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doi:10.2204/iodp.sp.320321.2008

Background

Geologic setting, previous drilling, and site survey

To develop a detailed history of the equatorial Pacific current system, the strategy pursued during the most recent ODP leg (199; also during Leg 138) was to drill along a line of equal oceanic crustal age, thus obtaining an approximate north–south transect across the major east–west currents during time intervals of particular interest.

In the Paleocene and Eocene, the shallow carbonate compensation depth (CCD) prevents deposition of carbonate except at shallow ocean crust. Drilling at the paleoposition of the ridge crest at the critical time interval allows recovery of the shallowest sections available in the pelagic oceans and thereby assures the best possible preservation of the carbonate sediments recovered. As the crust cools and sinks, the seafloor on which the sediments are deposited becomes deeper and deeper—and closer to the lysocline and CCD. Thus, the best preserved part of the sections recovered in such "time-line" transects is restricted by the depth at which carbonate dissolution significantly increases, as well as by the northward movement of sediment sections out of the region of high equatorial productivity. This limitation was exemplified by the results from ODP Leg 199, during which only limited amounts of carbonate prior to the Eocene/Oligocene boundary (e.g., at ODP Site 1218 on 42 Ma crust) were recovered.

For the PEAT science program, we plan to overcome this limitation of the time-line strategy by pursuing an equatorial age transect, or "flow-line" strategy (Figs. F3, F4) to collect well-preserved equatorial sections through the Cenozoic. The Pacific plate motion over this time will cause the sites to also form an oblique latitudinal transect across all time slices.

We will drill a series of sites on the paleoequator at key intervals in the evolution of the Cenozoic climate. These intervals span from the very warm early Eocene, through the cooling of the late Eocene and Oligocene and the early Miocene time of relatively warm climates (or low ice volume), and into sections deposited during the development of the major Southern and Northern Hemisphere ice sheets in the middle and late Miocene (Fig. F5). There are very few previous drill sites that match our site selection criteria. Each site will be close to the geographic paleoequator and on crust aged slightly older than the age intervals of particular interest.

In this way, we will be able to track paleoceanographic conditions at the paleoequator in the best preserved sediments obtainable. We can also make use of the high level of correlation between tropical sediment sections and seismic stratigraphy to develop a more complete model of equatorial productivity and sedimentation.

Selecting target ages

The time slices to be drilled during this campaign were chosen to cover the overall climatic history of the Cenozoic and to target particular times of marked changes in the climatic regime. The spacing of the sites was determined by what we know of the Cenozoic evolution of the lysocline from previous drilling. Where the CCD is particularly shallow, the spacing in time of age-transect sites needs to be closer than where the CCD is deep (Fig. F4). As a guide, Site 1218 was drilled on 42 Ma crust during times when the CCD was near 3.3 km. Nannofossil oozes were deposited at this location up to ~37 Ma before the crust at this site sank below the CCD. An age separation between drill sites of 5 m.y. is a maximum for the shallow CCD of the Eocene; for good preservation of foraminifers, an even closer spacing should be used. The results of our paleoequator reconstruction and proposed drill site locations are shown in Figure F6.

Site location strategy

In pursuing the history of the equatorial Pacific Ocean through both time-line and flow-line transects, we have two major advantages over the efforts that took place in the earlier days of scientific ocean drilling. Although previous drill sites have targeted the general area, they mostly do not fulfill all of our criteria in terms of (1) a sufficient number of holes to obtain a continuous record, (2) modern coring technology to obtain undisturbed sediments, (3) a location inside the paleoequatorial zone, or (4) a location on the right crustal age to ensure the presence of calcium carbonate at the targeted time slice.

We have positioned proposed Sites PEAT-1C through 8C somewhat to the south of the estimated paleoequatorial position at their target ages in order to maximize the time that drill sites remain within the equatorial zone (i.e., ±2° of the Equator), to allow for some error in positions (evidence suggests a southward bias of the equatorial sediment mound relative to the hotspot frame of reference; Knappenberger, 2000), and to place the interval of maximum interest above the basal hydrothermally altered sediments. In order to plan site surveys we used the digital age grid of seafloor age from Müller et al. (1997; based on Cande and Kent, 1995), heavily modified and improved with additional magnetic anomaly picks from Petronotis (1991) and Petronotis et al. (1994), and DSDP/ODP basement ages. For this grid, each point is then backrotated in time to zero age, using the fixed-hotspot stage-poles from Koppers et al. (2001) and Engebretson et al. (1985) and the paleopole data from Sager and Pringle (1988). Each latitude–longitude gridpoint was backrotated according to age. After each point was mapped to a new location by this transform, all point ages were regridded by contouring the set of all backrotated points for a given age.

The supporting site survey data for Expeditions 320 and 321 are archived at the IODP Site Survey Databank (ssdb.iodp.org).

Eocene (Sites PEAT-1C to 4C)

The Eocene was a time of extremely warm climates that reached a global temperature maximum near 52 Ma (Fig. F5). From this maximum there was a gradual climatic cooling through the Eocene to the Eocene/Oligocene boundary. There appears to have been a slight reversal to this trend within the middle Eocene, near 43 Ma, and in the late Eocene at 34–36 Ma, just prior to the pronounced drop in oxygen isotopes that marks the Eocene/Oligocene boundary and one of the most dramatic changes of the CCD (Fig. F4). Throughout the Eocene, the CCD lay near 3.2–3.3 km depth, albeit with potentially significant short-term fluctuations (Lyle et al., 2005). Thus, recovering well-preserved carbonate sediments from the equatorial region is a substantial challenge, but it is not impossible if the depth of the East Pacific Rise lay near the global average of 2.7 km. The Eocene equatorial upwelling system appears to differ from the modern equatorial upwelling regime by having strong secondary upwelling lobes ~10° in latitude away from the primary equatorial region (Figs. F7, F8). These upwelling lobes produce a much broader region of (relatively) high productivity than is present today.

Early and middle Eocene (Sites PEAT-1C, 2C; ~53 Ma and 50 Ma crust)

During Leg 199 a north–south transect was drilled across the equatorial region on oceanic crust of ~56 Ma age. Sites on this transect had generally drifted below the CCD by 52–53 Ma. Thus we presently lack calcareous sediments from the region of the equatorial circulation system during the time of maximum Cenozoic warmth. Proposed Site PEAT-1C has been located on crust with an estimated age of ~53 Ma in order to intercept the interval between 53 and 50 Ma in basal carbonate sediments above the shallow early Eocene CCD (4200–4300 m). This interval was poorly sampled during Leg 199.

Average (noncarbonate) accumulation rates in the early Eocene were moderate, showing only slight increases in some of the more northern sites on the Leg 199 transect (Sites 1215 and 1220). What is particularly interesting in the records of Leg 199 is that the very shallow CCD of this early Eocene time appears to deepen to the north, perhaps suggesting a northern source for the bottom waters. Sites targeting this time interval would ideally give us sediments with sufficient carbonate material to better constrain the isotopic and biotic characteristics of the near-surface equatorial waters.

During the early Eocene, a very shallow CCD and typical rapid tectonic plate subsidence of young crust near the shallow ridge-crest conspire to make the time window during which carbonate is preserved short (~2–5 Ma). Thus, although good records of pelagic carbonates during and just after the Paleocene/Eocene Thermal Maximum (PETM) were recovered at Leg 199 sites (Lyle, Wilson, Janecek, et al., 2002; Raffi et al., 2005; Nuñes and Norris, 2006), the time period of the early Eocene Climatic Optimum (Zachos et al., 2001a) is not well sampled.

Proposed Sites PEAT-1C (52 Ma crust) and 2C (49–50 Ma crust) aim to provide the sedimentary archive to address causes and responses of the true Cenozoic "Greenhouse" world: the Eocene was a time of extremely warm climates that reached maximum temperatures near 52 Ma (Zachos et al., 2001a). From this maximum there was a gradual climatic cooling to the Eocene/Oligocene boundary. We have positioned the sites to the south of the estimated paleoequatorial position at the target age in order to maximize the time that drill sites remain within the equatorial zone (i.e., ±2° of the Equator), to allow for some southward bias of the equatorial sediment mound relative to the hotspot frame of reference (Knappenberger, 2000) and to place the interval of maximum interest above the basal hydrothermal sediments.

Middle and late Eocene (Site PEAT-3C; 46 Ma crust)

Good paleomagnetic stratigraphy at Leg 199 sites allowed a much improved calibration of nannofossil and radiolarian biostratigraphic datums (Moore et al., 2004; Raffi et al., 2005; Pälike et al., 2005, 2006b; Nigrini et al., 2006). From the combined information, a more detailed picture emerged of temporal variations in sediment accumulation through the middle and upper Eocene of the tropical Pacific. These data show an increase of up to 2–3 times in accumulation rates of siliceous ooze within the middle Eocene (41–45 Ma). There are also several notable periods of highly fluctuating CCD associated with intervals in which carbonate is preserved to 4000 m, or ~700 m deeper than the average Eocene CCD (Lyle, Wilson, Janecek, et al., 2002; Lyle et al., 2005; Rea and Lyle, 2005). Such fluctuations in the CCD are similar in magnitude to those at the Eocene/Oligocene boundary (Coxall et al., 2005).

High siliceous sedimentation occurs near an apparent short reversal in the middle Eocene cooling trend (Fig. F5). It is difficult to interpret the cause of such a substantial change in silica flux during a very warm climatic regime. At the very least we need good carbonate recovery during this interval in order to apply the substantial array of carbonate-based proxies to this interval in order to evaluate the temperature and structure of the near-surface ocean.

Eocene/Oligocene boundary (Site PEAT-4C; 38 Ma crust)

Proposed Site PEAT-4C targets the events bracketing the Eocene–Oligocene transition, with the specific aim to recover carbonate-bearing sediments of latest Eocene age (Kennett and Shackleton, 1976; Miller et al., 1991; Zachos et al., 1996; Coxall et al., 2005; Exon, Kennett, Malone, et al., 2001) and testing the hypothesized magnitude of Eocene glaciation events (Lyle et al., 2005; Tripati et al., 2005). Site PEAT-4C is located on upper middle Eocene crust with an estimated age of ~38 Ma. The Eocene–Oligocene transition marks the most dramatic deepening of global CCD in the Cenozoic (van Andel, 1975). Coxall et al. (2005) (Fig. F9) demonstrate that the change in CCD coincides with a rapid step-wise increase in benthic oxygen stable isotope ratios, reflecting the growth of the Antarctic ice sheet.

The apparent latest Eocene climate cooling and increased primary productivity at low latitudes seems somewhat at odds with the apparent slight warming indicated by the oxygen isotopes (Fig. F5). These oddities, together with the major changes in planktonic assemblages, suggest an important restructuring of the upper mixed layer and thermocline waters in the Pacific that continues into the lower Oligocene. Well-recovered and well-preserved equatorial sections across this relatively short transition between warm and cold global climates will be very valuable in determining the impact of high-latitude ocean boundary changes on climate, circulation, and productivity in the equatorial region.

So far the most complete Eocene/Oligocene boundary section recovered from the equatorial Pacific has been ODP Site 1218 on 42 Ma crust; however, it is far from pristine. Carbonate percentages drop markedly below the boundary and go to zero near 34 Ma (Lyle et al., 2005; Coxall et al., 2005). This prevented the recovery of information about paleoceanographic conditions prior to the Eocene–Oligocene transition and also has implications for the interpretation of paleotemperature proxies such as Mg/Ca ratios in foraminiferal shells that were bathed in waters with very low carbonate ion concentrations (Lear and Rosenthal, 2006; Elderfield et al., 2006). Data from Site 1218 allowed the astronomical time calibration of the entire Oligocene (Coxall et al., 2005; Wade and Pälike, 2004; Pälike et al., 2006b), but the lack of carbonate in the uppermost Eocene at this site made the detailed time control now available for the Oligocene much less certain for the late Eocene. Site PEAT-4C is located on estimated crustal basement age of ~38 Ma and crossed the paleoequator shortly thereafter. It was located to provide the missing information about the crucial chain of events prior and during the Eocene–Oligocene transition.

Oligocene (Site PEAT-5C; ~32 Ma crust)

Proposed Site PEAT-5C targets the Oligocene and is located on lower Oligocene crust. This interval of time is noted for its markedly heavy benthic oxygen isotopes (Fig. F5) and its relatively deep CCD (Fig. F4). There was probably ice on Antarctica during this interval, but not the large ice sheets found there later in the middle Miocene. There is no compelling evidence for ice sheets in the Northern Hemisphere during the Oligocene and early Miocene. Thus, there was apparently a relatively low global ice volume, relatively cold bottom waters, a relatively cold South Pole, and a relatively warm North Pole. This scenario of a "one cold pole" world has given rise to speculation on the impact of interhemispheric temperature imbalance on pole-to-Equator temperature gradients and on the symmetry of the global wind systems. The extent to which such an imbalance may have affected the trade winds, the position of the Intertropical Convergence Zone, and the seasonal shifts in this zone should be seen in the wind-driven currents of the equatorial region.

The older, low-resolution DSDP data indicate relatively high but variable sediment accumulation rates during this interval and better carbonate preservation south of the Equator (van Andel et al., 1975). In the Leg 199 equatorial transect, the highest accumulation rates encountered (>15 m/m.y.) occurred in the lower part of the Oligocene, but these were in sites north of the Oligocene Equator or on relatively old (and therefore deep) crust. Thus we expect a better preserved, thicker carbonate section at the Oligocene Equator. Studies of Oligocene sections from Leg 199 and from other ODP sites (e.g., Paul et al., 2000; Zachos et al., 2001b; Billups et al., 2004; Pälike et al., 2006a) indicate the presence of strong eccentricity and obliquity cycles in carbonate preservation and suggest a strong (southern) high-latitude influence on the carbonate record. These cycles are leading to the development of an orbitally tuned timescale that reaches back to the base of the Oligocene (Pälike et al., 2006b). Such a timescale will make it possible to develop a very detailed picture of equatorial geochemical fluxes and of the degree of variability in the equatorial system of the Oligocene.

Latest Oligocene–earliest Miocene (Site PEAT-6C; 27 Ma crust)

Proposed Site PEAT-6C will focus on paleoceanographic events in the late Oligocene and into the early and middle Miocene, including the climatically significant Oligocene–Miocene transition and its recovery. In conjunction with proposed Sites PEAT-5C and 7C, Site PEAT-6C is also designed to provide a latitudinal transect for early Miocene age slices.

At the end of the Oligocene there is a significant multimillion-year-long rise in the oxygen isotope record (Lear et al., 2004), which is closely followed by a relatively short, sharp increase in oxygen isotope values that has been interpreted as a major glacial episode ("Mi-1") (Fig. F5) (Paul et al., 2000; Zachos et al., 1997, 2001a, 2001b; Pälike et al., 2006a) and correlated to a pronounced drop in sea level (Miller et al., 1991). This event is very close to the Oligocene/Miocene boundary and has now been astronomically age calibrated in several ocean basins (Shackleton et al., 2000; Billups et al., 2004). Although there are clear periodic isotopic signals indicating major changes in ice volume, ocean temperatures, and/or ocean structure, this biostratigraphic boundary has always been somewhat of an enigma. Unlike the major changes in the isotopic stratigraphy, the biostratigraphies of the planktonic microfossils show very little change at all across this boundary. In fact it is one of the most difficult epoch boundaries to pick using solely the microfossil biostratigraphies.

At Sites 1218 and 1219 of ODP Leg 199 this interval was well recovered; however, carbonate preservation still presented a problem for classic foraminiferal stratigraphy. Both sites were deep and well within the lysocline, making the application of temperature proxies such as Mg/Ca ratios in foraminiferal tests more difficult (C.H. Lear, pers. comm., 2006). At the time mid-Oligocene sediments were deposited, Site 1218 already resided on 18 m.y. old crust and was ~4100 m deep. Site 1219 was on ~34 m.y. old crust and was ~4500 m deep. There was a relative increase in the large diatoms near this boundary in the siliceous coarse fraction, suggesting increased productivity; however, detailed, high-resolution flux rates across this interval have yet to be determined. A well-recovered section on the latest Miocene–Oligocene Equator, near the late Oligocene ridge-crest as targeted by Site PEAT-6C, should provide both the resolution and the preservation required to better describe the changes in the equatorial ocean taking place at this time.

Miocene (Site PEAT-7C; 24 Ma crust)

Site PEAT-7C is proposed for drilling to focus on the paleoceanographic events in the early and middle Miocene. The latest Oligocene through the middle Miocene appears to have been a time of relative warmth comparable to the latest Eocene. However, the variability in the isotopic record of the early to middle Miocene is larger than that of the Eocene and may indicate more variability in climate and in global ice volume. The climatic "optimum" at ~15 Ma comes just before the major development of ice sheets on Antarctica and the marked increase in ice-rafted debris in circum-Antarctic sediments. The early Miocene also marks a major evolutionary change from the relatively static Oligocene planktonic biota. In the equatorial Pacific, the late Oligocene to early Miocene marks the beginning of abundant diatoms in the stratigraphic record (Barron et al., 2004) and thus may represent a major change in carbon cycling as well.

The only major ocean boundary change proposed for the time near the Oligocene/Miocene boundary was the opening of the Drake Passage to deep flow; however, there is some debate as to the exact timing of this event (Barker, 2001; Pagani et al., 1999; Lawver and Gahagan, 2003; Scher and Martin, 2006), and its direct impact on the tropical ocean is uncertain. It may be that, as in the Eocene/Oligocene boundary section, the link lies in the shallow intermediate waters that provide nutrients to lower latitude upwelling regions. For the equatorial region, an even more pertinent question is what changes were occurring in the Miocene tropical ocean that led to this burst of Miocene evolution?

Middle Miocene (Site PEAT-8C; ~18 Ma crust)

In principle, the age-transect strategy of this proposal would not be complete without data from the Pliocene–Pleistocene. However, in addition to the logistical reasons of cruise length, near-paleoequatorial records have already been targeted by ODP Legs 138 (Pisias, Mayer, Janecek, Palmer-Julson, and Van Andel, 1995) and 202 (Mix, Tiedemann, Blum, et al., 2003), and these provide information about the development of Northern Hemisphere glaciation. Our last proposed site focuses instead on the interesting events following a middle Miocene maximum in deposition (van Andel et al., 1975).

Site PEAT-8C is proposed for drilling to focus on the paleoceanographic events following a middle Miocene maximum in deposition (van Andel et al., 1975). In addition, large changes in the glaciation state and frequency have recently been described in the middle Miocene (Holbourn et al., 2005; Abels et al., 2005), in the interval following ~14 Ma. There is a wide latitude range of CaCO₃ deposition during the earliest Neogene, with a relatively sharp transition to a narrower CaCO₃ belt after 20 Ma (Lyle, 2003). CaCO₃ mass accumulation rates in the central equatorial Pacific recovered from the 18–19 Ma "famine" and in the period between 14 and 16 Ma reached a second maximum in carbonate deposition, which is also evident in the seismic stratigraphy of the equatorial sediment bulge (Knappenberger, 2000; Mitchell et al., 2003). We designed Site PEAT-8C to recover an equatorial record at the early middle Miocene sedimentation maximum.

Understanding the interplay between the CCD, CaCO₃ dissolution, and productivity

The Pacific, specifically the equatorial upwelling zone, is the largest oceanic source of CO₂ to the atmosphere and controls atmospheric CO₂ levels (Dore et al., 2003). The release and uptake of CO₂ is the direct consequence of calcium carbonate deposition and the interplay between nutrient delivery, carbonate dissolution, surface water productivity, and export of biogenic carbonate from surface waters to the sediment pile. Distinguishing between the effects of carbonate dissolution and productivity has been a field of intense study in the past. An important objective of this proposal is to address the detailed workings of depth-dependent carbonate dissolution, which is intricately linked to the climate system and paleoceanography. In the standard model for carbonate dissolution, accumulation rates locally decrease linearly from a lysocline to a CCD, reflecting a linearly increasing rate of dissolution. The depth of both of these mapable surfaces varies spatially and temporally, with a result of climatic and physical processes. The equatorial Pacific is one of the classical areas where the lysocline–CCD model was first developed, but there has been little subsequent effort to test it, a necessary step, considering that the functional form of dissolution is now known to depend in a more complex way on organic carbon burial and water mass properties. The age transect will provide the necessary additional data with which to test the carbonate paradigm and to recover previously unavailable carbonate material from important Paleogene time slices in the Pacific.

Specifically, the recovery of shallowly buried carbonate sediments from near the paleoequatorial upwelling zone would contribute significantly to separating the various processes that affect carbonate deposition and preservation and would reduce some of the processes that affect climatic proxy records, such as diagenetic recrystallization (Pearson et al., 2001). Neogene productivity has been strongly oriented parallel to the Equator, so differences in carbonate thicknesses at a common latitude but differing depths will permit the effect of dissolution to be isolated following Lyle (2003), Mitchell et al. (2003), and Mitchell and Lyle (2005). In addition, the strategy adopted in this program will provide new data throughout the Cenozoic with which it will be possible to map the spatial evolution of the equatorial CCD with time. This is because the northward component of the Pacific plate movement results in the multiple recovery of the same time slice in different sites but with a slightly different paleolatitude.

Recovering more detailed records from the best possible material will also allow a better understanding of physical processes that might affect or hinder our interpretation of carbonate proxy records, such as the "carbonate ion effect"—an observed and modeled influence of the carbonate ion concentration on stable isotope fractionation in carbonate (Spero et al., 1997; Zeebe and Wolf-Gladrow, 2001).

Preliminary work with Ewing seismic data (Mitchell et al., 2003) has revealed a surprising lack of correlation between dissolution and depth in the westerly region of this study area. Our aim is to develop a more extensive three-dimensional model for the stratigraphy of equatorial Pacific deposits that links all existing core data using a grid of high-resolution seismic reflection profiles, including our new data from the PEAT site survey onboard the Roger Revelle in 2006 (AMAT03). The numerical stratigraphic model will then be used to assess carbonate dissolution, in particular the spatial pattern of sharp changes in dissolution such as the extremely abrupt change in the CCD at the Oligocene/Eocene boundary, which has been linked to a possible abrupt onset of continental weathering. The sediment archive recovered at the PEAT will allow the application of the substantial array of carbonate-based proxies with which the wider regional seismic study can be ground-truthed.

Reconstructing paleoceanographic properties and sea-surface temperatures

A large number of paleoceanographic interpretations rely on obtaining proxy data (stable isotope measurements, elemental ratios such as Mg/Ca, sea-surface temperature [SST] estimates from faunal distributions and isotope data, alkenone proxies, geochemical productivity, and burial indicators, etc.). In turn, a very large number of these measurements rely on the presence of biogenic calcium carbonate. For the Pacific, the drilling strategy we propose is conceptually the best approach to recover this important material with the best possible preservation and the least amount of diagenetic effects for a long intervals throughout the Cenozoic and will thus contribute to the objectives of the IODP Extreme Climates Initiative.

Spatial range considerations

In order to recover the best preserved and most complete carbonate record from the paleoequatorial Pacific, the age-transect siting strategy necessarily implies a restricted north–south transect, even though the northward movement of the Pacific plate does allow us to recover identical time slices multiple times at different paleolatitudes (separated by several degrees). However, we note that the regional seismic study to be developed as part of our site survey work gives us the opportunity to integrate data from older drill sites with new drilling. The site survey linked the new sites to key drill sites from ODP Legs 9, 85, 138, and 199. The combination will give us more detailed knowledge concerning the age transect so that a more complete model of the evolution of the equatorial Pacific can be developed.

Paleomagnetic objectives

One important aspect of the PEAT science program is the recovery of high-quality paleomagnetic data so that attempts to improve existing geological timescales (Gradstein et al., 2004) can be extended further back in time. Results from ODP Leg 199 demonstrate that these records can be recovered from near-equatorial carbonate (e.g., Lanci et al., 2004, 2005). During Leg 199 we succeeded in recovering almost all magnetic reversals from the Paleogene through to the present. However, biogenic carbonate sediments through most of the Eocene, nor for ages younger than the lower Miocene, were not recovered during Leg 199. Thus, although the paleomagnetic record during these times was of high quality, global stratigraphic correlation is hindered by the lower mass accumulation rate, the absence of a detailed isotope stratigraphy, and sparser biostratigraphic control. In order to facilitate the development of an integrated magneto- and biostratigraphic framework with a stable isotope stratigraphy (necessary to enable global correlation), recovery of magnetic reversals within carbonate sediment is desirable. This prerequisite contributed to the strategy described in this proposal.

In addition, further detailed paleomagnetic, magnetostratigraphic, and magnetic rock fabric data, most importantly from the Eocene, will help to resolve the suggestion that the geographic Equator, as determined from the biogenic sediment bulge, might not coincide with the paleoequator position backtracked with a fixed-hotspot reference frame (Moore et al., 2004; Tarduno, 2003; Acton and Gordon, 1994; Parés and Moore, 2005).

Ancillary benefits (MORB, basement)

Our proposed drilling aims to recover basement samples at all sites. A transect of mid-ocean-ridge basalt (MORB) samples from a fixed location in the absolute mantle reference frame would be a unique sample suite and, although not one of the primary objectives of this proposal, should be of strong potential interest to mantle geochemists. In addition, a transect of basalt samples along a flow line that have been erupted from similar environments should be of interest for low-temperature alteration studies (see, e.g., Elderfield and Schultz, 1996).

Constructing the age transect

The location of drill sites has been accomplished through seismic survey during the site survey Cruise AMAT03 onboard the Roger Revelle in 2006. We employed our current knowledge of seafloor age, plate rotation models, and the history of the CCD to initially locate the surveys.

The determination of possible areas for drill sites depends on three types of information: (1) a map of seafloor age, determined from magnetic lineations and previous drilling results; (2) plate rotation models that allow us to hindcast the position of the paleoequator; and (3) a detailed history of the CCD. The reconstruction of the CCD was pioneered by van Andel (1975) and can now be supplemented with additional results from recent drilling. Figure F4 illustrates the resulting CCD history for the Pacific. Importantly, the early Cenozoic time interval coincides with a very shallow CCD, which makes the location of drill sites more critical. In addition, a shallow CCD implies that one can only obtain calcareous sediments over much shorter time intervals.

The reconstructed history of the CCD was then used as a guide as to how time intervals of particular interest (Fig. F5) can be drilled. This approach is shown in Figure F4, where we have plotted "ideal" crustal ages needed to drill each of the eight time intervals of interest while remaining above the CCD. In order to implement our age-transect approach, crustal ages shown in Figure F4 have to be translated to specific locations on today's ocean floor. In particular, for each crustal age shown in Figure F4, we attempt to locate those sites that were positioned at the paleoequator during the time interval of interest.

Results are shown in Figure F6, together with proposed drill sites, which take into account plate rotation with respect to the paleoequator and which are shifted slightly toward the south to accommodate a potential error in the fixed-hotspot rotation model (Moore et al., 2002, 2004) and to maximize the time in the equatorial zone (±2° of the Equator). The model presented in Figure F6 is partly corroborated by comparison with previous drilling results. For example, DSDP Site 78, near proposed survey area PEAT-5, has a basement age as predicted.

The equatorial grid of seismic reflection lines needed to extrapolate from our borehole data to the region has been amassed primarily from site surveys for scientific drilling. Digital seismic reflection data have been collected from site surveys for DSDP Leg 85 and ODP Legs 138 and 199, as well as the new survey for the PEAT science program.

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