Award: OCE-2205278

The global burial of biogenic silica (bSi) in river-dominated regions is disproportionately large relative to their oceanic area. In shallow river-plume coastal systems like the northern Gulf of Mexico, bSi burial affects biological processes in the overlying waters by reducing the availability of a critical nutrient (silicon) for diatoms, which are important group of planktonic producers in the food web. Furthermore, chemical transformation of bSi in the sediments can produce CO₂ and consume alkalinity and other elements required for biological organisms; therefore, understanding the factors affecting bSi burial are important. Hurricane Ida moved through the Mississippi Delta and adjacent Louisiana shelf (northern Gulf of Mexico) in August 2021. Our group collected sediment cores at three sites less than two weeks prior to the storm. All sites experienced Category 4 hurricane conditions with maximum sustained winds of 130 knots (67 m/s) and Ida's wind speed was stronger than other recent notable regional hurricanes (e.g. Hurricane Harvey, 2017). Using a multi-proxy approach, this project analyzed how the major storm event altered bSi burial in the following 12 months after the storm.

This is the first study to capture regional dynamics in the sediment Si cycle in response to a tropical cyclone. The storm response was variable among sites but appeared to be related to sediment grain size. At a clay dominated site, there was a decline in bSi and apparent reworking of sediments (increase in grain size proportion) post storm. The decline in bSi was concurrent with a massive pulse of dissolved silicate (DSi) in the porewaters. The magnitude of the integrated porewater DSi pool among the sites samples was significant, a quantity that could support the bSi production in the overlying water column. In the year after the storm, DSi in the porewater continued to decline but never reached the lower levels observed prior to the storm. bSi in deeper sediments increased throughout the year after the initial post-storm decline, although bSi in the surface sediments remained reduced. In a silt dominated site, there was a decrease in the fraction of clay and fine silt in surface sediments and a corresponding decline in bSi. However, unlike the clay dominated site, DSi in the porewater declined. Results suggest that the storm-driven injection of oxygen into the surface sediments stimulated bSi dissolution and potentially enhanced liberation of particle-bound Si from diagenetic products. Thus, tropical cyclones may facilitate water-column diatom production via benthic flux in the short term but bSi sequestration in deeper sediments may increase over time. These results help us understand how element sequestration may be altered regionally in response to storm events.

This project also provided student development opportunities and training. Three graduate students from the PI's laboratory joined the science party to help meet the project objectives; however, each was able to sample sediments and the water column for their own thesis research. Beyond graduate students, the field campaigns provided a platform to train undergraduate interns (e.g. Research Experience for Undergraduate -REU- student) and bring outreach professionals (e.g. Alabama Aquarium Educator) to sea for communicating the project goals to the public. Analysis of the sampled materials during our cruises facilitated the training of the PI's lab members in multiple types of sediment analyses. All students were supported by non-project funding sources, and this work was leveraged to provide professional development opportunities while helping meet the project objectives. Data generated are available for use without restrictions through the Biological & Chemical Oceanography Data Management Office under project 905459.

Last Modified: 10/01/2023
Modified by: Jeffrey W Krause