Physical properties of 4 cores from the Bengal Fan

New stratigraphic and high-resolution seismic data from the Bengal Fan indicate that the world's 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 14C yr B.P.) and 1b (9700 14C 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.

Physical properties of sediment cores of the Bengal Fan

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.

Physical properties of 39 sediment cores from the Bengal Fan and northeast Indian Ocean

We obtained sediment physical properties and geochemical data from 47 piston and gravity cores located in the Bay of Bengal, to study the complex history of the Late Pleistocene run-off from the Ganges and Brahmaputra rivers and its imprint on the Bengal Fan. Grain-size parameters were predicted from core logs of density and velocity to infer sediment transport energy and to distinguish different environments along the 3000-km-long transport path from the delta platform to the lower fan. On the shelf, 27 cores indicate rapidly prograding delta foresets today that contain primarily mud, whereas outer shelf sediment has 25% higher silt contents, indicative of stronger and more stable transport regime, which prevent deposition and expose a Late Pleistocene relic surface. Deposition is currently directed towards the shelf canyon 'Swatch of No Ground', where turbidites are released to the only channel-levee system that is active on the fan during the Holocene. Active growth of the channel-levee system occurred throughout sea-level rise and highstand with a distinct growth phase at the end of the Younger Dryas. Coarse-grained material bypasses the upper fan and upper parts of the middle fan, where particle flow is enhanced as a result of flow-restriction in well-defined channels. Sandier material is deposited mainly as sheet-flow deposits on turbidite-dominated plains at the lower fan. The currently most active part of the fan with 10-40 cm thick turbidites is documented for the central channel including inner levees (e.g., site 40). Site 47 from the lower fan far to the east of the active channel-levee system indicates the end of turbidite sedimentation at 300 ka for that location. That time corresponds to the sea-level lowering during late isotopic stage 9 when sediment supply to the fan increased and led to channel avulsion farther upstream, probably indicating a close relation of climate variability and fan activity. Pelagic deep-sea sites 22 and 28 contain a 630-kyear record of climate response to orbital forcing with dominant 21- and 41-kyear cycles for carbonate and magnetic susceptibility, respectively, pointing to teleconnections of low-latitude monsoonal forcing on the precession band to high-latitude obliquity forcing. Upper slope sites 115, 124, and 126 contain a record of the response to high-frequency climate change in the Dansgaard-Oeschger bands during the last glacial cycle with shared frequencies between 0.75 and 2.5 kyear. Correlation of highs in Bengal Fan physical properties to lows in the d18O record of the GISP2 ice-core suggests that times of greater sediment transport energy in the Bay of Bengal are associated with cooler air temperatures over Greenland. Teleconnections were probably established through moisture and other greenhouse-gas forcing that could have been initiated by instabilities in the methane hydrate reservoir in the oceans.

Clay mineralogy of sediments from the Bengal Fan

We have measured the relative abundances of smectite, illite, chlorite, and kaolinite in a composite section of the distal Bengal Fan. Two sources of sediment appear to dominate, a smectite-poor, illite-rich source associated with rapid denudation of the Himalayas and a smectite-rich, illite-poor source probably on the continental margin of the Indian sub-continent. Changes in source appear to be related to uplift in the Himalayas and Tibetan Plateau both directly and through the climatic and oceanographic consequences of uplift.

Nd and Pb isotope data for DSDP and ODP sediments from the Bengal Fan

Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ~40% of the Indus Fan predates the middle Miocene, and ~35% predates uplift of the Murray Ridge (early Miocene, ~22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ~40% of the Neogene sediment in the fan is derived from the Indian plate.

Sedimentary history and chemical characteristics of clay minerals from the Bengal Fan

The purpose of this study is to clarify the sedimentary history and chemical characteristics of clay minerals found in sediments deposited in the distal part of the Bengal Fan since the Himalayas were uplifted 17 m.y. ago. A total of seventy-eight samples were collected from three drilled cores which were to be used for the clay mineral analyses by means of XRD and ATEM. The results obtained from the analyses show that individual clay mineral species in the sediment samples at each site have similar features when the samples are of the same age, whereas these species have different features in samples of differing geological ages. Detrital clay minerals such as illite and chlorite were deposited in greater amounts than kaolinite and smectite during the Early to Middle Miocene. This means that the Himalayan uplift was vigorous at least until the Middle Miocene. In the Pliocene chemical weathering was more prevalent so that instead, in the distal part of the Bengal Fan, kaolinite shows the highest concentrations. This would accord with weaker uplift in the Himalayas. In the Pleistocene period, vigorous Himalayan uplift is characterized by illite-rich sediment in place of kaolinite. In the Holocene, smectite shows the highest concentration in place of the illite and kaolinite which were the predominant clay minerals of the earlier periods. Increasing smectite concentration suggests the Himalayan uplift to have been stable after the Pleistocene period. The smectite analyzed here is found to be dioctahedral Fe-beidellite, and it originated largely from the augite-basalt on the Indian Deccan Traps. The tri-octahedral chlorite is subdivided into three sub-species, an Fe-type, a Mg-type and an intermediate type. The mica clay mineral can be identified as di-octahedral illite which is rich in potassium. The chemical composition and morphology of each clay mineral appears to exhibit no change with burial depth in the sedimentary columns. This implies that there was no systematic transformation of clay minerals with time.

Osmium isotope composition of leachates and bulk sediment from ODP Holes in the Bengal Fan

Os isotopic compositions and OS and Re concentrations were measured in H2O2-H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios (Pegram et al., 1994, doi:10.1016/0012-821X(94)90172-4) much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y. Bengal Fan leachate 187Os/188Os ratios increase with increasing depositional age, in contrast to the seawater Os isotopic ratio, which decreases with increasing age. Several lines of evidence suggest that, at the time of sediment burial, the leachate Os compositions most likely reflected the seawater values. Thus, the current divergence is probably the result of post-depositional processes. One such process, in situ radiogenic ingrowth of 187Os, can be excluded because the measured Re concentrations of these sediments are too low. Similarly, since most of the bulk rock Os isotopic ratios were lower than those of the associated leachates, the high leachate 187Os/188Os values cannot be explained by in situ sediment alteration. Instead, it is proposed that the increase with age results from radiogenic OS brought in by thermoconvective circulation from further upslope in the Fan. The ultimate source of this 187Os would then be alteration of radiogenic sediments or post-depositional radioactive decay of Re in sediments rich in organic carbon. Finally, the divergence between the results obtained on Bengal Fan sediments and those obtained in the open ocean (Pegram et al., 1992, doi:10.1016/0012-821X(92)90132-F) by the same leaching technique suggest that Os sediment leachate data must be interpreted with caution.

Mineralogy of silts from the Bengal Fan

Early Miocene to Quaternary sediments drilled from the Bengal Fan are divided into six zones by modal proportions of heavy minerals. The sediments were mostly derived from the Himalayas. Detritus from the Indian subcontinent is found sporadically in clay-rich sediments that were deposited during periods of slow sedimentation, when the deep-sea channel migrated away from the drilled sites. The oldest sediments, ranging from 17 to about 15 Ma, were derived mostly from the Precambrian and Paleozoic sedimentary rocks of the lower Himalayas. At about 15 Ma, metamorphic terrains were eroded in the source area. Further large-scale unroofing of metamorphic rocks occurred around 11 Ma. After 10 Ma, the major constituents in the drainage basin or the drainage pattern changed a few times. Between 3.5 and 0.5 Ma, a large peridotite body was unroofed by uplift and successive erosion of the central Himalayas. At this time, the single large river that had supplied detritus to the early Bengal Fan was divided into the Indus and Ganges rivers.

Consolidation characteristics of outer Bengal Fan deposits (Table 1)

The state of consolidation of outer Bengal Fan deposits is closely related to their depositional history. Early Pleistocene sediments are underconsolidated, i.e., fabric strength is lower than under equilibrium with the present overburden stress, at depths greater than 60-80 mbsf. This may be due to rapid accumulation (>15-20 cm/k.y.) and overlying low-permeability (<10**-7 cm/s) deposits. The underlying Pliocene sediments are overconsolidated, i.e., fabric strength is higher than in equilibrium with the present overburden stress; at Sites 717 and 719 the sediments are slightly overconsolidated, whereas at Site 718 the overconsolidation is significant. Overconsolidation is explained by erosion that occurred during the early Pleistocene for which a stratigraphic gap was recorded in the drilled cores. The eroded section is estimated to be in the range of 10-40 m at Sites 717 and 719, respectively, and 130-150 m at Site 718. Below 250-300 mbsf the most sediments are normally consolidated. The amount of erosion seems to be related to block rotation and uplift due to intraplate deformation.

(Table 1) Sr, Nd and Os isotopic composition from marine sediments of the Bengal Fan

Sr, Nd, and Os isotopic data are presented for sediments from diverse locations in the Bay of Bengal. These data allow the samples to be divided into three groups, related to their sedimentary contexts. The first group, mainly composed of sediments from the shelf off Bangladesh and the currently active fan, has Sr and Nd characteristics consistent with a dominantly Himalayan source. Their 187Os/188Os ratios (~1.2-1.5) show that the average detrital material delivered by the Ganga-Brahmaputra (G-B) river system is not unusually radiogenic. A large difference in 187Os/188Os ratio exists between these Bengal Fan sediments and Ganga bedloads (187Os/188Os ~2.5, Pierson-Wickmann et al. (2000, doi:10.1016/S0012-821X(00)00003-0)). This difference mainly reflects addition of a less radiogenic Brahmaputra component, though mineralogical sorting and loss of radiogenic Os during transport may also play some role. The second sample group contains sediments from elsewhere in the Bay, particularly those located on the continental slope. They display Os isotopic compositions (0.99-1.11) similar to that of present seawater and higher Os and Re concentrations. These characteristics suggest the presence of a large hydrogenous contribution, consistent with the lower sedimentation rate of these samples. Sr and Nd ratios indicate that a significant fraction of these sediments is derived from erosion of non-Himalayan sources, such as the Indo-Burman range. These observations could be explained by the deflection of sediments from the G-B river system by westward currents in the head of the Bay. The third group contains only one sample, but shows that in addition to a Himalayan source, sediment discharge from Sri Lanka may influence the detrital component in the distal part of the fan. The similarity between the isotopic compositions of the group I R/V Sonne samples and those of Ocean Drilling Program Leg 116 (France-Lanord et al., 1993; Reisberg et al., 1997, doi:10.1016/S0012-821X(00)00003-0) suggests that the material eroding in the Himalayas has been roughly constant since the Miocene. The high Os isotopic ratios of leachates of both Sonne group I and Miocene Leg 116 sediments imply that much of the leachable highly radiogenic Os component was conserved during transport through the estuary or interaction with seawater. In constrast, samples with lower, but still relatively high, sedimentation rates (Sonne groups II and III and Pliocene Leg 116) seem to have significantly adsorbed or exchanged Os and Re with seawater. This suggests that in some cases the Os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient marine Os record, or conversely, to date unfossiliferous sediments.

Bulk and amino acid parameters of core SO188-342KL from the Northern Bay of Bengal for the last 18 ka

The Northern Bay of Bengal (NBoB) is a globally important region for deep-sea organic matter (OM) deposition due to massive fluvial discharge from the Ganges-Brahmaputra-Meghna (G-B-M) rivers and moderate to high surface productivity. Previous studies have focused on carbon burial in turbiditic sediments of the Bengal Fan. However, little is known about the storage of carbon in pelagic and hemipelagic sediments of the Bay of Bengal over millennial time scales. This study presents a comprehensive history of OM origin and fate as well as a quantification of carbon sediment storage in the Eastern Bengal Slope (EBS) during the last 18 ka. Bulk organic proxies (TOC, TIC, TN, d13CTOC, d15NTN) and content and composition of total hydrolysable amino acids (THAA) in a sediment core (SO188-342KL) from the EBS were analyzed. Three periods of high OM accumulation were identified: the Late Glacial (LG), the Bölling/Alleröd (B/A), and the Early Holocene Climatic Optimum (EHCO). Lower eustatic sea level before 15 ka BP allowed a closer connection between the EBS and the fluvial debouch, favoring high terrestrial OM input to the core site. This connection was progressively lost between 15 and 7 ka BP as sea level rose to its present height and terrestrial OM input decreased considerably. Export and preservation of marine OM was stimulated during periods of summer monsoon intensification (B/A and EHCO) as a consequence of higher surface productivity enhanced by cyclonic-eddy nutrient pumping and fluvial nutrient delivery into the photic zone. Changes in the THAA composition indicate that the marine plankton community structure shifted from calcareous-dominated before 13 ka BP to siliceous-dominated afterwards. They also indicate that the relative proportion of marine versus terrestrial OM deposited at site 342KL was primarily driven by relative sea level and enlarged during the Holocene. The ballasting effect of lithogenic particles during periods of high coastal proximity and/or enhanced fluvial discharge promoted the export and preservation of OM. The high organic carbon accumulation rates in the EBS during the LG (18-17 ka BP) were 5-fold higher than at present and comparable to those of glacial upwelling areas. Despite the differences in sediment and OM transport and storage among the Western and Eastern sectors of the NBoB, this region remains important for global carbon sequestration during sea level low-stands. In addition, the summer monsoon was a key promotor of terrestrial and marine OM export to the deep-ocean, highlighting its relevance as regulator of the global carbon budget.