Exchange times in the Pacific Equatorial Current system

Tritium data from the GEOSECS eastern and western track stations sampled in the thermocline of the North Equatorial Counter Current (NECC) showed high values, which are indicative of the contribution of the North Equatorial Current (NEC) to the equatorial region. A tritium front at 6°N is evidence of the penetration of northern latitude water into the NECC. The results of a two-? model show no net flux between NEC and NECC and a lower bound estimate of five years for an exchange time scale between the NEC and NECC in the thermocline; this is consistent with a horizontal diffusivity of 5 × 10⁷ cm²/s.     

Tritium and90Sr in North Atlantic surface water

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of measured North Atlantic surface water tritium concentrations which is part of this paper and includes hitherto unpublished measurements, and on available⁹⁰Sr compilations.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.     

Future use of tritium in mapping pre-bomb groundwater volumes

The tritium input to groundwater, represented as volume-weighted mean tritium concentrations in precipitation, has been close to constant in Tucson and Albuquerque since 1992, and the decrease in tritium concentrations at the tail end of the bomb tritium pulse has ceased. To determine the future usefulness of tritium measurements in southwestern North America, volume-weighted mean tritium levels in seasonal aggregate precipitation samples have been gathered from 26 sites. The averages range from 2 to 9 tritium units (TU). Tritium concentrations increase with site latitude, and possibly with distance from the coast and with site altitude, reflecting local ratios of combination of low-tritium moisture advected from the oceans with high-tritium moisture originating near the tropopause. Tritium used alone as a tool for mapping aquifer volumes containing only pre-bomb recharge to groundwater will become ambiguous when the tritium in precipitation at the end of the bomb tritium pulse decays to levels close to the analytical detection limit. At such a time, tritium in precipitation from the last one to two decades of the bomb pulse will become indistinguishable from pre-bomb recharge. The threshold of ambiguity has already arrived in coastal areas with a mean of 2 TU in precipitation and will follow in the next three decades throughout the study region. Where the mean tritium level is near 5 TU, the threshold will occur between 2025 and 2030, given a detection limit of 0.6 TU. Similar thresholds of ambiguity, with different local timing possible, apply globally.     

Residence times in river basins as determined by analysis of long-term tritium records

The US Geological Survey has maintained a network of stations to collect samples for the measurement of tritium concentrations in precipitation and streamflow since the early 1960s. Tritium data from outflow waters of river basins draining 4500–75000 km² are used to determine average residence times of water within the basins. The basins studied are the Colorado River above Cisco, Utah; the Kissimmee River above Lake Okeechobee, Florida; the Mississippi River above Anoka, Minnesota; the Neuse River above Streets Ferry Bridge near Vanceboro, North Carolina; the Potomac River above Point of Rocks, Maryland; the Sacramento River above Sacramento, California; the Susquehanna River above Harrisburg, Pennsylvania. The basins are modeled with the assumption that the outflow in the river comes from two sources—prompt (within-year) runoff from precipitation, and flow from the long-term reservoirs of the basin. Tritium concentration in the outflow water of the basin is dependent on three factors: (1) tritium concentration in runoff from the long-term reservoir, which depends on the residence time for the reservoir and historical tritium concentrations in precipitation; (2) tritium concentrations in precipitation (the within-year runoff component); (3) relative contributions of flow from the long-term and within-year components. Predicted tritium concentrations for the outflow water in the river basins were calculated for different residence times and for different relative contributions from the two reservoirs. A box model was used to calculate tritium concentrations in the long-term reservoir. Calculated values of outflow tritium concentrations for the basin were regressed against the measured data to obtain a slope as close as possible to 1. These regressions assumed an intercept of zero and were carried out for different values of residence time and reservoir contribution to maximize the fit of modeled versus actual data for all the above rivers. The final slopes of the fitted regression lines ranged from 0.95 to 1.01 (correlation coefficient > 0.96) for the basins studied. Values for the residence time of waters within the basins and average relative contributions of the within-year and long-term reservoirs to outflow were obtained. Values for river basin residence times ranged from 2 years for the Kissimmee River basin to 20 years for the Potomac River basin. The residence times indicate the time scale in which the basin responds to anthropogenic inputs. The modeled tritium concentrations for the basins also furnish input data for urban and agricultural settings where these river waters are used.     

Interpretation of groundwater flow patterns through a reconstruction of the tritium precipitation record in the Cochabamba Valley, Bolivia

Regional behaviour of the groundwater flow system in the Cochabamba Valley, Bolivia, is evaluated through the interpretation of tritium (³H) distributions in groundwater samples from wells and springs. In order to interpret groundwater ³H concentrations in Cochabamba Valley, where no historical record of ³H concentrations in rainfall exists, a reconstructed ³H precipitation record is developed. The record of ³H concentrations in precipitation is fairly extensive in the Amazon Basin and this record was extrapolated to the neighbouring Cochabamba Valley. Tritium concentrations in rainfall have been observed to increase under natural conditions with increasing latitude and with increasing distance from the ocean. By considering these trends, a linear relationship for increasing ³H concentration in precipitation is developed, based on data from the Amazon Basin, that realistically predicts regional ³H distributions from the northeast Brazilian coast to Cuzco, Peru. This ³H precipitation record is then extrapolated to the Cochabamba Valley and, after correction for radiogenic decay, is used to interpret trends in groundwater ³H concentrations within the valley.

The groundwater flow system in one of the principal alluvial fans, which serves as an important groundwater resource for the city, is studied in detail. Tritium concentrations drop from approximately 8–10 tritium units (TU) in the recharge area to concentrations below the detection limit of 0.8 TU further out in the valley. Groundwater velocities of approximately 0.3 to 0.9 m d⁻¹ are estimated from distributions of groundwater ³H concentrations along the alluvial fan with the use of the reconstructed precipitation ³H record. Regional characteristics of the groundwater flow system are discussed with respect to future development and protection of the groundwater resources.     

A theoretical investigation of tropospheric ozone and stratospheric-tropospheric exchange processes

Summary Based on the global distribution of various surface types the mean tropospheric residence time of ozone is estimated as a function of latitude. Due to the land-sea distribution varies from 50 days in the northern hemisphere to 190 days in the southern hemisphere. For the stratospheric-tropospheric exchange a sinusoidal variation with season is assumed. The annual variation of tropospheric ozone thus gets a sine function from mean, amplitude and phase of whch the injection function for the particular latitude can be determined.     

BIBLIOGRAPHY / BIBLIOGRAPHIE

Summary

In 1958 the U.S. Geological Survey and the U.S. Weather Bureau jointly conducted a study of tritium fallout distribution over the continental United States, Alaska, and Puerto Rico. The observation period extended from early April to late July in most areas. While this was insufficient to cover the decay side of the curve, it was sufficiently long to establish general trends of tritium activity contours. Tritium activities were found to be maximum in the north central part of the country and minimum in coastal areas. The tritium activities show an inverse correlation with chloride concentrations in precipitation. Tritium activities in precipitation appear to be influenced by several “low altitude effects” particularly in storms of brief duration. Ground level tritium activity measurements are not directly representative of tritium in cloud moisture because of these effects.     

Time dependent thermospheric neutral response tothe 2–11 November 1993 storm period

Many satellite and ground-based observations from 2–11 November 1993 werecombined in the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure toderive realistic time dependent global distributions of the auroral precipitation and ionosphericconvection. These were then used as inputs to the Thermosphere–Ionosphere–ElectrodynamicsGeneral Circulation Model (TIEGCM) to simulate the thermospheric and ionospheric responseduring the storm period. The November 1993 storm was an unusually strong storm associatedwith a recurring high speed stream of solar plasma velocity in the declining phase of the solarcycle. Significant gravity waves with phase speeds of about 700 m/s caused by Joule heating werepresent in the upper thermosphere as perturbations to the neutral temperature and wind fields,especially on 4 November. The observed gravity waves in the meridional wind and in the height ofthe electron density peak at several southern hemisphere stations were generally reproduced bythe model using the AMIE high latitude inputs. Both model and observed equatorward windswere enhanced during the peak of the storm at Millstone Hill and at Australian ionosondestations. The observed neutral temperature at Millstone Hill increased about 400 K during thenight on 4 November, returning to normal on 9 November, while the model increased 300 K thefirst night at that location but was still elevated on 11 November. Enhanced westward windsduring the storm were evident in the UARS WIND Imaging Interferometer (WINDII) data. Theenhanced westward winds in the model were largest around 40–45° magnetic latitude at night,and also tended to be largest in the longitudes containing the magnetic poles. The peak westwardwind enhancements at 0 LT reached about 250 m/s at 300 km, and about 100 m/s at 125 km thefirst day of the storm at 40° magnetic latitude. At 20° magnetic latitude, the maximum westwardwind enhancements at 125 km at 0 LT appeared 2–4 days after the major part of the storm,indicating very long time constants in the lower thermosphere. The model showed global averageneutral temperature enhancements of 188 K after the peak of the storm that decayed with time,and which correlated with variations 8 h earlier in the Dst index and in the electric potential dropinput from AMIE. The global average temperature enhancement of 188 K corresponded to apotential drop increase of only about 105 kV. The results showed that the TIEGCM usingrealistic AMIE auroral forcings were able to reproduce many of the observed time dependentfeatures of this long-lived geomagnetic storm. The overall global average exospheric temperaturevariation correlated well with the time variation of the cross-tail potential drop and the Dst indexduring the storm period. However, the enhanced westward winds at mid-latitudes were stronglyrelated to the corrected Joule heating defined by the time dependent AMIE inputs.     

Overview of tritium records from precipitation and surface waters in Germany

Tritium is one of the most important environmental tracers in isotope hydrology for understanding the dynamics of groundwater and connected surface water and has been used in a wide range of applications at different scales. A key requirement for using tritium as a tracer is the knowledge of its spatial and temporal distribution in different water types. As a fundamental input, quantity long-term time series of tritium in precipitation are of particular importance. In this paper, the authors present an overview of tritium data sets of the Federal Institute of Hydrology (BfG), the Helmholtz Zentrum München (HMGU) and the German Weather Service (DWD). Since the 1970s, all three institutions have monitored the tritium concentration at 53 surface water and 37 precipitation stations on a monthly basis. The primary purpose of the data set was to provide baseline information for different water types all over Germany as an integral part of the German radiation protection monitoring system. Additionally, as geochemically inert tracer, tritium provides a unique tool to different user groups in a wide range of research questions and applications.     

Tritium in the Dead Sea

Tritium data in the Dead Sea for the period 1960–1979 are given. Tritium levels have increased until 1965 in the upper layers of the Dead Sea reaching a level of 170 TU, in response to the atmospheric buildup of tritium from thermonuclear testing. The levels have been decreasing ever since, both because of rapidly declining atmospheric concentrations of tritium and because of mixing of the surface layers with tritium deficient, deeper water masses. The depth of penetration of the tracer delineated the depth of meromictic stratification and successfully monitored the deepening of the pycnocline, until the overturn in 1979 homogenised the entire tritium profile. Modelling the changing tritium inventory over this period showed the predominance of the direct exchange across the air/sea interface, both in the buildup of tritium in the lake and also in its subsequent removal from it. The good fit between calculated and measured tritium inventories confirmed the evaporation estimate of 1.46 m/yr (the mean value for the period) with a precision unattained by other methods.     

Aerosol deposition velocities on the Pacific and Atlantic oceans calculated from7Be measurements

The concentrations of⁷Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface.⁷Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation⁷Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of⁷Be surface water concentrations,⁷Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of⁷Be across the ocean surface was calculated to be 0.027 atoms cm^?2 s^?1 which is probably not significantly greater than the worldwide average⁷Be flux across land and ocean surfaces of 0.022 atoms cm^?2 s^?1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s^?1. This value may be 10–50% too low, since it was calculated using atmospheric⁷Be concentrations which were measured at continental stations. Measurements of atmospheric⁷Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.     

行星际磁场对极尖区位形变化的统计研究

利用Cluster卫星数据,选取2001—2010年期间的616个极尖区穿越事件,研究了行星际磁场（IMF）的大小和方向对极尖区位形的影响.结果表明：当B_z为北向时,随着B_x负向的增大,极尖区的磁不变纬度向高纬方向（极区）移动;当B_z为南向时,随着B_x负向增大,极尖区的磁不变纬度略微向低纬度方向（赤道）移动.B_x正向增大时,极尖区并没有明显移动.B_x对极尖区影响在南半球较为显著,在北半球没有明显规律性变化.此外,随着行星际磁场锥角的增大（>90°）,极尖区也随之向高纬移动.当B_z南向时,随着B_y的负向增大,极尖区在北半球向晨侧移动,在南半球向昏侧移动.而当B_z南向增加时,南北半球两个极尖区的磁不变纬度都朝赤道方向移动;但北向B_z时几乎没有移动.     

The effects of vertical viscosity coefficients with different distribution characteristics on classical Ekman spiral structure

The classical Ekman theory tells us that the ocean surface current turns to the right(left) side of wind direction with 45° in the north(south) hemisphere,but the observation and research results show that the surface current deflexion angle is smaller than 45° in the Arctic and high latitude areas while larger than 45° in the low latitude areas.In order to explain these phenomena,a series of idealized numerical experiments are designed to investigate the influence of vertical viscosity coefficients with different vertical distribution characteristics on the classical and steady Ekman spiral structure.Results show that when the vertical viscosity coefficient decreases with water depth,the surface current deflexion angle is larger than 45°,whereas the angle is smaller than 45° when the vertical viscosity coefficient increases with water depth.So the different observed surface current deflexion angles in low latitude sea areas and the Arctic regions should be attributed to the different vertical distribution characteristics of vertical viscosity coefficients in the upper ocean.The flatness of the Ekman spiral is not equal to one and does not show regular behaviors for the numerical experiments with different distribution of vertical viscosity.However,the magnitudes and directions of volume transport of Ekman spirals are almost the same as the results of classical Ekman theory,i.e.,vertical viscosity coefficient distributions have no effect on the magnitudes and directions of volume transport.     

The tritium and carbon-14 profiles at the Geosecs I (1969) and GOGO I (1971) North Pacific stations

Tritium and¹⁴C data of the “GOGO I” station at 28.5°N, 121.6°W in November 1971 are reported. The tritium decline between 150 and 350 m depth is as pronounced as was observed on a previous occupation of the same position, station “Geosecs I” in September 1969, and the tritium concentrations below 200 m are unchanged.¹⁴C data from the depth range of tritium decline are corrected for fallout¹⁴C contribution. The correcting procedure requires simultaneous measurements of¹⁴C, ΣCO₂, and tritium. It is concluded that the natural absolute¹⁴C concentration attains a maximum near 400 m depth, of 7.5% excess over that of surface water.     

用氚讨论中国西部新疆干旱区的水循环率

Tritium concentrations are used to trace water circulation in the Urumqi and Turfan basins in the Xinjiang, western China. Tritium analyses were made for 77 water samples of river waters, groundwaters, spring waters, lake waters and glacier ice collected in summers in 1992 and 1994. The tritium concentrations in the waters are in a wide range from 0 to 125 TU, most of which are considerably high compared with those of most waters in Japan, because tritium levels in precipitation in the area are over ten times as high as those in Japan. River waters originating in glacier regions contain melt glacier, the proportion of which is over 0.5 to river water. The mean resi-dence time of circulating meteoric water in the mountain regions is estimated to be about 15 years. Most groundwaters and spring waters in the flat regions are mainly derived from river waters originating in glacier regions. The groundwater of greatest tritium concentrations in wells in Urumqi City is derived from Urumqi River about 25 years ago. It takes several ten years for river water to pass the underground to many springs. Some groundwaters and spring waters have taken a long time more than 40 years to travel under the ground. Enrichment of tritium in lake water by evaporation is considered to estimate the contribution of groundwater flow to the recharge of lake. Various contributions of groundwater to lakes are inferred for the various type of salinity in closed or semi-closed lakes. The inflow rates of groundwater to salt lakes are small as against fresh water lakes.     

Paleomagnetic constraints on the tectonic history of the major blocks of China duing the Phanerozoic

Paleomagnetic study of China and its constraints on Asia tectonics has been a hot spot. Some new paleomagnetic data from three major blocks of China. North China Block (NCB), Yangtze Block (YZB) and Tarim Block (TRM) are first reported, and then available published Phanerozoic paleomagnetic poles from these blocks with the goal of placing constraints on the drift history and paleocontinental reconstruction are critically reviewed. It was found that all three major blocks were located at the mid-low latitude in the Southern Hemisphere during the Early Paleozoic. The NCB was probably independent in terms of dynamics. its drift history was dominant by latitudinal placement accompanying rotation in the Early Paleozoic. The YZB was close to Gondwanaland in Cambrian, and separated from Gondwanaland during the Late-Middle Ordovician. The TRM was part of Gondwanaland, and might be close to the YZB and Australia in the Early Paleozoic. Paleomagnetic data show that the TRM was separated from Gondwanaland during the Late-Middle Ordovician, and then drifted northward. The TRM was sutured to Siberia and Kazakstan blocks during the Permian, however, the composite Mongolia-NCB block did not collide with Siberia till Late Jurassic. During Late Permian to Late Triassic, the NCB and YZB were characterized by northern latitudinal placement and rotation on the pivot in the Dabie area. The NCB and YZB collided first in the eastern part where they were located at northern latitude of about 6°—8°, and a triangular oceanic basin remained in the Late Permian. The suturing zone was located at northern latitude of 25° where the two blocks collided at the western part in the Late Triassic. The collision between the two blocks propagated westward after the YZB rotated about 70° relative to the NCB during the Late Permian to Middle Jurassic. Then two blocks were northward drifting (about 5°) together with relative rotating and crust shortening. It was such scissors-like collision procedure that produced intensive compression in the eastern part of suturing zone between the NCB and YZB, in which continental crust subducted into the upper mantle in the Late Permian, and then the ultrahigh-pressure rocks extruded in the Late Triassic. Paleomagnetic data also indicate that three major blocks have been together clockwise rotating about 20° relative to present-day rotation axis since the Late Jurassic. It was proposed that Lahsa Block and India subcontinent successively northward subducted and collided with Eurasia or collision between Pacific/Philippines plates and Eurasia might be responsible for this clockwise rotating of Chinese continent.     

A continuous record of artificial tritium fallout at the South Pole (1954–1978)

Snow samples were taken from a 5-m-deep pit located near the South Pole station in January 1975 and continuous deuterium, tritium and β activity profiles have been obtained from them. These three measurements and the stratigraphic observations allow us to deduce a precise chronology of the pit from 1950 to 1975, providing a continuous record of artificial tritium fallout in the southern hemisphere; this record has been extended to 1978 using samples from a second pit taken this last year. Taking advantage of the unusual 1957–1958 stable isotope content in the snow, we have demonstrated that an important part of the isotopic signal in the precipitation is retained in the snow in spite of the low accumulation rate (8.2 g cm^?2 yr^?1).The first artificial tritium, due to the 1952 Ivy experiments was detected during 1954. A two-year delay between explosions and fallout is well established and allows us to relate the tritium fallout to the main nuclear tests from 1952 to 1960. This delay appears longer for the large 1961–1962 devices. A stratospheric half residence time equal to 20 months is deduced from the fallout decrease occurring after the 1966 peak. For the French southern hemisphere experiments, it is about one year. A sharp tritium decrease is observed after a high 1973 peak, providing a new tritium reference level for future glaciological studies in Antarctica.The β and tritium peaks occur respectively during the Antarctic summer and the Antarctic winter, showing different injection mechanisms. This winter input and the high tritium values registered at the South Pole indicate a preferential tritium transfer over the pole area. Two mechanisms, stratospheric-tropospheric exchange and direct stratospheric cloud precipitation could account for this injection.     

Higher tritium concentrations measured in permafrost thaw lakes in northern Alberta

Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical modelling of floating debris in the world's oceans

A global ocean circulation model is coupled to a Lagrangian particle tracking model to simulate 30 years of input, transport and accumulation of floating debris in the world ocean. Using both terrestrial and maritime inputs, the modelling results clearly show the formation of five accumulation zones in the subtropical latitudes of the major ocean basins. The relative size and concentration of each clearly illustrate the dominance of the accumulation zones in the northern hemisphere, while smaller seas surrounded by densely populated areas are also shown to have a high concentration of floating debris. We also determine the relative contribution of different source regions to the total amount of material in a particular accumulation zone. This study provides a framework for describing the transport, distribution and accumulation of floating marine debris and can be continuously updated and adapted to assess scenarios reflecting changes in the production and disposal of plastic worldwide.