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Terrestrial sediment export to the coastal ocean is a major driver of long-term climate variability

Summary

Terrestrial sediment export to the coastal ocean is a major driver of long-term climate variability.  Understanding past fluxes recorded in sediment deposits requires a thorough knowledge of sediment sources within a river basin and accurate flux and composition estimates in modern systems, a problem which is impeded by logistical and analytical challenges.  I am investigating these issues, in collaboration with colleagues conducting detailed surveys of sediment transport, in the Fraser River in British Columbia.  A lack of sediment impoundments in this basin makes it an ideal site for source-to-sink studies of sediment provenance.  I have collected depth profiles of suspended sediments during two flow stages.  I propose to analyze these sediments for inorganic radioisotopes (87Sr/86Sr and 143Nd/144Nd) in order to make quantitative estimates of sediment sources throughout the basin.

 My thesis research to this point has focused on the dissolved inorganic geochemistry of the Fraser River.  In particular, I have used radiogenic strontium isotopes (87Sr/86Sr) to identify variability in the sources of weathering products from across the basin, and how the relative proportions of these sources change throughout the year (Voss et al., in press).  I am now working to extend this work to the sedimentary load of the Fraser River.  Sedimentary material across the basin exhibits the same trend as the dissolved load of more radiogenic material deriving from the headwaters versus less radiogenic material from the lower portions of the basin (Fig. 1).

While the isotope composition of sediments from across the basin is informative for characterizing the sources of river sediments, it does not provide a quantitative tool for estimating the average composition of the total sediment flux from the Fraser River to the coastal ocean.  Transport of river sediments is affected by flow dynamics, which cause settling and resuspension in different reaches, as well as vertical segregation of particles of different densities.  Sediments mobilized across the basin may be stored for thousands of years in terrestrial fluvial deposits.  Furthermore, careful sampling is required in order to yield a representative snapshot of sedimentary material being transported at a single place and time.  While I have collected suspended sediments from the Fraser during previous sampling campaigns, more sophisticated techniques were necessary to collect proper depth profiles.

I have made preliminary analyses of other suspended sediment samples from the Fraser.  A previous sampling campaign yielded a 3-point depth profile near the Fraser mouth.  The Sr isotope values of these samples (Fig. 2) show a significant trend from more radiogenic values near the surface to less radiogenic values at depth.  Surface sediments are also finer grained than sediments from near the river bottom.  These trends suggest two possible mechanisms: 1) Sediments carried near the surface contain a greater proportion of material deriving from the distant headwaters of the basin compared to sediments carried near the river bed, and 2) Minerals which are more abundant in fine sediments contain more radiogenic Sr than minerals which partition into coarser grains. 

Finally, from the perspective of geochemical budgets, quantifying this heterogeneity with depth and across hydrologic conditions is critical to characterizing the overall composition of terrestrial material delivered to the coastal ocean.

This year, I had two opportunities to collect such samples.  Colleagues at Simon Fraser University (SFU) and the Water Survey of Canada (WSC) routinely sample depth profiles of suspended sediments in the Fraser River, which they analyze for sediment load and grain size composition.  I joined the SFU/WSC team for two of their sampling excursions this year in order to collect the larger volumes of water required for geochemical analyses of river sediments.  I was able to borrow sampling gear (specially designed water samplers and filtration units) from colleagues in France and at the University of Southern California to rapidly travel to British Columbia and participate in the sampling trips on June 20 and October 3.

During both the June and October sampling expeditions, I collected 5–10 L water samples from 6 depths in the center of the river channel near the mouth of the Fraser.  I brought these samples to Jeremy Venditti’s lab at SFU and filtered them to collect the suspended sediments.  Back at WHOI, I have begun processing them for various geochemical analyses.  The most critical of these are the inorganic radioisotope measurements of 87Sr/86Sr and 143Nd/144Nd.  These two isotope quantities are sensitive source tracers of terrestrial material, which are also commonly studied in coastal margin marine sediments.

My opportunities to collect these samples were fortuitous, however the financial support for this project has come from a patchwork of sources intended for other purposes.  I am requesting funding for one day of analyses on the Neptune multicollector ICPMS at WHOI ($1,470), which will allow me to measure both 87Sr/86Sr and 143Nd/144Nd on my 12 samples.  Funding from the COI will allow me to make the final isotope analyses on these precious samples, thus completing a major component of my thesis research.

References

Voss B. M., Peucker-Ehrenbrink B., Eglinton T. I., Fiske G., Montluçon D. B., LeCroy C., Pal S., Marsh S., Gillies S. L., Janmaat A., Bennett M., Downey B., Fanslau J., Fraser H., Macklam-Harron G., and Martinec M. (in press) Tracing river chemistry in space and time: a high-resolution study of variability in dissolved inorganic constituents of the Fraser River, Canada. Geochim. Cosmochim. Acta. doi:10.1016/j.gca.2013.09.006.

Last updated: December 4, 2013