Preparation of Low-Carbon Sediments from the Mississippi River and Certain Tributaries for Solid-state CPMAS 13C NMR Analysis

Abstract

The nature of organic carbon in aquatic sediments and soils with low carbon contents and significant contents of paramagnetic elements such as iron and manganese is difficult to assess by solid-state, cross-polarization magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectrometry because of the inherent low sensitivity of 13C NMR analyses, and band broadening and sensitivity losses caused by paramagnetic elements. Other investigators have addressed this problem in the analysis of soils by enriching the organic carbon content by flotation, by magnetic separation of paramagnetic minerals, and by chemical reduction of iron by stannous chloride and sodium dithionite. In this study, they found that satisfactory 13C NMR spectra could be obtained if the C/Fe ratio was greater than 1 wt%. Each of the physical and chemical treatments used to increase the C/Fe ratio resulted in losses of organic matter and changes in the nature of organic matter through physical fractionation and chemical alteration. Suspended stream sediments frequently have equivalent contents of organic carbon and sesquioxide coatings with which the organic matter is associated. These sesquioxide coatings consist predominantly of iron and manganese oxyhydroxides that cause problems with NMR analyses. In this chapter we describe a method to enrich organic matter and remove iron and manganese from low-carbon sediments sampled from the Mississippi, Illinois, and Ohio Rivers with minimal loss and alteration of the organic matter. The second objective is to characterize the sedimentary organic matter by 13C NMR using recent advances that increase instrument sensitivity. Suspended and bed sediments were collected during a sampling cruise on the Mississippi River during May–June 1990. Fine bed sediments were collected in depositional regions of the river or tributaries with a pipe dredge. Suspended silts were collected using a continuous-flow centrifuge operated on board the Research Vessel Acadiana. Both bed sediments and suspended silts were freeze-dried prior to additional treatment procedures and NMR analyses. A flow chart of selective mineral dissolution procedures is presented in Figure 17.1. The acid pyrophosphate treatment6 was placed first in the sequence to remove calcium and magnesium minerals that would form insoluble oxalates in the following extraction.