Pelagic Cores Research Articles

Partitioning of eolian and hemipelagic sediment in eastern Equatorial Pacific core TR 163-31B and the late Quaternary paleoclimate of the Northern Andes

ABSTRACT We apply simple eolian-hemipelagic mixing model to characterize the terrigenous sediment component in well-dated deep-sea core TR163-31B raised from south of the Carnegie Ridge, 300 km west of the coast of Ecuador. The terrigenous component forms 11-19% of the sediment in this pelagic core. The relative amount of hemipelagic and eolian material was determined by comparison of the 1-30 m grain size distribution of the terrigenous fraction to that from a mixing model constructed from eolian and hemipelagic end members. Results indicate that hemipelagic deposition dominated this region from approximately 21.5 to 28 ka (14C age) and 15 to 17.5 ka, that both components are important from 17.5 to 21.5 ka, and that the eolian component dominated sediment younger than 15 ka. This depositional pattern generally matches the paleoclimatology of the northern Andes, specifically cooler and wetter conditions during stadials and warmer and drier conditions during interstadials and the Holocene.

Late Quaternary paleoproductivity changes off the Congo deduced from stable carbon isotopes of planktonic foraminifera

A detailed record (≈2-kyr intervals) of the difference in stable carbon isotopes ( Δδ 13C) between Globigerina bulloides and Globigerinoides ruber (pink) is used to reconstruct changes in upwelling intensity off the Congo River for the last 190,000 yr. Comparisons of the oxygen and carbon isotope data from this core with records from the Niger Fan and from pelagic cores in the eastern equatorial South Atlantic indicate that the Congo Fan isotope records do not contain a strong freshwater signal as is described off other major rivers. The temporal pattern of the Δδ 13C signal correlates with the marine organic carbon record from the Congo Fan. Thus the planktonic Δδ 13C record, reflecting past changes in upwelling intensity and nutrient content, corroborates the signal provided by sedimentary organic carbon, which is presumed to indicate changes in the amount of biological productivity and export flux to the seafloor. The planktonic Δδ 13C signal is characterized by a dominant 23-kyr periodicity which provides evidence for a strong response of upwelling fluctuations off the Congo to precessional forcing. Minima in the Δδ 13C record are aligned with periods of minimum boreal summer insolation over Central Africa reflecting an increase of upwelling and biological productivity off the Congo at periods of enhanced zonal intensity of southeast trades and corresponding weak southeast monsoon over the eastern South Atlantic. A strong response to changes in river discharge probably did not occur, indicating that fertilization by river-derived nutrients has played only a minor role with respect to Late Quaternary changes in the total amount of primary productivity off the Congo.

Discrete barite particles and barium as tracers of paleoproductivity in south Atlantic sediments

Geochemical analyses as well as X ray diffraction measurements were carried out on five sediment cores from the eastern Angola Basin and the equatorial divergence of the South Atlantic. Barite concentrations were calculated from total barium concentrations by subtracting the estimated barium background supplied by “nonbarite” barium carriers. Barite concentrations assessed by this geochemical method show a good correspondence to barite concentrations determined by quantitative X ray diffraction measurements. Barite proved to be an important carrier of barium in the pelagic cores, contributing up to 90% of the total barium concentrations in the sediment, while clastic material provides an important source of barium at sites closer to the African continent. Barite accumulation rates were calculated in order to eliminate the diluting effects of varying inputs of terrigenous and biogenic material. Barite accumulation rates show cyclic variations with maxima corresponding to glacial and minima to interglacial stages. Absolute paleoproduction rates were computed from barite accumulation rates. At the Congo fan and the equatorial divergence they are consistent with calculations based on total organic carbon (TOC) accumulation. At the Walvis Ridge, glacial‐interglacial cycles contrast to a constant paleoproductivity computed from TOC accumulation.

Pyrolysis-mass spectrometry and multivariate data analysis of Venezuela Basin sediments

Pyrolysis-mass spectrometry was used to analyze the distribution of the organic material in the upper 40 cm of Venezuela Basin sediments. Box cores from three different provinces were obtained: pelagic (carbonate), turbidite and hemipelagic. With the aid of a relatively straightforward multivariate technique, the spectrographic data were investigated and the geological layers differentiated. These layers were in general agreement with those determined through the visual, X-ray and geochemical analyses of the same cores. The organic material in the pelagic core was poorly mixed, and a gradual alteration of this material with depth had taken place. The spectral pattern characteristic of the uppermost (youngest) layers was similar to those obtained from carbohydrate and/or fatty acid standards, whereas those from the deepest (oldest) layers resembled non-proteinaceous nitrogen containing material. The turbidite and pelagic layers in the turbidite core could be well differentiated according to their origin. A so-called iron-enriched layer in the core could readily be identified as consisting of pelagic rather than turbidite material. Analysis of the organic material from the hemipelagic core showed a strong change in the material at a depth of 12 cm.

Early diagenetic processes affecting nutrients in the pore waters of Central Indian Ocean cores

Pore-water nutrients, nitrite, nitrate, phosphate, silicate, pH and solid-phase organic carbon were analysed for one core from the Arabian Sea and three cores from the manganese nodule area in the Central Indian Ocean Basin. Possible denitrification and unoxidized organic carbon are observed in the Arabian Sea core, while nitrification with intermediate low nitrate and high organic carbon are found in the mottled zones of the pelagic cores. Mobilization of iron-host oxyhydroxides and peaked profiles of organic carbon are related to a gradual increase in the phosphate profiles. The asymptotic silica values are less than the 100% saturation levels of amorphous silica and they fall outside the stability field of sepiolite.

Terrigenous supply of 10Be and dating with 14C and 10Be in sediments of the Angola basin (SE Antlantic)

10Be concentrations were measured in 20 (hemi)pelagic core top samples with an age control partly based on AMS 14C datings. The 10Be concentrations do not show any dependence on the bulk mass accumulation rates (MAR), but there is a significant linear correlation between 10Be accumulation rates and MAR. This correlation is best explained by terrigenous 10Be supply modified by biologic scavenging and, of secondary importance, adsorption on particulate matter in the ocean surface water in the Zaire plume. The variable term of the linear regression function indicates a maximum 10Be concentration for the terrigenous component of 5 × 10 9 at.g −1, the constant suggests an oceanic 10Be precipitation of (100–530) × 10 6 at.cm −2 ka −1. A 500 ka long record of 10Be concentrations is in agreement with the predicted values, but short-term variations in MAR obscure the expression of a long-term break in MAR about 350 ka ago.

Early diagenetic reactions in interbedded pelagic and turbiditic sediments in the Nares Abyssal Plain (western North Atlantic): Consequences for the composition of sediment and interstitial water

Abstract Measured pore-water concentrations of iron in interbedded pelagic and turbiditic sediments from the Nares Abyssal Plain are in excellent agreement with sediment colour and measured redox potential. The organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid-phase concentration of constituents that are involved in early diagenetic reactions. In the turbiditic sediments the concentration of NO 3 − generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area the concentration of NO 3 − is still higher than it is in the bottom water. The pore-water concentration of Mn 2+ in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to Mn, Ca-CO 3 . The pore water of the turbiditic sediments is also saturated with respect to calcite. The few “diagenetic spikes” in the pore-water concentration of NO 3 − and Mn 2+ and the concentration/depth profile of dissolved iron, H 4 SiO 4 and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Linked with these reactions is the irregular pore-water concentration of phosphate, which is shown to originate partly from the oxidation of organic matter, but mainly from the desorption of phosphate from iron oxide. Potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the Nares Abyssal Plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits.

326Ra in Bering Sea sediment and its application as a geochronometer.

232Th, 230Th and 226Ra were determined for three pelagic cores collected in the northern North Pacific and the western and eastern basins of the Bering Sea. The surface sediment collected in the Pacific contains much 232Th and 230Th decreasing in concentration with depth indicating a sedimentation rate of 2.2 × 10-3 cm/yr. The Bering Sea sediments do not show vertical profiles of decreasing concentration with depth. This is due to a fast sedimentation rate in the Bering Sea. The excess radioactivity of 226Ra relative to that of 230Th was observed in the surface sediment collected in the eastern basin of the Bering Sea where the sediment was of siliceous ooze, showing a vertical profile decreasing in concentration with depth. The excess 226Ra is caused by the biogenic silicate material and has been used as a geochronometer. The sedimentation rate in the Bering Sea basin turns out to be 5 × 10-2cm/yr, which shows good agreement with that obtained by the 14C method. Therefore, 226Ra chronology is applicable to pelagic or hemi-pelagic sediment of siliceous ooze.

Quaternary climate change as revealed by calcium carbonate fluctuations in western Equatorial Atlantic sediments

Fluctuations of total calcium carbonate content in eight western Equatorial Atlantic cores are used to evaluate Quaternary climate change. Both pelagic Mid-Atlantic Ridge and terrigenous-rich continental-rise cores that span the last 130,000 years show identical carbonate fluctuations which accurately reflect climatic oscillations during the Holocene, Wisconsin Glacial, and Last Interglacial. The carbonate fluctuations reveal a 20,000-year periodicity for warm-cold (interstadial-stadial) cycles with carbonate maxima reflecting interstadials and minima reflecting stadials. The carbonate fluctuations correlate in detail with other time scales for climatic oscillations deduced previously from radiometrically-dated sea-level maxima, solar insolation fluctuations, and North American ice-margin fluctuations. Pelagic cores spanning the last 500,000 years do not clearly reflect the 20,000-year cold-warm cycles but do show the 100,000-year glacial-interglacial cycles previously revealed by oxygen-isotope variations. The carbonate fluctuations in another core, which contains a continuous record of the last 1,800,000 years, indicates that 15 to 20 of these 100,000-year cycles have occurred during this period. The carbonate fluctuations of two cores suggest that the Atlantic circulated faster during glacials than during interglacials. A core from the Demerara Plateau (2000-m depth) has anomalous carbonate fluctuations plus expanded interglacial and shortened glacial sections that suggest faster circulation of surface water masses during glacials. The sediments of a pelagic core, which were deposited beneath the Antarctic Bottom Water, have undergone severe carbonate dissolution during glacials. These cycles apparently indicate an increase in production and faster circulation of the Antarctic Bottom Water during glacials.

The trace element geochemistry of a North Pacific pelagic clay core

The distribution of MnO, Fe 2O 3, Cu, Pb, Co, Ni, Cr and V is studied in eighteen samples of a North Pacific pelagic core. Five of the samples were subjected to a chemical separation technique and the resulting partition patterns are examined. It is shown that the hydrogenous fraction of all the elements is partitioned between nodular (micro-nodules) and non-nodular (chiefly iron oxide) phases, and that the concentration between the two phases changed during the sedimentary history of the core. The trace element partition changes are related to variations in the particle size distribution of quartz, and probably occurred at the Tertiary-Quarternary boundary.