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Drilling has been essential in illuminat- previously identified Ice Ages and hinted at have been debated ever since (Ryan, 2009).
ing the origin and evolution of convergent a cyclicity later tied to orbital forcing. Additional drilling in the Mediterranean,
margins. Not only are crust and sediment Longer cores were needed to extend these with better coring capabilities and refine-
being subducted, but material from the records. Today, compilations of oxygen ment of the areas to be drilled, is needed.
overlying plate is scraped off by the under- (δ O) and carbon isotope values (δ C) Among the new IODP proposals is “The
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lying plate, termed subduction erosion. from calcite secreted by benthic foramin- Demise of a Salt Giant,” to examine the
This process is ubiquitous at convergent ifera are essential for past climate recon- climate and environmental transitions dur-
margins, recycling material deep into the structions. Data compiled by Zachos et al. ing the terminal Messinian salinity crisis.
lower crust and mantle (e.g., Scholl and (2001), Lisiecki and Raymo (2005), and
von Huene, 2009). Trace elements and iso- Cramer et al. (2009), include tens of thou- Paleocene–Eocene Thermal Maximum
topic tracers in volcanic rocks substantiate sands of analyses from hundreds of cores (PETM)
the subduction and hence chemical recy- collected by many scientists over decades. At the start of the ODP, no one knew
cling of ocean sediments (e.g., Plank and These compilations and many new oceano- about the PETM. This global warming
Langmuir, 1993). graphic proxies document long- and short- event challenges our understanding of the
Crucial to the processes at subduction term climate change and rates of change. carbon cycle and is discussed in more than
margins is illuminating the cause of large But the changes go far beyond “just” 500 refereed papers. Its discovery led to
earthquakes, identifying what happens at climate. SOD has allowed us to identify identification of multiple “hyperthermal
the seismogenic zone when the release of changes in ocean chemistry, better under- events” of lesser magnitude in the
pressure causes a major rupture. From stand the effect of the asteroid impact at the Paleocene–Eocene, thus a new view of
cores and borehole measurements, varia- K/Pg boundary, and, through continuous climate instability in a greenhouse world.
tions in lithologies, fluids, pore pressure, ocean records, better understand evolution. First identified in Site 690 (Leg 113,
and heat flow have been documented. All of these advances benefit from Weddell Sea, Antarctica), it was character-
These observations, in conjunction with dip excellent resolution, provided by continu- ized by extinction of benthic foraminifera
angle, subducting seafloor topography, and ing developments in paleomagnetics, bio- (Thomas, 1989). Stable isotope data told an
results from ocean bottom seismometers, stratigraphy, X-ray core scanners, and incredible story, a rapid deep-sea warming
highlight this environment’s complexity. orbitally tuned age models. Early in SOD, combined with a massive negative carbon
Multiple Legs (DSDP, ODP) and holes were often spot cored. Then the com- isotope excursion (Kennett and Stott, 1991)
Expeditions (IODP) have focused on sub- munity decided that holes generally should and—as later recognized—widespread
duction zones and backarc basins. The be continuously cored. After the advent of deep-sea carbonate dissolution due to
Nankai Trough Seismogenic Zone the HPC, higher resolution work became ocean acidification (Zachos et al., 2005).
Experiment (NanTroSEIZE) on board the possible, but cores in a single hole do not The details of this event are still debated,
Chikyu is a multiyear project to core provide a truly complete record. The top but the deep ocean warmed ~5–8 °C in a
through a plate boundary in an active fault and bottom of each core is disturbed. few tens of thousands of years, ca. 55.8 Ma
area. In addition to cores, it is providing the Today, multiple, overlapping holes at a (Mudelsee et al., 2014). Even in the Arctic
opportunity to monitor hydrologic and geo- single site allow creation of a continuous Ocean, already ~18 °C, temperatures rose
dynamic properties of the subduction zone composite core (Fig. 2). Here, highlights of an additional 5–6 °C during the PETM
through borehole observatories. All of this two paleoceanographic achievements are (Sluijs et al., 2006).
information will help to develop models briefly summarized. Acidification and a δ C excursion were
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that describe subduction zone process and linked to a massive discharge of carbon.
should allow geoscientists to better explain Messinian Salinity Crisis Calculating how much carbon was released
and predict earthquake behavior and Leg 13 departed Lisbon, Portugal, depends upon the source and its isotopic
tsunami generation. in 1973 to explore the origins of the composition. This has been highly debated
Mediterranean Sea and, in particular, to and may be some combination of decom-
Revolution #2: Climate and Ocean identify reflectors on seismic profiles. posing clathrates (Dickens et al., 1995),
Change: Paleoceanography Coring the pervasive and prominent burning of organic matter (Kurtz et al.,
The science of pre-Quaternary paleo- M-Reflector proved problematic. The 2003), mantle-sourced volcanic CO 2, and
ceanography arguably did not exist when problem was caused by the unexpected interaction of magma with organic matter
the Glomar Challenger set sail in 1968. presence of gravel, which jammed the drill (Svensen et al., 2004; Gutjahr et al., 2017).
The first International Congress of pipe, and salt (dolomite, gypsum, anhy-
Paleoceanography took place in Zurich, drite, and halite), which was slow to Revolution #3: Biosphere Frontiers:
Switzerland, in 1983, and the American impossible to penetrate. The M-reflector Deep Marine Biosphere
Geophysical Union began publishing was caused by the top of a thick salt layer From its initiation, there have been
Paleoceanography (now Paleocean- that accumulated in the late Miocene, serendipitous discoveries by SOD, but
ography and Paleoclimatology) in 1986. 3 km below the surface of the Atlantic probably none was as unexpected as that of
Early paleoceanographic research, based Ocean. The initial interpretation was that the deep biosphere. In the late 1980s, some
on short cores, showed a marine expression the Mediterranean dried up several times researchers began to discover microbial
of the Ice Ages recognized on land. Oxygen during the Messinian (Hsü et al., 1973). activity in deep marine sediments (e.g.,
isotope records of foraminifera (Emiliani, The details about the causes and extent of Whelan and Tarafa, 1986). This discovery
1957) documented more than the four the desiccation and the role of tectonics didn’t get celebrated the same way as the

8 GSA Today | March-April 2019