ALBERT

Description: The Cenozoic volcanic activity on Iceland has been recorded in North Atlantic sediments drilled during several Ocean Drilling Program (ODP)/Deep Sea Drilling Project legs (Legs 104, 151, 152, 162, and 163). Leg 162 (North Atlantic-Arctic Gateways II) recovered ash layers at Sites 982, 985, and 907 (Jansen, Raymo, Blum, et al., 1996, doi:10.2973/odp.proc.ir.162.1996). The revisited Site 907 was first drilled during Leg 151, and the ash from this site has been described in detail by Lacasse et al. (1996, doi:10.2973/odp.proc.sr.151.122.1996) and Werner et al. (1996, doi:10.2973/odp.proc.sr.151.123.1996). Site 982 is located within the Hatton-Rockall Basin on the Rockall Plateau, which is situated west of the British Isles. Site 985 is located northeast of Iceland at the foot of the eastern slope of the Iceland Plateau, adjacent to the Norwegian Basin. Here we report chemical analyses of Neogene tephra layers from Holes 982A, 983B, 982C, 985A, and 985B. The sedimentary sequence at Site 982 spans the lower Miocene-Holocene; Site 985 recovered sediments spanning the upper Oligocene-Holocene. Twenty-two distinct ash layers and ash-bearing sediments were sampled in Holes 982A-982C (Cores 162-982A-16H through 24H, 162-982B-14H through 56X, and 162-982C-15H through 27H), and 59 ash layers were sampled in Holes 985A and 985B (Cores 162-985A-11H through 59X, and 162-985B-11H through 14H). Almost 50% of the sampled ash is strongly altered (predominantly from Site 985). A cluster of altered thin layers in the lower Pliocene of Site 985 (top of Unit III) is remarkable.

Description: Sediment spectral reflectance measurements were generated aboard the JOIDES Resolution during Ocean Drilling Program Leg 162 shipboard operations. The large size of the raw data set (over 1.3 gigabytes) and limited computer hard disk storage space precluded detailed analysis of the data at sea, although broad band averages were used as aids in developing splices and determining lithologic boundaries. This data report describes the methods used to collect these data and their shipboard and postcruise processing. These initial results provide the basis for further postcruise research.

Description: To understand the late Cenozoic glacial history of the Northern Hemisphere, continuous long-term proxy records from climatically sensitive regions must be examined. Ice-rafted debris (IRD) from Ocean Drilling Program (ODP) Site 918, located in the Irminger Basin, is one such record. IRD in marine sediments is a direct indicator of the presence of glacial ice extending to sea level on adjacent landmasses, and, therefore, is an important paleoclimatic signal from the mid- to high latitudes. The IRD record at Site 918 is the first long-term ice-rafting record available for southeast Greenland, a region that may have been a key nucleation area for widespread glaciation during the late Cenozoic (Larsen et al, 1994, doi:10.2973/odp.proc.ir.152.1994). This data report presents the results of coarse sand-size IRD mass accumulation rate (MAR) analyses for Site 918 from the late Miocene through the Pleistocene. In addition, a preliminary analysis of IRD compositions is included. Detailed discussions of the local, regional, and global paleoclimatic implications of this data, and of the companion Site 919 Pleistocene IRD MAR data (Krissek, 1999, doi:10.2973/odp.proc.sr.163.118.1999), are in preparation. Such future work will include comparisons of these IRD MAR data sets to the Site 919 oxygen isotope stratigraphy developed by Flower (1998, doi:10.2973/odp.proc.sr.152.219.1998).

Description: Coring during Ocean Drilling Program and Deep Sea Drilling Project Legs 163, 152, 104, 81, and 38 recovered sequences of altered basalt from North Atlantic seaward-dipping reflector sequences (SDRS) erupted during the initial rifting of Greenland from northern Europe and likely associated with excessive mantle temperatures caused by an impacting mantle plume head. Cr-rich spinel is found abundantly as inclusions and groundmass crystals within the olivine-rich lavas of Hole 917A (Leg 152) cored into the Southeast Greenland SDRS, but only rarely as inclusions within plagioclase in the lavas of the Vøring Plateau SDRS, and it is absent from other cored SDRS lavas from the Rockall Plateau and Southeast Greenland. Eruptive melt compositions determined from inferred, thermodynamically-defined, spinel-melt exchange equilibria indicate that the most primitive melts represented by Hole 917A basalts have Mg/(Mg + Fe2+) at least as high as 0.70 and approach near-primary mantle melt compositions. In contrast, Cr-rich spinels from Hole 338 (Leg 38) lavas on the Vøring Plateau SDRS give evidence for melt with Mg/(Mg + Fe2+) only as high as 0.64. This study underlines that primitive melts similar to those from Hole 917A comprise only a small fraction of the eruptive North Atlantic SDRS melts, and that most SDRS basalts were, in fact, too evolved to have precipitated Cr-rich spinel, with true melt Mg/(Mg + Fe2+) likely below 0.60. The evolved nature of the SDRS basalts implies large amounts of fractionation at the base of the crust or deep within it, consistent with seismic results that indicate an abnormally thick Layer 3 underlying the SDRS.

Description: The magnetic polarity stratigraphy at Site 907 obtained from the shipboard pass-through magnetometer and from discrete samples is readily interpretable back to the onset of the Gilbert Chron (5.89 Ma). From this level to the base of the section at ~14 Ma, the interpretation is corroborated by silicoflagellate datums with predictable correlation to polarity chrons. The resulting magnetostratigraphic interpretation differs from those proposed in the Leg 151 (Hole 907A) and 162 (Holes 907B and 907C) Initial Reports volumes. An important hiatus in the 7-10 Ma interval at Site 907 caused sedimentation to slow or cease for ~2.7 m.y. We have revised the shipboard correlation among the three holes at Site 907, resulting in a new composite section splice and recalculation of composite depths. For Site 985, magnetostratigraphic interpretation is possible down to ~150 meters below seafloor (mbsf) (C3An/C3Ar) at ~6 Ma. There are no useful biostratigraphic datums from Site 985 to support this interpretation; however, the interpretation is supported by the correlation of Sites 985 and 907 using natural gamma data from the shipboard multisensor track. Below ~150 mbsf at Site 985, drilling-related deformation at the onset of extended core barrel drilling precluded magnetostratigraphic interpretation.

Description: Cores recovered at Sites 986 and 987 comprise glacial fan sedimentation associated with the Svalbard-Barents Sea and Greenland Ice Sheets, respectively. At Site 986, the top 150 m and the basal 250 m yielded interpretable magnetic stratigraphies. The record from the intervening 550 m is compromised by drilling-related core deformation, poor recovery, and numerous debris flows. The uppermost 150 m appears to record the Brunhes/Matuyama boundary and the Jaramillo Subchron. The base of the drilled section (at ~950 meters below seafloor [mbsf]) is interpreted to lie within the Matuyama Chron (age 〈2.58 Ma) with an apparent normal polarity interval in the ~730-750 mbsf interval. Dinoflagellate cyst biostratigraphy and Sr isotopic ratios are consistent with a Matuyama age for the base of the drilled section and with the normal polarity interval as the Olduvai Subchron. On the other hand, the last occurrence of Neogloboquadrina atlantica (sinistral) and the last common occurrence of the warm-dwelling Globigerina bulloides at 647-650 mbsf in Hole 986D indicate an age for this level of ~2.3 Ma, inconsistent with the designation of the Olduvai Subchron in the ~730-750 mbsf interval. If the age at 647-650 mbsf in Hole 986D is taken as 2.3 Ma and the base of the hole lies within the Matuyama Chron, then the sedimentation rate in the basal 300 m of the cored section averages 1 m/k.y. At Site 987, the magnetic stratigraphy is fairly unambiguous throughout the section and yields an age of 7.5 Ma (Chron 4n) for the base of the drilled section. The paucity of calcareous and siliceous microfossils precludes biostratigraphic corroboration of the magnetostratigraphic interpretation, although dinoflagellate cysts provide general support, particularly at the base of the section. The age model indicates relatively low sedimentation rates (~5 cm/k.y.) at the base of the section with rates at least four to five times greater during intervals of debris flows at ~5-4.6 and ~2.6 Ma.

Description: Diffuse reflectance records from Feni Drift in the North Atlantic faithfully record sediment percent carbonate. A high-resolution, reflectance-based age model for these sediments derived from an orbitally tuned age model for western equatorial Atlantic, Ceara Rise sediments was generated by spectral frequency mapping. Power spectra of the Feni Drift record indicate statistically significant sub-Milankovitch cyclicity at 7.6-8.4 and 4.8-6.1 kyr. We infer that these ~8 and ~5 kyr cycles document a linkage between North and equatorial Atlantic climate given our ability to correlate these records. These climate cycles influence Atlantic basin carbonate prior to the intensification of Northern Hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets. The presence of these peaks, which could be related to equatorial clipped precession, implies a possible non-linear response to Milankovitch forcing.

Description: A series of 1-atm. melting experiments on basaltic flows collected from Holes 918D and 989B in the oceanic succession of the East Greenland continental margin can be used to define possible phase equilibria and liquid lines of descent. A sample from Site 918 (Section 152-918D-108R-2) shows the melting order low-Ca pyroxene (1153ºC), augite (1182ºC), olivine (1192ºC), and plagioclase (1192ºC). A sample from Site 989 (Section 163-989B-10R-7) melts in the order of low-Ca pyroxene (1113ºC), augite (1167ºC), plagioclase (1177ºC), and olivine (1184ºC). In particular, the relatively early appearance of low-Ca pyroxene distinguishes the melting relations for the oceanic succession from those observed for the basaltic continental succession at Site 917. A basaltic andesite flow from the Middle Series at Site 917 (Section 152-917A-27R-4) shows the melting order of low-Ca pyroxene (1142ºC), plagioclase (1173ºC), and olivine (1173ºC). This melting order is difficult to reconcile with the observed large compositional variations in SiO2 and FeO for the Middle Series, which imply early magnetite fractionation. Major element considerations and rare-earth element modeling of the dacites of the Middle Series suggest that they formed by low extent of melting (〈20%) of continental hydrated gabbroic or mafic amphibolite at pressures 〈8 kbar. These crustally derived melts represent possible contaminants of basaltic magmas of the Lower and Middle Series at Site 917.

Description: Thirty-five samples from the drill core of the three Leg 163 sites (Sites 988, 989, and 990) off the southeast coast of Greenland were analyzed for 27 major, minor, and trace elements by X-ray fluorescence (XRF) and for 25 trace elements, including 14 rare-earth elements (REEs), by an inductively coupled plasma source mass spectrometer (ICP/MS). Sr- and Nd-isotope data are reported for seven samples and oxygen-isotope data are reported for 19 plagioclase separates. In addition, a reconnaissance survey of the composition of the main mineral phases, plagioclase, pyroxene, and oxides was determined on an electron microprobe to provide the basic information required for petrogenetic modeling. Olivine pseudomorphs are present in many of the samples, but in no case was an olivine grain found that was fresh enough to give a reliable analysis. The chemical and isotopic data recorded here were determined to provide a comparison with the larger data sets acquired by the Edinburgh, Copenhagen, and Leicester laboratories from both Legs 152 and 163 drill cores. This will permit a detailed comparison of the North Atlantic flood basalt province as a whole with the better known Columbia River, Deccan, and Karoo continental flood basalt provinces, for which substantial chemical data sets are already available at Washington State University.

Description: Voluminous, subaerial magmatism resulted in the formation of extensive seaward-dipping reflector sequences (SDRS) along the Paleogene Southeast Greenland rifted margin. Drilling during Leg 163 recovered basalts from the SDRS at 66ºN (Site 988) and 63ºN (Sites 989 and 990). The basalt from Site 988 is light rare-earth-element (REE) enriched (La(n)/Yb(n) = 3.4), with epsilon-Nd(t=60) = 5.3, 87Sr/86Sr = 0.7034, and 206Pb/204Pb = 17.98. It is similar to tholeiites recovered from the Irminger Basin during Leg 49 and to light-REE-enriched tholeiites from Iceland. Drilling at Site 989, the innermost of the sites on the 63ºN transect, was proposed to extend recovery of the earliest part of the SDRS initiated during Leg 152. These basalts are, however, younger than those from Site 917 and are compositionally similar to basalts from the more seaward Sites 990 and 915. Many of the basalts from Sites 989 and 990 show evidence of contamination by continental crust (e.g., epsilon-Nd(t=60) extends down to -3.7, 206Pb/204Pb extends down to 15.1). We suggest that the contaminant is a mixture of Archean granulite and amphibolite and that the most contaminated basalts have assimilated ~5% of crust. Uncontaminated basalts are isotopically similar to basalts from Site 918, on the main body of the SDRS, and are light-REE depleted. Consistent with previous models of the development of this margin, we show that at the time of formation of the basalts from Sites 989 and 990 (1) melting was at relatively shallow levels in a fully-fledged rift zone; (2) fragments of continental crust were present in the lithosphere above the zones of melt generation; and (3) the sublithospheric mantle was dominated by a depleted Icelandic plume component.

Description: This data report provides a systematic documentation of the low-temperature alteration associated with the formation of a volcanic-rifted margin by the quantification of alteration effects and vein mineralogy and distributions in basalts recovered on Leg 152 (Larsen, Saunders, Clift, et al., 1994, doi:10.2973/odp.proc.ir.152.1994). Basaltic rocks from Holes 917A and 918D have been investigated to provide a quantitative description of the extents of recrystallization and secondary mineral abundance resulting from low-temperature alteration and weathering. Only limited descriptions of alteration and secondary mineral distributions were undertaken on board ship during Leg 152, and the data presented here provide an essential complement to the shipboard logs of the limited amount of basalt recovered during Leg 163 from Sites 988, 989, and 990 (Duncan, Larsen, Allan, et al., 1996, doi:10.2973/odp.proc.ir.163.1996).

Description: During Ocean Drilling Program (ODP) Leg 162, five sites were drilled in the subpolar North Atlantic (Sites 980-984), three sites in the Nordic Seas (Sites 985-987), and two holes at Iceland Sea Site 907 (first drilled during ODP Leg 151). Carbonate sediments at the subpolar sites have generally common to abundant and well-preserved planktonic foraminifers, especially at Feni Drift Sites 980/981 and Rockall Plateau Site 982. Gardar Drift Site 983 and Bjorn Drift Site 984 featured greater concentrations of clay material and ice-rafted debris, diluting carbonate material in some intervals (particularly before ~1.8 Ma at Site 984). Nordic Seas Sites 907 and 985-987 feature generally rare to common and moderately well-preserved planktonic foraminifers only within the past 1 m.y., although Pliocene taxa are sparsely recorded at Site 986 on the Svalbard margin. Planktonic foraminifer datum levels are located to the section level where possible for the subpolar North Atlantic sites. Comparison to an integrated magnetostratigraphy and calcareous nannofossil stratigraphy shows that several datum levels are synchronous to within 5% of their published ages. In particular, the start of the Neogloboquadrina pachyderma (sinistral) Acme Zone (1.8 Ma), the first occurrence (FO) of Globorotalia inflata (2.09 Ma), the last occurrence (LO) of Globorotalia cf. crassula (3.3 Ma), and the FO of Globorotalia puncticulata (4.5 Ma) are judged synchronous in eastern sections of the subpolar North Atlantic. The LO of Neogloboquadrina atlantica (sinistral) occurs ~100-200 k.y. later relative to its mid-latitude North Atlantic age (2.41 Ma).

Description: Silicoflagellate assemblages were analyzed for the Miocene intervals recovered during Ocean Drilling Program (ODP) Leg 162. The middle and late Miocene are represented in Holes 907B (Iceland Plateau) and 982B (Rockall Plateau) by well-preserved silicoflagellates. A zonal scheme is presented, which improves previous attempts to establish a comprehensive silicoflagellate stratigraphy for this realm, based on an increased resolution given by the analysis of this largest-ever number of samples for the given interval (about three times as many as those studied for ODP Leg 104). The most important change is the replacement of the last appearance of Corbisema triacantha by the first appearance of Bachmannocena circulus apiculata as the top of the upper C. triacantha Zone/bottom of the B. c. apiculata Zone. In addition, two important bioevents (the first appearance of Bachmannocena diodon nodosa and the last appearance of Distephanus crux stradneri) allow the subdivision of the B. c. apiculata Zone into three new subzones: Caryocha spp. Subzone, D. c. stradneri Subzone, and B. d. nodosa Subzone (oldest to youngest, respectively). This zonal scheme is correlated with the standard calcareous nannoplankton zonation and tentatively correlated with the magnetic polarity time scale. A qualitative analysis of the changes in surface-water characteristics is given, based on the variation in abundances of the several silicoflagellate groups. This shows a replacement of silicoflagellates that are adapted to warm surface waters by those adapted to temperate surface waters during the middle Miocene. In turn, the latter are gradually replaced by those adapted to cool surface waters during the late Miocene. The taxonomy of the taxa encountered has been revised in an effort to bring together differing schools. Only one new taxon, Distephanus crux lockerii, is described herein.

Description: Magnetic polarity stratigraphies for Sites 980-984 are based on shipboard measurements from the pass-through magnetometer after alternating field (AF) demagnetization at a peak field of 25 mT and shore-based stepwise AF demagnetization of discrete samples. The characteristic magnetization component was determined after AF demagnetization removed the steep downward drill-string-related magnetic overprint. Peak AF fields in the 20-30 mT range were required to resolve the component, carried by magnetite, that was used to produce unambiguous Pliocene-Pleistocene magnetic stratigraphies at all five sites. At Sites 980 and 983, magnetic stratigraphies were resolved to the base of the recovered advanced hydraulic piston corer (APC) section, which lies in the Matuyama Chron (1r.2r) and Olduvai Subchron (2n), respectively. At Sites 981 and 982, magnetization intensities decrease sharply in the normal polarity zone corresponding to the Gauss Chron (2An), and magnetic stratigraphies below this level could not be resolved. At Site 984, the resolution of magnetic stratigraphy was curtailed at ~250 meters below seafloor (Olduvai Subchron) by core deformation at the base of the APC section and in the underlying extended core barrel section. As the magnetic stratigraphies at all four sites are unequivocal, polarity chron interpretations can be made without aid from the biostratigraphy. Mean sedimentation rates within polarity chrons have been calculated and Pliocene-Pleistocene biomagnetostratigraphic correlations tested.

Description: Multisensor track data, including magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE) wet bulk density, and natural gamma emission, were collected on all cores recovered during Ocean Drilling Program Leg 162. Data from the upper Pliocene and lower Pleistocene of Sites 981 and 984 are here compared to results from analyses of a limited set of discrete samples, including benthic foraminiferal isotopic composition, grain size, carbonate content, abundance of foraminifers and lithic particles, and clay mineralogy. Natural gamma emission most closely monitors the input of felsic terrigenous material to these two sites. Magnetic susceptibility also tracks felsic terrigenous input at Site 981 but appears to reflect a separate, more mafic, terrigenous component at Site 984. The GRAPE record does not correlate well with any discretely measured variable at Sites 981 or 984.

Description: Ocean Drilling Program Site 975 is located near the base of the Menorca Rise in the South Balearic Basin of the western Mediterranean Sea. Coring at this site penetrated the Pliocene/Miocene boundary and recovered a sequence of sediments that represent the final stages of salt deposition and the transition from evaporitic to open marine conditions at the end of the Miocene (Messinian). Detailed petrographic observations and bulk mineralogical analyses by X-ray diffraction form the basis for preliminary interpretations of depositional environments for this section. Gypsum is thought to have been deposited in an evaporating basin below wave base. Cycles consisting of a clay layer overlain by gypsiferous chalk, laminated gypsum, and finally pinch-and-swell gypsum suggest upsection increases in salinity. The gypsum section is overlain by two exotic sand layers thought to mark events of fresher water (marine or meteoric) inflow to the basin. Gypsum deposition terminated and was replaced by inorganic precipitation of micritic calcite with periodic, variable dilution by fine-grained terrigenous sediment. The micritic sediments have fine, slightly wavy, laminations indicating either an algal/microbial mat origin, or varve-like fluctuations in deposition, perhaps in a deep basin. The Pliocene/Miocene boundary falls within an interval of banded micritic silty clays that reflect the final environmental fluctuations during the transition to the open marine conditions of the Pliocene.

Description: Upper Miocene to Pleistocene hemipelagites and resedimented facies recovered at Holes 976B and 977A (Leg 161) in the Alboran Basin consist mainly of biogenic and detrital components, with a minor contribution of neoformed mineral phases. Diagenetic processes have not obliterated the primary deposition signal, and therefore detrital components (quartz, feldspar, detrital dolomite, rock fragments, and clays) provide information about source rocks and provenances. No major bulk or clay mineralogy differences were recognized between resedimented and hemipelagic facies; in fact, similar mineral assemblages in both types of facies suggest common source rocks. However, mineral abundance fluctuations can be related to climate variations and tectonic factors, as the main controls of sediment fill of this basin. A marked increase in smectites in Messinian sediments suggests an extensive development of soils during that time, probably favored by the alternation of wet and dry climate episodes and the relative aridification of the Mediterranean borderlands. A notable increase in detrital components suggests a sea-level fall and/or tectonic uplift during the late Pliocene. The significant increase in detrital dolomite in the uppermost Pliocene deposits suggests the uplift of dolomite-rich rocks as source areas. Mineral components in Pleistocene sediments indicate increasing tectonic stability, and clay-mineral fluctuations during the Pleistocene can be related not only to tectonic events, but also to alternating cooling and warming periods.

Description: Water column and core-top d15N data show that Arabian Sea denitrification produces large nitrogen isotopic enrichments that are regionally recorded with fidelity in the sediments. These results facilitate interpretation of a 1 m.y. d15N record for Ocean Drilling Program site 722B on the Owen Ridge in terms of climatically linked oscillations in denitrification at the major orbital periods. As at present, denitrification was greatest during interglacial periods and, apparently, was not active during most glacial intervals. Cross-spectral analysis of d15N with foraminiferal d18O (global climate/sea level index) and lithogenic grain size (monsoon strength index) suggests forcings by changes in hydrography and productivity acting through the extent and intensity of the oxygen minimum zone. The data suggest that denitrification may be an internal forcing mechanism for climate change during major glacial/interglacial transitions through influence on marine N inventory and atmospheric CO2. However, compensation or amplification may occur from other sinks (sediment denitrification) or sources (nitrogen fixation).

Description: A newly constructed record of eolian dust accumulation from the central North Pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3.6 Ma. We associate this sudden drying with the uplift of at least the northern portion of the Tibetan Plateau, shutting off the Indian Ocean moisture source to central and western China. This ten-fold increase in atmospheric dust loading appears to be associated with the 1-m.y.-long shift toward heavy d18O values that occurred at 3.6-2.6 Ma. The dust grain-size record of wind intensity begins its late Cenozoic coarsening a million years before the drying event, at ~4.5 Ma. The northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric CO2 in the latest Miocene and the early Pliocene closing of the Panamanian Seaway.

Description: Free and 'bound' long-chain alkenones (C37:2 and C37:3) in oxidized and unoxidized sections of four organic matter-rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of Uk'37. The profiles of both alkenones across the redox boundary show a preferential degradation of the C37:3 compared to the C37:2 compound. Because of the high initial Uk'37 values and the way of calculating the Uk'37 this degradation hardly influences the Uk'37 profiles. However, for lower Uk'37 values, measured selective degradation would increase Uk'37 up to 0.17 units, equivalent to 5°C. For most of the Uk'37 band-width, much smaller degradation already increases Uk'37 beyond the analytical error (0.017 units). Consequently, for interpreting the Uk'37 record in terms of past sea surface temperatures, selective degradation needs serious consideration.

Description: Analyses of Re, Os, and Ir concentrations, as well as Os-isotopic compositions, are reported for a suite of sediments from Ocean Drilling Program Site 959. These samples vary in age from late Neogene to Late Cretaceous, and represent a range in depositional oxidation-reduction conditions from suboxic in the Neogene to anoxic in the Late Cretaceous. Age assignments based on shipboard biostratigraphic data are used to calculate initial 187Os/186Os ratios of Neogene nannofossil/foraminifer oozes and Eocene to upper Oligocene laminated diatomites. These calculated initial ratios are in general agreement with published data constraining the Os-isotopic evolution of seawater through time, indicating that the Os-isotopic composition of these sediments is controlled largely by the Os isotopic composition of contemporaneous seawater. Results from analyses of Upper Cretaceous to lower Paleocene claystones do not exhibit elevated Ir concentrations and exhibit Re-Os systematics that are highly consistent with closed-system production of 187Os by in situ 187Re decay. Scatter in both the Cretaceous and Cenozoic data sets is likely the result of the influence of nonhydrogenous Os, carried by clastics, on the bulk sediment Os-isotopic composition, or post-depositional mobility of Re and/or Os.

Description: Stable oxygen isotope data from four holes drilled at the Ocean Drilling Program Site 967, which is located on the lower northern slope of the Eratosthenes Seamount, provide a continuous record of Eastern Mediterranean surface-water conditions during the last 3.2 Ma. A high-resolution stratigraphy for the Pliocene-Pleistocene sequence was established by using a combination of astronomical calibration of sedimentary cycles, nannofossil stratigraphy, and stable oxygen isotope fluctuations. Sapropels and color cycles are present throughout the last 3.2 Ma at Site 967, and their ages, as determined by calibration against the precessional component of the astronomical record, are consistent with those estimated for the sapropels of the classical land-based marine sequences of the Punta Piccola, San Nicola, Singa, and Vrica sections (southern Italy). The Site 967 oxygen isotope record shows large amplitude fluctuations mainly caused by variations in surface water salinity throughout the entire period. Spectral analysis shows that fluctuations in the d18O record were predominantly influenced by orbital obliquity and precessional forcing from 3.2 to 1 Ma, and all main orbital frequencies characterize the d18O record for the last million years. The start of sapropel formation at 3.2 Ma indicates a possible link between sapropel formation and the build up of northern hemisphere ice sheets. The dominance of the obliquity cycle in the interval from 3.2-1 Ma further points to the sensitivity of Eastern Mediterranean climate to the fluctuations in the volume of Arctic ice sheets. An intensification of negative isotope anomalies at Site 967, relative to the open ocean, supports a link between high run-off (during warm periods) and sapropel formation. freshwater input would have inhibited deep-water formation, which led to stagnation of deeper waters. Comparison with the land sections also confirms that differential preservation and diagenesis play a key role in sapropel occurrence.

Description: A simple, untuned "constant sedimentation rate" timescale developed using three radiometric age constraints and eleven d18O records longer than 0.8 Myr provides strong support for the validity of the SPECMAP timescale of the late Quaternary (Imbrie et al., 1984). In particular, the present study independently confirms the link between major deglaciations (terminations) and increases in northern hemisphere summer radiation at high latitudes and shows that this correlation is not an artifact of orbital tuning. In addition, the excess ice characteristic of late Quaternary "100-kyr" climate cycles typically accumulates when July insolation at 65°N has been unusually low for more than a full precessional cycle, or 〉21 kyr, and once established does not last beyond the next increase in summer insolation. Thus, the timing of the growth and decay of large 100-kyr ice sheets, as depicted in the deep sea d18O record, is strongly (and semipredictably) influenced by eccentricity through its modulation of the orbital precession component of northern hemisphere summer insolation.

Description: Rocks of the lower sheeted dike complex of Hole 504B sampled during Leg 140 were analyzed for major and trace element compositions to investigate the effects of igneous processes and hydrothermal alteration on the compositions of the rocks. The rocks are relatively uniform in composition and similar to the shallower dikes. They are moderately evolved mid-ocean-ridge basalts (MORB) with relatively high MgO (7.9-10 wt%) and Mg# (0.60-0.70), and have unusually low incompatible element contents (TiO2 = 0.42-1.1 wt%, Zr = 23-62 ppm). Discrete compositional intervals in the hole reflect varying degrees of differentiation, and olivine and plagioclase accumulation in the rocks, and may be related to injection of packets of dikes having similar compositions. Systematic depletions of total REE, Zr, Y, TiO2, and P2O5 in centimeter-size patches are most likely attributed to exclusion of highly differentiated, late-stage interstitial liquids from small portions of the rocks. The rocks exhibit increased H2O+ reflecting hydrothermal alteration. Replacement of primary plagioclase by albite and oligoclase led to local gains of Na2O, losses of CaO, and slightly positive Eu anomalies. Some mobility of P2O5 led to minor increases and decreases in P2O5 contents, and some local mobility of Ti may have occurred during alteration of titanomagnetite to titanite. Higher temperatures of alteration in the lower sheeted dikes led to breakdown of pyroxene and sulfide minerals and losses of Zn, Cu, and S to hydrothermal fluids. Later addition of anhydrite to the rocks in microfractures and replacing plagioclase caused local increases in sulfur contents. The lower sheeted dikes are a major source of metals to hydrothermal fluids for the formation of metal sulfide deposits on and within the seafloor.

Description: Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504B have been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aphyric basalts to moderately phyric (2%-10% phenocrysts) diabase that are typically multiply saturated with plagioclase, olivine, and clinopyroxene, in order of relative abundance. All analyzed samples are classified as Group D compositions with moderate to slightly elevated compatible elements (MgÆ-value = 0.65% ± 0.03%; Al2O3 = 15.5% ± 0.8%; CaO = 13.0% ± 0.3%; Ni = 114 ± 29 ppm), and unusually depleted levels of moderate to highly incompatible elements (Nb 〈 1 ppm; Zr = 44 ± 7 ppm; Rb 〈 0.5 ppm; Ba ~ 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal ocean ridge basaltic mantle source followed by extensive fractionation of olivine, plagioclase, and clinopyroxene. Leg 140 aphyric to sparsely phyric (0%-2% phenocrysts) basalts and diabases are compositionally indistinguishable from similarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanics through the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of Costa Rica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot be identified, although compositional patterns reflecting variable amounts of phenocryst addition are apparent at various depths. Below ? 1700 mbsf to the bottom of the Leg 140 section, there is a broadly systematic pattern of Zn depletion with depth, the result of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for the hydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located in the transition between pillow lavas and sheeted dikes. Localized zones of intense alteration (60%-95% recrystallization) are present on a centimeter to meter scale in many lithologic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted compared with "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primary igneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystallization, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alteration zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids of mid-ocean ridges.

Description: Geochemical data from plagioclase-hosted silicate melt inclusions from Leg 140, Hole 504B diabase dikes are reported. Hand-picked plagioclase grains were heated to 1260°-1280°C to remelt the glass inclusions and to infer trapping temperatures. The samples were then polished to expose the inclusions, which were analyzed by electron and ion microprobes. Inclusion compositions are mainly in equilibrium with the host plagioclase and are more depleted in incompatible elements than the host rock. Simple crystal-liquid equilibrium calculations show that the melt inclusions could have been in equilibrium with depleted abyssal peridotite diopsides, whereas whole-rock basalt compositions generally could not have been. The melt inclusions are significantly more depleted than normal (N-type) mid-ocean-ridge basalt (MORB) and are consistent with being produced by 8%-16% incremental or open-system melting with 2% residual porosity in the peridotite source. These magmas were formed during pressure-release melting of the mantle over a range of depths between 30 and 15 km.