Changes in dynamic viscosity of sea water with depth in the Pacific Ocean at Stations DM9-613W and DM9-618W

To check on the assumption that metabolic products of planktonic organisms can affect the coefficient of dynamic viscosity of seawater, viscosity was measured in water samples taken from depths of 0 to 1843 m, west of the Hawaiian Islands. Obtained results showed that plankton has no effect on viscosity of water in regions of low productivity and that viscosity can be determined with high degree of accuracy from the appropriate tables.

Small Changes in pH Have Direct Effects on Marine Bacterial Community Composition: A Microcosm Approach

As the atmospheric CO2 concentration rises, more CO2 will dissolve in the oceans, leading to a reduction in pH. Effects of ocean acidification on bacterial communities have mainly been studied in biologically complex systems, in which indirect effects, mediated through food web interactions, come into play. These approaches come close to nature but suffer from low replication and neglect seasonality. To comprehensively investigate direct pH effects, we conducted highly-replicated laboratory acidification experiments with the natural bacterial community from Helgoland Roads (North Sea). Seasonal variability was accounted for by repeating the experiment four times (spring, summer, autumn, winter). Three dilution approaches were used to select for different ecological strategies, i.e. fast-growing or low-nutrient adapted bacteria. The pH levels investigated were in situ seawater pH (8.15-8.22), pH 7.82 and pH 7.67, representing the present-day situation and two acidification scenarios projected for the North Sea for the year 2100. In all seasons, both automated ribosomal intergenic spacer analysis and 16S ribosomal amplicon pyrosequencing revealed pH-dependent community shifts for two of the dilution approaches. Bacteria susceptible to changes in pH were different members of Gammaproteobacteria, Flavobacteriaceae, Rhodobacteraceae, Campylobacteraceae and further less abundant groups. Their specific response to reduced pH was often context-dependent. Bacterial abundance was not influenced by pH. Our findings suggest that already moderate changes in pH have the potential to cause compositional shifts, depending on the community assembly and environmental factors. By identifying pH-susceptible groups, this study provides insights for more directed, in-depth community analyses in large-scale and long-term experiments.

Chemical composition of plagioclase from DSDP Legs 45 and 46, Atlantic Ocean

Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal. It is proposed that the plagioclase cores formed during injection of primitive magma into a previously existing magma chamber, that the mantle formed during mixing of a partially mixed magma and the remaining magma already in the chamber, and that the inner rim formed when the mixed magma was in a sheeted dike system. The large plateau at the outside of the mantle may have formed during the injection of the next batch of primitive magma into the main chamber, which may trigger an eruption. This model is consistent with fluid dynamic calculations and geochemically based magma mixing models, and is suggested to be the major mechanism for generating the disequilibrium conditions in the magma.

Diversity and faunal composition of benthic foraminifera in Hole 44-390A on the Blake Nose Plateau

Benthic foraminiferal assemblages are a widespread tool to understand changes in organic matter flux and bottom-water oxygenation and their relation to paleoceanographic changes in the Upper Cretaceous oceans. In this study, assemblage data (diversity, total number, and number per species and gram) from Deep Sea Drilling Project (DSDP) Site 390 (Blake Nose, western North Atlantic) were processed for the lower Maastrichtian (Globotruncana falsostuarti - Gansserina gansseri Planktic Foraminiferal Zone). These data document significant changes in nutrient flux to the sea floor as well as bottom-water oxygenation during this time interval. Parallel to the observed changes in the benthic foraminiferal assemblages the number of inoceramid shells decreases, reflecting also a significant increase in bottom-water oxygenation. We speculate, that these data could reflect the onset of a shift from warmer low-latitude to cooler high-latitude deep-water sources. This speculation will predate the major reorganization of the oceanic circulation resulting in a circulation mode similar to today at the Early/Late Maastrichtian boundary by ~1 Ma and therefore improves our understanding of Late Cretaceous paleoceanography.

(Table 1) Calcium isotope composition, Mg/Ca, Sr/Ca and U/Ca ratios of calcareous cysts of Thoracosphaera heimii

We investigated the influences of temperature, salinity and pH on the calcium isotope as well as trace and minor element (uranium, strontium, magnesium) to Ca ratios on calcium carbonate cysts of the calcareous dinoflagellate species Thoracosphaera heimii grown in laboratory cultures. The natural habitat of this species is the photic zone (preferentially at the chlorophyll maximum depth) of temperate to tropical oceans, and it is abundant in deep-sea sediments over the entire Cenozoic. In our experiments, temperatures ranged from 12 to 30 °C, salinity from 36.5 to 38.8 and pH from 7.9 to 8.4. The delta44/40Ca of T. heimii cysts resembles that of other marine calcifiers, including coccolithophores, foraminifers and corals. However, its temperature sensitivity is considerably smaller and statistically insignificant, and T. heimii might serve as a recorder of changes in seawater delta44/40Ca over geologic time. The Sr/Ca ratios of T. heimii cysts show a pronounced temperature sensitivity (0.016 mmol/mol °C**-1) and have the potential to serve as a palaeo-sea surface temperature proxy. No clear temperature- and pH-dependences were observed for Mg/Ca. U/Ca seems to be influenced by temperature and pH, but the correlations change sign at 23 °C and pH 8.2, respectively.

Changes of dissolved organic matter compositions in artifical and natural seawater during Pseudovibrio growth experiment

To study the consumption of dissolved organic matter (DOM) by bacteria living in untra-oligotrophic artificial or natural seawater, we analyzed the composition of DOM before (timepoint t0, directly after inoculation) and after (timepoint t2, 3 weeks of incubation) growth of the bacteria using Fourier transform ion cyclotron mass spectrometry (ESI FT-ICR-MS). The oligotrophic natural seawater used originates from the South Pacific Gyre. Our data show that the bacteria were able to utilize a variety of different organic compounds. These compounds belong to different chemical compound groups and likely fuel the bacterial energy, carbon and nitrogen requirements under the ultra-oligotrophic conditions.

Description and chemical composition of ferromanganese encrustations and deposits in the Atlantic Ocean from R/V Atlantis II cruises 20, 42, 60, 85; R/V Chain cruise 35; R/V Knorr cruise 61 and RRS Shackleton cruise 75/1 stations

Chemical and mineralogical compositions of ferromanganese oxide coatings on rocks dredged from the New England Seamounts, the Sierra Leone Rise and the Mid-Atlantic Ridge near the Equator have been determined in an investigation of regional differences in Atlantic ferromanganese deposits. Most encrustations are clearly of hydrogenous origin, consisting mainly of todorokite and delta MnO2, but several recovered from the equatorial fracture zones may be hydrothermal accumulations. Differences in the chemistry of the water column and in growth rates of the ferromanganese coatings may be important in producing this regional contrast in composition. Fine-scale changes in element abundances within the encrustations indicate that the nature of the substrate has little influence on compositional variations.

Stable carbon isotope composition of benthic foraminifera from sediments of the Skagerrak, North Sea

The sediment cores 225514 and 225510 were recovered from 420 and 285 m water depth, respectively. They were investigated for their benthic foraminiferal delta13C during the last 500 years. Both cores were recovered from the southern flank of the Skagerrak. The delta13C values of Uvigerina mediterranea and other shallow infaunal species in both cores indicate that organic matter rain rates to the seafloor varied around a mean value until approximately AD 1950 after which they increased. This increase might result from changes in the North Atlantic Current System and a co-occurring persistently high North Atlantic Oscillation index state in the 1980s to 1990s, rather than from anthropogenic eutrophication. Using delta13C mean values of multiple species, we reconstruct delta13C gradients of dissolved inorganic carbon (DIC) within pore waters for the time periods AD 1500 to 1950 and AD 1950 to 2000. The calculated delta13CDIC ranges, interpreted as indicating total organic matter remineralization due to respiration, are generally bigger in Core 225514 than in Core 225510. Since mean delta13C values of U. mediterranea suggest that organic matter rain rates were similar at both locations, differences in total organic matter remineralization are attributed to differing oxygen availability. However, oxygen concentrations in the overlying bottom water masses are not likely to have differed significantly. Thus, we suggest that organic matter remineralization was controlled by oxygen availability within the sediments, reflecting strong differences in sedimentation rates at the two investigated core sites. Based on the assumptions that tests of benthic foraminiferal species inhabiting the same microhabitat depth should show equal delta13C values unless they are affected by vital effects and that Globobulimina turgida records pore water delta13CDIC, we estimate microhabitat-corrected vital effects for several species with respect to G. turgida: >0.7 per mil for Cassidulina laevigata, >1.3 per mil for Hyalinea balthica, and >0.7 per mil for Melonis barleeanus. Melonis zaandami seems to closely record pore water delta13CDIC.

Geochemistry nd isotopic composition of Indian Ocean sediments

A down-core 231Pa/230Th record has been measured from the southwestern Indian Ocean to reconstruct the history of deep water flow into this basin over the last glacial-interglacial cycle. The (231Paxs/230Thxs)0 ratio throughout the record is nearly constant at approximately 0.055, significantly lower than the production ratio of 0.093, indicating that the proxy is sensitive to changes in circulation and/or sediment flux at this site. The consistent value suggests that there has been no change in the inflow of Antarctic Bottom Water to the Indian Ocean during the last 140 ka, in contrast to the changes in deep circulation thought to occur in other ocean basins. The stability of the (231Paxs/230Thxs)0 value in the record contrasts with an existing sortable silt (SS) record from the same core. The observed equation image variability is attributed to a local geostrophic effect amplifying small changes in circulation. A record of authigenic U from the same core suggests that there was reduced oxygen in bottom waters at the core locality during glacial periods. The consistency of the (231Paxs/230Thxs)0 record implies that this could not have arisen by local changes in productivity, thus suggesting a far-field control: either globally reduced bottom water oxygenation or increased productivity south of the Opal Belt during glacials.

(Table 1) Maceral composition and organic carbon chemistry of ODP Leg 104 samples

The Cenozoic sediments sampled in ODP Leg 104 on the Vøring Plateau show a distinct variability of the total organic carbon content (TOC) and the accumulation rates of TOC. Based on the geochemical and organic-petrographic characterization of the sedimentary organic matter (OM), the allochthonous and autochthonous proportion of the OM could be quantified. The results clearly demonstrate that high TOC percentages and TOC accumulation rates in Cenozoic sediment sections display a generally high input of allochthonous organic matter. Oxidized and partly well-rounded organic particles built up the main portion of OM within the Miocene, TOC-rich sediments. The most probable source of this oxidized OM are reworked sediments from the Scandinavian shelf. Changes in the input of these organic particles are to some degree correlative with sea-level changes. The Cenozoic accumulation of autochthonous OM is low and does not reveal a clear variation during the Miocene and early Pliocene. In spite of a high accumulation rate of biogenic opal during the Early Miocene, the accumulation rate of autochthonous TOC is low. The autochthonous particle assemblage is dominated by relatively inert OM, like dinoflagellate cysts. This points to an intensive biological and/or early diagenetic degradation of the marine OM under well oxidized bottom water conditions during the last 23 Myr. Nevertheless, a continuation of marine OM degradation during later stages of diagenesis cannot be excluded. A prominent dominance of allochthonous OM over autochthonous is documented with the beginning of the Pliocene. At 2.45 Ma the episodic occurrence of ice-rafted, thermally mature OM reflects the onset of the glacial erosion of Mesozoic, coal and black shale bearing sediments on the Scandinavian and Barents Sea shelves. The first occurrence of these, in view of the actual burial depth, thermally overmature OM particles is, therefore, a marker for the beginning of the strong Scandinavian glaciation and the advance of the glacial front toward the shelves.

Geochemical composition of the Trincheira Complex in the southwestern Amazon craton, Brazil

We document the first-known Mesoproterozoic ophiolite from the southwestern part of the Amazon craton, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation (BIFs), pelites, psammitic and a smaller proportion of calc-silicate rocks. This sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary orogen. The rocks were deformed by a single tectonic event, which included isoclinal folding and metamorphism of the granulite-amphibolite facies. Layered magmatic structures were preserved in areas of low strain, including amygdaloidal and cumulate structures. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. The mafic-ultramafic rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of mafic granulites and porphyroblastic amphibolites in the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB), although they locally display small Ta, Ti and Nb negative anomalies, indicating a small subduction influence. This behavior changes to an island arc tholeiites (IAT) signature in the upper units of fine-grained amphibolites and amphibole rich-amphibolites, characterized by progressive depletion in the incompatible elements and more pronounced negative Ta and Nb anomalies, as well as common Ti and Zr negative anomalies. Tectono-magmatic variation diagrams and chondrite-normalized REE and primitive mantle normalized patterns suggest a back-arc to intra-oceanic island arc tectonic regime for the eruption of these rocks. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Accordingly, the Trincheira Complex is a record of oceanic crust relics obducted during the collision of the Amazon craton and the Paraguá block during the Middle Mesoproterozoic. Thus, the recognition of the Trincheira ophiolite and suture significantly changes views on the evolution of the southern margin of the Amazon craton, and how it can influence the global tectonics and the reconstruction of the continents.

Geochemical and nannofossil analyses of Cenomanian/Turonian sediments of DSDP Hole 75-530A

One of the key objectives of Deep Sea Drilling Project (DSDP) Leg 75 was to shed light on the underlying causes of Cretaceous oceanic anoxia in the South Atlantic by addressing two major hypotheses: productivity productivity-driven anoxia vs. enhanced ocean stratification leading to preservation of organic matter and black shale deposition. Here we present a detailed geochemical dataset from sediments deposited during the Cenomanian/Turonian (C/T) transition and the global oceanic anoxic event 2 (OAE 2) at DSDP Site 530A, located off-shore Namibia (southeast Angola Basin, north of Walvis Ridge). To characterise the succession of alternating black and green shales at this site and to reconstruct the evolution of their paleoenvironmental setting, we have combined data derived from investigations on bulk organic matter, biomarkers and the inorganic fraction. The location of the C/T boundary itself is biostratigraphically not well constrained due to the carbonate-poor (but organic matter-rich) facies of these sediments. The bulk d13Corg record and compound-specific d13C data, in combination with published as well as new biostratigraphic data, enabled us to locate more precisely the C/T boundary at DSDP Site 530A. The compound-specific d13C record is the first of this kind reported from C/T black shales in the South Atlantic. It is employed for paleoenvironmental reconstructions and chemostratigraphic correlation to other C/T sections in order to discuss the paleoceanographic aspects and implications of the observations at DSDP Site 530A in a broader context, e.g., with regard to the potential trigger mechanisms of OAE 2, global changes in black shale deposition and climate. On a stratigraphic level, an approximation and monitoring of the syndepositional degree of oxygen depletion within the sediments/bottom waters in comparison to the upper water column is achieved by comparing normalised concentrations of redox-sensitive trace elements with the abundance of highly source specific molecular compounds. These biomarkers are derived from photoautotrophic and simultaneously anoxygenic green sulphur bacteria (Chlorobiacea) and are interpreted as paleoindicators for events of photic zone euxinia. In contrast to a number of other OAE 2 sections that are characterised by continuous black shale sequences, DSDP Site 530A represents a highly dynamic setting where newly deposited black shales were repeatedly exposed to conditions of subtle bottom water re-oxidation, presumably leading to their progressive alteration into green shales. The frequent alternation between both facies and the related anoxic to slight oxygenated conditions can be best explained by variations in vertical extent of an oxygen minimum zone in response to changes in a highly productive western continental margin setting driven by upwelling.