Pacific Centre for Isotopic and Geochemical Research
Department of Earth, Ocean and Atmospheric Sciences,
The University of British Columbia
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Research Spotlight Archives

Pb Isotopes in the Ocean

Applications of the nUBC AttoM
Jason McAlister, Kristin Orians, Dominique Weis

Billions of litres of seawater undulate against our shores, ferry rides to islands penetrating the sea surface are set against the backdrop of a seemingly endless ocean horizon, we enjoy days at the beach, and feast on salmon, halibut, and shellfish. Perhaps you have stared off over the ocean, considered the origins of these waters, pondered the processes sustaining life within the three dimensional structure of our ocean, or contemplated the role of the ocean in climate change. Oceanography explores these issues, and contrary to the iconic Jacques Cousteau diving beneath the ocean, much of our knowledge of the sea results from interpretation of data acquired in the laboratory.

This work conducted at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia in partnership with Nu Instruments (nUBC) employing the Nu AttoM mass spectrometer (Figure 1) reports oceanographic isotopic ratios of lead (Pb) and supports more routine oceanographic study of Pb isotopes based on smaller seawater sample volume requirements. Results identify regional and atmospheric sources of elements important to understanding ocean food webs and processes inherent to climate change.

Fig 1Figure 1: The Nu AttoM located in the nUBC facility of the PCIGR at UBC. Within the seemingly calm white exterior of the Nu AttoM mass spectrometer is a plasma generating temperatures of 10,000oC and an enormous magnet coupled with highly sensitive electronics that steer atoms around a high speed track, all while recording one million results every second.

Seawater contains every naturally occurring chemical element, and while the characteristically salty ocean is dominated by familiar elements comprising table salt, low concentration trace elements such as aluminum (Al), gallium (Ga), iron (Fe), copper (Cu), and lead (Pb) reveal much more regarding biogeochemical processes occurring in the ocean. Lead (Pb) is a particularly interesting element as sources can be determined by distinct isotopic ratios, providing identification analogous to our fingerprints.

Analysis of trace metals in the ocean is a challenging endeavour, accurate measurements have been made for only ~30 years. Large sample volumes (10L) required for early analysis made global studies difficult. Analytical advances have allowed increasingly routine analysis of elements such as Fe and Al utilizing smaller volumes (5 – 25mL) of seawater. However, Pb isotope analysis has required sample volumes up to 100 times larger. Engineering advances present in the Nu AttoM allow utilization of seawater sample volumes in this work of only 15 – 50mL, requiring 0.2-0.5ng Pb (equivalent to only 0.1% the mass of a grain of sand) and produce results that are able to statistically distinguish isotopic ratios of samples that differ by less than 0.1%.

Utilization of the Nu AttoM provided the opportunity to conduct the first oceanographic Pb isotope depth profile transect. Samples were acquired in the Pacific Ocean along ‘Line P’ (Figure 2), one of the most historically important transects in the field of Oceanography, extending 1000km from UBC and Vancouver.

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Figure 2: Map of the north Pacific Ocean showing the location of ocean water samples acquired along ‘Line P’ for analysis of Pb isotopes. Locations of Vancouver, Hawaii, and China are provided as reference.

Results indicate heterogeneous Pb isotopes within the biologically important shallow mixed layer, indicating distinct sources of both water masses and atmospheric deposition (Figure 3). Calculations reveal terrestrial contributions as well as dust sources crossing the Pacific Ocean from Asia, results important to investigating CO2 and carbon cycling in the ocean and vital to investigation of climate change. Opportunities provided by the Nu AttoM further the advancement of Pb isotopic ratios in oceanographic study.

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Figure 3: Depth profile transect of the Pb isotopic ratio 206Pb / 207Pb along Line P in the northeast Pacific Ocean, white circles indicate sampling depths. Relatively high values of 206Pb / 207Pb indicated in red are consistent with Vancouver Island coastal values. Comparatively low 206Pb / 207Pb values in blue are indicative of atmospheric sources originating in Asia.