Diatoms and Silicoflagellates from ODP Hole 114-699A and Site 114-704

The stratigraphic ranges and relative abundances of selected diatoms and silicoflagellates are presented from three Neogene sedimentary sequences from the subantarctic South Atlantic. These data were compiled from Hole 699A in the southwest South Atlantic and Holes 704A and 704B in the southeast South Atlantic. Thirty-five samples were examined from a 67.5-m section of Hole 699A, which is mostly late Miocene or younger in age. A total of 225 samples was examined from the upper 569.1-m lower Miocene to Quaternary section in Holes 704A and 704B. Although the partial census of the Site 704 sequences is only preliminary, it reveals that the Neogene is remarkably complete and serves as a reference for further detailed examination of an important biostratigraphic-magnetostratigraphic reference section for the Neogene record of the Southern Ocean.

Paleomegnetic of basaltic sequenzes from DSDP Site 462 in the Nauru Basin

The voluminous volcanic eruptions in the Nauru Basin, Western Pacific, have long been regarded as important research targets for tectonic history of the Pacific Plate and for the widespread Cretaceous volcanic activity in the Western Pacific. The Nauru Basin volcanic rocks were recovered at Site 462 by Deep Sea Drilling Project (DSDP) Legs 61 and 89, where more than 600 m of lavas and sills were drilled, thereby making it the deepest penetration into crust of Cretaceous age in the Pacific Ocean. For paleomagnetism, this section represents a unique possibility for averaging out secular variation to obtain a reliable paleolatitude estimate. However, previous paleomagnetic studies have only been subjected to alternating field (AF) demagnetization on several core samples, thus, unable to provide comprehensive understanding on the paleolatitude of the basin. The work reported here aims to determine the Cretaceous paleomagnetic paleolatitude for the Pacific Plate and define the magnetostratigraphy for the basaltic sections drilled in the Nauru Basin. A total of 391 basaltic rock samples were carefully re-sampled from DSDP Sites 462 and 462A. Stepwise thermal and AF demagnetizations have isolated characteristic components in the majority of the samples. The most important findings from this study include: (1) Two normal and one reversed polarity intervals are identified in Site 462, and six normal and six reversed polarity intervals are found in Site 462A, although possible erroneous markings of the opposite azimuth for some reversed polarity cores during the DSDP coring cannot be completely ruled out. (2) Based on previous radiometric ages, the magnetostratigraphic correlations with the Geomagnetic Polarity Time Scale (GPTS) indicate that the lower-basaltic flow unit in Site 462A began to erupt at least before 130 Ma. No correlation is available for the upper-sill unit. (3) Paleosecular variation for the lower-flow unit has been sufficiently averaged out; whereas bias may exist for that of the upper-sill unit; (4) The calculated mean inclination of ~50° for the lower-flow unit yields a paleolatitude of 30.8°S for the Nauru Basin at the time of emplacement. This value is well to the north of suggested location in plate reconstruction models, suggesting that there has been a significant amount of apparent polar wander of the Nauru Basin and Pacific plate since 130 Ma. In addition, the paleolatitude for the Nauru Basin is ~7° further south and the basin's age is more than 10 my older than those of the Ontong Java Plateau (OJP), which suggest that the volcanic eruptions of the lower flows in the Nauru Basin are unlikely related to the emplacement of the Ontong Java Plateau.

Late Eocene-earliest Miocene Sr isotopic reference section of DSDP Site 73-522

We present a revised calibration of Sr isotopes to the geomagnetic polarity timescale (GPTS) using closely spaced (~0.15 m.y. resolution) samples from the classic uppermost Eocene through lowermost Miocene section at Site 522, eastern South Atlantic. The Sr isotopic data are fit with two linear segments with a sharp change in slope at circa 27.5 Ma from 0.000038/m.y. (27.5 to 34.4 Ma) to 0.000051/m.y. (23.8 to 27.5 Ma). Regression analysis indicates that stratigraphic resolution ranges from ±1 m.y. (for one analysis) to ±0.6 m.y. (for three analyses) for the younger interval and ±1.2 m.y. (for one analysis) to ±0.7 m.y. (for three analyses) for the older interval, representing an increase in resolution from previous studies of ±1-2 m.y. The paleoceanographic significance of this change in slope is unclear. It occurs during an interval of intermittent Antarctic glaciation, between the Oi2a and Oi2b glaciations. The subsequent interval from circa 27 to 24 Ma appears to be an interval of minimal glaciation. Thus this observation does not support previous suggestions that increases in rates of Sr isotopic change are directly associated with the frequency of Antarctic glaciations. Rather, the increase in slope may be related to increased weathering associated with the "mid-Oligocene" glaciation.

Chemical and isotopic compositions of basalts and glasses from lavas of the Sierra Leone fault site

According to detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial rift zone of the MAR between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts, but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in basalts and their glasses. It was shown that some samples have significant nonsystematic differences in the 87Sr/86Sr ratio between basalts and their chilled glasses and less significant difference in e-Nd; higher Sr ratios can be observed both in glasses and basalts of the same lava fragments. No significant correlation is observed between isotope characteristics of samples and their geochemistry; it was also shown that seawater did not affect Sr and Nd isotope compositions of the chilled glasses from the studied pillow lavas. It is suggested that such differences in isotope ratios are related to small-scale heterogeneity of melts owing to incomplete homogenization during their rapid ascent to the surface. Heterogeneity of basaltic melts is explained by their partial contamination by older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the surface of the ocean floor. The wider scatter of the Sr isotopic ratios relative to Nd ones is related to presence of xenocrysts of calcic plagioclase; correspondingly, absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. Causes of this phenomenon remain unclear.

Major element oxides in ODP Site 164-997 sediments

Since being first discovered in the Blake-Bahama region of the west Atlantic in the 1970s (Hollister, Ewing, et al., 1972, doi:10.2973/dsdp.proc.11.1972), submarine gas hydrates have been identified in the continental margin worldwide. Ocean Drilling Program (ODP) Leg 164 was the first drilling designated to study the occurrence and distribution of natural gas hydrates in Blake Ridge where a well developed, distinct BSR (Bottom Simulating Reflector) has been identified (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). It has been reported there is a prominent discrepancy between the BSR and the base of gas hydrate stability (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996; Ruppel, 1997, doi:10.1130/0091-7613(1997)025<0699:ACTOAT>2.3.CO;2), though theoretically they should be at the same depth. Natural gas hydrate in marine sediments coexists with sediment particles, so detailed delineation of sediment geochemistry will be of benefit to solve this apparent discrepancy. The main objectives of this study are to supply background data of the major chemical compositions of sediments from a hydrated sediment section.

Results of analyses of fluid inclusions from ODP Site 735B, Leg 118 and 176

Microthermometric and isotopic analyses of fluid inclusions in primitive olivine gabbros, oxide gabbros, and evolved granitic material recovered from Ocean Drilling Program Hole 735B at the Southwest Indian Ridge provide new insights into the evolution of C-O-H-NaCl fluids in the plutonic foundation of the oceanic crust. The variably altered and deformed plutonic rocks span a crustal section of over 1500 m and record a remarkably complex magma-hydrothermal history. Magmatic fluids within this suite followed two chemically distinct paths during cooling through the subsolidus regime: the first path included formation of CO2+CH4+H2O+C fluids with up to 43 mole% CH4; the second path produced hypersaline brines that contain up to 50% NaCl equivalent salinities. Subsequent to devolatilization, respeciation of magmatic CO2, attendant graphite precipitation, and cooling from 800°C to 500°C promoted formation of CH4-enriched fluids. These fluids are characterized by average d13C(CH4) values of -27.1+/-4.3 per mil (N=45) with associated d13C(CO2) compositions ranging from -24.9 per mil to -1.9 per mil (N=39), and average dD values of exsolved vapor of -41+/-12 per mil (N=23). In pods, veins, and lenses of highly fractionated residual material, hypersaline brines formed during condensation and by direct exsolution in the absence of a conjugate vapor phase. Entrapped CO2+CH4+H2O-rich fluids within many oxide-bearing rocks and felsic zones are significantly depleted in 13C (with d13C(CO2) values down to about -25 per mil) and contain CO2 concentrations higher than those predicted by equilibrium devolatilization models. We hypothesize that lower effective pressures in high-temperature shear zones promoted infiltration of highly fractionated melts and compositionally evolved volatiles into focused zones of deformation, significantly weakening the rock strength. In felsic-rich zones, volatile build-up may have driven hydraulic fracturing of gabbroic wall rocks resulting in the formation of magmatic breccias. Comparison of isotopic compositions of fluids in plutonic rocks from 735B, the MARK area of the Mid-Atlantic Ridge, and the Mid-Cayman Rise indicate (1) that the carbon isotope composition of the lower oceanic crust may be far more heterogeneous than previously believed and (2) that carbon-bearing species in the oceanic crust and their distribution at depth are highly variable.

Element concentrations and Osmium, Platinum and Rhenium isotope ratios of ODP Site 153-920 abyssal peridotites

Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt-186Os and 187Re-187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353 +/- 7, identical to the mean of 0.1198340 +/-12 for Os-Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about +/- 30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing D(Re), reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128-0.129, consistent with estimates for the primitive upper mantle.

Mineralogy and chemistry of basalts and secondary minerals from DSDP Site 417, Atlantic Ocean

Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H20, increase of Fe 3 +/FeT and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H20, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3 b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe 3 +/FeT and H20 occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H20 , and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.

Composition of sediment pore water and isotopes in carbonate and benthic foraminifera at DSDP Holes 68-501, 69-505 and Site 69-504

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of seawater. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of basement recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher basement topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90% of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity approach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicated by large decreases in d18O, Mg, alkalinity, Na, and K and an increase in Ca with increasing depth. The size of the changes in d18O, Mg, alkalinity, Ca, Sr, and SO4 varies laterally over 500 meters, indicating lateral gradients in pore water chemistry that are nearly as large as the vertical gradients. The lateral gradients are believed to result from similar lateral gradients in the composition of the basement formation water, which propagate upward through the sediments by diffusion. A model of the d18O profile suggests that the basement at Site 501/504 was sealed off from advection about 1 m.y. ago, so that reaction rates began to dominate the basement pore water chemistry. A limestone-chert diagenetic front began to move upward through the lower sediments less than 200,000 yr. ago.

Organic matter in sediments of ODP Site 131-808

Microscopic studies reveal a predominance of terrestrial organic matter in sediments of Site 808. Terrestrial vitrinite and inertinite are more abundant (73% to 100%) than marine organic matter (alginite, 0% to 27%), which increases from open oceanic deposits of the Shikoku Basin sediments to sediments of the outer trench wedge. The abundance of terrestrial organic matter is also reflected through carbon isotope values of -23 per mil to -25.9 per mil. Mass accumulation rates of organic carbon are low in hemipelagic sediments of the Shikoku Basin (<0.2 g/cm**2/k.y.) but increase significantly in sediments of the Nankai Trench (0.2 to 1.7 g/cm**2/k.y.). Although the organic mass accumulation is high in sediments of the Nankai Trench, a comparison of sedimentation rates and total organic carbon suggests relative dilution of organic carbon through turbidite flows. Calculated marine paleoproductivity of organic carbon is low in sediments of the open ocean (Shikoku Basin) and increases closer to the shore (Nankai Trench). Thermal evolution of organic matter is obtained from vitrinite reflectance measurements. Two populations of vitrinites have been observed between 600 and 1234 mbsf. Reflectance values change with increasing depth and temperature in both groups of vitrinite (0.3% to 0.68% in group 1; 0.6% to 1% in group 2).