Estimating Paleoproductivity from Organic Carbon Accumulation Rates--Problem or Panacea?
M. A. Arthur, M. S. Leinen, D. Cwienk
Organic carbon accumulation rates (OCAR; g/cm2/ky), calculated as the product of the weight fraction of organic carbon (OC), dry bulk density (DBD; g/cc), and sedimentation rate (SR; cm/ky), are the most uniform method of expressing variations in OC flux and/or preservation in marine environments. Several factors influence OC preservation; these include rate of OC production (flux from surface waters or other sources), water depth (WD), rates of OC decomposition in the water column, and SR (i.e., residence time of OC at the sediment/water interface). Understanding the patterns and variations of paleoproductivity (PP) that resulted in accumulation of relatively OC-rich strata depends upon differentiating between factors that enhance OC preservation and OC flux.
Assuming that OC preservation is not a function of dissolved oxygen (DO) at levels above about 0.5 ml/l, Muller and Suess in 1979 related OCAR to PP in modern marine settings by correcting for the effects of OC preservation as a function of SR. Zahn et al in 1986 improved on the Muller-Suess equation, using a more comprehensive data set and taking into account the progressive diminution of the OC flux during transit through the water column. We have tested the efficacy of these PP equations as applied to several independently derived data sets for which OCAR could be calculated for the last 12 ky in cores from a variety of depths and a range of productivity in the Pacific and Atlantic Ocean basins. We also attempted to construct an equation that best predicted PP by fitting all availa le data in our data set for the modern ocean using stepwise multiple linear regression analysis of OC, DBD, SR, and WD data to predict PP and by using several methods of estimating productivity at each locale. Inclusion of larger data sets than previously used reduced the ability of any equation to predict PP. The most significant problem is estimating modern surface productivity. Although this method of estimating PP has some utility, satisfactory application to the ancient record will depend on more rigorous estimates of modern productivity, better understanding of the influence of low DO levels on rates of OC degradation, and consideration of diagenetic loss of labile OC during subsequent burial.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.