Geochemistry and diatom abundances in sediment trap samples of Effingham Inlet, Canada

Sediment traps were deployed inside the anoxic inner basin of Effingham Inlet and at the oxygenated mouth of the inlet from May 1999 to September 2000 in a pilot study to determine the annual depositional cycle and impact of the 1999-2000 La Niña event within a western Canadian inlet facing the open Pacific Ocean. Total mass flux, geochemical parameters (carbon, nitrogen, opal, major and minor element contents, and stable isotope ratios) and diatom assemblages were determined and compared with meteorological and oceanographic data. Deposition was seasonal, with coarser grained terrestrial components and benthic diatoms settling in the autumn and winter, coincident with the rainy season. Marine sedimentary components and abundant pelagic diatoms were coincident with coastal upwelling in the spring and summer. Despite the seasonal differences in deposition, the typical temperate-zone Thalassiosira-Skeletonema-Chaetoceros bloom succession was muted. A July 1999 total mass flux peak and an increase in biogenous components coincided with a rare bottom-water oxygen renewal event in the inlet. Likewise, there were cooler-than-average sea surface temperatures (SSTs) just outside the inlet, and unusually high abundances of a previously undescribed cool-water marine diatom (Fragilariopsis pacifica sp. nov.) within the inlet. Each of these occurrences likely reflects a response to the strong La Niña that followed the year after the strongest-ever recorded El Niño event of 1997-1998. By the autumn of 1999, SSTs had returned to average, and F. pacifica had all but disappeared from the remaining trap record, indicating that oceanographic conditions had returned to normal. Oxygenation events were not witnessed in the inlet in the years before or after 1999, suggesting that a rare oceanographic and climatic event was captured by this sediment trap time series. The data from this record can therefore be used as a benchmark for identifying anomalous environmental conditions on this coast.

Particulate organic matter (POM) of samples from the Lena River delta

Particulate organic matter (POM) derived from permafrost soils and transported by the Lena River represents a quantitatively important terrestrial carbon pool exported to Laptev Sea sediments (next to POM derived from coastal erosion). Its fate in a future warming Arctic, i.e., its remobilization and remineralization after permafrost thawing as well as its transport pathways to and sequestration in marine sediments, is currently under debate. We present one of the first radiocarbon (14C) data sets for surface water POM within the Lena Delta sampled in the summers of 2009 - 2010 and spring 2011 (n = 30 samples). The bulk D14C values varied from -55 to -391 per mil translating into 14C ages of 395 to 3920 years BP. We further estimated the fraction of soil-derived POM to our samples based on (1) particulate organic carbon to particulate nitrogen ratios (POC : PN) and (2) on the stable carbon isotope (d13C) composition of our samples. Assuming that this phytoplankton POM has a modern 14C concentration, we inferred the 14C concentrations of the soil-derived POM fractions. The results ranged from -322 to -884 per mil (i.e., 3060 to 17 250 14C years BP) for the POC : PN-based scenario and from -261 to -944 per mil (i.e., 2370 to 23 100 14C years BP) for the d13C-based scenario. Despite the limitations of our approach, the estimated D14C values of the soil-derived POM fractions seem to reflect the heterogeneous 14C concentrations of the Lena River catchment soils covering a range from Holocene to Pleistocene ages better than the bulk POM D14C values. We further used a dual-carbon-isotope three-end-member mixing model to distinguish between POM contributions from Holocene soils and Pleistocene Ice Complex (IC) deposits to our soil-derived POM fraction. IC contributions are comparatively low (mean of 0.14) compared to Holocene soils (mean of 0.32) and riverine phytoplankton (mean of 0.55), which could be explained with the restricted spatial distribution of IC deposits within the Lena catchment. Based on our newly calculated soil-derived POM D14C values, we propose an isotopic range for the riverine soil-derived POM end member with D14C of -495 ± 153 per mil deduced from our d13C-based binary mixing model and d13C of -26.6 ± 1 per mil deduced from our data of Lena Delta soils and literature values. These estimates can help to improve the dual-carbon-isotope simulations used to quantify contributions from riverine soil POM, Pleistocene IC POM from coastal erosion, and marine POM in Siberian shelf sediments.

Geochemistry of secondary carbonates in ODP Leg 115 basalts

This report presents the results of a study of the stable isotopic and chemical composition of secondary carbonate minerals precipitated within basalts at Ocean Drilling Program Sites 707 and 715. At Site 715, the secondary carbonates are all composed of calcite and display a narrow range of carbon and oxygen stable isotope ratios, with values ranging from -2.75 per mil to 1.95 per mil PDB and -0.27 per mil to 2.86 per mil PDB, respectively. Strontium, iron, and manganese values of the samples are generally low. The geochemistry of Site 715 samples indicates that they precipitated from seawater-domi- nated fluids, at low temperatures, as is typical of secondary carbonates from most Deep Sea Drilling Project sites. In contrast, at Site 707, aragonite, siderite, and manganese-rich calcite occur as secondary carbonates in addition to calcite. The carbon isotopes of the Site 707 carbonates of all rock types are depleted in 13C. Values range from -2.79 per mil to -16.43 per mil PDB. Oxygen isotope values do not show a wide variation, ranging from -1.78 per mil to 1.17 per mil. The strontium contents of the samples range from 5200 to 8100 ppm for aragonites, and from 145 to 862 ppm for calcites. Iron and manganese contents are high in calcites and siderites and low in aragonites. Site 707 carbonates precipitated at low temperatures in a fairly closed system, in which basalt-seawater interaction has greatly influenced the chemistry of the pore fluids. The reactions occurring within the system before and in conjunction with secondary carbonate precipita- tion include oxidation of isotopically light methane, derived from fluids circulating within the basalts, and reduction of substantial amounts of iron and manganese oxides from the basalts.

Pedogeomorphology, geochronology and paleobotany of soil profiles obtained in the Nagqu area, central Tibet

Vast areas on the Tibetan Plateau are covered by alpine sedge mats consisting of different species of the genus Kobresia. These mats have topsoil horizons rich in rhizogenic organic matter which creates turfs. As the turfs have recently been affected by a complex destruction process, knowledge concerning their soil properties, age and pedogenesis are needed. In the core area of Kobresia pygmaea mats around Nagqu (central Tibetan Plateau, ca. 4500 m a.s.l.), four profiles were subjected to pedological, paleobotanical and geochronological analyses concentrating on soil properties, phytogenic composition and dating of the turf. The turf of both dry K. pygmaea sites and wet Kobresia schoenoides sites is characterised by an enrichment of living (dominant portion) and dead root biomass. In terms of humus forms, K. pygmaea turfs can be classified as Rhizomulls mainly developed from Cambisols. Wet-site K. schoenoides turfs, however, can be classified as Rhizo-Hydromors developed from Histic Gleysols. At the dry sites studied, the turnover of soil organic matter is controlled by a non-permafrost cold thermal regime. Below-ground remains from sedges are the most frequent macroremains in the turf. Only a few pollen types of vascular plants occur, predominantly originating from sedges and grasses. Large amounts of microscopic charcoal (indeterminate) are present. Macroremains and pollen extracted from the turfs predominantly have negative AMS 14C ages, giving evidence of a modern turf genesis. Bulk-soil datings from the lowermost part of the turfs have a Late Holocene age comprising the last ca. 2000 years. The development of K. pygmaea turfs was most probably caused by an anthropo(zoo)-genetically initiated growth of sedge mats replacing former grass-dominated vegetation ('steppe'). Thus the turfs result from the transformation of pre-existing topsoils comprising a secondary penetration and accumulation of roots. K. schoenoides turfs, however, are characterised by a combined process of peat formation and penetration/accumulation of roots probably representing a (quasi) natural wetland vegetation.

Descriptions, isotopes and minerals of sediments at DSDP Leg 64 Holes

Mineralogical and oxygen isotopic analyses of samples from Deep Sea Drilling Project Sites 477, 481, and 477 in the Guaymas Basin indicate the existence of two distinct hydrothermal systems. In the first, at Sites 481 and 478, hot dolerite sills intruded into highly porous hemipelagic siliceous mudstones that were moderately rich in organic matter, thermally altered the adjacent sediments, and expelled hydrothermal pore fluids. The second, at Site 477 and active at present, is most probably caused by a recent igneous intrusion forming a magma chamber at shallow depth. In the first hydrothermal system, the main thermal reactions above and below the sills are dissolution of opal-A and formation of quartz, either directly or through opal-CT; formation of smectite; formation of analcime only above the sills; dissolution and recrystallization of calcite and occasional formation of dolomite or protodolomite. The d18O values of the hydrothermally altered sediments range from 9.9 to 12.2 per mil (SMOW). The d18O values of recrystallized calcites above the first sill complex, Site 481, indicate temperatures of 140° to 170°C. No fluid recharge is required in this system. The thickness of the sill complexes and the sequence and depth of intrusion into the sediment column determine the thickness of the alteration zones, which ranges from 2 or 3 to approximately 50 meters. Generally, the hydrothermally altered zone is thicker above than below the sill. In the second type, the sediments are extensively recrystallized. The characteristic greenschist-facies mineral assemblage of quartz-albite-chlorite-epidote predominates. Considerable amounts of pyrite, pyrrhotite, and sphene are also present. The lowest d18O value of the greenschist facies rocks is 6.6 per mil, and the highest d18O value of the associated pore fluids is +1.38 per mil (SMOW). The paragenesis and the oxygen isotopes of individual phases indicate alteration temperatures of 300 ± 50°C. On the basis of the oxygen isotopes of the solids and associated fluids, it is concluded that recharge of fluids is required. The water/rock ratio in wt.% is moderate, approximately 2/1 to 3/1 - higher than the calculated water/rock ratio of the hydrothermal system at the East Pacific Rise, 21 °N.

Mineralogy and stable isotopic composition of carbonates and sulfide minerals from ODP Leg 164 sites

During Ocean Drilling Program Leg 164, gas hydrates were recovered in the Blake Ridge where the top of the gas hydrate zone lies at about 200 meters below seafloor (mbsf) and the bottom-simulating reflector (BSR) is located at about 450 mbsf. There is no sedimentological discontinuity crossing the BSR. The BSR is disrupted by the salt piercement of the Cape Fear Diapir. The authigenic carbonates (dolomite and siderite) are always present in small amounts (a few weight percent) in the sediments; they are also concentrated in millimeter- to centimeter-sized nodules and layers composed of dolomite above the top of the gas hydrate reservoir, and of siderite below the BSR. In the Blake Ridge, the dolomite/siderite boundary is located near 140 mbsf. The distribution with depth of the d18O values of dolomite and siderite shows a sharp decrease from high values (maximum 7.5 per mil) in the topmost 50 m, to very low values (minimum -2.7 per mil) at 140 mbsf, and at greater depth increase to positive values within the range of 1.8 per mil to 5.0 per mil. The d13C distribution is marked by the rapid increase with greater depth from low values (-31.3 per mil to -11.4 per mil) near 50 mbsf to positive values at 110 mbsf, which remain in the range of 1.7 to 5.4 down to 700 mbsf. Diagenetic carbonates were precipitated in pore waters in which d18O and d13C values were highly modified by strong fractionation effects, both in the water and in the CO2-CH4 systems associated with the formation and dissociation of gas hydrates.