Radiolarian biostratigraphy and chronology of radiolarian events of ODP Leg 199 sites and EW9709 sediment cores

A generally rich radiolarian fauna ranging in age from Quaternary to early Eocene (Zone RP7) was found at five of the eight sites drilled during Ocean Drilling Program (ODP) Leg 199. Of particular interest are the stratigraphically complete assemblages that range in age from middle Miocene (Zone RN5) to early Eocene (Zone RP7), composites of Sites 1218, 1219, and 1220. At the same sites, multisensor track (MST) data show consistent cycles in gamma ray attenuation density, color, and carbonate content that can be correlated on a submeter scale from the early Miocene to early Eocene. In addition, the magnetic reversal records from these three sites allow construction of an absolute timescale. A series of 305 radiolarian morphologic first and last occurrences and evolutionary transitions for radiolarians were determined and correlated directly with the accompanying MST and paleomagnetic data, resulting in a detailed and accurate dating of events. Since many of the bioevents are found at more than one site, it was also possible to test their reliability within the study area. Twelve new species are described: Calocycletta (Calocycletta) anekathen, Dorcadospyris anastasis, Dorcadospyris copelata, Dorcadospyris cyclacantha, Dorcadospyris ombros, Dorcadospyris scambos, Eucyrtidium mitodes, Theocyrtis careotuberosa, Theocyrtis perpumila, Theocyrtis perysinos, Theocyrtis setanios, and Thyrsocyrtis (Pentalacorys) orthotenes.

Age models and foraminifer isotopes of four ODP Leg 202 sites

We present benthic isotope stratigraphies for Sites 1236, 1237, 1239, and 1241 that span the late Miocene-Pliocene time interval from 6 to 2.4 Ma. Orbitally tuned timescales were generated for Sites 1237 and 1241 by correlating the high-frequency variations in gamma ray attenuation density, percent sand of the carbonate fraction, and benthic d13C to variations in Earth's orbital parameters. The astronomical timescales for Sites 1237 and 1241 are in agreement with the one from Atlantic Site 925/926 (Ocean Drilling Program Leg 154). The comparison of benthic d18O and d13C records from the east Pacific sites and Atlantic Site 925/926 revealed a surprising clarity of the "41-k.y. signal" in d13C records and a remarkably good correlation between their d13C records. This suggests that the late Miocene-Pliocene amplitudes of obliquity-related d13C cycles reflect a magnitude of global response often larger than that provided by obliquity-related d18O cycles. At Site 1237, the orbitally derived ages of Pliocene magnetic reversal boundaries between the base of Réunion and the top of Thvera confirm astronomical datings of the generally accepted ATNTS2004 timescale, except for the top of Kaena and the base of Sidufjall. Our astronomical age for the top of Kaena is about one obliquity cycle older. The base of Sidufjall appears to be about one precession cycle younger. The age models of Sites 1236 and 1239 were established by correlating their benthic d18O and d13C records directly to the orbitally tuned isotope record of Site 1241.

40Ar-39Ar geochronological studies on rocks at DSDP Leg 58 Holes

The technique of 40Ar-39Ar step-heating dating was applied to three rock samples from core of DSDP Site 443, one sample from Site 445, and four samples at Site 446. All sites were drilled during DSDP Leg 58. At Site 443 (Shikoku Basin), about 116 meters of basalt basement was drilled. Three samples were chosen for dating from different levels in the basalt; two samples are aphyric basalt, and the other is subophitic dolerite. At Site 445 (Daito Ridge), no basement rock was drilled; however, conglomeratic sandstone was cored in the lower part of the hole. 40Ar-39Ar dating was applied to a basalt pebble in the conglomerate. At Site 446 (Daito Basin), the lower cored sequence is clay stone interlayered with 16 basalt sills. Four samples were chosen from sills at different levels.

Consolidation and strength assessment of ODP Leg 123 sediments

Twenty-one samples, ranging in depth from 0 to 150 meters below seafloor (mbsf), were obtained from Leg 123 Sites 765 and 766. All samples were tested for Atterberg limits: 14 for laboratory vane shear strength and seven for uniaxial consolidation. Based on the determined Atterberg limits, along with shipboard measurements of water content, the sediment appears to be underconsolidated from 0 to 40 mbsf at Site 765 and from 0 to 80 mbsf at Site 766. Normal consolidation trends were observed for the sediments below these depths. Vane shear strengths, when compared with calculated values for a normally consolidated clay, indicate underconsolidated sediment at both sites. However, the use of Atterberg limit and vane shear strength data to assess consolidation state is complicated by the presence of silt-sized calcium carbonate in the form of nannofossil ooze. Thus, uniaxial-consolidation test data were analyzed to determine the overconsolidation ratios (OCR) and sediment compressibilities. OCR values were found to be less than one (underconsolidated) at both sites, using two separate methods of analysis.

Chemistry of interstitial waters of samples from DSDP Leg 25

Interstitial water analyses from sediments collected during Leg 25 of the Deep Sea Drilling Project have revealed that in the southwest Indian Ocean, great chemical activity exists in sediments in various depositional environments. Variable sedimentation rates allow us to set some interesting boundary conditions on chemical and transport processes in these interstitial waters, particularly with regard to the distribution of dissolved sulfate. In terrigenous rapidly deposited sediments, large depletions are observed in magnesium and potassium, whereas relatively small decreases in dissolved calcium occur. In slowly deposited detrital sediments, also, large decreases in potassium and magnesium coincide with very large calcium increases. In truly pelagic sediments, a one to one replacement of magnesium by calcium is observed in the interstitial waters, presumably due to reactions in the basal sediment layers. Biogenous deposits have great influence on dissolved silica (sponge spicules and radiolarians) and on dissolved strontium (carbonate recrystallization). Otherwise, dissolved silica reflects the clay mineralogy and shows variations which seem particularly dependent on the presence or absence of kaolinite. Variable dissolved manganese values reflect reducing conditions and/or availability of manganese in the solid phases for mobilization in reducing sediments.

Late Neogene to Pleistocene palynology of ODP Leg 112 samples

The main objectives of this study are (1) to characterize the spatial and temporal variations in organic matter deposited in upwelling and related sediments (manifest in the palynoclast and organic-walled microplankton assemblages) and (2) to relate these variations to paleoenvironmental changes. A total of 40 samples from Holes 679D, 680B, 681B, 684B, 686B, and 687B were analyzed. Without exception, amorphogen dominates the palynoclast assemblages overwhelmingly. Influx of terrestrial particulate organic matter to the marine realm was extremely low. Levels of amorphogen swamp other palynoclast categories, and little significance can be attached to any variations observed. Microplankton dominate the palynomorph assemblages, with variable levels of subordinate foraminiferal test linings. Miospores are rare and are absent in most samples. Foraminiferal test linings are particularly abundant in the shallowest samples, which may reflect low surface-water paleotemperatures. Cysts of heterotrophic peridiniacean dinoflagellates (P-cysts) dominate the microplankton assemblages, with variable levels of cysts of autotrophic gonyaulacacean dinoflagellates (G-cysts). Samples dominated by P-cysts are derived largely from laminated, unbioturbated units deposited under the influence of strong upwelling. A lower abundance of P-cysts in some samples is restricted to unlaminated, bioturbated units deposited under oxygenated conditions. We conclude that the ratio of P-cysts to G-cysts is a useful indicator of variable upwelling strength. Detailed study of the variations in the microplankton assemblages offers one the greatest potential for palynological characteriztion and understanding of the upwelling system.

Calcareous nannofossils from ODP Leg 105 holes

Neogene calcareous nannofossils were examined from 10 holes at three sites cored during ODP Leg 105. Sediment recovered in Baffin Bay at Site 645 is virtually barren of calcareous nannofossils, with the exception of a sparse lower Miocene assemblage. Sites 646 and 647 in the Labrador Sea contain upper Miocene to Holocene sediments having numerous barren intervals. Upper Pleistocene fossil coccolithophorid floras in the Labrador Sea indicate alternations of cold subpolar with transitional (subpolar/subtropical) assemblages. Extreme variations in the abundance of Coccolithus pelagicus were observed at Sites 646 and 647. These variations are correlated with stable isotopic data to interpret oceanographic responses to warming and cooling trends. The climatic history indicated by the changes of these assemblages closely approximates the past climatic fluctuations recorded in other North Atlantic cores. One new taxon, Discoaster bergenii, is described.

Mineralogy and inorganic geochemistry of sediments at DSDP Leg 65 Holes

During Leg 65, 15 holes were drilled at four sites located on young crust in the mouth of the Gulf of California. Quaternary to upper Pliocene hemipelagic sediments above and interlayered within the young basaltic basement were cored. The influence of hot lava, high temperature gradients, and hydrothermal activity on the mineralogy and geochemistry of the terrigenous sediments near contacts with basalts might therefore be expected. The purpose of the present study was to determine the mineralogy and inorganic geochemistry of these sediments and to analyze the nature and extent of low temperature alteration. To this end we studied the mineralogy and inorganic geochemistry of 75 sediment samples, including those immediately overlying uppermost basalts and those from layers alternating with basalts within the basement. We separated three size fractions - <2 µm (clay), 2-20 µm (intermediate), and >20 µm (coarse) - and applied the following mineralogical determinations: x-ray diffraction (XRD), infrared spectroscopy, transmission and scanning electron microscopy, and optical microscopy (for coarse fractions, using thin sections and smear slides). We calculated the percentages of clay minerals using Biscaye's (1964) method, and used routine wet chemical analyses to determine bulk composition and quantitative spectral analyses for trace elements.

Silicoflagellate abundances in sediments of ODP Leg 207 sites

The Ocean Drilling Program (ODP) drilled at five sites in the western Atlantic Ocean during Leg 207. The objective of the drilling was to recover samples from the shallow buried Cretaceous and Paleocene sediments on the Demerara Rise off Suriname, South America. These sediments are being studied for a number of paleoceanographic studies of the low-latitude Atlantic off the coast of Suriname (this volume). For this report two sites, Sites 1257 and 1258, were selected for silicoflagellate study because shipboard results suggested these two sites as the only ones with siliceous microfossils of Paleocene-Eocene age. The Demarara Rise is a predominant submarine plateau located off the coast of Suriname and French Guyana. This plateau stretches 380 km along the coast and is 220 km wide. The depth to seafloor along the depth transect drilled during ODP Leg 207 ranges from 1000 to 4500 m, but most of the remainder of the plateau lies in shallow water of 700 m. Much of this area is covered with 2-3 km of sediments. The Demerara Rise is built on rifted Precambrian continental crust. The plateau was one of the last places to be in contact with West Africa during the opening of the Atlantic Ocean (see Shipboard Scientific Party, 2004). Site 1257 (9°27'N, 54°20'W; water depth = 2951 m) is located on a terrace on the northwestern Demerara Rise ~400 km from Suriname. This is the second deepest water depth location drilled during Leg 207. Sediments from this area range in age from Miocene to Albian. This area is part of the transform fault that separated from Central America and western Africa. Three holes were drilled at Site 1257. Site 1258 (9°26'N, 54°43'W; water depth = 3192 m) is located on the western slope of the Demerara Rise ~380 km north of Suriname. This site is the distal and deepest site of the paleoceanographic depth transect drilled across Demerara Rise during Leg 207. The area is located on a ridge of Paleocene sediments cropping out on the seafloor. Three holes were drilled at Site 1258, but only one is studied.

Petrography and mineral compositions of gabbros at DSDP Leg 60 Holes

The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate that they are the cumulus assemblages of calc-alkalic crystal fractional on that occurred beneath the West Mariana Ridge. They are among a class of gabbros known only from other calc-alkalic associations (e.g., the Lesser Antilles and the Peninsular Ranges batholith of Southern California) and differ from gabbros of stratiform complexes, ophiolites, and the ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic Plagioclase (An94-97) but with fairly iron-rich olivines (Fo76-77). Other gabbros contain biotite and amphibole and occur in breccias with fairly high-grade greenschist facies (amphibole-chlorite-stilpnomelane) metabasalts. One unusual gabbro has experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende, anorthite, and green hercynitic spinel. This reaction, the extremely calcic Plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar (about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia formation, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C - and probably 350°C - were reached, and most likely could not have been attained without extrusion or intrusion of magmas nearby, even though no such rocks were cored.

Volcaniclastic sediments of ODP Leg 136 sites

Sediment cores recovered from three holes drilled during Ocean Drilling Program Leg 136 include volcaniclastics probably derived from the Hawaiian islands. The volcaniclastics shallower than 10 meters below seafloor are fresh and are composed of basaltic glass (sideromelane), basaltic fragments (mainly tachylite), plagioclase, olivine, pyroxene, and opaque minerals. Most of these glasses are probably products of hydrovolcanism. Visibly, some of these volcaniclastics are recognized as bedded ash layers having thicknesses that range from 5 to 10 cm. However, many volcaniclastics are disrupted by bioturbation to some degree, and are sometimes totally mixed with ambient brown clays. No visible correlative ash layer among these holes was found. It seems that many ash layers thinner than the bedded layers were disrupted by bioturbation because of the low sedimentation rate of volcaniclastics. The volcaniclastics were probably transported one of two ways: through air fall and pelagic settling or through turbidity-current transport. Other archipelagic apron volcaniclastic sediments of volcanic seamounts suggest that turbidite transport is the favored explanation of origin.

Radiolarians from ODP Leg 116 holes

Cores recovered from three sites of Leg 116 were studied for radiolarians. Generally, radiolarians were absent from most samples prepared for examination. Moderate to well-preserved radiolarian assemblages are found only in the uppermost one or two cores that were the focus of this study. All of the radiolarian assemblages in the upper cores belong to the Buccinosphaera invaginata Zone of latest Quaternary age. However, there is one stratum where a few Miocene radiolarians are reworked into the modern assemblages. Local seamounts are suggested sources for the reworked radiolarians.

Diatom abundances in ODP Leg 114 holes

The practically continuous, paleomagnetically dated late Gauss-Brunhes sediment profiles of ODP Sites 699 and 701, south of the present Polar Front Zone (PFZ), and Site 704, north of the present PFZ, are used for a high-resolution study of abundance fluctuations of eight stratigraphic marker species in space and time. Ecological restrictions and preferences of the diatom species Hemidiscus karstenii, Actinocyclus ingens f. planus, Thalassiosira elliptipora, Thalassiosira kolbei, Thalassiosira vulnifica, Simonseniella barboi, Cosmiodiscus insignis, and Nitzschia weaveri are deduced. The ages of their first abundant appearance datums (FAAD), last-appearance datums (LAD), and last abundant appearance datums (LAAD) at the three sites are determined. The interpolated datum ages agree relatively well with those determined by other authors, if one interprets most of their LADs as LAADs. FAADs and LAADs produce more accurate datums than LADs. For the late Matuyama (younger than approximately 2.0 Ma), when PFZ fluctuations effected all three site sites, the datum ages determined agree within the methodically caused limits of accuracy for each datum. For the early Matuyama (older than approximately 2.0 Ma) the results can be interpreted as either that the ages of the FAAD of T. kolbei and LAAD of T. vulnifica datums determined at Sites 699 and 701 are more reliable or that these datums are diachronous between these two sites and Site 704. Such a diachroneity could be caused by different paleoceanographic conditions (stable subantarctic conditions over Site 704 and stable antarctic conditions over Sites 699 and 701). A few taxonomic changes were necessary. One new genus is defined (Simonseniella gen. nov.) and five new combinations are proposed: Simonseniella barboi (Brun) comb, nov., Simonseniella praebarboi (Schrader) comb, nov., Simonseniella curvirostris (Jousé) comb, nov., Thalassiosira elliptipora (Donahue) comb, nov., and Thalassiosira vulnifica (Gombos) comb. nov.

Mineral chemistry of basalts at DSDP Leg 55 Holes

Microprobe mineral compositions of olivine, plagioclase, clinopyroxene, chrome spinel, ilmenite, and titanomagnetite are presented for 7 samples from 4 flows of hawaiite and one flow of tholeiitic basalt from Hole 430A at Ojin Seamount, 4 samples from 3 flows of alkalic basalt from Hole 432A at Nintoku Seamount, and 29 samples from 2 flows of alkalic basalt and 24 flows of tholeiitic basalt from Holes 433A, 433B, and 433C at Suiko Seamount. The four hawaiite flows from Hole 430A on Ojin Seamount have nearly identical mineralogy. The plagioclase phenocrysts and calculated equilibrium olivine appear to have crystallized at about 1175°C; the groundmass plagioclase crystallized from about 1135° to 1010°C; and the Fe-Ti oxides equilibrated at temperatures from 1000°C to 720°C under oxygen fugacities of 10**-11 to 10**-17. The single tholeiitic flow contains glomerocrysts of plagioclase (An80 to An65) and clinopyroxene (Wo43En46Fsn to Wo42En45Fs13). The plagioclase phenocrysts give calculated temperatures as high as 1400°C, indicating that they were not equilibrated with a magma having the bulk rock composition. The plagioclase groundmass crystallized at 1120° to 1070°C, and the Fe-Ti oxides equilibrated at 1070° to 930°C under oxygen fugacities of 10**-10 to 10**-12. Using mineral compositions of Hawaiian basalts as a guide, we infer that the hawaiite flows were erupted during the post-caldera alkalic eruptive stage and the tholeiite was erupted during the shield-building or caldera collapse stage. The three alkalic basalt flows from Hole 432A on Nintoku Seamount have similar mineralogy, although Flow Units 1 and 2 contain much more abundant plagioclase phenocrysts. The groundmass plagioclase crystallized at temperatures between 1175° and 1000°C. The olivine and plagioclase phenocrysts do not appear to be in equilibrium with the enclosing magmas. The mineral compositions suggest that these samples are intermediate between alkalic basalt and hawaiite; they probably erupted during the post-caldera alkalic stage of eruption. The two analyzed alkalic basalt flows are the two youngest flows recovered at Holes 433A, 433B, and 433C. Flow Unit 1 contains abundant sector-zoned clinopyroxene, and Flow Unit 2 contains rare kink-banded olivine xenocrysts. The plagioclase phenocrysts yield calculated temperatures of 1440° to 1250°C, indicating that they are probably not cognate. Calculated-equilibrium olivine indicates crystallization of olivine at about 1170°C. The Fe-Ti oxides equilibrated at temperatures of 1140° to 870°C under oxygen fugacities of 10**-9 to 10**-14. The groundmass plagioclase crystallized at temperatures of 1178° to 1035 °C. The mineral compositions indicate that these alkalic basalts erupted during the post-caldera alkalic eruptive stage. The 24 analyzed tholeiitic basalts are subdivided on the basis of phenocryst abundances into olivine tholeiites, plagioclase tholeiites, and tholeiites. The crystallization sequence appears to have been chrome spinel, olivine, plagioclase, and clinopyroxene as phenocryst phases, followed by and overlapping with groundmass crystallization of plagioclase (1180° to 920°C), clinopyroxene, and Fe-Ti oxides (1140° to 670°C). At least three flows contain pigeonite. The mineral compositions indicate that all the samples from Flow Unit 4 downward are tholeiitic basalts, although Flow Unit 64 has mineral compositions transitional to those in alkalic basalts.

Plio-Pleistocene magnetic polarity stratigraphies of ODP Leg 177 Sites from the South Atlantic

Magnetic polarity stratigraphies from ODP Leg 177 'high resolution' sites indicate Brunhes sedimentation rates in the 12-25 cm/kyr range, with a trend of decreasing sedimentation rates with increasing age. Magnetite is the principal remanence-carrying mineral. Downcore alteration of magnetite and authigenic growth of iron sulfides introduces a high coercivity diagenetic remanence carrier (pyrrhotite). The change in pore water sulfate with depth in the sediment tends to be in step with the decrease in magnetization intensity, indicating the link between sulfate reduction and magnetite dissolution. Shipboard pass-through magnetometer data are generally very noisy due to a combination of weak magnetization intensities, drilling-related core deformation, and the influence of authigenic iron sulfides. Post-cruise progressive demagnetization of discrete samples aids the magnetostratigraphic interpretation, as these measurements are less influenced by low magnetization intensities and drilling-related deformation. The magnetostratigraphic interpretations provide much-needed calibration for biostratigraphic events in the high latitude southern oceans. Apart from the ODP Hole 745B (Kerguelen Plateau), published Plio-Pleistocene magnetostratigraphies from ODP sites in the Southern Ocean are poorly constrained. For this reason, we compare interpolated ages of 11 radiolarian events and one diatom event that occur at Hole 745B and Leg 177 sites.