Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

Sulphur chemistry of the ODP Site 206-1256 volcanic section

Sulfide mineralogy and the contents and isotope compositions of sulfur were analyzed in a complete oceanic volcanic section from IODP Hole 1256D in the eastern Pacific, in order to investigate the role of microbes and their effect on the sulfur budget in altered upper oceanic crust. Basalts in the 800 m thick volcanic section are affected by a pervasive low-temperature background alteration and have mean sulfur contents of 530 ppm, reflecting loss of sulfur relative to fresh glass through degassing during eruption and alteration by seawater. Alteration halos along fractures average 155 ppm sulfur and are more oxidized, have high SO4/Sum S ratios (0.43), and lost sulfur through oxidation by seawater compared to host rocks. Although sulfur was lost locally, sulfur was subsequently gained through fixation of seawater-derived sulfur in secondary pyrite and marcasite in veins and in concentrations at the boundary between alteration halos and host rocks. Negative d34S[sulfide-S] values (down to -30 per mil) and low temperatures of alteration (down to ~40 °C) point to microbial reduction of seawater sulfate as the process resulting in local additions of sulfide-S. Mass balance calculations indicate that 15-20% of the sulfur in the volcanic section is microbially derived, with the bulk altered volcanic section containing 940 ppm S, and with d34S shifted to -6.0 per mil from the mantle value (0 per mil). The bulk volcanic section may have gained or lost sulfur overall. The annual flux of microbial sulfur into oceanic basement based on Hole 1256D is 3-4 * 10**10 mol S/yr, within an order of magnitude of the riverine sulfate source and the sedimentary pyrite sink. Results indicate a flux of bacterially derived sulfur that is fixed in upper ocean basement of 7-8 * 10**-8 mol/cm**-2/yr1 over 15 m.y. This is comparable to that in open ocean sediment sites, but is one to two orders of magnitude less than for ocean margin sediments. The global annual subduction of sulfur in altered oceanic basalt lavas based on Hole 1256D is 1.5-2.0 * 10**11 mol/yr, comparable to the subduction of sulfide in sediments, and could contribute to sediment-like sulfur isotope heterogeneities in the mantle.

Geochemistry of Hawaiian volcanics

The "Ko'olau" component of the Hawaiian mantle plume represents an extreme (EM1-type) end member of Hawaiian shield lavas in radiogenic isotope space, and was defined on the basis of the composition of subaerial lavas exposed in the Makapu'u section of Ko'olau Volcano. The 679 m-deep Ko'olau Scientific Drilling Project (KSDP) allows the long-term evolution of Ko'olau Volcano to be reconstructed and the longevity of the "Ko'olau" component in the Hawaiian plume to be tested. Here, we report triple spike Pb isotope and Sr and Nd isotope data on KSDP core samples, and rejuvenation stage Honolulu Volcanics (HV) (together spanning ~2.8 m.y.), and from ~110 Ma basalts from ODP Site 843, thought to be representative of the Pacific lithosphere under Hawai'i. Despite overlapping ranges in Pb isotope ratios, KSDP and HV lavas form two distinct linear arrays in 208Pb/204Pb-206Pb/204Pb isotope space. These arrays intersect at the radiogenic end indicating they share a common component. This "Kalihi" component has more radiogenic Pb, Nd, Hf, but less radiogenic Sr isotope ratios than the "Makapu'u" component. The mixing proportions of these two components in the lavas oscillated through time with a net increase in the "Makapu'u" component upsection. Thus, the "Makapu'u" enriched component is a long-lived feature of the Hawaiian plume, since it is present in the main shield-building stage KSDP lavas. We interpret the changes in mixing proportions of the Makapu'u and Kalihi components as related to changes in both the extent of melting as well as the lithology (eclogite vs. peridotite) of the material melting as the volcano moves away from the plume center. The long-term Nd isotope trend and short-term Pb isotope fluctuations seen in the KSDP record cannot be ascribed to a radial zonation of the Hawaiian plume: rather, they reflect the short length-scale heterogeneities in the Hawaiian mantle plume. Linear Pb isotope regressions through the HV, recent East Pacific Rise MORB and ODP Site 843 datasets are clearly distinct, implying that no simple genetic relationship exists between the HV and the Pacific lithosphere. This observation provides strong evidence against generation of HV as melts derived from the Pacific lithosphere, whether this be recent or old (100 Ma). The depleted component present in the HV is unlike any MORB-type mantle and most likely represents material thermally entrained by the upwelling Hawaiian plume and sampled only during the rejuvenated stage. The "Kalihi" component is predominant in the main shield building stage lavas but is also present in the rejuvenated HV. Thus this material is sampled throughout the evolution of the volcano as it moves from the center (main shield-building stage) to the periphery (rejuvenated stage) of the plume. The presence of a plume-derived material in the rejuvenated stage has significant implications for Hawaiian mantle plume melting models.

Biogeochemistry measurements in a mesocosm experiment in the Bay of Hopavagen, Norway, in August 2012

The goal of this work has been to examine the influence of upper ocean food web structure and functioning on both the natural and artificially enhanced sequestration of carbon within the ocean. Data obtained in the mesocosm experiment run in the Bay of Hopavågen in August 2012 are used to assess the extent to which organic matter produced within four different food webs is retained in the upper ocean food web versus remineralized back to carbon dioxide and inorganic nutrients (ammonium, dissolved silicon, phosphate) versus exported from the system in the form of rapidly sinking particles. The experiment was carried out in a set of 12 mesocosms covering, in triplicate, 2 different phytoplankton communities (diatom versus non-diatom) exposed to 2 different zooplankton communities (-copepod and +copepod). These starting conditions were established by first filling the bags, roughly simultaneously, with seawater from the Bay of Hopavågen. Mesozooplankton were then removed to the most complete extent possible immediately removed from half of the mesocosms through repeated vertical hauls of a plankton net (200 µm mesh). Nitrate and phosphate was added to half mesocosms daily to promote the growth of non-siliceous phytoplankton (e.g. dinoflagellates or coccolithophores). To the other half of the mesocosms, nitrate, phosphate, and silicate were added to promote the growth of diatoms. Material was allowed to settle and the two distinct phytoplankton populations were allowed to develop for 4 days, after which copepods collected from the Bay of Hopavågen were added back to the half of the N+P mesocosms and to the half of the N+P+Si mesocosms from which mesozooplankton had not been removed at the beginning. This yielded a set of four initial starting conditions (N+P-copepods, N+P+copepods, N+P+Si-copepods, and N+P+Si+copepods). In the primary mesocosms, samples for a set of core parameters were taken every time the mesocosms were sampled. Samples for particulates (PIC, BSi, POC, PON) were collected on GF/F or 0.4 µm polycarbonate.

Neodymium isotopes in benthic foraminifera from the South Atlantic

Records of the past neodymium (Nd) isotope composition of the deep ocean can resolve ambiguities in the interpretation of other tracers. We present the first Nd isotope data for sedimentary benthic foraminifera. Comparison of the epsilon-Nd of core-top foraminifera from a depth transect on the Cape Basin side of the Walvis Ridge to published seawater data, and to the modern dissolved SiO2- epsilon-Nd trend of the deep Atlantic, suggests that benthic foraminifera represent a reliable archive of the deep water Nd isotope composition. Neodymium isotope values of benthic foraminifera from ODP Site 1264A (Angola Basin side of the Walvis Ridge) from the last 8 Ma agree with Fe-Mn oxide coatings from the same samples and are also broadly consistent with existing fish teeth data for the deep South Atlantic, yielding confidence in the preservation of the marine Nd isotope signal in all these archives. The marine origin of the Nd in the coatings is confirmed by their marine Sr isotope values. These important results allow application of the technique to down-core samples. The new epsilon-Nd datasets, along with ancillary Cd/Ca and Nd/Ca ratios from the same foraminiferal samples, are interpreted in the context of debates on the Neogene history of North Atlantic Deep Water (NADW) export to the South Atlantic. In general, the epsilon-Nd and delta13C records are closely correlated over the past 4.5 Ma. The Nd isotope data suggest strong NADW export from 8 to 5 Ma, consistent with one interpretation of published delta13C gradients. Where the epsilon-Nd record differs from the nutrient-based records, changes in the pre-formed delta13C or Cd/Ca of southern-derived deep water might account for the difference. Maximum NADW-export for the entire record is suggested by all proxies at 3.5-4 Ma. Chemical conditions from 3 to 1 Ma are totally different, showing, on average, the lowest NADW export of the record. Modern-day values again imply NADW export that is about as strong as at any stage over the past 8 Ma.

Seawater carbonate chemistry, POC, PIC, TPC, SPM, N, TEP and growth rate during experiments with coccolithophores Emiliania huxleyi (AC472), Calcidiscus leptoporus (AC370) and Syracosphaera pulchra (AC418) during experiments, 2011

The response of three coccolithophores (Emiliania huxleyi, Calcidiscus leptoporus and Syracosphaera pulchra) to elevated partial pressure (pCO2) of carbon dioxide was investigated in batch cultures. For the first time, we also report on the response of the non calcifying (haploid) life stage of these three species. The growth rate, cell size, inorganic (PIC) and organic carbon (POC) of both life stages were measured at two different pCO2 (400and 760 ppm) and their organic and inorganic carbon production calculated. The two lifestages within the same species generally exhibited a similar response to elevated pCO2, theresponse of the haploid stage being often more pronounced than that of the diploid stage. Thegrowth rate was consistently higher at higher pCO2 but the response of other processes varied among species. The calcification rate of C. leptoporus and of S. pulchra did not change at elevated pCO2 while increased in E. huxleyi. The POC production as well as the cell size of both life stages of S. pulchra and of the haploid stage of E. huxleyi markedly decreased at elevated pCO2. It remained unaltered in the diploid stage of E. huxleyi and C. leptoporus and increased in the haploid stage of the latter. The PIC:POC ratio increased in E. huxleyi and was constant in C. leptoporus and S. pulchra. These results suggest that the non-calcifying stage, is more responsive than the calcifying stage and that the most versatile genera will proliferate in a more acidic ocean rather than all coccolithophores will decline.

Palynological record of Lake Nero in Russia

This collection prepared to IX Congress of INQUA containes 25 articles concerning general and regional problems of Pleistocene. The chronological scale of the Late Pliocene and Pleistocene, climatical cycles and methods of the absolute dating are considered. Some data obtained by means ef paleomagnetic, thermoluminescence and radiocarbon methods at several point sections (Likhvin, Rostov-Jarosiavsky, Priasovje, Ob-garm, Chagan, Pryobskoje Plateau, Lower Volga) are given.

Sea surface temperature reconstruction of sediment core GeoB6518-1

Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer insolation (Kutzbach and Liu, 1997, doi:10.1126/science.278.5337.440; Partridge et al., 1997, doi:10.1016/S0277-3791(97)00005-X). Conversely, short-term precipitation changes in the northern African tropics have been linked to North Atlantic sea surface temperature anomalies, affecting the northward extension of the Intertropical Convergence Zone and its associated rainbelt (Hastenrath, 1990, doi:10.1002/joc.3370100504, Street-Perrott and Perrott, 1990, doi:10.1038/343607a0). Our knowledge of large-scale hydrological changes in equatorial Africa and their forcing factors is, however, limited (Gasse, 2000, doi:10.1016/S0277-3791(99)00061-X). Here we analyse the isotopic composition of terrigenous plant lipids, extracted from a marine sediment core close to the Congo River mouth, in order to reconstruct past central African rainfall variations and compare this record to sea surface temperature changes in the South Atlantic Ocean. We find that central African precipitation during the past 20,000 years was mainly controlled by the difference in sea surface temperatures between the tropics and subtropics of the South Atlantic Ocean, whereas we find no evidence that changes in the position of the Intertropical Convergence Zone had a significant influence on the overall moisture availability in central Africa. We conclude that changes in ocean circulation, and hence sea surface temperature patterns, were important in modulating atmospheric moisture transport onto the central African continent.