- The primary objective of Leg 138 was to provide detailed information about the ocean's response to global climate change
during the Neogene. Two north south transects were drilled (95° and 110°W) within the region of equatorial divergence driven
upwelling (and thus high accumulation rates and resolution) and spanning the major equatorial ocean current boundaries (and thus
recording a high amplitude signal of the response of the sediment to climatically and/or tectonically driven changes in ocean
circulation). The Neogene is marked by a number of well known climatic and tectonic events (the closing of the Isthmus of
Panama, the onset of North Atlantic Deep Water (NADW), the rapid uplift of the Himalayas, the major intensification of Northern
Hemisphere glaciation), and the response of the ocean before and after these events was a key focus of Leg 138 drilling.
To address these objectives at the highest resolution possible, the Leg 138 scientific staff developed a number of new shipboard
strategies and analytical procedures. These included the real time analysis of the near continuous gamma ray attenuation porosity
evaluator (GRAPE) and susceptibility profiles produced by the multisensor track (MST) on unsplit cores to monitor core recovery
and, if necessary, to modify the drilling strategy to ensure proper offset of coring gaps; the collection of near continuous color
reflectance data on split cores; the logging of the first hole drilled at each site to optimize drilling and sampling strategies for
subsequent holes; and the use of multiple continuous records to unambiguously construct complete composite sections for each
site. The complete, continuous records provided by the GRAPE (with a temporal resolution of often yr), in conjunction with an
excellent microfossil stratigraphy and often excellent magnetostratigraphy, allowed for astronomical tuning of the stratigraphic
record and resulted in a set of internally consistent, high resolution age models that provide a secure, absolute time scale for the
past 6 m.y. For the period before 6 m.y., the absolute time calibration is less secure, but it is still better than any previously offered.
The high resolution stratigraphic framework of Leg 138 provided new insight into the previously ambiguous tectonic history
of the region. By assuming that maximum sedimentation rates along the north south transect would be expected at the equator, the
Leg 138 stratigraphy supports the 1985 work of Cox and Engerbretson, which calls for two different poles of rotation of the Pacific
Plate during the interval 0-20 Ma. The Leg 138 plate reconstructions also support several previously hypothesized ridge crest
jumps and a slowing of the absolute motion of the Nazca Plate at about 5 Ma.
Although Leg 138 data that predates about 13 Ma is limited, the impression that one can gain from these data is that the eastern
equatorial Pacific was characterized by relatively high carbonate concentrations and accumulation rates before about 11 Ma. This
pattern was interrupted occasionally by rapid massive outpourings of near monospecific laminated diatom oozes that probably
represent the formation of massive mats along strong surface water fronts. The laminated diatom oozes (LDO) continue to be
present in the Leg 138 record (many of them being expressed as seismic reflections) until about 4.4 Ma. Carbonate accumulation
rates begin to decline slowly between 11 and 9.8 Ma, when, at about 9.5 Ma, a near complete loss of carbonate (the "carbonate
crash") takes place everywhere in the Leg 138 region (and beyond), except at the westernmost sites close to the equator.
The "carbonate crash" was a time of fundamental change for the eastern equatorial Pacific, and perhaps for most of the ocean
basins. Unlike many of the carbonate variations that precede and postdate it, this "crash" represents a major dissolution event
whose effects can be traced seismically in the central and western Pacific. The changes in bottom water chemistry associated with
this event (or series of events) appear to be related to the early phases of the closing of the Panama Gateway. The role of NADW
initiation and intensification for controlling carbonate accumulation in the eastern equatorial Pacific is still not resolved; however,
ocean modeling demonstrates that the closing of the Panama Gateway may also have a direct influence on NADW production.
Therefore, the effects of changes in the Panama Gateway sill depth and the production of NADW may be manifested in the history
of eastern equatorial Pacific sedimentation.
The "carbonate crash" was followed by a recovery of the carbonate system (except in the Guatemala and Peru basins, which
never recovered) that led up to the late Miocene/ early Pliocene sedimentation rate maxima, during which equatorial sedimentation
rates are as much as five times greater than those of the late Pliocene or Pleistocene. Examination of modern productivity/ preser
vation relationships implies that the sedimentation rate maximum was the result of enhanced productivity. The distribution of
eolian sediments and isotopic gradients, along with an analysis of the modes of variance in carbonate deposition over the last 6
m.y., suggest a more northerly position of the Intertropical Convergence Zone (ITCZ), a stronger north south gradient across the
equator, and a more zonal circulation focused along the equator during the time of maximum sedimentation. The mechanisms
suggested for these changes in circulation patterns include the response of the eastern equatorial Pacific to the closing of the
Isthmus of Panama, as well as a global increase in the flux of Ca and Si into the oceans, a possible response to evolution of the
Himalayas and the Tibetan Plateau.
In an effort to understand the response of the climate system to external (orbital) forcing, 6-m.y.-long, continuous records of
carbonate (derived from GRAPE), δ¹⁸O and insolation were analyzed and compared. Evolutionary spectral calculations of the
variance and coherence among these records indicate that the insolation record is dominated by precessional frequencies, but that
the relative importance of the two precessional frequencies has changed significantly over the last 6 m.y. In general, precessional
forcing is not found in the carbonate or isotopic records. In the tilt band, however, a linear response is present between solar forcing and the carbonate and isotope records over some intervals. The carbonate record appears to be tightly coupled to the tilt component
of insolation before about 1.9 Ma; however, the isotope record does not begin to show sensitivity to orbital tilt until about 4.5 Ma,
the time of significant changes in sedimentation patterns in the eastern equatorial Pacific. Only during the last 500,000 yr do all
frequencies respond in a similar manner; we also see a marked increase in the response of the isotopic record to orbital forcing
(including 100,000- and 400,000-yr periods).
- Pisias, N.G., L. Mayer, A.C. Mix (1995). Paleoceanography of the Eastern Equatorial Pacific during the Neogene: Synthesis of Leg 138 Drilling Results <http://www.coas.oregonstate.edu/facultypages/mix/Pisias_etal_1995_ODP138_SR .pdf>. In: Pisias, N.G., L. Mayer, T. Janecek, A. Palmer-Julson, T.H. van Andel (eds.), /Proceedings of the Ocean Drilling Program, Scientific Results,/ 138, College Station, TX (Ocean Drilling Program), 5-24.