2018 AGU Talk given by Bob Stern and Randy Keller

Hi, I’m Bob Stern and along with my co-author Randy Keller, we’d like to discuss some ideas about how to stimulate geoscientific research in the part of the US that borders the Gulf of Mexico. This is a low-lying region that marks the coastal plain of the Gulf of Mexico. This region will play a big role in the 21st century US economy because of its prolific natural resources, especially oil and gas, and because it will experience more natural disasters in the form of hurricanes, flooding, and sealevel rise. In many ways, the future of the United States is tied to the future of this region.

The Gulf of Mexico coastal plain covers parts of seven states that encompass one and a half million square kilometers, about 20% of the area of the contiguous United States. Much of this region is experiencing rapid population growth, especially Texas – the second most populous state - Florida – the third most populous state - and Georgia – the eighth most populous state. This rapidly growing region is also increasingly vulnerable to natural hazards such as hurricanes, flooding, sealevel rise, and even tsunamis. We need to better understand the crustal and lithospheric structure of this region and its tectonic evolution in order to prepare for the future. However, it is not easy to carry out these studies because the crust of this region is mostly buried beneath kilometers of sediment and sedimentary rocks.

We need to devise new strategies for geoscientific study of the Northern Gulf coastal plain. Such studies the buried thick sediments and underlying basement of this key region are needed in order to better understand the controls on subsidence and thus mitigate hazards and to understand the subsurface system and better exploit natural resources. Understanding the causes of subsidence in this region is a key focus because this worsens the damage done by hurricanes, floods, and sea level rise. If we better understand the rates and causes of subsidence in this region, we can better prepare for hurricanes, floods and sea level rise by engineering ways to slow or reverse the sinking. Better understanding subsidence will require studying the entire sedimentary column and its fluids and the transitional lithosphere it is built on. We will need to better understand the full range of subsidence mechanisms as well as the structure of the lithosphere and sediments and how these contribute to subsidence. The role of the asthenosphere, mantle plumes, and dynamic topography should also be considered.

First, a few words about what we know. The northern Gulf of Mexico, including the coastal plain (where people and hazards are) and the continental shelf (where the resources are) are onshore and offshore parts of a textbook passive continental margin. This region has one of the thickest accumulations of sediment on Earth. This huge pile of sediments was deposited in the 165 million years after the Gulf of Mexico opened during the breakup of the supercontinent Pangea. As the small ocean basin opened and new crust formed by seafloor spreading, thick sequences of Jurassic salt were deposited before normal marine sediments began to be deposited in latest Jurassic and into Cretaceous time. Cenozoic deposition varied from clay and sand-rich deposits west of the Mississippi delta, shed from the growing mountains in far western Texas, New Mexico and NE Mexico, to carbonate sediments deposited in clearer waters east of the Mississippi. Hydrocarbon deposits are found in the top few kilometers of the sedimentary pile, the deeper two-thirds of the pile is largely unknown. The entire passive margin system needs to be studied in detail in order to better understand subsidence, from the asthenosphere to Quaternary sediments. A geodetic system for monitoring subsidence, using satellite interferometry and GPS what is happening in the deep subsurface mechanisms we can develop realistic numerical models of subsidence. If we better understand subsidence and we can better understand the fluids generated by the sedimentary pile, including hydrocarbons.

We need geophysical data to image the crust and lithosphere and to choose where to conduct more detailed geophysical surveys, drilling, and coring. We need to drill as deep as possible. The deepest continental drilling to date reached 11 km in the Kola Peninsula of Russia, but it may be possible with modern technology to reach 15 or 20 km to the base of the sediment pile and into the transitional crust beneath the Gulf of Mexico coastal plain. We need to sample and study the fluids, metamorphism, and microbes that exist all the way down, as deep as we can drill. This pile of sediments is the world’s best natural laboratory for an integrated study of burial metamorphism. As we drill through it, we will encounter a wide range of diagenetic and metamorphic environments as we penetrate down into greenschist and maybe even amphibolite facies. We will want to see how the distinctive horizons or zones seen in seismic reflection lines correspond to features observed in the core. This will be the first integrated study of an active site of burial diagenesis and metamorphism.

The geomicrobiological opportunities alone warrant the effort to image and sample. We will see the bottom of the biosphere and into Earth’s sterile zone. The top 5 km of the sedimentary pile has a normal geotherm of about 25° per kilometer, will this persist all the way to the base of the sediment pile ten kilometers below? What kinds of communities live at the margins of life? What are the most extremophilic of the extremophiles. All these aspects must be considered in order to build a robust subsidence model for the region, with all models required to fit the geodetic results for the regional network.

A community effort along the lines of RIDGE or GeoPRISMS is needed to organize the academic community. In this effort, AGU can play a leading role. The diverse interdisciplinary community to carry out this effort already exists in our Union, so it should start here.

The work will be expensive. Geophysical experiments and deep drilling are expensive. Geodetic networks and laboratory studies of cores, fluids, and microbes, and numerical models are not cheap. The financial resources needed to do this cannot come from NSF alone. This work will require other partners including NASA, NOAA, USGS, and DOE. Partners in the state and local governments will need to be recruited. Even so, the academic-government partnership lacks the expertise or resources to do this work, and a third leg of support is needed. The US hydrocarbon industry has a big footprint in this region and we need to involve this geoscientific community. One way to do this would be through a new NSF program encouraging research partnerships, with funding from both companies and NSF. Such a program would differ from many industry-university consortia in that data and results would not be proprietary but would be subject to the same rules as any other NSF-funded project. An example of how this might work would be that an industry active source seismic experiment could “listen” longer than industry needs to image the sediments for use by industry and collect non-proprietary data on crust and lithosphere. Academic geoscientists and students could be funded by NSF to work on the nonproprietary data.

Ok, thanks for listening. There is a strong need, opportunity, and challenge to carry out a systematic, integrated, interdisciplinary study of subsidence of the US Gulf of Mexico coastal plain. We’re sure this effort needs to be undertaken but we’re much less sure how to move this opportunity and challenge forward. The project is likely to take some time to put together and Randy and I may not be around to see our dream realized. We do know that a very wide ranges of scientists will be needed to plan and carry out this work and that AGU is the best place to start the discussion. We hope you agree.