Sedimentation at Manop Site H in the eastern equatorial Pacific

Gravity cores recovered from Manganese Nodule Project site H (6°33'N, 92°49'W) show marked downcore variations in the abundance of calcium carbonate, organic carbon, opal, manganese, and other components deposited over the past 400,000 years. Variations in the downcore abundance of organic carbon, which ranges from 0.2 to 1.0%, can be used to hindcast redox conditions in the surface sediments over this time. The results indicate that the depth to the manganese redox boundary varied from about 5 to 25 cm below the seafloor during four major cycles. Downcore variations in solid phase Mn, Ni, and Cu can be produced by such changes in redox conditions. A model which predicts that solid phase Mn can be trapped and buried when the Mn redox boundary migrates rapidly upward is consistent with the observed organic carbon and Mn records and supports the reconstructed redox variations. The history of redox variations at site H can be explained by changes with time in surface water productivity. Major productivity variations at the site occur over 100-kyr cycles, with relatively higher productivity occurring during glacial stages. Thus Quaternary climate changes influence surface water productivity, redox conditions in sediments, and the cycling of transition metals.

Lower to middle Miocene calcareous nannofossil events from the Ontong Java Plateau

Uppermost Oligocene through middle Miocene calcareous nannofossil events that were considered potentially useful from a biostratigraphic point of view have been investigated from Ocean Drilling Program Sites 806 and 807 in the western equatorial Pacific Ocean. Comparisons have been made to the corresponding events from other equatorial regions and the mid-latitude North Atlantic. In terms of biostratigraphic reliability, defined by the ability of the pertinent species to provide distinctive marker events and synchroneity over geographic distance, the investigated events can be classified into four general categories: The good markers: last occurrence (LO) Sphenolithus ciperoensis, first occurrence (FO) S. delphix, LO S. delphix, FO S. belemnos, LO S. belemnos, FO S. heteromorphus, termination acme (TA) Discoaster deflandrei, and LO Sphenolithus heteromorphus. The poor markers: LO Helicosphaera recta, TA Cyclicargolithus abisectus, LO Triquetrorhabdulus carinatus, and FO Calcidiscus macintyrei. Ecologically controlled markers with regional value: LO Dictyococcites bisectus, LO Helicosphaera ampliaperta, FO Reticulofenestra pseudoumbilica, LO Cyclicargolithus floridanus, and LO Coronocyclus nitescens. The low abundance markers: FO Discoaster druggii, gradational form of Sphenoliths dissimilis/Sphenolithus belemnos, FO Triquetrorhabdulus rugosus, and FO T. rioensis.

Chemistry and age determination of basement fluids from Juan de Fuca Ridge

The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80 km transect across the eastern flank of the Juan de Fuca Ridge at 48°N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from not, vert, similar16° to 63°C. Although thermal effects of advective heat exchange are only seen within 20 km east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca2+ and decreases in alkalinity, Mg2+, Na+, K+, SO42- and delta18O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember 87Sr/86Sr ~ 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with 14C ages less than 10,000 yr and particle velocities of ~1-5 m/yr, in agreement with thermally constrained volumetric flow rates through a ~600 m thick permeable layer of ~10% porosity. Delta(element)/Delta(heat) extraction ratios are similar to values for ridge-crest hydrothermal systems.

Major and rare earth element composition of ODP Site 191-1179 sediments, northwest Pacific

Depth profiles of major element and rare earth element (REE) abundances in sediment samples (mainly siliceous ooze and clay) recovered from Holes 1179B and 1179C at Site 1179, Ocean Drilling Program Leg 191 (41.4°N, 159.6°E) were determined. The oxidation states of Mn and Ce were determined by X-ray absorption near-edge structure. Some geochemical indicators were tested, including the MnO/TiO2 ratios, a bivariate diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), and other discrimination diagrams. The oxidation state of Mn is reduced Mn(II) in the depth profile below 0.60 meters below seafloor (mbsf), which is consistent with relatively low and high abundances of Mn in the sediments and pore waters, respectively. It is possible that the diagenetic effect on the oxidation state and abundance of Mn makes it difficult for the MnO/TiO2 ratio to reflect the depositional environment. The normalized ratio of La and Ce does not change very much with depth, suggesting that the diagenetic effect does not affect the REE signature in the sediments. On the diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), the sediments studied here plot at the boundary of the pelagic and continental margin fields. This suggests that continental material has contributed to the sediment to some degree, even though Site 1179 is in a pelagic region of the northwestern Pacific Ocean, >1600 km from Japan.

Paleomagnetic properties of igneous rocks from several DSDP and ODP sites, western Pacific

Oceanic basalts and other related igneous rocks are considered excellent recorders of the Earth's paleomagnetic field. Consequently, basalt core paleomagnetic data are valuable for the constraints they provide on plate tectonic motions, especially for oceanic plates such as the Pacific. Unfortunately, few Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) boreholes have been cored very deeply into the ocean crust. The result is that there are only a few sites at which a large enough number of basalt flows have been cored to properly average secular variation (e.g., Kono, 1980, doi:10.2973/dsdp.proc.55.135.1980; Cox and Gordon, 1984, doi:10.1029/RG022i001p00047). Furthermore, there are a number of sites where basaltic core samples were retrieved but the cores were not measured. Often this occurs because leg scientists had more important sections to work on, or the section was ignored because it was too short to record enough time to average secular variation and obtain a reliable paleolatitude. Even though it may not be possible to determine a precise paleolatitude from such short sections, measurements from a small number of flows are important because they can be combined with other coeval paleomagnetic data from the same plate to calculate a paleomagnetic pole (Gordon and Cox, 1980, doi:10.1111/j.1365-246X.1980.tb02642.x; Cox and Gordon, 1984, doi:10.1029/RG022i001p00047). For this reason, I obtained samples for paleomagnetic measurements from eight Pacific sites (169, 170, 171, 581, 597, 800, 803, and 865), most of which have not been previously measured for paleomagnetism.

Stable isotope record and planktonic foraminiferal fragmentation of the late Pleistocene section of ODP Hole 134-828A

A high-resolution record of foraminiferal fragmentation (a dissolution indicator) for the last 250 k.y. (isotopic Stages 1 to 7) is identified in the upper 61.9 m of Ocean Drilling Program (ODP) Hole 828A, west Vanuatu. This record is comparable in detail to the atmospheric CO2 record and the d18O stack. Phase shifts between preservation spikes and maximum ice volumes (d18O of Globigerinoides sacculifer) are analogous to those on Ontong Java Plateau. Mass spectrometer (AMS14C) dating of a sample taken at the base of dissolution cycle B1 and the position of the last glacial maximum indicates a lag in time of ~8 k.y. in the Vanuatu region for the last glacial termination. When dissolution spikes are compared with minimum ice volumes there is no phase shift for the last two glacial terminations. The difference between Vanuatu and Ontong Java Plateau may be explained by local CO2 sinks and the interplay between intermediate and deep water masses. Terrigenous input increasingly affected sediment of Hole 828A on the North d'Entrecasteaux Ridge (NDR) as it approached Espiritu Santo Island. Mud and silt suspended in mid-water flows become important after 125 ka, while turbidites bypass the New Hebrides Trench only towards the last glacial maximum (LGM). Terrigenous supply seems to affect the lysocline profile that changed from an "open ocean" to a "near continent" type, thus favoring dissolution. Fragmentation of planktonic foraminifers is a more sensitive indicator of lysocline variations than is foraminiferal susceptibility to dissolution, the foraminiferal dissolution index, the abundance of benthic foraminifers, or CaCO3 content. A modern foraminiferal lysocline for the neighboring area (between 10°S and 30°S, and 160°E and 180°E) is found at 3.1 km below sea level, compared to west Vanuatu where it is shallower. The past lysocline level was deeper than 3086 m during intervals of dissolution minima, and ranged from ~2550 to 3000 m during intervals of dissolution maxima. The high sedimentation rates (in the order of 10 to 50 cm/k.y.) found in Hole 828A offer a great potential for future high-resolution studies either in this hole or other western localities along the NDR. Areas of high sedimentation near continental regions have been discarded for paleoceanographic and/or paleoclimatic studies. Nonetheless, conditions analogous to those found in Hole 828A are expected to occur in many trench areas around the world where mid-water flows have preserved as yet undiscovered fine high-resolution sedimentary records.

Element concentrations and isotopic composition of tholeiite and whole rocks samples from the east Mariana Basin

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40Ar/39Ar ages determined in this study for two Leg 129 basalts average 114.6 +/- 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites (87Sr/86Sr init = 0.70360-0.70374; 143Nd/144Nd init = 0.512769-0.512790; 206Pb/204Pb meas = 18.355-18.386) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 +/- 1.0 Ma 40Ar/39Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Distribution and abundance of calcareous-nannoplankton species in the Cretaceous sediments of DSDP Leg 62 Holes

Cretaceous sediments were recovered at all four sites (Sites 463-466) of the central North Pacific drilled during Leg 62 of the Deep Sea Drilling Project. One of the objectives was to get more information about the development of ocean plankton communities and early evolution of planktonic groups of the Mesozoic. In this article, the Cretaceous calcareous nannofossils from two areas of the central North Pacific (Mid-Pacific Mountains and Hess Rise) are listed and discussed. (The Cenozoic calcareous nannofossils are discussed by R. Schmidt 1981). Coring was continuous at all sites. Mesozoic calcareous nannoplankton assemblages range on the Mid-pacific Mountains from Barremian to Early Maastrichtian, and on Hess Rise from Albian to Late Maastrichtian. (No calcareous nannofossils older than Barremian or Albian respectively were found).

Late Quateranry dinoflagellate cysts of ODP Site 202-1233

The late Quaternary organic-walled dinoflagellate cyst record of Site 1233 (41°S, offshore Chile) was studied with a ?200 year resolution spanning the last 25,000 years. The study provides the first continuous record of sub-recent and recent dinoflagellate cysts in the Southeast (SE) Pacific. Major changes in the composition of the cyst association, cyst concentration and morphology of Operculodinium centrocarpum reflect changes in sea surface temperature (SST), sea surface salinity (SSS), palaeoproductivity and upwelling intensity. These changes can be associated with latitudinal shifts of the circumpolar frontal systems. The high cyst concentration, high Brigantedinium spp. abundances, low species diversity and the occurrence of certain cold water species are supportive for a 7-10° equatorward shift of the Antarctic Circumpolar Current (ACC) during the coldest phase of the Last Glacial Maximum (LGM) between 25 and 21.1 cal ka BP. Deglacial warming initiated at ~18.6 cal ka BP. Termination I (18.6-11.1 cal ka BP) is interrupted by an unstable period of extreme seasonality, rather than a cooling event, between 14.4 and 13.2 cal ka BP, synchronous with the Antarctic Cold Reversal (ACR). The Holocene Maximum is observed between 11.6 and 9.8 cal ka BP and is typified by the most southward position of the northern margin of the ACC. A cooling phase occurred during the early Holocene (until ~7 cal ka BP) and during the last ~0.8 ka. Our data indicates that the SE Pacific (41°S) climate has been influenced over the whole record by changes in the Southern Hemisphere (SH) high-latitudes, while during the mid to late Holocene, also a tropical forcing mechanism was involved, including the El Niño Southern Oscillation and the variable Hadley cell intensity. Furthermore, this study showed a relationship between the variable morphology of the spines/processes of O. centrocarpum and the combined variation of sea surface salinity and temperature (SSS/SST-ratio).

Oligocene magnetostratigraphy for equatorial Pacific ODP Sites

An Oligocene magnetostratigraphy from ODP Sites 1218 and 1219 (Equatorial Pacific) has been obtained by measurements made on u-channel samples, augmented by about 221 discrete samples. U-channel samples were measured at 1 cm intervals and were stepwise demagnetized in alternating fields (AF) up to a maximum peak field of 80 mT. The magnetization directions were determined at 1 cm intervals by principal component analysis of demagnetization steps in the 20 to 60 mT peak field range. A similar treatment was carried out on the discrete samples, which confirmed the results obtained with u-channel measurements. Sites 1218 and 1219 were precisely correlated based on multisensor track, paleontological and shipboard magnetostratigraphic data; this correlation is substantiated by u-channel measurements. Although the magnetostratigraphy obtained from the u-channels is similar to the interpretation deduced from shipboard measurements based on blanket demagnetization at peak AF of 20 mT, the u-channel results are substantially more robust since many interpretative uncertainties are resolved by the stepwise demagnetization and higher stratigraphic resolution. The temporal resolution of u-channel-based magnetic stratigraphy in the Oligocene section of Sites 1218 and 1219 is better than 5 kyr, and it is therefore suitable for detection of brief polarity subchrons. However, in spite of the high resolution, we did not find any reversals corresponding to the numerous cryptochrons identified in this time span by Cande and Kent (1995, doi:10.1029/94JB03098).

Stratigraphic correlation of ODP Site 199-1218 and 199-1219

During Ocean Drilling Program Leg 199 a high-resolution (~1-2 cm/k.y.) biogenic sediment record from the late Paleocene to the early Miocene was recovered, containing an uninterrupted set of geomagnetic chrons as well as a detailed record of calcareous and siliceous biostratigraphic datum events. Shipboard lithologic proxy measurements and shore-based determinations of CaCO3 revealed regular cycles that can be attributed to climatic forcing. Discovering drill sites with well defined magneto- and biostratigraphic records that also show clear lithologic cycles is rare and valuable and creates the opportunity to develop a detailed stratigraphic intersite correlation, providing the basis to study paleoceanographic processes and mass accumulation rates at high resolution. Here we present extensive postcruise work that extends the shipboard composite depth stratigraphy by providing a high-resolution revised meters composite depth (rmcd) scale to compensate for depth distortion within individual cores. The depth-aligned data were then used to generate stacked records of lithologic proxy measurements. Making use of the increased signal-to-noise ratio in the stacked records, we then proceeded to generate a detailed site-to-site correlation between Sites 1218 and 1219 in order to decrease the depth uncertainty for magneto- and biostratigraphic datums. Stacked lithologic proxy records in combination with discrete measurements of CaCO3 were then exploited to calculate high-resolution carbonate concentration curves by regression of the multisensor track data with discrete measurements. By matching correlative features between the cores and wireline logging data, we also rescaled our core rmcd back to in situ depths. Our study identifies lithology-dependent core expansion due to unloading as the mechanism of varying stratigraphic thicknesses between cores.