Reports: DNI853891-DNI8: Interactions of Fluvial Delta Island Aggradation with Organic Matter Deposition and Coastal Nutrient Export
Kevan B. Moffett, PhD, Washington State University
This research has helped to advance our understanding of the co-evolution of delta island aggradation, organic matter (OM) deposition, and delta nutrient export, which can then fuel adjacent marine productivity and OM deposition. Specific project goals were to: (1) Identify the co-depositional history of organic and clastic contributions to delta island sediments at the Wax Lake Delta, Louisiana, (2) Use remote sensing image analysis to relate historical island and vegetation extents as a step toward spatially-explicit sediment-and-OM deposition modeling, (3) Quantify the hydraulic and nutrient dynamics that support OM production and preservation in delta island sediments and so also improve estimates of the unconsumed nutrients exported to marine systems.
Field work at the young, prograding Wax Lake Delta was completed, consisting of elevation surveys, vegetation mapping, and sediment coring. Sediment cores were analyzed for the grain size distribution, OM content, bulk density, and also depositional age via lead-210 dating. Cores (50-100 cm deep) were collected along a 2500 m long transect along the western levee of Pintail Island, capturing the full range of island elevations and the transition from bare sediment to herbaceous plants and trees. At all but the youngest sampling site, sediment cores showed a significant decrease in OM content and a significant increase in grain size with depth. The total OM contribution to vertical growth was determined to be less significant than the contribution of mineral sedimentation to island accretion and emergence. Mineral accumulation rates suggested that elevation growth was accelerating or holding steady over time, in contrast to deltaic theory suggesting rates should slow as elevation increases. It is possible that sediment compaction in deeper layers of the island is creating accommodation space for continued high-rate mineral sediment deposition. This portion of the study concludes that the young, prograding delta island sediments are notable but not substantial sinks for long-term preservation of OM, in both the dominant Salix nigra and Colocasia esculenta vegetation zones. Instead, the fate of the carbon fixed by the highly productive marsh on the islands is likely dominated by a combination of degradation, remineralization and export.
Landsat imagery of Wax Lake Delta were analyzed to derive maps of decades of historical changes in island topography and flooding frequency. Methods to relate time-lapse island exposure to elevation and flooding were developed for the delta sub-area Pintail Island. A whole-delta analysis then related how island growth (i.e. sediment accretion) and marsh development (i.e. hotspots of OM production and burial) have interacted over recent decades, and variations with position along the delta. The manuscript combining these sediment and remote sensing analyses has been reviewed and was recommended by the journal to be split into two papers; this revision is currently in progress. The work was also presented at a previous American Geophysical Union annual meeting.
Additional field data collection and numerical modeling focusing on the island pore fluids and their carbon and nutrient compositions was conducted. Sediment permeability variations were determined, a fully coupled 3D numerical model of surface and subsurface fluid flow through the delta island sediments was developed, and the first inventory of carbon and dissolved nutrient pools within the deeper sediments of the delta system not accessed by prior studies (i.e., deeper than the top 10 cm) was completed. A significant difference in dissolved organic carbon (DOC) stores between Salix nigra and Colocasia esculenta vegetation zones, as well as in pH and salinity. Coupled water balance and chloride balance modeling demonstrated that the degree of observed salinization of the island sediments was consistent with concentration by evapotranspiration water loss, and at different rates due to the differing leaf area between the two dominant vegetation zones. Numerical flow modeling identified stagnation areas within the island sediments where mobile constituents might be expected to accumulate, and identified multiple zones of contrasting surface-subsurface exchange regimes. The relative abundance of DOC despite predominantly reducing conditions supports the prior hypothesis that degradation may outweigh preservation of OM in the young delta sediments. The numerical model was also useful for simulating the routing of dissolved carbon and nitrogen through the channel-island-lagoon network as coupled to the island porous medium, which is a necessary step to better constrain the mass fluxes of water, salt, nutrients, and dissolved organic carbon across surface-ground and delta-marine interfaces and move toward an improved mass balance of C&N for the delta that accounts for previously neglected accumulations and consumptions within island sediments. The subsurface carbon pool and its relation to fluid exchange between the sediments and island channels are the subjects of manuscripts in preparation, and results were presented at the Geological Society of America 2016 meeting and the Coastal and Estuarine Research Federation meeting in 2017. The integration of sediment, remote sensing, pore fluid, and modeling analyses will be presented at the American Geophysical Union Ocean Sciences meeting in 2018.
Career and Student/Training Impacts:
This grant has had substantial positive impact on the careers of the PI and participating students. Project participants have included: the PI, a female, early-career STEM tenure-track faculty member, initially at the University of Texas at Austin (UT) and now at Washington State University (WSU); and Peter Zamora, a minority postdoctoral scholar, formerly earning a PhD at UT, supported by this grant as a postdoc at WSU, and now a tenure-track faculty member at the University of North Carolina at Wilmington. The professional growth and integrative thinking opportunities this grant afforded the postdoc was instrumental in broadening his research skills and aiding him in landing a tenure-track faculty job to continue his research and teaching contributions on coastal-margin topics. This project has also enabled synergy between the PI, postdoc, and ongoing collaboration with geomorphologists studying petroleum-related topics at coastal margins.
In sum, this grant: supported the development of a minority (Philippine) postdoctoral scholar into a tenure-track faculty position; supported the work of an early-career female PI on research related to the origins of fossil fuel reserves, both deltaic and offshore; 5 conference presentations, and will result in 4-5 related publications in top journals.