K.A. Orlandini | USGS Science

USGS2000

Time-averaged fluxes of lead and fallout radionuclides to sediments in Florida Bay

Recent, unmixed sediments from mud banks of central Florida Bay were dated using 210 Pb/ 226 Ra, and chronologies were verified by comparing sediment lead temporal records with Pb/Ca ratios in annual layers of coral ( Montastrea annularis ) located on the ocean side of the Florida Keys. Dates of sediment lead peaks (1978±2) accord with prior observations of a 6 year lag between the occurrence of maximum atmospheric lead in 1972 and peak coral lead in 1978. Smaller lags of 1–2 years occur between the maximum atmospheric radionuclide fallout and peaks in sediment temporal records of 137 Cs and Pu. Such lags are consequences of system time averaging (STA) in which atmospherically delivered particle-associated constituents accumulate and mix in a (sedimentary?) reservoir before transferring to permanent sediments and coral. STA model calculations, using time-dependent atmospheric inputs, produced optimized profiles in excellent accord with measured sediment 137 Cs, Pu, lead, and coral lead distributions. Derived residence times of these particle tracers (16±1, 15.7±0.7, 19±3, and 16±2 years, respectively) are comparable despite differences in sampling locations, in accumulating media, and in element loading histories and geochemical properties. For a 16 year weighted mean residence time, STA generates the observed 6 year lead peak lag. Evidently, significant levels of nondegradable, particle-associated contaminants can persist in Florida Bay for many decades following elimination of external inputs. Present results, in combination with STA model analysis of previously reported radionuclide profiles, suggest that decade-scale time averaging may occur widely in recent coastal marine sedimentary environments.

Florida↗

USGS1996

Uranium-series disequilibrium, sedimentation, diatom frustules, and paleoclimate change in Lake Baikal

The large volume of water, approximately one-fifth of the total surface fresh water on the planet, contained in Lake Baikal in southeastern Siberia is distinguished by having a relatively high concentration of uranium (ca. 2 n M ), and, together with the surface sediments, an unusually high 234 U 238 U alpha activity ratio of 1.95. About 80% of the input of uranium to the lake, with a 234 U 238 U ratio of 2.0, comes from the Selenga River. Profiles of uranium, as well as the extent of isotopic disequilibrium in a 9 m sediment core collected on Academic Ridge, generally show high values during interglacial periods corresponding to high diatom frustule numbers (DiFr) and biogenic silica (BSi) data that have been reported elsewhere. During glacial periods (low DiFr and BSi), uranium progeny ( 234 U and 230 Th) were in secular equilibrium with low concentrations of their parent 238 U. Radionuclide distributions were interpreted in terms of a quantitative model allowing for adsorption of riverine inputs of uranium onto two classes of sedimenting particles with differing 238 U 232 Th ratios and uranium progeny in secular equilibrium. If the 234 U 238 U activity ratio of adsorbed uranium has remained constant, mean sedimentation rates can be independently estimated as 3.6 ± 0.6 and 3.7 ± 0.9 cm · kyr −1 for the decay of 234 U and in-growth of 230 Th, respectively. These rates are consistent with a mean rate of 3.76 cm · kyr −1 , calculated by optimization of the correspondence between adsorbed 238 U and δ 18 O in dated oceanic sediments. The adsorbed uranium apparently tracks variable river flow during interglacials and is drastically reduced during periods of glaciation. Evidently, uranium has not been significantly redistributed within Baikal sediments over at least the past 250 kyr and is a unique, biologically non-essential, tracer for climate-sensitive processes, which provide their own internal geochronometers, potentially useful for ages up to 1 Myr BP.

Earth and Planetary Science Letters↗