Extractible phospholipid fatty acids of abyssal sediment cores from three stations in the Venezuela Basin, transects between them, and a station in the Puerto Rico Trench were analyzed to determine microbial biomass and community composition. Results were compared to abyssal sediments from an area of high-energy boundary currents in the North Atlantic, and estuarine sediments from Apalachee Bay, Florida. Venezuela Basin and Puerto Rico Trench sediments were characterize by low microbial biomass, measured as phospholipid palmitic acid. Venezuela Basin sediments of three different sedimentary regimes showed a remarkably similar microbial community structure, as characterized by fatty acid profiles. Prokaryotic organisms dominated the microbial community, and fatty acids believed to be signatures of anaerobic organisms were present in greater proportions in Venezuela Basin and Puerto Rico Trench sediments than in either the North Atlantic abyssal sediments or shallow-water estuarine sediments.
We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of Denmark and Norway. Rates of NH4+ and Sigma CO2 liberation sediment incubations were used with O2 penetration depths to conclude that O2 respiration accounted for only between 3.6-17.4% of the total organic carbon oxidation. Dentrification was limited to a narrow zone just below the depth of O2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn reduction and sulfate reduction dominated organic carbon mineralization, but their relative significance varied depending on the sediment. Where high concentrations of Mn-oxide were found (3-4 wt% Mn), only Mn reduction occurred. With lower Mn oxide concentrations more typical of coastal sediments, Fe reduction and sulfate reduction were most important and of a similar magnitude. Overall, most of the measured O2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon. We suspect that the importance of O2 respiration in many coastal sediments has been overestimated, whereas metal oxide reduction (both Fe and Mn reduction) has probably been well underestimated.
Time resolved data on the carbon isotopic composition of carbonate minerals and the sulfur isotopic composition or sulfate minerals show a strong negative correlation during the Cretaceous. Carbonate minerals are isotopically heavy during this period while sulfate minerals are isotopically light. The implication is that carbon is being transferred from the oxidized, carbonate reservoir to the reservoir of isotopically light reduced organic carbon in sedimentary rocks while sulfur is being transferred from the reservoir of isotopically light sedimentary sulfide to the oxidized, sulfate reservoir. These apparently oppositely directed changes in the oxidation state of average sedimentary carbon and sulfur are surprising because of a well-established and easy to understand correlation between the concentrations of reduced organic carbon and sulfide minerals in sedimentary rocks. Rocks rich in reduced carbon are also rich in reduced sulfur. The isotopic and concentration data can be reconciled by a model which invokes a significant flux of hydrothermal sulfide to the deep sea, at least during the Cretaceous.
Based on a multiparameter approach including organic carbon, biogenic opal, carbonate and the species composition of planktic foraminifera, this paper provides the first qualitative assessment of the history of paleoproductivity on glacial–interglacial time scales in the southern Peru–Chile Current (PCC) off Chile, which belongs to the least studied parts of the world ocean. During the Last Glacial Maximum (LGM) highest relative paleoproductivity of the last 33 kyr, indicated by high accumulation rates of organic carbon, biogenic opal and carbonate, has been found, contrasted by lowest values during the early and Middle Holocene. This shift from high to low productivity is accompanied by a major change in the species composition of planktic foraminifera from a dominance of Neogloboquadrina pachyderma (sin.) to the dominance of N. pachyderma (dex.). The temporal pattern of paleoproductivity off Chile is very similar to the history of the continental paleoclimate in the region, which is supposed to be driven by the position of the Southern Westerlies. This observation points to a functional relationship between the position of the Southern Westerlies and the history of paleoproductivity off Chile. Assuming that atmospheric and oceanographic circulation are closely linked, a northward displacement of the Antarctic Circumpolar Current during the LGM, in line with the northward movement of the Southern Westerlies, would bring the main nutrient source closer to our core sites resulting in increased productivity. However, based on the available data, it is not clear if the higher productivity in the southern PCC during the LGM reflects generally higher productivity during this time or only a regional displacement of the main productivity centers.
Sediment cores located along the Eurasian continental margin (Arctic Ocean) have been studied to reconstruct the environmental changes in terms of waxing and waning of the Barents/Kara Sea ice-sheets, Atlantic water inflow, and sea-ice distribution over the last 150 kyr. The stratigraphy of the cores is based on stable oxygen isotopes, AMS , and paleomagnetic data. We studied variations in marine and terrigenous input by a multiproxy approach, involving direct comparison of sedimentological and organo-geochemical data. Extensive episodes of northern Barents Sea ice-sheet growth during marine isotope stages (MIS) 6 and 2 have been supported by, at least, subsurface Atlantic water inflow, moisture-bearing storms, low summer insolation, and minimal calving of ice. Ice advance during MIS 4 was probably restricted to the shallow shelf. Between MIS 4 and MIS 2, large ice-sheet fluctuations correspond to contemporary Laurentide surging events and indicate short-term climatic changes in the Arctic Ocean as has been recorded in lower latitudes. In contrast, in low precipitation areas in eastern Eurasia, glacial activity was rather limited. Only distinct ice-rafted debris (IRD) input during Termination II and early MIS 3 reflects severe glaciations on the northern Severnaya Semlya margin during MIS 6 and MIS 4. We conclude that (1) oscillations of ice-sheets are less frequent along the eastern Eurasian margin than in areas with continuous moisture supply like the western Eurasian margins and that (2) major fluctuations of the Kara Sea ice-sheet during the last 150 kyr apparently followed the major interglacial/glacial MIS 5/4 and MIS 7/6 transitions rather than the precession (23 kyr) and the tilt (41 kyr) cyclicity of the Earth's orbit as observed for the Scandinavian (SIS) and the Svalbard ice-sheets, respectively [Mangerud, J., Jansen, E., Landvik, J.Y., 1996. Late Cenozoic history of the Scandinavian and Barents Sea ice-sheets. In: Solheim, A., Riis, F., Elverhøi, A., Faleide, J.J., Jensen, L.N., Cloetingh, S. (Eds.), Impact of Glaciations on Basin Evolution: Data and Models from the Norwegian Margins and Adjacent Basins. Global and Planetary Chance, Special Issue 12, pp. 11-26.]. Surface and/or subsurface Atlantic water masses coupled with seasonally ice-free conditions penetrated continuously to at least the Franz Victoria Trough during the last 150 kyr. However, sustained periods of open water were largely restricted to substages 5.5, 5.1, and the Holocene as indicated by distinct carbonate dissolution and higher accumulation of marine organic matter (MOM). Signals of periodic open-water conditions along the northern margin of Severnaya Semlya are of less importance. Higher production of foraminifera, probably due to Atlantic water inflow occurred between 38 and 12 kyr and corresponds to periodic Atlantic water advection penetrating into the Arctic Ocean. However, marine organic proxies indicate a continuous decrease of surface-water productivity from the western to the eastern Eurasian continental margin due to a more extensive sea-ice cover over the last 150 kyr.
Detailed interpretation of single channel seismic reflection and Huntec deep-tow boomer and sparker profiles demonstrates that the southwestern Black Sea shelf formed by a protracted shelf-edge progradation since the Miocene–Pliocene. Five seismic–stratigraphic units are recognized. Unit 1 represents the last phase of the progradational history, and was deposited during the last glacial lowstand and Holocene. It is divided into four subunits: Subunit 1A is interpreted as a lowstand systems tract, 1B and 1C are interpreted as a transgressive systems tract, and Subunit 1D is interpreted as a highstand systems tract. The lowstand systems tract deposits consist of overlapping and seaward-prograding shelf-edge wedges deposited during the lowstand and the subsequent initial rise of sea level. These shelf-edge wedges are best developed along the westernmost and easternmost segments of the study area, off the mouths of rivers. The transgressive systems tract deposits consist of a set of shingled, shore-parallel, back-stepping parasequences, deposited during a phase of relatively rapid sea-level rise, and include a number of prograded sediment bodies (including barrier islands, beach deposits) and thin veneers of seismically transparent muds showing onlap onto the flanks of older sedimentary features. A number of radiocarbon dates from gravity cores show that the sedimentary architecture of Unit 1 contain a detailed sedimentary record for the post-glacial sea-level rise along the southwestern Black Sea shelf. These data do not support the catastrophic refilling of the Black Sea by waters from the Mediterranean Sea at 7.1 ka postulated by [Ryan, Pitman, Major, Shimkus, Maskalenko, Jones, Dimitrov, Görür, Sakınç, Yüce, Mar. Geol. 138 (1997) 119–126], [Ryan, Pitman, Touchstone Book (1999) 319 pp.], and [Ballard, Coleman, Rosenberg, Mar. Geol. 170 (2000) 253–261].
Sediments of the North Atlantic contain between 40 ° and 55 °N a series of layers rich in ice-rafted material (Heinrich layers, HL's). In core KS01 from the northeastern Atlantic (46 °00.2′N, 17 ° 12.1′W) we identified HL1 to 5. Important parts of these layers are abnormally dense. In order to unravel the depositional history of the HL's and the environmental conditions which have allowed such induration, we investigated the 18O and 13C contents of planktonic foraminifers and studied the bulk physical and lithological properties of the layers. The detailed investigation of HL1, HL2 and HL4 shows that, at this site, drastic changes in the surface water and at the sediment-water interface occurred during the HL deposition. We distinguished two phases. The first phase, which includes two steps, is characterised by glacial conditions and high productivity (not reflected by planktonic foraminifers). During the second step the detrital carbonate supply increased and organic carbon content maxima occur in these HL's. The second phase is characterised by an important decrease of the surface water salinity and an important reduction of the productivity. The diagenetic precipitation of calcite within microburrows in relation with anoxic and microbial mineralisation of organic matter may have contributed to the induration of the layers.
Foraminiferal assemblage changes, size and mineralogy of lithic grains, oxygen isotopes, CaCO3, and dolomite content were studied along a 9-m-long core from the Meriadzek Terrace to gain insight into climate, productivity, and sediment source changes at this part of the Northeast Atlantic margin during the late Quaternary. An age model has been generated on the basis of radiocarbon dating, downcore foraminiferal assemblages, and δ18O values. High sedimentation rates at this site allow very detailed records for the last glacial period down to late isotopic stage 3. Sea surface temperature (SST) inferred from the foraminiferal assemblages, δ18O curve, and the temperature estimation by the SIMMAX analog technique reveal three distinctive periods during isotopic stage 2, with late stage 2 (15.3–13.4 ka) being the coldest period of the last 26 ka. A northward retreat of the polar front at 13.4 ka based on the SST record coincides with the strongest peak of primary productivity as indicated by the foraminiferal fluxes. Levels rich in large lithic grains (LLG) interpreted as ice-rafted debris (IRD) correspond to periods of low SST and are coeval with Heinrich layers 1, 2 and 3. However, the hematite-stained quartz found in the detrital fraction and the scarce dolomite and detrital carbonate content in our core point to an Iceland and/or Fenno-Scandian rather than a Laurentian or Greenland origin of this material.
Cores raised during IODP Expedition 303 off southern Greenland (Eirik Ridge site 1305) and off the Labrador Coast (Orphan Knoll site 1302/1303) were analyzed to establish an isotope stratigraphy, respectively for the “inner” and “outer” basins of the Labrador Sea (LS). These isotopic data also provide information on the Atlantic Meridional Overturning Circulation (AMOC), notably with regard to the intensity of the Western Boundary Under Current (WBUC), which is tightly controlled by the production of Denmark Strait Overflow Water (DSOW), and the production of Labrador Sea Water (LSW) in the inner basin through winter cooling and convection. The upper 184 m of sediment at Eirik Ridge spans marine isotope stages (MIS) 32 to 1. At this site, two distinct regimes are observed: prior to MIS 20, the isotopic record resembles that of the open North Atlantic records of the interval, whereas a more site-specific pattern is observed afterwards. This later pattern was characterized by i) high DSOW production rates and strong WBUC during interglacial stages, as indicated by sedimentation rates, ii) large amplitude δ18O-shifts from glacial stages to interglacial stages (> 2.5‰) and iii) an overall range of δ18O-values significantly more positive than before. At Orphan Knoll, the 105 m record spans approximately 800 ka and provides direct information on linkages between the northeastern sector of the Laurentide Ice Sheet and the North Atlantic. At this site, a shift towards larger amplitude glacial/interglacial ranges of δ18O-values occurred after MIS 13, although isotopic records bear a typical North Atlantic signature, particularly during MIS 5, in contradiction to those of Eirik Ridge, where substages 5a to 5c are barely recognized. Closer examination of δ18O-records in planktic and benthic foraminifera demonstrates the presence of distinct deep-water masses in the inner vs. outer LS basins during MIS 11 and more particularly MIS 5e. Data confirm that the modern AMOC, with LSW formation, seems mostly exclusive to the present interglacial, and also suggest some specificity of each interglacial with respect to the production rate of DSOW and the AMOC, in general.
A detailed magnetic analysis has been done on the upper 145 m of the splice of ODP Site 1146 drilled during Leg 184 in the South China Sea. First, a magnetostratigraphic record was obtained. The Bruhnes–Matuyama reversal, preceded by a precursor, and the two Jaramillo reversals were identified at 115, 133 and 137.5 mcd (meter composite depth), respectively. Several known excursions during the Brunhes period, as well as the Kamikatsura event during the Matuyama chron, were also identified. Coupled with the oxygen isotope record obtained from benthic foraminifera, this allows us to establish a precise age model, showing that the studied interval extends to 1.18 Myr. Long-term and short-term variations have been documented in the magnetic parameters, which appear at least in part related to changes in monsoon activity. On the long-term trend, three main intervals with different magnetic trends were identified: 1.2–0.7, 0.7–0.2 and the most recent 0.2 Myr. During the oldest period, the magnetic content decreases, while the grain size of the magnetic mineral increases with large fluctuations. The coercivity of the magnetic minerals also increases consistently with data obtained by other authors from Pacific cores. During the following 500 kyr all the magnetic properties are rather uniform, with rather low abundance and relatively large grain sizes and low S-ratio. This probably illustrates a period of enhanced winter monsoon. Then, in the most recent period, the magnetic grain size decreases again with pronounced fluctuations. On the short-term scale, a 23 kyr component is identified by power spectrum analysis in all magnetic parameters in the bottom part of the studied interval. This component is correlated with the oxygen isotope record, as shown by coherence analysis. Except for the most recent 200 kyr, where the pattern changes, cold/warm periods coincide with low/high magnetic content, and magnetic grains are coarser during cold stages and finer during warm periods. This is consistent with the clay mineral analysis and illustrates changes in the balance between arid/humid periods corresponding to dominant winter/summer monsoon and giving rise to physical erosion and wind blowing/chemical weathering in the main fluvial plains and transport by oceanic current.
Sand and gravel deposits from the Atlantic coasts of southwest Britain, Brittany and western Ireland are identified as potential indicators of historic (post-AD 1000) ocean-sourced high-energy events, such as storm surges and tsunami. This is an important historic period as it includes the climatic perturbations of the Medieval Warm Period and the Little Ice Age, and also seismogenic events, such as the Lisbon tsunami of 1755. Ten new sites are identified from various coastal settings and dated using eight new radiocarbon dates alongside previously published data. Generally, sites do not appear to record multiple high-energy events, suggesting that either only the most extreme and/or recent events are registered. A number of radiocarbon dates from marine shell yield modern ages when corrected for the marine reservoir effect. Rather than necessarily indicating recent deposition, this may reflect a poor understanding of terrestrial carbon input into coastal and estuarine waters, and the practice of applying broad regional ΔR values at the local scale. Two groupings of radiocarbon dates are recognised; a Medieval Group and a Post-Medieval Group, which might reflect events within known climatic perturbations and/or tsunami occurrences. These events may include the Lucia Flood of 1287, the All Saint's Day Flood of 1570, the 1607 Flood, the Great Storm of 1703, and the Lisbon tsunami. Some older (pre-AD 1000) deposits indicate the potential to construct frequency/magnitude records of high-energy events throughout the Holocene. Data presented here support the view that salt marshes within the Bristol Channel and Severn Estuary were completely eroded away early in the 17th century.
Variations in the clay mineral composition of mid-late Quaternary sediments recovered from the Cariaco basin are interpreted in terms of changes in paleoclimate. The results indicate the changing contribution of regional and local sediment sources during interglacial high stands vs. glacial low stands. The relative contribution of illite increases during interglacial stages, and decreases during glacial stages. Illite–smectite shows a much weaker negative relationship to glacial eustacy. The abundances of kaolinite and chlorite show no correlation with paleoclimate. Sediments sourced from rivers adjacent to the Cariaco basin contain low illite and provide a significant influx of illite–smectite during low sea level stands, but during interglacial periods, they are probably deposited on the shallow continental shelf bordering the local coastline. Clays sourced from the Amazon and Orinoco rivers are richer in illite, and their contribution is reduced in times of low sea level stands, as the continental shelf becomes much narrower and sediment is discharged directly onto the continental rise. During the last glacial period, changes in clay mineral composition were magnified relative to previous glaciations, possibly due to tectonic evolution of the region. The sill depth between the Cariaco basin and the open Caribbean Sea was reduced to a minimum, decreasing further the relative importance of the Amazon/Orinoco contribution.
The western flank of the Great Bahama Bank, drilled during ODP Leg 166 at seven sites, represents a prograding carbonate sequence from late Oligocene to Holocene [Eberli et al., Proc. ODP Init. Reports 166 (1997)]. The signatures of the detrital input and of diagenetic alteration are evident in clay enriched intervals from the most distal Sites 1006 and 1007 in the Straits of Florida. Mineralogical and chemical investigations (XRD, TEM, SEM, ICP-MS) run on bulk rocks and on the clay fractions enable the origin and evolution of silicate parageneses to be characterized. Plio–Pleistocene silt and clay interbeds contain detrital clay assemblages comprising chlorite, illite, interstratified illite–smectite, smectite, kaolinite and palygorskite. The greater smectite input within late Pliocene units than in Pleistocene oozes may relate either varying source areas or change in paleoclimatic conditions and weathering intensity. The clay intervals from Miocene–upper Oligocene wackestone sections are fairly different, with prevalent smectite in the fine fraction, whose high crystallinity and Mg contents that point towards an authigenic origin. The lower Miocene section, below 1104 mbsf, at depths where compaction features are well developed, is particularly characterized by abundant authigenic Na-K-clinoptilolite filling foraminifer tests. The authigenic smectite and clinoptilolite paragenesis is recorded by the chemical trends, both of the sediment and the interstitial fluid. This diagenetic evolution implies Si- and Mg-rich fluids circulating in deeper and older sequences. For lack of any local volcaniclastic input, the genesis of zeolite and the terms of water–rock interaction are discussed. The location of the diagenetic front correlates with that of the seismic sequence boundary P2 dated as 23.2 Ma. This correspondence may allow the chronostratigraphic significance of some specific seismic reflections to be reassessed.
During ODP Leg 166, the recovery of cores from a transect of drill sites across the Bahamas margin from marginal to deep basin environments was an essential requirement for the study of the response of the sedimentary systems to sea-level changes. A detailed biostratigraphy based on planktonic foraminifera was performed on ODP Hole 1006A for an accurate stratigraphic control. The investigated late middle Miocene–early Pliocene sequence spans the interval from about 12.5 Ma (Biozone N12) to approximately 4.5 Ma (Biozone N19). Several bioevents calibrated with the time scale of Berggren et al. (1995a,b) were identified. The ODP Site 1006 benthic oxygen isotope stratigraphy can be correlated to the corresponding deep-water benthic oxygen isotope curve from ODP Site 846 in the Eastern Equatorial Pacific (Shackleton et al., 1995. Proc. ODP Sci. Res. 138, 337–356), which was orbitally tuned for the entire Pliocene into the latest Miocene at 6.0 Ma. The approximate stratigraphic match of the isotopic signals from both records between 4.5 and 6.0 Ma implies that the paleoceanographic signal from the Bahamas is not simply a record of regional variations but, indeed, represents glacio-eustatic fluctuations. The ODP Site 1006 oxygen and carbon isotope record, based on benthic and planktonic foraminifera, was used to define paleoceanographic changes on the margin, which could be tied to lithostratigraphic events on the Bahamas carbonate platform using seismic sequence stratigraphy. The oxygen isotope values show a general cooling trend from the middle to late Miocene, which was interrupted by a significant trend towards warmer sea-surface temperatures (SST) and associated sea-level rise with decreased ice volume during the latest Miocene. This trend reached a maximum coincident with the Miocene/Pliocene boundary. An abrupt cooling in the early Pliocene then followed the warming which continued into the earliest Pliocene. The late Miocene paleoceanographic evolution along the Bahamas margin can be observed in the ODP Site 1006 δ13C values, which support other evidence for the beginning of the closure of the Panama gateway at 8 Ma followed by a reduced intermediate water supply of water from the Pacific into the Caribbean at about 5 Ma. A general correlation of lower sedimentation rates with the major seismic sequence boundaries (SSBs) was observed. Additionally, the SSBs are associated with transitions towards more positive oxygen isotope excursions. This observed correspondence implies that the presence of a SSB, representing a density impedance contrast in the sedimentary sequence, may reflect changes in the character of the deposited sediment during highstands versus those during lowstands. However, not all of the recorded oxygen isotope excursions correspond to SSBs. The absence of a SSB in association with an oxygen isotope excursion indicates that not all oxygen isotope sea-level events impact the carbonate margin to the same extent, or maybe even represent equivalent sea-level fluctuations. Thus, it can be tentatively concluded that SSBs produced on carbonate margins do record sea-level fluctuations but not every sea-level fluctuation is represented by a SSB in the sequence stratigraphic record.
For correlating a turbidite record with a detailed land record of natural disasters during the last ca. 100 years, the depositional age of the surface hemipelagic sediments of the Kumano Trough located in central Japan were estimated. We attempted to extract mass accumulation rate of “hemipelagic fallout” from hemipelagic intervals on the basis of 137Cs geochronology, X-radiograph, dry bulk density, and grain-size distribution. Based on assumptions of the biogenic mixing effect on the 137Cs appearance horizon and hemipelagite erosion by turbidity current, the reconstructed turbidite record from hemipelagic fallout mass accumulation rate is well correlated with the record of major natural disasters in the Kumano area. Based on the correlation, most of turbidite layers are flood/storm origin except for a well-sorted seismo-turbidite induced by the Tonankai Earthquake that occurred the mid 20th century. Although the wide distribution of a flood-induced turbidite layer due to the 19th century Tostukawa flood, which was a huge disaster in the area caused by heavy rain and dams breaching, it does not show a hyperpycnite succession. The flood by dams breaching may cause surge-like flow. Angular to subangular sand-sized lithic fragments in the turbidite show that sand grains were transported from the Kumano River directly. Plant debris and microfossils are common in both seismo-turbidite and flood/storm induced turbidites.
Little is known of long-term processes affecting microbial abundance in buried marine sediments. In collaboration with geochemists and sedimentologists involved in ODP Leg169S, we undertook a study of bacterial and viral abundance throughout the entire Holocene sediment section in Saanich Inlet, British Columbia, Canada. Sediments were sampled at 1.5-m intervals from the sediment surface down into Pleistocene sequences at depths of >100 m. Preparations of formalin-fixed sediment were stained with the nucleic acid stain Yo-Pro and bacteria and viruses were enumerated using epifluorescence microscopy. Viral presence was confirmed by electron microscopy. More widely spaced measurements of adenosine triphosphate (ATP) and the biogenic pore water gases H2, CH4 and CO were used as indices of microbial metabolic activity. Bacterial and viral abundances were high (>109 gdw−1) in these organic-rich sediments relative to oceanic areas, and were highly correlated, indicating a probable close functional dependence characteristic of predator–prey relations. The upper Holocene section showed a significant subsurface peak in microbial abundance that was correlated negatively with sediment organic matter content, but corresponded with biogenic gas accumulation. The interpretation of these and other significant trends is discussed in relation to the Holocene/late Pleistocene history of organic matter sedimentation and diagenetic processes.
High-resolution pollen analysis of laminated marine sediments from ODP Hole 1034B in Saanich Inlet, British Columbia reveals changes in vegetation and inferred climate during the Holocene. Four main pollen zones are discerned using constrained cluster analysis. Although the timing of major vegetation changes at the Saanich Inlet is similar to other study sites in the Pacific Northwest, the composition of pollen assemblage zones is different from the mainland sites. Vegetation assemblages reconstructed from the pollen and spore record include a Douglas-fir (Pseudotsuga menziesii) parkland with abundant grass (Poaceae) and bracken (Pteridium) between 11,450 and 8300 BP (all ages are calibrated calendar years), oak (Quercus) savanna or parkland with high grass and bracken (8300–7040 BP), a mixed deciduous/coniferous forest with oak, western hemlock (Tsuga heterophylla) and Douglas-fir (7040–5750 BP), and the development of modern coastal temperate forest with the marked expansion of cedar (Cupressaceae), western hemlock, spruce (Picea) and Douglas-fir (5750–1050 BP). Climatic periods inferred from the cores include an early Holocene warm/dry interval (11,450–8300 BP), a warm period with mild winters (8300–7040 BP), a period of transitional mid-Holocene climate (7040–5750 BP), and the advent of a relatively cool/wet neoglacial climate after 5750 BP. Modern conifer forests and oak savannas became established by about 3800 BP. The Saanich Inlet pollen record indicates that vegetation and inferred climate change was particularly rapid between 8700 and 8300 BP when grass and bracken abruptly decrease and oak becomes a significant component of the paleovegetation. Because neoglacial conditions have prevailed from 3800 years to present in the Pacific Northwest, factors other than climate, such as anthropogenic modification of the landscape, may be responsible for the persistence of oak savannas.
Upper Paleocene (zone CP8b) cyclic sediment from Ocean Drilling Program (ODP) Site 1051 on Blake Nose, western North Atlantic, alternates from white carbonate-rich to green carbonate-poor in response to precessional forcing. Compositional differences between the two bed types are minor: mineral and nannofossil composition of the beds vary subtly, but grain size of the terrigenous component and biogenic silica content remain constant. Iron content determined by sediment magnetic susceptibility and iron intensity determined by a core-scanning XRF correlates negatively with carbonate content and are higher in green beds. Kaolinite content of green beds is slightly higher as well. Green beds exhibit lower evenness values than white beds for nannofossil assemblages and are more dominated by the species Coccolithus. Dominance by Coccolithus indicates more mesotrophic conditions over Blake Nose during deposition of the green beds, which correlates with slightly higher iron and kaolinite contents.
Ten ODP sites drilled in a depth transect (2164–4775 m water depth) during Leg 172 recovered high-deposition rate (>20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8–0.9 Ma has been developed. The time scales have a resolution of 10–20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (2164–2975 m water depth, Sites 1055–1058) while the deeper sites (2995–4775 m water depth, Sites 1060–1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma.
Sedimentation rates (SRs) off SW Africa were calculated by performing spectral analyses in the depth domain on borehole and gamma-ray attenuation (GRA) bulk density data from ODP Sites 1081–1084. Inversion and integration of SRs versus depth from spectral analysis yielded detailed SR profiles in the time domain. Our technique allowed the detection of excursions in calculated SRs that not only often differed from those established through coarse-scaled biostratigraphic data, but also revealed a greater regional variability in the sediment accumulation over time. High-resolution bulk density data exhibited distinct periodicity in the waveband of Milankovitch cycles (precession at 19–23 kyr; obliquity at 41 kyr; eccentricity at 100 kyr). The pronounced Milankovitch cyclicity suggests that climate variability and trends in SRs along the Benguela Current System (BCS) were responding to insolation patterns during the past 6 Myr. We find relatively low SRs when evolutive amplitude spectra are dominated by obliquity and eccentricity periods. In contrast, significant SR peaks at all sites often occur when strong precessional amplitudes coexist with obliquity and eccentricity cycles. Episodes of high SRs often coincide with peaks in organic carbon mass accumulation rates (MAR Corg) and reduced sea surface temperature (SST) in the southern Walvis Basin, which have been associated with increased regional upwelling. This suggests that the high SRs reflect high productivity (high MAR Corg) attributed to strong wind and upwelling intensity during cool climate periods (low SSTs) in accordance with orbital forcing patterns observed in our spectra.
High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ18O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ18O variations is followed by a period marked by maximum in the δ18O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ18O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage.
A late Miocene to early Pliocene sequence drilled on the continental slope of southwest Africa off the Orange river mouth (ODP Site 1085) has been investigated. Clay mineral assemblages, coarse siliciclastics and benthic foraminifer accumulation rates (BFAR) unravel a step by step evolution of marine and continental environments closely related to sea level variations, ocean circulation and global climate: (1) smectite is a typical tracer of the Orange river load, whereas illite is mostly transported by the Benguela current (like chlorite) and winds, and kaolinite is derived from low latitudes by the poleward undercurrent and the North Atlantic Deep Water (NADW); (2) increased erosion and influence of the Orange river after 9.6 Ma is linked to a sea level drop at a time of Antarctic ice-growth. This has been followed by an increased seasonality of precipitation and high productivity, but low oxygen content and associated dissolution of carbonates; (3) increased productivity and dissolution of carbonates, and coeval increase of continental aridity after 8.9 Ma express a further development of the Benguela current and upwelling; (4) better preservation of carbonates and increased contribution of terrigenous material from northern sources at 6.9 Ma are related to increased circulation of NADW after an early stage of northern hemisphere glaciation; (5) increased erosion and contribution from the Orange river and westward shift of the area of higher productivity from 5.9–5.8 Ma to 5.3–5.2 Ma are related to a significant fall of sea level, and encompass the time of the entire Mediterranean salinity crisis; (6) short-term variations of the smectite/illite ratio (S/I) and BFAR suggest a major control of productivity by wind and current activities (and related upwelling), but may express brief variations of sea level in specific intervals before 8.9 Ma and during the late Messinian especially.
Pliocene age sediments from Ocean Drilling Program Leg 175, Site 1085-A and B in the Cape Basin were analyzed to investigate the impact of the intensification of Northern Hemisphere glaciation (INHG) on the South Atlantic Benguela Current system from 4 to 2 Ma. Proxies for productivity (concentrations and mass accumulation rates of total organic carbon, carbon to nitrogen ratios, percent calcium carbonate, and percent biogenic silica) as well as weight percent sand (a proxy for preservation or winnowing) peak at 3.2, 3.0, 2.4, and 2.25 Ma. Normative calculations of allied trace and major elemental determinations indicate synchronous increases in productivity peaks, as well as high concentrations and accumulations of terrigenous sediments. Coeval increases in hemipelagic sedimentation and productivity indicators could be the result of enhanced eolian sedimentation resulting from strengthened winds, leading to elevated rates of upwelling and enhanced productivity. However, rapid burial, as indicated by high sedimentation rates, could also enhance preservation. The very high concentrations (>30%) and accumulations (up to 60 g/cm2/kyr) limit the likelihood that eolian sedimentation was the only transport mechanism, invoking an additional fluvial source. Rapid burial by either eolian or fluvial transport links these intervals of enhanced preservation and productivity with continental climate changes resulting from (1) increased winds and/or dust availability due to higher aridity in the Namibia/northern South Africa region; (2) lowered sea-level related to increased ice volume; (3) increased sediment load due to wetter conditions in the continental interior; or (4) some combination. Peaks at 3.2, 2.4 and 2.25 Ma are coincident with maximum precession, suggesting a link between hemipelagic sedimentation and enhanced monsoonal circulation over southern Africa. The Site 1085 sedimentary record during the INHG seems to be controlled by low-latitude processes linked to precession rather than hig-latitude processes.
Experimental substitutions of transition and alkaline earth cations into synthetic 10Å(Na⁺)-manganate show that cation uptake and the stability of the cation-substituted mineral increase with stability of the hydroxide of the cation. Hydrothermal treatment of synthetic 10Å-manganates with different metal contents as well as marine diagenetic and hydrothermal 10Å-manganates shows that the stabilities of their structures are enhanced with increasing temperature. The stabilization is due to reinforcement of the “tunnel” walls supporting the [Mn⁴⁺O²⁻6] octaheral layers.
The stratigraphy of several radioisotopes has been determined in an undisturbed core from the central north Atlantic (14C, 238U, 230Th, 232Th, 226Ra, 210Pb, 10Be). Carbon-14 indicates a sedimentation rate of 1 cm 1000 yrs−1. The core represents about 30,000 years. The oxygen isotope signal agrees with this scale. The range is about 1.5 ‰, from glacial to Holocene. The maximum change is near 11,000 years B.P. Mixing effects are minor because of the low fertility of the overlying waters. Excess lead-210 shows a minimum between 1 and 2 cm depth, and a maximum immediately below. It does not yield a diffusion coefficient for the Goldberg and Koide (1962) mixing model.The 10Be concentration in the Holocene sediment (on a CaCO3-free basis) is somewhat higher than that deposited during the previous cold period. However, both clay and 10Be deposition rates are lower by a factor of over two during the Holocene. The source of the excess 10Be in the glacial section could be a continental reservoir or increased cosmic ray production.The U and Th concentrations as well as the excess activity ratios of the sediment deposited during the Holocene are higher (by 30–100%) than in glacial sediments. Again, this is in contrast to the deposition rates of U and Th, which are lower in the Holocene by a factor of about two. The most reliable signals are the radiocarbon and the oxygen isotope stratigraphy.
Records of 10Be, 9Be, mineralogy and grain size were obtained from two cores collected by the Polarstern Expedition 1991 in the southern Nansen Basin (Core 2213-6) and the Yermak Plateau (Core 2208-2). The accumulation of sediments examined started from about 350 ka (BP), and includes relatively well defined trends of Be isotopes coincident with interglacial/glacial climatic cycles. Sediment accumulation rates (g/cm2 ka) were higher during glacial periods and our estimates of 1.0 and 2.5cm/ka sedimentation rates during the Holocene agree with other estimates for the southern Nansen Basin and the Yermak Plateau, respectively. The variations in 10Be concentration (atoms/g) and flux (atoms/cm2 ka) are inverse to sediment flux, where high 10Be concentration and flux are associated with generally low sedimentation/accumulation rates during interglacial periods. We hypothesize that climate plays an important role in 10Be records from the Arctic sediments, reflecting the intensity and distribution of the ice mass on land and the ocean.
A 10Be/9Be-based chronostratigraphy has been determined for ODP 181, Site 1121 sediment core, recovered from the foot of the Campbell Plateau, Southwest Pacific Ocean. This core was drilled through the Campbell ‘skin drift’ in ca. 4500 m water depth on the mid-western margin of the extensive Campbell Nodule Field, beneath the flow of the major cold-water Deep Western Boundary Current (DWBC). In the absence of detailed biostratigraphy, beryllium isotopes have provided essential time information to allow palaeo-environmental interpretation to be undertaken on the upper 7 m of the core. Measured 10Be/9Be ratios of sediment, and of ferromanganese nodules entrapped in the sediment, decrease systematically with depth in the core, in accordance with radioactive decay. However, the 10Be/9Be data diverge from ca. 3 m below the seafloor (mbsf) to the top of the core, giving rise to several possible geochronological models. The preferred model assumes that the measured 10Be/9Be ratios of the nodule rims reflect initial 10Be/9Be ratios equivalent to contemporary seawater, and that these can be used to derive the true age of the sediment where the nodules occur. The nodule rim ages can be then used to interpret the sediment 10Be/9Be data, which indicate an overall age to ca. 7 mbsf of ca. 17.5 Ma. The derived chronology is consistent with diatom biostratigraphy, which indicates an age of 2.2–3.6 Ma at 1 mbsf. Calculated sedimentation rates range from 8 to 95 cm m.y.−1, with an overall rate to 7 mbsf of ca. 39 cm m.y.−1. The lowest rates generally coincide with the occurrence of entrapped nodules, and reflect periods of increased bottom current flow causing net sediment loss. Growth rates of individual nodules decrease towards the top of the sediment core, similar to the observed decrease in growth rate from core to rim of seafloor nodules from the Campbell Nodule Field. This may be related to an overall increase in the vigour of the DWBC from ca. 10 Ma to the present.
The interval of time represented by marine isotope stages 11 and 12 (∼360–470 ka) contains what may be the most extreme glacial and interglacial climate conditions of the Late Pleistocene. It has been suggested that sea level rose by ∼160 m at the termination of glacial stage 12. This is 30% greater than the sea level rise that followed the most recent glacial maximum. There have been few detailed studies of the unique conditions that existed during the stage 11–12 time period because of the lack of high-quality core material. This problem has been addressed by the collection of high deposition rate cores from sediment drifts in the western North Atlantic during Ocean Drilling Project Leg 172. Benthic foraminiferal δ13C data from cores collected between ∼4600 and 1800 m were used to reconstruct bathymetric gradients in deep and intermediate water properties for selected time slices during this glacial–interglacial cycle. During glacial stage 12, the deep western North Atlantic was filled by a water mass that was more nutrient-enriched than modern Antarctic Bottom Water. Above 2000 m, a more nutrient-depleted water mass existed during this glacial stage. Such an intermediate water mass has been described for more recent glacial periods and presumably forms in a more proximate region of the North Atlantic. Interglacial stage 11 water mass properties closely resemble those of the present-day western North Atlantic. A nutrient-depleted water mass (δ13C of 0.75–1.0‰), similar to modern North Atlantic Deep Water existed between 3500 and 2000 m. This was underlain by a water mass with lower δ13C values (<0.75‰) that probably was derived from a southern source. Using Leg 172 data, along with previously published results from the Atlantic and Pacific oceans, we estimate a mean global δ13C change of 0.95‰ from stage 12 to stage 11. This is twice the whole ocean δ13C change reported for the transition from the last glacial maximum to the Holocene.
Nova Scotia's position near the margin of large Pleistocene ice sheets makes the Scotian Shelf region critical for evaluation of glaciation models and sea-level change. The sea floor of the inner Scotian Shelf was mapped using multibeam bathymetry, a combination of conventional seismic and sonar techniques, and sampling. Multibeam bathymetry provides an areal image of the sea floor. Combining this unique image with sub-bottom seismic imaging, the relationships between sea floor topography and the underlying strata were explored.
Armouring phenomena are common in river beds, gravel beaches and spits. The gravel segregation recognized at depth in three different beaches of Patagonia and Tierra del Fuego, Argentina, allows us to propose a mechanism for formation.A mixed population of gravel set into motion by waves is deposited progressively. Granules and fine gravels with higher pivoting angles in the beach slope are easily trapped within the bed, while rounded pebbles continue rolling over (overpassing) it. Finally, decreasing flow velocity allows the deposition of the larger pebbles, thus armouring the beach.A carpet of round clasts of uniform size is more stable than the same clasts in a mixed population armouring the bed and covering a uniform layer of rounded granules and fine gravels.
A SEM-based petrographic study of Miocene to Quaternary phosphorites recovered during ODP Leg 112 shows that they contain a variety of apatitic nannostructures. In porous types, apatite appears as ovoid, dumbbell-like, botryoid, micron-sized, and inframicron apatite particles and flakes. These structures are probably due to microbially mediated apatite precipitation. In compact varieties, diagenesis reduced the porosity obscuring the original apatite microparticles. Macroscopic aspects of phosphorites (colour, brightness, hardness, density) are essentially linked to the porosity of apatitic structures but independent of the apatite particles and structures; generally they reflect diagenetic stages in evolution. The variety and distribution of the apatite particles and structures suggest that phosphorites mainly formed in three different ways: phosphatization of (1) fish excrements forming copronodules, (2) diatom muds producing nodules and (3) faecal pellets resulting in rounded grains. These phosphorite particles formed in low energy conditions; many of them were subsequently reworked.
A census of Plio–Pleistocene (5–0 Ma) terrestrial palynomorph assemblages from ODP Site 1123, located 1100 km offshore eastern New Zealand and in a water depth of 3290 m, reveals marked variations in warm- (Cyathea, tall tree Podocarpus/Prumnopitys, Dacrydium cupressinum) and cold- (Halocarpus, Phyllocladus, Nothofagus fusca type, Coprosma) climate indicator species at Milankovitch-scale periodicities. Time series analysis indicates that the vegetation record is covariant with marine climate proxies (carbonate content) and is strongly coherent at the 40-ka and 100-ka orbital frequency. A pronounced increase in amplitude and a coeval decrease in frequency of climate cycles from 40 to 100 ka occurs between 0.92 and 0.62 Ma, and provides a rare vegetation record of a fundamental reorganisation of Earth’s climate system – the Mid-Pleistocene Climate Transition. Despite the long distance from land and great water depth, terrestrial palynomorphs are relatively abundant in this high-resolution marine sediment record, where they reflect a southern North Island source. The predominance of robust spores and buoyant pollen (Cyathea, Podocarpus/Prumnopitys) indicates considerable current sorting and degradation of more delicate morphotypes during transport across the eastern North Island shelf. Little evidence is found for sorting and transport processes being significantly modulated by climate.
ODP Site 1124, located 600 km east of the North Island of New Zealand, records post-middle Oligocene variations in the Pacific Deep Western Boundary Current (DWBC) and New Zealand’s climatic and tectonic evolution. Sediment parameters, such as terrigenous grain size, flux, magnetic fabric, and non-depositional episodes, are used to interpret DWBC intensity and Antarctic climate. Interpretations of DWBC velocities indicate that the Antarctic Circumpolar Current reached modern intensities at ∼23 Ma, as the tectonic seaways expanded, completing the thermal isolation of Antarctica. Periods of more intense bottom water formation are suggested by the presence of hiatuses formed under the DWBC at 22.5–17.6, 16.5–15, and 14–11 Ma. The oldest interval of high current intensity occurs within a climatically warm period during which the intensity of thermohaline circulation around Antarctica increased as a result of recent opening of circum-Antarctic gateways. The younger hiatuses represent glacial periods on Antarctica and major fluctuations in the East Antarctic Ice Sheet, whereas intervals around the hiatuses represent times of relative warmth, but with continued current activity. The period between 11 to 9 Ma is characterized by conditions surrounding a high velocity DWBC around the time of the formation and stabilization of the West Antarctic Ice Sheet. The increased terrigenous input may result from either changing Antarctic conditions or more direct sediment transport from New Zealand. The Pacific DWBC did not exert a major influence on sedimentation at Site 1124 from 9 Ma to the present; the late Miocene to Pleistocene sequence is more influenced by the climatic and tectonic history of New Zealand. Despite the apparent potential for increased sediment supply to this site from changes in sediment channeling, increasing rates of mountain uplift, and volcanic activity, terrigenous fluxes remain low and constant throughout this younger period.
Microbial mats characteristically are dominated by a few functional groups of microbes: cyanobacteria, colorless sulfur bacteria, purple sulfur bacteria, and sulfate-reducing bacteria. Their combined metabolic activities result in steep environmental microgradients, particularly of oxygen and sulfide.The driving force of most microbial mats is photosynthesis by cyanobacteria and algae. Subsequently, sulfate-reducing bacteria, using excretion-, lysis-, and decomposition products of cyanobacteria, produce sulfide by the dissimilatory reduction of sulfate. The sulfide can be reoxidized to sulfate by colorless and purple sulfur bacteria.Colorless sulfur bacteria are chemotrophic organisms primarily oxidizing sulfide and other reduced forms of sulfur with oxygen to obtain energy. The oxidation of reduced sulfur species also provides reducing equivalents for the reduction of carbon dioxide to cellular carbon. The final product of sulfide oxidation is sulfate, with elemental sulfur, deposited extracellularly, as the principal intermediate.Purple sulfur bacteria primarily are anaerobic phototrophic organisms using sulfide and other reduced forms of sulfur exclusively as the electron donor for the reduction of CO2 to cellular carbon. Usually, sulfur is temporarily stored intracellularly. The final product of the oxidation of reduced forms of sulfur is sulfate.The niches for these metabolically different groups of microbes in ecosystems with steep, often non-overlapping, gradients of oxygen and sulfide appear to be spatially separated. However, maximum viable counts of colorless sulfur bacteria and purple sulfur bacteria were both found in the top 5–10 mm of mats. Unexpectedly, viable counts of sulfate-reducing bacteria also peaked at the same depth horizon.Sulfide is inhibitory for most oxygenic phototrophs. Sulfide production immediately underneath the layer of cyanobacteria might inhibit their growth, and, consequently, that of the entire ecosystem. In microbial mats this effect is minimized by the combined action of colorless and purple sulfur bacteria. Colorless sulfur bacteria generally have a much higher affinity for sulfide than purple sulfur bacteria, however, in microbial mats, their activity is hampered by low oxygen supply rates. As shown by pure culture studies with colorless sulfur bacteria, sulfide is incompletely oxidized when oxygen is short in supply, resulting in the production of potential electron donors for purple sulfur bacteria, such as sulfur, thiosulfate and polysulfides. In the absence of purple sulfur bacteria, colorless sulfur bacteria would not be able to maintain a low sulfide concentration due to shortage of oxygen, which in turn would result in increased inhibition of oxygenic photosynthesis.It thus appears that the combined action of all four groups of functional microbes mentioned effectively results in optimal growth of these recent “stromatolites”.
In this paper we describe two sensors for measurement of particle size-distribution and settling-velocity distribution. These measurements are critical to the correct estimation of the true sediment concentration in the field, as well as to evaluating models for transport rates of sediments. A multi-angle measurement of laser scattering is made and inverted to obtain the particle size distribution. Since small-angle scattering is relatively insensitive to particle composition, the size distribution measurements are robust, and do not require particle refractive index. It is shown that with a knowledge of the size distribution, true particulate volume concentration can be obtained, unaffected in calibration by changes in particle size distribution. The data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer. The importance of these observations lies in the implication that historical data acquired with single-parameter optical or other sensors needs to be revisited. In the second instrument, analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates. In this case, the history of concentration of each size class is examined to determine the settling velocity, without invoking any assumptions of settling regime. Settling velocity data from a field experiment off the New Jersey coast fit the model: where an is radius in microns and settling velocity is in cm/s.
Lithostratigraphy, grain sizes and down-hole logs of Site 1166 on the continental shelf, and Site 1167 on the upper slope, are analyzed to reconstruct glacial processes in eastern Prydz Bay and the development of the Prydz trough-mouth fan. In eastern Prydz Bay upper Pliocene–lower Pleistocene glaciomarine sediments occur interbedded with open-marine muds and grade upward into waterlaid tills and subglacial tills. Lower Pleistocene sediments of the trough-mouth fan consist of coarse-grained debrites interbedded with bottom-current deposits and hemipelagic muds, indicating repeated advances and retreats of the Lambert Glacier–Amery Ice Shelf system with respect to the shelf break. Systematic fluctuations in lithofacies and down-hole logs characterize the upper Pliocene–lower Pleistocene transition at Sites 1166 and 1167 and indicate that an ice stream advanced and retreated within the Prydz Channel until the mid Pleistocene. The record from Site 1167 shows that the grounding line of the Lambert Glacier did not extend to the shelf break after 0.78 Ma. Published ice-rafted debris records in the Southern Ocean show peak abundances in the Pliocene and the early Pleistocene, suggesting a link between the nature of the glacial drainage system as recorded by the trough-mouth fans and increased delivery of ice-rafted debris to the Southern Ocean.
Probable in-situ manganese deposits larger than 1 cm in diameter buried in ODP/DSDP cores were selected for study after examining previous descriptions of the manganese deposits in site reports and the ODP data base. Most of the selected samples from 11 cores occur at or just above sedimentary hiatuses or in slowly deposited sediments and are overlain by rapidly deposited sediments of biogenic, terrigenous or volcanogenic origin. The changes in sedimentation recorded in the lithostratigraphic sections around these deposits are closely related to changes in tectonic evolution, deep water circulation or biological productivity at the sites.The similarity in composition and structure of the buried deposits to those of the modern manganese nodules and crusts with no evidence of post-depositional change suggest that buried manganese deposits may be used as indicators of past sedimentary conditions during which they formed. Their major components are hydrogenetic and early-diagenetic manganese minerals as well as detrital minerals. The characteristics of these manganese deposits suggests that similar processes of deposition have taken place since the Paleogene or older.
A high resolution sedimentary record spanning the last deglacial and Holocene periods was obtained by studying the deep-sea sediment core GIK 17748-2 retrieved from 2545 m water depth in the Valparaiso Basin (Chilean continental slope, Southeast Pacific — 32°45.00′S; 72°02.00′W). AMS- measurements and oxygen-isotope analyses of the planktic foraminifera species Globigerina bulloides and Neogloboquadrina pachyderma (dex.) indicate an age of 13.3 AMS- kyr B.P. at the base of the core (383 cm) and sedimentation rates ranging between 9 and 92 cm/kyr. Foraminiferal assemblages and accumulation rates were analysed in order to investigate variations in paleoceanography and paleoproductivity in the southern Peru–Chile Current during the last 13 kyr. Changes in foraminiferal assemblages indicate a sequence of three major hydrographic regimes at the site of the core: (1) deglacial sediments are characterised by variable faunal compositions generally reflecting strong upwelling and high productivity. (2) During the early and middle Holocene upwelling and, thus, productivity decreased, as indicated by an elevated relative abundance of N. pachyderma (dex.), and the importance of the Subtropical Surface Water of the Peru–Chile Countercurrent increased, as indicated by higher relative amounts of N. dutertrei. (3) For the last 3 kyr, variable faunal assemblages indicate variable competing environmental conditions characterised on one hand by increased upwelling, reflected by an elevated relative abundance of N. pachyderma (sin.), and on the other hand, by warmer conditions, deduced from a higher relative abundance of N. dutertrei, probably due to more frequent and more intense El Niño events. In addition, hints for slightly cooler conditions between 11.4 and 10.7 kyr B.P. might be related to a Younger Dryas event in this region.
The northwestern Cascadia Basin of western North America accumulated high-sedimentation-rate sequences during the Pleistocene sea-level low-stands. The continental shelf was largely exposed at that time, and rivers and estuaries delivered large sediment fluxes directly to the deep ocean. The IODP EXP1301 core, which was taken from the middle portion of the Cascadia Basin, is well preserved and exhibits the deeper and — more distal sedimentary facies. The lithology in this location is composed of two units, 1) hemipelagic mud with a thin sand layer and 2) thick, coarsening upward silt–sand turbidites with a small proportion of granules at the top. We will focus on the detailed sand-grain proportions in order to understand the origin of these sediments. We determined the modal proportions of the heavy minerals, and the chemical composition of olivine and orthopyroxene in fourteen samples. These are characterized by an abundance of amphibole, pyroxenes and epidote, and the presence of minerals derived from peridotite. There is no drastic change in the modal and mineral compositions of the sands and silts between the turbidite and hemipelagic sequences. There were two probable drainage systems on the continent, the Frazer and Columbia rivers, which shed turbidites into the Cascadia Basin after 1.6 Ma, especially at 0.46–0.76 Ma. Based on a comparison of the modal and mineral compositions, the Northern Cascadia Basin has been supplied with sediments, mainly from the Frazer River, through the Straits of Juan de Fuca, by Pleistocene to Holocene turbidites.
A geochronology time series provides a powerful tool for elucidating sedimentary processes such as episodic deposition and diffusive mobility of particle-reactive constituents. Depth distributions of 210Pb and 137Cs from Skan Bay, Alaska were determined for sediment cores collected in 1980, 1984, 1987, and 1990. Sediment X-radiographs reveal distinct layers indicating that sediments were not continuously mixed by bioturbation. However, the geochronology time series is inconsistent with an undisturbed, steady-state sediment column. Profiles from 1980, 1984, and 1990 reveal subsurface regions in which 210Pb activity is relatively constant. In addition, the depth of the primary 137Cs maximum (reflecting the 1963 peak in atmospheric bomb testing) does not increase in a regular fashion between 1980 and 1990. The 210Pb and 137Cs geochronologies can be reconciled by removing the effects of an instantaneous depositional event. The average 210Pb sedimentation rate (corrected for episodic deposition) in cores that were collected over a ten year period (0.241 ± 0.006 g cm−2 yr−1) is in excellent agreement with the average 137Cs sedimentation rate (0.258 ± 0.008 g cm−2 yr−1) calculated from three stratigraphic markers [peak fallout (1963), first appearance in the sediment record (1952), and the Chernobyl accident (1986)]. The mobility of bomb-derived 137Cs under in situ conditions was evaluated by a time-dependent numerical model applied to the 137Cs time series. The model indicates that bomb-derived cesium is immobile in Skan Bay sediments with a solid-liquid distribution coefficient (Kd of ⩾ 105 (ml g−1).
A sediment core taken from a tidal mudflat in Ho Bugt in the northernmost part of the Wadden Sea in Denmark is used to explore the application of optically stimulated luminescence (OSL) dating to young fine-grained estuarine sediments, using 210Pb dating and the 137Cs Sellafield peak from 1980 as independent age control. Despite the anticipated difficulties of weak luminescence signals and incomplete resetting of residual radiation dose prior to deposition, the OSL ages from the sediment core ranged from 7.0±1.5 at the surface to 305±16 years (68 cm depth). OSL- and 210Pb-dates were in good agreement back to ∼1975, and even as far back as ∼1945 using the CRS-model. The average OSL age of 9±3 years for the surface mixing zone indicates that the OSL signal of the quartz grains was well zeroed at deposition. The OSL ages give new information on past deposition regimes on the Ho Bugt mudflat: fine-grained sediment started to deposit at ∼0.5 mm year−1 approximately 300 years ago, when a terrestrial reed-swamp was transgressed. About 100 years ago, the sedimentation rate increased to ∼4 mm year−1. Approximately 40 years ago, the sediment became more fine-grained, perhaps because of emergence of the tidal flat; the accretion rate at that time was already well above the local rate of sea-level rise of ∼1 mm year−1. The post-1980 accretion rate was found to be ∼16 mm year−1 with OSL dating and ∼9–12 mm year−1 with 210Pb and 137Cs dating. Advantages of the OSL method include applicability over a much larger time-span (∼125 ka compared to ∼120 years for 210Pb) and a lack of significant dependence on changes in mean grain size or 210Pb supply. With this method, it is also possible to date sediment cores from sand flats, providing a new approach to the problem of evaluation of stability and calculation of sediment budgets for estuaries and coastal lagoons.
Profiles of 210Pb, 137Cs and 239,240Pu measured in 83 sediment cores collected from various sedimentary regimes in the East China Sea were analyzed to elucidate the sources, routes and budgets of sediments as well as these radionuclides. Distributions of sedimentation rates and nuclide inventories reveal alongshore transport of sediments, 137Cs and 239,240Pu from the mouth of the Yangtze River toward the south, largely confined to the inner-shelf area (water depth <70 m). Mass balance calculations suggest that the East China Sea is a sink for the particle-reactive 210Pb and 239,240Pu, with about one-sixth of their sedimentary budgets supplied via boundary scavenging. In contrast, due to lower affinity of 137Cs for particles and rapid turnover of the shelf water, the East China Sea serves as a source for 137Cs. About two-thirds of the cumulative input of 137Cs have been transported out of the East China Sea, leaving the remaining one-third stored in the bottom sediments and the overlying water column. As for the sediment budget, mass balance cannot be established due to a shortfall in sediment supply of more than 30% based on a comparison between input terms documented thus far and the sedimentation flux derived from this study. It is very likely that we have overestimated the sediment burial flux or that long-distance transport from the Yellow River’s dispersal system to the East China Sea is underestimated. Alternatively, the imbalance could be explained by the discrepancy between sediment input and output on decadal to centennial timescales.
Sedimentation dynamics in the East China Sea was investigated based on profiles and inventories of , and in sediment cores collected from various settings in this marginal sea. The derived sedimentation rates vary by two orders of magnitude, from ∼2 to 0.02 cm yr−1, and generally decrease southward along the inner shelf and eastward offshore. This pattern is consistent with the dispersal of Yangtze River-derived sediments by the current and tidal systems. The weighted mean sedimentation rate over the shelf (ca. 0.3 g cm−2 yr−1) appears to be higher than fluvial inputs from the Yangtze River and Taiwan (0.14 g cm−2 yr−1). The discrepancy can be reduced by considering the effect of sediment mixing (which renders sedimentation rates overestimated) and the possibility of additional sediment input from the Yellow River's dispersal system. Spatial variations of the observed nuclide inventories in sediments suggest extensive sediment redistribution, boundary scavenging of , and large inputs of and from Yangtze River's drainage basin.
Organic δ13C and C/N analyses of estuarine deposits provide proxies for changes in the source of organic matter, which can be driven by fluctuations in relative sea level, river discharge, and catchment disturbance. Here we present the results of a comprehensive vegetation and sediment δ13C and C/N survey of Welwich Marsh (outer Humber Estuary, UK), together with high-resolution δ13C and C/N analyses of Holocene cores collected nine years previously from the Humber Estuary and the Lincolnshire Marshes, Eastern England, UK. The contemporary intertidal δ13C and C/N dataset shows a gradual increase in surface sediment δ13C with decreasing marsh height and suggests that δ13C is controlled by the degree of tidal inundation and thus reflects organic matter source. However, sediment C/N ratios are less sensitive to tidal changes and the recent introduction of C4 salt-marsh species complicates the contemporary analogue. The Holocene δ13C and C/N records are in general agreement with existing microfossil data and provide additional palaeoenvironmental information. This includes support for an estuary-wide expansion of marine conditions from c. 3.3 ka cal. yr BP, followed by a contraction of marine conditions after c. 2.7 ka cal. yr BP, and evidence for an increase in delivery of terrigenous organic matter to the inner estuary in the late Holocene. Bulk organic δ13C and C/N analysis is shown to be a reliable and independent indicator of coastal environmental change and is therefore a complementary technique to the more commonly used microfossil approach. However, this study also shows that in some circumstances the technique may be compromised when applied to sediments from cores that have been stored for a period of time.
Geochemical studies of Cretaceous strata rich in organic carbon (OC) from Deep Sea Drilling Project (DSDP) sites and several land sections reveal several consistent relationships among amount of OC, hydrocarbon generating potential of kerogen (measured by pyrolysis as the hydrogen index, HI), and the isotopic composition of the OC. First, there is a positive correlation between HI and OC in strata that contain more than about 1% OC. Second, percent OC and HI often are negatively correlated with carbon isotopic composition (σ 13C) of kerogen. The relationship between HI and OC indicates that as the amount of organic matter increases, this organic matter tends to be more lipid rich reflecting the marine source of the organic matter.
Recent full coverage bathymetric and geophysical surveys (Yokosuka 90 and 91), carried out in the framework of the starmer Japanese-French joint project, reveal details of the structure of the N160° segment of the North Fiji Basin Ridge. Despite its intermediate spreading rate this segment shows a “slow spreading” type morphology with steep 1000 m high walls reaching an axial depth of 4000 m. Its northern tip at 14°50′S is a complex area involving several plates boundaries. This domain can be interpreted either as a complex boundary between two plates, or as a new triple junction, symmetrical with the 16°50′S Triple Junction characterizing its southern tip. In either case, the complexity of the spreading ridge geometry in this area illustrates the instability of the accretionary geometry due to the deformation of the whole North Fiji Basin between the much larger Indo-Australian and Pacific plates.
A geochemical and Sr isotope study was carried out on sediment cores collected from the Logatchev hydrothermal vent site (14°45′N) on the Mid-Atlantic ridge, with the aim of identifying aeolian inputs into the overlying water column. The Logatchev vent site lies beneath the NE trade winds, which transport large amounts of detrital material across the North Atlantic. Sequential dissolution of the sediments has been used to discriminate the detrital fraction from other types of sediment. REE patterns for the acetic acid fraction at the Logatchev site record contributions from biogenic carbonate (seawater-like REE pattern) and oxy-hydroxides, which exhibit positive Eu and negative Ce anomalies, typical of hydrothermal origin. The Logatchev site leach fractions have similar major element compositions to terrigenous Al–Fe–Mn oxides, whose REE patterns show strong positive Ce and negative Eu anomalies that are comparable to those of “preformed” Fe–Mn oxides of continental origin. Based on major element and REE data and 87Sr/86Sr ratios (0.7197–0.7236), the residue fractions at the Logatchev site contain a significant contribution from aeolian sources, whose chemical composition is virtually identical to oligotrophic settling particles in the Equatorial Atlantic Ocean. In comparison, residues from the Rainbow hydrothermal site (36°14′N), have geochemical composition controlled by hydrothermal anhydrite [Chavagnac V., German C.R., Milton J.A., Palmer M.R., 2005. Sources of REE in sediment cores from the Rainbow vent site (36°14′N, MAR). Chem. Geol. 216, 329–352.]. They are characterised by 87Sr/86Sr ratios of 0.7087–0.7093, which are significantly less radiogenic than those of the Saharan dust. Hence, the data suggest (1) a significant contribution of aeolian material to the sediments at 14°45′N that is not present at 36°14′N, (2) that this material is most likely of North-African origin, and (3) that this source of aeolian material has remained relatively constant over the last 13 kyr.
14C-labelled methane, biologically prepared by Methanobacterium thermoautotrophicum, is widely used to determine methane oxidation rates. However, M. thermoautotrophicum synthesizes carbon monoxide as a by-product during methanogenesis. In this study, sulfate-reducing bacteria utilizing the acetyl-CoA/carbon monoxide-dehydrogenase pathway were able to form 14CO2 from 14CH4 containing 14CO. 14C-labelled carbon monoxide was removed from 14CH4 by oxidation over hopcalite to carbon dioxide and fixation in sodium hydroxide solution. Measurable formation of 14CO2 from purified 14C-labelled methane by sulfate-reducing bacteria was not observed. Therefore, reported anaerobic methane oxidation rates in marine habitats measured with 14CH4 from M. thermoautotrophicum are inclined to include carbon monoxide oxidation rates.Anaerobic oxidation of 14CH4 by sulfate-reducing and acetogenic bacteria and methanogenic archaebacteria was tested. Only methanogenic species produced up to 900 ppm 14CO2 from 14CH4 applied. This observation and the absence of methane oxidation by sulfate-reducing bacteria sustain the hypothesis that methanogenic archaebacteria in a syntrophic community might be responsible for the oxidation of methane in anaerobic habitats.
The δ18O and δ18C isotopic composition of Globigerinoides ruber, the concentration of C37 alkenones, n-nonacosane and n-hexacosan-1-ol and the populations of coccolith species in core 17961-2 have been used to characterize the climatic changes which occurred in the South China Sea (SCS) during the last climatic cycle. The relative composition of di- and triunsaturated C37 alkenones, the U37K index, has been used to estimate the variation in sea surface temperatures. The concentrations of the terrigenous markers n-nonacosane and n-hexacosan-1-ol have allowed to infer changes in continental water dynamics during the glacial and interglacial times. The stratigraphic record of these compounds has shown that the influence of continental waters (i.e., the Molengraaff River) and restricted water circulation in SCS gave rise to a marginal system of higher but slower response to climatic change. Both in terminations I and II, the change from glacial to interglacial conditions involves a considerable reduction of continental water input into this semi-enclosed system. The high resolution study of the biomarker proxies in Termination I has shown that the SST increase lagged about 2.3 ky the δ18O isotopic decrease of Cibicidoides wuellerstorfi. Probably, the SST increase corresponds to the period of the Sunda Shelf inundation onset.