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Sediment Grain Size Measurements: Is There a Differenc Between Digested and Un-digested Samples? And Does the Organic Carbon of the Sample Play a Role
Citation:
HO, K. T., R. Lazalere, H. Smith, M. C. PELLETIER, AND R. L. JOHNSON. Sediment Grain Size Measurements: Is There a Differenc Between Digested and Un-digested Samples? And Does the Organic Carbon of the Sample Play a Role. Presented at SETAC North American 29th Annual Meeting, Tampa, FL, November 16 - 20, 2008.
Impact/Purpose:
This research investigates the difference between undigested and digested sediment samples with respect to grain size. The grain size measurements may be used to differentiate eutrophic areas from non-eutrophic areas in marine ecosystems.
Description:
Grain size is a physical measurement commonly made in the analysis of many benthic systems. Grain size influences benthic community composition, can influence contaminant loading and can indicate the energy regime of a system. We have recently investigated the relationship between grain size and total organic carbon (TOC) in marine ecosystems as a measure of eutrophication or organic enrichment of an area. During the development of this relationship, we recognized that there are many methods of measuring grain size. Whether or not the sediment sample is digested prior to grain size measurement is a major decision. Digestion (usually treatment with hydrogen peroxide) is performed to remove organic carbon. Geologists often digest sediment samples prior to grain size analysis to arrive at the true geologic grain size without organic carbon playing a role. Biologists may want to know grain size under field conditions and therefore might not digest the sample prior to measurement. The literature contains grain size measurements from both digested and undigested samples and it is not clear if grain size measurements from both of these methods yield the same value. Our question was, “Can all of these measurements be used in the same way for determining the relationship between TOC and grain size in impacted and un-impacted marine areas?” We hypothesized that grain size measurements in sediments with low TOC would be less affected by digestion than those with higher TOC. The affect would be that sediments with higher TOC would appear to have more fine grained particles after digestion. Twenty-three marine sediments with TOC ranging from 0.22- 10 % were obtained from different areas of the northeastern United States. The TOC was measured on each sediment sample and the sample divided into two aliquots. One of the two aliquots was digested and then grain size was measured on both aliquots. Preliminary information indicates that as the TOC increases, there is a larger difference between digested and undigested samples. Further statistical analysis will be performed and presented to determine if there is a range of TOC for which digesting a sediment sample prior to grain size measurement significantly changes the grain size results.