1. PARTICLE ABSORPTION CHANGES ASSOCIATED WITH CDM REPARTITIONING IN SUSPENDED SEDIMENTS
Estapa, Margaret1; Boss, Emmanuel2; Mayer, Lawrence M.1 1School of Marine Sciences, Darling Marine Center, University of Maine, 193 Clark's Cove Rd., Walpole, ME, 04573, US; 2School of Marine Sciences, Aubert Hall, University of Maine, Orono, ME, 04469, US
Background
The organic carbon content of near-coastal suspended sediments is often used to provide information about sediment diagenesis processes occurring on continental shelves. Organic carbon contributes to light absorption by marine “non-algal particles”, so the light absorption properties of coastal mineral-organic aggregates should also vary with their organic matter content.
In this laboratory investigation, we allowed colored dissolved matter (CDM) in Maine streamwater to sorb to suspended Atchafalaya River sediments. By using an ac-9 to separately measure dissolved and particulate absorption, we directly demonstrate that the subsequent increase in the loading of colored material increases spectral light absorption by sediment particles.
Observations of color transfer to sediments
This illustrative example clarifies the idea of color transfer from the dissolved absorption spectrum (top panels) to the particulate absorption spectrum (bottom panels) during overnight mixing (symbolized by red arrows).
The transfer of color from the dissolved to the particulate phase is quantified in the linear relationship between change in dissolved absorption and change in particle absorption at 412 nm. (Robust linear regression, p < 0.05)
Initial specific absorption by sediments, prior to mixing with CDOM. Uncertainty in weight of sediments and volume of suspension, and different sediment dispersal methods, cause observed variation.
Initial dissolved absorption spectra, before the addition of sediments. Red lines represent streamwater collected in May, which absorbed much more strongly than in March (represented by blue lines). Streamwater was diluted with deionized water as indicated in the legend.
Sediment suspensions and optical measurements
We created suspensions of river sediment and measured their optical properties before and after a period of mixing with CDM.
During each experiment, optical properties of stirred suspensions were determined at four stages by gravity-feeding them through a 10-cm pathlength ac-9 (WETLabs). These stages were: 1) on 0.2 μm filtrate immediately prior to sediment addition, 2) the suspension immediately after sediment addition, 3) the suspension after overnight mixing to allow CDM to associate with particles, and 4) on the 0.2 μm filtrate of the suspension after overnight mixing.
Initial particle absorption spectra were calculated by subtracting the dissolved measurement collected in stage 1 from the suspension measurement of stage 2. Final particle absorption spectra were calculated by subtracting the dissolved measurement collected at stage 4 from the suspension measurement of stage 3.
To take home:
The linear relationship between absorption lost from CDM and gained by suspended sediments empirically shows that dissolved chromophores can be transferred to mineral-detrital particles, where they contribute to particle absorption.
The transfer of color between streamwater and riverine sediments has implications for the delivery of terrestrial CDM to the ocean, as well as light-driven biological and photochemical reactions occurring in sediment-bound colored detritus.
Change in dissolved phase (< 0.2 μm) absorption after overnight mixing with suspended sediments. Note that in some cases, deionized water suspensions gained color from suspended sediments, and CDM-only controls exhibited small absorption losses.
Acknowledgements
MLE thanks Lee Karp-Boss and Dave Townsend for generously providing lab space and supplies, Maura Thomas, Jim Loftin, and Linda Schick for assistance with instrumentation and sample preparation, and Wayne Slade for helping to find a running stream in Maine in March. Funding was provided by NSF/Chemical Oceanography and USGS.
Change in specific absorption of suspended sediments at end of experiment. Note that sediments suspended in deionized water lost color to the dissolved phase in some cases.