Preliminary scientific assessment

Expedition 349 achieved the vast majority of its scientific objectives as outlined in the Scientific Prospectus (Li et al., 2013). The expedition also obtained several unexpected findings that bear significant science implications. We set out with five broadly defined objectives, each of which has been addressed as follows at the five sites drilled, cored, and logged during this expedition:

  1. Date the timing of the opening of different subbasins of the SCS and correlate the ages from magnetic anomalies to biostratigraphic, magnetostratigraphic, and radiometric ages.
    1. All of the five drill sites were designed to address this objective, although only at Sites U1431, U1433, and U1434 we were able to penetrate into igneous basement. Site U1432 was designed to determine the onset age of seafloor spreading in the East Subbasin through coring the lower half of sediment layers and the uppermost 100 m of igneous basement. Hole U1432B was drilled to 800 mbsf with casing successfully installed to address this objective; however, a technical error during the final cementing operation forced us to abandon Hole U1432B. This prevented us from coring the oldest sediment and oceanic crust to determine the initiation age of seafloor spreading in the SCS. We urge the science community to work together toward a future IODP expedition to return to Site U1432.
    2. To determine the age of termination of seafloor spreading in the East Subbasin, we drilled Site U1431 and successfully obtained continuous sediment records and relatively fresh basaltic samples in the basement. Preliminary results for the sediment ages were obtained during the expedition through biostratigraphic and paleomagnetic analyses. Radiometric dating of basalt samples will be conducted during shore-based investigation.
    3. At Sites U1433 and U1434 we successfully obtained sediment and basalt cores that will allow us to determine the termination age of seafloor spreading in the Southwest Subbasin and compare it to the Site U1431 termination age in the East Subbasin. Preliminary biostratigraphic and paleomagnetic dating was conducted during the expedition, whereas shore-based analysis will provide radiometric ages for the basalts.
    4. At Site U1435, which is on a structural high near the ocean/continent boundary at the northern edge of the East Subbasin, we successfully obtained cores of Oligocene sediment above and pre-Oligocene sandstone below an unconformity. The obtained samples will provide us an opportunity to investigate the transition from rifting to seafloor spreading in the SCS.
  2. Measure the magnetization, mineralization, and geochemical compositions of basement rock to understand the causes of the sharp magnetic contrast between different subbasins.
    1. We successfully obtained igneous basement rock in the East Subbasin (Site U1431) and the Southwest Subbasin (Sites U1433 and U1434), allowing us to measure and fully characterize the magnetization, mineralization, and geochemical compositions of basement rock.
    2. The results of shipboard physical property measurements and shore-based analyses of the rock samples, integrated with regional geophysical and geological studies, will advance our understanding of the causes of the sharp magnetic contrast between the two subbasins.
  3. Evaluate the origin and source evolution of SCS basement rocks to better understand the formation of SCS oceanic crust and the deep mantle processes driving this formation.
    1. We successfully obtained basalt samples near the relict seafloor spreading centers in the East Subbasin (Site U1431) and the Southwest Subbasin (Sites U1433 and U1434). Preliminary petrological and geochemical studies were made during the expedition, and shore-based geochemical and petrological studies have already been coordinated and will be carried out by multiple laboratories. These studies will help to characterize geochemical compositions of the basement rocks to evaluate the origin and source evolution of the SCS oceanic crust and mantle.
    2. A surprising finding from the expedition is that we obtained abundant samples of volcaniclastic materials in sedimentary sections at sites near the relict spreading centers of the East and Southwest Subbasins. Shore-based analyses of these volcaniclastics will allow us to date both their eruption and sedimentation ages, as well as to characterize their geochemical compositions. Results will allow us to investigate the combined source evolution of the SCS spreading centers and the nearby seamounts, and how they interacted.
  4. Evaluate the paleoceanographic and climatic responses to the opening of the SCS and develop a 3-D sedimentation and subsidence model.
    1. We obtained several surprising results that were previously unknown to the SCS research community. For example, the sediments reveal abundant turbidite sequences at Sites U1431, U1432, and U1433, suggesting a history of turbulent sedimentary deposition events that were previously unknown.
    2. Thick carbonate turbidites in the form of deepwater carbonates and chalks found in the Southwest Subbasin (Site U1433) are indicators of provenance, and changes in the supply of these sediments to the site has implications for the paleoceonographic and paleoclimatic evolution in that region.
    3. We have obtained complete records of the sedimentary seamount aprons, which contain valuable information on the history of seamount eruption and associated volcaniclastic deposits in both the East Subbasin (Site U1431) and the Southwest Subbasin (Sites U1433 and U1434).
    4. Reddish brown to yellowish brown deep-sea claystone sequences were encountered immediately above the basement at Sites U1431, U1433, and U1434. The scientific implications of these sediment layers were debated and will become clearer after shore-based investigation.
    5. Preliminary ages were obtained through biostratigraphic and paleomagnetic analyses during the expedition. The results revealed a surprisingly wide range of sedimentation rates within individual holes and between different sites. Shore-based investigation will further improve the sedimentation models, identify potential hiatuses in the sedimentary records, and determine the provenance sources of the sediments.
  5. Obtain downhole geophysical logs to reveal physical properties of the sediment and the top oceanic basement and to provide a record of unrecovered intervals.
    1. We successfully conducted downhole geophysical logging in the East Subbasin (Hole U1431E) and the Southwest Subbasin (Hole U1433B). These were the first downhole geophysical data obtained in the deep basins of the SCS.
    2. In the Southwest Subbasin, downhole logging at Hole U1433B was accomplished using the triple combo and FMS-sonic tool strings. We successfully logged a significant proportion of the basement, revealing pillow basalt flow sequences. The downhole logging results provide a record of unrecovered intervals in both the sediment and basement cores, including two possible interbedded sedimentary layers up to 1.5 m thick in the pillow basalt sequence of Hole U1433B.
    3. In the East Subbasin, downhole logging at Hole U1431E was also accomplished using the triple combo and FMS-sonic tool strings. At this site, the downhole logging tools reached a significant depth of the sediment layers, also providing a record of unrecovered intervals in the sediment cores.