Organic carbon decomposition rates in sediments of the pacific manganese nodule belt dated by 230Th and 231Pa

Abstract

Oxic siliceous clay and calcareous ooze cores from the central North Pacific were analyzed for 232Th, 231Pa and230Th activities, and for organic carbon contents to establish Late Quaternary sedimentation rates and organic carbon decomposition rates during early diagenesis. High excess activities revealed Late Quaternary deposits in the upper sections of all cores but one. Sedimentation rates for the youngest phase of continuous deposition were between 3.2 and 5.8 mm/1000 yr for calcareous ooze and between 1.5 and 4.1 mm/1000 yr for siliceous clay. The ionium decrease with depth revealed characteristic discontinuities in five sediment cores, also shown by other parameters. The discontinuities date back to 15,000–100,000 years B.P. and are associated with pronounced facies changes in four cores. One core contained siliceous clay (1.9 mm/1000 yr) covered by more rapidly accumulating calcareous ooze (5.8 mm/1000 yr). In three siliceous clay cores the discontinuity marked the facies I/II boundary of von Stackelberg [31]. Fixed ammonium distributions in the sediment cores, reflecting varying amounts of illite, were used in lithostratigraphic correlations. Ages of around 1.3 million years, estimated by extrapolation of sedimentation rates to greater depths, suggest that the facies I/II boundary is equivalent to the Quaternary/Pliocene boundary in areas of continuous Quaternary (facies I) sedimentation. All sediments were typically low in organic carbon, i.e. less than 0.4 wt.%. Concentrations decreased exponentially with depth showing the same discontinuities as the ionium distribution. Organic carbon contents, however, reached constant carbon/alumina ratios of between 0.005 and 0.007 at depths of about 1 m below the surface. Thus small amounts of organic matter (i.e. First-order decay constants of metabolizable organic carbon ranged from 3.2 × 10−6 to 27.5 × 10−6 yr−1, corresponding to half-lifes between 217,000 and 25,000 years. Rate constants were found to be proportional to the 1.5th power of the sedimentation rate similar to the relationships established for anoxic sediments by Toth and Lerman [43] and Berner [44]. However, the constants derived here are higher by about three orders of magnitude if extrapolated to comparable sedimentation rates used by the cited authors. This probably reflects the higher efficiency of organic matter decomposition under oxygenated conditions. Our results suggest a sedimentation rate threshold of >1 to