Roger D. Flood

Morphology of abyssal mudwaves at project MUDWAVES sites in the Argentine Basin

Abstract Extensive fields of mudwaves are present in the Argentine Basin. Mudwaves and water parameters at three sites were studied to provide detailed information on bathymetry, mudwave structure, sediment parameters and bottom water flow characteristics. Two sites, 5 and 6, are discussed in this paper. Crests of mudwaves at Site 5 on the north flank of the Zapiola Drift are oriented 45° anti-clockwise from the measured westward flow direction and migrate to the left of the flow direction. Mudwaves at Site 6 on the south flank of the Zapiola Drift are oriented 20° anti-clockwise from the inferred southeast bottom flow direction, and also migrate to the left of direction. Evidence from current meters and surface sediments suggests that the mudwaves at Site 5 are active today, while those at Site 6 have been active in the Holocene, but perhaps not within the last 100 years. Mudwave orientation anti-clockwise to mean flow, preferential sediment accumulation on the left wave flank, and apparent sediment accumulation on the wave crest agree with a model for mudwave growth presented by Blumsack and Weatherly (1989, Deep-Sea Research, 36, 1327–1339). Cross-wave asymmetry in sediment accumulation pattern is sufficient to cause a 10% change in surface sediment bulk density over the wave profile. The record of mudwave migration obtained from Sites 5 and 6 in the southern Argentine Basin and from Site 7 in the northern Argentine Basin suggests that bottom water flow has changed over a number of time scales during the last 200,000 years. Thus the analysis of abyssal bed forms can provide independent evidence about past changes in bottom flow.

Astronomical age models for Pleistocene drift sediments from the western North Atlantic (ODP Sites 1055-1063)

Ten ODP sites drilled in a depth transect (2164–4775 m water depth) during Leg 172 recovered high-deposition rate (>20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8–0.9 Ma has been developed. The time scales have a resolution of 10–20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (2164–2975 m water depth, Sites 1055–1058) while the deeper sites (2995–4775 m water depth, Sites 1060–1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma.

Paleoceanographic significance of sediment color on western North Atlantic drifts: I. Origin of color

Reflectance spectra collected during ODP Leg 172 were used in concert with solid phase iron chemistry, carbonate content, and organic carbon content measurements to evaluate the agents responsible for setting the color in sediments. Factor analysis has proved a valuable and rapid technique to detect the local and regional primary factors that influence sediment color. On the western North Atlantic drifts, sediment color is the result of primary mineralogy as well as diagenetic changes. Sediment lightness is controlled by the carbonate content while the hue is primarily due to the presence of hematite and Fe 2þ /Fe 3þ changes in clay minerals. Hematite, most likely derived from the Permo-Carboniferous red beds of the Canadian Maritimes, is differentially preserved at various sites due to differences in reductive diagenesis and dilution by other sedimentary components. Various intensities for diagenesis result from changes in organic carbon content, sedimentation rates, and H2S production via anaerobic methane oxidation. Iron monosulfides occur extensively at all high sedimentation sites especially in glacial periods suggesting increased high terrigenous flux and/or increased reactive iron flux in glacials. 7 2002 Elsevier Science B.V. All rights reserved.

Abyssal bedforms as indicators of changing bottom current flow: Examples from the U.S. East Coast continental rise

Direct indications of circulation patterns in the deep sea (especially along continental rises and sediment drifts) can be obtained through the analysis of abyssal bedforms such as furrows and mud waves. Such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments. Water that sinks in the northern North Atlantic forms part of the Western Boundary Undercurrent along the continental rise of eastern North America and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time. Studies of furrows and mud waves along the Blake-Bahama Outer Ridges show that they are active sedimentary features today, and that bottom flow speeds near the time of the Younger Dryas were increased over later intervals. A more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern.

Paleoceanographic significance of sediment color on western North Atlantic Drifts: II. Late Pliocene–Pleistocene sedimentation

Abstract Color variations were interpreted in paleoceanographic terms for the late Pliocene–Pleistocene sediments recovered by ODP Leg 172 on deep-sea drifts at Blake–Bahama Outer Ridge and northeastern Bermuda Rise. The color-derived parameters used in interpretation included predicted carbonate content, terrigenous fluxes, and hematite content. Abundance of Upper Carboniferous spores indicates that the hematite is probably derived from the Permo-Carboniferous red beds of the Canadian Maritimes. In the last 800 kyr sedimentation pattern changes on the Blake–Bahama Outer Ridge were determined by the sediment delivery to the deep basin as well as circulation changes. Sediment delivery increased during glacials (especially during the last 500 kyr and particularly since Stage 6). A fundamental change in the thermohaline circulation occurred at about ∼500 ka corresponding to the end of the Mid-Pleistocene Transition period at the onset of the predominant 100-kyr climate cyclicity. Sedimentation related to WBUC had intensified at that time and had become more focused at depths below 3000 m. Changes in hematite content and sedimentation rate show a pulse of sediment via the St. Lawrence outlet at the Pliocene–Pleistocene boundary suggesting that a likely change in the hydrography/physiography of the Laurentide Ice Sheet could have been involved in the climatic and ocean circulation changes at that time.

Paleoflow history determined from mudwave migration: Argentine Basin

Abstract A mudwave site in the central Argentine Basin was investigated to determine its present and past mudwave activity. Site 7, on the south flank of the Ewing Drift in the northern Argentine Basin, has mudwaves that have migrated primarily throurgh preferential deposition, resulting in a relatively complete sediment on both wave flanks. Thus a history of mudwave migration can be determined for this site. The mudwave studied shows migration from before 420 ka until 32 ka, with paleocurrent speeds near 15 cm s−1 and with minor changes in current speeds around 110–120 ka. The wave migration rate became less from 32 ka to 26 ka with paleospeeds of 9 cm s−1. Wave migration ceased about 26 ka, suggesting a cessation in flow. This study indicates that bottom water velocities at this site were high during the last glacial period. but dropped abruptly near the end of the last glacial period. However, bottom current velocities appear to have been moderately high during the previous interglacial period (∼ 120 ka), suggesting that there is no simple relationship between flow speed and paleoclimate at this site.

Field experiments to study seafloor seismoacoustic response

Results of a 5-year program to measure the seismoacoustic properties of unlithified sediments in the seafloor are presented. A field technique utilizing an impulsive source and a geophone array located on the seafloor was used to obtain detailed travel-time curves which permit a number of different kinds of analysis. A new signal source powered by electrically detonated, 8-gauge shotgun shells was used to focus energy into the bottom and eliminate much of the high-frequency water-borne noise that accompanies the usual explosive sources. Both interface waves and wide-angle refractions were analyzed. A variety of data inversion techniques including slant stacking and cross multiplication were used to obtain dispersion curves, and constrained, least-squares inversion using partial derivatives was used to develop an iterative inversion method to obtain velocity and attenuation as a function of depth. Examples of data from a wide angle of different kinds of sediment ranging from soft Holocene mud to stiff over...

Migration history of a fine-grained abyssal sediment wave on the Bahama Outer Ridge

Abstract Abyssal mud waves (or fine-grained sediment waves) are often cited as evidence for deep current activity because subbottom profiles show that the wave form has migrated with time. The migration history of a fine-grained sediment wave on the Blake-Bahama Outer Ridge (ODP Site 1062) has been studied through the analysis of multiple ODP holes spaced across the wave. Additional information about wave migration patterns comes from 3.5-kHz records and watergun seismic profiles. These data suggest that wave migration has varied during the last ∼10 Myr, although the only sediments sampled are younger than 4.8 Ma. Seismic profiles suggest wave migration was initiated about 8–10 Ma, and wave migration was pronounced from about 5 Ma to about 1 Ma (with an episode of wave reorganization about 4.5 Ma). Analysis of ODP cores suggests that migration rates have been somewhat lower and more variable during the last 1 Myr. Intervals of no wave migration are observed for several time intervals and appear to characterize deglaciations, especially during the last 500 kyr. Comparisons between seismic profiles and the core record show that most of the seismic horizons correlate closely with time horizons, and thus that the seismic profiles give a reasonable representation of sediment wave migration. Models suggest that wave migration is more pronounced during periods of higher bottom current flow and less pronounced during periods of lower current flow. Thus the migration record is consistent with generally higher bottom flow speeds at this site prior to 1 Ma and lower bottom flow speeds after 1 Ma. The Mid-Pleistocene Transition from a dominant climatic periodicity of 40 kyr to a dominant climatic periodicity of 100 kyr starts at about this time, suggesting an overall reduction in bottom flow speed at this site coincident with changing climate patterns. These changes in flow speed could be related to changes in the depth of the Western Boundary Undercurrent as well as to changes in the speed of thermohaline circulation.

Shallow seismic experiments using shear waves

During the summer of 1986, a series of seismo‐acoustic experiments was carried out in shallow water off the New Jersey shore. The purpose of these experiments was to measure the geoacoustic properties of the ocean sediments that comprise the upper few hundred meters of the sediment column. Seismic sources and receivers were deployed at or very near the bottom in order to excite shear waves in the sediment and minimize the effects of interference from waterborne propagation. The experiments were performed at several sites where prior field work had established physical properties and a detailed profile of the sediments. By using conventional air guns deployed in an unconventional way, strong interface and diving shear waves were generated; these data were inverted to obtain shear wave velocity as a function of depth. The inversion results were then compared with the predictions of a geoacoustic model that accounts for the effects of voids ratio, overburden pressure, and other physical parameters. The in si...

Side-looking sonar backscatter response at dual frequencies

Dual-frequency side-looking sonars have the potential to be used as remote sensing tools to characterize subaqueous terrains. In one case study of the carbonate-ooze-coated Blake Plateau off-shore of Georgia, U.S.A., the difference in acoustic attenuation for 50 and 20 mm wavelengths (30 and 72 kHz frequency) permits the discrimination of sub-bottom scatterers from seabed surface textural features to reveal patchy regions where a buried hard ground had been pock-marked by karst-like depressions. In a second study of the Upper Hudson River in New York, U.S.A., related to environmental contaminates, the backscatter response at 15 and 3 mm acoustic wavelengths (100 and 500 kHz frequency) serves as a useful proxy for sediment grain size with coarser detritus distinguished from finer sediments. Sand and gravel regions inferred from the backscatter were confirmed by ground truth sampling.