CRAMP Sediment Analysis
Sediment samples are collected haphazardly along the transect by divers during the survey. Available loose material is scooped by hand into pre-labeled plastic sample bags (twirl or zip-topped) and sealed. Two samples (approximately 500 cc each) are collected at each of the two depths for each site, providing 4 samples from each site. At some sites the sediment is found in depressions and/or sand channels adjacent to the transect areas. Size of the largest fragments is limited by the size of the collector’s hand. Every attempt is made to collect a representative sample of local material. The samples are taken to the laboratory for analysis.
In the laboratory, the wet samples are individually shaken and manually mixed while in the collecting bags to assure homogeneity of each sample. This mixture is divided into 4 sub-samples (A, B, C, and D) of 60-120 grams of material.
Size Fraction Determination
Standard brass sieves (USA Standard Testing Sieve: A.S.T.M.E.-11 specifications), with opening diameters of 2.8 mm (.111 in.), 500 micrometers (.0197 in.), 250 micrometers (.0098 in.), and 63 micrometers (.0025 in.) and a brass catch pan are used to provide 5 size fractions: rubble, gravel, coarse, fines, and silt respectively.
Sediment sub-samples A and B are wet sieved through the stacked sieves, using deionized water from a hand-held wash bottle. All washings are collected for final filtering, providing the silt fraction. The sediment fraction remaining on each sieve is washed through pre-weighed filter paper (Whatman Brand 114 wet-strength, 25 micrometer) and air-dried for a minimum of 3 weeks.
The dried, filtered samples are then weighed on 3 separate days. All weights, including that of the filter, are entered into a spreadsheet. Using the average weight of each fraction, the percent by weight of each is determined by calculating the ratio of the various size fractions to the total sample weight. Because the weight of an occasional large piece of rubble can distort the final results, giving a misleading picture of the sediment composition, percentages are calculated both with and without the rubble component.
Inorganic-Organic Carbon Fraction Determination
Approximately 30 g of material from sub-samples C and D are air-dried to a constant weight. From each, 10 g is ground with mortar and pestle to a fine, homogenous material and placed in labeled crucibles. These are placed in a drying oven at 100 oC for 10 hours, cooled in a desiccator, and weighed (weight of crucible plus sample). Next, the crucibles are placed in a muffle furnace at a temperature of 500 oC for 12 hours, cooled in a desiccator, and re-weighed. Loss in weight at 500 oC is assumed to be due to burning off of organic fraction. The samples are then returned to the furnace and heated at 1000 oC for 2 hours, cooled in a desiccator, and re-weighed. Loss of weight at this temperature is assumed to represent loss of carbonate materials. Crucible weight, crucible plus dry sample weight, crucible plus sample after burning at 500 oC and crucible plus sample after burning at 1000 oC are entered into a spreadsheet. Percent organic and percent carbonate are calculated from these data and recorded in the CRAMP database.
Qualitative Composition Analysis
The components of the rubble portion of the dried sub-sample are examined under magnification for qualitative assessment of the sample composition. Fragments of mollusks, urchin spines, coral, basalt, Halimeda and other calcareous algae, etc. are noted and general descriptions of the major components are listed.
Purpose of Sediment Analysis
Sediment composition and grain size analyses are used as an indicator of the environment. They provide a characterization of the benthos, supplying information about benthic structure. Determination of sediment composition reflects type and abundance of coral coverage, and geologic structure.
Sediments also relate to other environmental variables measured by CRAMP, providing a quantitative link to biological parameters such as fish and algal populations. Some physical characteristics of sites can also be defined through sediment analyses including wave exposure and topographical relief.
Complex, dynamic processes are involved in the relationship sediment have with coral reefs. Sediment deposits may be influenced by bathymetry, wave regimes, and local currents that affect sediment deposition rates and size fractions. Silt (<63 um) is primarily comprised of land-based sediments. Sites with a high percentage of silt and organic content are subjected to large amounts of terrigeneous input. This could be indicative of a chronic, sustained stress to corals in the vicinity. Sediments with a high proportion of these fine-grained particles can also be indicative of sites with relatively low water motion.
|Photoquadrats -- Video Transects -- Rugosity Measurements -- Sediment Analysis|
Last Update: 04/21/2008
By: Lea Hollingsworth
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744
email: jokiel@Hawai‘i .edu