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.
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.
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.
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.
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.
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.
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.
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.
