Mark Baskaran

Handbook of environmental isotope geochemistry

Section-A: Foundational Chapters.- Volume-I: Section-B: Applications of Isotopes in Aquatic Systems.- Volume-I: Section-C: Applications of Isotopes in Groundwater System.- Volume-II: Section-D: Applications of Isotopes in Exploration Geochemistry.- Volume-II: Section-E: Applications of Isotopes in Atmospheric Studies.- Volume-II: Section-F: Applications to Weathering and Geomorphological Studies.- Volume-II: Section-G: Applications of Isotopes to Archaeology, Anthropology and Human Health.- Additional Chapters/Topics for consideration.- Applications of Isotopes to Climatic Research.

Atmospheric depositional fluxes of 7Be and 210Pb at Galveston and College Station, Texas

The bulk depositional fluxes of 210Pb and 7Be were measured at a coastal (Galveston) and an inland (College Station) station for about 3 years, between 1989 and 1991. The annual depositional fluxes of 7Be and 210Pb at Galveston during this period varied by a factor of about 2.5, between 8.9 and 23.2 disintegrations per minute (dpm) cm−2 yr−1, with a mean of 14.7 dpm cm−2 yr−1 for 7Be, and 0.67 and 1.71 dpm cm−2 yr−1, with a mean of 1.03 dpm cm−2 yr−1 for 210Pb, respectively. The precipitation-normalized 7Be flux increases with increasing amount of precipitation. There is no systematic and consistent seasonal trend in the depositional fluxes for 7Be or for 210Pb. The volume-weighted 210Pb concentrations, when normalized to the amount of precipitation, seem to be constant over the time period of this study. Four to six heavy rain events (> 5 cm) in a single day account for 20–30% of the annual deposition of 7Be and 210Pb. Such events account, however, for only about 4–6% of the total number of rainy days in a year. The dry depositional fluxes of these nuclides appear to be a significant fraction of the bulk depositional flux only during the months when there is very little rain. The fraction of dry to total depositional flux of 210Pb appears to be higher than that of 7Be. The strong positive correlation between 7Be and 210Pb depositional fluxes indicates that the flux of both nuclides is controlled by scavenging processes by local precipitation. This correlation also indicates that a major portion of the air masses that brings precipitation to Galveston and College Station is of continental origin. Our data therefore suggest that 7Be and 210Pb cannot be used as independent atmospheric tracers in our coastal station. This observation is consistent with those observed at many other continental and coastal stations.

The transport of U- and Th-series nuclides in sandy confined aquifers

Abundances of ^(238)U, ^(234)U, ^(232)Th, ^(226)Ra, ^(228)Ra, ^(224)Ra, and ^(222)Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that ^(232)Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of ^(232)Th and ^(230)Th on aquifer grains. Uranium in the aquifer waters has very high [^(234)U/^(238)U] ∼ 9 and low ^(238)U concentrations. These levels can be explained by low weathering rates in the aquifer (w_(238U) ∼ 2 × 10^(−18) to 2 × 10^(−17)s^(−1)) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The ^(222)Rn and ^(228)Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [^(234)U/^(238)U] and high ^(238)U concentrations. The flow model shows that it is not possible to obtain the high [^(234)U/^(238)U] and low [^(238)U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.

A search for the seasonal variability on the depositional fluxes of 7Be and 210Pb

Investigations of the atmospheric fallout of 7Be and 210Pb in many places around the globe indicate that there are seasonal variations on the depositional fluxes of these nuclides: high in winter in a few places, while in most places the high occurs in spring. However, these earlier studies did not address quantitatively the corresponding seasonal variations of the amount of precipitation during these periods. Data on the depositional fluxes of 7Be and 210Pb in Galveston, Texas, during the years 1989–1991 indicate that the seasonal variation is not uniform from year to year, and the amount of precipitation primarily controls the depositional fluxes. All the recently published data on the depositional fluxes of these radionuclides to date are synthesized. Also, the seasonal variations of the depositional fluxes of 7Be and 210Pb due to the variations in the amounts of precipitation during these seasons are discussed. This study clearly shows that the depositional fluxes of 7Be and 210Pb during fall were minimum, and most of the time the fallout maxima were confined to either spring and/or summer. Interseasonal variations of the depositional fluxes of these nuclides do not remain the same. The data presented in this paper in conjunction with earlier published data seem to indicate that the observed seasonal increase in the depositional fluxes of 7Be during spring is not due to the troposphere-stratosphere exchange of air masses during that season. The 7Be/ 210Pb activity ratio values remain the highest in summer and this is attributed to the increased rate of vertical transport of 7Be (from the upper troposphere to the middle and lower troposphere) and 210Pb (from the lower troposphere to upper troposphere) due to decreased stability of the troposphere during summer months.

Transport of U- and Th-series nuclides in a Baltic Shield watershed and the Baltic Sea

The transport of Th, Ra, and ^(210)Pb from a continental source region and through an estuarine environment was investigated. Unlike previous studies, here both short- and long-lived nuclide data were obtained for river inputs, river water, and estuarine waters. The mire-rich Kalix River drainage basin was chosen as a typical northern shield area because this river may represent typical waters flowing into the Arctic and northern seas. Groundwaters from bedrock and glacial tills have comparable Th isotope concentrations and do not exhibit significant Th isotopic shifts relative to host rocks. The extensive peat deposits of the basin receive groundwater discharges and concentrate Th and U (but not Ba and Ra), which cause high ^(230)Th/^(232)Th ratios in mire waters. However, mire outflows do not have a significant impact on Th and Ra isotopic compositions of the river. Overall weathering characteristics for the basin are obtained from the river data. The ^(230)Th/^(232)Th, ^(228)Ra/^(226)Ra, and ^(226)Ra/Ba river ratios are comparable to those of source rocks, consistent with similar release rates of these nuclides from U-, Th-, and Ba-bearing minerals. River ratios of (^(230)Th/^(238)U)_(AR) and (^(226)Ra/^(238)U)_(AR) are <1, so that Th and possibly Ra are accumulating in the weathering regions, and the weathering profile is still evolving. Low (^(228)Ra/^(232)Th)_(AR) and (^(226)Ra/^(230)Th)^(AR) ratios indicate that Th is preferentially retained over Ra. River (^(234)Th/^(238)U)_(AR) ratios are greater than (^(230)Th/^(238)U)_(AR) ratios and suggest that in systems where river inputs are well characterized, these ratios can be used to calculate Th transit times through the watershed. Filtration data indicate that although a dominant fraction of the Th is transported in the river on particles, the rest is almost entirely carried by colloids. The Kalix River discharges into the Baltic Sea. Model calculations for the transport of Th and Ra isotopes in the Baltic Sea show that the high ratios of (^(234)Th/^(238)U)_(AR) found here reflect long Th residence times relative to particle scavenging of ∼50 d. The water column ^(232)Th budget is dominated by eolian inputs. The ^(226)Ra concentrations may be higher than those of water inflows, with ≤60% derived from underlying sediments. A greater fraction of ^(228)Ra is derived from sediments to balance the decay of ^(228)Ra within the water column during the 35-yr residence time of water in the Baltic. The Baltic (^(228)Ra/^(226)Ra)_(AR) ratios, which are relatively constant over a range of salinities, are fortuitously similar to those of the river inflows.

Accumulation of anthropogenic and natural radionuclides in bottom sediments of the Northwest Pacific Ocean

Abstract Activities of anthropogenic radionuclides in bottom sediments of the Northwest-Central and West Caroline basins of the Northwest Pacific Ocean are reported. Down-core distributions of natural radionuclides were also investigated in order to elucidate the sedimentation processes of anthropogenic radionuclides in the seafloor. In the Northwest-Central Basin, the inventories of 239+240Pu and 241Am decrease from south to north while the opposite trend is observed for 137Cs and 90Sr. This latitudinal gradient in the inventories of artificial radionuclides reflects higher fluxes of 239+240Pu and 241Am near the weapons test sites and transport of 137Cs and 90Sr over longer distances, as well as the regional differences in sediment-mixing and accumulation rates. In the West Caroline Basin, relatively low inventories of 239+240Pu and 241Am were observed due to the topographic isolation of deep water. Sediment-mixing rates were calculated using down-core distributions of 239+240Pu, 241Am, 137Cs and 90Sr based on two distinct models (continuous and constant) of tracer input and with a third model with pulsed input during the peak fallout period. The continuous input model yielded mixing coefficients comparable to those derived from excess 210Pb profiles within an order of magnitude.

Problems with the dating of sediment core using excess 210Pb in a freshwater system impacted by large scale watershed changes

Pb-210 dating of freshwater and coastal sediments have been extensively conducted over the past 40 years for historical pollution reconstruction studies, sediment focusing, sediment accumulation and mixing rate determination. In areas where there is large scale disturbance of sediments and the watershed, the vertical profiles of excess (210)Pb ((210)Pbxs) could provide erroneous or less reliable information on sediment accumulation rates. We analyzed one sediment core from Hendrix Lake in southwestern Arkansas for excess (210)Pb and (137)Cs. There is no decrease in excess (210)Pb activity with depth while the (137)Cs profile indicates sharp peak corresponding to 1963 and the (137)Cs penetration depth of (137)Cs corresponds to 1952. The historical data on the accelerated mercury mining during 1931-1944 resulted in large-scale Hg input to this watershed. Using the peak Hg activity as a time marker, the obtained sediment accumulation rates agree well with the (137)Cs-based rates. Four independent evidences (two-marker events based on (137)Cs and two marker events based on Hg mining activity) result in about the same sedimentation rates and thus, we endorse earlier suggestion that (210)Pb profile always needs to be validated with at least one another independent method. We also present a concise discussion on what important factors that can affect the vertical profiles of (210)Pbxs in relatively smaller lakes.

Mobile mud dynamics in the East China Sea elucidated using 210Pb, 137Cs, 7Be, and 234Th as tracers

“Mobile mud” (MM), which has fine grain size distribution (>90% clay + silt, and 2.5 in south inshore indicates that the sediment focusing resulted in the increased mass flux. The residence time of MM is estimated as 3–6 years both by mass balance of MM and 210Pbex in MM.

Meteoric 7Be and 10Be as Process Tracers in the Environment

7Be (T1/2 = 53 days) and 10Be (T1/2 = 1.4 Ma) form via natural cosmogenic reactions in the atmosphere and are delivered to Earth’s surface by wet and dry deposition. The distinct source term and near-constant fallout of these radionuclides onto soils, vegetation, waters, ice, and sediments makes them valuable tracers of a wide range of environmental processes operating over timescales from weeks to millions of years. Beryllium tends to form strong bonds with oxygen atoms, so 7Be and 10Be adsorb rapidly to organic and inorganic solid phases in the terrestrial and marine environment. Thus, cosmogenic isotopes of beryllium can be used to quantify surface age, sediment source, mixing rates, and particle residence and transit times in soils, streams, lakes, and the oceans. A number of caveats exist, however, for the general application of these radionuclides as tracers in the environment, as steady deposition and geochemical immobility are not guaranteed in all systems. Here we synthesize and review scientific literature documenting the deposition and behavior of these nuclides at the Earth’s surface, focusing on current and potential applications for Earth scientists working to quantify terrestrial and marine processes.

Tracking the complete revolution of surface westerlies over Northern Hemisphere using radionuclides emitted from Fukushima

Massive amounts of anthropogenic radionuclides were released from the nuclear reactors located in Fukushima (northeastern Japan) between 12 and 16 March 2011 following the earthquake and tsunami. Ground level air radioactivity was monitored around the globe immediately after the Fukushima accident. This global effort provided a unique opportunity to trace the surface air mass movement at different sites in the Northern Hemisphere. Based on surface air radioactivity measurements around the globe and the air mass backward trajectory analysis of the Fukushima radioactive plume at various places in the Northern Hemisphere by employing the Hybrid Single-Particle Lagrangian Integrated Trajectory model, we show for the first time, that the uninterrupted complete revolution of the mid-latitude Surface Westerlies took place in less than 21 days, with an average zonal velocity of>60 km/h. The position and circulation time scale of Surface Westerlies are of wide interest to a large number of global researchers including meteorologists, atmospheric researchers and global climate modellers.