The Fohsella chronocline in middle Miocene sediments from the Ontong Java Plateau, Pacific Ocean

Micropaleontologists have traditionally recognized the mid-Miocene Fohsella lineage as a flagship for phyletic gradualism within the planktic foraminifera. However, study of a deep-sea record from the western equatorial Pacific (ODP Site 806) reveals that coiling ratios within this clade suddenly (<5 kyr) shift after a prolonged, ancestral state of near randomness (~50%) to a transient phase (13.42-13.43 Ma) of dextral dominance (~75%) immediately following the first common occurrence of keeled fohsellids. This brief period of dextral dominance was abruptly (<5 kyr) succeeded by an irreversible change to sinistral dominance (~96%). Fohsellid abundances decline markedly through the interval in which the sinistral preference is established. The shift to sinistrality (13.42 Ma) predated the deepening of fohsellid depth ecology by ~240-488 kyr, indicating that these two events were unrelated. This view is supported by a lack of delta 18O evidence for depth-habitat differences between the two chiral forms, which refutes the notion that sinistral fohsellids were "pre-adapted" for ensuing hydrographic change because they occupied a deeper depth habitat than their dextral counterparts. Planktic foraminiferal assemblages become strongly oligotrophic in character through the interval in which the fohsellid delta 18O increase is recorded, indicating that the migration to deeper depths was fostered by an expansion of the mixed layer in the western equatorial Pacific. Salient aspects of this brief, but conspicuous faunal change are a marked increase in the abundance of symbiont-bearing globigerinoidids, a concomitant collapse of local Jenkinsella mayeri/siakensis populations, and reduced fohsellid abundances. The rapid and permanent nature of the Fohsella sinistral shift provides a distinct, unequivocal datum that may prove useful for correlating mid-Miocene sections throughout the Caribbean Sea and tropical regions in the western sectors of the Pacific and Atlantic. The coiling ratio changes that occurred during the evolution of the Fohsella chronocline probably reflect changing population dynamics between cryptic genotypes with different coiling preferences.

Corg, CaCO3, bulk densities, accumulation rates and ages of ODP Leg 167 sites and three piston cores, California margin

Sediments from five Leg 167 drill sites and three piston cores were analyzed for Corg and CaCO3. Oxygen isotope stratigraphy on benthic foraminifers was used to assign age models to these sedimentary records. We find that the northern and central California margin is characterized by k.y.-scale events that can be found in both the CaCO3 and Corg time series. We show that the CaCO3 events are caused by changes in CaCO3 production by plankton, not by dissolution. We also show that these CaCO3 events occur in marine isotope Stages (MIS) 2, 3, and 4 during Dansgaard/Oeschger interstadials. They occur most strongly, however, on the MIS 5/4 glaciation and MIS 2/1 deglaciation. We believe that the link between the northeastern Pacific Ocean and North Atlantic is primarily transmitted by the atmosphere, not the ocean. Highest CaCO3 production and burial occurs when the surface ocean is somewhat cooler than the modern ocean, and the surface mixed layer is somewhat more stable.

Composition of phyllosilicates from hydrothermally altered basalts of DSDP Hole 83-504B, Costa Rica Rift, Pacific Ocean

Phyllosilicates occurring as replacements of olivine, clinopyroxene and interstitial materials and as veins or fracture-fillings in hydrothermally altered basalts from DSDP Hole 504B, Leg 83 have been studied using transmission and analytical electron microscopy. The parageneses of phyllosilicates generally change systematically with depth and with the degree of alteration, which in turn is related to permeability of basalts. Saponite and some mixed-layer chlorite/smectite are the dominant phyllosilicates at the top of the transition zone. Chlorite, corrensite, and mixed-layer chlorite/corrensite occur mainly in the lower transition zone and upper levels of the sheeted dike zone. Chlorite, talc, and mixed-layer talc/chlorite are the major phyllosilicates in the sheeted dike zone, although replacement of talc or olivine by saponite is observed. The phyllosilicates consist of parallel or subparallel discrete packets of coherent layers with packet thicknesses generally ranging from < 100 A to a few hundred A. The packets of saponite layers are much smaller or less well defined than those of chlorite, corrensite and talc, indicating poorer crystallinity of saponite. By contrast, chlorite and talc from the lower transition zone and the sheeted dike zone occur in packets up to thousands of A thick. The Si/(Si + A1) ratio of these trioctahedral phyllosilicates increases and Fe/(Fe + Mg) decreases in the order chlorite, corrensite, saponite, and talc. These relations reflect optimal solid solution consistent with minimum misfit of articulated octahedral and tetrahedral sheets. Variations in composition of hydrothermal fluids and precursor minerals, especially in Si/(Si+A1) and Fe/(Fe+Mg) ratios, are thus important factors in controlling the parageneses of phyllosilicates. The phyllosilicates are generally well crystallized discrete phases, rather than mixed-layered phases, where they have been affected by relatively high fluid/rock ratios as in high-permeability basalts, in veins, or areas adjacent to veins. Intense alteration in basalts with high permeability (indicating high fluid/rock ratios) is characterized by pervasive albitization and zeolitization. Minimal alteration in the basalts without significant albitization and zeolitization is characterized by the occurrence of saponite ± mixed-layer chlorite/smectite in the low-temperature alteration zone, and mixed-layer chlorite/corrensite or mixed-layer talc/chlorite in the high-temperature alteration zone. Textural non-equilibrium for phyllosilicates is represented by mixed layering and poorly defined packets of partially incoherent layers. The approach to textural equilibrium was controlled largely by the availability of fluid or permeability.

Geochemistry and mineralogy of Eocene-Oligocene sediments of IODP Hole 302-M0002A

In 2004, Integrated Ocean Drilling Program Expedition 302 (Arctic Coring Expedition, ACEX) to the Lomonosov Ridge drilled the first Central Arctic Ocean sediment record reaching the uppermost Cretaceous (~430 m composite depth). While the Neogene part of the record is characterized by grayish-yellowish siliciclastic material, the Paleogene part is dominated by biosiliceous black shale-type sediments. The lithological transition between Paleogene and Neogene deposits was initially interpreted as a single sedimentological unconformity (hiatus) of ~26 Ma duration, separating Eocene from Miocene strata. More recently, however, continuous sedimentation on Lomonosov Ridge throughout the Cenozoic was proclaimed, questioning the existence of a hiatus. In this context, we studied the elemental and mineralogical sediment composition around the Paleogene-Neogene transition at high resolution to reconstruct variations in the depositional regime (e.g. wave/current activity, detrital provenance, and bottom water redox conditions). Already below the hiatus, mineralogical and geochemical proxies imply drastic changes in sediment provenance and/or weathering intensity in the hinterland, and point to the existence of another, earlier gap in the sediment record. The sediments directly overlying the hiatus (the Zebra interval) are characterized by pronounced and abrupt compositional changes that suggest repeated erosion and re-deposition of material. Regarding redox conditions, euxinic bottom waters prevailed at the Eocene Lomonosov Ridge, and became even more severe directly before the hiatus. With detrital sedimentation rates decreasing, authigenic trace metals were highly enriched in the sediment. This continuous authigenic trace metal enrichment under persistent euxinia implies that the Arctic trace metal pool was renewed continuously by water mass exchange with the world ocean, so the Eocene Arctic Ocean was not fully restricted. Above the hiatus, extreme positive Ce anomalies are clear signs of a periodically well-oxygenated water column, but redox conditions were highly variable during deposition of the Zebra interval. Significant Mn enrichments only occur above the Zebra interval, documenting the Miocene establishment of stable oxic conditions in the Arctic Ocean. In summary, extreme and abrupt changes in geochemistry and mineralogy across the studied sediment section do not suggest continuous sedimentation at the Lomonosov Ridge around the Eocene-Miocene transition, but imply repeated periods of very low sedimentation rates and/or erosion.

Petrographic description and geochemistry of basalts and altered volcanic rocks from ODP Leg 183 sites

The basalts recovered during Legs 183 and 120 from the southern, central, and northernmost parts of the Kerguelen Plateau (Holes 1136A, 1138A, 1140A, and 747C, respectively), as well as those recovered from the eastern part of the crest of Elan Bank (Hole 1137A), represent derivates from tholeiitic melts. In the northern part of the Kerguelen Plateau (Hole 1140A), basalts may have formed from two sources located at different depths. This is reflected in the presence of both low- and high-titanium basalts. The basalts are variably altered by low-temperature hydrothermal processes (at temperatures up to 120°C), and some are affected by subaerial weathering. The hydrothermal alteration led mainly to the formation of smectites, chlorite minerals, mixed-layer hydromica-smectite and smectite-chlorite minerals, hydromica, serpentine(?), clinoptilolite, heulandite, stilbite, analcime, mordenite, thomsonite, natrolite(?), calcite, quartz, and dickite(?). Alteration of extrusive basalts is mainly related to horizontal fluid flow within permeable contact zones between lava flows. Under a nonoxidizing environment of alteration, the tendency to lose most of elements, including rare earth elements, from basalts dominates. Under on oxidizing environment, basalts accumulate many elements.

Sedimentary mineralogy of argillaceous sediments at DSDP Legs 56-57 Holes

Mineral assemblages of DSDP Holes 436 and 438A and the upper section of Hole 439 (871.5-911.0 m sub-bottom) resemble each other and are composed of montmorillonite (probably a small portion of montmorillonite/illite mixed-layer clays), illite, chlorite, kaolinite, quartz, plagioclase, hornblende, calcite, dolomite, siderite, gypsum, pyrite, and halite. In the middle section of Hole 439 (933.5-1041.0 m), clinoptilolite is also found. In the lower section of Hole 439 (1077.5-1150.0 m), montmorillonite is not confirmed, and clinoptilolite and mixed-layer illite are found. These assemblages, which also contain detrital kaolinite, are generally found in sediments from brackish-water environments. At Site 439, more than 1000 meters of sediment might have been removed by erosion at the base.

Radiochemical analyses and contents of organic carbon and CaCO3 in equatorial Pacific Ocean sediments

Uranium series nuclide concentrations have been measured on sediments from five box cores from an equatorial Pacific transect. 230Thexcess activities show discontinuities at the Holocene-glacial boundary as dated by 14C. The glacial sedimentation rates determined by 230Th and 14C are 2.5-3.0 cm/kyr. The Holocene rates from 230Th are much lower than those dated by 14C (1.9-2.3 cm/kyr) because of carbonate dissolution. 230Th sedimentation fluxes exceed water column supply by factors of 1.2-1.8 in the Holocene and 1.8-3.0 in the glacial sections. A number of models have been applied to calculate carbonate dissolution rates. The results show that carbonate dissolution rates in the Holocene (in g/cm**2 kyr) equal 1.5 * 10**-3 exp (1.4D) where D is water depth in kilometers. A point-by- point estimation of sediment fluxes through time show that clay accumulation rates in the area have been near constant at 0.1-0.2 g/cm**2 kyr over the past 20 kyr whereas carbonate accumulation rates have decreased dramatically from 0.6-1.0 g/cm**2 kyr in the glacial sections of the cores to 0.2-0.6 g/cm**2 kyr in the Holocene. The errors caused by the uncertainties in the age of the termination of the last glacial period have been investigated and results show that a range of 11-14 kyr leads to an error upper limit of about 30% in the estimation of CaCO3 dissolution rates. The response time of CaCO3 and 230Thex concentrations in the mixed layer of sediments due to an impulse of change in CaCO3 dissolution rate has also been discussed, showing that the observed changes in carbonate dissolution may be explained in terms of a single or a continuous change, depending upon the thickness of the mixed layer.

Carbon and Lead chemistry of sediments from the Middle Atlantic Bight

A mass budget was constructed for organic carbon on the upper slope of the Middle Atlantic Bight, a region thought to serve as a depocenter for fine-grained material exported from the adjacent shelf. Various components of the budget are internally consistent, and observed differences can be attributed to natural spatial variability or to the different time scales over which measurements were made. The flux of organic carbon to the sediments in the core of the depocenter zone, at a water depth of 1000 m, was measured with sediment traps to be 65 mg C m**-2 day**-1, of which 6-24 mg C m**-2 day**-1 is buried. Oxygen fluxes into the sediments, measured with incubation chambers attached to a free vehicle lander, correspond to total carbon remineralization rates of 49-70 mg C m**-2 day**-1. Carbon remineralization rates estimated from gradients of Corg within the mixed layer, and from gradients of dissolved ammonia and phosphate in pore waters, sum to only 4-6 mg C m**-2 day**-1. Most of the Corg remineralization in slope sediments is mediated by bacteria and takes place within a few mm of the sediment-water interface. Most of the Corg deposited on the upper slope sediments is supplied by lateral transport from other regions, but even if all of this material were derived from the adjacent shelf, it represents <2% of the mean annual shelf productivity. This value is further lowered by recognizing that as much as half of the Corg deposited on the slope is refractory, having originated by reworking from older deposits. Refractory Corg arrives at the sea bed with an average 14C age 600-900 years older than the pre-bomb 14C age of DIC in seawater, and has a mean life in the sediments with respect to biological remineralization of at least 1000 years. Labile carbon supplied to the slope, on the other hand, is rapidly and (virtually) completely remineralized, with a mean life of < 1 year. Carbon-14 ages of fine-grained carbonate and organic carbon present within the interstices of shelf sands are consistent with this material acting as a source for the old carbon supplied to the slope. Winnowing and export of reworked carbon may contribute to the often-described relationship between organic carbon preservation and accumulation rate of marine sediments.