Ba, Al and SO4 concentrations in sediments and pore fluid of ODP Leg 127/128 samples

The barium distribution in sediments and pore fluids from five sites drilled in the Japan Sea have been used to illustrate the geochemical behavior of this element as it pertains paleoproductivity reconstructions, diagenetic remobilization, and barite precipitation in authigenic fronts. Sites where sulfate is depleted in the pore fluids also show high concentrations of dissolved barium, reflecting dissolution of biogenic barite. The high rate of sedimentation at Sites 798 and 799 results in a rapid sulfate depletion, which in turn leads to barite dissolution and reprecipitation in diagenetic fronts. The dissolved barium distribution at these sites has been used to quantify the rate of barite dissolution; we estimate a first-order rate constant for barite dissolution to be 2*10**-6/s at Site 799 and 2*10**-7/s at Site 798. Authigenic barite has been documented in sediments from Site 799 at 323 meters below seafloor by scanning electron microscopy and X-ray fluorescence analysis. These results indicate barite precipitation in a diagenetic front near the zone of sulfate depletion by upward migration of dissolved barium and downward diffusion of sulfate. Barite precipitation has also been inferred at Sites 796 and 798 based on sedimentary and dissolved barium distributions. Sulfate is not depleted in the pore fluids of Site 794. The lack of diagenetic remobilization of biogenic barium at this site preserves the high barium signal associated with the high-productivity sequences deposited during the late Miocene to Pliocene. Significantly, the organic carbon distribution does not indicate high accumulation rates during the periods of high opal and barium deposition. Instead, higher organic carbon accumulations are recorded in the Quaternary and middle Miocene sequences; intervals that are also characterized by deposition of siliciclastic turbidites. The presence of a terrestrial component in the organic carbon record renders barium a more useful indicator than organic carbon for paleoproductivity reconstructions in this marginal sea.

Strontium isotope ratios of fish teeth from ODP sites in the central North Pacific

Strontium isotopic compositions of ichthyoliths (microscopic fish remains) in deep-sea clays recovered from the North Pacific Ocean (ODP holes 885A, 886B, and 886C) are used to provide stratigraphic age control within these otherwise undatable sediments. Age control within the deep-sea clays is crucial for determining changes in sedimentation rates, and for calculating fluxes of chemical and mineral components to the sediments. The Sr isotopic ages are in excellent agreement with independent age datums from above (diatom ooze), below (basalt basement) and within (Cretaceous-Tertiary boundary) the clay deposit. The 87Sr/86Sr ratios of fish teeth from the top of the pelagic clay unit (0.7089891), indicate an Late Miocene age (5.8 Ma), as do radiolarian and diatom biostratigraphic ages in the overlying diatom ooze. The 87Sr/86Sr ratio (0.707887) is consistent with a Cretaceous-Tertiary boundary age, as identified by anomalously high iridium, shocked quartz, and sperules in Hole 886C. The 87Sr/86Sr ratios of pretreated fish teeth from the base of the clay unit are similar to Late Cretaceous seawater (0.707779-0.7075191), consistent with radiometric ages from the underlying basalt of 81 Ma. Calculation of sedimentation rates based on Sr isotopic ages from Hole 886C indicate an average sedimentation rate of 17.7 m/Myr in Unit II (diatom ooze), 0.55 m/Myr in Unit IIIa (pelagic clay), and 0.68 m/Myr in Unit IIIb (distal hydrothermal precipitates). The Sr isotopic ages indicate a period of greatly reduced sedimentation (or possible hiatus) between about 35 and 65 Ma (Eocene-Paleocene), with a linear sedimentation rate of only 0.04 m/Myr The calculated sedimentation rates are generally inversely proportional to cobalt accumulation rates and ichthyolith abundances. However, discrepancies between Sr isotope ages and cobalt accumulation ages of l0-15 Myr are evident, particularly in the middle of the clay unit IIIa (Oligocene-Paleocene).

Nannofossil biostratigraphy of ODP Hole 206-1256B sediments

Site 1256 of Ocean Drilling Program Leg 206 to the Guatemala Basin on the eastern flank of the East Pacific Rise yielded a near-complete, middle Miocene-Quaternary carbonate-rich section that provides an opportunity to study low-latitude biostratigraphic and paleoceanographic events. The sedimentary sequence in Hole 1256B has been zoned using calcareous nannofossils according to the biostratigraphic schemes by Martini of 1971 (modified by Martini and Müller in 1986) and Okada and Bukry of 1980. The nannofossil assemblage is characteristic of the low latitudes, with abundant Gephyrocapsa, Discoaster, and Sphenolithus, and is in general moderately to well preserved, depending on nannofossil abundance and the presence of diatoms. Age estimates for the first occurrence and last occurrence of Reticulofenestra rotaria were derived from biostratigraphy and magnetostratigraphy independently and assigned to 7.18 and 6.32 Ma, respectively. Linear sedimentation rates, calculated using 28 nannofossil datums and age estimates, are high in the middle Miocene, decrease from the late Miocene to the Pliocene, then increase upsection. The abrupt drop in carbonate mass accumulation rates during the early late Miocene is referred to as the "carbonate crash." This pattern reflects (1) the long-trend decrease of productivity as the site moves away from the upwelling system at the equatorial divergence as well as (2) fluctuation in the chemistry of the bottom waters associated with production of the North Atlantic Bottom Water and ventilation via the Panama Gateway. A basement age of 14.5 Ma was obtained by extrapolating the 39.1-m/m.y. rate in the middle Miocene to the basement at 250.7 meters below seafloor, and is consistent with the ~15-Ma age of the oceanic crust estimated from marine magnetic anomalies. Reworked nannofossils and lithologic changes were used to unravel postdepositional history, and three episodes were recognized, one of which in the latest Miocene can be widely correlated.

Age determination of sediment cores from the continental shelf and slope off North-West Africa

Three sediment cores from the continental shelf and slope off NW Africa (Banc d'Arguin; 52 m, 665 m and 973 m water depth) have been investigated by means of a coarse fraction analysis. The two shallower cores have been deposited during less than 10,000 years, the deeper one during the last 36,000 years. The Holocene sedimentation ( 4000 years) in the deeper part of core 79 the edge of the Banc d'Arguin is strongly influenced by reworking of Late Glacial dune sands and biogenic particles from shallower ware (<40 m), as well as eroding current influence. A decrease in grain size of silicate material and a decrease in lateral supply, correlated to a doubling of accumulation rates in the upper part of the core, indicates a more autochthonous sedimentation with less sorting influence in the youngest Holocene. The depth of provenance of the allochttonous material can be assumed in 100-300 m water depth as indicated by various biogenous particles. Small amounts of shallow water particles in the autochthonous layers indicate a supplay from shallow water, which probably occured b ythe mechanism of "particle by particle supply". None of the three cores indicates upwelling influence, although occanographers found intense upwelling in the area of the Banc d'Arguin. The Holocene climate in that area probably has been arid, small variations in terrigenous matter composition and grain size in the Early Holocene might be due to decreased wind strength or to an increase in rain fall. The Peak Glacial section (14,000-22,000 y. B.P.) of the deepest core 88 indicates a very much intensified eolian silt supply and an additional bottom supply of quartz sand In the interval 22,000-36,000 y. B.P. wind strength decreased, but probably no increase in humidity occurred. So this area in about 19° 40' N had an arid climate in the Late Holocene and in the Peak Glacial. The fragmentation of planktonic foraminifers and the abundance of aragonitic tests of pteropods in core 88 indicate an Early Holocene (8330 y. B.P.) preservation spike. Two minima in fragmentation correlated to maxima in pteropod content at about 15,700 and 21,000 y. B.P. are correlated to maxima in shallow water supply and thus do not reflect preservation conditions, but only lateral supply from the carbonate dissolution minimum zone in about 300 m water depth.

Composition of the sand fraction of sediment cores from the West-African continental margin

Seven cores from the West African continental margin in 12-18° N have been investigated by means of a coarse fraction analysis. Four of the seven cores contain allochthonous material: turbidites and debris flow deposits. The source of the allochthonous material is in about 300-600 m water depth. The age of the slide induced debris flow deposits is at the end of oxygen isotope stage 2. One debris flow deposit is covered by a turbidite (core GIK13211-1). The turbidites in the deep-sea core GIK13207-3 originate from river-influenced sediments from the West-African continental margin, whereas the autochthonous sequences are influenced by volcanic material from the Cape Verde Islands. Particle by particle supply from upper slope areas has been found in all four cores from the continental slope. Current sorting occurs on the submarine diapir (core GIK13289-3), whereas core GIK13291-1 on the NW-flanc, 200 m below core GIK13289-3, has no current sorting, except for stage 1 and parts of stage 5. The current sorting is reflected by parallel variations of median diameters of whole tests and of fragments of planktonic foraminifers, by higher median diameters of foraminifers on top of the diapir, by reduced accumulation rates and increased sand fraction percentages in core GIK13289-3 compared to core GIK13291-1. The Late Quarternary climatic history of the West-African near coastal area (12-18° N) has been redrawn: - in oxygen isotope stage 1 a humid climate is found in 12-18° N (This "humid impression" in 18° N, which is actually an arid area, is due to the poleward directed undercurrent, which transports Senegal river material to the north). - in oxygen isotope stage 2 an arid climate existed in 14-18° N, whereas in 12° N river discharfe persisted. But within stage 2 dune formation occured in 12° N on the (dry) shelf, additionally to fluviatile sediment input. - Older periods are preserved in autochthonous sediments of core GIK13289-3 and GIK13291-1, where oxygen stage 3,5 and 7 (the latter only in core GIK13289-3 present) show a humid climate (as well as in stage 5 of core GIK13255-3), interrupted by short arid intervals in core GIK12389-3, and stage 4 and 6 show an arid climate, interrupted by short humid periods The allochthonous stage 5 sediment in core GIK13211-1 also reflects a humid climate. The dissolution of planktonic foraminifers is strongest in th eLate Holocene and shows a minimum in the early Holocene, where also pteropods are preserved. The degree of carbonate dissolution is related mainly to the fine matter content (< 63 µm) whereas water depth is a less decisvive factor.

Eocene to Quaternary planktonic foraminifers from the Northwest Pacific

Eocene through Quaternary planktonic foraminifers were identified in cores recovered during Leg 126. Turbidites and volcanic ash beds are intercalated with hemipelagic sediments. Preservation of foraminifers is variable, ranging from excellent to poor and appears to have been affected by fluctuations in the carbonate compensation depth (CCD), depth of burial, changes in bottom water temperature, current velocity, sediment accumulation rates and seafloor topography. Preservation of foraminifers in Quaternary sediments is generally good, however, species abundance varies by a factor of I05-106 and reflects dilution by volcanogenic as well as terrigenous constituents and cannot be used for paleoceanographic reconstructions. In pre-Quaternary deposits planktonic foraminiferal tests frequently exhibit dissolution effects; biostratigraphic zonation and placement of zonal boundaries is difficult owing to hiatuses, dissolution facies, extraneously deposited sediments, and discontinuous coring. The Eocene foraminiferal faunas include specimens of the Globorotalia cerroazulensis plexus, markers of Zone P16 as well as Globigerina senni and Globigerinatheka spp., which became extinct before the end of the Eocene. Six hiatuses and/or dissolution periods, probably reflecting global cooling events and/or changes in oceanic circulation patterns were recorded at Site 792. Recrystallized, poorly preserved, possibly reworked Eocene species (Globigerina senni and Globigerapsis sp.) were recorded in sediments at Site 793.

Biogenic and terrigeneous components in Pliocene-Pleistocene sediments of the equatorial Atlantic

High-resolution analyses of sediments at equatorial Atlantic Sites 662, 663, and 664 define the accumulation rates of biogenically produced CaC03 and opal and of eolian dust from North Africa over the last 3.7 m.y. The mean flux of opal increased abruptly by 60%-70% near 2.5 Ma (2.65 to 2.3 Ma), reflecting pulses of increased opal productivity along the equator due mainly to increased upwelling. The mean winter-plume dust influx from Sahelian and Saharan Africa also increased at this time by between 35% and 75%, following smaller increases earlier in the late Pliocene. The increased opal flux implies a stronger zonal component of the southern trade winds in Southern Hemisphere winter. Consistent with this wind configuration, the stronger dust flux suggests a weaker southwesterly monsoonal flow into Africa in Northern Hemisphere summer, thus increasing Sahelian aridity and winter-plume dust fluxes. Dust fluxes to the equator may possibly have also been enhanced by stronger Northern Hemisphere winter trade winds and a more southerly position of the Intertropical Convergence Zone over Africa. These late Pliocene biogenic and terrigenous flux changes coincided with the appearance of Northern Hemisphere ice sheets, implying an ultimate causal link. The immediate control on changes in tropical circulation may, however, have been changes in the Atlantic sector of the Southern Ocean. A steady background trend of increasing winter-plume dust flux occurred from the late Pliocene until the middle Pleistocene. This may reflect a progressive, tectonically induced aridification of northern and eastern Africa because of the gradual uplift of the Tibetan Plateau.

Sedimentation patters in the eastern equatorial Pacific Ocean

The post-middle Miocene evolution of sedimentary patterns in the eastern equatorial Pacific Ocean has been deduced from a compilation and synthesis of CaCO3, opal, and nannofossil assemblage data from 11 sites drilled during Leg 138. Improvements in stratigraphic correlation and time scale development enabled the construction of lithostratigraphic and chronostratigraphic frameworks of exceptional quality. These frameworks, and the high sedimentation rates (often exceeding 4 cm/k.y.) provided a detailed and synoptic paleoceanographic view of a large and highly productive region. The three highlights that emerge are: (1) a middle late Miocene "carbonate crash" (Lyle et al., this volume); (2) a late Miocene-early Pliocene "biogenic bloom"; and (3) an early Pliocene "opal shift". During the carbonate crash, an interval of dissolution extending from -11.2 to 7.5 Ma, CaCO3 accumulation rates declined to near zero over much of the eastern equatorial Pacific, whereas opal accumulation rates remained substantially unchanged. The crash nadir, near 9.5 Ma, was marked by a brief shoaling of the regional carbonate compensation depth by more than 1400 m. The carbonate crash has been correlated over the entire tropical Pacific Ocean, and has been attributed to tectonically-induced changes in abyssal flow through the Panamanian seaway. The biogenic bloom extended from 6.7 to 4.5 Ma, and was characterized by an overall increase in biogenic accumulation and by a steepening of the latitudinal accumulation gradient toward the equator. The bloom has been observed over a large portion of the global ocean and has been linked to increased productivity. The final highlight, is a distinct and permanent shift in the locus of maximum opal mass accumulation rate at 4.4 Ma. This shift was temporally, and perhaps causally, linked to the final closure of the Panamanian seaway. Before 4.4 Ma, opal accumulation was greatest in the eastern equatorial Pacific Basin (near 0°N, 107°W). Since then, the highest opal fluxes in the equatorial Pacific have occurred in the Galapagos region (near 3°S, 92°W).

Antarctic shallow water benthos from three stations at Potter Cove, King George Island, West Antarctic Peninsula

The West Antarctic Peninsula is one of the fastest warming regions on the planet. Faster glacier retreat and related calving events lead to more frequent iceberg scouring, fresh water input and higher sediment loads which may affect benthic marine communities. On the other hand, the appearance of newly formed ice-free areas provides new substrates for colonization. Here we investigated the effect of these conditions on four benthic size classes (microbenthos, meiofauna and macrofauna) using Potter Cove (King George Island, West Antarctic Peninsula) as a case study. We identified three sites within the cove experiencing different levels of glacier retreat-related disturbance. Our results showed the existence of different communities at the same depth over a relatively small distance (about 1 km**2). This suggests glacial activity structures biotic communities over a relatively small spatial scale. In areas with frequent ice scouring and higher sediment accumulation rates, a patchy community, mainly dominated by macrobenthic scavengers (such as Barrukia cristata), vagile organisms, and younger individuals of sessile species (such as Yoldia eigthsi) was found. Meiofauna organisms such as cumaceans are found to be resistant to re-suspension and high sedimentation loads. The nematode genus Microlaimus was found to be successful in the newly exposed ice-free site, confirming its ability as a pioneering colonizer. In general, the different biological size classes appear to respond in different ways to the ongoing disturbances, suggesting that adaptation processes may be size related. Our results suggest that with continued deglaciation, more diverse but less patchy macrobenthic assemblages can become established due to less frequent ice scouring events.

Physical properties and abundance of diatoms of the ANDRILL AND1-1B drill core

In the austral summer of 2006/7 the ANDRILL MIS (ANtarctic geological DRILLing- McMurdo Ice Shelf) project recovered a 1285 m sediment core from beneath the Ross Ice Shelf near Hut Point Peninsula, Ross Island, Antarctica in a flexural moat associated with the volcanic loading of Ross Island. Contained within the upper ~600 m of this core are sediments recording 38 glacial to interglacial cycles of Early Pliocene to Pleistocene time, including 13 discrete diatomite units (DU). The longest of these, DU XI, is ~76 m thick, contains two distinct unconformities marked by layers of volcanic brecciated sands, and has been assigned an Early to Mid-Pliocene age (5-3 Ma). A detailed record (avg. sample spacing of 33 cm) of the siliceous microfossil assemblages have been generated for DU XI and used in conjunction with geochemical and sedimentological data to subdivide DU XI into four discrete subunits of continuous sedimentation. Within each unit, changes in diatom assemblages have been correlated with the d18O record, providing a temporal resolution as high as 600 yr, and allowing for the construction of a detailed age model and calculation of associated sediment accumulation rates within DU XI. Results indicate a productivity-dominated sedimentary record with higher sediment accumulation rates containing a greater proportion of hemipelagic mud occurring during relatively cool periods and reduced accumulation during warmer intervals. This implies that even during periods of substantial warmth, Milankovitch-paced changes in Antarctic ice volume can be linked to ecological changes recorded as shifts in diatom assemblages.

(Table 1) Sapropel analysis from ODP Sites 160-964, 160-966, 160-967 and 160-969

Seventeen eastern Mediterranean Pliocene sapropels from ODP Sites 964, 966, 967 and 969, some of which are coeval, have been analysed for their geochemistry. The sapropels are characterized by very high organic carbon contents (up to 30%) which are reported to be the result of both increased productivity and improved preservation. Although the organic matter in the sapropels is mainly of marine origin, the d13Corg values and C/N ratios appear "terrestrial". This is the result of anaerobic organic matter degradation which preferentially removed nitrogen- and 13C-rich organic components. A comparison with Ti/Al profiles, which mimic the precession index, and a calculation of organic carbon accumulation rates indicate that sedimentation rates were at most 30% lower or at most 50% higher during sapropel formation. Thus, sapropel formation lasted from between 2000 and 10,000 years at Site 964 to between 4500 and 12,000 years at Site 967. A synthesis of new data and a comparison with existing models indicates that productivity, which increased due to extra nutrients supplied as a result of winter mixing and as a result of enhanced input by the Nile, was the driving mechanism behind sapropel formation. The resulting sapropel formation was simultaneous at different depths, but lasted longer in the part of the basin closest to the Nile.

(Table 1) Density values of ODP Leg 126 samples

Sediment dry-bulk density values are essential components of mass accumulation rate calculations. This manuscript presents three equations to calculate dry-bulk density from laboratory measurements of physical properties that have been corrected for the salt content of the pore fluid. In addition, two equations for use with values not corrected for salt content are included. Derivations of the equations from first principles are presented. The second part of the manuscript briefly examines laboratory measurements of the various properties used in the dry-bulk density equations. A discussion of the problems inherent in the density measurements and recommendations are included. This work represents the first comprehensive compilation of equations of dry-bulk density and should prove useful to all scientists who investigate accumulation rates.

Pollen record and age determination of a sediment core from the Kara Sea

AMS-14C dated sediment cores from the Ob and Yenisei estuaries and the adjacent inner Kara Sea were investigated to determine the siliclastic and organic carbon fluxes and their relationship to paleoenvironmental changes. The variability of sediment fluxes during Holocene times is related to the post-glacial sea-level rise and changes in river discharge and coastal erosion input. Whereas during the late/middle Holocene most of the terrigenous sediments were deposited in the estuaries and the areas directly off the estuaries, huge amounts of sediments accumulated on the Kara Sea shelf farther north during the early Holocene before about 9 Cal. kyrs. BP. The maximum accumulation at that time is related to the lowered sea level, increased coastal erosion, and increased river discharge due to the final stage of mountain deglaciation of the Putoran Massif. Increased supply of Yenisei-derived material indicated by peak magnetic susceptibility values probably occurred in climate-related pulses culminating near 11, 10, and 9 Cal. kyrs. BP. As sea level rose, the main Holocene depocenter migrated southward. Based on hydrogen index values and n-alkanes, the organic matter is predominantly of terrigenous origin. Maximum accumulation rates of 1.5 to more than 6 g/cm2/y occurred in the early Holocene sediments, suggesting more humid climatic conditions with an increased vegetation cover in the source area at that time. In general, high organic carbon accumulation rates characterize the estuaries and the inner Kara Sea as important sink for terrigenous organic carbon. A high-resolution record of Holocene variability of magnetic susceptibility (MS) in an AMS14C-dated sediment core from the northern Yenisei estuary may indicate natural variability of Arctic climate change and river discharge on a centennial to millenial time scale. Short-term maxima in MS probably related to warmer climate, enhanced precipitation, intensified weathering/erosion and increased river discharge, display a frequency of about 300 to 700 years.