Assessing Past and Present Lead and Thorium Inventories and MARs Following the Deepwater Horizon Blowout
- The 2010 Deepwater Horizon (DWH) blowout led to a depositional pulse in the Northern Gulf of Mexico (NGoM) known as the Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) event (Figure 1).
- A time-series (2010-2022) of sediment cores were taken and analyzed to characterize the sedimentary response to the event, post event, and the stabilization and recovery of the time-series sites (Figure 2).
- During the 2022 time-series core collections, excess 210Pbxs and 234Thxs profiles were used to determine sedimentation and accumulation rates of each coring site.
Figure 1: Depiction of depositional pulse due to DWH blowout (Daly et al. 2016). For the purpose of this study only the seafloor and sedimentation component were focused on (D).
Figure 2: Locations of past and current time-series core sites
- Gamma Spectroscopy to measure short-lived radioisotope activities to determine 210Pbxs and 234Thxs
- Constant Rate of Supply model (CRS) to determine Inventory, sum of all activity in core, and Mass Accumulation Rates (MARs) to calculate average MAR of entire profile for 210Pbxs and 234Thxs.
- Compared 234Thxs Inventory and Apparent Mass Accumulation Rates (A-MARs) to determine if bioturbation is present at time-series sites. A-MARs include sedimentation and influence of bioturbation, mixing of the inventory down core.
- Used the concept of comparing 234Thxs Inventory and A-MAR and applied it to the comparison of 201Pbxs Inventory and MAR to determine if there are any apparent changes in the deepening of the profile at the time-series sites over time.
- Compared 210Pbxs MARs to 234Thxs A-MARs in order to determine the difference between sedimentation and accumulation
Figure 3: 210Pbxs activity profiles and Mass Accumulation Rate for time-series site DSH10 in 2022. A) The sum of the activity in the profile gives the 210Pbxs Inventory for the core. B) The average MAR for the core is utilized to compare to the Inventories.
234 Thxs and 210Pbxs Mass Accumulation Rates (AMAR) and Inventories
Figure 4: 2010-2022 time series sites (A) 234Thxs A-MAR, (B) 234Thxs Inventory. (Modified from Larson et al. 2018)
Figure 5: 2010-2022 time series sites (A) 210Pbxs MAR, (B) 210Pbxs Inventory.
- Multicores taken in May 2022 continued to show signs of stabilization after the Deep-Water Horizon Event.
- Only one multicore from the time series set was collected in May 2022
- A) MARs from sites DSH10 and PCB06 tend to decrease after the DWH event, while site DSH08 is stable over the time series and site MC04 seems to have an increase in MAR in 2016, possible indication of Bioturbation.
- B) Initial depositional pulse can be seen in the 210Pbxs Inventory, as well as the stabilization of the inventory over time.
- B) Sites DSH10 and DSH08 are seen to have a higher inventory than sites MC04 and PCB06 most likely due to higher sediment accumulation from the Mississippi River Delta.
Multicore site DSH10 continued to show no signs of bioturbation.
Multicore site DWH01 started to show first signs of bioturbation in 2022 since the DWH event. *Arrow
Figure 6.1: 234Thxs Activity profiles for time series site DSH10
Figure 6.2: 234Thxs Activity profiles for time series site DWH01
234 Thxs Activity profiles for Time Series Sites DSH10 & DWH01
- Site DSH10 continues to show no signs of bioturbation. This could mean that the site never had any bioturbation to begin with or the site has yet to recover from the DWH event.
- Site DWH01 has its first signs of bioturbation. This indicates that it has taken nearly 12 years for this site to have measurable bioturbation from the DWH event. Further core collections and analysis will help determine if bioturbations levels will continue to increase or not.
- The trends seen when comparing 210Pbxs Inventory and MARs may be due to spatial relationships. DSH10 and DSH08 are on the western side of the Desoto canyon where they are perceived to have a higher sediment accumulation due to their closer proximity to the Mississippi River Delta.
- Further core collections and analysis would be beneficial in helping to determine what would cause changes in the 210Pbxs Inventory and MARs.
Larson RA, Brooks GR, Schwing PT, Carter S, Hollander DJ (2018) High resolution investigation of event-driven sedimentation: northeastern Gulf of Mexico. Anthropocene 24:40–50. https://doi.org/10.1016/j.ancene.2018.11.002
Daly KL, Passow U, Chanton J, Hollander DJ (2016) Assessing the impacts of oil-associated marine snow formation and sedimentation during and after the Deepwater Horizon oil spill. Anthropocene 13:18–33. https://doi.org/10.1016/j.ancene.2016.01.006
I would like to acknowledge the professors of the SAS program, the crew of the Weather Bird II for assisting on the cruise, the members of the SAS program, and NSF (Award 2119864) for providing funding for this project and program. I would also like to acknowledge funding I received from NSSRP during the summer of 2022.
Alexsandra Henes1, Kattey Pass1, Avi Kaupler1, Rebekka Larson1, Gregg Brooks1, Patrick Schwing1
1 Eckerd College, 4200 54th Ave. S., St. Petersburg, FL 33711, alhenes@eckerd.edu