Investigating Influences of Natural and Anthropogenic Lead Sources on Environmental Archives at Multiple Spatial and Temporal Scales: Evidence from Oysters and Ice Cores
Dissertation Abstract: Environmental archives provide records of natural and anthropogenic variation in the environment at different temporal and spatial scales. Lead (Pb) concentrations and isotopic ratios are useful tracers of anthropogenic emissions from industrial activities including mining, smelting, and leaded gasoline use, however, methods of measuring Pb are largely limited in low concentration media. In this study, measurement protocols and methods are developed in two environmental archives – oyster shells and ice – to improve geochemical analyses of Pb. Additionally, oyster fitness is examined to determine best management practices for culturing bivalves under worsening ocean acidification (OA) conditions. As a result of OA, periods of decreased calcite saturation state have increased in duration, intensity, and magnitude, making calcareous organisms such as oysters, vulnerable to changes in carbonate chemistry.
In Chapter 2 condition indices in oyster shells were measured to examine the impacts of culturing practices on oyster fitness to mitigate impacts of worsening OA conditions. Oysters were cultured at varying tidal heights, either on rebar platforms, or on top of shell plantings. Results indicated that growing oysters off bottom without shell plantings produced the most positive influence on oyster fitness. Shell plantings negatively influenced summertime condition index results, but showed higher than predicted shell weights, likely suggesting these cultures produced higher quality shell compared to oysters cultured on rebar platforms. Thus, this work demonstrated that traditional methods of measuring oyster fatness, using condition indices, may be problematic in carbonate poor environments due to their reliance on shell weights, which may be impacted by dissolution of calcium carbonate shell. While laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) offers a low cost, efficient, and powerful tool for direct elemental analysis of solid samples, Pb isotopic analyses are limited in environmental samples as a result of low Pb concentrations, and precision.
To increase signal-to-noise ratios and improve precision on Pb isotopic analyses using LA-Q-ICP-MS, new measurement protocols were established using an Aerosol Rapid Introduction System (ARIS), and collisional focusing. Lead isotopic ratios were measured in homogeneous reference glasses (USGS basaltic BCR-2G and BHVO-2G, and MPI-DING komatiite GOR128-G) and heterogeneous environmental samples (i.e., Pacific oyster shells). Internal precision on Pb isotopic ratios were better in reference materials run at higher count rates (~ 0.5% RSE), compared to reference glass GOR128-G run at lower count rates (~2% RSE). Internal precision and average count rates were similar between environmental samples and GOR128-G run at 25 μm, indicating signal intensity greatly influences precision. Results indicated significant variability within environmental samples, suggesting changes in sources, however, uncertainties (≫ 0.2% RSD) prevented their identification. The study indicated that ARIS and collisional focusing can be used to improve Pb isotopic measurements, but that higher signal intensities (e.g., analysis of samples with higher concentrations) may be required to improve uncertainties and allow source identification and apportionment.
Ice cores provide potentially high-resolution archives of (Pb) pollution; however, due to ultra-low (ppt level) concentrations found in ice cores and limits of instrumentation, studies have struggled to develop a long-term global record of sources of Pb pollution. Lead isotopic ratios were measured using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) from meltwater samples collected from a 250-year (83 m) section of an ice core from Summit Station in Greenland (Summit-2010). Results indicate that using HR-ICP-MS produced results with uncertainties better than those achieved in previous studies using thermal ionization mass spectrometry. Using a statistical fingerprinting model, isotopic Pb pollution sources were quantified and demonstrated changes in emissions from coal mining and combustion, and Pb ore smelting in England, Wales, and Scotland between 1759 and 1817. Significant influence from emissions of Australian Broken Hill Pb ores was observed beginning in the 1890s, likely stemming from Pb ore smelting in Europe. Sources indicated increased influence of Chinese emissions in recent decades following legislation on Pb emissions in European and North American. The 250-year high-resolution reconstruction of Pb isotopes allowed for attribution of Pb sources to Greenland ice with unprecedented detail. These studies demonstrate the effectiveness of using geochemical tools in order to answer questions regarding the manifestation and impact of anthropogenic influence on natural systems. In particular, high-resolution ice core records may inform atmospheric transport models, historical interpretation, and understanding of Pb cycling.