April 3, 2022
DSDP Leg 3 – a legacy expedition for plate tectonics
Posted by Laura Guertin
I’m writing up posts relating to IODP Expedition 390 before I join JOIDES Resolution as an outreach education officer. These blog entries focus on varying topics, such as what to know before we go. And it’s important to spend some time reviewing the “why” of Expedition 390. To understand why this expedition at this South Atlantic location, we need to take a trip back in time. And our journey starts with the Deep Sea Drilling Project (DSDP).
The Deep Sea Drilling Project (1968–1983) was a 15 year program of exploration of the sediments on the ocean floor and the upper part of the underlying crust, recovering cores from 624 sites throughout the oceans. In 1981, Emiliani described it as “unquestionably … the largest and most successful program of geological investigation ever undertaken by man.” — from Hay (2009)
A Quick Look at DSDP
There are some excellent summaries about the development of scientific ocean drilling and the Deep Sea Drilling Project, from About the Deep Sea Drilling Project (history from IODP) to Dr. Suzanne O’Connell’s GSA Today article, Holes in the Bottom of the Sea: History, Revolutions, and Future Opportunities.
The purpose of DSDP was scientific exploration through the collection of seafloor core samples from around the world. What started as scientific ideas in 1966 (Phase I of DSDP) led to the building and launch of D/V Glomar Challenger in 1968 (*note that Glomar Challenger was named in honor of H.M.S. Challenger). In her recent EOS piece, An Ambitious Vision for the Future of Scientific Ocean Drilling, Dr.Paula Bontempi noted that these cores collected by Glomar Challenger would help researchers study seafloor compositions and ages, explore for natural resources, and otherwise inform a variety of questions about Earth’s deep-ocean environments.
The Deep Sea Drilling Project was the first of three international scientific ocean drilling programs that have operated for decades (DSDP, ODP, IODP). Glomar Challenger docked for the last time with DSDP in November 1983. Parts of the ship, such as its dynamic positioning system, engine telegraph, and thruster console, are stored at the Smithsonian Institution in Washington, DC (see an image of the engine power stand in the archive collection at the Smithsonian’s National Museum of American History).
For those that want to do a deeper dive with DSDP’s 624 sites, NOAA National Centers for Environmental Information has DSDP core data archived and accessible online. One can search by data type, geographic area, and leg/site/hole.
DSDP Leg 3
Glomar Challenger set out on its third expedition from December 1968 to January 1969, drilling 17 holes at 10 sites in the equatorial and South Atlantic Ocean between Dakar, Senegal, and Rio de Janeiro, Brazil. Fast-forward 50+ years to after the completion of Deep Sea Drilling Project (DSDP) Leg 3, and a new group of scientists and crew members are heading to the same location, but on a new ship with new technologies and questions to address.
DSDP Leg 3 focused on drilling holes in a transect across the Mid-Atlantic Ridge at 30oS latitude and set out to verify the theories of seafloor spreading and plate tectonics. I found online a report that provides the scientific highlights of all completed 55-day legs of DSDP from Leg 1 through Leg 18, courtesy of UCSD Library archives. Here is what the report summarizes for Leg 3:
Leg III -Dakar to Rio de Janeiro, December 1, 1968, to January 24, 1969. Dr. Arthur E. Maxwell, Associate Director, and Dr. Richard P. Von Herzen, of Woods Hole Oceanographic Institution, were Cruise Co-Chief Scientists.
Two primary objectives were engaged and met during Leg III: Investigate the tectonic development of the Mid-Atlantic Ridge (structure and movement of the crust) and examine the history of sedimentation in the South Atlantic. Efficiency also was high on this leg as 92% core recovery was achieved -2,536 ft. in all-and drilling at each site was deep enough to penetrate the sedimentary column and reach the basaltic basement.
1. Almost unassailable evidence confirms seafloor spreading. The magnetic anomaly pattern is relatively uniform across the South Atlantic, which indicates a steady spreading of the seafloor over the past 150 millions years. This is, South American and Africa –probably a single geographic unit at one time –are drifting apart at the rate of 2 inches per year.
2. Sediment depths are uniform but age distributions vary widely. The constancy of sediment thickness formerly was regarded as “proof” against the seafloor spreading hypothesis. However, cores taken from points across the ridge showed that age distribution –from Recent to Upper Cretaceous –varied widely. Sedimentation rates are high near the ridge and become markedly lower as one moves away from the axis. Sediment age versus distance plottings correlate closely with the magnetic anomaly findings and also indicate a spreading rate of about 2 inches per year.
3. Cores reveal a wide variety of sediment types. Most consisted of fossilized skeletons of marine organisms –primarily nannoplankton chalk oozes and some foraminiferal ooze sediments. Sequences were similar in many of the holes, indicating wide variety of climatic conditions or oceanic circulation.
With this knowledge and basis, IODP joint Expeditions 390/393 are sailing in 2022 to revisit these Leg 3 sites and will seek to accomplish the following objectives:
- Quantify the timing, duration and extent of ridge flank hydrothermal fluid-rock exchange;
- Investigate sediment and basement-hosted microbial community variation with substrate composition and age; and
- Investigate the response of Atlantic Ocean circulation patterns and Earth’s climatic system to rapid climate change during the Late Cretaceous and Cenozoic.
See the Expedition 390 page to check out blog posts and additional information from the time JOIDES Resolution spent at sea during the collection of the new cores and scientific data.
Citations and Additional Sources
Emiliani, C. (1981). A new global geology. In Emiliani, C. (ed.), The Oceanic Lithosphere, The Sea, Vol. 7. New York, NY: John Wiley and Sons, pp. 1685–1716.
Hay W.W. (2009) Deep Sea Drilling Project (DSDP). In: Gornitz V. (eds) Encyclopedia of Paleoclimatology and Ancient Environments. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-4411-3_63
About the Deep Sea Drilling Project [Deep Sea Drilling Project Reports and Publications] – http://deepseadrilling.org/about.htm
DSDP Phase: Glomar Challenger [International Ocean Discovery Program, JOIDES Resolution Science Operator, Outreach/Program History] – http://www.iodp.tamu.edu/publicinfo/glomar_challenger.html
Deep Sea Drilling Project (1989). Archive of Core and Site/Hole Data and Photographs from the Deep Sea Drilling Project (DSDP). National Geophysical Data Center, NOAA. doi:10.7289/V54M92G2 [Accessed April 2, 2022]
National Research Council (2011). Scientific Ocean Drilling: Accomplishments and Challenges. Washington, DC: The National Academies Press. https://doi.org/10.17226/13232
In my time leading up to sailing on Expedition 390, I became fascinated with the history and significance of DSDP Leg 3. I created a quilt as a nod to this historic milestone in the development of and evidence for plate tectonics theory. This is a blog post that details more about the quilt, including the significance of the colors and fabric – especially as they connect to the initial reports volume for the expedition. (photo taken during my on-site onboard outreach officer training at the IODP/Texas A&M facility, August 2021).