Tides, Marsh Tales, and Mud Pies?

As a child my mother told me I would never have a career that kept me playing in the mud…..well she was only partially right. The mud of Texas coastal marshes hold the secrets of times gone past. We are currently starting a study of sediment depositions and elevation levels to understand past and current perturbations from widespread weather events such as hurricanes and long-term trends of climate change.

The Gulf Coast has suffered extensive loss of coastal marshes since human settlement. Where marsh loss has not been directly caused by rising sea level and other anthropogenic activities, it has been caused by a shift from emergent wetland vegetation to mudflats and open water due to salt water intrusion. Sea level rise, subsidence, and anthropogenic hydrological alterations have changed the environmental parameters that historically regulate coastal marsh function, resulting in their degradation. Global climate change additionally poses significant long-term threats to coastal marsh habitats and species that are dependent on those habitats along the Texas coast.

So how do we measure changes in marsh elevations? We measure elevation changes in coastal marsh with a Surface Elevation Table (SET for short).

SET Diagram

SET Diagram

SETs use a stable platform and a rotating horizontal arm to detect changes in elevation of the marsh surface. The arm is a portable mechanical leveling device for measuring relative sediment elevation changes. The arm has a series of pins that are dropped to the soil surface and then provide researchers with accurate and precise measurements (mm resolution) that are always taken in the exact location.

Douglas Head gently moves vegetation in preparation for SET measurement.

Douglas Head gently moves vegetation in preparation for SET measurement.

Over time these measurements can be mapped and then can provide trends of surface change over time.

This work is important because most climate change models are predicting appreciable sea-level rise along the Texas Gulf Coast. NOAA National Water Level Observation Network tide stations indicate that sea-level rise is not occurring at congruent levels along the entirety of the Texas Coast. Sea-level rise estimates from the Port Isabel tide station indicate a sea-level rise of 1.97 mm, well under the global average of 3.1 mm per year. However, tide stations near Galveston, Texas estimate sea-level rise to be occurring at 6.84 mm per year, over twice the global average. Because the rate of sea level rise for the upper Texas Coast is greater than the global average, it likely reflects the additional impact of local land subsidence due to oil and gas activities or ground water extraction. This information will allow the I&M program to improve climate modeling efforts on National Wildlife Refuges to improve species specific management such as those for whooping cranes and mottled ducks. Additionally, it will allow biologists and managers to better plan and carry out future conservation efforts.

Ches Vervaeke (USGS) measures inidvidual pins on SET measuring arm.

Ches Vervaeke (USGS) measures inidvidual pins on SET measuring arm.

In addition to SETs, coastal I&M staff have assisted in the collection of soil cores for vertical profiling of Cesium-137. Cesium occurs in distinct layers due to fall-out from nuclear testing in the 1940s-early 1960’s. In 1963 the Test Ban Treaty outlawed nuclear testing, which now is allowing geologists to use this unique signature within the soil profile to map long-term soil changes, aggradation and subsidence rates in coastal marshes.

Douglas Head and Tom Doyle extrude a soil core for Cesium-137 dating.

Douglas Head and Tom Doyle extrude a soil core for Cesium-137 dating.

We have collected several samples from Chenier National Wildlife Refuges Complex, Texas Mid-Coast Refuges Complex, and Aransas National Wildlife Refuge for analysis.  This project aims to further our understanding of the impacts of sea-level rise, altered hydrology, and subsidence on coastal marshes by establishing a series of SET benchmarks and vegetation monitoring sites on coastal National Wildlife Refuges.  The collected data will be compared to local rates of sea-level rise as observed through the National Water Level Observation Network tide stations.  he data collected for this project should be able to measure minimum detectable changes in aggradation and subsidence that are significantly less than this observed rate of sea-level rise, and will provide valuable site level information regarding the effects of these processes in a variety of marsh types as well as insight to potential impacts on their future condition. Information collected will be used to improve conservation planning and management actions (e.g., restorative efforts, prescribed burning, etc.) within the coastal zone and to improve climate models (e.g., Sea-level Affecting Marshes Model).

A SET will be installed at this location on Brazoria NWR in the Fall.

A SET will be installed at this location on Brazoria NWR in the Fall.

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