The effects of water content on diffuse reflectance spectrophotometry studies of deep-sea sediment cores

Abstract Sediment cores recovered during ODP drilling operations are commonly scanned using a Minolta CM-2002 spectrophotometer to obtain spectral data. Core processing procedures aboard the JOIDES Resolution require that these spectral data be measured on wet cores. However, the effects of changing water content on the spectra of marine sediments is unknown. We examined the effects of changing water content on visible light (VIS) diffuse reflectance spectra for core samples composed of clay to foraminiferal ooze. The spectra of dry powdered samples were measured, then the samples were completely saturated (∼35% water content) and spectra were measured at set time intervals as the samples progressively dried. The fully saturated samples were appreciably darker than the initial dry samples. The samples continued to darken during the initial phases of drying until the water content had decreased from 35% to ∼20%. As the water content continued to decrease below 20%, the samples became progressively lighter and attained spectral values similar to the fully saturated samples at water contents of ∼17%. Samples continued to lighten until they were totally dry; however, values never became as light as those of the dry samples prior to initial wetting. Changes in reflectance with decreasing water content are not uniform across the VIS. If the water content is greater than 5%, the reflectance decrease relative to a dry sample is greater at the red end of the spectrum. However, at water contents of less than 5%, the spectral curves of the wet and dry sediments are similar in shape, but the wet sediments are still darker. Comparison of these data with data obtained from similar saturation experiments using a coarse-grained beach sand suggest that the initial darkening of the saturated carbonate sediments apparently results from changing of the physical properties of the sediment; decreasing the water content apparently rearranges the grains. These studies suggest that maximum information will be obtained when spectral measurements are taken on samples that are allowed to dry as much as possible.