Metallic Fluence Monitors and J-value standards

Utilizing the neutron-irradiation parameter J is one of the major uncertainties in 40Ar/39Ar dating. The associated error of the individual J-value for a sample of unknown age depends on the accuracy of the age of the geological standards, the fast-neutron fluence distribution in the reactor and the distances between standards and samples during irradiation. While it is generally assumed that rotating irradiation evens out radial neutron fluence gradients, we observed axial and radial variations of the J-values in sample irradiations in the rotating channels of two reactors. To quantify them, we included three-dimensionally distributed metallic fast- (Ni) and thermal- (Co) neutron fluence monitors in three irradiations and geological age standards in three more. Two irradiations were carried out under Cd-shielding in the FRG1 reactor in Geesthacht, Germany, and four without Cd-shielding in the LVR-15 reactor in Rez, Czech Republic. The 58Ni(nf,p)58Co activation reaction and ?-spectrometry of the 811 keV peak associated with the subsequent decay of 58Co to 58Fe allow to calculate the fast-neutron fluence. The fast-neutron fluences at known positions in the irradiation container correlate with the J-values determined by mass-spectrometric 40Ar/39Ar measurements of the geological age standards. Ra-dial neutron fluence gradients are up to 1.8 %/cm in FRG1 and up to 2.2 %/cm in LVR-15; the corre-sponding axial gradients are up to 5.9 and 2.1 %/cm. We conclude that sample rotation might not al-ways suffice to meet the needs of high-precision dating and gradient monitoring can be crucial.

Age determination of sediment core SO75-26KL, Portuguese margin

A benthic isotope record has been measured for core SO75-26KL from the upper Portuguese margin (1099 m water depth) to monitor the response of thermohaline overturn in the North Atlantic during Heinrich events. Evaluating benthic delta18O in TS diagrams in conjunction with equilibrium deltac fractionation implies that advection of Mediterranean outflow water (MOW) to the upper Portuguese margin was significantly reduced during the last glacial (< 15% compared to 30% today). The benthic isotope record along core SO75-26KL therefore primarily monitors variability of glacial North Atlantic conveyor circulation. The 14C-accelerator mass spectrometry ages of 13.54±.07 and 20.46±.12 ka for two ice-rafted detritus (IRD) layers in the upper core section and an interpolated age of 36.1 ka for a third IRD layer deeper in the core are in the range of published 14C ages for Heinrich events H1, H2, and H4. Marked depletion of benthic delta13C by 0.7-1.1 per mil during the Heinrich events suggests reduced thermohaline overturn in the North Atlantic during these events. Close similarity between meltwater patterns (inferred from planktonic delta18O) at Site 609 and ventilation patterns (inferred from benthic delta13C) in core SO75-26KL implies coupling between thermohaline overturn and surface forcing, as is also suggested by ocean circulation models. Benthic delta13C starts to decrease 1.5-2.5 kyr before Heinrich events Hl and H4, fully increased values are reached 1.5-3 kyr after the events, indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor's full strength well after the events. Benthic delta13C changes in the course of the Heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation, a distinct feature of thermohaline instability in numerical models. Inferrred gradual spin-up of thermohaline circulation after Hl and H4 is in contrast to abrupt wanning in the North Atlantic region that is indicated by sudden increases in Greenland ice core delta18O and in marine faunal records from the northern North Atlantic. From this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the Greenland ice core record.

Geochemisty at DSDP Hole 62-465A

Very significant enhancements of the element iridium have been observed in association with the Cretaceous/ Tertiary boundary in marine sediments laid down 65 m.y. ago and subsequently uplifted above the ocean's surface. If our hypothesis for the origin of the iridium and the cause of the Cretaceous/Tertiary life extinctions (the asteroid-impact theory) (Alvarez et al., 1980) is correct, the Ir anomaly should be associated with the Cretaceous/ Tertiary boundary region wherever it is intact. The present work was undertaken to search for the Ir anomaly in a deep-sea-drilling core, in order to check this aspect of the asteroid-impact theory.

Stable isotope record of benthic and planktonic foraminifera from ODP Site 175-1087 in the southern Cape Basin, Atlantic Ocean

High-resolution planktonic and benthic stable isotope records from Ocean Drilling Program Site 1087 off southeast Africa provide the basis for a detailed study of glacial-interglacial (G-IG) cycles during the last 500 k.y. This site is located in the Southern Cape Basin at the boundary of the coastal upwelling of Benguela and close to the gateway between the South Atlantic and the Indian Oceans. It therefore monitors variations of the hydrological fronts associated with the upwelling system and the Atlantic-Indian Ocean interconnections, in relation to global climate change. The coldest period of the last 500 k.y. corresponds to marine isotope Stage (MIS) 12, when surface water temperature was 4°C lower than during the last glacial maximum (LGM) as recorded by the surface-dwelling foraminifer Globigerinoides ruber. The warmest periods occurred during MISs 5 and 11, a situation slightly different to that observed at Site 704, which is close to the Polar Front Zone, where there is no significant difference between the interglacial stages for the past 450 k.y., except the long period of warmth during MIS 11. The planktonic and benthic carbon isotope records do not follow the G-IG cycles but show large oscillations related to major changes in the productivity regime. The largest positive 13C excursion between 260 and 425 ka coincides with the global mid-Brunhes event of carbonate productivity. The oxygen and carbon isotopic gradients between surface and deep waters display long-term changes superimposed on rapid and high-frequency fluctuations that do not follow the regular G-IG pattern; these gradients indicate modifications of the temperature, salinity, and productivity gradients due to changes in the thermocline depth, the position of the hydrological fronts, and the strength of the Benguela Current.

Chemical element concentrations in sediments from DSDP Holes 60-458 and 60-459

The development of laser ablation-inductively coupled plasma-mass spectrometry has revolutionized the analysis of tephras by providing (1) an efficient and precise method for determining abundances of a wide variety of trace elements at low concentrations in individual glass shards and (2) assessment of geochemical heterogeneities within individual ash horizons. This development is important for petrogenetic studies of intraoceanic arc systems, where tephras provide the most complete temporal record of magmatism. Results from the Izu-Bonin and Mariana arc systems indicate that despite close geographical proximity and similar tectonic evolution, they contrast strongly in terms of geochemical evolution since 35 Ma. Whereas the Mariana tephras have exceptional compositional diversity, ranging from low-K (Oligocene), to high-K (Miocene), and subsequently medium-K compositions (Pliocene-Quaternary), the Izu-Bonin arc has been dominated by low-K compositions throughout. The Mariana increases in K are paralleled by increases in abundances of incompatible trace elements and by increased values of diagnostic ratios (e.g., Nb/yb and Th/yb) regarded as monitors of potential mantle-source fertility. The relative uniformity of Nb/yb and Nb/Zr ratios in Izu-Bonin tephras indicates that cyclic processes of backarc basin development and mantle depletion do not necessarily induce large-scale temporal geochemical variations in the associated arc. Temporal variability within the Mariana arc, and its divergence from the Izu-Bonin arc ca. 13 Ma, can be traced to a major injection of subducted sediment in the Mariana system at this time.

Grain characteristics and XRD raw data of ODP Sites 162-981 and 162-984

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.

Pleistocene stable isotope record of planktonic foraminifera of ODP Site 138-847

We present Pleistocene oxygen and carbon isotope records from two planktonic foraminifer species (Globigerinoides sacculifer and Neogloboquadrina dutertrei) from Ocean Drilling Program Site 847 (0°16'N, 95°19'W; 3334 m water depth). An average sample resolution of 4500 yr was obtained by sampling at an interval of 15 cm through a continuous 35-m section from 0 to 1.15 Ma. Our d18O-based chronology is similar to that derived independently by astronomically tuning the gamma-ray attenuation porosity evaluator (GRAPE) record (Shackleton et al., 1995), though offsets as large as ± 30 k.y. occur on occasion. The surface waters at eastern equatorial Pacific Site 847, 380 km west of the Galapagos, are characterized by strong and constant upwelling, elevated nutrient concentrations, and high productivity. The isotopic composition of G. sacculifer (300-355 µm) reflects conditions in the thin-surface mixed layer, and the composition of N. dutertrei (355-425 µm) monitors the subsurface waters of the permanent shallow (10-40 m) thermocline. The Pleistocene d18O difference (N. dutertrei minus G. sacculifer, Dd18Od-s) averages 0.9 per mil and ranges from 0 per mil to 1.7 per mil. Neglecting species effects and shell size, the average Pleistocene d13C difference (G. sacculifer minus N. dutertrei, Dd13Cs-d) is 0.0 per mil and ranges from -0.5 per mil to 0.5 per mil. The Dd18Od-s and Dd13Cs-d records are used to infer vertical contrasts in upper ocean water temperature and nutrient concentration, though d13C may also be influenced by other factors, such as CO2 gas exchange. Variations in the isotopic differences are often synchronous with glacial/interglacial climate change. Glacial periods are characterized by smaller vertical contrasts in both temperature and nutrient concentration, and by notably greater accumulation rates of N. dutertrei and CaCO3. We attribute these responses to greater upwelling at the equatorial divergence. Superimposed on the glacial/interglacial Dd18Od-s pattern is a long-term trend possibly associated with the advection of Peru Current waters. The temporal fluctuations in the isotopic contrasts are strikingly similar to those observed at Site 851 (Ravelo and Shackleton, this volume), suggesting that the inferred changes in thermal and chemical profiles occurred over a broad region in the equatorial Pacific.

Biogenic opal and carbonate concentration in late Neogene sediments of ODP Site 114-704

A preliminary composite depth section was generated for Site 704 by splicing Holes 704A and 704B together over the interval 0-350 mbsf (0-9 m.y.). High-resolution carbonate and opal data from the cores were correlated with the calcium and silicon signals from the GST logging run in Hole 704B to identify missing and disturbed intervals in the cores. Paleomagnetic and biostratigraphic age boundaries were then transferred to the composite depth records to obtain an age model, and sedimentation rates were calculated by linear interpolation between datums. Algorithms relating measured dry-bulk density to carbonate content and depth were generated to produce predicted values of density for every sample. Accumulation rates of bulk, carbonate, opal, and terrigenous sediment components were then computed to generate a record of sediment deposition on the Meteor Rise that has a resolution of better than 200,000 yr for the period from 8.6 to 1.0 m.y. From 8.6 to 2.5 m.y., bulk-accumulation rates on the Meteor Rise averaged less than 2 g/cm**2/1000 yr and were dominated by carbonate deposition. The first significant opal deposition (6.0 m.y.) punctuated a brief (less than 0.6 Ma) approach of the Polar Front Zone (PFZ) northward that heralded a period of increasing severity of periodic carbonate dissolution events (terrigenous maxima) that abruptly terminated at 4.8 m.y. (base of the Thvera Subchron), synchronous with the reflooding of the Mediterranean after the Messinian salinity crisis. From 4.8 to 2.5 m.y., carbonate again dominated deposition, and the PFZ was far south except during brief northward excursions bracketing 4.2-3.9, 3.3-2.9, and 2.8-2.7 m.y. At 2.5 m.y., all components of bulk-accumulation rates increased dramatically (up to 15 g/cm2/1000 yr), and by 2.4 m.y., a pattern of alternating, high-amplitude carbonate and opal cyclicity marked the initiation of rapid glacial to interglaci·l swings in the position of the PFZ, synchronous with the "onset" of major Northern Hemisphere glaciation. Both mass-accumulation rates and the amplitude of the cycles decreased by about 2 m.y., but opal accumulation rates remained high up through the base of the Jaramillo (0.98 m.y.). From 1.9 to 1 m.y., the record is characterized by moderate amplitude fluctuations in carbonate and opal. This record of opal accumulation rates is interpreted as a long-term "Polar Front Indicator" that monitors the advance and retreat of the opal-rich PFZ northward (southward) toward (away from) the Meteor Rise in the subantarctic sector of the South Atlantic Ocean. The timing of PFZ migrations in the subantarctic South Atlantic Ocean is remarkably similar to Pliocene-Pleistocene climate records deduced from benthic oxygen isotope records in the North Atlantic Ocean (Raymo et al., 1989, doi:10.1029/PA004i004p00413; Ruddiman et al., 1989, doi:10.1029/PA004i004p00353). These include northward migrations during "cold" intervals containing strong glacial isotope stages (2.4-2.3, 2.1-2.0, 1.95-1.55, 1.45-1.30 m.y. and at about 1.13 and 1.09 m.y.) and southward migrations during "warm" intervals containing weak glacial and/or strong interglacial stages (2.45-2.40, 2.30-2.10, 2.00-1.95, 1.52-1.45, 1.30-1.18, 1.11, and 1.06-0.93 m.y.). Although our preliminary composite record is not continuous (some stages are obviously missing), there is hope that future work will identify these missing intervals in the as yet incomplete Hole 704B and will extend this high-resolution Southern Hemisphere climate record back to 8.6 m.y.

Major and trace element abundances at DSDP Hole 72-516F

Anomalously high values of Ir have been detected in association with the Cretaceous/Tertiary boundary in about a dozen marine or continental sections laid down about 65 Ma in many different parts of the world (Alvarez et al., 1981). One possible exception is the Cretaceous/Tertiary boundary region in DSDP Site 356, in the South Atlantic off the coast of South America. In a detailed study of that section, the maximum Ir abundance was about 0.2 ppb, much lower than the 3-90 ppb found in most of the other marine locations (authors' unpublished data on the Cretaceous/Tertiary region of Site 356, 1982). A study of Hole 516F, which is not too far from Site 356, could show whether the very low (or missing) Cretaceous/Tertiary Ir anomaly at Site 356 is characteristic of this region of the South Atlantic.

Changes in the vegetation and trade winds in equatorial Northwest Africa 140,000-70,000 yr B.P., as deduced from two marine pollen records

The deep-sea cores M 16415-2 and M 16416-2 at about 9°N off Sierra Leone were analysed palynologically for the time interval 140,000-70,000 yr B.P. Results were presented in absolute (pollen concentration and pollen influx) and relative diagrams (pollen percentage). In a previous study it was evidenced that in northwest Africa pollen is mainly transported to the Atlantic by wind, so that the efficiency of aeolian pollen transport (pollen flux) could be used to evaluate changes in the intensity of the northeast trade winds. The glacial episodes (represented by the oxygen isotope stages 6 and 4) are characterized by strong northeast trade winds, whereas the last interglacial (stage 5) is characterized by weak trade winds. The pollen influx diagram shows that the intensity of the trade winds increased slightly during the relatively cool intervals of stage 5 (viz. 5.4 and 5.2). Tropical forest had maximally expanded around 124,000 yr B.P. (stage 5.5), around 98,000 yr B.P. (transition of stage 5.3 to 5.2), and around 70,000 yr B.P. (first part of stage 4): an increasing delay of the response of tropical forest to global intervals with maximum temperature is apparent during the last interglacial. As tropical forests need continuous humidity, the record of tropical forest monitors changes in climatic humidity south of the Sahara. During the last interglacial, the southern boundary of the Sahara shifted only little: expansions and contractions of the tropical forest area are correlated with contra-oscillations of the grass-dominated savanna zone. Great latitudinal shifts of the desert savanna boundary, on the contrary, occurred during the penultimate glacial interglacial transition (around 128,000 yr B.P.) to the north, and during the last interglacial-glacial transition (around 65,000 yr B.P.) to the south.

Benthic foraminiferal assemblages in ODP Hole 167-1017E

A strong oxygen-minimum zone (OMZ) currently exists along the California margin because of a combination of high surface-water productivity and poor intermediate-water ventilation. However, the strength of this OMZ may have been sensitive to late Quaternary ocean-circulation and productivity changes along the margin. Although sediment-lamination strength has been used to trace ocean-oxygenation changes in the past, oxygen levels on the open margin are not sufficiently low for laminations to form. In these regions, benthic foraminifera are highly sensitive monitors of OMZ strength, and their fossil assemblages can be used to reconstruct past fluctuations. Benthic foraminiferal assemblages from Ocean Drilling Program Site 1017, off Point Conception, exhibit major and rapid faunal oscillations in response to late Quaternary millennial-scale climate change (Dansgaard-Oeschger cycles) on the open central California margin. These faunal oscillations can be correlated to and are apparently synchronous with those reported from Santa Barbara Basin. Together they represent major fluctuations in the strength of the OMZ which were intimately associated with global climate change-weakening, perhaps disappearing, during cool periods and strengthening during warm periods. These rapid, major OMZ strength fluctuations were apparently widespread on the Northeast Pacific margin and must have influenced the evolution of margin biota and altered biogeochemical cycles with potential feedbacks to global climate change.

Impact of atmospheric pCO2, seawater temperature, and calcification rate on the delta 18O and delta 13C composition of echinoid calcite (Echinometra viridis)

The tropical echinoid Echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20°C and 30°C to mimic winter and summer temperatures and at carbon dioxide (CO2) partial pressures of approximately 487 ppm-v and 805 ppm-v to simulate current and predicted-end-of-century levels. Spine material produced during the experimental period and dissolved inorganic carbon (DIC) of the corresponding culture solutions were then analyzed for stable oxygen (delta 18Oe, delta 18ODIC) and carbon (The tropical echinoid Echinometra viridis was reared in controlled laboratory experiments at temperatures of approximately 20°C and 30°C to mimic winter and summer temperatures and at carbon dioxide (CO2) partial pressures of approximately 487 ppm-v and 805 ppm-v to simulate current and predicted-end-of-century levels. Spine material produced during the experimental period and dissolved inorganic carbon (DIC) of the corresponding culture solutions were then analyzed for stable oxygen (delta18Oe, delta18ODIC) and carbon (delta13Ce, delta13CDIC) isotopic composition. Fractionation of oxygen stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (delta18O = delta18Oe - delta18ODIC) was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pCO2. Fractionation of carbon stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (Delta delta13C = delta13Ce - delta13CDIC) was significantly positively correlated with pCO2 and significantly inversely correlated with temperature, with pCO2 functioning as the primary factor and temperature moderating the pCO2-delta13C relationship. Echinoid calcification rate was significantly inversely correlated with both delta18O and delta13C, both within treatments (i.e., pCO2 and temperature fixed) and across treatments (i.e., with effects of pCO2 and temperature controlled for through ANOVA). Therefore, calcification rate and potentially the rate of co-occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines. Study results suggest that echinoid delta18O monitors seawater temperature, but not atmospheric pCO2, and that echinoid delta13C monitors atmospheric pCO2, with temperature moderating this relationship. These findings, coupled with echinoids' long and generally high-quality fossil record, supports prior assertions that fossil echinoid delta18O is a viable archive of paleo-seawater temperature throughout Phanerozoic time, and that delta13C merits further investigation as a potential proxy of paleo-atmospheric pCO2. However, the apparent impact of calcification rate on echinoid delta18O and delta13C suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate.13Ce, delta13CDIC) isotopic composition. Fractionation of oxygen stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (delta18O = delta18Oe - delta18ODIC) was significantly inversely correlated with seawater temperature but not significantly correlated with atmospheric pCO2. Fractionation of carbon stable isotopes between the echinoid spines and DIC of their corresponding culture solutions (delta13C = delta13Ce - delta13CDIC) was significantly positively correlated with pCO2 and significantly inversely correlated with temperature, with pCO2 functioning as the primary factor and temperature moderating the pCO2-delta13C relationship. Echinoid calcification rate was significantly inversely correlated with both delta18O and delta13C, both within treatments (i.e., pCO2 and temperature fixed) and across treatments (i.e., with effects of pCO2 and temperature controlled for through ANOVA). Therefore, calcification rate and potentially the rate of co-occurring dissolution appear to be important drivers of the kinetic isotope effects observed in the echinoid spines. Study results suggest that echinoid delta18O monitors seawater temperature, but not atmospheric pCO2, and that echinoid delta13C monitors atmospheric pCO2, with temperature moderating this relationship. These findings, coupled with echinoids' long and generally high-quality fossil record, supports prior assertions that fossil echinoid delta18O is a viable archive of paleo-seawater temperature throughout Phanerozoic time, and that delta13C merits further investigation as a potential proxy of paleo-atmospheric pCO2. However, the apparent impact of calcification rate on echinoid delta18O and delta13C suggests that paleoceanographic reconstructions derived from these proxies in fossil echinoids could be improved by incorporating the effects of growth rate.

Radiogenic Hf isotopic composition of continental eolian dust in Pacific Ocean sediments

The inorganic silicate fraction extracted from bulk pelagic sediments from the North Pacific Ocean is eolian dust. It monitors the composition of continental crust exposed to erosion in Asia. 176Lu/177Hf ratios of modern dust are subchondritic between 0.011 and 0.016 but slightly elevated with respect to immature sediments. Modern dust samples display a large range in Hf isotopic composition (IC), -4.70 < epsilon-Hf < +16.45, which encompasses that observed for the time series of DSDP cores 885/886 and piston core LL44-GPC3 extending back to the late Cretaceous. Hafnium and neodymium isotopic results are consistent with a dominantly binary mixture of dust contributed from island arc volcanic material and dust from central Asia. The Hf-Nd isotopic correlation for all modern dust samples, epsilon-Hf= =0.78 epsilon-Nd = +5.66 (n =22, R**2 =0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in epsilon-Hf of Asian dust exceeds that predicted on the basis of corresponding epsilon-Nd values (34.76 epsilon-Hf < +2.5; -10.96< epsilon-Nd <-10.1). This is attributed to: (1) the fixing of an important unradiogenic fraction of Hf in zircons, balanced by radiogenic Hf that is mobile in the erosional cycle, (2) the elevated Lu/Hf ratio in chemical sediments which, given time, results in a Hf signature that is radiogenic compared with Hf expected from its corresponding Nd isotopic components, and (3) the possibility that diagenetic resetting of marine sediments may incorporate a significant radiogenic Hf component into diagenetically grown minerals such as illite. Together, these processes may explain the variability and more radiogenic character of Hf isotopes when compared to the Nd isotopic signatures of Asian dust. The Hf-Nd isotope time series of eolian dust are consistent with the results of modern dust except two samples that have extremely radiogenic Hf for their Nd (epsilon-Hf =+8.6 and +10.3, epsilon-Nd =39.5 and 39.8). These data may point to a source contribution of dust unresolved by Nd and Pb isotopes. The Hf IC of eolian dust input to the oceans may be more variable and more radiogenic than previously anticipated. The Hf signature of Pacific seawater, however, has varied little over the past 20 Myr, especially across the drastic increase of eolian dust flux from Asia around 3.5 Ma. Therefore, continental contributions to seawater Hf appear to be riverine rather than eolian. Current predictions regarding the relative proportions of source components to seawater Hf must account for the presence of a variable and radiogenic continental component. Data on the IC and flux of river-dissolved Hf to the oceans are urgently required to better estimate contributions to seawater Hf. This then would permit the use of Hf isotopes as a monitor of past changes in erosion.