Foraminifera abundances and biostratigraphy of Cenozoic sediments from the Ontong Java Plateau

The Ontong Java Plateau in the western equatorial Pacific contains a deposition record of biserial planktonic foraminifers concentrated in the Paleogene, in which frequencies up to 67% of the planktonic foraminifers are reported, and in the late Neogene, in which a maximum frequency of 48% is reported. Biserial planktonic foraminifers are rare or absent in the latest Oligocene and early Miocene, an interval characterized by warm bottom water and low temperature gradients. These conditions supported a surface assemblage rather than the biserial planktonic foraminifers, whose Neogene species inhabited the oxygen minimum at intermediate depths in the upper water column. Biserial planktonic foraminifers tend to be of high frequency during high sea stands and low frequency during low sea level, presumably in response to the strengthening or weakening of the oxygen minimum. Species extinction and evolution events occur during low sea stands in the Neogene and sometimes correspond to strong reflection horizons of the plateau's seismic stratigraphy. The biserial species are useful biostratigraphic indexes in the plateau section. The last occurrence (LO) of Streptochilus martini corresponds with the Eocene/Oligocene boundary; S. subglobigerum without Neogloboquadrina acostaensis indicates Zone N15; S. latum occurs from the middle of Zone N16 to near the top of Zone N17; S. globigerum ranges from near the top of Zone N17 to the middle of Zone N19/N20; and the S. globulosum continuous range begins just before the first left-to-right coiling change of Pulleniatina, but the species becomes rare in the Pleistocene section.

Stable isotopic record of late Pliocene and Pleistocene foraminifera of ODP Site 114-704

We studied the stable isotopic and carbonate stratigraphy of ODP Hole 704A to reconstruct the paleoceanographic evolution of the eastern subantarctic sector of the South Atlantic Ocean. Site 704 is well positioned with respect to latitude (46°52.8'S, 7°25.3'E) and bathymetry (2532 m) to monitor past migrations in the position of Polar Front Zone (PFZ) and changes in deep-water circulation during the late Pliocene-Pleistocene. Several important changes occurred in proxy paleoceanographic indicators across the Gauss/Matuyama boundary at 2.47 Ma: (1) accumulation rates of biogenic sedimentary components increased by an order of magnitude (Froelich et al., this volume); (2) planktonic d1 8O values increased by an average of 0.5 per mil; (3) the amplitude of the benthic d18O signal increased; (4) the accumulation rate of ice-rafted detritus increased several fold (Warnke and Allen, this volume); and (5) carbon isotopic ratios of benthic foraminifers decreased by 0.5 per mil, as did the d13C of the fine-fraction carbonate by 1.5 per mil (Mead et al., 1991, doi:10.2973/odp.proc.sr.114.152.1991), but no change occurred in planktonic foraminiferal d13C values. Most of these changes are consistent with more frequent expansions and contractions of the PFZ over Site 704 after 2.47 Ma, bringing cold, nutrient-rich waters to 47°S that stimulated both carbonate and siliceous productivity. The synchronous increase in d18O values and ice-rafted detritus accumulation in Hole 704A indicates that the 2.4 Ma paleoceanographic event included ice volume growth on both Antarctica and Northern Hemisphere continents. The decrease in benthic d13C values indicates that the ventilation rate of Southern Ocean deep water decreased and the nutrient content increased during glacial events after 2.5 Ma. At the Gauss/Matuyama boundary, benthic d13C values of the Southern Ocean shifted toward those of the Pacific end member, indicating a decrease in the relative mixing ratio of Northern Component Water and Circumpolar Deep Water. During the early Matuyama (~2.3 to 1.7 Ma), the PFZ generally occupied a southerly position with respect to Site 704 and carbonate productivity prevailed. Exceptions to these general conditions occurred during strong glacial events of the early Matuyama (e.g., isotopic stages 82, 78, 74, and 70), when the PFZ migrated to the north and opal sedimentation predominated at Site 704. At 1.7 Ma, the PFZ migrated toward the equator and occupied a more northerly position for a prolonged interval between ~1.7 and 1.5 Ma. Beginning at ~1.5-1.4 Ma, surface and bottom water parameters (d18O, d13C, %CaCO3, and %opal) in the subantarctic South Atlantic became highly correlated such that glacial events (d18O maxima) corresponded to d13C and carbonate minima and opal maxima. This pattern is typical of the correlation found during the latest Pleistocene in the Southern Ocean (Charles and Fairbanks, in press). This event coincided with increased suppression of Northern Component Water during glacial events after 1.5 Ma (Raymo et al., 1990, doi:10.1016/0012-821X(90)90051-X), which may have influenced the climatology of the Southern Hemisphere by altering the flux of heat and salt to the Southern Ocean).

Organic carbon, freely extractable lipids and ketone recovery of ODP Leg 199 sediments

Six samples from Sites 1219 and 1221 ranging in age from early Eocene to early Oligocene were analyzed for freely extractable lipids to determine whether the low organic carbon (Corg) sediments of the Eocene equatorial Pacific (Corg content typically 0.03%) are appropriate for biomarker studies. Only one sample from the Oligocene equatorial Pacific (Sample 199-1219A-13H-3, 50-54 cm) contained any biomarkers of interest to paleoceanography. The only lipids identified in the remaining samples appear to be contaminants from drilling or subsequent handling. Sample 199-1219A-13H-3, 50-54 cm, contained alkenone biomarkers specific to haptophyte algae that are used for estimating past mean annual sea-surface temperature (maSST). If the Holocene calibration of maSST is appropriate for the Oligocene, the estimated equatorial temperature is >=28.3°C, or at least 3°C warmer than modern equatorial maSST at a similar longitude.

Composition of Eocene claystones of ODP Hole 105-647A

Authigenic carbonates were recovered in lower to middle Eocene claystones at Ocean Drilling Program Site 647 in the Labrador Sea. Detailed chemical, petrographic, and X-ray investigations reveal that these diagenetic carbonates have a complex mineralogical composition. At least five different carbonate phases are identified: calcium-rich rhodochrosite, rhodochrosite, manganosiderite, siderite, and calcite. Manganese carbonates are the dominant carbonate phases formed throughout the section. Textural analyses show two major generations of carbonate formation. Early cementation of micritic carbonate in burrow structures was followed by carbonate cementation forming microsparry to sparry crystals. At approximately 620 meters below seafloor (mbsf), three concretions of iron carbonates occur, which indicates a special pore-water chemistry. Thin section analyses from this level show (1) several generations of diagenetic carbonates, (2) widespread secondary cavity formation in burrow structures, and (3) various cement precipitations in voids. We suggest that this level represents a hiatus or highly condensed sequence, as indicated by (1) the low carbonate content in host sediments, (2) carbonate dissolution reflected by the high ratio of benthic to planktonic foraminifers, and (3) complex diagenetic alteration in the carbonate concretions. Iron and manganese enrichments observed in lithologic Unit IV may have been derived from a hydrothermal source at the adjacent, then active, Labrador Sea mid-ocean ridge. Authigenic smectites forming numerous pseudomorphs of siliceous microfossils are precipitated in burrow structures. We propose that diagenetic smectite formation from biogenic opal and iron oxyhydroxide (analogous to smectite formation in surface sediments of the East Pacific area) occurred in the Labrador Sea during the early and middle Eocene.

Element ratios and stable isotopes of foraminifers from ODP Site 202-1237

Downcore records of magnesium/calcium, strontium/calcium, manganese/calcium, and oxygen and carbon isotopes of planktonic and benthic foraminifers from Ocean Drilling Program (ODP) Site 1237 on the Peru-Chile margin provide critical information regarding the history of climate in the region over the past 6 m.y. Specifically, these records can be used to infer the sea-surface temperature (SST) and sea-surface salinity (SSS) history of a region that today is associated with substantial wind stress curl-driven upwelling (Shipboard Scientific Party, 2003, doi:10.2973/odp.proc.ir.202.108.2003). This report provides data tables and other supporting information for measurements made on planktonic and benthic foraminifers from Site 1237. Items included in this report are (1) oxygen and carbon isotopic measurements of planktonic and benthic foraminifers and (2) Mg/Ca, Sr/Ca, and Mn/Ca ratio measurements of planktonic foraminifers from Holes 1237B, 1237C, and 1237D.

Rock magnetic properties of ODP Hole 109-670A

The magnetic properties of 11 samples from Site 670 of Leg 109, 3 harzburgites and 8 highly serpentinized peridotites, have been studied. Reflected light microscopy and Curie temperatures confirm that magnetite is the dominant magnetic mineral in all samples. However, both rock types show different magnetic behavior. Susceptibility, saturation magnetization, and NRM are higher for the serpentinites, because of the higher magnetite content. The hysteresis parameters indicate magnetite particles with pseudosingle domain structure for both rock types. For the remarkable anisotropy of the magnetic susceptibility no definite explanation could be found, because of the complex texture of the samples. In both rock types the presence of maghemite, a product of low temperature oxidation of magnetite, has been indicated by reflected light microscopy and by thermomagnetic analysis. As the maghemite converts to hematite at temperatures above 350°C, the temperature during the serpentinization was below this value assuming that the maghemitization took place at the same time.

Geochemistry and mineralogy of ultramafic rocks from conical seamount

The Mariana arc-trench system, the easternmost of a series of backarc basins and intervening remnant arcs that form the eastern edge of the Philippine Sea Plate, is a well-known example of an intraoceanic convergence zone. Its evolution has been studied by numerous investigators over nearly two decades (e.g., Kang, 1971; Uyeda and Kanamori, 1979; LaTraille and Hussong, 1980; Fryer and Hussong, 1981; Mrosowski et al., 1982; Hussong and Uyeda, 1981; Bloomer and Hawkins, 1983; Karig and Ranken, 1983; McCabe and Uyeda, 1983; Hsui and Youngquist, 1985; Fryer and Fryer, 1987; Johnson and Fryer, 1988; Johnson and Fryer, 1989; Johnson et al., 1991). The Mariana forearc has undergone extensive vertical uplift and subsidence in response to seamount collision, to tensional and rotational fracturing associated with adjustments to plate subduction, and to changes in the configuration of the arc (Hussong and Uyeda, 1981; Fryer et al., 1985). Serpentine seamounts, up to 2500 m high and 30 km in diameter, occur in a broad zone along the outer-arc high (Fryer et al., 1985; Fryer and Fryer, 1987). These seamounts may be horsts of serpentinized ultramafic rocks or may have been formed by the extrusion of serpentine muds. Conical Seamount, one of these serpentine seamounts, is located within this broad zone of forearc seamounts, about 80 km from the trench axis, at about 19°30'N. The seamount is approximately 20 km in diameter and rises 1500 m above the surrounding seafloor. Alvin submersible, R/V Sonne bottom photography, seismic reflection, and SeaMARC II studies indicate that the surface of this seamount is composed of unconsolidated serpentine muds that contain clasts of serpentinized ultramafic and metamorphosed mafic rocks, and authigenic carbonate and silicate minerals (Saboda et al., 1987; Haggerty, 1987; Fryer et al., 1990; Saboda, 1991). During Leg 125, three sites were drilled (two flank sites and one summit site) on Conical Seamount to investigate the origin and evolution of the seamount. Site 778 (19°29.93'N, 146°39.94'E) is located in the midflank region of the southern quadrant of Conical Seamount at a depth of 3913.7 meters below sea level (mbsl) (Fig. 2). This site is located in the center of a major region of serpentine flows (Fryer et al., 1985, 1990). Site 779 (19°30.75'N, 146°41.75'E), about 3.5 km northeast of Site 778, is located approximately in the midflank region of the southeast quadrant of Conical Seamount, at a depth of 3947.2 mbsl. This area is mantled by a pelagic sediment cover, overlying exposures of unconsolidated serpentine muds that contain serpentinized clasts of mafic and ultramafic rocks (Fryer et al., 1985, 1990). Site 780 (19°32.5'N, 146°39.2'E) is located on the western side of Conical Seamount near the summit, at a depth of 3083.4 mbsl. This area is only partly sediment covered and lies near active venting fields where chimney structures are forming (Fryer et al., 1990).

Lithology, stratigraphy, organic carbon content, extract and liquid chromatography yields, and Rock-Eval pyrolysis at DSDP Holes 75-530A and 75-532

Seventeen sediment samples of Albian-Cenomanian to early Pliocene age from DSDP Hole 530A in the Angola Basin and six sediment samples of early Pliocene to late Pleistocene age from the Walvis Ridge were investigated by organic geochemical methods, including organic carbon determination, Rock-Eval pyrolysis, gas chromatography and combined gas chromatography/mass spectrometry of extractable hydrocarbons, and kerogen microscopy. The organic matter in all samples is strongly influenced by a terrigenous component from the nearby continent. The amount of marine organic matter present usually increases with the total organic carbon content, which reaches an extreme value of more than 10% in a Cenomanian black shale from Hole 530A. At Site 530 the extent of preservation of organic matter in the deep sea sediments is related to mass transport down the continental slope, whereas the high organic carbon contents in the sediments from Site 532 reflect both high bioproductivity in the Benguela upwelling regime and considerable supply of terrigenous organic matter. The maturation level of the organic matter is low in all samples.

Geochemistry of serpentine muds of ODP Leg 125 holes

The <63-µm fractions of serpentinite muds from two seamounts on the Mariana and Izu-Bonin forearcs were analyzed for mineral composition by X-ray diffraction and for chemical composition by X-ray fluorescence. The silt fraction of the muds consists predominantly of chrysotile, brucite, and ample amorphous constituents. Chlorite and smectite are less abundant components. Of special interest is the occurrence of iowaite, a brucite-like, Cl-bearing mineral with a layered structure. Iowaite was not found in the samples from the summit site of one of the seamounts drilled; however, it is scattered throughout the strata, composing the flanks of both seamounts investigated. No systematic change of the iowaite abundance with depth was observed. The distribution of iowaite is confined to the surface of the flanks of the seamount. Based on the distribution on the mineral and its chemical composition, we suggest that the iowaite formed by oxidation of some of the ferrous iron in brucite contained in the serpentine mud as it contacted abyssal seawater during protrusion onto the seafloor. The resulting positive charge imparted to the brucite was compensated by the uptake of seawater chloride. Consequently, the formation of iowaite is restricted to the seafloor where oxygen and chloride are available for these reactions. The availability of oxygen is considered the limiting factor. We conclude that iowaite formation cannot be a major cause for the low chlorinity of pore fluids inside the seamounts.

Age determination and stable isotope record of ODP Hole 162-980B

The conversion of surface water to deep water in the North Atlantic results in the release of heat from the ocean to the atmosphere, which may have amplified millennial-scale climate variability during glacial times (Broecker et al., 1990, doi:10.1029/PA005i004p00469) and could even have contributed to the past 11,700 years of relatively mild climate (known as the Holocene epoch) (Bond et al., 2001, doi:10.1126/science.1065680; Alley et al., 1997, doi:10.1130/0091-7613(1997)025<0483:HCIAPW>2.3.CO;2; Keigwin and Boyle, 2000, doi:10.1073/pnas.97.4.1343). Here we investigate changes in the carbon-isotope composition of benthic foraminifera throughout the Holocene and find that deep-water production varied on a centennial-millennial timescale. These variations may be linked to surface and atmospheric events that hint at a contribution to climate change over this period.

Calculated surface water paleotemperatures for the lower Pliocene at DSDP Site 18-173 from the California continental slope (Table I)

The oxygen isotope record of the planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma from Pliocene and early Pleistocene sediments at both DSDP site 173 and the Centerville Beach section in California suggests a large influx of isotopically light water in this area during late Pliocene and early Pleistocene time. Salinity may have been reduced by as much as 2 to 4 ?. Surface sea water paleotemperatures for the lower Pliocene range from 9.5°C to 15.5°C. The oxygen isotope record of the benthonic genus Uvigerina shows little variation indicating environmental stability at depth. At DSDP site 173 the small variation in Uvigerina is due to variation in the oxygen isotopic composition of the oceans as glaciers waxed and waned. At the Centerville Beach section the oxygen isotopic composition of Uvigerina reflects the gradual shoaling of the Humboldt Basin. Carbon and oxygen isotope ratios in G. bulloides and N. pachyderma are inversely correlated at the 95% confidence level. This may indicate that the oxygen and carbon isotopic composition of foraminifera are influenced by the same factors. On the other hand, the inverse correlation may be due to metabolic fractionation. No correlation was found between oxygen and carbon isotopic composition in Uvigerina.

Geochemical and palynological measurements from the ODP Leg 189 at Site 1172 on the East Tasman Plateau

A brief (~150 kyr) period of widespread global average surface warming marks the transition between the Paleocene and Eocene epochs, ~56 million years ago. This so-called "Paleocene-Eocene thermal maximum" (PETM) is associated with the massive injection of 13C-depleted carbon, reflected in a negative carbon isotope excursion (CIE). Biotic responses include a global abundance peak (acme) of the subtropical dinoflagellate Apectodinium. Here we identify the PETM in a marine sedimentary sequence deposited on the East Tasman Plateau at Ocean Drilling Program (ODP) Site 1172 and show, based on the organic paleothermometer TEX86, that southwest Pacific sea surface temperatures increased from ~26 °C to ~33°C during the PETM. Such temperatures before, during and after the PETM are >10 °C warmer than predicted by paleoclimate model simulations for this latitude. In part, this discrepancy may be explained by potential seasonal biases in the TEX86 proxy in polar oceans. Additionally, the data suggest that not only Arctic, but also Antarctic temperatures may be underestimated in simulations of ancient greenhouse climates by current generation fully coupled climate models. An early influx of abundant Apectodinium confirms that environmental change preceded the CIE on a global scale. Organic dinoflagellate cyst assemblages suggest a local decrease in the amount of river run off reaching the core site during the PETM, possibly in concert with eustatic rise. Moreover, the assemblages suggest changes in seasonality of the regional hydrological system and storm activity. Finally, significant variation in dinoflagellate cyst assemblages during the PETM indicates that southwest Pacific climates varied significantly over time scales of 103 - 104 years during this event, a finding comparable to similar studies of PETM successions from the New Jersey Shelf.

Rock magnetic of serpentinite seamounts in the Mariana and Izu-Bonin regions

During Leg 125, scientists drilled two serpentinite seamounts: Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc. Grain densities of the serpentinized peridotites range from 2.44 to 3.02 g/cm**3. The NRM intensity of the serpentinized peridotites ranges from 0.01 to 0.59 A/m and that of serpentine sediments ranges from 0.01 to 0.43 A/m. Volume susceptibilities of serpentinized peridotites range from 0.05 * 10**-3 SI to 9.78 * 10**-3 SI and from 0.12 * 10**-3 to 4.34 * 10**-3 SI in the sediments. Koenigsberger ratios, a measure of the relative contributions of remanent vs. induced magnetization to the magnetic anomaly, vary from 0.09 to 80.93 in the serpentinites and from 0.06 to 4.74 in the sediments. The AF demagnetization behavior of the serpentinized peridotites shows that a single component of remanence (probably a chemical remanence carried by secondary magnetite) can be isolated in many samples that have a median destructive field less than 9.5 mT. Multiple remanence components are observed in other samples. Serpentine sediments exhibit similar behavior. Comparison of the AF demagnetization of saturation isothermal remanence and NRM suggests that the serpentinized peridotites contain both single-domain and multidomain magnetite particles. The variability of the magnetic properties of serpentinized peridotites reflects the complexity of magnetization acquired during serpentinization. Serpentinized peridotites may contribute to magnetic anomalies in forearc regions.

Composition of organic carbon at ODP Site 128-798

Organic geochemical and sedimentological investigations have been performed on sediments from ODP Sites 798 and 799 in order to reconstruct the depositional environment in the Japan Sea through late Cenozoic times. The Miocene to Quaternary sediments from Site 798 (Oki Ridge) and Site 799 (Kita-Yamato Trough) are characterized by high organic carbon contents of up to 6%. The organic matter is mainly a mixture of marine and terrigenous material. The dominant factors controlling marine organic carbon enrichment in the sediments of Hole 798A are probably an increased surface-water productivity and/or an increased preservation rate of organic carbon under anoxic deep-water conditions. In lower Pliocene sediments at Site 798 and Miocene to Quaternary sediments at Site 799, rapid burial of organic matter in turbidites may have been important, too. Remarkable cycles of dark, laminated sediments distinctly enriched in (marine) organic carbon by up to 5% and light, bioturbated to homogeneous sediments with reduced organic carbon contents indicate dramatic short-term paleoenvironmental variation.