Stephen J. Culver, East Carolina University
Radionuclide analysis and X-radiography of ca. 30 cm-long box-cores from the offshore Mississippi delta has revealed sediment event layers formed by Hurricanes Ivan, Katrina, and Rita. We compared the nature and provenance of foraminiferal assemblages contained within each known hurricane deposit with assemblages from pre- and post-storm deposits to obtain a better understanding of hurricane transport of sediment in this neritic deltaic environment. Analysis of ca. 2 m-long kasten cores indicates the low preservation potential of distinctive foraminiferal hurricane signatures over multi-decadal time scales.
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; several stations were reoccupied in two or all three years. Eight 30 cm 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 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 32 common taxa and hypothesis tests (Scheffé’s method) revealed that 10 of these species differed significantly between hurricane and non-hurricane deposits. While the abundances of most of these species were lower in hurricane versus non-hurricane deposits, the abundances of four species (Ammonia parkinsoniana, Bolivina lowmani, Eggerella advena, and Textularia earlandi) were higher in hurricane deposits. These taxa, as well as very rare marsh taxa (Arenoparrella mexicana, Jadammina macrescens) found only in hurricane deposits indicate some marsh or lagoonal sediment input in addition to locally-resuspended material in the hurricane deposits. Discriminant analysis using the same 32 common taxa as were used in the ANOVAs showed that the foraminiferal assemblages of the hurricane deposits were statistically distinguishable from the non-hurricane deposits. Some of the key species most important for the discrimination of these two groups (which tended to be the less common taxa) occurred only in the hurricane deposits, indicating transport from outside the study area. Other species were found in situ only at the shallow end of the sample transect and occurred only in the hurricane deposits at the deeper end of the transect, thus indicating downslope transport.
Foraminiferal analysis of three 2 m kasten cores, taken in 2007 along the same transect as the 2004, 2005 and 2007 box cores, does not indicate the presence of distinctive hurricane units. This suggests that the foraminiferal signature of hurricanes is destroyed by bioturbation soon after deposition. X-radiography of the sediments confirms this interpretation. However, rare specimens of Textularia earlandi hint at the prior presence of hurricane deposited sedimentary units. Work continues on a kasten core taken from the head of the Mississippi Canyon to investigate whether hurricane-caused downslope transport, recognized in box cores, extends to the canyon.
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 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 learned from another committee member how to date sediment via radionuclide analysis. They have already had six abstracts accepted on this PRF-funded research, one at a regional Geological Society of America conference in 2011, two at a national Geological Society of America conference in 2011, two at a regional Geological Society of America conference in 2012 and one at the national Geological Society of America conference in 2012. One graduate student has successfully defended her research and is currently preparing a manuscript for publication. The other graduate student is expected to defend in early 2013.
Five undergraduate students 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. Three 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.