Box 2.1
Con rmation of Seafloor Spreading

One of the earliest and most significant accomplishments of scientific ocean drilling was to provide data that confirmed seafloor spreading. Data and samples retrieved by the drillship Glomar Challenger in the early 1970s were used to confirm that new ocean crust was being generated at mid-ocean ridges, supporting the complete theory of plate tectonics, the paradigm for solid Earth science. Data from survey ships that provided geophysical evidence for the existence of symmetric, alternating patterns of magnetic polarity (“magnetic anomalies”) on either side of the world’s mid-ocean ridges led to suggestions by various scientists in the early 1960s that new ocean crust was being created there. However, these interpretations could not be conclusively substantiated in the absence of data directly obtained from the seafloor. In 1970, Glomar Challenger drilled a series of boreholes across the Mid-Atlantic Ridge in the South Atlantic and retrieved basal sediments that had been deposited on ocean crust. The biostratigraphic ages of those samples increased nearly linearly with distance from the ridge crest, in close agreement with the ages predicted by analysis of the magnetic anomalies on the seafloor. Confirmation of the theory of seafloor spreading had direct consequences for the development of new fields of scientific inquiry and cross-disciplinary geoscientific research.

SOURCES: Dietz, 1961; Hess, 1962; Vine and Matthews, 1963; Heirtzler et al., 1968; Maxwell etal., 1970.

recovered from ocean sediments at ODP Sites 677 and 846, where benthic and planktonic foraminiferal dates were correlated with Earth’s orbital variations (e.g., Shackleton et al., 1990; also see Chapter 4). These results, in turn, have had implications for understanding of the composition and behavior of oceanic lithosphere, processes occurring at subduction zones, and the formation of large igneous provinces (LIPs) (see later sections of this chapter).

The development of hydraulic piston coring (HPC) and advanced piston coring (APC; see Box 2.2) allowed sampling of continuous undisturbed sediment cores from below the sediment-water interface to the crystalline bedrock (Prell et al., 1980; Ruddiman et al., 1986; also see Box 4.3). This unique achievement allowed the creation of a high-fidelity Cenozoic paleoceanographic/paleoclimatic time series by cross-validating magnetic dipole reversal stratigraphy from ocean core samples with higher resolution biostratigraphy and astronomically forced climate variations recorded in finely laminated ocean sediments (e.g., Hilgen, 1991). The basis for such an approach was first proposed by Shackleton and Opdyke (1973) when they dated high-resolution δ18O stratigraphy with fixed-point magnetic reversal ages to yield multi-millennial-scale paleoclimate (i.e., global ice volume) records. When Hilgen (1991) extended his earlier cyclostrati-graphic work to the Miocene-Pliocene boundary, it became possible to obtain three-way cross-validation of dated paleoclimate (biostratigraphy and lithostratigraphy based) records whose ultimate fixed-point dating scheme rested on the GPTS, synthesized from sediments of the world’s oceans. A global approach to developing a precise astronomical time scale used many DSDP and ODP cores and enabled oxygen isotope excursions to be precisely dated (e.g., Lisiecki and Raymo, 2005; Box 2.2; also see Chapter 4).

Another method to correlate oxygen isotope changes with age and geomagnetic reversals or excursions uses the relative paleointensity of sediment cores (e.g., Channell et al., 2009; Figure 2.1). This method has provided independent age calibrations to which climate changes can be compared and is also valuable for understanding the behavior of Earth’s magnetic field through time. Measurements of continuous relative geomagnetic paleointensity from long sediment cores have confirmed occurrence of intensity minima during dipole reversals and helped discover short-term dipole excursions.

Another accomplishment in the area of global plate motions and reconstructions came when cores collected on DSDP Leg 55 led to the initial recognition that the Hawaiian hotspot had not always been fixed at the latitude of Hawaii (Kono, 1980). In 1992, ODP Leg 145 cored a thickness of ocean floor basalt opposite Detroit Seamount, one of the oldest seamounts in the Hawaiian-Emperor chain, and determined that its paleomagnetic latitude of formation was well north of Hawaii (Tarduno et al., 2003). ODP Leg 197 provided compelling paleomagnetic evidence that from about 76 to 45 myr the Hawaiian hotspot was rapidly migrating southward to reach its present position, a finding that held implications for reconstructing past Pacific plate motions and also for the concepts of plume stability and mantle dynamics and circulation.

Fields of Inquiry Enabled

Research derived from scientific ocean drilling has fundamentally influenced and advanced the fields of plate tectonics, paleomagnetism and geomagnetism, and geologic time scales. The combination of high-resolution magnetostratigraphy and relative paleointensity; oxygen isotope records as a proxy for changes in climate; and bio-, cyclo-, and lithostratigraphy have helped further the study of paleoclimate and paleoceanography (discussed in further detail in Chapter 4), which attests to the importance of cross-disciplinary approaches in advancing research of the ocean floor and Earth processes.

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