Measurements of trace metal abundances and isotopes in carbonate sediments, Exuma, Bahamas

In order to validate the use of 238U/235U as a paleoredox proxy in carbonates, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1 ppm) than in primary precipitates (avg. 1.5 ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H2S, and thus indirectly to organic matter content. d238/235U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, d238/235U of bulk sediments from the push cores was 0.2-0.4 per mil heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and 238U/235U ratios. We also report the occurrence of dolomite in a tidal pond core which contains low 234U/238U and 238U/235U ratios and discuss the possibility that the dolomitization process may result in sediments depleted in 238U. From this initial exploration, it is clear that 238U/235U variations in ancient carbonate sediments could be driven by changes in global average seawater, by spatial and temporal variations in the local deposition environment, or subsequent diagenesis. To cope with such effects, proxies for syndepositional pore water redox conditions (e.g., organic matter content, iron speciation, and trace metal distributions) and careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of 238U/235U data from ancient carbonate sediments.

Figure 1 Geomorphological map of proglacial area of Yala (Dakpatsen) Glacier, Langtang Himal, Nepal (pdf 500 kB)

A series of minor moraine ridges are observed on the till surface in the proglacial area of Yala Glacier, Nepal Himalaya. The till surface, which is often fluted, consists ofsix different till sheets. It lies present glacial margin and the bulky terminal moraine ridges. These till sheets correspond to six re-advance stages during the general retreat which followed Little Ice Age ad- vance which formed the bulky terminal moraine ridges. Field observations and till fabric analysis suggest that the minor moraine ridges of Yala Glacier seem to be formed annually, by ice push. On the assumption that their annual character was maintained for a long time, and that the time span needed for each re-advance was proportional to the height of terminal moraine of each till sheet, the dating of Little fee Age moraines was attempted. The results indicate that Little Ice Age advances occurred in 1815 and in 1843, roughly simultanously with those in Europe.

Radiocarbon ages of sediment core PG1414-2 from Beall Lake, Windmill Islands

A mid-Holocene climate optimum is inferred from a palaeosalinity reconstruction of a closed saline lake (Beall Lake) from the Windmill Islands, East Antarctica using an expanded diatom salinity weighted averaging (WA) regression and calibration model. The addition of 14 lakes and ponds from the Windmill Islands, East Antarctica, to an existing weighted averaging regression and calibration palaeosalinity model of 33 lakes from the Vestfold Hills, East Antarctica expands the number of taxa and lakes and the range of salinity in the existing model and improves the model's predictive ability. This improved model was used to infer Holocene changes in lake water salinity in Beall Lake, Windmill Islands. Six changes in diatom-inferred salinity in Beall Lake are put into broad chronological context based on three radiocarbon dates: as the East Antarctic Ice Sheet (EAIS) retreated from the Windmill Islands during the early Holocene (~9000-8130 corr. yr BP), Beall Lake formed as a melt water-fed freshwater lake, which gradually became more saline as marine influence increased from ~8000 corr. yr BP. Between ~8000 and 4800 corr. yr BP, the diatom assemblage included planktonic marine taxa such as Chaetoceros spp. and cryophilic taxa such as Fragilariopsis cylindrus, which indicate favourable summer growth conditions. A mid-Holocene warm period produced a climate that was warmer and more humid with increased precipitation and snow accumulation. This is reflected in the Beall Lake core as a reduction in the salinity of the lake diatom assemblage from ~4800-4600 corr. yr BP. Holocene isostatic uplift rates in the Windmill Islands vary from 5-6 m/1000 yr. By applying this uplift rate, it is calculated that the bedrock would have risen above sea level by ~4000 yr BP. The Beall Lake core diatom assemblage from ~4600-2900 corr. yr BP includes both marine cryophilic and planktonic taxa together with freshwater benthic and planktonic lacustrine taxa. This mix of species indicates the emergence of the lake from the sea around ~4600 corr. yr BP. From ~2800 corr. yr BP, retreat of the ice margin led to increasing melt water inputs and associated freshening of the lake basin until ~1900 corr. yr BP. The lake basin had no oceanic influence by this time, allowing a terrestrial freshwater flora to establish and thrive for the next ~1000 yr. At ~1850 corr. yr BP, a sudden and rapid salinity change is evident in Beall Lake. A late Holocene warm period between 2000 and 1000 yr BP has been observed in ice core records from Law Dome (an ice cap abutting the Windmill Islands to the east and north). It is therefore inferred that, at ~1850 corr. yr BP, summer temperatures within the Beall Lake catchment area were much higher than present summer temperatures. The climate optimum identified in the Beall Lake core ~4800 yr BP confirms mid-Holocene warming of the Windmill Islands and suggests a synchronous mid-Holocene climate optimum occurred across coastal East Antarctica. In addition, the abrupt climate change inferred at ~1850 yr BP suggests that higher resolution sampling of sediment cores from coastal East Antarctic limnological oases will provide more evidence of rapid climate change events over coastal East Antarctica in future.

Rock magnetic properties and radiocarbon dates of samples obtained on Ile de Possession, South Indian Ocean

A 6200 year old peat sequence, cored in a volcanic crater on the sub-Antarctic Ile de la Possession (Iles Crozet), has been investigated, based on a multi-proxy approach. The methods applied are macrobotanical (mosses, seeds and fruits) and diatom analyses, complemented by geochemical (Rock-Eval6) and rock magnetic measurements. The chronology of the core is based on 5 radiocarbon dates. When combining all the proxy data the following changes could be inferred. From the onset of the peat formation (6200 cal yr BP) until ca. 5550 cal yr BP, biological production was high and climatic conditions must have been relatively warm. At ca. 5550 cal yr BP a shift to low biological production occurred, lasting until ca. 4600 cal yr BP. During this period the organic matter is well preserved, pointing to a cold and/or wet environment. At ca. 4600 cal yr BP, biological production increased again. From ca. 4600 cal yr BP until ca. 4100 cal yr BP a 'hollow and hummock' micro topography developed at the peat surface, resulting in the presence of a mixture of wetter and drier species in the macrobotanical record. After ca. 4100 cal yr BP, the wet species disappear and a generally drier, acidic bog came into existence. A major shift in all the proxy data is observed at ca. 2800 cal yr BP, pointing to wetter and especially windier climatic conditions on the island probably caused by an intensification and/or latitudinal shift of the southern westerly belt. Caused by a stronger wind regime, erosion of the peat surface occurred at that time and a lake was formed in the peat deposits of the crater, which is still present today.

Methane and sulfide fluxes in permanent anoxia: in situ studies at the Dvurechenskii mud volcano (Sorokin Trough, Black Sea)

The Dvurechenskii mud volcano (DMV), located in permanently anoxic waters at 2060 m depth (Sorokin Trough, Black Sea), was visited during the M72/2 cruise with the RV Meteor to investigate the methane and sulfide release from mud volcanoes into the Black Sea hydrosphere. We studied benthic fluxes of methane and sulfide, and the factors controlling transport, consumption and production of both compounds within the sediment. The pie shaped mud volcano showed temperature anomalies as well as solute and gas fluxes indicating high fluid flow at a small elevation north of the geographical center. The anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was excluded from this zone due to fluid-flow induced sulfate limitation and a fresh mud flow and consequently methane escaped into the water column with a rate of 0.46 mol/m**2/d. In the outer center of the mud volcano fluid flow and total methane flux were decreased, correlating with an increase in sulfate penetration into the sediment, and with higher SR and AOM rates. Here between 50-70% of the methane flux (0.07-0.1 mol/m**2/d) was consumed within the upper 10 cm of the sediment. Also at the edge of the mud volcano fluid flow and rates of methane and sulfate turnover were substantial. The overall amount of dissolved methane released from the mud volcano into the water column was significant with a discharge of 1.4x10**7 mol/yr. The DMV maintains also high areal rates of methane-fueled sulfide production of on average 0.05 mol/m**2/d. However, we concluded that sulfide and methane emission into the hydrosphere from deep water mud volcanoes does not significantly contribute to the sulfide and methane inventory of the Black Sea.

Pore water concentrations and solid phase concentrations of sediment cores from the Ligurian continental slope

In October 1979, a period of heavy rainfall along the French Riviera was followed by the collapse of the Ligurian continental slope adjacent to the airport of Nice, France. A body of slope sediments, which was shortly beforehand affected by construction work south of the airport, was mobilized and traveled hundreds of kilometers downslope into the Var submarine canyon and, eventually, into the deep Ligurian basin. As a direct consequence, the construction was destroyed, seafloor cables were torn, and a small tsunami hit Antibes shortly after the failure. Hypotheses regarding the trigger mechanism include (i) vertical loading by construction of an embankment south of the airport, (ii) failure of a layer of sensitive clay within the slope sequence, and (iii) excess pore fluid pressures from charged aquifers in the underground. Over the previous decades, both the sensitive clay layers and the permeable sand and gravel layers were sampled to detect freshened waters. In 2007, the landslide scar and adjacent slopes were revisited for high-resolution seafloor mapping and systematic sampling. Results from half a dozen gravity and push cores in the shallow slope area reveal a limited zone of freshening (i.e. groundwater influence). A 100-250 m wide zone of the margin shows pore water salinities of 5-50% SW concentration and depletion in Cl, SO4, but Cr enrichment, while cores east or west of the landslide scar show regular SW profiles. Most interestingly, the three cores inside the landslide scar hint towards a complex hydrological system with at least two sources for groundwater. The aquifer system also showed strong freshening after a period of several months without significant precipitation. This freshening implies that charged coarse-grained layers represent a permanent threat to the slope's stability, not just after periods of major rainfall such as in October 1979.

Radiocarbon measurements and loose-sediment quantification of seafloor sediment samples off northern Mauritania

Heterozoan carbonates are typical for extratropical sedimentary systems. However, under mesotrophic to eutrophic conditions, heterozoan carbonates also form in tropical settings. Nevertheless, such heterozoan tropical sedimentary systems are rare in the modern world and therefore are only poorly understood to date. Here a carbonate depositional system is presented where nutrient-rich upwelling waters push onto a wide shelf. These waters warm up in the shelf, giving rise to the production and deposition of tropical heterozoan facies. The carbonate facies on this shelf are characterized by a mixture of tropical and cosmopolitan biogenic sedimentary grains. Study of facies and taxonomy are the key for identifying and characterizing tropical heterozoan carbonates and for distinguishing them from their coolwater counterparts, in particular in the past where the oceanography cannot be determined directly.

Distribution of benthic foraminifers in surface layer of bottom sediments from the littoral zone of the Kunashir Island, South Kuriles

Composition and abundance of modern benthic foraminifers in the littoral zone of the Kunashir Island (South Kuriles) were studied. This littoral zone was examined on the sides of the Sea of Okhotsk, the Pacific Ocean, and the Izmena Bay. In the littoral zone of the Izmena Bay benthic foraminifers were not found. The highest biodiversity and maximal density of foraminifers were observed at a bench among rocks and blocks, in depressions of various size and depth (baths), at places where algae and water plants were attached, on silty sands, and on sands with admixture of broken shells, silt, and clastic matter composing the coast. The lowest density and biodiversity were found in mouths of creeks and rivers, on rock plates free from sediments and attached algae and water plants, as well as in places not protected from wind and wave activity. It was established that on both sides of the Sea of Okhotsk and of the Pacific Ocean foraminiferal complexes vary both in biodiversity and in density of their distribution in the littoral zone.

(Appendix 1-4) Geochemistry, desorption methods, TOC, TC, and mineral content of different settings

Sorption of volatile hydrocarbon gases (VHCs) to marine sediments is a recognized phenomenon that has been investigated in the context of petroleum exploration. However, little is known about the biogeochemistry of sorbed methane and higher VHCs in environments that are not influenced by thermogenic processes. This study evaluated two different extraction protocols for sorbed VHCs, used high pressure equipment to investigate the sorption of methane to pure clay mineral phases, and conducted a geochemical and mineralogical survey of sediment samples from different oceanographic settings and geochemical regimes that are not significantly influenced by thermogenic gas. Extraction of sediments under alkaline conditions yielded higher concentrations of sorbed methane than the established protocol for acidic extraction. Application of alkaline extraction in the environmental survey revealed the presence of substantial amounts of sorbed methane in 374 out of 411 samples (91%). Particularly high amounts, up to 2.1 mmol kg**-1 dry sediment, were recovered from methanogenic sediments. Carbon isotopic compositions of sorbed methane suggested substantial contributions from biogenic sources, both in sulfate-depleted and sulfate-reducing sediments. Carbon isotopic relationships between sorbed and dissolved methane indicate a coupling of the two pools. While our sorption experiments and extraction conditions point to an important role for clay minerals as sorbents, mineralogical analyses of marine sediments suggest that variations in mineral composition are not controlling variations in quantities of sorbed methane. We conclude that the distribution of sorbed methane in sediments is strongly influenced by in situ production.

Sedimentological, biogeomcial and geochronological data of thermokarst lake sediment core PG1967

Ice-rich permafrost landscapes are sensitive to climate and environmental change due to the melt-out of ground ice during thermokarst development. Thermokarst processes in the northern Yukon Territory are currently not well-documented. Lake sediments from Herschel Island (69°36'N; 139°04'W) in the western Canadian Arctic provide a record of thermokarst lake development since the early Holocene. A 727 cm long lake sediment core was analyzed for radiographic images, magnetic susceptibility, granulometry, and biogeochemical parameters (organic carbon, nitrogen, and stable carbon isotopes). Based on eight calibrated AMS radiocarbon dates, the sediment record covers the last ~ 11,500 years and was divided into four lithostratigraphic units (A to D) reflecting different thermokarst stages. Thermokarst initiation at the study area began ~ 11.5 cal ka BP. From ~ 11.5 to 10.0 cal ka BP, lake sediments of unit A started to accumulate in an initial lake basin created by melt-out of massive ground ice and thaw subsidence. Between 10.0 and 7.0 cal ka BP (unit B) the lake basin expanded in size and depth, attributed to talik formation during the Holocene thermal maximum. Higher-than-modern summer air temperatures led to increased lake productivity and widespread terrain disturbances in the lake's catchment. Thermokarst lake development between 7.0 and 1.8 cal ka BP (unit C) was characterized by a dynamic equilibrium, where lake basin and talik steadily expanded into ambient ice-rich terrain through shoreline erosion. Once lakes become deeper than the maximum winter lake ice thickness, thermokarst lake sediments show a great preservation potential. However, site-specific geomorphic factors such as episodic bank-shore erosion or sudden drainage through thermo-erosional valleys or coastal erosion breaching lake basins can disrupt continuous deposition. A hiatus in the record from 1.8 to 0.9 cal ka BP in Lake Herschel likely resulted from lake drainage or allochthonous slumping due to collapsing shore lines before continuous sedimentation of unit D recommenced during the last 900 years.