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1.
Concentrations of lead were measured in a surface transect and at two vertical profile stations (15°N and 20°S) in the Central Pacific. These measurements complement similar measurements made earlier in the North Pacific at 33°N and in the Northwest Atlantic at 34°N [1,2], as well as recent measurements of eolian lead input fluxes near each of these locations [3]. The new transect of surface water concentrations of lead corroborates previous measurements, which decrease from 13 ng/kg at 30°N to 4 ng/kg at 17°S in the Central Pacific [4]. This transect gradient is shown to overlie a similar geographic gradient of subsurface maximum concentrations of lead in the three Pacific vertical profile stations, decreasing from 14 ng/kg at 33°N to 11 ng/kg at 14°N to 2.5 ng/kg at 20°S. Lead concentrations at each of those locations exhibit maxima at 400 m, decreasing concentrations to 2500 m and approximately concentrations of 0.8–1.1 ng/kg below that depth. The subsurface maximum at the northwest Atlantic profile station (36 ng/kg at 34°N) is also congruent with surface water lead concentrations which decrease from 806 ng/kg to 32 ng/kg in an offshore transect from Rhode Island to 34°N, 66°W [5], and the shape of the Atlantic profile is congruent with those in the Pacific. There is a positive correlation between the magnitudes of eolian lead input fluxes and the magnitudes of the upper water maxima in lead concentration profiles at corresponding locations as follows: South Pacific easterlies 3 ng/cm2 yr vs. 2.5 ng/kg; North Pacific easterlies 6 ng/cm2 yr vs. 11 ng/kg; North Pacific westerlies 50 ng/cm2 yr vs. 14 ng/kg; and North Atlantic westerlies 170 ng/cm2 yr vs. 36 ng/kg.This relationship enables one to view the anthropogenic perturbations of the marine lead cycle on a global scale, since the industrial origin of eolian and seawater lead has been established by correlations between geographic patterns of industrial lead emissions to the atmosphere and isotopic ratios of industrial leads [3] and by geographic patterns of Pb/silicate-dust ratios and lead isotopic ratios in ocean surface waters [3–5]. These new data coupled with earlier biogeochemical data indicate that surface water concentrations of lead in the North Pacific and North Atlantic are now conservatively estimated to be 8 to 20-fold greater and those in the South Pacific are 2-fold greater than natural concentrations because of industrial emissions of lead to the atmosphere.  相似文献   

2.
The evolution of interocean carbon isotopic gradients over the last 2.5 m.y. is examined using high-resolution δ13C records from deep sea cores in the Atlantic and Pacific Oceans. Over much of the Northern Hemisphere ice ages, relative reductions in North Atlantic Deep Water production occur during ice maxima. From 2.5 to 1.5 Ma, glacial reductions in NADW are less than those observed in the late Pleistocene. Glacial suppression of NADW intensified after 1.5 Ma, earlier than the transition to larger ice sheets around 0.7 Ma. At a number of times during the Pleistocene, δ13C values at DSDP Site 607 in the North Atlantic were indistinguishable from eastern equatorial Pacific δ13C values from approximately the same depth (ODP Site 677), indicating significant incursions of low δ13C water into the deep North Atlantic. Atlantic/Pacific δ13C values converge during glaciations between 1.13-1.05 m.y., 0.83-0.70 m.y., and 0.46-0.43 m.y. This represents a pseudo-periodicity of approximately 300 kyr which cannot easily be ascribed to global ice volume or orbital forcing. This partial decoupling, at low frequencies, of the δ18O and δ13C signals at Site 607 indicates that variations in North Atlantic deep water circulation cannot be viewed simply as a linear response to ice sheet forcing.  相似文献   

3.
3He/4He ratios in dissolved helium at GEOSECS stations 115, 117, 30, and 120 provide an east-west section across the Mid-Atlantic Ridge at 30°N. Below 1500 m depth, the3He/4He profiles show little structure and have values within a few percent of the atmospheric ratio, indicating that the Mid-Atlantic Ridge is not a significant source of injected3He at this latitude. Mean3He/4He ratios calculated for the deep water at each station show that the3He/4He ratio in the western Atlantic at 30°N is 2–3% higher than in the eastern basin, probably due to mixing between a3He-rich boundary current in the western basin and low-3He deep water to the east.  相似文献   

4.
Primordial neon,helium, and hydrogen in oceanic basalts   总被引：3，自引：0，他引：3  
A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess20Ne; relative to atmospheric neon the20Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The20Ne anomalies are associated with high3He/4He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic21Ne and4He. The21Ne/4He ratios in crustal gases are consistent with calculated values based on G. Wetherill's18O (α,n) reaction; the lack of20Ne enrichment in these gases shows that the mantle20Ne anomalies are not radiogenic.21Ne enrichments in Kilauea neon and “high-3He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower4He/Ne ratios.Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on3He/4He ratios. The He isotopic and concentration measurements on “low-3He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic4He, and may provide a method for dating basalt glasses.Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.  相似文献   

5.
Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ14C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in14C and3H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both14C and3H might be associated with the Antarctic intermediate water.  相似文献   

6.
3He/4He ratios up to 3.5 times the ratio of atmospheric He were found in groundwater samples. The3He enrichment can be attributed to radiogenic3He produced by in-situ beta-decay of3H. This shows that tritiogenic3He is accumulating in confined waters. From tritiogenic3He and3H concentrations, ages of groundwaters can be calculated. Detection of tritiogenic3He gives a tool to trace a tritium contamination which occurred in the past and cannot be assessed only by the3H counting method.  相似文献   

7.
A large data base has recently accumulated on the concentrations of helium isotopes in diamonds mined from various regions. It was noted earlier (Ozima et al. (1985) [1]; Lal et al. (1989) [2]) that the frequency distribution of the4He concentrations is a fairly narrow one, whereas that of3He concentrations is a broad one with no pronounced peaks. The ratios 3He/4He, on the other hand show a broad maximum around 2 Ra (Ra equals atmospheric 3He/4He ratio, = 1.40 × 10−6) with a slow decrease over two orders of magnitude on either side. Does this imply that the diamonds sample a wide variety of helium reservoirs having a range of 3He/4He ratios but somehow attain similar4He concentrations? We propose that in a majority of the diamonds studied,4He is primarily due to implantation of radiogenic alpha particles from the host material after emplacement in the crust, usually kimberlite, and that the concentrations of4He in diamonds often get appreciably altered by this process. Thus the4He trapped in the diamond at the time of its crystallization is usually overwhelmed by the implanted helium and the measured 3He/4He ratios do not generally correspond to any “sources” in the mantle. However, the implanted4He resides in the outer 16 μm of the diamond, and the intrinsic4He and 3He/4He ratios in the diamond can be studied if its outer layers are removed.The wider implications of diamond being the “target” material for nuclear reaction products from the host material are discussed. Radiogenic3He produced in the host material is also implanted in the diamond, but this contribution is small on a gross basis. However, since the depth of implantation of3He is greater than that of4He, some of the very high 3He/4He ratios observed in diamonds could be due to the “implantation” of radiogenic3He. The radiogenic reactions in the host material can also contribute to appreciable21Ne in diamonds.  相似文献   

8.
We have performed systematic analyses of both cosmogenic 3He (3Hec) and cosmogenic 21Ne (21Nec) in ultramafic xenoliths from Central Asia and in a quartz sample from Antarctica. Five xenoliths, which show no or insignificant 21Nec excesses, were used to estimate the initial 4He/3He ratio of 90,470 in the subcontinental lithospheric mantle under the Baikal extension zone. Seven xenoliths show large 21Ne/22Ne anomalies ranging up to 0.204 and 4He/3He down to 31,000, due to the presence of cosmogenic 21Ne and 3He. The (3He/21Ne)c ratio is 1.41 ± 0.22 in the xenoliths and 2.76 in the quartzite. This difference is due to the dependence of the 21Nec production rate on the elemental composition of the target material. We estimated the 3Hec and 21Nec production rates at different locations worldwide and calculated the 3Hec and 21Nec exposure ages. These ages range between 7100 and 28,000 years for the xenoliths, and we determined their relative positions within the volcanic tuff layer. The mean 3Hec and 21Nec exposure ages of the quartz sample are 1.35 ± 0.07 and 2.21 ± 0.12 Ma, respectively. This difference is most probably related to 3Hec diffusive losses from the quartz mineral grains, even at low temperatures, due to the relatively high diffusion coefficient for cosmogenic 3He.  相似文献   

9.
Areal distributions and complete time histories since 1952 are presented of the tritium and90Sr concentrations in North Atlantic surface water. The distributions are based on a compilation of measured North Atlantic surface water tritium concentrations which is part of this paper and includes hitherto unpublished measurements, and on available90Sr compilations. To reconstruct the insufficiently represented early concentrations, a two-box North Atlantic mixing model with tropospheric input is employed, for which the input is specified (on relative scales), and which is fitted to the available surface water observations. This procedure gives a natural tritium concentration of 0.2 TU (±30%), and furthermore suggests that part of the old oceanic tritium and90Sr measurements are high. The fit requires characteristic model mixing times of 2.5 years (exchange with an intermediate-depth reservoir about three times the size of the surface box) and 30 years (loss into the deep ocean), and a tritium/90Sr input ratio of 310 Ci/Ci. The areal distributions and time histories can serve as boundary value data for evaluations of subsurface tritium and90Sr measurements.  相似文献   

10.
Water samples collected at the 21°N hydrothermal site on the East Pacific Rise crest, including Deep-Tow and hydrocast samples collected in 1977 and three hot vent water samples collected recently with the submersible “Alvin”, contain significant additions of3He,4He, and Mn. Although the vent water collections were at least 50-fold diluted with ambient seawater, they are up to 53 times enriched in3He and 7.4 times enriched in4He relative to saturated seawater, with concentrations of total dissolvable manganese (TDM) up to 310 μg/kg.3He and4He covary in the vent samples, with3He/4He about 8 times the atmospheric ratio, reflecting a mantle helium source. In contrast to the helium isotopes the Mn/3He ratio in the vent samples is variable, ranging from 4.3 × 104 up to 1.0 × 105 g/cm3. Profiles of3He/4He and TDM in the water column at 21°N show a sharp maximum ofδ(3He) = 47%and TDM= 0.69 μg/kg, much higher than the average values of 34% and 0.2 μg/kg for the deep water in this region. This spike in3He and Mn occurs at 2400 m depth, 200 m above the level of the 21°N vents, and 100 m higher than any local bathymetry, evidence for upward transport of the hydrothermal discharge via rising plumes of hot vent water. Two of the 21°N Deep-Tow samples associated with small (?0.010°C) temperature anomalies hadδ(3He) = 38%and TDM= 0.28 and 0.58 μg/kg, also slightly elevated relative to background. The Deep-Tow and hydrocast samples have lower Mn/3He ratios than average vent samples due to Mn removal by scavenging. Comparison of vent samples and water column measurements at 21°N indicate that the pure vent water could be detected using3He and Mn even when diluted ～105 times with seawater, confirming that these two tracers are extremely sensitive indicators of submarine hydrothermal activity.  相似文献   

11.
Vertical eddy diffusivities (Kv's) have been estimated at fourteen widely separated locations from fourteen222Rn profiles and two228Ra profiles measured near the ocean floor as part of the Atlantic and Pacific GEOSECS programs. They show an inverse proportionality to the local buoyancy gradient [(g/?)(??pot/?z)] calculated from hydrographic measurements. The negative of the constant of proportionality is the buoyancy flux [?Kv(g/?)(??pot/?z)] which has a mean of ?4 × 10?6 cm2/sec3. Our results suggest that the buoyancy flux varies very little near the ocean floor. Kv's for the interior of the deep Pacific calculated from the relationship Kv = (4 × 10?6cm2/sec3)/[(g/?)(??pot/?z)] agree well with published estimates. Kv's calculated for the pycnocline are one to two orders of magnitude smaller than upper limits estimated from tritium and7Be distributions.Heat fluxes calculated with the model Kv's obtained from the222Rn profiles average 31 μcal cm?2 sec?1 in the Atlantic Ocean and 8 μcal cm?2 sec?1 in the Pacific Ocean.  相似文献   

12.
228Ra concentrations were measured in 12 subsurface water samples collected during August–September 1970 in the North Atlantic at the Second Geosecs Intercalibration Station.228Ra concentrations ranged from 1.5 to 2.1 dpm/100 kg in the water layer between 100 and 600 m depth, above the main thermocline, but decreased with increasing depth to less than 0.5 dpm/100 kg at 1.5 km, below the main thermocline. The two deepest samples, collected below 4 km depth, indicated that the228Ra concentration increased again to at least 1.2 dpm/100 kg as the interface with the sea floor sediments was approached. Above the 4 km level, the228Ra profile was roughly similar to the tritium profile measured by Roether and Münnich.  相似文献   

13.
Helium trapped in the chilled glass rims of Pacific Ocean basalts is highly enriched in 3He; the 3He/4He and 3He/Ne ratios are respectively 10 and 1000 times the atmospheric ratios. We interpret these large enrichments as further evidence that primordial 3He is still present in the interior of the earth. The 3He/4He ratio in basalt glass is the same as the isotope ratio of the “excess helium” in Pacific Ocean deep water, supporting the theory that the atmospheric escape rate of 3He is balanced by a flux of primordial 3He from the mantle.  相似文献   

14.
Detailed quantitative cathodoluminescence (CL) imaging analysis was carried out for radiation-damage halos observed by CL (CL halo) created in natural quartz by implantation of 4 MeV He+ ions. The band of CL halo was approximately 14 μm in width and was constant for any He+ ion dose. The width of the halo is consistent with the theoretical range of 4He ions in quartz. A quantitative response of CL intensity to He+ ion dose was obtained, leading to the application of CL halos to geodosimetrical use. The CL intensity increases exponentially in the luminescent band from the implantation surface to the inside, until it reaches a maximum at 14 μm depth, with a rapid decrease beyond this point. This result is as predicted by Bragg's law, although we find some differences between the CL intensity and the theoretical stopping power.  相似文献   

15.
Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ13C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ13C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ13C problem’. One explanation for this glacial distribution of δ13C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ13C values. We show here, however, that glacial-interglacial δ13C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ13C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ13C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ13C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.  相似文献   

16.
This paper presents the results of226Ra intercalibration measurements made by three groups of investigators (LDGO, USC and SIO) on the seawater samples collected in profile at three Geosecs intercalibration stations. A common radium standard prepared from an NBS bulk standard for the Geosecs program has been adopted by all groups. At Geosecs-I station in the Northeast Pacific, the new226Ra results obtained from reoccupation of the station show that the agreement of the three groups has been significantly improved over the initial comparison made in 1970.At Geosecs-II in the Northwest Atlantic, the initial comparison of the226Ra profiles showed that the USC data were systematically higher than the SIO data by 1 radium unit of 10?14 g/kg. This corresponds to a relative difference of 20% due to the very low radium content of Atlantic waters. The new results obtained from reoccupation of the station show that both the USC and SIO data are consistent with their previous data. Thus, a systematic difference of 1 radium unit still exists. However, the new LDGO profile falls in between those of USC and SIO, lying closer to the SIO profile.At Geosecs-III station in the Southwest Pacific, the226Ra measurements show that the LDGO data are systematically higher than the SIO data by about 10% above 3.4 km depth. Below this depth, both sets of data agree and show a sharp decrease in radium concentration. This radium discontinuity corresponds to the benthic front which is a density discontinuity separating the Deep and Bottom Water in the South Pacific.  相似文献   

17.
Manganese in the North Pacific   总被引：1，自引：0，他引：1  
A quantitative and precise method for determination of dissolved Mn at the nanomole(nmol)/kg level in seawater has been developed and used to study the distribution of Mn in the northeast Pacific. Mn concentrations in the surface mixed layer decrease from 1.0 to 0.6 nmol/kg between the central gyre and the western boundary of the California Current, then increase to values from 2 to 6 nmol/kg near the coastal boundary (in contrast to the distribution of210Pb). Particulate Mn in the surface waters accounts for only about 1% of the total.Vertical distributions of Mn are characterized by surface maxima, minima near 300 m, maxima at mid-depth coinciding with the oxygen minimum and the labile nutrient maxima, and concentrations in Pacific bottom waters of approximately 0.2 nmol/kg. The oceanic distribution of Mn appears to be dominated by external inputs superimposed upon overall scavenging which can lead to Mn maxima in (1) the surface waters due to riverine and atmospheric sources; (2) the deep ocean as a result of hydrothermal injection and/or sediment resuspension; and (3) the oxygen minimum region resulting from in-situ breakdown of organic matter, in-situ MnO2 reduction, and/or advective-diffusive transport of dissolved Mn from anoxic slope sediments.  相似文献   

18.
The3He/4He results of the Atlantic GEOSECS program are interpreted in terms of a simple mixing model which allows the contribution of the3He-rich southern component of deep water to be separated from that of the3He-poor northern component. The anomalies from the mixing line connecting these components reveal a3He-rich zone over the Mid-Atlantic Ridge 16° south of the equator and one against the North American continental margin off New Jersey at about the same elevation. The origin of the latter feature remains a mystery.  相似文献   

19.
We performed a complete noble gas study on eight different josephinites and one oregonite. The 4He/3He ratios range between 100,000 and 330,000 and are probably due to a combination of a MORB He-component from the Josephinite Peridotite massif, where these nickel-iron specimens are found, and either atmospheric He or radiogenic He from the underlying continental or subcontinental basement. The 40Ar/36Ar ratios of 302 to 381 are slightly higher than the ratio of air-argon. The neon, krypton and xenon isotopic ratios are identical to the corresponding air ratios. We cannot confirm large3He and21Ne excesses published earlier. The observed noble gas isotopic signatures are in agreement with a formation of josephinites near the surface. The data do not favour a deep mantle origin or a formation at the mantle-core boundary as proposed before.  相似文献   

20.
A14C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 105 m3/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on14C concentrations at a station in the Venezuelan Basin which average Δ14C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a14C concentration of the Atlantic inflow of Δ14C= ?71%. as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm2/sec. The oxygen consumption, and silica and CO2 regeneration, rates below 2500 m depth are obtained as ?0.18, + 0.08, and + 0.2 μmole kg?1 yr?1, respectively. The CO2 regeneration has but a negligible effect on the14C balance.  相似文献   

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