Geochemistry of recent sediments from the Indian Ocean

During the International Indian Ocean Expedition (1964/65) sediment cores were taken on six profiles off the western coast of the Indian Subcontinent. These profiles run approximately perpendicular to the coast, from the deep-sea over the continental slope to the continental shelf. Additional samples and cores were taken in a dense pattern in front of the delta of the Indus River. This pattern of sampling covered not only marine sediments, but also river and beach sediments in Pakistan. The marine samples were obtained with piston, gravity and box corers and by a Van Veen grab sampler. The longest piston core is about 5 meters long. 1. Distribution of the elements on the sediment surface The area of maximal carbonate values (aprox. 80-100% CaCO3) essentially coincides with the continental shelf. The highest Sr values were observed largely within this area, but only in the vicinity of the Gulf of Cambay. Mainly the aragonitic coprolites are responsible for those high Sr contents. The Mg contents of the carbonates are comparatively low; surprisingly enough the highest Mg concentrations were also measured in the coprolites. The maximum contents of organic matter (Core) were found along the upper part of the continental slope. They coincide with the highest porosity and water content of the sediments. Frequently the decomposition of organic matter by oxydation is responsible for the measured Corg contents. On the other side the quantity of originally deposited organic material is less important in most cases. The enrichment of the "bauxitophile" elements Fe, Ti, Cr and V in the carbonate- and quartz-free portions of the sediments is essentially due to the influence of coarse terrigenous detritus. For the elements Mn, Ni and Cu (in per cent of the carbonateand quartz-free sediment) a strong enrichment was observed in the deep-sea realm. The strong increase in Mn toward the deep-sea is explained by authigenesis of Mn-Fe-concretions. Mn-nodules form only under oxydizing conditions which obviously are possible only at very low rates of deposition. The Mg, B and, probably also Mn contents in the clay minerals increase with increasing distance from the continent. This can be explained by the higher adsorption of those elements from sea water because of increasing duration of the clay mineral transport. The comparison of median contents of some elements in our deep-sea samples with deep-sea sediments described by TUREKIAN & WEDEPOHL (1961) shows that clear differences in concentration exist only in the case of "bauxitophile" elements Cr and Be. The Cr and Be contents show a clear increase in the Indian Ocean deep-sea samples compared to those described by TUREKIAn & WEDEPOHL (1961) which can obviously be attributed to the enrichment in the lateritic and bauxitic parent rocks. The different behaviour of the elements Fe, Ti and Mn during decomposition of the source rocks, transport to the sea and during oxydizing and reducing conditions in the marine environment can be illustrated by Ti02/Fe and MnO/Fe ratios. The different compositions of the sediments off the Indus Delta and those of the remaining part of the area investigated are characterized by a different distribution of the elements Mn and Ti. 2. Chemical inhomogenities in the sediments Most longer cores show 3 intervals defined by chemical and sedimentological differences. The top-most interval is coarse-grained, the intermedial interval is fine grained and the lower one again somewhat coarser. At the same time it is possible to observe differences from interval to interval in the organogenic and detrital constituents. During the formation of the middle interval different conditions of sedimentation from those active during the previous and subsequent periods have obviously prevailed. Looking more closely at the organogenic constituents it is remarkable that during the formation of the finer interval conditions of a more intensive oxydation have prevailed that was the case before and after: Core decreases, whereas P shows a relative increase. This may be explained by slower sedimentation rate or by a vertical migration of the oxygen rich zone of the sea-water. The modifications of the elements from minerals in detrital portion of the sediments support an explanation ascribing this fact to modifications of the conditions of denudation and transportation which can come about through a climatic change or through tectonic causes. The paleontological investigations have shown (ZOBEL, in press) that in some of the cores the middle stratum of fine sedimentation represents optimal conditions for organic life. This fact suggests also oxydizing conditions during the sedimentation of this interval. In addition to the depositional stratification an oxydation zone characterized by Mn-enrichment can be recognized. The thickness of the oxidation zone decreases towards the coast and thins out along the middle part of the continental slope. At those places, where the oxydation zone is extremely thin, enrichment of Mn has its maximum. This phenomenon can probably be attributed to the migration of Mn taking place in its dissociated form within the sediment under reducing conditions. On the other side this Mn-migration in the sediment does not take place in the deep-sea, where oxydizing conditions prevail. 3. Interstitial waters in the sediments Already at very small core depths, the interstitial waters have undergone a distinct modification compared with the overlying sea water. This distinct modification applies both to total salinity and to the individual ions. As to the beginning of diagenesis the following conclusions can be drawn: a) A strong K-increase occurs already at an early stage. It may be attributable to a diffusion barrier or to an exchange of Mg-ions on the clays. Part of this increase may also originate from the decomposition of K-containing silicates (mica and feldspars). A K-decrease owing to the formation of illite (WEAVER 1967), however, occurs only at much greater sediment depth. b) Because of an organic protective coating, the dissolution of carbonate is delayed in recent organogenic carbonates. At the same time some Ca is probably being adsorbed on clay minerals. Consequently the Ca-content of the interstitial water drops below the Ca-content of the sea water. c) Already at an early stage the Mg adsorption on the clays is completed. The adsorbed Mg is later available for diagenetic mineral formations and transformations.

Structural group composition of low-polarity organic matter dissolved in waters of the Northwest Indian Ocean and the Red Sea

Data on amounts of various functional groups, i.e. aldehyde, acid, ester, alcohol, thiol and aromatic groups in several fractions of low-polarity dissolved organic matter are presented. An assumption that this organic matter is part of the lipid fraction is not confirmed. Amount of aromatic compounds in waters of the Northwest Indian Ocean is estimated to be about 1000 times higher than quantity of aromatic hydrocarbons discharged into the ocean each year in petroleum and petroleum products.

Description and illustrations of Mn deposits from the John Murray Expedition (R/V Mabahiss)

During the cruise of the" Mabahiss" from Zanzibar to Colombo at Station 133 (1° 25' 54" S. to 1° 19' 42" S. and 66° 34' 12" E. to 66° 35' 18" E.) several small rock fragments were brought up in the Monegasque net; and, since at this position there is no possibility of the material being transferred by floating Ice, these specimens are of some interest as samples of oceanic rock foundations. All the rocks have a black appearance, but in the majority this skin is of negligible thickness. Exceptionally, however, it may attain to 1/3 in. (St. 133, 8), and then the specimens are rounded. The coating is made of dark opaque manganese material. At Station 166 one or two similar specimens of angular basalt were found in the trawl consisting mainly of manganese nodules.

Description and chemical composition of manganese nodules sampled during the R/V "Vityaz" Expedition Vityaz-35

This paper presents data on the chemical composition of iron-manganese nodules and associated sediments collected during the 35th voyage of the R/V "Vityaz" in 1962. The samples were made available to the author by Prof, P. L. Bezrukov. Data on the general distribution of manganese nodules at the bottom of the Indian Ocean were already given by P. L. Bezrukov (1962, 1963). Here the author analyzed the geochemistry of nodules samples from seven stations and four samples from the associated sediments. The analysis separates the outer layer of nodules from their apparent internal core.

Analysis of ferromanganiferous sedimentary rocks from Timor

Seven manganese nodules, eight ferromanganiferous shales from the Cretaceous Wai Bua Formation of Timor, and a pelagic limestone with four ferromanganese enriched layers from the Middle Eocene of Timor have been analysed. The nodules are compared with modern deep-sea nodules, and the ferromanganiferous shales are contrasted with relatively shallow marine manganiferous shales. The conclusion is reached that these rocks from Timor were probably deposited in a bathypelagic environment. There is a total absence of any indication that volcanic material has contributed to these deposits. The chemical composition of the ferromanganiferous rocks are discussed and some indications of biogenic influences are noted. The Middle Eocene pelagic limestone is compared with a similar modern sediment described from the Easter Island Rise in the Pacific.

Seasonal changes in labile organic matter composition on the continental shelf and bathyal sediments of the Cretan Sea

Bacterial abundance, biomass and cell size were studied in the oligotrophic sediments of the Cretan Sea (Eastern Mediterranean), in order to investigate their response to the seasonal varying organic matter (OM) inputs. Sediment samples were collected on a seasonal basis along a transect of seven stations (ranging from 40 to 1570 m depth) using a multiple-corer. Bacterial parameters were related to changes in chloroplastic pigment equivalents (CPE), the biochemical composition (proteins, lipids, carbohydrates) of the sedimentary organic matter and the OM flux measured at a fixed station over the deep basin (1570 m depth). The sediments of the Cretan Sea represent a nutrient depleted ecosystem characterised by a poor quality organic matter. All sedimentary organic compounds were found to vary seasonally, and changes were more evident on the continental shelf than in deeper sediments. Bacterial abundance and biomass in the sediments of the Cretan Sea (ranging from 1.02 to 4.59 * 10**8 cells/g equivalent to 8.7 and 38.7 µgC/g) were quite high and their distribution appeared to be closely related to the input of fresh organic material. Bacterial abundance and biomass were sensitive to changes in nutrient availability, which also controls the average cell size and the frequency of dividing cells. Bacterial abundance increased up to 3-fold between August '94 and February '95 in response to the increased amount of sedimentary proteins and CPE, indicating that benthic bacteria were constrained more by changes in quality rather than the quantity of the sedimentary organic material. Bacterial responses to the food inputs were clearly detectable down to 10 cm depth. The distribution of labile organic compounds in the sediments appeared to influence the vertical patterns of bacterial abundance and biomass. Cell size decreased significantly with water depth. Bacterial abundance and biomass were characterised by clear seasonal changes in response to seasonal OM pulses. The strong coupling between protein flux and bacterial biomass together with the strong bacterial dominance over the total biomass suggest that the major part of the carbon flow was channelled through the bacteria and the benthic microbial loop.

Ice-rafted debris in sites of ODP Leg 114

All holes drilled during Leg 114 contained ice-rafted debris. Analysis of samples from Hole 699A, Site 701, and Hole 704A yielded a nearly complete history of ice-rafting episodes. The first influx of ice-rafted debris at Site 699, on the northeastern slope of the Northeast Georgia Rise, occurred at a depth of 69.94 m below seafloor (mbsf) in sediments of early Miocene age (23.54 Ma). This material is of the same type as later ice-rafted debris, but represents only a small percentage of the coarse fraction. Significant ice-rafting episodes occurred during Chron 5. Minor amounts of ice-rafted debris first reached Site 701, on the western flank of the Mid-Atlantic Ridge (8.78 Ma at 200.92 mbsf), and more arrived in the late Miocene (5.88 Ma). The first significant quantity of sand and gravel appeared at a depth of 107.76 mbsf (4.42 Ma). Site 704, on the southern part of the Meteor Rise, received very little or no ice-rafted debris prior to 2.46 Ma. At this time, however, the greatest influx of ice-rafted debris occurred at this site. This time of maximum ice rafting correlates reasonably well with influxes of ice-rafted debris at Sites 701 (2.24 Ma) and 699 (2.38 Ma), in consideration of sample spacing at these two sites. These peaks of ice rafting may be Sirius till equivalents, if the proposed Pliocene age of Sirius tills can be confirmed. After about 1.67 Ma, the apparent mass-accumulation rate of the sediments at Site 704 declined, but with major fluctuations. This decline may be the result of a decrease in the rate of delivery of detritus from Antarctica due to reduced erosive power of the glaciers or a northward shift in the Polar Front Zone, a change in the path taken by the icebergs, or any combination of these factors.

Description and chemical composition of manganese nodules during R/V "Tyro" cruise APNAP 1 to the Madeira Abyssal Plain in 1986

The porewater and sediment composition of two boxcores and of a small gravity core, taken on a manganese-nodule-covered hill and in the Madeira Abyssal Plain proper respectively, are compared. The pore-water study of the two boxcores indicates that oxic conditions prevail in both cores. In addition, it indicates that no detectable fluxes of Mn or Fe occur from the porewater to the ocean bottom water. Variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate, which acts as a diluting agent. A clear carbonate minimum is observed at 20-22 cm depth in the two cores. This minimum is likely to be associated with the last glacial period (10-20 kyr B.P.). This association is supported by the sediment accumulation rate of 15 mm/kyr as found by extrapolation from the rate for pelagic sediments in the Madeira Abyssal Plain. The bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of Atlantic nodules. The trace metal and Rare Earth Elements composition indicate a hydrogenous origin for the manganese nodules of this study. On the basis of the chemical composition, and that of nodules relative to that of the adjacent sediments, an average nodule accretian rate of 2.8-3.3 mm/myr has been calculated. Although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition, internal structures of the nodules reveal great inhomogeneity, both visually and chemically. These fluctuations may be related to variations in the fluxes of Mn and Fe, which in turn could be climate-related.

Description and chemical composition of ferromanganese nodules from Station 123, Oneida Lake, New York

226Ra is used to document the growth histories of six manganese nodules from Oneida Lake, New York. Detailed sectioning and analysis reveal that there are discontinuous gradients in 226Ra content in these samples. These gradients result from periods of rapid growth (>1 mm/100 years) separated by periods of no growth of erosion. Although the 226Ra 'age' of the nodules approximates the age of Oneida Lake, the nodules are not sediment-covered because they occur only in areas of the lake where fine-grained sediments are not accumulating.

Concentration of the rare-earth elements in metalliferous sediments from DSDP Leg 92 holes

DSDP Leg 92 drilled at four sites along an east-west transect at 19°S on the western flank of the East Pacific Rise (EPR), in an area where sediments are essentially a mixture of hydrothermal and biogenic components, with only a minimal contribution of clastic material. Rare-earth element (REE) data on the metalliferous (non-carbonate) fraction of samples ranging in age from ~2 to ~27 Ma indicate the existence of two distinct groups of patterns corresponding to two broad age groups, one <=8 Ma, the other >=10 Ma. Within each group, REE patterns have characteristics which are near-uniform, despite large variations in total REE abundances. Sediments of the younger group are enriched in light REE (LREE) relative to deep bottom waters influenced by the hydrothermal plume extending west from the EPR at 19°S. Sediments of the older groups show further relative LREE enrichment and/or heavy REE (HREE) depletion. Surficial sediments deposited beneath the lysocline have high Sum REE concentrations resulting from slow accumulation rates, and patterns resembling older sediments due to early diagenetic effects. A correlation between the mass accumulation rates (MAR) of Sum REE and Fe + Mn suggests that ferromanganese particulate matter supplied by the hydrothermal plume scavenges REE; during this process the LREE are preferentially removed from plume seawater. The MAR of Fe + Mn shows a general decrease with age above basement, whereas Sum REE concentrations in the metalliferous component increase with age above basement. This supports the Ruhlin and Owen model wherein limited scavenging of REE, due to rapid burial of sediment near the palaeo-axis, leads to low concentrations (but high MAR-values) for the REE. Following deposition and burial of the hydrothermal component, further relative flattening of the REE pattern takes place, probably the result of diagenetic reactions over several million years. Phase partitioning data indicate that the proportion of REE residing in more poorly crystalline phases tends to increase with age (from ~45% to 90% of Sum REE). This suggests that as initial ferromanganese precipitates undergo diagenetic recrystallization, REE are transferred to the poorly crystalline phases, and/or are scavenged from pore waters by these phases. Because of the various modifications to REE patterns apparently produced both in the water column and post-depositional settings, the REE patterns of metalliferous sediments will not reflect fine-scale REE variations in associated oceanic water masses.

Distribution of marine litter at bathyal and abyssal depths in the Mediterranean Sea

The distribution, type and quantity of marine litter accumulated on the bathyal and abyssal Mediterranean seafloor has been studied in the framework of the Spanish national projects PROMETEO and DOS MARES and the ESF-EuroDEEP project BIOFUN. Litter was collected with an otter trawl and Agassiz trawl while sampling for megafauna on the Blanes canyon and adjacent slope (Catalan margin, north-western Mediterranean) between 900 and 2700 m depth, and on the western, central and eastern Mediterranean basins at 1200, 2000 and 3000 m depth. All litter was sorted into 8 categories (hard plastic, soft plastic, glass, metal, clinker, fabric, longlines and fishing nets) and weighed. The distribution of litter was analysed in relation to depth, geographic area and natural (bathymetry, currents and rivers) and anthropogenic (population density and shipping routes) processes. The most abundant litter types were plastic, glass, metal and clinker. Lost or discarded fishing gear was also commonly found. On the Catalan margin, although the data indicated an accumulation of litter with increasing depth, mean weight was not significantly different between depths or between the open slope and the canyon. We propose that litter accumulated in the canyon, with high proportions of plastics, has predominantly a coastal origin, while litter collected on the open slope, dominated by heavy litter, is mostly ship-originated, especially at sites under major shipping routes. Along the trans-Mediterranean transect, although a higher amount of litter seemed to be found on the Western Mediterranean, differences of mean weight were not significant between the 3 geographic areas and the 3 depths. Here, the shallower sites, also closer to the coast, had a higher proportion of plastics than the deeper sites, which had a higher proportion of heavy litter and were often affected by shipping routes. The weight of litter was also compared to biomass of megafauna from the same samples. On the Blanes slope, the biomass of megafauna was significantly higher than the weight of litter between 900 and 2000 m depth and no significant differences were found at 2250 and 2700 m depth. Along the trans-Mediterranean transect, no significant differences were found between biomass and litter weight at all sites except in two sites: the Central Mediterranean at 1200 m depth, where biomass was higher than litter weight, and the Eastern Mediterranean at 1200 m depth, where litter weight was higher than biomass. The results are discussed in the framework of knowledge on marine litter accumulation, its potential impact on the habitat and fauna and the legislation addressing these issues.

Detailed description, geochemistry and radioactivity of manganese nodules taken during the R/V Vityaz cruise to the Indian Ocean

During the 33th voyage of the R/V "Vityaz" in the Indian Ocean iron-manganese nodules were collected at several stations. Both nodules and associated sediments were analysed by spectral analysis over 30 chemical elements. Radioactivity measurements were also performed on these samples.

Description and geochemical composition of manganese nodules retrieved from Lake Ontario, Canada

A large deposit of ferromanganese oxide coated sands and scattered manganese nodules occurs in the northern portion of Lake Ontario. The Mn and Fe contents of the concretions are similar to those in concretions from other environments, while their Ni, Cu, and Co contents are lower than in deep-sea nodules, but higher than in most previously described lacustrine concretions. Pb and Zn are high in the coatings and exceed the concentrations found in many previously analyzed Mn deposits. Within the deposit, Mn, Ni, Co, and Zn contents are correlated, and they vary inversely with Fe. Mn, Fe, Ni, Cu, and Pb are present in the interstitial waters of the sediments underlying the deposit in higher concentrations than in the overlying lake waters, thus providing a potential source of metals for concretion formation.The origin and compositional variations in the deposit possibly can be explained in terms of the fractionation and precipitation of Fe and Mn as a result of redox variations in the lake sediments. Eh increases from south to north across the deposit in such a way that iron may be selectively oxidized and precipitated in the south and manganese, in the north. The upward diffusion of Mn, Fe, and associated elements from the underlying sediments probably provides the principal source of the metals in the south of the deposit, while metal-enriched bottom waters are probably the principal source in the north.