Oxygen and carbon isotopic composition of shells from Radix sp. in lakes across the Tibetan Plateau

The Tibetan Plateau (TP), including its surrounding mountain ranges, represents the largest store of ice outside the polar regions. It hosts numerous lakes as well as the head waters of major Asian rivers, on which billions of people depend, and it is particularly sensitive to climate change. The moisture transport to the TP is controlled by the Indian and Pacific monsoon and the Westerlies. Understanding the evolution of the interaction of these circulation systems requires studies on climate archives in different spatial and temporal contexts. The objective of this study is to learn more about the interannual variability of precipitation patterns across the TP and how different hydrologic systems react to different climatic factors. Aragonite shells of the aquatic gastropod Radix, which is widely distributed in the region, may represent suitable archives for inferring hydrologic and climatic signals in particularly high resolution. Therefore, sclerochronological studies of d18O and d13C ratios in Radix shells from seven lakes were conducted, each representing a different hydrologic and climatic setting, on a transect from the Pamirs across the TP. The shell patterns exhibit an increasing influence of precipitation and a decreasing influence of evaporation on the isotope compositions from west to east. d18O values of shells from lakes on the eastern and central TP (Donggi Cona, Yamdrok Yumco, Tarab Co) mirror monsoon signals, indicated by more negative values and higher variabilities compared to the more western lakes (Karakul, Bangong/Nyak, Manasarovar). In Yadang Co, located on the central southern TP, the monsoon rains did not reach the lake in the sampling year, although it is located in a region which is usually affected by monsoon circulation. The d18O values are used to differentiate the annual hydrological cycle into ice cover period, melt water period, precipitation period and evaporation period. d13C compositions in the shells particularly depend on specific habitats, which vary in biological productivity and in carbon sources. d18O and d13C patterns show a positive covariance in shells originating from large closed basins. The results show that Radix shells mirror general climatic differences between the seven lake regions. These differences reflect both regional and local climate signals in sub-seasonal resolution, without noticeable dependence on the particular lake system.

Geochemical studies of surface sediment samples off North-West Africa

One hundred and twenty-eight surface-sediment samples collected off North-West Africa were studied geochemically to detect the expressions of different meridional climate regimes and zonal productivity gradients in the surface sediments. This geochemical multi-tracer approach, coupled with additional information on the bulk carbonate and TOC contents makes it possible to characterise the sedimentological regime in detail. Typical terrigenous elements like Al, K and Fe mirror the importance of the humid (fluvial) influence in the north of the study area and the dominance of aeolian input in the south. Furthermore, the distributions of Ti and Fe in the surface sediments serve as tracers for the supply of eolian volcanic material from the Canary Islands. The spatial variability of the TOC contents in the surface sediments closely follows the ocean surface productivity patterns, whereas the CaCO3 contents are mainly controlled by dilution with terrigenous matter. The potential productivity proxy Ba is not a reliable tracer for productivity in this region, since it is mainly supplied by terrigenous input (coupled with aluminosilicates).

Magnetic properties, grain sizes, XRF Scanner Data of sediment core GeoB9307-3

Geochemical and mineralogical proxies for paleoenvironmental conditions have the underlying assumption that climate variations have an impact on terrestrial weathering conditions. Varying properties of terrigenous sediments deposited at sea are therefore often interpreted in terms of paleoenvironmental change. Also in gravity core GeoB9307-3 (18° 33.99' S, 37° 22.89' E), located off the Zambezi River, environmental changes during Heinrich Stadial 1 (HS 1) and the Younger Dryas (YD) are accompanied by changing properties of the terrigenous sediment fraction. Our study focuses on the relationship of variability in the hydrological system and changes in the magnetic properties, major element geochemistry and granulometry of the sediments. We propose that changes in bulk sedimentary properties concur with environmental change, although not as a direct response of climate driven pedogenic processes. Spatial varying rainfall intensities on a sub-basin scale modify sediment export from different parts of the Zambezi River basin. During humid phases, such as HS 1 and the YD, sediment was mainly exported from the coastal areas, while during more arid phases sediments mirror the hinterland soil and lithological properties and are likely derived from the northern Shire sub-basin. We propose that a de-coupling of sedimentological and organic signals with variable discharge and erosional activity can occur.

Stable isotope ratios from foraminfera in Kara Sea sediments

Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.-min.) in the isotope values when measuring several (>=5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of >2per mil of d18O and d13C at low absolute levels (d18O: <3per mil, d13C: <-3per mil) near the river estuaries of Ob and Yenisei and low ranges of not, vert, similar1per mil at higher absolute levels (d18O: 2-5.4per mil, d13C: -3 per mil to -1.5per mil) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8-7 cal. ka BP) of a sediment core north of Yenisei estuary revealed d18O and d13C values which on average are lower by 0.6? in both, d18O and d13C, than in the upper core section (<5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.-min.) in the single-valve d18O and d13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise

Benthic foraminifera in surface sediments of the Gulf of Lions

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Response of marine palynomorphs to Neogene climate cooling in the Iceland Sea, ODP Hole 151-907A

The present study on ODP Leg 151 Hole 907A combines a detailed analysis of marine palynomorphs (dinoflagellate cysts, prasinophytes, and acritarchs) and a low-resolution alkenone-based sea-surface temperature (SST) record for the interval between 14.5 and 2.5 Ma, and allows to investigate the relationship between palynomorph assemblages and the paleoenvironmental evolution of the Iceland Sea. A high marine productivity is indicated in the Middle Miocene, and palynomorphs and SSTs both mirror the subsequent long-term Neogene climate deterioration. The diverse Middle Miocene palynomorph assemblages clearly diminish towards the impoverished assemblages of the Late Pliocene; parallel with a somewhat gradual decrease of SSTs being as high as 20 °C at ~13.5 Ma to around 8 °C at ~3 Ma. Superimposed, palynomorph assemblages not only reflect Middle to Late Miocene climate variability partly coinciding with the short-lived global Miocene isotope events (Mi-events), but also the initiation of a proto-thermohaline circulation across the Middle Miocene Climate Transition, which led to increased meridionality in the Nordic Seas. Last occurrences of species cluster during three events in the Late Miocene to Early Pliocene and are ascribed to the progressive strengthening and freshening of the proto-East Greenland Current towards modern conditions. A significant high latitude cooling between 6.5 and 6 Ma is depicted by the supraregional "Decahedrella event" coeval with lowest Miocene productivity and a SST decline. In the Early Pliocene, a transient warming is accompanied by surface water stratification and increased productivity that likely reflects a high latitude response to the global biogenic bloom. The succeeding crash in palynomorph accumulation, and a subsequent interval virtually barren of marine palynomorphs may be attributed to enhanced bottom water oxygenation and substantial sea ice cover, and indicates that conditions seriously affecting marine productivity in the Iceland Sea were already established well before the marked expansion of the Greenland Ice Sheet at 3.3 Ma.

Carbon isotopic composition of dissolved CO2, CO2 gas and methane in sediments from ODP Leg 172 sites

Carbon isotopic data of interstitial dissolved CO2 (CO2), CO2 gas, and methane show that a variety of microbial diagenetic processes produce the observed isotopic trends. Anaerobic methane oxidation (AMO) is an important process near the sulfate-methane interface (SMI) that strongly influences the isotopic composition of CO2 in the sulfate reduction and upper methanogenic zones, which in turn impacts methane isotopic composition. Dissolved CO2 and methane are maximally depleted in 13C near the SMI, where 13C values are as light as -31.8 and -101 PDB for CO2 and methane, respectively. CO2 reduction links the CO2 and methane pools in the methanogenic zone so that the carbon isotopic composition of both pools evolves in concert, generally showing increasing enrichments of 13C with increasing depth. These isotopic trends mirror those within other methane-rich continental rise sediments worldwide.

Mineralogy of southern Kerguelen Plateau sediments

Kaolinite, goethite, minor hematite, and gibbsite were found in fluvial upper Lower Cretaceous basal sediment from the Southern Kerguelen Plateau, Sites 748 and 750, 55°S latitude. This mineral assemblage, derived from the weathering of basalt, indicates near-tropical weathering conditions with high orographic rainfall, at least 100 cm per year. The climate deteriorated by the Turonian or Coniacian, as indicated by the decline in kaolinite content of this sediment. The Upper Cretaceous sediment at Site 748 consists of 200 m of millimeter-laminated, sparsely fossiliferous, wood-bearing glauconitic siltstone and clay stone with siderite concretions deposited on a shelf below wave base. Some graded and cross beds indicate that storms swept over the shelf and reworked the sediment. Overlying this unit is 300 m of intermittently partly silicified, bryozoan-inoceramid-echinoderm-rich glauconitic packstones, grainstones, and wackestones. The dominant clay mineral in both units is identical to the mineral composition of the glauconite pellets: randomly interstratified smectite-mica. The clay fraction has a higher percent of expandable layers than the mineral of the glauconite pellets, and the clay of the underlying subunit has a higher percentage of expandable layers than the clay of the carbonate subunit. Potassium levels mirror these mineral variations, with higher K levels in minerals that have a lower percentage of expandable layers. The decrease in expandability of the mineral in the upper subunit is attributed to diagenesis, the result of higher porosity.

AFLP data based on presence (1) and absence (0) of AFLP loci (A4-F171) in Gonyostomum semen

Many recent studies have found genetically differentiated populations in microorganisms despite potentially high dispersal. We designed a study to specifically examine the importance of physical dispersal barriers, i.e. geographic distance and lack of hydrological connectivity, in restricting gene flow and enhancing divergence in limnic microorganisms. We focused on the nuisance microalga Gonyostomum semen, which has recently expanded in northern Europe and differentiated into genetically distinct populations. Gonyostomum semen was sampled from six lakes distributed in two adjacent watersheds, which thereby comprised, both connected and non-connected lakes. The individual isolates were genotyped by Amplified Fragment Length Polymorphism. Several lake populations were differentiated from each other, but connectivity within watersheds could not explain the observed population genetic pattern. However, isolation by distance was moderate and might limit the gene flow among distant populations. In addition, we found low, but significant linkage disequilibrium, which indicates regular sexual recombination in this species, despite its high degree of asexual reproduction. Therefore, we conclude that the genetic properties of microalgae with occasional sexual reproduction essentially mirror regularly recombining species. Furthermore, the data indicated bottlenecks supporting the hypothesized recent range expansion of this species.

Ostracodes in bottom sediments of the Laptev Sea

This work is the first detailed description of the Late Pleistocene-Holocene and Recent Ostracoda of the Laptev Sea. A total of 45 species in 22 genera and 13 families have been identified. All these species are described monographically. Three different ecological assemblages of ostracodes corresponding to different combinations of environmental parameters have been established; they are restricted to three regions of the sea: western-central, eastern, and southern. The recent ostracode assemblages of the Laptev Sea have been compared with those from other Arctic areas and are most similar to those of the Beaufort and Kara seas. Data on recent Ostracoda are used for paleoenvironmental reconstructions on the eastern shelf and western continental slope of the Laptev Sea. For this purpose, ostracodes from five sections obtained from these parts of the sea have been examined. The oldest sediments, which are of Late Pleistocene age (15.8 cal. ka BP), have been recovered in a core from the western continental slope. These yielded five ostracode assemblages, which correspond to different paleoenvironments and replaced each other in the course of the rapid postglacial sea-level rise, thus showing variations in the Atlantic water inflow from the west and freshwater discharge from the subaerially exposed shelf. On the outer shelf of the eastern part of the sea, the rapid sea-level rise in the Early Holocene (lowermost dating 11.3 cal. ka BP) led to a rapid transition from assemblages of brackish-water nearshore environments to those of modernlike normal marine environments; modern environments were established about 8.2 cal. ka ago. Since core sections from the inner shelf correspond to the time when the level of the sea had already reached its modern values, changes in taxonomic composition of ostracode assemblages primarily mirror variations in river runoff.

Shell size variation of Neogloboquadrina pachyderma sin. in the Norwegian-Greenland Sea during the last 1.3 Myr

We present measurements of the maximum diameter of the planktonic foraminifer Neogloboquadrina pachyderma sin. from six sediment cores (Ocean Drilling Program sites 643, 644, 907, 909, 985 and 987) from the Norwegian-Greenland Sea. Our data show a distinct net increase in mean shell size of N. pachyderma sin. at all sites during the last 1.3 Ma, with largest shell sizes reached after 0.4 Ma. External factors such as glacial-interglacial variability and carbonate dissolution alone cannot account for the observed variation in mean shell size of N. pachyderma sin. We consider the observed shell size increase to mirror an evolutionary trend towards better adaptation of N. pachyderma sin. to the cold water environment after 1.1-1.0 Ma. Probably, the Mid Pleistocene climate shift and the associated change of amplitude and frequency of glacial-interglacial fluctuations have triggered the evolution of this planktonic foraminifer. Oxygen and carbon stable isotope analyses of different shell size classes indicate that the observed shell size increase could not be explained by the functional concept that larger shells promote increasing sinking velocities during gametogenesis. For paleoceanographic reconstructions, the evolutionary adaptation of Neogloboquadrina pachyderma sin. to the cold water habitat has significant implications. Carbonate sedimentation in highest latitudes is highly dependent on the presence of this species. In the Norwegian-Greenland Sea, carbonate-poor intervals before 1.1 Ma are, therefore, not necessarily related to severe glacial conditions. They are probably attributed to the absence of this not yet polar-adapted species. Further, transfer function and modern analog techniques used for the reconstruction of surface water conditions in high latitudes could, therefore, contain a large range of errors if they were applied to samples older than 1.1-1.0 Myrs.

On the effects of circulation, sediment resuspension and biological incorporation by diatoms in an ocean model of aluminium, link to model data

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ~ 40° N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al / Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging.

Sedimentological and palynological analysis of sediment cores from the northeastern Vietnamese Mekong River Delta

Three radiocarbon-dated sediment cores from the northeastern Vietnamese Mekong River Delta have been analysed with a multiproxy approach (grain size, pollen and spores, macro-charcoal, carbon content) to unravel the palaeoenvironmental history of the region since the mid Holocene. During the mid-Holocene sea-level highstand a diverse, zoned and widespread mangrove belt (dominated by Rhizophora) covered the extended tidal flats. The subsequent regression and coeval delta progradation led to the rapid development of a back-mangrove community dominated by Ceriops and Bruguiera but also represented locally by e.g. Kandelia, Excoecaria and Phoenix. Along rivers this community seems to have endured even when the adjoining floodplain had already shifted to freshwater vegetation. Generally this freshwater vegetation has a strong swamp signature but locally Arecaceae, Fabaceae, Moraceae/Urticaceae and Myrsinaceae are important and mirror the geomorphological diversity of the delta plain. The macro-charcoal record implies that natural burning of vegetation occurred throughout the records, however, the occurrence of the highest amounts of macro-charcoal particles is linked with modern human activity.

Boron concentrations and isotope ratios of authigenic carbonates from the Oregon margin

Boron contents and boron, carbon and oxygen stable isotopes were determined for authigenic carbonates recovered from Ocean Drilling Program Leg 146, Oregon margin. Carbonate precipitates are the most widespread authigenic phase in the shallow accretionary wedge and carry chemical information about long-term variations in pore fluid origin and flow paths in the Cascadia subduction zone. Drilling the first ridge (toe area including the frontal thrust) and the second ridge (or Hydrate Ridge) of the prism demonstrated different fluid regimes, with higher B contents in the authigenic precipitates at the toe. The delta11B of 18 authigenic precipitates analysed ranges from 13.9 per mil to as high as 39.8 per mil, extending the upper range of previously reported carbonate delta11B values considerably. When related to the delta11B ratio of their parent solutions, these data are characteristic of fluid-related processes in accretionary prisms. Together with delta13C and delta18O, delta11B ratios of the carbonate concretions, nodules and crusts allow one to distinguish between precipitation influenced by (i) seawater, (ii) fluid reservoirs at different depth levels within the accretionary prism and (iii) cage water from dissociated gas hydrates, the latter possibly indicating a fluctuation of the bottom simulating reflector during most recent Earth's history. From this first systematic boron study on authigenic precipitates from an accretionary prism it is suggested that B contents of such carbonate crusts and concretions exceed those reported for other marine carbonates. Given the abundance of such precipitates at convergent margins, they represent a significant B sink in geochemical cycling. Isotopic compositions of the parent fluids to the carbonates mirror B chemistry of modern pore waters from convergent margins. The precipitates carry information of different subduction-related fluid processes over a certain period of time, and hence are a crucial tracer in the investigation of palaeo-fluid flow.

Biogeochemical parameters (oxygen, nitrate, phosphate, TOC and strontium) at three sites from the Clarion-Clipperton Fracture Zone

The Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of an exchange of dissolved constituents between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m towards the basaltic basement. NO32- concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. We performed transport reaction modeling and determined at one site the 87Sr/86Sr ratio of the pore water and the bottom water overlying the sediments, which indicated that the 87Sr/86Sr ratio of the pore water at the bottom of the sediment column is similar to the seawater Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. Our results suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. The oxygen profiles presented here represent the first of their kind ever measured in the Pacific Ocean, as they indicate an upward flux of molecular oxygen from a basaltic aquifer, something that has so far only been documented - at one other location worldwide - the North Pond site in the Atlantic Ocean. We show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.