Data from PhD thesis

The ocean quahog, Arctica islandica is the longest-lived non-colonial animal known to science. A maximum individual age of this bivalve of 405 years has been found in a population off the north western coast of Iceland. Conspicuously shorter maximum lifespan potentials (MLSPs) were recorded from other populations of A. islandica in European waters (e.g. Kiel Bay: 30 years, German Bight: 150 years) which experience wider temperature and salinity fluctuations than the clams from Iceland. The aim of my thesis was to identify possible life-prolonging physiological strategies in A. islandica and to examine the modulating effects of extrinsic factors (e.g. seawater temperature, food availability) and intrinsic factors (e.g. species-specific behavior) on these strategies. Burrowing behavior and metabolic rate depression (MRD), tissue-specific antioxidant and anaerobic capacities as well as cell-turnover (= apoptosis and proliferation) rates were investigated in A. islandica from Iceland and the German Bight. An inter-species comparison of the quahog with the epibenthic scallop Aequipecten opercularis (MLSP = 8-10 years) was carried out in order to determine whether bivalves with short lifespans and different lifestyles also feature a different pattern in cellular maintenance and repair. The combined effects of a low-metabolic lifestyle, low oxidative damage accumulation, and constant investment into cellular protection and tissue maintenance, appear to slow-down the process of physiological aging in A. islandica and to afford the extraordinarily long MLSP in this species. Standard metabolic rates were lower in A. islandica when compared to the shorter-lived A. opercularis. Furthermore, A. islandica regulate mantle cavity water PO2 to mean values < 5 kPa, a PO2 at which the formation of reactive oxygen species (ROS) in isolated gill tissues of the clams was found to be 10 times lower than at normoxic conditions (21 kPa). Burrowing and metabolic rate depression (MRD) in Icelandic specimens were more pronounced in winter, possibly supported by low seawater temperature and food availability, and seem to be key energy-saving and life-prolonging parameters in A. islandica. The signaling molecule nitric oxide (NO) may play an important role during the onset of MRD in the ocean quahog by directly inhibiting cytochome-c-oxidase at low internal oxygenation upon shell closure. In laboratory experiments, respiration of isolated A. islandica gills was completely inhibited by chemically produced NO at low experimental PO2 <= 10 kPa. During shell closure, mantle cavity water PO2 decreased to 0 kPa for longer than 24 h, a state in which ROS production is supposed to subside. Compared to other mollusk species, onset of anaerobic metabolism is late in A. islandica in the metabolically reduced state. Increased accumulation of the anaerobic metabolite succinate was initially detected in the adductor muscle of the clams after 3.5 days under anoxic incubation or in burrowed specimens. A ROS-burst was absent in isolated gill tissue of the clams following hypoxia (5 kPa)-reoxygenation (21 kPa). Accordingly, neither the activity of antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT), nor the specific content of the ROS-scavenger glutathione (GSH) was enhanced in different tissues of the ocean quahog after 3.5 days of self-induced or forced hypoxia/anoxia to prepare for an oxidative burst. While reduced ROS formation compared to routine levels lowers oxidative stress during MRD and also during surfacing, the general preservation of high cellular defense and the efficient removal and replacement of damaged cells over lifetime seem to be of crucial importance in decelerating the senescent decline in tissues of A. islandica. Along with stable antioxidant protection over 200 years of age, proliferation rates and apoptosis intensities in most investigated tissues of the ocean quahog were low, but constant over 140 years of age. Accordingly, age-dependent accumulations of protein and lipid oxidation products are lower in A. islandica tissues when compared to the shorter-lived bivalve A. opercularis. The short-lived swimming scallop is a model bivalve species representing the opposite life and aging strategy to A. islandica. In this species permanently high energy throughput, reduced investment into antioxidant defense with age, and higher accumulation of oxidation products are met by higher cell turnover rates than in the ocean quahog. The only symptoms of physiological change over age ever found in A. islandica were decreasing cell turnover rates in the heart muscle over a lifetime of 140 years. This may either indicate higher damage levels and possibly ongoing loss of functioning in the heart of aging clams, or, the opposite, lower rates of cell damage and a reduced need for cell renewal in the heart tissue of A. islandica over lifetime. Basic physiological capacities of different A. islandica populations, measured at controlled laboratory conditions, could not explain considerable discrepancies in population specific MLSPs. For example, levels of tissue-specific antioxidant capacities and cell turnover rates were similarly high in individuals from the German Bight and from Iceland. Rather than genetic differences, the local impacts of environmental conditions on behavioral and physiological traits in the ocean quahog seem to be responsible for differences in population-specific MLSPs.

Investigations on sediments from Lake Nam Co (Tibetan Plateau)

This study focuses on the analysis of lake sediments retrieved from the deepest part of Lake Nam Co (Tibetan Plateau). One gravity core of 115 cm length, covering the last ~ 4000 cal BP, was analyzed for geochemical and biological parameters. High organic content at ~ 4000 cal BP and the coinciding presence of pyrite framboids until ~ 2000 cal BP point to hampered decomposition of organic material due to anoxic conditions within the lake sediments. At the same time sedimentological and biological proxies suggest a rather high lake level, but still ~ 5 m below the recent one, with less saline lake water due to enhanced monsoonal activity. During this time a change in the source of organic matter to lowered input of terrestrial components is observed. A rather quick shift to a dry environment with less monsoonal influence and a lake level ~ 15 m lower than today at ~ 2000 cal BP lead to the oxygenation of sediment, the degradation of organic matter and the absence of pyrite. Oscillations of the lake level thereafter were of minor amplitude and not able to establish anoxia at the lake bottom again. A wet spell between ~ 1500 cal BP and ~ 1150 cal BP is visible in proxies referring to catchment hydrology and the ostracod-based water depth transfer function gives only a slightly elevated lake level. The last ~ 300 years are characterized by low TOC and rising TN values reflecting enhanced nutrient supply and hence an advancing influence of human activity in the catchment. Decreasing TOC/TN values point to a complete shift to almost solely aquatic biomass production. These results show that hydrological variations in terms of lake level change based on monsoonal strength can be linked to redox conditions at the lake bottom of Nam Co. Comparison with other archives over larger parts of the Tibetan Plateau and beyond exhibits a rather homogeneous climatic pattern throughout the late Holocene.

Stable isotope (carbonates), organic carbon, calcium carbonate and occurrence of coccoliths at DSDP Hole 71-511

Thin but discrete pelagic limestone beds intercalated among the black mudstones near the top of the extensive Mesozoic black shale sequence of the Falkland Plateau are reminiscent of similar occurrences in the central and North Atlantic and may be cyclic in nature. They have been studied via carbonate, organic carbon, stable isotope, nannofloral, and ultrastructural analysis in an attempt to determine their mode of origin. Nannofossil diversity and preservation suggest that selective dissolution or diagenesis did not produce the interbedded coccolith-rich and coccolith-poor layers, nor did blooms of opportunistic species play a role. Stable isotope measurements of carbonate do not adequately constrain the origin of the cyclicity; however, the d13C data suggest that the more nannofossil-rich intervals may be due to higher nutrient supply and overturn of deeper waters at the site rather than influxes of well-oxygenated waters into an otherwise anoxic environment. Such an explanation is in accord with the nannofloral evidence

Organic matter composition of Cretaceous sediments of the central Pacific Ocean

Complete records of organic-carbon-rich Cretaceous strata were continuouslycored on the flanks of the Mid-Pacific Mountains and southern Hess Rise in the central North Pacific Ocean during DSDP Leg 62. Organic-carbon-rich laminated silicified limestones were deposited in the western Mid-Pacific Mountains during the early Aptian, a time when that region was south of the equator and considerably shallower than at present. Organic-carbon-rich, laminated limestone on southern Hess Rise overlies volcanic basement and includes 136 m of stratigraphic section of late Albian to early Cenomanian age. This limestone unit was deposited rapidly as Hess Rise was passing under the equatorial high-productivity zone and was subsiding from shallow to intermediate depths. The association of volcanogenic components with organic-carbon-rich strata on Hess Rise in the Mid-Pacific Mountains is striking and suggests that there was a coincidence of mid-plate volcanic activity and the production and accumulation of organic matter at intermediate water depths in the tropical Pacific Ocean during the middle Cretaceous. Pyrolysis assays and analyses of extractable hydrocarbons indicate that the organic matter in the limestone on Hess Rise is composed mainly of lipid-rich kerogen derived from aquatic marine organisms and bacteria. Limestones from the Mid-Pacific Mountains generally contain low ratios of pyrolytic hydrocarbons to organic carbon and low hydrogen indices, suggesting that the organic matter may contain a significant proportion of land-derived material, possibly derived from numerous volcanic islands that must have existed before the area subsided. The organic carbon in all samples analyzed is isotopically light (d13C -24 to -29 per mil) relative to most modern rine organic carbon, and the lightest carbon is also the most lipid-rich. There is a positive linear correlation between sulfur and organic carbon in samples from Hess Rise and from the Mid-Pacific Mountains. The slopes and intercepts of C-S regression lines however, are different for each site and all are different from regression lines for samples from modern anoxic marine sediments and from Black Sea cores. The organic-carbon-rich limestones on Hess Rise, the Mid-Pacific Mountains, and other plateaus and seamounts in the Pacific Ocean are not synchronous but do occur within the same general middle Cretaceous time period as organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean. Strata of equivalent age in the deep basins of the Pacific Ocean are not rich in organic carbon, and were deposited in oxygenated environments. This observation, together with the evidence that the plateau sites were considerably shallower and closse to the equator during the middle Creataceous suggests that local tectonic and hydrographic conditions may have resulted in high surface-water productivity and the preservation of organic matter in an oxygen-deficient environment where an expanded mid-water oxygen minimum developed and impinged on elevated platforms and seamounts.

Geochemistry and soil characteristics of sediment core GeoB4234

Stable isotopes of sedimentary nitrogen and organic carbon are widely used as proxy variables for biogeochemical parameters and processes in the water column. In order to investigate alterations of the primary isotopic signal by sedimentary diagenetic processes, we determined concentrations and isotopic compositions of inorganic nitrogen (IN), organic nitrogen (ON), total nitrogen (TN), and total organic carbon (TOC) on one short core recovered from sediments of the eastern subtropical Atlantic, between the Canary Islands and the Moroccan coast. Changes with depth in concentration and isotopic composition of the different fractions were related to early diagenetic conditions indicated by pore water concentrations of oxygen, nitrate, and ammonium. Additionally, the nature of the organic matter was investigated by Rock-Eval pyrolysis and microscopic analysis. A decrease in ON during aerobic organic matter degradation is accompanied by an increase of the 15N/14N ratio. Changes in the isotopic composition of ON can be described by Rayleigh fractionation kinetics which are probably related to microbial metabolism. The influence of IN depleted in 15N on the bulk sedimentary (TN) isotope signal increases due to organic matter degradation, compensating partly the isotopic changes in ON. In anoxic sediments, fixation of ammonium between clay lattices results in a decrease of stable nitrogen isotope ratio of IN and TN. Changes in the carbon isotopic composition of TOC have to be explained by Rayleigh fractionation in combination with different remineralization kinetics of organic compounds with different isotopic composition. We have found no evidence for preferential preservation of terrestrial organic carbon. Instead, both TOC and refractory organic carbon are dominated by marine organic matter. Refractory organic carbon is depleted in 13C compared to TOC.

Geochemistry and diatom proxy record of ODP Site 127-797

Late Quaternary sediments of the Japan Sea are characterized by centimeter- to meter-scale alternations of dark and light layers which are synchronous basinwide. High-resolution analyses of the sediments from Ocean Drilling Program site 797 reveal that deposition of the meter-scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio-eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the Tsushima Strait. The centimeter- to decimeter-scale alternations reflect millennial-scale variations which are possibly associated with Dansgaard-Oeschger (D-O) cycles, with each dark layer appearing to correspond to an interstadial. This variability is attributed to the development of a humid climate in central to eastern Asia and the consequent increase in discharge from the Huanghe and Changjiang Rivers during interstadials. This caused expansion of the East China Sea coastal water (ECSCW), which penetrated more strongly into the Japan Sea. The increased influence of the lower-salinity, nutrient-enriched ECSCW reduced deep water ventilation and enhanced the surface productivity, leading to the development of anoxic bottom waters and deposition of the dark layers. Thus the centimeter- to decimeter-scale alternations of the dark and light layers record wet and dry cycles in central to eastern Asia possibly associated with D-O cycles.

Investigations of Cretaceous and Tertiary kerogens in sediments of ODP Leg 113 holes

Seventy-one samples from nine sites were analyzed for total organic carbon (TOC). Fifty-six samples, containing 0.2% or more TOC, were evaluated by Rock-Eval to assess the nature of their kerogen and its petroleum source potential. Visual kerogen studies were carried out. Petroleum potential was encountered only in Valanginian calcareous claystones at Hole 692B close to the margin of Dronning Maud Land. A section of 44.7 m was penetrated. The unit possesses a revised mean TOC of 9.8% and petroleum potential of 43.2 kg/Mg, relatively high values in comparison to other Cretaceous anoxic oceanic sections and the totality of petroleum source rocks. At Sites 689 and 690, extremely low TOC levels, mean 0.07%, preclude kerogen analysis. Kerogens in Eocene to Pliocene sediments of the central and western Weddell Sea (Sites 694, 695, 696, and 697) are similar everywhere, largely comprising brown to black, granular, amorphous material of high rank, and generally possessing several reflectance populations of vitrinite particles. The latter are interpreted as indicative of the recycling of sediments of a variety of levels of thermal maturity.

Micropaleontological investigation of Late Albanian Col de Palluel

The Niveau Breistroffer black shale succession in the Vocontian Basin (SE France) is the regional equivalent of the widely distributed Late Albian Oceanic Anoxic Event 1d. The studied black shale-rich interval at the Col de Palluel section is 6.28 m thick and comprises four black shale units with up to 2.5 wt% total organic carbon (TOC) intercalated with marlstones. Calcareous nannofossil, palynomorph, planktic Foraminifera and stable isotopic data from the Niveau Breistroffer succession suggest that short-term climate changes influenced its deposition, with relatively warm and humid climate during black shale formation in comparison with relatively cool and dry climatic conditions during marlstone deposition. An increase in the terrigenous/marine ratio of palynomorphs indicates enhanced humidity and higher runoff during black shale formation. A nutrient index based on calcareous nannofossils and the abundance pattern of small (63-125 µm) hedbergellid Foraminifera show short-term changes in the productivity of the surface water. Surface-water productivity was reduced during black shale formation and increased during marlstone deposition. A calcareous nannofossil temperature index and bulk-rock oxygen isotope data indicate relative temperature changes, with warmer surface waters for black shale samples. At these times, warm-humid climate and reduced surface-water productivity were accompanied by greater abundances of 'subsurface'-dwelling calcareous nannofossils (nannoconids) and planktic Foraminifera (rotaliporids). These taxa presumably indicate more stratified surface-water conditions. We suggest that the formation of the Niveau Breistroffer black shales occurred during orbitally induced increase in monsoonal activity that led to increasing humidity during periods of black shale formation. This, in turn, caused a decrease in low-latitude deep-water formation and probably an increase in surface-water stratification. The combination of these two mechanisms caused depleted O2 concentrations in the bottom water that increased the preservation potential of organic matter

Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

Nd, Sm and carbon isotopic data for samples from ODP Legs 115, 145, 171, and 207

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 e-Nd units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low e-Nd(t) values (-14 to -16.5) observed throughout the rest of the studied interval. The OAE2 e-Nd excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over <20 k.y. Low background e-Nd values on Demerara Rise are explained as the Nd isotopic signature of the South American craton, whereas eruptions of the Caribbean large igneous province or enhanced mixing of intermediate waters in the North Atlantic could have caused the excursion.

Organic geochemical character and hydrocarbon source potential of selected black shales at DSDP Hole 93-603B

LECO analysis, pyrolysis assay, and bitumen and elemental analysis were used to characterize the organic matter of 23 black shale samples from Deep Sea Drilling Project Leg 93, Hole 603B, located in the western North Atlantic. The organic matter is dominantly gas-prone and/or refractory. Two cores within the Turonian and Cenomanian, however, contained significant quantities of well-preserved, hydrogen-enriched, organic matter. This material is thermally immature and represents a potential oil-prone source rock. These sediments do not appear to have been deposited within a stagnant, euxinic ocean as would be consistent with an "oceanic anoxic event." Their organic geochemical and sedimentary character is more consistent with deposition by turbidity currents originating on the continental shelf and slope.

High-resolution in situ oxygen microprofiles, porewater and solid phase geochemistry from the Crimean shelf (Black Sea) from Maria S. Merian cruise MSM15/1

The outer western Crimean shelf of the Black Sea is a natural laboratory to investigate effects of stable oxic versus varying hypoxic conditions on seafloor biogeochemical processes and benthic community structure. Bottom-water oxygen concentrations ranged from normoxic (175 µmol O2/L) and hypoxic (< 63 µmol O2/L) or even anoxic/sulfidic conditions within a few kilometers' distance. Variations in oxygen concentrations between 160 and 10 µmol/L even occurred within hours close to the chemocline at 134 m water depth. Total oxygen uptake, including diffusive as well as fauna-mediated oxygen consumption, decreased from 15 mmol/m**2/d on average in the oxic zone, to 7 mmol/m**2/d on average in the hypoxic zone, correlating with changes in macrobenthos composition. Benthic diffusive oxygen uptake rates, comprising respiration of microorganisms and small meiofauna, were similar in oxic and hypoxic zones (on average 4.5 mmol/m**2/d), but declined to 1.3 mmol/m**2/d in bottom waters with oxygen concentrations below 20 µmol/L. Measurements and modeling of porewater profiles indicated that reoxidation of reduced compounds played only a minor role in diffusive oxygen uptake under the different oxygen conditions, leaving the major fraction to aerobic degradation of organic carbon. Remineralization efficiency decreased from nearly 100 % in the oxic zone, to 50 % in the oxic-hypoxic zone, to 10 % in the hypoxic-anoxic zone. Overall, the faunal remineralization rate was more important, but also more influenced by fluctuating oxygen concentrations, than microbial and geochemical oxidation processes.

Carbon and nitrogen isotopic composition of Early Toarcian sediments

Bulk sedimentary nitrogen isotope (d15Ntot) data have been generated from Lower Jurassic black, carbon-rich shales in the British Isles and northern Italy deposited during the early Toarcian oceanic anoxic event. A pronounced positive d15Ntot excursion through the exaratum Subzone of the falciferum Zone (defined by characteristic ammonites in the British Isles) broadly correlates with a relative maximum in weight percent total organic carbon and, in some sections, with a negative d13Corg excursion. Upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of d15Ntot, and parallels may be drawn with Quaternary sediments of the Arabian Sea, Gulf of California, and northwest Mexican margin. The development of Early Toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity. Approximately coincident phenomena include the following: a relative climatic optimum, realignment of major oceanic current systems, and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event.

Petrography, minerals and isotopic composition at DSDP Leg 66 Holes

We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs. The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed. Carbon isotopic composition shows an unusually wide range - from -42.9 to +13.5 per mil - in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18O from +3.4 to +7.60 per mil in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative d13C value was derived from biochemical oxidation of methane with a negative d13C value. On the other hand, carbon with positive d13C value was probably formed during methane production in an anoxic condition.