Summary:
Dr. Richard Peterson at Coastal Carolina University’s Department of Coastal & Marine Systems Science was awarded an RFP-VI grant at $671,806 to conduct the RFP-VI project titled, “Radium Isotope Release from Oil Degradation: Development of an ‘Oil Clock’”. The project consisted of 1 institution (Coastal Carolina University); 1 principal investigator (Peterson); 2 PhD students (Jian’an Liu and Leigha Peterson); 3 master’s degree students (Elana Ames, Charlotte Kollman, Matthew Kurpiel); and 4 undergraduate students (Jiana Walkel, Tillmon Ancrum, Rebecca Hight, Alexander Villafana).
Published studies regarding microbial respiration dynamics in a deep, laterally-spreading hydrocarbon plume months after the Deepwater Horizon blowout report contradictory findings because they lacked a tool to unambiguously constrain the amount of time that in-situ microbial communities were exposed to the hydrocarbons. These contradictory findings have led to uncertainties regarding the potential rates of natural remediation of hydrocarbons in the marine environment from the Deepwater Horizon event, and perhaps more importantly, for future oil spill scenarios. Time is the central theme of this proposal--specifically, the residence time that hydrocarbons spend in the marine environment and therefore exposure time related to the kinetically and microbially-controlled processes of natural attenuation. Many studies interpreting results of the Deepwater Horizon blowout attempted to infer exposure time based on potentially misleading timelines (e.g., the blowout timeline), but few, if any, have proposed methods for age-dating the material once released. Fewer still have the specialized tool set to do so both in the deep sea and at the surface. The radioisotope approach proposed here is well established for coastal and continental shelf settings, and applying those geochemical tracers to hydrocarbon dynamics in the open ocean would be of immense value in understanding the complex evolution of hydrocarbon degradation within an oceanic plume. These efforts do not duplicate any existing efforts in the Gulf of Mexico, but may certainly augment the interpretations of various ongoing studies.
This proposal is based on exciting new data demonstrating that as hydrocarbons degrade within a water parcel, radium isotopes (specifically 224Ra and 228Ra) are released into the surrounding water. The ultimate goal of this proposal is to use this characteristic to develop an ‘oil clock’ geochronometer of hydrocarbon residence time in the marine system based on this released radium isotopic signature. The objectives of this proposal include: (1) collect and geochemically characterize discharged hydrocarbons from natural seep vents at Green Canyon lease block 600 (GC-600) to assess the degree of hydrocarbon source homogeneity across a Gulf of Mexico lease block; (2) use these collected hydrocarbons to perform a series of time-course radium release experiments, in which natural and dispersant-influenced degradation processes under conditions simulating deep water and surface hydrocarbon plumes will be used to determine the radium isotopic release signatures; and (3) create and validate a conceptual model of hydrocarbon residence time in the marine environment based on these isotopic release signatures.
These objectives and final deliverable of the project (i.e., the geochronometer conceptual model) meet the goal of Theme #2: Chemical Evolution and Biological Degradation of the Petroleum/Dispersant System and Subsequent Interactions with Coastal, Open-Ocean, and Deep- Water Ecosystems. The experimental approach of this project aims to simulate real-world conditions at the ocean surface as well as at depth to allow these degradation processes to occur such that the radium isotopic release signatures can be assessed as a function of hydrocarbon degradation. The goal of this project is not to examine the specific mechanisms, rates, or processes by which microbial and/or photodegradation of petroleum hydrocarbons occur, but rather how the chemical evolution of the degrading petroleum releases the radiotracers of interest. These results will ultimately produce a much needed tool available to the broader research and management communities that will constrain exposure times of oceanic microbial communities to hydrocarbons, and therefore allow a more comprehensive assessment of the efficiency of natural remediation processes in the event of another significant spill.
This project is led by an early career scientist at Coastal Carolina University and benefits from a strong collaboration with the ongoing GoMRI consortium on Ecosystem Impacts of Oil & Gas Inputs to the Gulf (ECOGIG). Valuable graduate and undergraduate student involvement will be incorporated into this project, as well as public educational components via blogs-from-sea. The fundamentally significant results from this work will be disseminated to oil spill researchers and managers via presentations at the GoMRI annual meetings, as well as to the broader scientific community through peer-reviewed journal publications and presentations at a national conference.
Research Highlights
Dr. Peterson’s research, which included 2 outreach activities, resulted in 4 scientific conference presentations to date and 9 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public. Significant outcomes of their research (all related to GoMRI Research Theme 2) are highlighted below.
A novel instrument (Figure 1) was designed and built during this project to directly collect oil discharging from the seabed and transport it to a surface ship via a remotely operated vehicle (ROV). The oil sampler is made from glass to prevent sorption to the apparatus and contains eight individual 1-L samplers in a carousel. The ROV manipulator can actuate the indexer to select between each of the eight glass samplers. Each sampler also has a pressure relief valve at the top to allow off-gassing during ascent. One important lesson learned during sampling was to collect the oil slowly and allow significant time for gas to separate and escape from the liquid hydrocarbon phase to prevent the formation of methane hydrate in the sampler (Figure 1).
During this project, the investigators executed two research cruises to the Gulf of Mexico to collect oil discharging from the seabed at GC-600. The first cruise was from 29 August to 5 September, 2018 and the second cruise was from 24-31 January, 2019. The cruises were aboard the R/V Point Sur and utilized the ROV Odysseus from Pelagic Research Services. ROV dives during the first cruise captured 200 mL of pure liquid hydrocarbon for experiments, and those during the second cruise collected 615 mL of liquid hydrocarbon (Figure 2).
These oil samples, along with archived oil samples from the Deepwater Horizon blowout, were used in a variety of incubation experiments to ultimately measure dissolved activities of 224Ra in surrounding waters as a function of hydrocarbon degradation conditions. The experimental design involved using 10 mL of these oil samples with 10-L of Ra-free seawater for each time-course experiment to examine the effect of degradation mechanisms on radium isotopic release to the water:
- under dark, cold conditions, simulating the deep ocean, wherein hydrocarbons were incubated with deep ocean, radium-free seawater – this manipulation would be expected to undergo only microbial degradation;
- under dark, cold conditions, serving as a control, wherein collected hydrocarbons were mixed with ambient, radium-free seawater poisoned with HgCl2 – this manipulation was not expected to undergo any hydrocarbon degradation;
- outside in natural light, simulating surface ocean conditions, wherein collected hydrocarbons were incubated with radium-free surface ocean water – this manipulation was expected to undergo both microbial and photodegradation; and
- outside in natural light, simulating surface ocean conditions, wherein collected hydrocarbons were incubated with radium-free surface ocean water poisoned with HgCl2 – this manipulation was expected to undergo only photodegradation.
Each incubation was destructively terminated at various time points between 0 days (representing initial Ra release upon mixing oil and seawater) and 21 days – the maximum amount of time over which 224Ra ingrowth/decay should occur prior to reaching secular equilibrium with its radioactive parent 226Ra in seawater. These incubations were also replicated using an addition of COREXIT9527 to assess whether the presence of oil dispersant affects 224Ra release from hydrocarbons.
The most significant finding from these experiments is that the short-lived 224Ra isotope (T1/2: 3.54 days) is definitely and conclusively released to surrounding water through time as a function of oil residence within that water, as exemplified in Figure 3 from several incubations of archived Deepwater Horizon oil as well as GC-600 oil across multiple points through time. Relative to system blanks (not shown but generally averaged ~0.1 dpm), 224Ra is significantly enriched in water samples containing oil, with peak activities reached between 1-2 days and subsequent decreases due to radioactive decay. This finding represents a significant step forward for the field; this represents the first time that radium has been shown to be directly associated with the hydrocarbon matrix and released to surrounding seawater as a function of residence time.
However, in contrast to the project’s original hypothesis, this release does not seem to be moderated by the type or degree of oil degradation. The magnitude of 224Ra release was of similar magnitude and timing for all forms of degradation pathway that were isolated. In some cases, it seems that 224Ra release signatures were somewhat lower in poisoned treatments (e.g., Figure 4), but that result may due to some form of chemical sorption effects with the heavy metals used in the poisons. In general, release of radium from the oil matrix into the aqueous phase is thought to be due to ion exchange processes rather than oil degradation. This finding is actually more promising for ultimately developing radium as a geochronometer for oil residence in the ocean as no form of degradation would be required to produce an aqueous signal of 224Ra.
These experiments also reveal significant differences in the magnitude of 224Ra released to the aqueous phase associated with different sources of oil. Even 10 years after recovery, archived Deepwater Horizon oil yielded generally higher 224Ra activities than freshly collected oil from GC-600. Over 10 years of holding time, all excess 224Ra associated with the Deepwater Horizon oil matrix would have long ago decayed, so the fact that it remains suggests that it is supported by radioactive parent isotopes (either 228Ra or 232Th). Both of these isotopes were found in this oil source through independent analysis. These findings suggest that primordial 232Th (T1/2: 14 billion years) was likely scavenged out of the water column by phytoplankton that was buried and ultimately converted into the oil. The 232Th remains associated with that organic matter, and supports 228Ra and subsequently 224Ra in the oil matrix.
The originally hypothesized behavior of 224Ra was proven correct (Figure 4). After introducing crude oil to a seawater system, 224Ra is released from the oil and reaches its highest activities after ~24 hours. After this release, activities decrease at a rate commiserate with radioactive decay, suggesting no further input. Some minimal release later in the incubations (11-16 days) is frequently observed, but the mechanisms for this later release remain elusive. These findings are very promising for developing radium as a geochronometer for oil residence in the water column.
However, for that purpose, using the activity of a single isotope in the field will not be sufficient. Activity of 224Ra may be altered by physical mixing processes independent of residence time of the oil in that seawater parcel. Therefore, a suitable stable surrogate must be used to normalize 224Ra to in order to account for physical processes. Despite extensive efforts through this project to develop either 226Ra or 228Ra as such a surrogate, low analytical resolution through this project prevents a sound conclusion of the suitability for either of those isotopes to serve as that stable proxy. Either the ability to analytically resolve low activities of these isotopes must improve, or an alternative surrogate must be found which is similarly sourced from oil and input to seawater as a function of residence time. Barium may offer such an alternative and its potential should be explored in future studies.
Proposal Abstract - RFP-VI PI Richard Peterson