Michael E Weber

Calibration and application of marine sedimentary physical properties using a multi-sensor core logger

Abstract We examined geophysical data from a Multi-Sensor Core Logger (MSCL), a logging device providing continuous measurements of gamma-ray attenuation, p-wave travel time, and magnetic susceptibility on marine sediment cores. In the first part we focused on the gamma-ray system and compared two different calibration methods. From the gamma-ray attenuation, we calculated densities and porosities by incorporating mass weighted attenuation coefficients. The application of an iteration method reduces the error of the density and porosity estimates compared to GRAPE data. In addition, we derived equations to calculate water content and dry bulk density from gamma-ray attenuation measurements. Comparison with physical properties determined on discrete samples revealed a very good correlation of both data sets (r = 0.99). This correlation is valid for sediments from substantially different geological settings (e.g., turbidites, hemipelagic muds, and opal-rich sediments). In the second part we applied our data to marine geological questions. For sediments from the Antarctic Polar Frontal Zone, there is indication that the content of biogenic opal can be assessed using a correlation of density and p-wave velocity. For sediments from the Bengal Fan, the relationship between the MSCL acoustic impedance (the product of density and p-wave velocity) and the grain-size distribution in discrete samples can be used to predict clay and sand/silt ratios for sediment cores from the shelf and upper continental slope.

Active growth of the Bengal Fan during sea-level rise and highstand

New stratigraphic and high-resolution seismic data from the Bengal Fan indicate that the world9s largest fan shows active growth during the most recent sea-level rise and the recent highstand. This unique phenomenon contradicts common sequence-stratigraphic models, and the sediment preserved provides new insight into the sedimentological response of a fan system to sea-level rise, climatic terminations, and monsoon intensity during the past climatic cycle. We present a detailed dated sequence of turbidite sedimentation based on a core transect perpendicular to the active channel-levee system in the upper mid-fan area. Between the two major terminations 1a (12 800 14 C yr B.P.) and 1b (9700 14 C yr B.P.), and especially at the end of the Younger Dryas, a 13-km-wide channel built up levees 50 m high. With decreasing sediment supply, continued sea-level rise, and increasing monsoon intensity during the early Holocene, turbidity currents were confined to the channel and gradually filled it. The canyon “Swatch of No Ground,” a shelf depocenter that serves as the source for frequent turbidity currents, and the channel-levee system provide the unique opportunity for studying an active highstand system. Many fans showed this behavior only during lowered sea-level.

The youngest channel-levee system of the Bengal Fan: results from digital sediment echosounder data

Abstract A high-resolution seismic survey focused on the youngest channel-levee system of the Bengal Fan was carried out during January/February 1994 from the RV Sonne . Acoustic strata patterns and the downslope development of the channel-levee system were examined with the parametric sediment echosounder parasound . Determination of the age of the sedimentary strata shows turbiditic activity during sea level rise and highstand. The initial formation of the system in the middle fan occurred in the late glacial and outer levee growth stopped with glacial termination. Several vertical, aggradational segments constitute the inner levees created in the Holocene. The formation of the inner levee segments indicates the constriction of a formerly 14 km wide channel in discrete phases. The top of the segments form topographic pinnacles, explaining the morphology of other channel-levee systems from other fans in the worlds oceans. Some cross-sections from the lower fan reveal lenticular channel-levee systems with common reflection characteristics. Prograding distinct reflections on the outer side of the upper levees terminate with a downlap against an unconformity, which separates the upper part of the overbank deposits from a mostly diffusely reflecting lower part. Channel constriction has also been observed in the lower fan. Two models are introduced to explain the generation of the most common acoustic strata patterns by sea level fluctuations or downslope shifting of the channel mouth.

Acoustical characterization of sediments by Parasound and 3.5 kHz systems: Related sedimentary processes on the southeastern Weddell Sea continental slope, Antarctica

Subbottom profiling was performed during four Polarstern expeditions to the southeastern Weddell Sea continental slope, northwest of Lyddan Ice Rise, Antarctica. Analogue records from high resolution, seismic reflection systems (3.5 kHz and Parasound) were used to classify sediment echo types. A comparison between the two systems shows the advantage of the Parasound system. The smaller beam angle produces a higher spatial resolution, which reduces the diffraction hyperbolae considerably. The higher vertical resolution and deeper penetration provide more detailed information concerning seismic stratification. Using four basic echo categories (P = Prolonged, L = Layered D = Diffraction hyperbolae, W = Wedging subbottoms), ten discrete sediment echo patterns were distinguished. Their regional distribution and the depth of acoustic signal penetration into the sediment were mapped. SW-NE trending sediment ridges with channels running parallel on the southeastern side of the ridges were traced in the eastern part of the study area. The prolonged and highly reflective echo types of the channel bottoms indicate deposition of predominantly coarse-grained sediments affected by erosional processes. These channels are interpreted as drainage systems for bottom water currents derived from the shelf and steep upper slope. The flow direction is towards the northeast, counter to the Weddell Gyre. In contrast, the sediment ridges show up to 80 m subbottom penetration and an echo type of distinct, multi-layered reflectors running parallel to subparallel with the surface reflector. The sediments of the ridges are interpreted to be “levee”-sediments, overspilled from the channels and preferentially deposited on the northwestern flank due to the Coriolis force. Sedimentary units separated by wedge shaped reflectors are widespread on the gently inclined terrace in the middle part of the continental slope. These sediments are presumed to be produced by syn-sedimentary slumping and proximal debris flows having a northeastern direction of movement.

Interhemispheric ice-sheet synchronicity during the last glacial maximum

Part of the East Antarctic Ice Sheet advanced and retreated in synchrony with Northern Hemispheric ice sheets. The timing of the last maximum extent of the Antarctic ice sheets relative to those in the Northern Hemisphere remains poorly understood. We develop a chronology for the Weddell Sea sector of the East Antarctic Ice Sheet that, combined with ages from other Antarctic ice-sheet sectors, indicates that the advance to and retreat from their maximum extent was within dating uncertainties synchronous with most sectors of Northern Hemisphere ice sheets. Surface climate forcing of Antarctic mass balance would probably cause an opposite response, whereby a warming climate would increase accumulation but not surface melting. Our new data support teleconnections involving sea-level forcing from Northern Hemisphere ice sheets and changes in North Atlantic deep-water formation and attendant heat flux to Antarctic grounding lines to synchronize the hemispheric ice sheets.

Sedimentation processes within channel-ridge systems, southeastern Weddell Sea, Antarctica

On the continental margin of the southeastern Weddell Sea, Antarctica, several channel-ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest-northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 – 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross-stratified coarse-silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice-rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross-stratified coarse-silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 – 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4–7 cm/s), the water masses of the Weddell Gyre, supplying fine-grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice-rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment-starved during interglacial intervals.

Pore water profiles and numerical modelling of biogeochemical processes in Peru Basin deep-sea sediments

Abstract An extensive set of pore water profiles, including O 2 , NO 3 − , NO 2 − , NH 4 + , PO 4 3− , Fe 2+ , Mn 2+ , SO 4 2− , SiO 4 4− , pH, and alkalinity as well as solid phase data on organic carbon content and porosity, in 18 sediment cores obtained from five areas in the Peru Basin (RV Sonne 106 expedition in 1996) is presented. The pore water profiles indicate that early diagenesis progresses through oxygen respiration, denitrification, and manganese(IV) reduction, while the subsequent redox states, iron(III) reduction, sulphate reduction and methanogenesis, cannot be observed from the dissolved phase in these deep-sea sediments. A numerical diagenetic model was applied to the data in order to derive parameters, such as the depositional flux of metabolizable organic matter or the bioturbation intensity, that control the geochemical conditions in the deep-sea floor and could not be obtained directly by measurements. Moreover, the numerical simulations show that even those pore water NO 3 − profiles that decline non-linearly to zero concentration are not compatible with a steady state assumption. They can be explained only by transient downward progression of a nitrate front.

Carbonate preservation history in the Peru Basin: Paleoceanographic implications

We studied preservation/dissolution cycles and paleoproductivity in eight sediment cores from the Peru Basin south of the highly productive surface waters of the eastern equatorial Pacific. Stratigraphy is based on stable oxygen isotopes and on combined magnetostratigraphy and biostratigraphy. Sediment cores which span the last 8 m.y., were retrieved during cruise 79 with RV SONNE close to the carbonate compensation depth (CCD). In general, sediments show Pacific-type carbonate cycles. We interpret a pronounced carbonate peak between 6 and 7 Ma as the result of a western and northern extension of the highly productive Peru Current. Decreased carbonate contents from the late Miocene to the late Pliocene might be associated with a slow contraction of the latitudinal extent of the high-productivity belt north of the study areas. During the Pliocene, carbonate variations showed 400 kyr cycles indicating the growth and decay of ice sheets, which should have been associated with pulsations of the Antarctic ice cap. An abrupt collapse of the carbonate system occurred at 2.4 Ma. Higher frequency variations of the carbonate record indicate the major increase of the northern hemisphere glaciation. During the Quaternary, carbonate fluxes are high during glacials and low during interglacials. Large amplitude variations with long broad minima and maxima, associated with small migrations of the lysocline and the CCD (< 200 m), are indicative of the preservation/dissolution history in the Peru Basin. During the early Pleistocene, climatic forcing by the 41 kyr obliquity cycle is not observed in the carbonate record. During the last 800 kyr, variability in the carbonate record was dominated by the 100 kyr eccentricity cycle. Fluxes of biogenic material (calcium carbonate, organic carbon, opal, and barium) were greatest during glacials, which imply higher productivity and export production of the Peru Current during cold climatic periods. Dissolution was greatest during interglacials as inferred from the relatively poor preservation of planktonic foraminifera and from the low accumulation rate of carbonate. After the Mid-Brunhes Event (400 ka), we observe a plateaulike shift to enhanced dissolution and to intensified productivity.

BMPix and PEAK tools: new methods for automated laminae recognition and counting Application to glacial varves from Antarctic marine sediment

We present tools for rapid and quantitative detection of sediment lamination. The BMPix tool extracts color and gray-scale curves from images at pixel resolution. The PEAK tool uses the gray-scale curve and performs, for the first time, fully automated counting of laminae based on three methods. The maximum count algorithm counts every bright peak of a couplet of two laminae (annual resolution) in a smoothed curve. The zero-crossing algorithm counts every positive and negative halfway-passage of the curve through a wide moving average, separating the record into bright and dark intervals (seasonal resolution). The same is true for the frequency truncation method, which uses Fourier transformation to decompose the curve into its frequency components before counting positive and negative passages. We applied the new methods successfully to tree rings, to well-dated and already manually counted marine varves from Saanich Inlet, and to marine laminae from the Antarctic continental margin. In combination with AMS14C dating, we found convincing evidence that laminations in Weddell Sea sites represent varves, deposited continuously over several millennia during the last glacial maximum. The new tools offer several advantages over previous methods. The counting procedures are based on a moving average generated from gray-scale curves instead of manual counting. Hence, results are highly objective and rely on reproducible mathematical criteria. Also, the PEAK tool measures the thickness of each year or season. Since all information required is displayed graphically, interactive optimization of the counting algorithms can be achieved quickly and conveniently.

Variability of surface sediments in the Peru basin: dependence on water depth, productivity, bottom water flow, and seafloor topography

Abstract We investigated surficial sediments for physico-chemical composition from numerous sites of seven study areas in the manganese nodule field of the northern Peru Basin as part of a deep-sea environmental study. Major results from this study are strong variability with respect to water depth, productivity in surface waters, locality, bottom water flow, and seafloor topography. Sediment sites are located mostly in 3900 to 4300 m water depth between the lysocline and the carbonate compensation depth (CCD). Large fluctuations in carbonate content (0% to 80%) determine sediment density and compressional-wave velocity, and, by dilution, contents of opal and non-biogenic material. Mass accumulation rates of biogenic components as well as geochemical proxies (barium and phosphorus) distinguish areas of higher productivity in the northwest near equatorial upwelling and in the northeast close to coastal upwelling, from areas of lower productivity in the west and south. Comparisons between the central Peru Basin area (Discol) and western Peru Basin area (Sediperu) reveals, for the Sediperu area, a shallower CCD, more carbonate but less opal, organic carbon, and non-biogenic material in sediments at the same water depth as well as larger down-core fluctuations of organic carbon and MnO2. Bottom water flow in the abyssal hill topography causes winnowing of material from summits of seamounts and ridges, where organic carbon preservation is poor, to basins where organic carbon preservation is better. Down-core measurements in box cores indicate a three-fold division in the upper 50 cm of the sediment column. An uppermost semi-liquid top layer is dark brown, 5–15 cm thick and contains most of the ferro-manganese nodules. A 5–15 cm thick transition zone of light sediment color has increasing shear strength, lowest opal contents and compressional-wave velocities, but highest carbonate contents and sediment densities. The lowermost layer contains stiffer light gray sediments.