Bobby C. Deaton

Modern and Last Glacial Maximum eolian sedimentation patterns in the Atlantic Ocean interpreted from sediment iron oxide content

Eolian dust derived from the desert regions of North Africa is blown far into the tropical Atlantic Ocean by persistent easterly and northeasterly winds. In this paper, we demonstrate that the iron oxides, hematite and goethite, are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical Atlantic. Iron oxides are identified by diffuse reflectance spectrophotometry, a technique capable of identifying these minerals in concentrations as low as 0.01% by weight. We analyze samples from both the modern and last glacial maximum (LGM) synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the Atlantic Ocean. To determine the relative contribution of the iron oxides, we factor analyzed the modern and LGM levels as a single data set. The iron oxide factor explains about 25% of the variance in the combined core top and LGM data set. Mapped factor scores for the LGM and modern ocean indicate high iron oxide values are present in just two regions, one off eastern North America and the other off northwest Africa. In the region off eastern North America, iron oxide occurs primarily during the LGM as the previously noted “brick red lutite,” a unique sediment type derived from the erosion of Permo-Carboniferous red beds in Atlantic Canada. A larger, lobe-shaped area of iron oxide rich sediment is present off northwest Africa in both the modern and LGM levels. The modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships, satellites, and analysis of air samples. During the LGM, iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases.

Evaluating optical lightness as a proxy for carbonate content in marine sediment cores

Abstract Optical lightness is increasingly being used as a proxy for the carbonate content of marine sediment. We compared three measurements of optical lightness, gray scale, brightness and L*, to carbonate content to test the reliability of these measurements as carbonate content proxies. In five piston cores from isotope stages 1 though 6 (0–∼130 ka), all measures of optical lightness are reasonable proxies of relative, but not absolute, changes in carbonate content. In contrast, in the upper sediments of ODP Hole 997A, which are Holocene to Pliocene in age, lightness values in only the top 10 m of the 180 m analyzed exhibit a strong correlation to carbonate content. To better understand which factors control the optical lightness of sediments, we analyzed samples in which carbonate content remained constant while the composition of three common clay minerals — chlorite, illite and kaolinite was varied. Kaolinite, even at a concentration of 20%, did not significantly change optical lightness. However, 20% illite reduced optical lightness by 30%, and 20% chlorite reduced lightness by 20%. We conclude that regional and temporal changes in the composition of the non-carbonate sediment fraction may significantly alter the relationship between lightness and carbonate content. We recommend that care must be taken when applying these measures to sediment sequences which span long time intervals, or to sediment samples taken over a broad geographical areas.

Quantitative reassessment of brick red lutites: Evidence from reflectance spectrophotometry

Abstract Brick red lutites are distinctive, bright red sediments deposited in the western North Atlantic. These unique sediments were derived from Permo-Carboniferous outcrops in the Canadian Maritime Provinces, transported out the Gulf of St. Lawrence, through the Cabot Straits, and redistributed by the Western Boundary Undercurrent along the North American continental margin. In this study we used fourteen cores taken off eastern North America to more precisely examine the input and depositional history of brick red lutites. Our study differs from previous studies in the manner in which the red sediment was identified. Unlike previous studies which used color charts to assess the presence of brick red lutites, we used visible light reflectance spectrophotometry. The primary mineral imparting a red color to brick red lutites is hematite. Experiments with known quantities of hematite in a matrix mineralogically similar to western North Atlantic sediment indicate that the presence of hematite can be detected at weight percentages of less than 0.1%. Using spectral reflectance data we characterized the distribution of brick red lutites for two broadly defined time levels — latest glacial and early Holocene. As expected, the maximum input of brick red lutite was during late glacial time and input decreased reaching a minimum during the early Holocene. Since the early Holocene there has been little change in the distribution of this unique red sediment on the North American margin. However, our reflectance spectra indicate renewed accumulation of hematite-rich sediment at the present time in the Cabot Straits.

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.

Determining hematite content from NUV/Vis/NIR spectra: Limits of detection

Abstract Hematite occurs in various geologic settings including igneous, metamorphic, and sedimentary rocks as well as in soils. However, it frequently occurs at low concentrations, especially in soils, where it may be <1% by weight. Because hematite has the potential to be an indicator of oxidizing and climatic conditions in soils and paleosols, it is important to understand its limit of detection. In this paper we examine the limits of detection of hematite visually and with diffuse reflectance spectrophotometry (DRS) and X‑ray diffraction (XRD). To accomplish this we used a sample set consisting of “knowns” or calibration samples. These known samples consisted of 15 different matrices of varying mineral composition into which hematite in 7 different concentrations ranging from 0.01 to 4% by weight were mixed. Including the 0% hematite, our calibration data set consisted of 120 samples. Visually, hematite can be detected at a concentration of 0.01% by weight in a light matrix and 0.5% in the darkest of our matrices. However, because of metamerism, visual techniques cannot specifically identify hematite. We find that for both DRS and XRD the limit of detection is also dependent on the matrix. For XRD the limit of detection for hematite in bulk samples is about 1%. For DRS the limit of detection depends on the data reduction technique used. The commonly used Kubelka-Munk remission function and its first and second derivatives can easily identify hematite at the 0.5% level. However, the first derivative of the percent reflectance curve can detect hematite at 0.01% by weight in a light matrix and 0.05% in a dark matrix. We suggest that the first derivative of DRS curves is the best currently available method for qualitatively detecting the mineral hematite at low concentrations found in soils, sediments, and rocks. Work described in this paper may be applied in several situations. Our study of visual limits of hematite detection should aid field geologists in assessing hematite content. Analysis of color wavelength bands may also have application in remote sensing by indicating which bands are most sensitive to hematite, reported to be an important constituent of the martian surface. Furthermore, this study could help clarify remotely sensed terrestrial albedo changes, especially the Sahara/Sahel transition where the sediments change from light, quartz-dominated to dark, hematite-dominated. Our study also points out that with laboratory-based spectra the first derivative of the reflectance curve is the most sensitive transform for processing spectral data for hematite, thereby allowing concentrations as low as 0.01% to be detected.

Marine sediment components : identification and dispersal assessed by diffuse reflectance spectrophotometry

Analysis of deep-sea sediments presents a number of exceptional challenges. Generally, the amount of material available is small and has to be shared among investigators. Also, deep-sea sediment contains a diverse range of components from a variety of sources including lithogenous, biogenous and hydrogenous. Finally, large number of samples is frequently analysed making rapid analysis a necessity. Given these limitations, a technique that is rapid, non-destructive or uses a very little sample, and is capable of identifying a large number of sediment components with a single analysis would be ideal. Diffuse Reflectance Spectrophotometry (DRS) has the potential to meet many of these criteria. In this paper, we discuss the application of DRS to deep-sea sediments from a variety of marine settings. We examine the modern sediment distribution in the Atlantic Ocean, Gulf of Mexico and Argentine Basin and compare the distribution of modern with the last glacial maximum sediment in the Atlantic Ocean. DRS has also proven useful in times series studies. DRS is especially useful for producing time series from Ocean Drilling Program (ODP) holes that require thousands of samples to produce a single time series. In this paper, time series from both the Atlantic and Pacific Oceans are examined.

Determination of metalloporphyrins in rocks, sediments and other geological materials using total reflectance spectrophotometry

This paper presents a high resolution method for obtaining diagnostic spectra of metalloporphyrins from geologic materials using total reflectance spectrophotometry. The technique is simple, straightforward and inexpensive, and does not require extracting the porphyrin from the material being investigated. Diffuse reflectance spectra from 250 to 850 nm have been measured for nickel, vanadium and copper porphyrins. Spectra of pure porphyrins, porphyrins mixed with geologic matrices and several hydrocarbon reservoir rocks are presented. These measurements may be valuable in determining the origin of organic material in rocks containing hydrocarbons.