8.3 South Atlantic Ocean and the Theory of Plate Tectonics
Glomar Challenger set out on its third expedition (Leg 3) 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 headed to the same location, but this time on JOIDES Resolution with new technologies and questions to address for Expeditions 390 and 393.
Image 1: Location map of drill sites for DSDP Leg 3; Image 2: Location map of drill sites for IODP EXP 390 & 393, with the sites from Leg 3 on the same map. Images from Initial Reports – Volume 3, IODP JRSO, CC BY 4.0.
As reported… scientific highlights from DSDP Leg 3
Deep Sea Drilling Project 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. A report that provides the scientific highlights of each completed 55-day leg 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.
Objectives
Two primary objectives were engaged and met during Leg III:
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- Investigate the tectonic development of the Mid-Atlantic Ridge (structure and movement of the crust);
- 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.
Findings
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 the knowledge and basis from DSDP Leg 3, IODP Expeditions 390/393 sailed in 2022 to revisit the region of these Leg 3 sites and set out 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;
- Investigate the response of Atlantic Ocean circulation patterns and Earth’s climatic system to rapid climate change during the Late Cretaceous and Cenozoic.
Scientists are still working on processing and analyzing samples from the 2022 expeditions, but the Initial Reports and Proceedings of both expeditions are available to explore online.
