (i.e., HT; cf., BA: benthic assemblage) and their temporal and spatial changes have been documented from the same succession, and an initial estimate of the primary paleoproductivity has been produced using a forward method (i.e., R_pp-forward). Being bounded mainly by the peritidal to inner-shelf environment shelly-facies or mixed-facies successions with BA1 to BA3 faunas both at the top and the base, which indicates the habitat types from HT II₁ to HT III₂, the biohabitat type of the two graptolite-bearing black shale intervals can be limited to HT III to HT IV, corresponding to the inner shelf to the outer shelf, with depths from roughly 60 m to 200—300 m. Based on the current data from the South China Sea and the southern part of the East China Sea, values of R_pp-forward should be about 100 to 400 gC/(m²·a). The difference in the results via the two methods suggests that paleoproductivity estimates from the geological strata need to be made cautiously, with particular attention paid to the paleogeographic setting, oxic-anoxic conditions, as also the preservation factor of organic carbon.