William Balsam

Organic storage of CO2 on the continental slope off the mid-Atlantic bight, the southeastern Bering Sea, and the Peru Coast

A comparison is made of organic content, sedimentation rates derived from 14C and 210Pb analyses, 13C and 15N isotope ratios, amorphous silica, particle size, and calcium carbonate within sediments from slopes off the mid-Atlantic bight, the southeastern Bering Sea, and the Peru coast. These sediments are mainly marine, diatom-rich, and about one-third of the organic carbon is recent, reflecting a possible transient of shelf export in response to mans increased activities since the industrial revolution. Using a combination of sedimentation and mixing rates of carbon, the C:N ratio of sediments within the upper 50 cm, and the amount of nitrogen thought to be released from the coastal zone, independent estimates suggest a carbon loading to world slopes of ∼0.3 to 0.5 × 109 tons C y−1. The Bering slope exhibits no anthropogenic transients, however, while increased carbon loading may have occurred off Peru in response to overfishing and off the mid-Atlantic bight in response to eutrophication. The generality of our results depends on which of the three systems is most representative of world slopes.

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.

RAPID AND QUANTITATIVE MEASUREMENT OF HEMATITE AND GOETHITE IN THE CHINESE LOESS-PALEOSOL SEQUENCE BY DIFFUSE REFLECTANCE SPECTROSCOPY

The long, continuous deposition of dust in the Chinese loess plateau offers a unique opportunity to study the nature of Fe oxide formation in a wide range of climatic conditions. A technique to obtain quantitative estimates of the concentration of hematite and goethite in loess and paleosol samples is reported. Experiments using diffuse reflectance spectroscopy on sets of laboratory mixed and natural loess and paleosol samples show that it is possible to obtain rapid and quantitative estimates of the absolute concentration of hematite and goethite in the Chinese loess sediments. Typical loess and paleosol samples were deferrated using the CBD procedure to produce a natural matrix material to which hematite and goethite in known weight percentages were added to produce a set of calibration standards. Spectral violet, blue, green, yellow, orange, red and brightness of standards were calculated from the reflectance data and served as independent variables for a multiple linear regression analysis. The effect of changing matrix from loess to paleosol was overcome by including a variety of different loess and paleosol samples in the regression equations. The resulting calibration equations provide estimates of wt.% hematite and goethite and have correlation coefficients >0.93. The total measured hematite and goethite concentrations exhibited consistent variations with CBD extractable iron. Tests of the equations for buffering changes in matrix composition were run with samples of varying mineralogical composition (calcite, illite, etc.) and demonstrated that the equations are well buffered for changes in matrix composition from loess to paleosol.

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.

Paleoceanography of the Deep Western Boundary Undercurrent on the North American continental margin for the past 25 000 yr

Temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the Deep Western Boundary Undercurrent (DWBUC) on the North American continental margin. From 25 to 17 ka the DWBUC axis was 500 m shallower and farther inshore than its present position on the lower rise; current speed was high from 25 to 20 ka and then waned until 17 ka. Between 17 and 7 ka the major axis of the DWBUC shallowed to 4000 m; current speed increased from 17 to 11 ka but slowed from H to 7 ka. Since 7 ka the DWBUC axis has deepened to its modern position at 4900 m. The zone of maximum carbonate dissolution remained below the DWBUC; the most intense dissolution was from 22 to 9 ka and 5 to 2 ka. Highest abundance of Uvigerina and highest percent benthic foraminifera were immediately above the DWBUC from 22 to 7 ka, after which values of both parameters fell to near zero.

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.

Mid-Pliocene Asian monsoon intensification and the onset of Northern Hemisphere glaciation

The late Pliocene onset of major Northern Hemisphere glaciation (NHG) is one of the most important steps in the Cenozoic global cooling. Although most attempts have been focused on high-latitude climate feedbacks, no consensus has been reached in explaining the forcing mechanism of this dramatic climate change. Here we present a key low-latitude climate record, the high-resolution Asian monsoon precipitation variability for the past fi ve million years, reconstructed from South China Sea sediments. Our results, with supporting evidence from other records, indicate signifi cant mid-Pliocene Asian monsoon intensifi cation, preceding the initiation of NHG at ca. 2.7 Ma ago. This 1.4-million-year-long monsoon intensifi cation probably enhanced monsoon-induced Asian continental erosion and chemical weathering and in the process left fi ngerprints in marine calcium isotopes. Furthermore, increased rock weathering and/or organic carbon burial probably lowered the contemporary atmospheric CO 2 and may have triggered the NHG onset.

Characterization of the Chinese loess–paleosol stratigraphy by whiteness measurement

Abstract Changing color is one of the most striking features of loess–paleosol sequences in central China: the loess is light yellow and the interstratified paleosols are reddish-brown. Until a few years ago, conventional color description of loess sediments was made using a qualitative visual method, the Munsell system. To better understand the character of the color changes and to quantify results, whiteness and magnetic susceptibility were compared from four loess–paleosol sequences. Results show that the two parameters are highly correlated in the four sequences studied. Grain-size analysis and chemical extraction experiments indicate that the concentration and species of fine-grained iron oxides are major factors determining color changes in loess and paleosols. Whiteness and latitude values from the last glacial–interglacial sequence exhibit a linear relationship suggesting that whiteness will be a useful tool for reconstructing past atmospheric circulation, especially the East Asian summer monsoon. Whiteness and magnetic susceptibility in the Luochuan profile over the past 0.9 Myr display a strong correlation indicating that whiteness can be also used to reveal a long climatic history from Chinese loess–paleosol sequences.

Sediment dispersal in the Argentine Basin: evidence from visible light spectra

Abstract Visible light reflectance spectra were determined for each of 236 core top samples from the Argentine Basin and surrounding areas. The spectra were analyzed by taking the first derivative of the raw spectral curves and factor analyzing the first derivative values using wavelengths as variables. We used a four-factor solution that explains about 90% of the cumulative variance. Factors were evaluated by comparing factor loadings to first derivative curves from known minerals and mineral combinations, by the mapped distributions of the factor scores, and by comparison to factors from other areas. The four factors represent (in order of decreasing variance) goethite, terrigenous vs pelagic sedimentation, organic matter, and chlorite. The goethite and chlorite factors are useful for interpreting sediment influx and dispersal. Goethitecomes from the Falkland Plateau and the Argentine continental margin. Once introduce to the deep ocean, goethite appears to be transported primarily by isobath-parallel currents. Chlorite, on the other hand, is transported into the Argentine Basin by Antarctic Bottom Water (AABW) and from the Falkland Plateau. However, along the Falkland Plateau chlorite is transported normal to the slope, probably by gravitational processes.