Matthew K. Reuer

Lead isotope analysis of marine carbonates and seawater by multiple collector ICP-MS

Abstract A consistent method for stable lead isotope analysis of marine carbonates and seawater is presented, utilizing multiple collector ICP-MS (MC-ICP-MS). This study presents new observations of the large (0.7% amu −1 ), time-dependent mass bias determined by thallium normalization, including preferential light ion transmission induced by the acceleration potential and plasma interface ( β =−1.3 to 0.9). These experiments show equivalent results for three empirical correction laws, and the previously proposed β Pb / β Tl correction does not improve isotope ratio accuracy under these conditions. External normalization to SRM-981 following the thallium correction provides one simple alternative, and a rationale is provided based on secondary bias effects. With current intensities less than 1.5×10 −12 A, external isotope ratio precision better than 250 ppm for SRM-981 207 Pb/ 206 Pb and 208 Pb/ 206 Pb ratios is observed (2 σ ). From reconstructed lead isotopic variability in the North Atlantic, this instrumental precision results in a signal-to-noise ratio greater than 100. Matrix effects are significant with concomitant calcium in SRM-981 (−280 ppm at 257 μM [Ca]). With the appropriate corrections and minimal concomitants, MC-ICP-MS can reliably determine 207 Pb/ 206 Pb and 208 Pb/ 206 Pb ratios of marine carbonates (30 mg) and seawater (160–200 g).

A mid-twentieth century reduction in tropical upwelling inferred from coralline trace element proxies

Abstract The Cariaco Basin is an important archive of past climate variability given its response to inter- and extratropical climate forcing and the accumulation of annually laminated sediments within an anoxic water column. This study presents high-resolution surface coral trace element records (Montastrea annularis and Siderastrea siderea) from Isla Tortuga, Venezuela, located within the upwelling center of this region. A two-fold reduction in Cd/Ca ratios (3.5–1.7 nmol/mol) is observed from 1946 to 1952 with no concurrent shift in Ba/Ca ratios. This reduction agrees with the hydrographic distribution of dissolved cadmium and barium and their expected response to upwelling. Significant anthropogenic variability is also observed from Pb/Ca analysis, observing three lead maxima since 1920. Kinetic control of trace element ratios is inferred from an interspecies comparison of Cd/Ca and Ba/Ca ratios (consistent with the Sr/Ca kinetic artifact), but these artifacts are smaller than the environmental signal and do not explain the Cd/Ca transition. The trace element records agree with historical climate data and differ from sedimentary faunal abundance records, suggesting a linear response to North Atlantic extratropical forcing cannot account for the observed historical variability in this region.

Anthropogenic lead dynamics in the terrestrial and marine environment

Human activities have greatly altered the natural geochemical cycles of several heavy metals, most notably lead derived from leaded-petrol and metal-smelting emissions. This inadvertent geochemical tracer experiment poses two challenges: understanding how anthropogenic lead affects human health and the environment, and quantifying its time-dependent distribution within terrestrial and marine systems. Accurate assessment of the latter relies on well-constrained historical and modern lead fluxes from proxy records and direct observations, lead source estimates from stable lead isotopes, and transport rate estimates from radionuclides. Numerous studies support the global-scale atmospheric lead fluxes principally derived from anthropogenic activities, the short lead residence time in the atmosphere and surface ocean, and the predominance of North American and European lead emissions. Emerging observations and models are currently addressing the time-dependent evolution of this reactive tracer in the atmosphere and oceans.

Spatial and temporal evolution of lead isotope ratios in the North Atlantic Ocean between 1981 and 1989

29� –68� W). In the early 1980s, 206 Pb/ 207 Pb ratios in the North African Basin averaged 1.193 ± 0.005 (1 s). Similar radiogenic ratios within the level of analytical precision (average 0.29%) were found in the Labrador and Iceland Basins (1.198 ± 0.006) and in the Norwegian Sea (1.196 ± 0.008). These radiogenic mixed layer signatures along with atmospheric global lead emission patterns suggest that most North Atlantic lead in the early 1980s was derived from North American leaded gasoline. Samples in the East Iberian Basin near Portugal and France showed lower 206 Pb/ 207 Pb ratios, between 1.167 and 1.182, indicating a significant influence of less radiogenic atmospheric lead transported from Europe and possibly the influence of the Rio Tinto acid mine drainage very close to shore in the Gulf of Cadiz. [Pb] across the entire North Atlantic Basin ranged between 54 and 145 pmol/kg, with the lowest values (54–74 pmol/kg) found at high latitudes (>65� N). In the late 1980s, surface waters in the western subtropical North Atlantic (North American Basin/ Sargasso Sea, >47� W) and in the eastern subtropical North Atlantic (North African Basin/Central Iberian Basin, 53%). These elemental and isotopic results demonstrate the strongly variable isotopic and elemental signatures of North American and European lead throughout the North Atlantic Ocean, frequently dominated by high 206 Pb/ 207 Pb and [Pb] North