Age model and grain size distribution of the fine fraction of ODP Site 114-704

The physical properties of sediments beneath an upwelling area in the southern part of the Atlantic Ocean (ODP Hole 704A) were investigated. Highly significant correlations characterize the relationship of carbonate content to bulk density (R = 0.85), carbonate content to porosity (R = 0.84), and carbonate content to impedance (R = 0.84). No relationship exists between carbonate content and compressional-wave velocity (R = 0.24), indicating that amplitude variations in impedance are primarily controlled by variations in bulk density, which, in turn, are controlled by climatically driven biogenic opal and carbonate deposition. In general, maxima in impedance correspond to maxima in carbonate content (minima in opal content). The impedance record exhibits its most drastic change at about 2.4 Ma, marking dramatic increases in the average content of biogenic opal and the beginning of large-amplitude fluctuations. Between 0.7 and 0.4 Ma carbonate content, bulk density, and grain density decrease while opal content drastically increases. Similar changes have been observed in sediments beneath an upwelling cell off northwest Africa, indicating an oceanwide enhancement in upwelling or in the calcite corrosiveness of bottom water that appears to be isochronous.

Calcium carbonate, biogenic opal, and organic carbon content and d15N ratios of sediments from the Bering Sea

Millennial-scale paleoceanographic changes in the Bering Sea during the last 71 kyrs were reconstructed using geochemical and isotope proxies (biogenic opal, CaCO3, and total organic carbon (TOC), nitrogen and carbon isotopes of sedimentary organic matters) and microfossil (radiolaria and foraminifera) data from two cores (PC23A and PC24A) which were collected from the northern continental slope area at intermediate water depths. Biogenic opal and TOC contents were generally high with high sedimentation rates during the last deglaciation. Laminated sediment depositions during the Early-Holocene (EH) and Bølling-Allerød (BA) were closely related with the increased primary productivity recorded by high biogenic opal and TOC contents and high d15N values. Enhanced surface-water productivity was attributed to increased nutrient supply from strengthened Bering Slope Current (BSC) and from increased amount of glacial melt-water, resulting in high C/N ratios and low d13C values, and high proportion of Rhizoplegma boreale during the last deglaciation. In contrast, low surface-water productivity during the last glacial period was due to depleted nutrient supply caused by strong stratification and to restricted phytoplankton bloom by extensive sea ice distribution under cold climates. Extensive formation of sea ice produces more oxygen-rich intermediate-water, leading to oxic bottom-water conditions due to active ventilation, which favored good preservation of oxic benthic foraminifera species. Remarkable CaCO3 peaks coeval with high biogenic opal and TOC contents in both cores during MIS 3 to MIS 4 are most likely correlated with Dansgaard-Oeschger (D-O) events. High d15N and d13Corg values during D-O interstadials support increased surface-water productivity resulting from nutrients supplied mainly by intensified BSC. During the EH, BA and D-O interstadials, dominant benthic foraminifera species indicate dysoxic bottom-water conditions as a result of increased surface-water productivity and weak ventilation of intermediate-water with mitigated sea ice development caused by strengthening of the Alaskan Stream. It is of note that the bottom-water conditions and formation of intermediate-water in the Bering Sea during the last glacial period are related to the variation of dissolved oxygen concentration of the bottom-water in the northeastern Pacific and to strong ventilation of intermediate-water in the northwestern Pacific. Thus, the millennial-scale paleoceanographic events in the Bering Sea during the D-O interstadials are closely associated with the intermediate-water ventilation, ultimately leading to weakening of North Pacific Intermediate Water.

Silicate content in sediment ofthe Santa Monica Basin, California

Seafloor recycling of organic materials in Santa Monica Basin, California was examined through in situ benthic chamber experiments, shipboard whole-core incubations and pore water studies. Mass balance calculations indicate that the data are internally consistent and that the estimated benthic exchange rates compare well with those derived from deep, moored conical sediment traps and hydrographic modeling. Pore water and benthic flux observations indicate that the metabolizable organic matter at the seafloor must be composed of at least two fractions of very different reactivities. While the majority of reactive organic compounds degrade quickly, with a half-life of <=6.5 years, 1/4 of the total metabolizable organic matter appears to react more slowly, with a half-life on the order of 1700 years. Down-core changes in pore water sulfate and titration alkalinity are not explained by stoichiometric models of organic matter diagenesis and suggest that reactions not considered previously must be influencing the pore water concentrations. Measured recycling and burial rates indicate that 43% of the organic carbon reaching the basin seafloor is permanently buried. The results for Santa Monica Basin are compared to those reported for other California Borderland Basins that differ in sedimentation rate and bottom water oxygen content. Organic carbon burial rates for the Borderland Basins are strongly correlated with total organic carbon deposition rate and bulk sedimentation rate. No significant correlation is observed between carbon burial and bottom water oxygen, extent of oxic mineralization and sediment mixing. Thus, for the California Borderlands, it appears that carbon burial rates are primarily controlled by input rates and not by variations in preservation.

Silicate in marine sediments from the Emerland Basin, North Atlantic

A method was developed to measure porosity and dissolved interstitial silicate at millimeter intervals or less in a sediment core. In cores from Emerald Basin (Scotian Shelf), interstitial concentrations near the sediment surface did not drop rapidly to bottom-water concentrations as measured in bottle casts (28 µM) but remained as high as 166 µM in the upper 0.5 mm of sediment High rates of benthic silicate release were measured which could not be accounted for by interstitial concentration gradients or by ventilation of macro-invertebrate burrows. The silicate discontinuity observed between the sediments and water column suggests that a diffusive sublayer exists in a zone of viscous flow above the sediment surface. This is possible only if a surface reaction is primarily responsible for silicate release. By assuming a linear concentration gradient across this diffusive sublayer, the silicate release rates were used to estimate the thickness of the sublayer to be about 2 mm.

Geochemical data from the USGS #1 Portland core, Canon City, Colorado

During the Cretaceous, widespread black shale deposition occurred during a series of Oceanic Anoxic Events (OAEs). Multiple processes are known to control the deposition of marine black shales, including changes in primary productivity, organic matter preservation, and dilution. OAEs offer an opportunity to evaluate the relative roles of these forcing factors. The youngest of these events-the Coniacian to Santonian OAE 3-resulted in a prolonged organic carbon burial event in shallow and restricted marine environments including the Western Interior Seaway. New high-resolution isotope, organic, and trace metal records from the latest Turonian to early Santonian Niobrara Formation are used to characterize the amount and composition of organic matter preserved, as well as the geochemical conditions under which it accumulated. Redox sensitive metals (Mo, Mn, and Re) indicate a gradual drawdown of oxygen leading into the abrupt onset of organic carbon-rich (up to 8%) deposition. High Hydrogen Indices (HI) and organic carbon to total nitrogen ratios (C:N) demonstrate that the elemental composition of preserved marine organic matter is distinct under different redox conditions. Local changes in d13C indicate that redox-controlled early diagenesis can also significantly alter d13Corg records. These results demonstrate that the development of anoxia is of primary importance in triggering the prolonged carbon burial in the Niobrara Formation. Sea level reconstructions, d18O results, and Mo/total organic carbon ratios suggest that stratification and enhanced bottom water restriction caused the drawdown of bottom water oxygen. Increased nutrients from benthic regeneration and/or continental runoff may have sustained primary productivity.

Description and chemical composition of manganese nodules during R/V "Tyro" cruise APNAP 1 to the Madeira Abyssal Plain in 1986

The porewater and sediment composition of two boxcores and of a small gravity core, taken on a manganese-nodule-covered hill and in the Madeira Abyssal Plain proper respectively, are compared. The pore-water study of the two boxcores indicates that oxic conditions prevail in both cores. In addition, it indicates that no detectable fluxes of Mn or Fe occur from the porewater to the ocean bottom water. Variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate, which acts as a diluting agent. A clear carbonate minimum is observed at 20-22 cm depth in the two cores. This minimum is likely to be associated with the last glacial period (10-20 kyr B.P.). This association is supported by the sediment accumulation rate of 15 mm/kyr as found by extrapolation from the rate for pelagic sediments in the Madeira Abyssal Plain. The bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of Atlantic nodules. The trace metal and Rare Earth Elements composition indicate a hydrogenous origin for the manganese nodules of this study. On the basis of the chemical composition, and that of nodules relative to that of the adjacent sediments, an average nodule accretian rate of 2.8-3.3 mm/myr has been calculated. Although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition, internal structures of the nodules reveal great inhomogeneity, both visually and chemically. These fluctuations may be related to variations in the fluxes of Mn and Fe, which in turn could be climate-related.

Amino acid geochronology of foraminifera test from north Queensland margin

In this study, we demonstrate the utility of amino acid geochronology based on single-foraminiferal tests in Quaternary sediment cores from the Queensland margin, Australia. The large planktonic foraminifer Pulleniatina obliquiloculata is ubiquitous in shelf, slope, and basin sediments of north Queensland as well as pantropical oceans. Fossil tests are resistant to dissolution, and retain substantial concentrations of amino acids (2-4 nmol/mg of shell) over hundreds of thousands of years. Amino acid D and L isomers of aspartic acid (Asp) and glutamic acid (Glu) were separated using reverse phase chromatography, which is sensitive enough to analyze individual foraminifera tests. In all, 462 Pulleniatina tests from 80 horizons in 11 cores exhibit a systematic increase in D/L ratios down core. D/L ratios were determined in 32 samples whose ages are known from AMS 14C analyses. In all cases, the Asp and Glu D/L ratios are concordant with 14C age. D/L ratios of equal-age samples are slightly lower for cores taken from deeper water sites, reflecting the sensitivity of the rate of racemization to bottom water temperature. Beyond the range of 14C dating, previously identified marine oxygen-isotope stage boundaries provide approximate ages of the sediments up to about 500,000 years. For this longer time frame, D/L ratios also vary systematically with isotope-correlated ages. The rate of racemization for Glu and Asp was modeled using power functions. These equations can be used to estimate ages of samples from the Queensland margin extending back at least 500,000 years. This analytical approach provides new opportunities for geochronological control necessary to understand fundamental sedimentary processes affecting a wide range of marine environments.

Mg/Ca and stable oxygen measurement of the past 27 Ma of ODP Hole 120-747A

We explore the applicability of paired Mg/Ca and 18O/16O measurements on benthic foraminifera from Southern Ocean site 747 to paleoceanographic reconstructions on pre-Pleistocene timescales. We focus on the late Oligocene through Pleistocene (27-0 Ma) history of paleotemperatures and the evolution of the d18O values of seawater (d18Osw) at a temporal resolution of ~100-200 kyr. Absolute paleotemperature estimates depend on assumptions of how Mg/Ca ratios of seawater have changed over the past 27 Myr, but relative changes that occur on geologically brief timescales are robust. Results indicate that at the Oligocene to Miocene boundary (23.8 Ma), temperatures lag the increase in global ice-volume deduced from benthic foraminiferal d18O values, but the smaller-scale Miocene glaciations are accompanied by ocean cooling of -1°C. During the mid-Miocene phase of Antarctic ice sheet growth (~15-13 Ma), water temperatures cool by ~3°C. Unlike the benthic foraminiferal d18O values, which remain relatively constant thereafter, temperatures vary (by 3°C) and reach maxima at ~12 and ~8.5 Ma. The onset of significant Northern Hemisphere glaciation during the late Pliocene is synchronous with an ~4°C cooling at site 747. A comparison of our d18Osw curve to the Haq et al. (1987, doi:10.1126/science.235.4793.1156 ) sea level curve yields excellent agreement between sequence boundaries and times of increasing seawater 18O/16O ratios. At ~12-11 Ma in particular, when benthic foraminiferal d18O values do not support a further increase in ice volume, the d18Osw curve comes to a maximum that corresponds to a major mid-Miocene sea level regression. The agreement between the character of our Mg/Ca-based d18Osw curve and sequence stratigraphy demonstrates that benthic foramaniferal Mg/Ca ratios can be used to trace the d18Osw on pre-Pleistocene timescales despite a number of uncertainties related to poorly constrained temperature calibrations and paleoseawater Mg/Ca ratios. The Mg/Ca record also highlights that deep ocean temperatures can vary independently and unexpectedly from ice volume changes, which can lead to misinterpretations of the d18O record.

Texture analyses of ODP Site 131-808 sediments

The microfabric of 11 mudrock specimens from ODP Site 808 (Nankai accretionary prism) was quantitatively analyzed using X-ray texture goniometry and optical petrography. The objectives of the study were to learn about rock strain and to detect a component of bulk lateral shortening in the deformation of the mudstones. Strain evaluation is based on the predictions of March theory, and on distortions of initially homogeneous marker particle distributions (the Fry technique). The main results are as follows. The specimens underwent a strain path of progressive flattening, which is closely related to loss of pore space by vertical loading. A component of bulk lateral shortening is detectable in the top 550 mbsf at Site 808, but compared with the amount of uniaxial vertical shortening, its relative magnitude is probably small. Moreover, it cannot be said with confidence whether this is caused by toe contraction of the accretionary wedge or by gravitationally induced downslope movement of the sediment pile. The mudstones examined were deposited in a marine environment with an oxic bottom water column. Micropore collapse is an important fabric building mechanism, but below 400 mbsf its effects are at least partly overridden by recrystallization of smectite. We conclude that mud microfabrics are not very precise deformation gauges, but can be used for rough estimations of strain.

Sr, C and O isotope ratios of Neogene sediments from DSDP Hole 38-341, Voring Plateau

The Neogene sediments from DSDP site 341 on the Voring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show a delta18O shift of + 1? during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawater delta18O. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb-Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 +/- 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of ~15 m/Ma. The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate (~0.2 m/Ma). The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, ~45 m/Ma. This is the first application of C, O and Sr isotopic stratigraphy combined with Rb-Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.

Early Eocene benthic foraminifera of DSDP Hole 48-401

Within the last decade, several early Eocene hyperthermals have been detected globally. These transient warming events have mainly been characterized geochemically - using stable isotopes, carbonate content measurements or XRF core scanning - yet detailed micropaleontological records are sparse, limiting our understanding of the driving forces behind hyperthermals and of the contemporaneous paleoceanography. Here, detailed geochemical and quantitative benthic foraminiferal records are presented from lower Eocene pelagic sediments of Deep Sea Drilling Project Site 401 (Bay of Biscay, northeast Atlantic). In calcareous nannofossil zone NP11, several clay-enriched levels correspond to negative d13C and d18O bulk-rock excursions with amplitudes of up to ~0.75 per mil, suggesting that significant injections of 12C-enriched greenhouse gasses and small temperature rises took place. Coeval with several of these hyperthermal events, the benthic foraminiferal record reveals increased relative abundances of oligotrophic taxa (e.g. Nuttallides umbonifera) and a reduction in the abundance of buliminid species followed by an increase of opportunistic taxa (e.g. Globocassidulina subglobosa and Gyroidinoides spp.). These short-lived faunal perturbations are thought to be caused by reduced seasonality of productivity resulting in a decreased Corg flux to the seafloor. Moreover, the sedimentological record suggests that an enhanced influx of terrigenous material occurred during these events. Additionally, the most intense d13C decline (here called level d) gives rise to a small, yet pronounced long-term shift in the benthic foraminiferal composition at this site, possibly due to the reappraisal of upwelling and the intensification of bottom water currents. These observations imply that environmental changes during (smaller) hyperthermal events are also reflected in the composition of deep-sea benthic communities on both short (<100 kyr) and longer time scales. We conclude that the faunal patterns of the hyperthermals observed at Site 401 strongly resemble those observed in other deep-sea early Paleogene hyperthermal deposits, suggesting that similar processes have driven them.

Strontium-isotope stratigraphy and oxygen- and carbon-isotope record of Middle Jurassic to Early Cretaceous belemnites in the South Atlantic

87Sr/86Sr data of belemnites are presented from a Middle Jurassic-Early Cretaceous succession from the Falkland Plateau (Deep Sea Drilling Project Sites 511 and 330) that was deposited in a periodically anoxic, semi-enclosed shallow water basin. Diagenetically screened strontium-isotope values of 0.706789 rise to 0.707044 before increasing sharply to 0.707428 in the uppermost part of the sampled succession. Comparison with published strontium calibration curves suggests that the oldest samples were Callovian to Oxfordian in age, whilst the remainder of the Jurassic part of the succession consisted of Kimmeridgian and Early Tithonian age sediments. The nannofossil, dinoflagellate and molluscan assemblages provide comparable age determinations. The strontium-isotope analysis of the youngest belemnites points to a Hauterivian-Barremian age, whilst age interpretations based upon the fauna provide a wide age range from the Barremian to early Albian. Strontium-isotope stratigraphy of this succession hence offers increased age resolution providing data regarding the timing of episodes of bottom water anoxia which have been recorded throughout the South Atlantic Basin. Well-preserved belemnite specimens display an oxygen-isotope range between +0.08 and -2.22? (PDB, Peedee belemnite international standard) and a carbon-isotope range from +2.35 to -1.33? (PDB). Delta13C values become increasingly negative through the Late Jurassic-Early Cretaceous and in concert with the 87Sr/86Sr data reveal a trend that could be accounted for by increasing levels of weathering and erosion. The oxygen-isotope data if interpreted in terms of palaeotemperature are consistent with warm palaeotemperatures in the Kimmeridgian and slightly cooler temperatures for the Tithonian and Early Cretaceous parts of the succession. The proposed relative Kimmeridgian warmth (based upon strontium-isotope age assignments) is thus in good agreement with other published palaeotemperature records.

Mg/Ca, Sr/Ca and Ca isotope ratios in benthonic foraminifers of surface sediment samples

We analysed Mg/Ca, Sr/Ca and Ca isotope ratios of benthonic foraminifers from sediment core tops retrieved during several research cruises in the Atlantic Ocean, in order to improve the understanding of isotope fractionation and element partitioning resulting from biomineralisation processes and changes in ambient conditions. Species include foraminifers secreting tests composed of hyaline low magnesium calcite, porcelaneous high magnesium calcite as well as aragonite. Our results demonstrate systematic isotope fractionation and element partitioning patterns specific for these foraminiferal groups. Calcium isotope fractionation is similar in porcelaneous and hyaline calcite tests and both groups demonstrate the previously described anomaly with enrichment of heavy isotopes around 3 - 4 °C (Gussone and Filipsson, 2010). Calcium isotope ratios of the aragonitic species Hoeglundina elegans, on the other hand, are about 0.4 per mil lighter compared to the calcitic species, which is in general agreement with stronger fractionation in inorganic aragonite compared to calcite. However, the low and strongly variable Sr content suggests additional processes during test formation, and we propose that transmembrane ion transport or a precursor phase to aragonite may be involved. Porcelaneous tests, composed of high Mg calcite, incorporate higher amounts of Sr compared to hyaline low Mg calcite, in agreement with inorganic calcite systematics, but also porcelaneous tests with reduced Mg/Ca show high Sr/Ca. While calcium isotopes, Sr/Ca and Mg/Ca in benthonic foraminifers primarily appear to fractionate and partition with a dominant inorganic control, d44/40Ca temperature and growth rate dependencies of benthonic foraminifer tests favour a dominant contribution of light Ca by transmembrane transport relative to unfractionated seawater Ca to the calcifying fluid, thus controlling the formation of foraminiferal d44/40Ca and Sr/Ca proxy signals.

Mg/Ca and Sr/Ca ratios of Hoeglundina elegans from surface sediments

Core top samples from Atlantic (Little Bahama Banks (LBB)) and Pacific (Hawaii and Indonesia) depth transects have been analyzed in order to assess the influence of bottom water temperature (BWT) and aragonite saturation levels on Mg/Ca and Sr/Ca ratios in the aragonitic benthic foraminifer Hoeglundina elegans. Both the Mg/Ca and Sr/Ca ratios in H. elegans tests show a general decrease with increasing water depth. Although at each site the decreasing trends are consistent with the in situ temperature profile, Mg/Ca and Sr/Ca ratios in LBB are substantially higher than in Indonesia and Hawaii at comparable water depths with a greater difference observed with increasing water depth. Because we find no significant difference between results obtained on "live" and "dead" specimens, we propose that these differences are due to primary effects on the metal uptake during test formation. Evaluation of the water column properties at each site suggests that in situ CO3 ion concentrations play an important role in determining the H. elegans Mg/Ca and Sr/Ca ratios. The CO3 ion effect is limited, however, only to aragonite saturation levels ([DeltaCO3]aragonite) below 15 µmol/kg. Above this level, temperature exerts a dominant effect. Accordingly, we propose that Mg/Ca and Sr/Ca in H. elegans tests can be used to reconstruct thermocline temperatures only in waters oversaturated with respect to the mineral aragonite using the following relationships: Mg/Ca = (0.034 ± 0.002)BWT + (0.96 ± 0.03) and Sr/Ca = (0.060 ± 0.002)BWT + (1.53 ± 0.03) (for [DeltaCO3]aragonite > 15 µmol/kg). The standard error associated with these equations is about ±1.1°C. Reconstruction of deeper water temperatures is complicated because in undersaturated waters, changes in Mg/Ca and Sr/Ca ratios reflect a combination of changes in [CO3] and BWT. Overall, we find that Sr/Ca, rather than Mg/Ca, in H. elegans may be a more accurate proxy for reconstructing paleotemperatures.