Iron isotope and trace metal record of Cenomanian-Turonian sediments

he global carbon cycle during the mid-Cretaceous (~125-88 million years ago, Ma) experienced numerous major perturbations linked to increased organic carbon burial under widespread, possibly basin-scale oxygen deficiency and episodes of euxinia (anoxic and H2S-containing). The largest of these episodes, the Cenomanian-Turonian boundary event (ca. 93.5 Ma), or oceanic anoxic event (OAE) 2, was marked by pervasive deposition of organic-rich, laminated black shales in deep waters and in some cases across continental shelves. This deposition is recorded in a pronounced positive carbon isotope excursion seen ubiquitously in carbonates and organic matter. Enrichments of redox-sensitive, often bioessential trace metals, including Fe and Mo, indicate major shifts in their biogeochemical cycles under reducing conditions that may be linked to changes in primary production. Iron enrichments and bulk Fe isotope compositions track the sources and sinks of Fe in the proto-North Atlantic at seven localities marked by diverse depositional conditions. Included are an ancestral mid-ocean ridge and euxinic, intermittently euxinic, and oxic settings across varying paleodepths throughout the basin. These data yield evidence for a reactive Fe shuttle that likely delivered Fe from the shallow shelf to the deep ocean basin, as well as (1) hydrothermal sources enhanced by accelerated seafloor spreading or emplacement of large igneous province(s) and (2) local-scale Fe remobilization within the sediment column. This study, the first to explore Fe cycling and enrichment patterns on an ocean scale using iron isotope data, demonstrates the complex processes operating on this scale that can mask simple source-sink relationships. The data imply that the proto-North Atlantic received elevated Fe inputs from several sources (e.g., hydrothermal, shuttle and detrital inputs) and that the redox state of the basin was not exclusively euxinic, suggesting previously unknown heterogeneity in depositional conditions and biogeochemical cycling within those settings during OAE-2.

Oxygen and carbon isotope data from late calcite and zeolite pore-fill cements at DSDP Site 445

Oxygen isotopic composition of zeolite pore-fill cements in andesitic volcaniclastic sandstones recovered from DSDP Site 445 ranges from +30.1 to +17.8? (SMOW) downhole. This change is controlled by large heat flow from the basement which caused early diagenetic emplacement of zeolites during early basin rifting. d18O-values of late calcite cements range from +25.1 to +27.4? (SMOW); their petrographic relation and inferred temperature of formation suggest that calcite cements were formed during late stages of diagenesis. Isotopic composition in these sandstones is in agreement with mineral paragenesis determined microscopically.

Lithium sources in pore fluids of Peru slope

High Li concentrations, up to a maximum of 1155 µM are observed in the pore fluids of the Peru convergent margin slope sediments. At Ocean Drilling Program Sites 683 and 685 (ca. 9°S), the Li concentration depth gradients are twice as steep as at Site 682 and 688 (ca. 11°S). Within the sediments, the most important Li sources are from aluminosilicate minerals. Biogenic opal-A contains little Li and thus dilutes the Li concentration of the bulk sediments. The sediment compositions and the thermal regimes are similar at 9° and 11°S, suggesting there is an additional, non-sedimentary source for the observed high Li concentrations in the northern pore fluids. At 9°S, the 87Sr/86Sr ratios reach a maximum value of 0.709958. The observed radiogenic 87Sr/86Sr values in the pore fluids support the suggestion that the additional Li may derive from exchange reactions with underlying continental crust. The high concentrations of Li at 11°S may derive from basalt alteration at moderate to high temperatures, as suggested by the non-radiogenic 87Sr/86Sr ratios in these pore fluids, which reach a minimum value of 0.707218. Based on (1) Li concentrations in the pore fluids in slope sediments from Peru and several other margins, and (2) an approximate estimate of fluid flux from continental margins into the ocean, continental margins provide an estimated 1 to 3 * 10**10 moles Li/yr to the ocean. This source of oceanic Li, which has not been considered previously, is of the same order of magnitude as some estimates of hydrothermal and river Li fluxes and may have important consequences for the oceanic Li isotope budget. The sink is unknown for this newly discovered and possibly large Li source, but it may be more pervasive low-temperature alteration of oceanic basement than previously estimated, or burial of mineral phases, such as authigenic clay minerals, or metal oxyhydroxides which may be Li-rich.

Stable isotope composition of early Danian sediments from the Atlantic Ocean

A long-standing question in Paleogene climate concerns the frequency and mechanism of transient greenhouse gas-driven climate shifts (hyperthermals). The discovery of the greenhouse gas-driven Paleocene-Eocene Thermal Maximum (PETM; ~55 Ma) has spawned a search for analogous events in other parts of the Paleogene record. On the basis of high-resolution bulk sediment and foraminiferal stable isotope analyses performed on three lower Danian sections of the Atlantic Ocean, we report the discovery of a possible greenhouse gas-driven climatic event in the earliest Paleogene. This event - that we term the Dan-C2 event - is characterized by a conspicuous double negative excursion in delta13C and delta18O, associated with a double spike in increased clay content and decreased carbonate content. This suggests a double period of transient greenhouse gas-driven warming and dissolution of carbonates on the seafloor analogous to the PETMin the early Paleocene at ~65.2 Ma. However, the shape of the two negative carbon isotope excursions that make up the Dan-C2 event is different from the PETM carbon isotope profile. In the Dan-C2 event, these excursions are fairly symmetrical and each persisted for about ~40 ky and are separated by a short plateau that brings the combined duration to ~100 ky, suggesting a possible orbital control on the event. Because of the absence of a long recovery phase, we interpret the Dan-C2 event to have been associated with a redistribution of carbon that was already in the biosphere. The Dan-C2 event and other early Paleogene hyperthermals such as the short-lived early Eocene ELMO eventmay reflect amplification of a regular cycle in the size and productivity of the marine biosphere and the balance between burial of organic and carbonate carbon.

Mineral composition and experimental results from XRD analyses of sediments from ODP Leg 190 sites

X-ray diffraction analyses of the clay-sized fraction of sediments from the Nankai Trough and Shikoku Basin (Sites 1173, 1174, and 1177 of the Ocean Drilling Program) reveal spatial and temporal trends in clay minerals and diagenesis. More detrital smectite was transported into the Shikoku Basin during the early-middle Miocene than what we observe today, and smectite input decreased progressively through the late Miocene and Pliocene. Volcanic ash has been altered to dioctahedral smectite in the upper Shikoku Basin facies at Site 1173; the ash alteration front shifts upsection to the outer trench-wedge facies at Site 1174. At greater depths (lower Shikoku Basin facies), smectite alters to illite/smectite mixed-layer clay, but reaction progress is incomplete. Using ambient geothermal conditions, a kinetic model overpredicts the amount of illite in illite/smectite clays by 15%-20% at Site 1174. Numerical simulations come closer to observations if the concentration of potassium in pore water is reduced or the time of burial is shortened. Model results match X-ray diffraction results fairly well at Site 1173. The geothermal gradient at Site 1177 is substantially lower than at Sites 1173 and 1174; consequently, volcanic ash alters to smectite in lower Shikoku Basin deposits but smectite-illite diagenesis has not started. The absolute abundance of smectite in mudstones from Site 1177 is sufficient (30-60 wt%) to influence the strata's shear strength and hydrogeology as they subduct along the Ashizuri Transect.

Geochemical records of sediment cores from the Sea of Japan

Intervals of organic C- and carbonate-rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal d18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.

Bulk composition and clay mineralogy at DSDP Legs 56 and 57

Altogether 513 samples from sediments of Cretaceous to Pleistocene age from DSDP Legs 56 and 57 were examined by x-ray methods. The main constituents are clay minerals, quartz, feldspar, opaline silica, and volcanic glass. The sediment composition reflects the position of the sites in relation to the main source area, the Japanese Island Arc. For example, relatively coarse-grained material rich in quartz and feldspar was deposited closest to the islands, whereas finer-grained material rich in clay minerals (mainly smectite and illite, with lesser amounts of kaolinite and chlorite) was deposited farther seaward. Vertical fluctuations in the composition of the sediments show the same trend in all sites and are caused mainly by a fluctuating contribution of biogenic silica with time. A trend reversal in the chlorite/kaolinite ratio at Site 438 supports the conclusion that the subsidence of the Oyashio ancient landmass took place during the middle Miocene. That ratio also indicates a northwest drift in the position of Site 436 by sea floor spreading. Oscillations of the illite/smectite ratio during the Pleistocene at Site 436 show the variations of climate during this period. During early diagenesis potassium is fixed in smectite. With increasing depth of burial a smectite-illite mixed layer is formed, with increasing illite layering. At Sites 434, 440, and 441, stepwise changes confirm intensive tectonic process at the midslope terrace and the lower inner slope of the Japan Trench.

Occurrence of interlayers in sediment cores collected in the East Indian Ocean

In the East Indian Ocean direct contribution of land volcanism to sedimentation appears as interlayers of tephra and tuffaceous sediments, pumice fragments, and dispersed volcanoclastic materials of silty grain size. Similarity of distribution of tephra, tuffaceous sediments, Ethmodiscus ooze, and turbidites in the Pleistocene section results from deposition of all these materials under controll of a single factor, namely synchronous redistribution owing to seismic activity on the ocean floor and on the Sunda Islands. Burial of layers of oxidized deposits and formation of iron-manganese nodules is at least partly related to global climate cooling and to circulation of ocean waters.

Stable oxygen isotope ratios of marine barite from Cenozoic sediments

We report new data on oxygen isotopes in marine sulfate (delta18O[SO4]), measured in marine barite (BaSO4), over the Cenozoic. The delta18O[SO4] varies by 6x over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The delta18O[SO4] does not co-vary with the delta18O[SO4], emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the delta18O[SO4] over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The delta18O[SO4] also does not co-vary with the d18O measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the delta18O[SO4] to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the delta18O[SO4]. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.

Geotechnical properties of sediments at DSDP Site 75-532

During Leg 75 of the Deep Sea Drilling Project (DSDP) from the D/V Glomar Challenger, a 200-m deep hole was drilled at Hole 532A on the eastern side of Walvis Ridge at a water depth of 1331 m. Sediment cores were obtained by means of a hydraulic piston corer. All of the cores from this boring were designated for geotechnical studies and were distributed among eight institutions. The results of laboratory studies on these sediment cores were compiled and analyzed. Sediment properties, including physical characteristics, strength, consolidation, and permeability were studied to evaluate changes as a function of depth of burial. It was concluded that the sediment profile to the explored depth of 200 m at Walvis Ridge consists of approximately 50 m of foram-nannofossil marl (Subunit 1a) over 64 m of diatom-nannofossil marl (Subunit 1b) over nannofossil marl (Subunit 1c) to the depth explored. All three sediment units appear to be normally consolidated, although some anomalies seem to exist to a depth of 120 m. No distinct differences were found among the sediment properties of the three subunits (1a, 1b, and 1c) identified at this site.

Geochemistry of pore water and sediments offshore Nicaragua and Costa Rica

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 ?M. The kinetic constant (AA) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying AA = 1.3 * 10**-U9 mol of Si/cm**2/ s. This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

Composition of organic carbon at ODP Site 128-798

Organic geochemical and sedimentological investigations have been performed on sediments from ODP Sites 798 and 799 in order to reconstruct the depositional environment in the Japan Sea through late Cenozoic times. The Miocene to Quaternary sediments from Site 798 (Oki Ridge) and Site 799 (Kita-Yamato Trough) are characterized by high organic carbon contents of up to 6%. The organic matter is mainly a mixture of marine and terrigenous material. The dominant factors controlling marine organic carbon enrichment in the sediments of Hole 798A are probably an increased surface-water productivity and/or an increased preservation rate of organic carbon under anoxic deep-water conditions. In lower Pliocene sediments at Site 798 and Miocene to Quaternary sediments at Site 799, rapid burial of organic matter in turbidites may have been important, too. Remarkable cycles of dark, laminated sediments distinctly enriched in (marine) organic carbon by up to 5% and light, bioturbated to homogeneous sediments with reduced organic carbon contents indicate dramatic short-term paleoenvironmental variation.

Os isotope record of ODP Site 159-959

The known temporal relationship between the benthic foraminiferal d18O record and the marine Os isotope record is used to reinterpret the absolute chronology and paleoceanographic context of an episode of organic carbon burial on the West African margin Ocean Drilling Program (ODP) Site 959. Although organic-rich sediments require significant corrections for in situ decay of 187Re to 187Os, these results demonstrate the utility of the marine Os isotope record for chemostratigraphic correlation of organic-rich sediments that are devoid of age diagnostic microfossils with pelagic carbonate sequences. Revision of the ODP Site 959 chronology shifts the age assignment of an interval of biosiliceous, organic-rich sediment deposition from the Oligocene to the late Eocene and earliest Oligocene, likely culminating with the first major glaciation of the Oligocene (Oi1). We speculate that enhanced organic carbon burial over much of the West African margin may have contributed to drawdown of atmospheric carbon dioxide before and during the Oi1 event and suggest that Os isotope chemostratigraphy provides a valuable tool for further exploring this possibility.

Carbonate sedimentology of the Propeller Mound, Northeast Atlantic

High resolution studies from the Propeller Mound, a cold-water coral carbonate mound in the NE Atlantic, show that this mound consists of >50% carbonate justifying the name "carbonate mound". Through the last ~300,000 years approximately one third of the carbonate has been contributed by cold-water corals, namely Lophelia pertusa and Madrepora oculata. This coral bound contribution to the carbonate budget of Propeller Mound is probably accompanied by an unknown portion of sediments buffered from suspension by the corals. However, extended hiatuses in Propeller Mound sequences only allow the calculation of a net carbonate accumulation. Thus, net carbonate accumulation for the last 175 kyr accounts for only <0.3 g/cm2/kyr, which is even less than for the off-mound sediments. These data imply that Propeller Mound faces burial by hemipelagic sediments as has happened to numerous buried carbonate mounds found slightly to the north of the investigated area.

Osmium isotope composition of leachates and bulk sediment from ODP Holes in the Bengal Fan

Os isotopic compositions and OS and Re concentrations were measured in H2O2-H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios (Pegram et al., 1994, doi:10.1016/0012-821X(94)90172-4) much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y. Bengal Fan leachate 187Os/188Os ratios increase with increasing depositional age, in contrast to the seawater Os isotopic ratio, which decreases with increasing age. Several lines of evidence suggest that, at the time of sediment burial, the leachate Os compositions most likely reflected the seawater values. Thus, the current divergence is probably the result of post-depositional processes. One such process, in situ radiogenic ingrowth of 187Os, can be excluded because the measured Re concentrations of these sediments are too low. Similarly, since most of the bulk rock Os isotopic ratios were lower than those of the associated leachates, the high leachate 187Os/188Os values cannot be explained by in situ sediment alteration. Instead, it is proposed that the increase with age results from radiogenic OS brought in by thermoconvective circulation from further upslope in the Fan. The ultimate source of this 187Os would then be alteration of radiogenic sediments or post-depositional radioactive decay of Re in sediments rich in organic carbon. Finally, the divergence between the results obtained on Bengal Fan sediments and those obtained in the open ocean (Pegram et al., 1992, doi:10.1016/0012-821X(92)90132-F) by the same leaching technique suggest that Os sediment leachate data must be interpreted with caution.