Biogenic Opal Research Articles

Adsorption and fractionation of thorium and protactinium on nanoparticles in seawater

Laboratory adsorption experiments were conducted to examine the role of particle composition in the scavenging and fractionation of 234Th and 233Pa on nanoparticles in seawater. Nanoparticles with known chemical composition and size were used, including SiO2, CaCO3, Fe2O3, Al2O3 and TiO2, representing biogenic opal, carbonate, metal oxides, and lithogenic particles, respectively. The results indicated that nanoparticles with the same size but different chemical compositions had different affinities for 234Th and 233Pa, giving rise to different partition coefficient (Kd) values. Compared to 234Th, log Kd values of 233Pa had a higher variability on different nanoparticles, ranging from 3.10 to 5.39. In general, the log Kd values for 234Th on CaCO3, Fe2O3, and Al2O3 were higher than those for 233Pa, while the opposite was true on SiO2 and TiO2 nanoparticles, resulting in a significant fractionation between 234Th and 233Pa during their adsorption on nanoparticle surfaces. Among the selected nanoparticles, Fe2O3 had the highest fractionation factor between 234Th and 233Pa (FTh/Pa) while SiO2 had the lowest, following the order of Fe2O3>CaCO3>Al2O3>TiO2>SiO2. In addition to the effect of chemical composition, the concentration of nanoparticles or micro-particles also significantly affected the partitioning of 234Th or 233Pa between dissolved and particulate phases, showing a strong particle concentration effect with a general decrease in log Kd values with increasing particle concentration. Interestingly, results from adsorption experiments with binary nanoparticles, containing both SiO2 and CaCO3, clearly demonstrated an increase in log Kd values of 233Pa with increasing SiO2/CaCO3 ratios of the adsorbent, but a decrease in the fractionation factor between 234Th and 233Pa. However, the enhanced adsorption of 233Pa on SiO2 nanoparticles or the increased log Kd value of 233Pa occurred only when the abundance of SiO2 reached 60% (in wt.) or higher. Thorium, on the other hand, was less sensitive to particle composition during its adsorption on nanoparticles in seawater. Thus, it seems that the Th/Pa ratios of oceanic particles could be largely controlled by the relative abundance of biogenic silica. Further studies are needed to examine the role of natural organic matter, which affects surface properties and chemical speciation of trace elements, in regulating the adsorption of radionuclides on nanoparticles.

Persistent millennial-scale link between Greenland climate and northern Pacific Oxygen Minimum Zone under interglacial conditions

Abstract. The intensity and/or extent of the northeastern Pacific Oxygen Minimum Zone (OMZ) varied in-phase with the Northern Hemisphere high latitude climate on millennial timescales during the last glacial period, indicating the occurrence of atmospheric and oceanic connections under glacial conditions. While millennial variability was reported for both the Greenland and the northern Atlantic Ocean during the last interglacial period, the climatic connections with the northeastern Pacific OMZ has not yet been observed under warm interglacial conditions. Here we present a new geochemical dataset, spanning the past 120 ka, for major components (terrigenous fraction, marine organic matter, biogenic opal, and carbonates) generated by X-ray fluorescence scanning alongside with biological productivity and redox sensitive trace element content (Mo, Ni, Cd) of sediment core MD02-2508 at 23° N, retrieved from the northern limit of the modern OMZ. Based on elemental ratios Si / Ti (proxy for opal), Cd / Al and Ni / Al, we suggest that biological productivity was high during the last interglacial (MIS5). Highly resolved opal reconstruction presents millennial variability corresponding to all the Dansgaard-Oeschger interstadial events over the last interglacial, while the Mo / Al ratio indicates reduced oxygenation during these events. Extremely high opal content during warm interstadials suggests high diatom productivity. Despite the different climatic and oceanic background between glacial and interglacial periods, rapid variability in the northeastern Pacific OMZ seems to be tightly related to Northern Hemisphere high latitude climate via atmospheric and possibly oceanic processes.

The late Pliocene Benguela upwelling status revisited by means of multiple temperature proxies

As compared to the late Pleistocene, Alkenone‐based sea surface temperature (SST) in the Benguela region revealed relatively warm and stable SST recorded between ∼3.5 and 2.0 Ma, and coincide with a period of increasing biological productivity as revealed by increasing deposition of biogenic opal. We assess how the hydrological patterns recorded in SST proxies are embedded in the geological record by performing a proxy‐proxy comparison. We used Laser‐Ablation Inductively Coupled Plasma‐Mass Spectrometry to measure the Mg/Ca on the planktonic foraminifera species Globigerina bulloides, allowing in situ measurements of Mg/Ca on individual foraminiferal tests. Mg/Ca‐derived temperatures provide much colder temperatures than alkenone‐derived SST by up to 10°C. We build a scenario involving contrasting sensitivities of paleothermometers upon the annual cycle, namely alkenones preferentially capturing SST when the surface ocean is warmer than the mean‐annual average SST, and G. bulloides capturing SST when upwelling intensifies. Multichamber analysis also suggests that G. bulloides migrates below the sea surface while calcifying its last chambers prior to gametogenesis, allowing the extraction of both surface and subsurface temperature from Mg/Ca measured on different chambers. The range of temperatures recorded between our multiple SST proxies is supported by the range of temperatures simulated with a general circulation model when different seasons, different water depth and different orbital configurations occurring during the late Pliocene are considered. A greater seasonal cycle in SST during the Pliocene can account for alkenone and Mg/Ca‐derived temperature contrast, pointing to a radically different mode of upwelling activity in the Benguela region compared to today.

New tools for the reconstruction of Pleistocene Antarctic sea ice

Based on the quantitative analysis of diatom assemblages preserved in 274 surface sediment samples recovered in the Pacific, Atlantic and western Indian sectors of the Southern Ocean we have defined a new reference database for quantitative estimation of late-middle Pleistocene Antarctic sea ice fields using the transfer function technique. The Detrended Canonical Analysis (DCA) of the diatom data set points to a unimodal distribution of the diatom assemblages. Canonical Correspondence Analysis (CCA) indicates that not only winter sea ice (WSI) but also summer sea surface temperature (SSST) represent the most prominent environmental variables that control the spatial species distribution. To test the applicability of transfer functions for sea ice reconstruction in terms of concentration and occurrence probability we applied four different methods, the Imbrie and Kipp Method (IKM), the Modern Analog Technique (MAT), Weighted Averaging (WA), and Weighted Averaging Partial Least Squares (WAPLS), using logarithm-transformed diatom data and satellite-derived (1981–2010) sea ice data as a reference. The best performance for IKM results was obtained using a subset of 172 samples with 28 diatom taxa/taxa groups, quadratic regression and a three-factor model (IKM-D172/28/3q) resulting in root mean square errors of prediction (RMSEP) of 7.27% and 11.4% for WSI and summer sea ice (SSI) concentration, respectively. MAT estimates were calculated with different numbers of analogs (4, 6) using a 274-sample/28-taxa reference data set (MAT-D274/28/4an, -6an) resulting in RMSEP ranging from 5.52% (4an) to 5.91% (6an) for WSI as well as 8.93% (4an) to 9.05% (6an) for SSI. WA and WAPLS performed less well with the D274 data set, compared to MAT, achieving WSI concentration RMSEP of 9.91% with WA and 11.29% with WAPLS, recommending the use of IKM and MAT. The application of IKM and MAT to surface sediment data revealed strong relations to the satellite-derived winter and summer sea ice field. Sea ice reconstructions performed on an Atlantic- and a Pacific Southern Ocean sediment core, both documenting sea ice variability over the past 150,000yr (MIS 1–MIS 6), resulted in similar glacial/interglacial trends of IKM and MAT-based sea-ice estimates. On the average, however, IKM estimates display smaller WSI and slightly higher SSI concentration and probability at lower variability in comparison with MAT. This pattern is a result of different estimation techniques with integration of WSI and SSI signals in one single factor assemblage by applying IKM and selecting specific single samples, thus keeping close to the original diatom database and included variability, by MAT. In contrast to the estimation of WSI, reconstructions of past SSI variability remain weaker. Combined with diatom-based estimates, the abundance and flux pattern of biogenic opal represents an additional indication for the WSI and SSI extent.

Decadal shift of biogenic sinking particle flux in the western North Pacific subpolar region

AbstractTime series of biogenic sinking particle flux in the western North Pacific subpolar region over two decades (1989–2008) revealed that the biogenic CaCO3 (CC) flux has shown a significant decreasing trend of 2.7% year−1 (annual average, −0.88 ± 0.13 mg m2 day−1 year−1) along with the decreasing particulate organic carbon (POC) flux of 0.7% year−1, while the biogenic opal (OP) flux had no long‐term trend. Comparing these results with the decreasing rate of satellite‐derived surface CC with −0.7% year−1, we concluded that three fourths of the decreasing trend of CC flux was derived from the strengthening of CaCO3 dissolution through seawater column due to the weakening of water ventilation and the rest was from the decline of CaCO3‐shelled species, indicating the enhancement of the efficiency in oceanic sequestration of atmospheric CO2 in the sea surface of this region due to the increase of OP/CC ratio.

Coccolithophore carbonate during the last 450 ka in the NW Pacific Ocean (ODP site 1209B, Shatsky Rise)

ABSTRACTThis study focuses on Pleistocene–Holocene sediments from the Shatsky Rise (Ocean Drilling Program Site 1209B, NW Pacific Ocean). We quantify the contribution of calcite made by coccoliths in oceanic sediments, investigating the role of calcareous nannoplankton during the last 450 ka. Coccolith carbonate constitutes 60–90% of bulk carbonate. Coccolith carbonate accumulation rates (CARs) and CaCO3 fine fraction confirm that coccolithophores are major contributors to the carbonate export and accumulation. Primary productivity shows highest values from marine isotope stage (MIS) 12 to 8. Thereafter, although coccolith calcite content remains high, other sediment components, such as as foraminifera and biogenic opal, seem to be favored, perhaps related to an increase in fertilization by eolian dust. Our results demonstrate the important role of coccolithophore production and sedimentation on the regulation of ocean carbonate chemistry on time scales >1000–100 000 years. On glacial–interglacial scales, coccolithophore productivity could have affected deep‐water saturation by buffering deep‐sea CO2 through increased carbonate dissolution episodes. Spectral and wavelet analyses are consistent with CARs primarily driven by glacial–interglacial variability and obliquity‐controlled changes. Coccolith‐based paleoceanographic reconstructions allow us to establish that during the last 450 ka the mid‐latitudes of the NW Pacific are controlled by the dynamics of the El Niño Southern Oscillation perturbations and Boreal Monsoon system.

Combined oxygen and silicon isotope analysis of diatom silica from a deglacial subarctic Pacific record

ABSTRACTWe present an SiF4 separation line, coupled to a laser fluorination system, which allows for an efficient combined silica δ18O and δ30Si analysis (50 min per sample). The required sample weight of 1.5–2.0 mg allows for high‐resolution isotope studies on biogenic opal. Besides analytical tests, the new instrumentation set‐up was used to analyse two marine diatom fractions (>63 µm, 10–20 µm) with different diatom species compositions extracted from a Bølling/Allerød–Holocene core section [MD01‐2416, North‐West (NW) Pacific] to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species‐related isotope effects (vital and environmental effects). While δ30Si offsets between the two fractions were not discernible, supporting the absence of species‐related silicon isotope effects, systematic offsets occur between the δ18O records. Although small, these offsets point to species‐related isotope effects, as bias by contamination can be discarded. The new records strengthen the palaeoceanographic history during the last deglaciation in the NW Pacific characterized by a sequence of events with varying surface water structure and biological productivity. With such palaeoceanographic evolution it becomes unlikely that the observed systematic δ18O offsets signal seasonal temperature variability. This calls for reconsideration of vital effects, generally excluded to affect δ18O measurements. Copyright © 2013 John Wiley & Sons, Ltd.

Oxygen isotope analyses of biogenic opal and quartz using a novel microfluorination technique

Measuring δ(18)O values in silicates is difficult and hazardous in comparison with measurements of carbonate minerals due to the difficulty in breaking Si-O-Si bonds. A novel method for measurement of δ(18)O values from quartz and biogenic silica utilizing high-temperature pyrolysis with continuous-flow isotope ratio mass spectrometry (CF-IRMS) is presented. Samples were prepared by offline dehydroxylation/dehydration under vacuum at 1060°C. The dehydroxylated samples are mixed with polytetrafluoroethylene (PTFE) powder (2.3:1 PTFE/Si) and graphite in silver foil capsules and reacted in a vario PYRO cube TC/EA system in a glassy carbon reaction tube. Quartz and biogenic opal samples react with available carbon in a microfluorination environment upon decomposition of the PTFE, producing CO for analysis via CF-IRMS. Silicate samples reacted quantitatively, producing data with yields ≥88% from ~400 µg samples. Multiple analyses with international standards demonstrated accuracy for replicate measurements (1σ range ±0.3-0.6‰), comparing favorably with previously published techniques. New data produced with the microfluorination technique are comparable with data from studies using existing methodologies. The microfluorination technique has the potential to greatly increase the number of laboratories producing silicate oxygen isotope data for mineralogic, paleoclimatic and biogeochemical applications.

The zinc isotopic composition of siliceous marine sponges: Investigating nature's sediment traps

The zinc (Zn) content and isotopic composition of marine biogenic opal have the potential to yield information about the nutrient availability, utilization and export of particulate organic matter from surface to deep waters. Here, we report the first measurements of the Zn isotopic composition of deep-sea marine sponge skeletal elements – spicules – collected in the Southern Ocean. Our results highlight different Zn uptake and isotopic fractionation behavior between the two major siliceous sponge clades (hexactinellids and demosponges), which is most likely linked to sponge feeding strategy. Hexactinellid spicule Zn isotopic compositions are not fractionated with respect to seawater, most likely due to Zn transport via the open internal structure of the sponges. In contrast, demosponge spicules exhibit a wide range of Zn isotopic compositions that are related to the opal Zn concentration, most likely reflecting variable Zn isotope compositions in the organic matter particles on which they feed, and internal fractionation processes.

Changes in biogenic opal productivity with Milankovitch cycles during the last 1.3 Ma at IODP Expedition 323 Sites U1341, U1343, and U1345 in the Bering Sea

Sediments from three Sites (U1341, U1343 and U1345) of IODP Expedition 323, located in the Bering Sea, were analyzed for variation of biogenic opal representing surface water paleoproductivity. Initial age models of three Sites (bottom age, U1341: 4.3 Ma, U1343: 2.4 Ma, U1345: 0.5 Ma) were established onboard by biostratigraphic and paleomagnetic investigations. The initial age models were tuned by δ18O stratigraphy, which was established on shore. Biogenic opal contents of sediments were analyzed for the interval of 1.3 Ma to the present. Generally, the %biogenic opal values at Site U1341 (Bowers Ridge) were significantly higher than those at the Beringian slope sites (U1343 and U1345). Higher natural gamma radiation (NGR) counts and magnetic susceptibility (MS) at the Beringian slope sites suggested a greater contribution of lithogenic matter than at Site U1341 on Bowers Ridge. The percentages of sea-ice diatoms at the Beringian slope sites are also higher than those at Bowers Ridge Sites. This is primarily attributed to the fact that the sediments from the Beringian slope area were significantly diluted by lithogenic matter derived from the Alaskan continent, and were largely affected by sea-ice cover. The cyclicity of biogenic opal accumulation rates at Site U1341 is determined to be about 160, 40, and 30 ky, whereas that at Site U1343 is determined to be about 330, 100, 40 and 20 ky. If these had been reflecting a global climate change, the cyclicity of biogenic opal contents at all sites would be consistently the same. However, the results do not show such a consistency. Obliquity is observed at both sites of the Bowers Ridge and Beringian slope. A strong eccentricity was observed only at U1343, but no signature for eccentricity was detected at U1341. Therefore, the Milankovitch cycle with 100 ky periodicity was only observed at the Beringian slope area. The results suggest that the %biogenic opal at Site U1341 indicated a local characteristic for biogenic productivity changes. Thus, the absence of the 100 ky cycle of biogenic opal at Site U1341 seems to be due to local phenomena such as water circulation, although the changes in biogenic opal productivity at Site U1341 are attributed to summer insolation.

Changes in silicate utilisation and upwelling intensity off Peru since the Last Glacial Maximum – insights from silicon and neodymium isotopes

We combine the stable silicon isotope composition (δ30Si) of diatoms and the radiogenic neodymium isotope compositions (εNd) of past seawater extracted from the authigenic fraction of the sediments (Mn–Fe coatings of particles and benthic foraminifers), as well as the radiogenic isotope compositions (Nd, Sr) of the detrital material itself to reconstruct silicic acid utilisation, water mass mixing, and upwelling intensity from the same marine sediments in the Peruvian upwelling region during the past 20,000 years. Additionally, the sedimentary signals were compared to the water column isotope compositions.Along the Peruvian shelf, the dissolved εNd in the water column ranges from −5.7 to +0.6. The corresponding εNd signatures of the coatings and the benthic foraminifers of the surface sediments range from −4.5 to +1.8 and from −2.5 to +2.2, respectively. The detrital εNd (87Sr/86Sr) signatures range from −6.3 to 0 (0.70508–0.71049). All phases show a trend from more radiogenic εNd values in the north towards less radiogenic values in the south broadly reflecting local weathering inputs and hinterland geology.The εNd signatures of the coatings extracted from sediment core SO147-106KL located in the present day centre of coastal upwelling near 12°S have been essentially constant (−1.5) throughout the past 20,000 years, while the detrital εNd (87Sr/86Sr) varied between values of −0.7 (0.70620) during the Last Glacial and −4.5 (0.70849) during the late Holocene reflecting changes in the origin of the sediment and current transport from a more northerly towards a more southerly source and from overall stronger to weaker upwelling. The δ30Si signature of both total biogenic opal (δ30Siopal) and of hand-picked diatoms (δ30Sidiatom) ranged from +0.3‰ (Last Glacial) to +1.4‰ (late Holocene) confirming large variations in upwelling intensity driving silicic acid utilisation by diatoms. Our combined bSiO2 MAR, δ30Siopal and detrital radiogenic isotope results demonstrate that the strongest El Niño-Southern Oscillation conditions off Peru of the past 20 ka have prevailed during the past 5 ka.

A new perspective on boundary scavenging in the North Pacific Ocean

Boundary scavenging, or the enhanced removal of adsorption-prone elements from the ocean in areas of high particle flux, is an often cited, though not well-quantified, concept used to understand the oceanic distribution of many trace metals. Because 230Th and 231Pa are produced uniformly from uranium decay and removed differentially by scavenging, the process of boundary scavenging can be elucidated by a more detailed knowledge of their water column distributions. To this end, filtered seawater was collected across the gradients in particle flux which span the subarctic Pacific: in the west during the Innovative North Pacific Experiment (INOPEX) and in the east along Line P. Lateral concentration gradients of dissolved 230Th are small throughout the subarctic Pacific at 12 sites of variable particle flux. This contradicts the prediction of the traditional boundary scavenging model. A compilation of water column data from throughout the North Pacific reveals much larger lateral concentration gradients for 230Th between the subarctic North Pacific and subtropical gyre, over lateral gradients in scavenging intensity similar to those found within the subarctic. This reflects a biogeochemical-province aspect to scavenging. Upper water column distributions of 231Pa and 231Pa/230Th ratio are consistent with the influence of scavenging by biogenic opal, while deep waters (>2.5km) reveal an additional 231Pa sink possibly related to manganese oxides produced at continental margins or ridge crests.

Large-scale depositional characteristics of the Ulleung Basin and its impact on electrical resistivity and Archie-parameters for gas hydrate saturation estimates

Gas hydrate saturation estimates were obtained from an Archie-analysis of the Logging-While-Drilling (LWD) electrical resistivity logs under consideration of the regional geological framework of sediment deposition in the Ulleung Basin, East Sea, of Korea. Porosity was determined from the LWD bulk density log and core-derived values of grain density. In situ measurements of pore-fluid salinity as well as formation temperature define a background trend for pore-fluid resistivity at each drill site. The LWD data were used to define sets of empirical Archie-constants for different depth-intervals of the logged borehole at all sites drilled during the second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2). A clustering of data with distinctly different trend-lines is evident in the cross-plot of porosity and formation factor for all sites drilled during UBGH2. The reason for the clustering is related to the difference between hemipelagic sediments (mostly covering the top ∼100 mbsf) and mass-transport deposits (MTD) and/or the occurrence of biogenic opal. For sites located in the north-eastern portion of the Ulleung Basin a set of individual Archie-parameters for a shallow depth interval (hemipelagic) and a deeper MTD zone was achieved. The deeper zone shows typically higher resistivities for the same range of porosities seen in the upper zone, reflecting a shift in sediment properties. The presence of large amounts of biogenic opal (up to and often over 50% as defined by XRD data) was especially observed at Sites UBGH2-2_1 and UBGH2-2_2 (as well as UBGH1-9 from a previous drilling expedition in 2007). The boundary between these two zones can also easily be identified in gamma-ray logs, which also show unusually low readings in the opal-rich interval. Only by incorporating different Archie-parameters for the different zones a reasonable estimate of gas hydrate saturation was achieved that also matches results from other techniques such as pore-fluid freshening, velocity-based calculations, and pressure-core degassing experiments. Seismically, individual boundaries between zones were determined using a grid of regional 2D seismic data. Zoning from the Archie-analysis for sites in the south-western portion of the Ulleung Basin was also observed, but at these sites it is linked to individually stacked MTDs only and does not reflect a mineralogical occurrence of biogenic opal or hemipelagic sedimentation. The individual MTD events represent differently compacted material often associated with a strong decrease in porosity (and increase in density), warranting a separate set of empirical Archie-parameters.

Cleaning methods for the isotopic determination of diatombound nitrogen in non‐fossil diatom frustules

Ground‐truthing the stable isotope ratio of diatom frustule‐bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical differences from sedimentary fossil diatom, for analysis. We tested and compared the performance of five oxidative treatment protocols in cleaning fresh diatom frustules of external organic material for isotopic analysis of diatom frustule‐bound N. We applied each treatment to diatom material from a T. weiss‐flogii culture and measured the diatom‐bound N content and isotopic composition (δ15N) of samples. A favored treatment is described that includes a multi‐step, progressive, oxidative cleaning. The results suggest that the application of H2O2 to fresh biogenic opal for more than 20 min increased the accessibility of the N within the opal matrix to subsequent chemical treatment, an effect not observed for diatom frustules from deep ocean sediments. We attribute this effect to structural alteration and dissolution of the diatom opal by the H2O2, consistent with previous observations regarding the high reactivity of fresh diatom opal and its subsequent chemical hardening in the sedimentary environment. In addition, the presence of high levels of organic matter upon treatment with H2O2 may exacerbate the chemical alteration of frustules. We find that the δ15N of diatom‐bound N is ~3 ‰ higher than that of the bulk culture biomass, consistent with expectations from surface sediment and down‐core measurements.

Chemical composition of plant silica phytoliths

Silica phytoliths are a subgroup of biogenic opal. Silica phytoliths are formed in many plant species and remain preserved in soil and sediments after plant decay. The chemical composition of fossil phytoliths may reveal ancient plant taxa, soil composition and climate. However, actually detailed knowledge on silica phytolith composition is scarce. Here we present result of instrumental neutron activation analysis of barley awns, stems and leaves, and barley phytoliths. The elements of interest were Na, Mg, Al, Si, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Zn, As, Br, Rb, Sb, Cs, Ba, La, Ce, Sm, Eu, Tb, Dy, Yb, Hf, Ta, W, Th, and U. We compared three phytolith extraction methods: dry ashing, acid digestion, and acid digestion followed by incineration. We found that sole acid digestion is inefficient to remove organic matter. By contrast both dry ashing and acid digestion followed by incineration are suitable for phytolith analysis. Comparison of phytoliths with their source plant material shows that phytoliths are enriched in terrigenous elements such as Al, Sc, Ti, V, Cs, Fe, rare earth elements, and depleted in the major inorganic constituents of plants such as K, Ca, Mg, Mn, Cl and Br.

Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean

Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12pmol/kg and 10pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as εNd=−18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to εHf=6.1 and εNd=−4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (εNd∼−8 to −8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between εHf=+6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from εNd=−18.9 to −4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.

Nitrogen isotope gradients off Peru and Ecuador related to upwelling, productivity, nutrient uptake and oxygen deficiency

We present new nitrogen isotope data from the water column and surface sediments for paleo–proxy validation collected along the Peruvian and Ecuadorian margins between 1°N and 18°S. Productivity proxies in the bulk sediment (organic carbon, total nitrogen, biogenic opal, C37 alkenone concentrations) and 15N/14N ratios were measured at more than 80 locations within and outside the present-day Peruvian oxygen minimum zone (OMZ). Microbial N-loss to N2 in subsurface waters under O2 deficient conditions leaves a characteristic 15N-enriched signal in underlying sediments. We find that phytoplankton nutrient uptake in surface waters within the high nutrient, low chlorophyll (HNLC) regions of the Peruvian upwelling system influences the sedimentary signal as well. How the δ15Nsed signal is linked to these processes is studied by comparing core-top values to the 15N/14N of nitrate and nitrite (δ15NNOx) in the upper 200 m of the water column. Between 1°N and 10°S, subsurface O2 is still high enough to suppress N-loss keeping δ15NNOx values relatively low in the subsurface waters. However δ15NNOx values increase toward the surface due to partial nitrate utilization in the photic zone in this HNLC portion of the system. δ15Nsed is consistently lower than the isotopic signature of upwelled NO3−, likely due to the corresponding production of 15N depleted organic matter. Between 10°S and 15°S, the current position of perennial upwelling cells, HNLC conditions are relaxed and biological production and near-surface phytoplankton uptake of upwelled NO3− are most intense. In addition, subsurface O2 concentration decreases to levels sufficient for N-loss by denitrification and/or anammox, resulting in elevated subsurface δ15NNOx values in the source waters for coastal upwelling. Increasingly higher production southward is reflected by various productivity proxies in the sediments, while the north–south gradient towards stronger surface NO3− utilization and subsurface N-loss is reflected in the surface sediment 15N/14N ratios. South of 10°S, δ15Nsed is lower than maximum water column δ15NNOx values most likely because only a portion of the upwelled water originates from the depths where highest δ15NNOx values prevail. Though the enrichment of δ15NNOx in the subsurface waters is unambiguously reflected in δ15Nsed values, the magnitude of δ15Nsed enrichment depends on both the depth of upwelled waters and high subsurface δ15NNOx values produce by N-loss. Overall, the degree of N-loss influencing subsurface δ15NNOx values, the depth origin of upwelled waters, and the degree of near-surface nitrate utilization under HNLC conditions should be considered for the interpretation of paleo δ15Nsed records from the Peruvian oxygen minimum zone.

Vertical fluxes of particulate biogenic material through the euphotic and twilight zones in the Cariaco Basin, Venezuela

Surface-tethered particle interceptor traps (PITs) were deployed at 50 and 100m (1–3 days) on ten occasions in the Cariaco Basin between March 2007 and November 2009 to measure the settling fluxes of biogenic particles at 50m (the base of the euphotic zone—Ez) and 100m. Fluxes at these two depths were compared to concurrent fluxes estimated with moored sediment traps at 150, 225 and 410m from the CARIACO Ocean Time-Series program. We measured particulate organic carbon (POC), particulate organic nitrogen (PON), calcium carbonate, biogenic silica and terrigenous material concentrations in samples collected with both the drifting and moored traps. We also estimated the fluxes of foraminifera shells and coccolithophore cells at 50 and 100m using drifting traps samples. Surface chlorophyll a and primary production observations during each sampling period were examined to quantify the relationship between the magnitude and geochemical composition of the vertical flux and overlying production. Surface chlorophyll a concentrations and primary production rates were highest during months of upwelling (2.58−1.35mgm−3 and 3.6−1.4g Cm−2d−1, respectively). The fluxes of POC, PON, calcite and silica measured during the upwelling season (December–May) were typically higher than during the period of non-upwelling (August–November), when surface waters are more strongly stratified. POC fluxes measured with the drifting traps (50 and 100m) varied between 0.95 (upwelling) and 0.14gm−2d−1 (non-upwelling), compared with those from the moored traps (150, 225 and 410m) which ranged from 0.21 to 0.01gm−2d−1. Similarly, the fluxes of biogenic opal in the upper 100m ranged from 1.12 and 0.18gm−2d−1, and those at greater depths varied from 0.27gm−2d−1 during upwelling to values near zero during stratification periods. The fluxes of POC, PON, calcite and silica in the upper 100m decreased by an order of magnitude at the depth of the oxic–anoxic interface (>200m). The sinking organic matter collected with the floating traps within the upper 100m was significantly correlated with surface chlorophyll a concentrations (r=0.68, p<0.05) as a result of close coupling between the flux of biogenic particles and primary production. In contrast, there was no clear relationship between surface chlorophyll/primary production and fluxes measured below 150m depth.

Trace metals in the NE Atlantic coastal zone of Finisterre (Iberian Peninsula): Terrestrial and marine sources and rates of sedimentation

The biogeochemical patterns of bioactive metals (Cd, Co, Cu, Ni, V and Zn) and Pb were studied in the Corcubión Sound, an area close to Cape Finisterre (NW Iberian Peninsula). Seasonal patterns of dissolved and particulate trace metals in freshwater and seawater, as well as in phytoplankton were determined during three upwelling events in the winter, late spring and summer of 2005. Metals were also analyzed in the surface and to 1m-depth in the sediments. Upwelling patterns permit the discrimination between those of the outer Sound and Corcubión Inlet. Dissolved trace metal concentrations in the seawater of the outer Sound were in the range of those reported for the Northeast Atlantic Ocean. Mean values of dissolved metal concentrations in the upwelled ENACW were: 0.07±0.03nMCd, 0.11±0.02nMCo, 2.6±1.6nMCu, 4.6±0.6nMNi, 0.20±0.10nMPb, 31±2nMV and 3.8±2.3nMZn. Higher values for dissolved Co, Cu and Zn were measured in two inflowing streams and in the Inlet. A severe–moderate contamination of these metals and Pb was detected in the sediments of the Inlet. Three different sediment layers were observed in cores located at the outer-Inlet boundary. The most recent showed the highest mass accumulation rates, mainly of Cu (e.g. 428mgCu·m−2·yr−1), Pb and Zn, during the last decade. The intermediate unit corresponds to the 20th century (e.g. 39mgCu·m−2·yr−1) and the deeper layer from the 19th to 14th centuries can be regarded as representing the pre-industrial period, and shows the lowest rates of metal accumulation (e.g. 7mgCu·m−2·yr−1). Diatoms were the main autochthonous source of metals in the sediments. The genera Chaetoceros and Lauderia dominate the overlaying seawater during winter and Pseudonitzschia and Leptocylindrus were typical of summer blooms while in the sediments Chaetoceros, Paralia, Leptocylindrus and Thalassionema were the main diatom genera. The comparison of the normalized metal vs. Si in the phytoplankton and vs. Si in the biogenic opal of the sediment shows a prevailing lithogenic–anthropogenic origin of metals such as Cu, Fe, Ni, Pb and V while the upwelling was the main source for Cd in the sediments because of the settling of diatoms. The continental influence and metal contamination are restricted to the innermost part of Sound during the last century where the anthropogenic activities supply Cd, Cu, Pb and Zn. Seasonally, dissolved trace metal concentrations in the outer Sound are within the normal range measured in the north-eastern Atlantic Ocean. The Sound system is pristine compared with the neighboring and anthropogenically disturbed rias, and may be a good reference of natural trace metal levels along the north-eastern Atlantic coast.

The annual particle cycle in Lake Van (Turkey)

The varved sediments of Lake Van provide a high-quality continental archive of seasonal to decadal-scale climate variability. In order to read the natural record, modern varve formation was studied on the basis of (1) remotely-sensed total suspended-matter (TSMrs) concentrations; (2) time-series of particle flux and water temperatures; and (3) turbidity, temperature, and oxygen profiles. TSMrs, validated by contemporaneous water-column sampling, shows great temporal and lateral variations (whitings and turbidity plumes). From 2006 to 2009, sequential sediment traps recorded high particle fluxes during spring and fall, medium fluxes during summer, and almost zero flux during winter. The mean total mass flux of 403mgm−2day−1 comprised 33% (seasonally up to 67%) calcium carbonate, 7% aquatic organic matter, 6% biogenic opal, and 54% detrital minerals. The CaCO3 fluxes are controlled by river discharge (precipitation and snowmelt) during spring, by high productivity during summer, and by river discharge (precipitation before snowfall starts) and mixing during fall. In November 2007, an anomalously high CaCO3 flux occurred as a result of a warm water surface supersaturated with calcite coinciding with an anomalous runoff event. The results demonstrate that the couplets of light and dark laminae in the short sediment cores are true varves representing spring–summer–fall and winter conditions, respectively. Consequently, varve formation can be linked to the seasonal climate pattern, providing a calibration that can be used to interpret the partially varved paleo-record of Lake Van and related environmental processes.