3He Data Research Articles

CO 2 and 3He in hydrothermal plumes: implications for mid-ocean ridge CO 2 flux

Measurements of pH and total carbon dioxide (ΣCO 2) from Axial Volcano in the 2 years following its eruption and from the Southern East Pacific Rise from 27°S to 32°S are used to demonstrate that decreases in pH in hydrothermal plumes at mid-ocean ridges are primarily caused by the emission of CO 2-rich hydrothermal fluids. As a result, changes in pH can be directly related to the amount of CO 2 added to the hydrothermal plumes. Because hydrothermal plumes integrate the hydrothermal output and chemical signatures from multiple sources in a vent field area, the chemistry in plumes reflects that in fluids being emitted from vent field areas as a whole. We use directly measured CO 2, CO 2 inferred from changes in pH, and 3He data to constrain the ratio of CO 2/ 3He in hydrothermal plumes along extensive segments of the mid-ocean ridge and over time following a volcanic eruption. There are a limited number of CO 2/ 3He values reported for mid-ocean ridge basalts and hydrothermal fluids, and thus the CO 2/ 3He ratios determined here greatly increase their geographic and temporal distribution and demonstrate that this ratio is fairly constant along the mid-ocean ridges (MORs), having a value of ∼2×10 9. These data suggest that a large degree of fractionation between 3He and CO 2 does not occur during magmatic degassing, eruption, and hydrothermal circulation. If a CO 2/ 3He ratio of 2×10 9 is representative of the ratio found in the mantle beneath the MOR, then hydrothermal fluxes of CO 2 can be estimated for the global oceans based on the flux of 3He from the mantle and MORs. These results suggest a MOR CO 2 flux of 0.5–2×10 12 mol year −1, which is consistent with other estimates.

Sources of nitrate contamination and age of water in large karstic springs of Florida

In response to concerns about the steady increase in nitrate concentrations over the past several decades in many of Florida’s first magnitude spring waters (discharge ≥2.8 m3/s), multiple isotopic and other chemical tracers were analyzed in water samples from 12 large springs to assess sources and timescales of nitrate contamination. Nitrate-N concentrations in spring waters ranged from 0.50 to 4.2 mg/L, and δ15N values of nitrate in spring waters ranged from 2.6 to 7.9 per mil. Most δ15N values were below 6 per mil indicating that inorganic fertilizers were the dominant source of nitrogen in these waters. Apparent ages of groundwater discharging from springs ranged from 5 to about 35 years, based on multi-tracer analyses (CFC-12, CFC-113, SF6, 3H/3He) and a piston flow assumption; however, apparent tracer ages generally were not concordant. The most reliable spring-water ages appear to be based on tritium and 3He data, because concentrations of CFCs and SF6 in several spring waters were much higher than would be expected from equilibration with modern atmospheric concentrations. Data for all tracers were most consistent with output curves for exponential and binary mixing models that represent mixtures of water in the Upper Floridan aquifer recharged since the early 1960s. Given that groundwater transit times are on the order of decades and are related to the prolonged input of nitrogen from multiple sources to the aquifer, nitrate could persist in groundwater that flows toward springs for several decades due to slow transport of solutes through the aquifer matrix.

Red Sea deep water circulation and ventilation rate deduced from the 3He and 14C tracer fields

3He is injected in the deep ocean at plate boundaries in relation with hydrothermal activity. Its oceanic distribution, which is at a steady-state, shows appreciable vertical and horizontal gradients. Hence, 3He conservation equations may be inverted to determine the flow field and mixing coefficients within the ocean. Here, we use this technique to investigate the deep circulation of the Red Sea, whose deep thermohaline circulation is comparable to that of a miniature world ocean. 3He data, which are a combination of the Geosecs, Meseda and Merou cruises, are inverted using a linear inverse box model. The present study allows us to draw the details of the internal circulation. The latter is characterized by (i) a descending branch in the northernmost part of the basin, which corresponds to the sinking and subsequent north–south movement of dense surface waters and of additional water entrained by the sinking plume, (ii) by an internal counterclockwise recirculating loop with a northward return flow at intermediate depth. In the literature, the rate and modes of renewal of the Red Sea deep waters are poorly constrained, with bulk residence time estimates ranging from a few decades to a few centuries. In our model, the deep water renewal rate is directly dependent on the magnitude of the 3He sources. The global 3He flux is estimated using two independent approaches. The first method is based on the calculation of the mean 3He transfer flux at the air–sea interface. The second approach relies on recent estimates of the global terrestrial 3He flux. Both methods agree within their respective uncertainties. The circulation scheme defined by the inversion is further constrained by simulating the bomb 14C distribution. Both isotopes, one steady-state tracer ( 3He) and one transient tracer ( 14C), lead to reasonable agreement. The renewal time corresponding to the sinking of newly formed deep water in the northern part of the basin is 60 years. However, the global ventilation rate, deduced from the simulation of the decay rate of a numerical tracer, is much faster (26 years). This result shows, as already pointed out by others, that deep water formation is not the only process by which the deep Red Sea is ventilated. Although this mechanism is important in our model, the basin wide deep circulation coupled with efficient vertical exchange between the deep basin and the upper layers appear to be an even more powerful mode of ventilation.

In-mediumρ0spectral function study via theH2,He3,C12(γ,π+π−)reaction

We report a helicity analysis of sub-threshold rho^0 production on 2H, 3He and 12C at low photo-production energies. The results are indicative of a large longitudinal rho^0 polarization (l=1, m=0) and are consistent with a strong helicity-flip production mechanism. This signature is used to extract in-medium rho^0_L invariant mass distributions for all three nuclei in a manner which is less model-dependent than previous measurements. The results are compared to kinematic and phenomenological models of the rho^0 spectral function. The 2H and 3He data distributions support the role of N*(1520) excitation in shaping the in-medium rho^0_L invariant mass distribution, while the 12C distributions are consistent with quasi-free rho^0_L production. The data support an in-medium modification of the rho^0_L invariant mass distribution.

An additional deep-water mass in Drake Passage as revealed by 3He data

We present 3He data from a repeat section across Drake Passage, from three sections off the South American continent in the Pacific, at 28°S, 35°S, and 43°S, and from three sections in the Atlantic, eastward of the Malvinas, close to 35°W, and near the Greenwich Meridian. In Drake Passage, a distinct high- 3He signal is observed that is centered just above the boundary of the Lower and the Upper Circumpolar Deep Water (LCDW, UCDW), and is concentrated towards the northern continental slope. 3He concentrations in the Antarctic Circumpolar Current (ACC) upstream of Drake Passage (World Ocean Circulation Experiment section P19 at 88°W) are markedly lower than those found in Drake Passage, and a regional source of primordial helium in the path of the ACC that might cause the high- 3He feature can be ruled out. We explain the feature by addition of high- 3He waters present at the 43°S Pacific section. This supports a previous, similar interpretation of a low-salinity anomaly in Drake Passage (Naveira Garabato et al., Deep-Sea Research I 49 (2002) 681), that is strongly related to the high- 3He feature. Employing multiparameter water mass analysis (including 3He as a parameter), we find that deep waters as met at the 43°S Pacific section, flowing south along the South American continental slope, contribute substantially to the ACC waters in Drake Passage (fractions exceed 50% locally). Lesser, but laterally more extended contributions are found east of the Malvinas, and still smaller ones are present at 35°W and at the Greenwich Meridian. Using velocity measurements from one of the two Drake Passage sections, we estimate the volume transport of these waters to be 7.0±1.2 Sv, but the average transport may be somewhat lower as the other realization had a less pronounced signal. The enhanced 3He signature in Drake Passage is essentially confined north of the Polar Front. Further downstream the signature crosses this front, to the extent that at 35°W the contributions south and north of it are of similar magnitude. At the same time, the 3He levels north of the front are reduced due to a substantial admixture of low- 3He North Atlantic Deep Water, such that 3He becomes highest south of the front. The flow of Southeast Pacific deep slope waters entering the ACC constitutes the predominant exit pathway of the primordial helium released in the deep Pacific, and represents a considerable fraction of the deep water return flow from the Pacific into the ACC. Therefore and also because the density range of the added deep slope waters is intermediate between those of UCDW and LCDW, they must be considered a distinct water mass.

Extraterrestrial accretion from the GISP2 ice core

The rate of extraterrestrial accretion for particles in the size range 0.45 μm to ∼20 μm was determined from dust concentrates extracted from Greenland Ice Sheet Project 2 (GISP2) ice core samples. Using instrumental neutron activation analysis (INAA), we determined the iridium (Ir) content of the dust. Following a core-specific correction for terrestrial Ir and assuming a chondritic Ir abundance of 500 ppb, we measure an average accretion rate for 0.45 μm to ∼20 μm particles over the entire Earth of 0.22 (± 0.11) × 10 9 g/yr (kton/yr) for 317 years of ice through the interval 6 to 20 ka. This is consistent with the interplanetary dust accretion rate of 0.17 (± 0.08) x 10 9 g/yr that we derive from published 3He data for the GISP2 core. Accounting for particles that are larger and smaller than those detected by or experiment, our best estimate of the total accretion rate (including particle sizes up to about 4 cm in diameter) is 2.5 × 10 9 g/yr. The uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error. Based on Monte Carlo simulations, we estimate the upper limit for total extraterrestrial accretion to Earth of 6.25 × 10 9 g/yr (95% confidence level). This accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice, estimates from ground-based radar studies, and with accretion estimates of 3He-bearing interplanetary dust particles, assuming that 3He is correlated with particle surface area. It is, however, lower than estimates based on platinum group element studies of marine sediments. The conflict may indicate systematic errors with either the marine or the non-marine samples, departures from the assumed particle spectrum of Grün and coauthors, or time-variable accretion rates, with the early Holocene period being characterized by low levels of accretion.

The geochemistry and mixing of leakage in a semi-confined aquifer at a municipal well field, Memphis, Tennessee, USA

The Memphis aquifer in southwestern Tennessee is confined to a semi-confined unconsolidated sand aquifer and is the primary municipal water source in the Memphis metropolitan area. Past studies have identified regions in the metropolitan area in which the overlying upper Claiborne confining unit lacks significant clay and provides a hydraulic connection between the shallow aquifer and the Memphis aquifer. In this study, major solute chemistry, 3H, and 3H/ 3He groundwater dating are used to investigate the extent and chemical effects of leakage through the confining unit to the Memphis aquifer in the vicinity of a municipal well field. The 3H/ 3He dates and geochemical modeling of the chemical data are used to constrain mixing fractions and the timing of modern recharge. Tritium activities of as much as 2.8 TU are observed in shallow production wells, but deeper production wells have 3H activities that approach the detection limit. Trends in water chemistry indicate vertical mixing in the aquifer of shallow Na–SO 4–Cl-rich water and deeper Ca–Mg–HCO 3-rich water. Water chemistry does not vary consistently with seasonal pumping, but 3H activity generally decreases during low use periods. Stable O and H isotopes show little variation and are not useful groundwater tracers for this study. The 3H-bearing, Na–SO 4–Cl-rich water is interpreted to reflect recharge of modern water through the upper Claiborne confining unit. The 3H/ 3He dates from 5 production wells indicate modern recharge, that infiltrated 15–20 a ago, is present in the shallow production wells. Geologic data and hydrologic boundary conditions suggest that the most likely source for continued leakage is a nearby stream, Nonconnah Creek. Geochemical reaction modeling using the NETPATH computer code suggests that proportions of shallow aquifer water leaking into the Memphis aquifer range from 6 to 32%. The 3H/ 3He dating and NETPATH modeling results correlate well, suggesting that these complementary analytical tools provide an effective means to evaluate proportions of modern water leaking into semi-confined aquifers. These results also indicate a need to carefully consider connections between surface water and semi-confined groundwater resources in wellhead protection programs.

Cosmogenic 36Cl ages of Quaternary basalt flows in the Mojave Desert, California, USA

Basalt flows provide excellent opportunities for calibration and intercomparison of Quaternary dating methods, remote sensing methods, and rates of geomorphic processes. The immediate motivation for this study was to provide chronology for a blind test of the utility of rock varnish microstratigraphy as an indicator of the age of flow emplacement. Five basaltic eruptive centers in the Mojave Desert of California were sampled for cosmogenic 36Cl analysis. Multiple samples were taken from most centers and, with one exception, produced good agreement. Assuming a surficial erosion rate of 1 mm/kyr −1, the flows yielded the following ages: Amboy Crater, 79±5 ka; Pisgah Crater, 22.5±1.3 ka; Cima field, I-Cone, 27±1.3 ka; Cima field, A-Cone, 21±1.6 ka and 11.5±1.5 ka; Cima field, flow of unidentified origin, 46±2 ka. The ages from the Cima I and A cones are in good agreement with previous cosmogenic 3He dating. Ages from the three previously undated flows are significantly older than previous estimates based on flow appearance. Tanzhou Liu performed varnish microstratigraphic analysis on samples collected from the same sites. His results were submitted for publication without knowledge of the 36Cl ages. His age estimates agree well with the 36Cl ages for the three previously undated flows, strongly supporting the validity of varnish microstratigraphy as a chronological correlation tool.

The cosmological density of baryons from observations of 3He+ in the Milky Way.

Primordial nucleosynthesis after the Big Bang can be constrained by the abundances of the light elements and isotopes 2H, 3He, 4He and 7Li (ref. 1). The standard theory of stellar evolution predicts that 3He is also produced by solar-type stars, so its abundance is of interest not only for cosmology, but also for understanding stellar evolution and the chemical evolution of the Galaxy. The 3He abundance in star-forming (H II) regions agrees with the present value for the local interstellar medium, but seems to be incompatible with the stellar production rates inferred from observations of planetary nebulae, which provide a direct test of stellar evolution theory. Here we develop our earlier observations, which, when combined with recent theoretical developments in our understanding of light-element synthesis and destruction in stars, allow us to determine an upper limit for the primordial abundance of 3He relative to hydrogen: 3He/H = (1.1 +/- 0.2) x 10(-5). The primordial density of all baryons determined from the 3He data is in excellent agreement with the densities calculated from other cosmological probes. The previous conflict is resolved because most solar-mass stars do not produce enough 3He to enrich the interstellar medium significantly.

Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach

Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August–September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 ( 3H/ 3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF 6), sulfur-35 ( 35S), and stable isotopes ( δ 18O and δ 2H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF 6 and 3H/ 3He, with most ages in the range of 0–3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35S (>1.5 years) and seasonal variation of δ 18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35S and δ 18O data reflect travel times through the unsaturated zone and, in the case of 35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3H/ 3He data (median age of 0.2 years) are lower than those obtained from SF 6 (median age of 4.3 years), and in contrast to median ages from 3H/ 3He (0.3 years) and SF 6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF 6 apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF 6 data. Water from springs has low excess air (0–1 cm 3 kg −1) and N 2–Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N 2–Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm 3 kg −1 of excess air with ages in the range of 0–25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions.

Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX 96 cruise

During the summer of 1996 the nuclear submarine USS Pogy occupied a line of stations extending through the middle of the Canadian Basin between about 88°N, 44°W (Lomonosov Ridge) and about 78°N, 144°W (center of the Canada Basin). CTD/Niskin bottle casts extending to 1600 m were carried out at eight stations, providing the first high‐quality temperature, salinity, CFC, tritium, and 3He data obtained from this region, although XCTD data had previously been collected in this region. These data, along with data from stations at the basin boundary to the south and west, reveal the presence of well‐ventilated intermediate water beneath the halocline in the center of the Canada Basin, indicating renewal times of the order of 1–2 decades. The least ventilated intermediate water was observed at the northern end of the Canada Basin along the southern flank of the Alpha Ridge. Intermediate water is derived from the Atlantic Ocean and enters the Arctic Ocean through Fram Strait and the Barents Sea. It flows around the Arctic basins in boundary currents and splits in the eastern Amundsen Basin with one branch crossing the Lomonosov Ridge and flowing along the East Siberian continental slope and the other flowing along the Eurasian flank of the Lomonosov Ridge. From the 1996 Scientific Ice Expedition (SCICEX 96) observations we conclude that the branch that flows along the East Siberian continental slope transports this water to the Chukchi Rise, where it apparently enters the central Canada Basin with some flow continuing along the boundary to the southern Canada Basin. The Fram Strait Branch Water mixes extensively with waters from the Canadian Basin during its transit along the East Siberian continental slope, being diluted by a factor of about 5 by the time it reaches the central Canada Basin. The Barents Sea Branch Water does not undergo such extensive mixing and is diluted by a factor of only about 2 when it reaches the central Canada Basin.

Presence of a primary solar-type atmosphere around the earth: evidence of dissolved noble gas

Recent noble gas data of mantle-derived samples show that there are two end members: PLUME-type and MORB-type. The estimated high 3He and 22Ne abundances of the PLUME source, possibly representing the lower mantle, should reflect the remnant of dissolved solar-type atmosphere. Calculations of the structure of the primary atmosphere and the noble gas dissolution into the magma ocean of the accreting planet suggest that the high 3He and 22Ne abundances can be explained if the primary atmosphere persisted until M∼0.4–0.6 M E ( M E being the present Earth mass). The PLUME source has higher 3He/ 4He and lower 21Ne/ 22Ne than the MORB source. This is explained by assuming that the lower mantle was less degassed during magma ocean cooling. The carbon abundance in the mantle can be constrained from the estimated abundance of mantle 3He and C/ 3He data of the present mantle-derived samples. Dissolved solar-type noble gas might explain high noble gas abundance in the present Venus, if the primary atmosphere persisted until the final stage of accretion under lower dust opacity of the atmosphere.

Characterizing a Sewage Plume Using the 3H‐3He Dating Technique

AbstractAn extensive 3 H −3He study was performed to determine detailed characteristics of a regional flow system and a sewage plume over a distance of 4 km in a sand and gravel aquifer at Otis Air Base in Falmouth, Massachusetts. 3 H −3He ages increase with depth in individual piezometer clusters and with distance along flowpaths. However, the age gradient with depth (Δt/Δ z) is smaller in the plume than that in the regional waters, due to the intense recharge in the infiltration beds. The 1960s bomb peak of tritium in precipitation is archived longitudinally along a flowline through the main axis of the plume and vertically in individual piezometer clusters. On the eastern side of the sampling area, where water from Ashumet Pond forces plume water deeper into the flow system, 3 H −3He ages are young at depth because the 3 H −3He “clock” is reset due to outgassing of helium in the pond. A reconstruction of the tritium input functions for the regional and plume samples shows that there is no offset in the peak [3H]+[3Hetrit] concentrations for the plume and regional water, indicating that the water from supply wells for use on the base is young. The 3 H −3He ages and detergent concentrations in individual wells are consistent with the beginning of use of detergents and the time period when their concentrations in sewage would have been greatest. Ages and hydraulic properties calculated using the 3 H −3He data compare well with those from previous investigations and from particle‐tracking simulations.

Isotope studies in the Caspian Sea

Oceanographic and isotopic investigations in the Caspian Sea and the analyses of the available data on the discharge to the sea and the observed sea level changes suggest that climatically caused changes of river inflow are the major cause of the sea level fluctuations over the last century. Hydrogen-3 and 3H– 3He data indicate that the deep basins of the sea are rapidly ventilated, although the hydraulic turnover time of the sea is approximately 200 years. The concentration levels of the anthropogenic radionuclides 90Sr, 137Cs and 239,240Pu in the water column can be explained by global fallout and therefore, at the sampling sites visited, there were no signs of dumping of radioactive wastes. The anthropogenic radionuclide data support the idea of fast exchange of water masses in the Caspian Sea. The isotopic and oceanographic data collected during the cruises have shown potential to allow for a better understanding of the water circulation in the Caspian Sea.

Flow of river water into a karstic limestone aquifer—2. Dating the young fraction in groundwater mixtures in the Upper Floridan aquifer near Valdosta, Georgia

Tritium/helium-3 ( 3H/ 3He) and chlorofluorocarbon (CFCs, CFC–11, CFC–12, CFC–113) data are used to date the young fraction in groundwater mixtures from a karstic limestone aquifer near Valdosta, Georgia, where regional paleowater in the Upper Floridan aquifer receives recharge from two young sources—the flow of Withlacoochee River water through sinkholes in the river bed, and leakage of infiltration water through post-Eocene semi-confining beds above the Upper Floridan aquifer. In dating the young fraction of mixtures using CFCs, it is necessary to reconstruct the CFC concentration that was in the young fraction prior to mixing. The 3H/ 3He age is independent of the extent of dilution with older ( 3H-free and 3He trit-free) water. The groundwater mixtures are designated as Type-1 for mixtures of regional paleowater and regional infiltration water and Type-2 for mixtures containing more than approximately 4% of river water. The fractions of regional paleowater, regional infiltration water, and Withlacoochee River water in the groundwater mixtures were determined from Cl − and δ 18O data for water from the Upper Floridan aquifer at Valdosta, Georgia. The chlorofluorocarbons CFC–11 and CFC–113 are removed by microbial degradation and/or sorption processes in most anaerobic (Type-2) groundwater at Valdosta, but are present in some aerobic Type-1 water. CFC–12 persists in both SO 4-reducing and methanogenic water. The very low detection limits for CFCs (approximately 0.3 pg kg −1) permitted CFC–11 and CFC–12 dating of the fraction of regional infiltration water in Type-1 mixtures, and CFC-12 dating of the river-water fraction in Type-2 mixtures. Overall, approximately 50% of the 85 water samples obtained from the Upper Floridan aquifer have CFC–12-based ages of the young fraction that are consistent with the 3H concentration of the groundwater. Because of uncertainties associated with very low 3H and 3He content in dilute mixtures, 3H/ 3He dating is limited to the river-water fraction in Type-2 mixtures containing more than about 10% river water. Of the 41 water samples measured for 3H/ 3He dating, dilution of 3H and low 3He concentration limited 3H/ 3He dating to 16 mixtures in which 3H/ 3He ages are defined with errors ranging from ±2 to ±7.5 a (1 σ). After correction for dilution with (assumed) CFC-free regional infiltration water and regional paleowater in the Upper Floridan aquifer, adjusted CFC–12 ages agree with 3H/ 3He ages within 5 a or less in 7 of the 9 co-dated Type-2 mixtures. Tritium data and dating based on both CFC–11 and CFC–12 in Type-1 mixtures indicate that travel times of infiltration water through the overlying Post-Eocene semi-confining beds exceed 35 a. The CFC and 3H/ 3He dating indicate that the river fraction in most groundwater entered the groundwater reservoir in the past 20 to 30 a. Few domestic and municipal supply wells sampled intercept water younger than 5 a. Calculated velocities of river water in the Upper Floridan aquifer downgradient of the sinkhole area range from 0.4 to 8.2 m/d. Radiocarbon data indicate that ages of the regional paleowater are on the 10 000-a time scale. An average lag time of approximately 10 to 25 a is determined for discharge of groundwater from the surficial and intermediate aquifers above the Upper Floridan aquifer to the Withlacoochee River.

An eastern Indian Ocean ³He section from Australia to Bali: Evidence for a Deep Pacific‐Indian Throughflow

The distribution of ³He has been measured on an Australia‐Bali transect, across the Pacific‐Indian throughflow region of the Eastern Indian Ocean, in SE and NW monsoonal regimes. Additional samples were also collected directly on the main sills over which water is exchanged between the Indonesian Seas and the Indian Ocean. The ³He data show that the South Banda Sea deep waters, between 800 and 1400 m, leave the Banda basin over the sills in the straits of Sumba, Savu, Roti and Timor and enter the equatorial Indian Ocean. The permanence of the ³He in both opposite monsoonal regimes, and the only weak dilution of the ³He signal between the straits and the Australia‐Bali section hundreds of kilometers apart, suggest that this deep outflow is not negligible.

A comparison of ocean tracer dating techniques on a meridional section in the eastern North Atlantic

Tritium and excess 3He data from a 1988 meridional section in the eastern North Atlantic are presented and related to hydrographic data for the region. The computed tritium - 3He ventilation ages are well correlated over a wide range of water masses with an independent tracer age estimate from a contemporaneous chlorolluorocarbon CFC-12 data set, with residuals only slightly greater than those expected from sampling error alone. The tritium - 3He ages are slightly younger (∼ 1 year) than the CFC-12 ages in recently ventilated water, and the disparity between the two tracer ages increases sharply for ages approaching the elapsed period since the bomb-tritium input in the early 1960s. The overall features of the tracer age-age curve are captured by a simple 2-D gyre circulation model and are related to the timescales of the transient for each tracer and the pre-anthropogenic and primordial backgrounds of 3He. Regions of anomalously low tritium - 3He age relative to the overall section tracer age-age curve are observed near the tropical-subtropical transition and in the deep thermocline in the southern part of the subtropical gyre. These areas contain steep tracer gradients on isopycnal surfaces and elevated non-linear tracer age mixing effects, based on diagnostic calculations.

Accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record

The flux of particulate extraterrestrial (ET) matter to the deep-sea has been calculated using a four-component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the Pacific Ocean. Nineteen published bulk-leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation. The calculated annual particulate flux, averaged over several 100 ka by slow accumulation and bioturbation, ranges from ∼18,000 to ∼67,000 tons, with a mean value of ∼37,000 ± 13,000 tons. The data indicate no significant variability in the flux with time, except at the K-T boundary and are thus compatible with results based on Ir accumulation in deep-sea sediments. The inferred constancy of the ET matter flux is in contrast to recent results based on the ET 3He flux that integrates only a small size fraction of the entire ET matter flux. 3He data indicate variability by a factor of 6 over the past 70 Ma and a fivefold increase in the ET flux between 3 Ma and 1 Ma that is not seen in the bulk ET matter flux based on osmium isotope data.The apparent constancy of the cosmic matter flux over the past 80 Ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record, except at the K-T boundary. A comparison between osmium isotope and Ir data from two pelagic sediment cores from the Pacific (DSDP Site 596 and LL44-GPC3) indicates that the seawater-soluble fraction of ET Os cannot exceed ∼36 kg/a and most likely is significantly smaller. The maximum value of 36 kg/a can account for approximately one half of the seawater-soluble ET matter flux necessary to balance the radiogenic continental runoff of Os without any additional unradiogenic source. Simple mass balance calculations indicate that an additional unradiogenic source of Os to the oceans, most likely alteration of oceanic crust, is required to balance the present-day seawater osmium-isotopic composition. This source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater.

Measurements of quasi-elastic pion single charge exchange on 3He at T π = 245 MeV

Differential cross sections from 3He( π ±, π 0) and 3He( π ±, π 0p) measurements are presented. The π 0 energy spectra from 3He( π ±, π 0) and 3He( π +, π 0p) are characteristic of quasi-free reactions. The proton angular distribution from 3He( π +, π 0p) also show evidence of a quasi-free mechanism. Indications of multi-nucleon processes are presented in the 3He( π −, π 0p) data. At least two nucleons must be involved. Comparisons between coincidence and single-arm measurements from this work and previous measurements of charged pion scattering on 3He provide evidence of an enhanced multiple scattering contribution to the charge exchange channel.

Modes of subsurface, intermediate, and deep water renewal in the Red Sea

A linear box model of the Red Sea is presented. The model, which inverts hydrographic and 3He data for the main features of the width‐integrated circulation, is able to describe a large part of the seasonal and annual mean circulation. By combining and reconciling major elements of current theories of the Red Sea circulation, available flow measurements, and tracer data the model provides a unified conceptual framework for the understanding of the circulation of the Red Sea. The model forms deep water in the extreme north in two modes, as first suggested by Cember (1988). The convective mode renews the bottom water at an annual mean rate of ∼0.04 Sv. In the winter the isopycnal mode water renews the uppermost part of the deep water mass. It flows to the south along the bottom of the pycnocline at an annual mean rate of 0.02–0.04 Sv. A new feature in the model's near‐surface circulation scheme is the ventilation of the thermocline in the winter at a mean rate of ∼0.04 Sv, also from the extreme north. There is also a middepth return flow to the north at an annual mean rate of ∼0.07 Sv. This flow has been suggested but never rigorously quantified before.