Reports: ND849560-ND8: The Foraminiferal Signature of Hurricanes: Northern Gulf of Mexico

Stephen J. Culver , East Carolina University

Analysis of ca. 30 cm-long box-cores from the offshore Mississippi delta using radionuclide analysis and X-radiography has revealed sediment event layers formed by Hurricanes Ivan, Katrina, and Rita.  By comparing the nature and provenance of foraminiferal assemblages contained within each known hurricane deposit with assemblages of pre- and post-storm deposits, a better understanding of hurricane transport of sediment in a neritic deltaic environment can be obtained.  Analyzing ca. 2 m-long kasten cores for distinctive foraminiferal hurricane signatures can help determine longer-term hurricane patterns in the northern Gulf of Mexico.

Box, kasten, and multicores as well as ponar surface grab samples were collected in a series of cruises off the Mississippi Delta in 2004, 2005, and 2007, reoccupying some stations in two or all three years.  Eight box and multicores from three of the revisited stations and five surface grab samples were selected from a transect trending southwest from Southwest Pass at about 20 meters water depth to the upper boundary of the Mississippi Canyon at 170 meters water depth.  Two 20 ml foraminiferal subsamples were taken from each storm deposit in the cores, as well as two from the pre- and post- storm sediments, where available.  Kasten cores (approximately 2m in length) were also selected from the 2007 cruise at three revisited stations.  X-radiographs of the kasten cores showed less well-defined hurricane units compared to the box and multicores taken in 2005. The kasten cores were sampled for foraminifera at 10 cm increments to determine if foraminiferal assemblages revealed hurricane units that were undetectable by grain size.

Foraminiferal taxa contained in known hurricane and non-hurricane units have been compared to type and figured specimens in the collections at the Smithsonian Institution, Washington, D.C. and identified to the species level.  ANOVAs were performed on transformed abundances of some common taxa in the box-core samples and they reveal that while the abundances of some species, such as Epistominella vitrea and Textularia earlandi, are significantly different in storm versus non-storm deposits, others, such as Buliminella morgani, show little to no difference.  Preliminary cluster analyses of box core and surface sample foraminiferal assemblages also show complex results. Some cluster groups contain both hurricane and non-hurricane samples, depending on the presence or absence of certain key species. The specific taxa that are significant in distinguishing between storm and non-storm deposits vary between stations; this is likely a function of the well documented depth-related distribution of benthic foraminiferal assemblages in the northern Gulf of Mexico.  Further analyses of box core and surface samples will investigate the provenance of individual storm beds based on foraminiferal distribution data.  Sediments in the kasten cores that were deposited prior to Hurricane Ivan have thus far yielded assemblages rich in foraminifera and dominated by Epistominella vitrea and Uvigerina peregrina. Intervals dominated by key species associated with hurricane event beds, such as T. earlandi, have yet to be recognized. 

The Petroleum Research Fund grant has provided the PI with the opportunity to conduct research in a new area of enquiry. It has facilitated collaboration with the three faculty members who collected the core material that this project is based upon during three NSF-sponsored oceanographic cruises. The opportunity to advise two graduate students (so far) during this project must also be acknowledged. The PRF funding has provided these two Masters students with research assistantships. It has allowed them to conduct foraminiferal research at the National Museum of Natural History, Smithsonian Institution and to have extensive discussions there with the scientific staff. A Senior Scientist in the Department of Paleobiology is serving as their external thesis committee member and is providing copious advice on statistical analysis of foraminiferal data. The students have also leaned from another committee member how to date sediment via radionuclide analysis. They have already had three abstracts accepted on this PRF-funded research, one at a regional Geological Society of America conference and two at a national Geological Society of America conference.

 Three undergraduate students, to date, have also been involved in the research of this project. They have learned how to process samples and how to pick and sort foraminifera. They have also learned about scientific integrity and issues such as sample contamination, communication with their advisor and time management. All of these undergraduates have become graduate students and one of them is working with foraminifera. Three additional graduate students have worked as laboratory technicians and have learned the intricacies of foraminiferal sample processing.

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