Subtask 3.2:  Geochemical Study of Northern Gulf Coast Ecosystem Change

Subtask Leader: Peter Swarzenski - USGS Florida Integrated Science Center, St. Petersburg

Subtask 3.2 Aim:

Communities along the northern Gulf of Mexico coastline have experienced unprecedented population growth within last few decades. This image was taken on the Pearl River, Louisiana. Photographer: Brady Couvillion (USGS/NWRC) Enlarge

Communities along the northern Gulf of Mexico coastline have experienced unprecedented population growth within last few decades. Important by-products of such accelerated industrialization and coastal development have been concomitant increase in the supply of contaminants and nutrients that can deleteriously impact estuaries. The exchange of sediments and materials between marsh/wetland environments and adjacent coastal systems have been altered.

Management issues related to coastal development, restoration, and river control must take into account the status of the ecosystem prior to natural and human-induced perturbations. Sediment/water supply and availability, as well as ecosystem health, require knowledge of the complex interaction of coastal wetlands with the adjacent coastal ocean.

The NGOM project requires improved documentation and understanding of the roles of human activities and natural processes over the last 100 years in changing the ecology of estuaries and marsh/wetland exchange with adjacent coastal systems.

Subtask 3.2 Activities:

Basin-level ecosystem change and/or deterioration can be best observed from a historic record preserved in unperturbed, fine-grained, deltaic sediments. We plan to collect representative cores in select coastal depositional environments, to develop 100-year geochronologies using multiple radionuclides (e.g., 7Be, xs210Pb, 137Cs), as geochemical signatures associated with ecosystem change. These include a suite of inorganic (i.e., contaminants and trace elements) and organic (i.e., stable carbon isotopes, sterols, fatty acids, lignin phenols) tracers. This effort will complement and extend similar efforts that, for example, use pollen or foraminifer assemblages to assess similar ecosystem change (see Subtask 3.1)

A tremendous wealth of data and information is already available on the deltaic Mississippi River. In contrast, still very little is known about the smaller fluvial systems of the NGOM, such as the Pearl River that divides lower Louisiana from Mississippi. We will first prepare a detailed literature review that highlights the relevant geology, hydrology and operating biogeochemical processes within these coastal systems. After this initial review, sampling will take place in the Pearl River estuary (spring/summer, FY2007). This effort will be coordinated with Drs. Karen McKee (USGS-National Wetlands Research Center) and Robert Twilley (Louisiana State University), as well as with other U.S. Geological Survey efforts within and external to this Coastal and Marine Geology Program NGOM project.

In addition to organic biomarkers, we will also analyze a full suite of trace elements; some of these trace elements are reverse redox indicators and other proxies for particle-reactive contaminants. We will use multiple radionuclide profiles (e.g., 210-Pb and 234-Th) to measure sedimentary processes on 100-day and 100-year time frames. This information will be used to trace the transport and deposition of particle reactive contaminants. The particle-reactive sedimentary information will be integrated with existing research efforts and physical observation platforms (i.e., Tulane, Louisiana State University, LUMCON, USGS NWC) that are ideally suited to address such exchange.

Briefly, we propose to work within a sophisticated tripod array deployed off a shelf-dominated coastal system (Atchafalaya River) by Dr. Allison's group (Tulane, Louisiana State University, LUMCON), which is examining the fine-grained sediment transport dynamics of this system. Such collaboration presents a very unique and cost-effective strategy to tie wetland exchange processes into ongoing local sediment work that would otherwise be beyond the scope of this project.

Subtask 3.2 Objectives:

The goal of Subtask 3.2 is to reconstruct the recent (100 year) record of ecosystem change within selected NGOM watersheds, and to quantify wetland/coastal system exchange processes over the same timeframe.

Specific objectives of this subtask are to:

• Develop 100 year-long records of ecosystem change from analyses of fine-grained deltaic sediments recovered from selected coastal depositional environments;
• Trace the transport and accumulation of particle-reactive contaminants on 100-day and 100-year time-scales in selected NGOM coastal watersheds as recorded in geochemistry of coastal sediments; and
• Link sediment studies with physical observations from existing instrument arrays deployed in NGOM.