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Major Lithologies of the Paleogene Equatorial Pacific

The sediments drilled during the Paleogene equatorial transect fall into the following five broad litho-chronostratigraphic units (Fig. F6): (1) a surficial clay unit, sometimes containing a basal radiolarian ooze with a basal age ranging from the middle Miocene in the south of the transect (Site 1218) to the early–middle Eocene in the north (e.g. Sites 1215 and 1216); (2) a nannofossil ooze/chalk unit whose base is at the E/O boundary and whose top is of early Miocene age in the south (Sites 1218 and 1219) and Oligocene age in the central part of the transect (Sites 1217, 1220, 1221, and 1222); Oligocene–Miocene carbonates are nonexistent in the north (Sites 1215 and 1216); (3) a middle–upper Eocene radiolarian ooze and radiolarian clay that is present at all sites except those in the north (Site 1215 and 1216); (4) a lower middle–lower Eocene unit composed of cherts, clays, and radiolarian ooze that is present in varying thicknesses at all Leg 199 sites along the 56-Ma transect except one in the extreme north (Site 1215); and (5) a lower Eocene–upper Paleocene nannofossil ooze or chalk resting upon basalt basement that is recovered at all sites except Site 1222 (where cherts overlie basement) and Site 1216 (where the relevant stratigraphic interval was not drilled).

Surficial Clay and Radiolarian Ooze

The upper clay unit is composed of wind-blown dust, clays, and radiolarians eroded and reworked from older sediment outcrops, authigenic minerals, and in sites marginal to the modern equatorial region, freshly deposited radiolarians.

The clay mineral composition of eolian dust transported to Leg 199 sites may provide a way to track the latitudinal position of the ITCZ (Fig. F7). Enhanced precipitation in the ITCZ helps to "wash out" dust particles from the atmosphere, forming an effective barrier to interhemispheric dust transport. In other words, dust generated in the Northern Hemisphere is restricted to latitudes north of the ITCZ. Asian dust (rich in quartz and illite clays) dominates the composition of dust deposited in the Pacific north of the ITCZ. Just south of the ITCZ, a clay mineralogy dominated by smectite is transported via the trade winds from Central and South American source regions. During Leg 199 we used light absorption spectroscopy (LAS) to produce shipboard estimates of clay mineralogy. In Figure F7, plots of mineralogical change downcore are shown vs. depth in core. Using paleoposition information, one can then identify the change in ITCZ through time by the change from an illite-rich to smectite-rich clay suite (Fig. F8).

Radiolarian ooze or clayey radiolarian ooze are important sediment lithologies in the basal part of the surficial clay unit in Leg 199 sites drilled near the modern equator. These oozes are found at Sites 1219–1221 (Fig. F6). In each case, the site occupied an off-equatorial position at the time the radiolarian ooze was deposited, in the range of 3°–6° north of the paleoequator. Thus, in the Neogene and latest Oligocene, radiolarian ooze is deposited on the fringes of the equatorial upwelling zone. The zone of radiolarian ooze deposition is bound on the south by the appearance of foraminifers and disappears to the north as primary productivity decreases.

Oligocene–Lower Miocene Nannofossil Ooze/Chalk

The second broad litho-chronostratigraphic unit along the Leg 199 transect is the nannofossil ooze that first appears at the base of the Oligocene. The clay or radiolarian ooze to carbonate transition is an outstanding marker for the E/O boundary because it is very abrupt (see "Eocene/Oligocene transition"). The unit has cyclic variations of carbonate content related to orbitally driven changes in insolation. This cyclic variability will be key to construct an orbitally tuned Oligocene age model of the sediments and to calibrate ages for magnetostratigraphy and biostratigraphy (see section on stratigraphic intercalibrations).

The nannofossil ooze/chalk unit is thickest at sites near the Oligocene equatorial region and thins to the north (Fig. F6). Nevertheless, traces of the lower Oligocene carbonate can be found as far north as Site 1217 (16°52'N). At the beginning of the Oligocene, Site 1217 was located ~9° north of the equator at a water depth of 4400 m.

The age of the uppermost nannofossil ooze/chalks depends on the paleoposition and water depth of each drill site. In the south (Sites 1218 and 1219), the uppermost nannofossil ooze unit has an age near the late Miocene. Traversing north, the uppermost nannofossil ooze at Site 1220 (10°11'N) is late Oligocene in age. Sites 1221 (12°02'N), 1222 (13°49'N), and 1217 (16°52'N) exhibit a carbonate interval only in the sediments of early Oligocene age.

Late–Middle Eocene Radiolarian Ooze

The third broad litho-chronostratigraphic unit along the Leg 199 transect consists of radiolarian ooze, clayey radiolarian ooze, and radiolarian clays (Fig. F6). Clay content tends to be higher at the top and bottom of the unit. The radiolarian oozes are characteristic of the Eocene section and are the most enigmatic facies from a modern perspective because of the lack of a true modern analog.

In the southern sites of the Leg 199 transect (Site 1219) and in shallower drill sites (Site 1218 and DSDP Site 162) carbonate can be found at low concentrations in the upper Eocene and at higher levels in the middle Eocene, especially within magnetic Chron C18 (~39–41 Ma). For the rest of the transect, CaCO3 is absent from this unit even in sites located at the contemporaneous paleoequator. The latitudinal width of the late–middle Eocene zone of radiolarian deposition varies through time. The middle Eocene, between ~40 and 45 Ma, represents the greatest latitudinal expansion—radiolarian oozes were found from the equator to a paleolatitude between 10° and 11°N (between DSDP Site 40 and Site 1216). In contrast, radiolarian deposition is confined to within a few degrees of the equator in the latest Eocene. Along the 56-Ma transect, radiolarians are deposited only to the south of Site 1221, located at a paleolatitude of ~°N. Sites 1221, 1222, and 162 all have a hiatus just below the Oligocene.

One of the striking features of a unit with such high biogenic silica content is the relative absence of diatoms. Only one major interval in the Eocene radiolarian ooze contains a relatively high diatom content (>10% diatoms in smear slides; Fig. F9). High numbers of diatoms appear in magnetic Chron C18 (radiolarian zone RP15; 39–41.5 Ma) and are associated with the appearance of carbonates in the same sediment interval at Sites 1218 and 1219.

Lower Middle–Lower Eocene Cherts, Clay and Radiolarian Ooze

The fourth broad litho-chronostratigraphic unit along the Leg 199 transect consists of cherts, clays, and radiolarian oozes (Fig. F6). Despite high numbers in the test of the Eocene, radiolarians are rare to absent in the basal carbonates. This unit proved predictably troublesome to recover. Leg 199 drilling results indicate that cherts are present at the boundary between the lower and middle Eocene throughout the transect with the exception of Site 1215 in the extreme north. A younger cherty interval is present at Site 1218 near the middle/upper Eocene boundary, but this interval was continuously cored and recovered with the extended core barrel (XCB) (Fig. F10).

The lower–middle Eocene chert interval is relatively thin at Site 1219 in the south (~8 m thick;
7°48'N) but thickens to the north. At Site 1220 (10°11'N) the unit, including recovered cores of early Eocene radiolarian ooze, spanned 40 m. Poorly recovered cherty intervals span 30–40 m at Sites 1221 (12°02'N), 1222 (13°49'N), and 1217 (16°52'N). The unit is chronologically extensive. It typically first appears in radiolarian Zone RP11-12 (~45–49 Ma) and continues into the lower chalk unit around the top of radiolarian Zone RP7 (~53 Ma). Its relationship with the lower nannofossil chalk is not yet clear. At Site 1219 a transition from zeolitic clay to nannofossil chalk was recovered, suggesting that the base of the unit may be marked by a gradational transition from clay to CaCO3. However, chert was recovered associated with most of the basal nannofossil chalk recovered on the leg, so this lower unit is not free from chertification.

Lower Eocene–Upper Paleocene Nannofossil Ooze or Chalk

The basal litho-chronostratigraphic unit along the Leg 199 transect consists of nannofossil ooze or chalk of early Eocene–upper Paleocene age overlying basement basalt. This unit is present at all sites except perhaps Site 1222, (where only chert was recovered in the one hole drilled to basement) and Site 1216 (where the relevant stratigraphic interval was not drilled). In three of the seven sites targeted to recover the P/E boundary (1215, 1220, and 1221), this objective was fulfilled (see "Paleogene/Eocene Transition"). Where present, the basal carbonate unit is most lithified in the south where overburden is greatest (~250 m burial depth; Sites 1218 and 1219). In contrast to the nannofossil chalks recovered from these two sites, at Site 1215, where the burial depth is significantly shallower (~25 m), we recovered a nannofossil ooze. At three of the sites containing basal chalks (Sites 1217, 1218, and 1220) the sediments are partially to extensively dolomitized. It is unclear how the dolomite was formed, but it appears to be related to proximity to basement. Dolomitization of the basal sediments bears no clear relationship to location along the latitudinal transect.

Paleogene CCD

One of the most striking features of the Paleogene equatorial Pacific sediments is the rapid appearance and disappearance of CaCO3 through the stratigraphic record. Leg 199 drilling has significantly increased stratigraphic control for many of the Paleogene and early Neogene carbonate–noncarbonate transitions.

Van Andel et al. (1975) compiled DSDP data from all the drill sites within the region and used subsidence curves of ocean crust to develop the Cenozoic history of calcium carbonate deposition. This compilation shows that the Eocene was marked by a shallow CCD or level in the water column where the pelagic rain rate of biogenic carbonate is equal to the rate of dissolution at depth. Specifically, the Eocene CCD in the equatorial Pacific is estimated to have been ~3400 m, shoaling to ~3200 m beyond 4° from the equator to either the north or south. Furthermore, one of the most prominent changes in the CCD through time captured in this early compilation was a pronounced deepening (by ~1600 m in the equatorial region) associated with the transition from the Eocene to the Oligocene. Shipboard results from Leg 199 support this early interpretation of events. In fact, our shipboard data demonstrate that the correlation between the Eocene–Oligocene transition and the CCD deepening is strikingly precise and consistent across wide tracts of the tropical Pacific. These findings, raise the intriguing possibility that this pronounced deepening of the CCD is in some way related to the first widely accepted sustained glaciation in Antarctica (Oi-1; Fig. F5). The nature of the relationship between these two important paleoceanographic signals will be an important component of shore-based Leg 199 research. In addition, our shipboard data also demonstrate that the transition from silica-rich/carbonate-poor Eocene to silica-poor/carbonate-rich Oligocene deep-sea sediments is remarkably rapid. The transition from carbonate-free to carbonate-bearing sediments is sharp, typically occurring over a 10- to 20-cm interval (Fig. F11) and implying that it took only a few tens of thousands of years to introduce the changes in ocean chemistry that depressed the CCD to >1 km below its Eocene depth.

Leg 199 sediments define other major CCD changes within the Eocene that will be better studied after the cruise. The Paleocene–lower Eocene basal chalks define a relatively shallow CCD in the beginning of the interval drilled during Leg 199 (Fig. F12). The transition from chalk to clay and radiolarian ooze occurs when near-equatorial sites passed through a water depth of only ~3200–3300 m (Sites 1219–1221), whereas the off-axis sites record a shift from carbonate to clay when they passed below 3400–3600 meters below sea level (mbsl) (Sites 1215 and 1217). In contrast, the Oligocene and Neogene CCD is ~4500–4600 m (Fig. F12). The difference in early Eocene CCD between equatorial and northern sites is greater than the errors associated with the estimation and is the reverse of the Neogene trend. Typically, in the Neogene the CCD is deeper beneath the equatorial region because of higher carbonate production in the equatorial region relative to periequatorial regions, but this situation seems not to have been the case in the early Eocene.

Although carbonate disappears from the equatorial Pacific in the lower Eocene, CaCO3 is not absent from the Eocene record. A prominent CaCO3 event appears within magnetic Chron C18r at ~40–41 Ma (defined by Sites 1218 and 1219 in the south) that can also be found at DSDP Site 162 (van Andel, 1973) (Fig. F12). Smaller carbonate events occur within magnetic Chron C20n (~43 Ma) and near the base of C17n (~38 Ma) at Site 1219. The younger event can also be found at Site 1218. In contrast, none of these carbonate events can be found further north, nearer the middle Eocene paleoequator (Site 1220). In fact, there is no CaCO3 in the middle and late Eocene section at Sites 1220, 1221, or 1222.

The transition from chalks to radiolarite at ~40 Ma in Site 1218 appears as abrupt as the transition from radiolarian clay to nannofossil oozes at the E/O boundary. The rapid transitions of sediment type in either direction suggest that there is a strong climate switch at work. Sediments with poor carbonate preservation have also been identified during each of the Oligocene intervals that correlate to warm intervals between the "Oi" glacial advances of Miller et al. (1991). Thus, the Leg 199 shipboard scientists have developed a working hypothesis that carbonate deposition in the tropical Pacific is in some way associated with continental glaciations, whereas tropical Pacific radiolarian oozes are associated with warm global climates. If this hypothesis is substantiated, one implication would be that the transition from cold to warm climates can be as abrupt as the transition from warm to cold climates.

Equatorial Position

Drill sites move with their respective plates and must be backtracked in space as well as depth. Van Andel et al. (1975) recognized the paramount importance of backtracking tropical Pacific drill sites with respect to the motion of the Pacific plate, because the equatorial upwelling system leaves a strong imprint on the sediment column when the site passes underneath the equator. This generalization holds for the Oligocene and Neogene. We can recognize the passage of Site 1219 under the equator in the Oligocene by the sedimentary changes, but we find confusing sedimentary signals in the earlier record.

We expected that sedimentation patterns might be different in the Eocene, and so we made it a primary leg objective to obtain independent evidence for the position of the paleoequator through the Paleogene. This objective is especially important because the backtrack paths we have used to find paleopositions of Leg 199 sites assume a fixed Hawaiian hotspot, and there is good evidence that the Hawaiian hotspot moved with respect to the Pacific plate prior to 42 Ma (Tarduno et al., in press). In addition, small errors in the plate tectonic model, when propogated over long periods of time, may lead to relatively large errors in a drill site position over long intervals of time.

Shipboard whole-core paleomagnetic analysis has been sufficient to make a preliminary definition of the position of the paleoequator and the change in paleolatitude of Leg 199 drill sites in the Oligocene and in the late middle Miocene (Fig. F13, F14). These data will be refined postcruise with discrete sample analyses. They suggest that the fixed hotspot backtrack of paleopositions is sufficiently accurate to estimate paleopositions back to the middle Eocene. Paleomagnetic analysis of older sediments should yield further evidence for equatorial positions in the early Eocene.

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