High-Resolution Th-230 Profiles in Carbonate Sediments: A Signal from Steady-State Dissolution? Book uri icon

abstract

  • Measurements of calcium carbonate dissolution rates in sediments lying above the calcite saturation horizon, based on pore water solute gradients and benthic flux chamber experiments, have yielded conflicting results. While interpretations of pore water data have implied that 25-60\% of the CaCO3 rain to sediments dissolves during early diagenesis, flux chamber measurements have found no dissolution in high-carbonate sediments. We have made measurements of Th-normalized carbonate accumulation rates in sediments on the Ontong-Java plateau in the western equatorial Pacific Ocean in order to apply a different approach to the question of whether or not there is CaCO3 dissolution above the calcite saturation horizon. At each site where we have applied the approach, we have measured Th-230 activities in sediments at a depth resolution of 2mm in the upper 2 cm of the sediments and at somewhat greater depth intervals below. These measurements would show the occurrence of dissolution through decreasing Th-normalized accumulation rates with increasing depth below the sediment-water interface in the upper 1-3 cm of the sediment column. The dissolution rates determined in this way would most likely be lower than the true dissolution rates, since sediments would tend to be homogenized to some degree by particle mixing. The sites where we have made Th-230 measurements range from 1600 to 4440m water depth and span calcite saturation states from moderately supersaturated (delta-carbonate of +7 micromoles / kg) to strongly undersaturated (delta-carbonate = -37 micromoles / kg). At the two shallowest sites, where sediments underlie supersaturated bottom water, Th-normalized accumulation rates decrease by 22 and 25\% from the 0-0.2cm depth interval to 1 cm depth. At a site lying just below the calcite saturation horizon, the decrease is 33\% and occurs somewhat closer to the sediment-water interface as dissolution is driven by both organic matter oxidation and bottom water undersaturation. At the strongly undersaturated site, there is no downcore decrease in the Th-normalized accumulation rate: dissolution driven by bottom water undersaturation is rapid enough to be reflected in the shallowest sediment interval (0-0.2cm). These results illustrate the utility of Th-normalized accumulation rate measurements for at least qualitative confirmation of the occurrence of carbonate dissolution during early diagenesis and show the occurrence of metabolically driven carbonate dissolution in high-carbonate sediments.

publication date

  • January 2001