Stable oxygen isotope record and lithogenic composition of ODP Hole 117-722B

The upper 38 m of Hole 722B sediments (Owen Ridge, northwest Arabian Sea) was sampled at 20 cm intervals and used to develop records of lithogenic percent, mass accumulation rate, and grain size spanning the past 1 m.y. Over this interval, the lithogenic component of Owen Ridge sediments can be used to infer variability in the strength of Arabian Sea summer monsoon winds (median grain size) and the aridity of surrounding dust source-areas (mass accumulation rate; MAR in g/cm**2/k.y). The lithogenic MAR has strong 100, 41, and 23 k.y. cyclicities and is forced primarily by changes in source-area aridity associated with glacial-interglacial cycles. The lithogenic grain size, on the other hand, exhibits higher frequency variability (23 k.y.) and is forced by the strength of summer monsoon winds which, in turn, are forced by the effective sensible heating of the Indian-Asian landmass and by the availability of latent heat from the Southern Hemisphere Indian Ocean. These forcing mechanisms combine to produce a wind-strength record which has no strong relationship to glacial-interglacial cycles. Discussion of the mechanisms responsible for production of primary Milankovitch cyclicities in lithogenic records from the Owen Ridge is presented elsewhere (Clemens and Prell, 1990, doi:10.1029/PA005i002p00109). Here we examine the 1 m.y. record from Hole 722B focusing on different aspects of the lithogenic components including an abrupt change in the monsoon wind-strength record at 500 k.y., core-to-core reproducibility, comparison with magnetic susceptibility, coherency with a wind-strength record from the Pacific Ocean, and combination frequencies in the wind-strength record. The Hole 722B lithogenic grain-size record shows an abrupt change at 500 k.y. possibly indicating decreased monsoon wind-strength over the interval from 500 k.y. to present. The grain-size decrease appears to be coincident with a loss of spectral power near the 41 k.y. periodicity. However, the grain-size decrease is not paralleled in the Globigerina bulloides upwelling record, an independent record of summer monsoon wind-strength (Prell, this volume). These observations leave us with competing hypotheses possibly involving: (1) a decrease in the sensitivity of monsoon windstrength to obliquity forcing, (2) decoupling of the grain size and G. bulloides records via a decoupling of the nutrient supply from wind-driven upwelling, and/or (3) a change in dust source-area or the patterns of dust transporting winds. Comparison of the lithogenic grain size and weight percent records from Hole 722B with those from a nearby core shows that the major and most minor events are well replicated. These close matches establish our confidence in the lithogenic extraction techniques and measurements. Further, reproducibility on a core-to-core scale indicates that the eolian depositional signal is regionally strong, coherent, and well preserved. The lithogenic weight percent and magnetic susceptibility are extremely well correlated in both the time and frequency domains. From this we infer that the magnetically susceptible component of Owen Ridge sediments is of terrestrial origin and transported to the Owen Ridge via summer monsoon winds. Because of the high correlation with the lithogenic percent record, the magnetic susceptibility record can be cast in terms of lithogenic MAR and used as a high resolution proxy for continental aridity. In addition to primary Milankovitch periodicities, the Hole 722B grain-size record exhibits periodicity at 52 k.y. and at 29 k.y. Both periodicities are also found in the grain-size record from piston core RC11-210 in the equatorial Pacific Ocean. Comparison of the two grain-size records shows significant coherence and zero phase relationships over both the 52 and 29 k.y. periodicities suggesting that the strengths of the Indian Ocean monsoon and the Pacific southeasterly trade winds share common forcing mechanisms. Two possible origins for the 52 and 29 k.y. periodicities in the Hole 722B wind-strength record are (1) direct Milankovitch forcing (54 and 29 k.y. components of obliquity) and (2) combination periodicities resulting from nonlinear interactions within the climate system. We find that the 52 and 29 k.y. periodicities show stronger coherency with crossproducts of eccentricity and obliquity (29 k.y.) and precession and obliquity (52 k.y.) than with direct obliquity forcing. Our working hypothesis attributes these periodicities to nonlinear interaction between external insolation forcing and internal climatic feedback mechanisms involving an interdependence of continental snow/ice-mass (albedo) and the hydrological cycle (latent heat availability).

Bacterial biomasses and activities in the Northwest African upwelling region

During the 'Meteor' expedition SUBTROPEX '82, sediment samples were taken at 14 stations in different water depths at 35, 29, 25, 21 and 17 °N, and measurements of bacterial biomasses and activities were carried out in these different upwelling-intensity areas. Highest densities and biomasses by AODC (2.2 x 10**8 cells, corresponding to 14.8 µg C/g sediment dry wt) were recorded at 21 °N, year-round upwelling, at 1200 and 800 m, but at 500 m biomass was still 4.3 µg C/g dry wt. Relatively high densities and biomasses (6.5 and 6.8 µg C/g dry wt) were found at 17 °N, upwelling mostly in winter and spring, at 1200 and 800 m. AODC were 2 to 3 orders of magnitude higher than viable counts, incubation at 2 or 20 °C. For deep-water sediments, counts at 2 °C were higher than at 20 °C. Biomass and ATP concentrations were highest in the 0 to 2 cm sediment layers; they decreased with sediment depth. Bacterial biomasses were correlated with organic carbon and ATP concentrations. The fractions of Bacterial ATP were calculated to be 2 to 24% of ATP-biomass. On the basis of organic carbon, however, fractions of Bacterial Organic Carbon were only 0.02 to 0.06%. For microbial communities, the conversion factor 0.004 for BOC to BATP seems 2 orders of magnitude too high. Maximum AEC ratios of 0.53 to 0.70 were found at 21 and 17 °N; the other stations had AEC ratios of 0.21 to 0.47. Numbers of bacteria with respiratory ETS were between 0.5 and 10.5 % of AODC. An exception was the shelf station at 35 °N with 34.2% of AODC.

(Table 1) Summary of horizon, lithology, and paleomagnetic of ODP Hole 126-791B basaltic mousse samples

A paleomagnetic study was made on the highly vesiculated basaltic tuff breccia (the basaltic mousse) drilled by Ocean Drilling Program Leg 126 from the Izu-Bonin backarc, Sumisu Rift, to estimate the mode of its emplacement. Thirty-four 10-cm**3 minicore samples were collected from almost all the horizons of the basaltic mousse. Stepwise thermal and alternating-field demagnetization experiments show that the natural remanent magnetizations of many samples are mainly composed of a single stable component. Although remanence inclinations are not expected to be disturbed by rotary drilling, the measured inclinations of remanence show a random directional distribution as a whole. The remanence inclinations, however, show directional consistency on a smaller scale. High-density sampling and measurements from a limited interval of drilled cores, and the measurement of small disks cut from a single minicore sample show that there is directional consistency over several centimeters. Strong and stable remanent magnetization, the existence of remanence direction consistency, and the fresh lithology of the samples suggest the thermal origin of remanence. Combining the paleomagnetic results with other geological, petrographical, and paleontological characters, the Hole 791B basaltic mousse can be interpreted as a subaqueous explosion breccia produced by deep-sea pyroclastic fountaining.

Oxygen isotopes and hydrocarbon composition of gas hydrate and hydrate water from ODP Leg 165 sites

Ocean Drilling Program (ODP) Leg 164 recovered a number of large solid gas hydrate from Sites 994, 996, and 997 on the Blake Ridge. Sites 994 and 997 samples, either nodular or thick massive pieces, were subjected to laboratory analysis and measurements to determine the structure, molecular and isotopic composition, thermal conductivity, and equilibrium dissociation conditions. X-ray computed tomography (CT) imagery, X-ray diffraction, nuclear magnetic resonance (NMR), and Raman spectroscopy have revealed that the gas hydrates recovered from the Blake Ridge are nearly 100% methane gas hydrate of Structure I, cubic with a lattice constant of a = 11.95 ± 0.05 angström, and a molar ratio of water to gas (hydration number) of 6.2. The d18O of water is 2.67 per mil to 3.51 per mil SMOW, which is 3.5-4.0 heavier than the ambient interstitial waters. The d13C and dD of methane are -66 per mil to -70 per mil and -201 per mil to -206 per mil, respectively, suggesting that the methane was generated through bacterial CO2 reduction. Thermal conductivity values of the Blake Ridge hydrates range from 0.3 to 0.5 W/(m K). Equilibrium dissociation experiments indicate that the three-phase equilibrium for the specimen is 3.27 MPa at 274.7 K. This is almost identical to that of synthetic pure methane hydrate in freshwater.

High resolved snow height measurements at Neumayer Station, Antarctica, 2013

Antarctica is a continent with a strong character. High wind speeds, very low temperatures and heavy snow storms. All these parameters are well known due to observations and measurements, but precipitation measurements are still rare because the number of manned stations is very limited in Antarctica. In such a polar snow region many wind driven phenomena associated with snow fall exist like snow drift, blowing snow or sastrugi. Snow drift is defined as a layer of snow formed by the wind during a snowstorm. The horizontal visibility is below eye level. Blowing snow is specified as an ensemble of snow particles raised by the wind to moderate or great heights above the ground; the horizontal visibility at eye level is generally very poor (National Snow And Ice Data Center (NSIDC), 2013). Sastrugi are complex, fragile and sharp ridges or grooves formed on land or over sea ice. They arise from wind erosion, saltation of snow particles and deposition. To get more details about these procedures better instruments than the conventional stake array are required. This small report introduces a new measuring technique and therefore offers a never used dataset of snow heights. It is very common to measure the snow height with a stake array in Antarctica (f.e. Neumayer Station, Kohnen Station) but not with a laser beam. Thus the idea was born to install a new instrument in December 2012 at Neumayer Station.

(Table 1) Strontium concentrations and isotope ratios of anhydrite from DSDP/ODP Hole 504B

In samples from 1575 to 1982 mbsf anhydrite leached from whole-rock powders. 87Sr/86Sr and Sr/Ca determined following methods described in (Teagle et al., 1996, doi:10.2973/odp.proc.sr.148.113.1996, Teagle et al., 1998, doi:10.2973/odp.proc.sr.158.223.1998). For the period of the analysis (Jan. 1994-Dec. 1995) NIST SRM 987 gave 87Sr/86Srs0.710244 +/- 0.000018 (2 sigma, n = 115). Full analytical procedural blanks were <50 pg for Sr. delta18O following (Pickthorn and O´Neil, 1985). Repeated extractions and measurements of samples and standards were reproducible within +/- 0.5.

Organic properties of sediments and amino acids in iterstitial waters of ODP Holes 111-677A and 111-678B

The basement at Ocean Drilling Program (ODP) Sites 677 and 678 originated from the Galapagos spreading center of the Costa Rica Rift and has moved about 200 km over the last 6 m.y. (Fig. 1) (Shipboard Scientific Party, 1987, 1988; Scientific Drilling Party, 1987). Sediments about 300 m thick cover basement so young that basal sediments at Sites 677 and 678 have been reheated up to 60?-70?C at Site 677 and altered to limestone and/or chert (Shipboard Scientific Party, 1988). Sediments from both sites indicate (1) a high sedimentation rate (about 48 m/m.y.) and (2) biogenic silica and carbonate as the main constituents of sediments (Table 1) (Shipboard Scientific Party, 1988). Heatflow observations and measurements of interstitial water chemistry around the sites show that Site 677 is in a lower heatflow zone (166 mW/m**2; 1°12.14'N, 83°44.22'W) whereas Site 678 is located in a zone of higher heat flow (250 mW/m**2; 1°13.01'N, 83°43.39'W) (Langseth et al., 1988; Shipboard Scientific Party, 1988). In the flank hydrothermal systems, circulating solution is moving upward through the sedimentary column in zones of higher heat flow while it is moving downward in zones of lower heat flow (Anderson and Skilbeck, 1981). The chemistry of the interstitial waters is modified by several processes such as (1) diagenetic reactions and (2) advective and (3) diffusive transports of dissolved constituents. Analyses of Ca2+ and Mg2+ in interstitial waters from Sites 677 and 678 show that their profiles are mainly controlled by advective transport (Shipboard Scientific Party, 1988). In contrast, the interstitial-water profiles for NH4+, Si, and PO4[3-] are highly affected by reactions in the sediments. Site 677 offers a good opportunity to investigate amino acids in the interstitial waters because sediments of similar compositions have been deposited at constant rates of sedimentation. There are few previous works on amino acid distributions in interstitial waters (Henrichs and Parrington, 1979; Michaelis et al., 1982; Henrichs et al., 1984; Henrichs and Farrington, 1987; Ishizuka et al., 1988). In this chapter, we report (1) Rock-Eval analysis and (2) the composition of total hydrolyzable and dissolved free amino acids (THAA and DFAA, respectively) in the interstitial waters. Our objectives are to discuss (1) the possible origin of organic materials, (2) the characteristics of THAA and DFAA, and (3) their relationships in interstitial waters.

Annotated record of the detailed examination of ferromanganese concretions from the Bothnia Gulf, the Baltic Sea and the Barents sea collected by Boström, K. and Ingri, J. (Technical University of Lulea)

The collection of ferromanganese nodules at Naturhistoriska Riksmuseet, Stockholm, Sweden has been donated by Pr. Boström, K. and Ingri, J. from the Technical University of Lulea. They have been collected in the Bothnia Gulf, the Baltic Sea anfd the Barents sea from 1976 until 1985. In 1997 it is was put to the care custody of the Laboratory for Isotope Geology (LIG) of NRM. As part of the Access Project at LIG, Curt Boman has gone through the collection and established a database with detailed information about the samples it contains. Ferromanganese nodules typically display a rounded shape and are formed by redox processes at the interface between the seabed sediment and water. In addition to iron and manganese they also contain other metal elements. Nodules chemical composition reflects the substances found in the sediment to which they are associated. Since the nodules grow continuously, they reflect changes in the sedimentary environment chemistry on a yearly basis, which makes them very interesting as environmental archives. The nodules can be found locally in large quantities and due to their metal content they are also economically interesting as a source of raw materials.

(Table 1) Temperature and foraminiferal Mg/Ca ratios of samples off Japan

We studied Mg/Ca in high-Mg, shallow-water benthic foraminifera in culture and in samples from natural environments, in order to evaluate the expression of latitudinal and seasonal temperature variability in Mg/Ca in their tests. We cultured Planoglabratella opercularis (d'Orbigny) and Quinqueloculina yabei Asano under controlled temperature (10°-25°C) and salinity (30-38) conditions. Both species show a linear correlation between Mg/Ca and temperature, but they differ in temperature sensitivity. Salinity does not significantly influence Mg/Ca. In the samples collected in nature, Mg/Ca and seawater temperatures are positively correlated, but there are more complexities than in the records for cultured specimens due to such factors as seasonal fluctuations in temperature. We conclude that Mg/Ca ratios in monospecific benthic foraminiferal samples may be used as a reliable temperature proxy, if the lifetime of the species is taken into account.

Geomicrobiology, bulk sediment characteristics and pore water chemistry of sediment cores from Laguna Potrok Aike, southern Patagonia

Authigenic minerals can form in the water column and sediments of lakes, either abiotically or mediated by biological activity. Such minerals have been used as paleosalinity and paleoproductivity indicators and reflect trophic state and early diagenetic conditions. They are also considered potential indicators of past and perhaps ongoing microbial activity within sediments. Authigenic concretions, including vivianite, were described in late glacial sediments of Laguna Potrok Aike, a maar lake in southernmost Argentina. Occurrence of iron phosphate implies specific phosphorus sorption behavior and a reducing environment, with methane present. Because organic matter content in these sediments was generally low during glacial times, there must have been alternative sources of phosphorus and biogenic methane. Identifying these sources can help define past trophic state of the lake and diagenetic processes in the sediments. We used scanning electron microscopy, phosphorus speciation in bulk sediment, pore water analyses, in situ ATP measurements, microbial cell counts, and measurements of methane content and its carbon isotope composition (d13C CH4) to identify components of and processes in the sediment. The multiple approaches indicated that volcanic materials in the catchment are important suppliers of iron, sulfur and phosphorus. These elements influence primary productivity and play a role in microbial metabolism during early diagenesis. Authigenic processes led to the formation of pyrite framboids and revealed sulfate reduction. Anaerobic oxidation of methane and shifts in pore water ion concentration indicated microbial influence with depth. This study documents the presence of active microbes within the sediments and their relationship to changing environmental conditions. It also illustrates the substantial role played by microbes in the formation of Laguna Potrok Aike concretions. Thus, authigenic minerals can be used as biosignatures in these late Pleistocene maar sediments.

Neogene planktonic foraminifera of ODP Site 130-806

The Neogene biostratigraphy presented here is based on the study of 230 samples through 737 m of pelagic sediment in Hole 806B. Sediment accumulation is interrupted only once in the uppermost lower Miocene (Zone N6), apparently coincident with a widespread deep-sea hiatus. Preservation of planktonic foraminifers through the section ranges from good to moderately poor. One hundred and ten species of planktonic foraminifers were identified; taxonomic notes on most species are included. All of the standard low-latitude Neogene foraminiferal zones are delineated, with the exceptions of Zones N8 and N9 because of a high first occurrence of Orbulina, and Zones N18 and N19 because of a high first occurrence of Sphaeroidinella dehiscens. Good agreement exists between the published account of the variation in planktonic foraminiferal species richness and the rates of diversification and turnover, and measurements of these evolutionary indexes in the record of Hole 806B. The global pattern of change in tropical/transitional species richness is paralleled in Hole 806B, with departures caused by either ecological conditions peculiar to the western equatorial Pacific or by inexactness in the estimation of million-year intervals in Hole 806B. Temporal changes in the relative abundance of taxa in the sediment assemblages, considered in light of their depth habitats, reveal a detailed picture of historical change in the structure of the upper water column over the Ontong Java Plateau. The dominance of surface dwellers (Paragloborotalia kugleri, P. mayeri, Dentoglobigerina altispira, Globigerinita glutinata, and Globigerinoides spp.) throughout the lower and middle Miocene is replaced by a more equitable distribution of surface (D. altispira and Globigerinoides spp.), intermediate (Globorotalia menardii plexus), and deep (Streptochilus spp.) dwellers in the late Miocene, following the closing of the Indo-Pacific Seaway and the initiation of large-scale glaciation in the Antarctic. The shoaling of the thermocline along the equator engendered by these climatic and tectonic events persisted through the Pliocene, when initial increases in the abundance of a new set of shallow, intermediate, and deep dwelling species of planktonic foraminifers coincide with the closing of the Panamanian Seaway.

Sediment facies of the Indian-Pakistan continental margin, Indian Ocean Expedition

The studies described here base mainly on sedimentary material collected during the "Indian Ocean Expedition" of the German research vessel "Meteor" in the region of the Indian-Pakistan continental margin in February and March 1965. Moreover,samples from the mouth of the Indus-River were available, which were collected by the Pakistan fishing vessel "Machhera" in March 1965. Altogether, the following quantities of sedimentary material were collected: 59.73 m piston cores. 54.52 m gravity cores. 33 box grab samples. 68 bottom grab samples Component analyses of the coarse fraction were made of these samples and the sedimentary fabric was examined. Moreover, the CaCO3 and Corg contents were discussed. From these investigations the following history of sedimentation can be derived: Recent sedimentation on the shelf is mainly characterized by hydrodynamic processes and terrigenous supply of material. In the shallow water wave action and currents running parallel to the coast, imply a repeated reworking which induces a sorting of the grains and layering of the sediments as well as a lack of bioturbation. The sedimentation rate is very high here. From the coast-line down to appr. 50 m the sediment becomes progressively finer, the conditions of deposition become less turbulent. On the outer shelf the sediment is again considerably coarser. It contains many relicts of planktonic organisms and it shows traces of burrowing. Indications for redeposition are nearly missing, a considerable part of the fine fraction of the sediments is, however, whirled up and carried away. In wide areas of the outer shelf this stirring has gained such a degree that recent deposits are nearly completely missing. Here, coarse relict sands rich in ooids are exposed, which were formed in very shallow stirred water during the time when the sea reached its lowest level, i.e. at the turn of the Pleistocene to the Holocene. Below the relict sand white, very fine-grained aragonite mud was found at one location (core 228). This aragonite mud was obviously deposited in very calm water of some greater depth, possibly behind a reef barrier. Biochemic carbonate precipitation played an important part in the formation of relict sands and aragonite muds. In postglacial times the relict sands were exposed for long periods to violent wave action and to areal erosion. In the present days they are gradually covered by recent sediments proceeding from the sides. On the continental margin beyond the shelf edge the distribution of the sediments is to a considerable extent determined by the morphology of the sea bottom. The material originating from the continent and/or the shelf, is less transported by action of the water than by the force of gravity. Within the range of the uppermost part of the continental slope recent sedimentation reaches its maximum. Here the fine material is deposited which has been whirled up in the zone of the relict sands. A laminated fine-grained sediment is formed here due to the very high sedimentation rate as well as to the extremely low O2-content in the bottom water, which prevents life on the bottom of the sea and impedes thus also bioturbation. The lamination probaly reflects annual variation in deposition and can be attributed to the rhythm of the monsoon with its effects on the water and the weather conditions. In the lower part of the upper continental slope sediments are to be found which show in varying intensity, intercalations of fine material (silt) from the shelf, in large sections of the core. These fine intercalations of allochthonous material are closely related to the autochthonous normal sediment, so that a great number of small individual depositional processes can be inferred. In general the intercalations are missing in the uppermost part of the cores; in the lower part they can be met in different quantities, and they reach their maximum frequency in the upper part of the lower core section. The depositions described here were designated as turbid layer sediments, since they get their material from turbid layers, which transport components to the continental slope which have been whirled up from the shelf. Turbidites are missing in this zone. Since the whole upper continental slope shows a low oxygen-content of the bottom water the structure of the turbid layer sediments is more or less preserved. The lenticular-phacoidal fine structure does, however, not reflect annual rhythms, but sporadic individual events, as e.g. tsunamis. At the lower part of the continental slope and on the continental rise the majority of turbidites was deposited, which, during glacial times and particularly at the beginning of the post-glacial period, transported material from the zone of relict sands. The Laccadive Ridge represented a natural obstacle for the transport of suspended sediments into the deep sea. Core SIC-181 from the Arabian Basin shows some intercalations of turbidites; their material, however, does not originate from the Indian Shelf, but from the Laccadive Ridge. Within the range of the Indus Cone it is surprising that distinct turbidites are nearly completely missing; on the other hand, turbid layer sediments are to be found. The bottom of the sea is showing still a slight slope here, so that the turbidites funneled through the Canyon of the Swatch probably rush down to greater water depths. Due to the particularly large supply of suspended material by theIndus River the turbid layer sediments show farther extension than in other regions. In general the terrigenous components are concentrated on the Indus Cone. It is within the range of the lower continental slope that the only discovery of a sliding mass (core 186) has been located. It can be assumed that this was set in motion during the Holocene. During the period of time discussed here the following development of kind and intensity of the deposition of allochthonous material can be observed on the Indian-Pakistan continental margin: At the time of the lowest sea level the shelf was only very narrow, and the zone in which bottom currents were able to stir up material by oscillating motion, was considerably confined. The rivers flowed into the sea near to the edge of the shelf. For this reason the percentage of terrigenous material, quartz and mica is higher in the lower part of many cores (e.g. cores 210 and 219) than in the upper part. The transition from glacial to postglacial times caused a series of environmental changes. Among them the rise of the sea level (in the area of investigation appr. 150 m) had the most important influence on the sedimentation process. In connection with this event many river valleys became canyons, which sucked sedimentary material away from the shelf and transported it in form of turbidites into the deep sea. During the rise of the sea level a situation can be expected with a maximum area of the comparatively plane shelf being exposed to wave action. During this time the process of stirring up of sediments and formation of turbid layers will reach a maximum. Accordingly, the formation of turbidites and turbid layer sediments are most frequent at the same time. This happened in general in the older polstglacial period. The present day high water level results in a reduced supply of sediments into the canyons. The stirring up of sediments from the shelf by wave action is restricted to the finest material. The missing of shelf material in the uppermost core sections can thus be explained. The laminated muds reflect these calm sedimentation conditions as well. In the southwestern part of the area of investigation fine volcanic glass was blown in during the Pleistocene, probably from the southeast. It has thus become possible to correlate the cores 181, 182, 202. Eolian dust from the Indian subcontinent represents probably an important component of the deep sea sediments. The chemism of the bottom as well as of the pore water has a considerable influence on the development of the sediments. Of particular importance in this connection is a layer with a minimum content of oxygen in the sea water (200-1500 m), which today touches the upper part of the continental slope. Above and beyond this oxygen minimum layer somewhat higher O2-values are to be observed at the sea bottom. During the Pleistocene the oxygen minimum layer has obviously been locatedin greater depth as is indicated by the facies of laminated mud occuring in the lower part of core 219. The type of bioturbation is mainly determined by the chemism. Moreover, the chemism is responsible for a considerable selective dissolution, either complete or partial, of the sedimentary components. Within the range of the oxygen minimum layer an alkaline milieu is developed at the bottom. This causes a complete or partial dissolution of the siliceous organisms. Here, bioturbation is in general completely missing; sometimes small pyrite-filled burrowing racks are found. In the areas rich in O2 high pH-values result in a partial dissolution of the calcareous shells. Large, non-pyritized burrowing tracks characterize the type of bioturbation in this environment. A study of the "lebensspuren" in the cores supports the assumption that, particularly within the region of the Laccadive Basin, the oxygen content in the bottom sediments was lower than during the Holocene. This may be attributed to a high sedimentation rate and to a lower O2-content of the bottom water. The composition of the allochthonous sedimentary components, detritus and/or volcanic glass may locally change the chemism to a considerable extent for a certain time; under such special circumstances the type of bioturbation and the state of preservation of the components may be different from those of the normal sediment.

Morphological plasticity of the coral skeleton under CO2-driven seawater acidification

Ocean acidification causes corals to calcify at reduced rates, but current understanding of the underlying processes is limited. Here, we conduct a mechanistic study into how seawater acidification alters skeletal growth of the coral Stylophora pistillata. Reductions in colony calcification rates are manifested as increases in skeletal porosity at lower pH, while linear extension of skeletons remains unchanged. Inspection of the microstructure of skeletons and measurements of pH at the site of calcification indicate that dissolution is not responsible for changes in skeletal porosity. Instead, changes occur by enlargement of corallite-calyxes and thinning of associated skeletal elements, constituting a modification in skeleton architecture. We also detect increases in the organic matrix protein content of skeletons formed under lower pH. Overall, our study reveals that seawater acidification not only causes decreases in calcification, but can also cause morphological change of the coral skeleton to a more porous and potentially fragile phenotype.