Sedimentology of surface samples from the Weddell Sea, Antarctica

Mineralogical and granulometric properties of glacial-marine surface sediments of the Weddell Sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from Antarctic sources. The silt fraction shows marked spatial differences in quartz contents. In the sand fractions heavy-mineral assemblages display low mineralogical maturity and are dominated by garnet, green hornblende, and various types of clinopyroxene. Cluster analysis yields distinct heavy-mineral assemblages, which can be attributed to specific source rocks of the Antarctic hinterland. The configuration of modern mineralogical provinces in the near-shore regions reflects the geological variety of the adjacent hinterland. In the distal parts of the study area, sand-sized heavy minerals are good tracers of ice-rafting. Granulometric characteristics and the distribution of heavy-mineral provinces reflect maxima of relative and absolute accumulation of ice-rafted detritus in accordance with major iceberg drift tracks in the course of the Weddell Gyre. Fine-grained and coarse-grained sediment fractions may have different origins. In the central Weddell Sea, coarse ice-rafted detritus basically derives from East Antarctic sources, while the fine-fraction is discharged from weak permanent bottom currents and/or episodic turbidity currents and shows affinities to southern Weddell Sea sources. Winnowing of quartz-rich sediments through intense bottom water formation in the southern Weddell Sea provides muddy suspensions enriched in quartz. The influence of quartz-rich suspensions moving within the Weddell Gyre contour current can be traced as far as the continental slope in the northwestern Weddell Sea. In general, the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice-rafted detritus beyond the shelves of the study area.

Characteristics of samples obtained during the expedition to Bolshoy Lyakhovsky Island in July/August 2014

The German-Russian project CARBOPERM - Carbon in Permafrost, origin, quality, quantity, and degradation and microbial turnover - is devoted to studying soil organic matter history, degradation and turnover in coastal lowlands of Northern Siberia. The multidisciplinary project combines research from various German and Russian institutions and runs from 2013 to 2016. The project aims assessing the recent and the ancient trace gas budget over tundra soils in northern Siberia. Studied field sites are placed in the permafrost of the Lena Delta and on Bol'shoy Lyakhovsky, the southernmost island of the New Siberian Archipelago in the eastern Laptev Sea. Field campaigns to Bol'shoy Lyakhovsky in 2014 (chapter 2) were motivated by research on palaeoenvironmental and palaeoclimate reconstruction, sediment dating, near surface geophysics and microbiological research. In particular the field campaigns focussed on: - coring Quaternary strata with a ages back to ~200.000 years ago as found along the southern coast; they allow tracing microbial communities and organic tracers (i.e. lipids and biomarkers, sedimentary DNA) in the deposits across two climatic cycles (chapter 3), - instrumenting a borehole with a thermistor chain for measuring permafrost temperatures (chapter 3), - sampling Quaternary strata for dating permafrost formation periods based on the optical stimulated luminescence (OSL) technique (chapter 4), - sampling soil and geologic formations for carbon content in order to highlight potential release of CO2 and methane based on incubation experiments (chapter 5), - profiling near surface permafrost using ground-penetrating radar and geoelectrics for defining the spatial depositional context, where the cores are located (chapters 6 + 7).

Tidal and spatial variations of DI13C and aquatic chemistry in a temperate tidal basin during winter time

Here, the pelagic carbonate system and the ?13C signature of dissolved inorganic carbonate (DIC) were investigated in a tidal basin of the southern North Sea, the Jade Bay, with respect to tidal cycles and a transect towards the North Sea in winter time (January and November, 2010). Physical parameters, major and trace elements, and nutrient concentrations were considered, too. Primary production and pelagic organic matter respiration were negligible during winter time. Both, the compositional variations on the transects as well as during the tidal cycles indicate the mixing of North Sea with fresh water. The combined spatial co-variations of different parameters indicate an introduction of fresh water that was enriched in DI12C, metabolites (e.g., ammonia), protons, and dissolved redox-sensitive elements (e.g., Mn2+). During the January campaign, the discharge via the flood gates was limited due to ice cover of the hinterland drainage ditches, allowing for an observation of tidal variations without significant mixing contributions from surface water discharges. Considering a binary mixing model with North Sea and fresh water as end-members, the extrapolated fresh water end-member composition for this campaign is estimated to contain about 3.8 mmol/kg DIC , and enhanced concentrations of NH4+, Mn2+, and protons compared to North Sea water. The fast temporal response of dissolved geochemical tracers on tidal variations in the Jade Bay indicates a continuous supply of a fresh water component. The measured composition of fresh waters entering the Jade Bay via flood gates (end of October, 2010) did not match the values estimated by the binary mixing model. Therefore, the overall fresh water component likely is a mixture between sources originating from flood gates and (in January) dominating submarine groundwater discharge entering the Jade Bay. This model is consistent with the results obtained during the November campaign, when a more important contribution from flood gates is expected and a more variable fresh water end-member is estimated. The co-variations of the concentrations and the stable carbon isotope composition of DIC are applied to evaluate possible superimposed sink-source-transformation processes in the coastal waters and a general co-variation scheme is suggested.

Mineralogical compositions of sediment samples from the San Francisco Bay coastal system

The mineralogical compositions of 119 samples collected from throughout the San Francisco Bay coastal system, including bayfloor and seafloor, area beaches, cliff outcrops, and major drainages, were determined using X-ray diffraction (XRD). Comparison of the mineral concentrations and application of statistical cluster analysis of XRD spectra allowed for the determination of provenances and transport pathways. The use of XRD mineral identifications provides semi-quantitative compositions needed for comparisons of beach and offshore sands with potential cliff and river sources, but the innovative cluster analysis of XRD diffraction spectra provides a unique visualization of how groups of samples within the San Francisco Bay coastal system are related so that sand-sized sediment transport pathways can be inferred. The main vector for sediment transport as defined by the XRD analysis is from San Francisco Bay to the outer coast, where the sand then accumulates on the ebb tidal delta and also moves alongshore. This mineralogical link defines a critical pathway because large volumes of sediment have been removed from the Bay over the last century via channel dredging, aggregate mining, and borrow pit mining, with comparable volumes of erosion from the ebb tidal delta over the same period, in addition to high rates of shoreline retreat along the adjacent, open-coast beaches. Therefore, while previously only a temporal relationship was established, the transport pathway defined by mineralogical and geochemical tracers support the link between anthropogenic activities in the Bay and widespread erosion outside the Bay. The XRD results also establish the regional and local importance of sediment derived from cliff erosion, as well as both proximal and distal fluvial sources. This research is an important contribution to a broader provenance study aimed at identifying the driving forces for widespread geomorphic change in a heavily urbanized coastal-estuarine system.

Dissolved iron measured along surface transects of the CARUSO-EISENEX experiment

Measurements of Fe(II) and H2O2 were carried out in the Atlantic sector of the Southern Ocean during EisenEx, an iron enrichment experiment. Iron was added on three separate occasions, approximately every 8 days, as a ferrous sulfate (FeSO4) solution. Vertical profiles of Fe(II) showed maxima consistent with the plume of the iron infusion. While H2O2 profiles revealed a corresponding minima showing the effect of oxidation of Fe(II) by H2O2, observations showed detectable Fe(II) concentrations existed for up to 8 days after an iron infusion. H2O2 concentrations increased at the depth of the chlorophyll maximum when iron concentrations returned to pre-infusion concentrations (<80 pM) possibly due to biological production related to iron reductase activity. In this work, Fe(II) and dissolved iron were used as tracers themselves for subsequent iron infusions when no further SF6 was added. EisenEx was subject to periods of weak and strong mixing. Slow mixing after the second infusion allowed significant concentrations of Fe(II) and Fe to exist for several days. During this time, dissolved and total iron in the infusion plume behaved almost conservatively as it was trapped between a relict mixed layer and a new rain-induced mixed layer. Using dissolved iron, a value for the vertical diffusion coefficient Kz=6.7±0.7 cm**2/s was obtained for this 2-day period. During a subsequent surface survey of the iron-enriched patch, elevated levels of Fe(II) were found in surface waters presumably from Fe(II) dissolved in the rainwater that was falling at this time. Model results suggest that the reaction between uncomplexed Fe(III) and O2? was a significant source of Fe(II) during EisenEx and helped to maintain high levels of Fe(II) in the water column. This phenomenon may occur in iron enrichment experiments when two conditions are met: (i) When Fe is added to a system already saturated with regard to organic complexation and (ii) when mixing processes are slow, thereby reducing the dispersion of iron into under-saturated waters.

Organic matter based proxies from surface sediments along three transects on the Pakistan continental margin

A valid assessment of selective aerobic degradation on organic matter (OM) and its impact on OM-based proxies is vital to produce accurate environmental reconstructions. However, most studies investigating these effects suffer from inherent environmental heterogeneities. In this study, we used surface samples collected along two meter-scale transects and one longer transect in the northeastern Arabian Sea to constrain initial OM heterogeneity, in order to evaluate selective aerobic degradation on temperature, productivity and alteration indices at the sediment-water interface. All of the studied alteration indices, the higher plant alkane index, alcohol preservation index, and diol oxidation index, demonstrated that they are sensitive indicators for changes in the oxygen regime. Several export production indices, a cholesterol-based stanol/stenol index and dinoflagellate lipid- and cyst-based ratios, showed significant (more than 20%) change only over the lateral oxygen gradients. Therefore, these compounds do not exclusively reflect surface water productivity, but are significantly altered after deposition. Two of the proxies, glycerol dibiphytanyl glycerol tetraether-based TEX86 sea surface temperature indices and indices based on phytol, phytane and pristane, did not show any trends related to oxygen. Nevertheless, unrealistic sea surface temperatures were obtained after application of the TEX86, TEX86L, and TEX86H proxies. The phytol-based ratios were likely affected by the sedimentary production of pristane. Our results demonstrate the selective impact of aerobic organic matter degradation on the lipid and palynomorph composition of surface sediments along a short lateral oxygen gradient and suggest that some of the investigated proxies may be useful tracers of changing redox conditions at the sediment-water interface.

230Th, 231Pa, and 10Be in particle size fractions of an opal-rich sediment of the Atlantic Southern Ocean

This study centers on the question: How sensitive are 231Pa/230Th and 10Be/230Th to sediment composition and redistribution? The natural radionuclides 231Pa, 230Th and 10Be recorded in deep sea sediments are tracers for water mass advection and particle fluxes. We investigate the influence of oceanic particle composition on the element adsorption in order to improve our understanding of sedimentary isotope records. We present new data on particle size specific 231Pa and 10Be concentrations. An additional separation step, based on settling velocities, led to the isolation of a very opal-rich phase. We find that opal-rich particles contain the highest 231Pa and 10Be concentrations, and higher 231Pa/230Th and 10Be/230Th isotope ratios than opal-poor particles. The fractionation relative to 230Th induced by the adsorption to opal-rich particles is more pronounced for 231Pa than for 10Be. We conclude that bulk 231Pa/230Th in Southern Ocean sediments is most suitable as a proxy for past opal fluxes. The comparison between two neighboring cores with rapid and slow accumulation rates reveals that these isotope ratios are not influenced significantly by the intensity of sediment focusing at these two study sites. However, a simulation shows that particle sorting by selective removal of sediment (winnowing) could change the isotope ratios. Consequently, 231Pa/230Th should not be used as paleocirculation proxy in cases where a strong loss of opal-rich material due to bottom currents occurred.

Uvigerina spp. d18O and Mg/Ca measurements from sediment cores JR244-GC528 and MD07-3076

Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected d18O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer d13C and foraminifer/coral 14C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.

Ba/Ca ratios in benthic foraminifera from recent core tops

The carbon isotope ratio (delta13C) and cadmium content (Cd/Ca) of benthic foraminifera shells have been used to reconstruct deep-water circulation patterns of the glacial oceans. These tracers co-vary with phosphorus in the modern ocean because they are nearly quantitatively regenerated from sinking biological debris in the upper water column. Hence they can be used to reconstruct the distribution of labile nutrients in glacial water masses. Independent constraints on glacial deep-ocean circulation patterns could be provided by a tracer of the distribution of silica and alkalinity, the deeply regenerated constituents of planktonic hard parts. Barium shares key aspects of its behaviour with these refractory nutrients because it is removed from solution in surface waters and incorporated into sinking particles which slowly dissolve deep in the water column and in the sediments. The fractionation of Ba between deep-water masses of the major ocean basins is largely controlled by thermohaline circulation patterns, so Ba conforms to different boundary conditions from Cd and 13C. As Ba substitutes into trigonal carbonates, it is a potential palaeoceano-graphic tracer if the Ba content of foraminifera shells reflects ambient dissolved Ba concentrations. Here we present data from Recent core-top benthic foraminifera which indicate that the Ba content of some recent calcitic benthic foraminifera does co-vary with bottom-water Ba.