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RESULTS (continued)

Eocene Radiolarian Ooze

The difference between equatorial Pacific Eocene sediments and those of the Oligocene and Neogene is striking. Everywhere along the Leg 199 transect, upper lower Eocene to uppermost Eocene sediments consist primarily of one microfossil group, the radiolarians, mixed with varying amounts of clay. In contrast, Oligocene sediments are dominantly nannofossil rich and also contain a diverse mixture of foraminifers, radiolarians, and diatoms. One of the primary tasks of Leg 199 is to decipher why radiolarian ooze is so common in the Eocene.

The task is made more difficult because analog Neogene and Holocene radiolarian oozes are rare and occupy a specific zone in the equatorial sedimentation regime that is not the same as the equivalent deposit in the Eocene. For example, the"type" Eocene radiolarian ooze (e.g. the middle Eocene ooze of Site 1220 at ~120 mbsf; more specifically, Core 199-1220B-9H, roughly Chron C20n, or 43 Ma) consists of 80%–90% radiolarians, <2% diatoms, and 10%–15% clay. The rest of the sediment is made up of opaque minerals. The sedimentation rate for this interval is 8.3 m/m.y. The total thickness of middle–upper Eocene radiolarian ooze at Site 1220 is 80 m, and it was deposited over 13 m.y. During that time, Site 1220 traversed from ~1°S to 1.5°N, based upon a fixed hotspot model to determine paleopositions.

In contrast, one of the best examples of a lower Miocene radiolarian ooze can be found in the upper sediments of Site 1220 as well. It also is composed dominantly of radiolarians (40%–80%), <2% diatoms, and has ~20% clay. The remainder of the sediment is made up of nannofossils. The sedimentation rate, however, is 2.3 m/m.y., the total thickness of the radiolarian ooze is 18 m, and the sediment was deposited when Site 1220 was between the latitudes of 4° and 6°N. This deposit was formed just beneath the early Miocene CCD but also at the edge of the equatorial productivity zone and as such is oceanigraphically somewhat similar to modern Pacific radiolarion deposits.

Neogene material deposited beneath the equatorial region, even without carbonate, would still contain large numbers of diatoms and would not be equivalent to the Eocene radiolarian oozes. Site 849, located at the modern equator, is roughly at the equivalent longitude and latitude as occupied by Site 1220 in the early Eocene. The sediments of Site 849 contain from two to ten times as many diatoms as radiolarians throughout the entire sediment section, based upon smear slides (Mayer, Pisias, Janecek, et al., 1992). An Eocene radiolarian ooze cannot simply be created by raising the CCD above the seafloor in a modern equatorial environment. Diatom production in the Eocene must have been significantly lower than Neogene production. The lack of diatoms is another indication, along with the low levels of organic carbon deposition (see "Fluxes and Paleoproductivity"), that primary productivity levels in the Eocene were significantly below Neogene levels.

Diatoms are occasionally found in the Eocene radiolarian oozes in relatively large abundances (Fig. F29). We checked whether occurrences of diatoms are related to when Leg 199 drill sites crossed the paleoequatorial region. Green arrows on the figure mark when diatom numbers increase to >5% in smear slide counts. In the Eocene section they occur primarily around one interval marked by radiolarian Zone RP15 (~39–41.5 Ma based on the preliminary Leg 199 age recalibration) rather than clustering near where we expect equator crossings. The RP-15 interval is also the time of maximum radiolarian ooze deposition at Site 1217, then at a paleolatitude of ~8°N. In contrast, maximum sedimentation rates of radiolarian ooze occurred roughly coincident with paleoequator crossings. The presence of diatoms seems to be related to a production event that is also associated with carbonate deposition at Sites 1218 and 1219, not with passage through the equatorial zone.

One of the important observations along the transect is that the radiolarian deposition zone in the uppermost Eocene is restricted to Sites 1218, 1219, and 1220 in the Leg 199 transect. These sites lie in a paleolatitudinal range from 1° to 2°N. All other sites exhibit a hiatus during this interval or have only clay deposition. The uppermost Eocene is marked by a hiatus of ~2 m.y. at Site 1221 (12°02'N), which expands to 5–6 m.y. at Site 1222 (13°49'N) and at DSDP Site 162 (14°52'N) based upon a recalibration of the DSDP biostratigraphy. Sediments deposited in the late Eocene at Site 1217 (16°52'N) are zeolitic clays. Assuming that radiolarian deposition marks some moderate level of productivity, the latest Eocene was a truly impoverished interval for regions beyond several degrees of latitude from the equator. In contrast, in middle Eocene time, significant radiolarian production occurred over a broad latitudinal range. Siliceous middle Eocene sediments are formed along the entire transect save only for the northernmost Sites 1215 and 1216. Therefore, the zone of radiolarian production may have been broader in middle Eocene time (reaching ~10°N latitude) than it was in the late Eocene (when it was only a few degrees wide). Therefore, not only was there a significant change in the type of sediments deposited in the equatorial Pacific during the later Paleogene but also a distinct narrowing of the focus of siliceous sedimentation between the middle Eocene and late Eocene.

Paleocene/Eocene Boundary

A principal objective of Leg 199 was to recover shallowly buried sections across the P/E boundary that could be used for paleoclimate and evolutionary studies. It has been known for several years (e.g. Thomas, 1990, 1998; Kaiho et al., 1996; Aubry et al., 1996, 1998; Kelly et al., 1996, 1998; Sanfilippo and Blome, 2001) that the negative carbon isotope anomaly and transient global warming associated with the P/E boundary are accompanied by carbonate-poor sediments overlying more calcareous sediments. A mass extinction of benthic foraminifers occurs at the same time as the carbon isotope anomaly, suggesting that the boundary records major changes in deep-ocean chemistry and habitats. Carbonate preservation tends to be poorest at the initiation of the carbon isotope anomaly and then improves over an interval several tens of centimeters to several meters thick that records several hundred thousand years of sedimentation. Two drill sites (ODP Sites 690 and 1051) have been reported with an interval of laminated sediment just above the base of the carbon isotope anomaly. Laminations have been interpreted to record low-oxygen environments that prevailed during the initial phases of the P/E boundary and may be partially responsible for the benthic foraminifer extinction (Kaiho et al., 1996; Thomas and Shackleton, 1996). Dickens et al. (1995) and Dickens (2001) have suggested that outgassing of submarine gas hydrate deposits could produce both low-oxygen conditions and carbonate dissolution by oxidation of methane in the water column. However, other than the changes in carbonate content and laminated sediments, most P/E boundary sections display a relatively simple internal stratigraphy.

Leg 199 drilled the first P/E boundary sections (Sites 1215, 1220, and 1221) ever to be sampled in the central tropical Pacific. Furthermore, these sites record a more complex stratigraphy for the P/E boundary than has been recognized previously (Fig. F30, F31). Sediments from both ODP Sites 1220 and 1221 change from calcareous ooze containing Paleocene microbiotas to a distinctive layered sedimentary unit in the vicinity of the benthic foraminifer extinction. The first beds at or above the benthic foraminifer extinction horizon consist of light yellowish brown sediments 8–12 cm thick. The yellow bed contains layers of volcanic ash at Site 1220 but is massive at Site 1221. The top of the yellow bed is burrowed and overlain by ~2 cm of rose-pink-colored sediment that displays laminations and a few discrete burrows. A layer of black, manganiferous sediments overlies the rose-pink bed and grades upward into brownish, partly laminated and burrowed calcareous clays. The sequence of yellow, pink, black, and brown beds all have low carbonate contents typical of the core of the P/E boundary at other localities. However, the interbedding of sediments with different color and sedimentary structures have not been observed in P/E boundary sections from elsewhere in the world. The presence of similar sequences of color bands in Sites 1220 and 1221 that are situated >200 km from one another suggests that the stratigraphy of these P/E boundaries has at least regional significance and may record changes in deep-ocean chemistry and sedimentation that reflect different phases of the development of the boundary interval on a global scale.

Biostratigraphy of the P/E Boundary Interval

Although the P/E boundary is marked by a major extinction of benthic foraminifers (~53% species extinction), surface ocean microbiotas display mostly temporary changes in taxonomic diversity. Unfortunately, most P/E boundary sections are highly condensed or have severely dissolved calcareous faunas, so the sequence of biotic events has not been worked out in detail. A few tropical–subtropical sites record the presence of three short-lived species of planktonic foraminifers ("excursion fauna") (Kelly et al., 1998) that are believed to have evolved shortly after the beginning of the global climate change associated with the P/E boundary and then become extinct ~200 k.y. later. Calcareous nannofossils show elevated rates of both extinction and speciation in the ~500 k.y. after the onset of Eocene time, but the sequence of evolutionary turnovers is still much in debate.

Results for Leg 199 drill sites suggest that, contrary to previous interpretations, the various biotic events associated with P/E boundary time (e.g., the extinction of the nannofossil species Fasciculithus tympaniformis and many benthic foraminifer taxa together with the evolution of the planktonic foraminifer "excursion fauna") are not synchronous within the P/E boundary interval. At both ODP Sites 1220 and 1221, Paleocene benthic foraminifers become extinct at the start of the deposition of the sequence of multicolored beds (Fig. F32). The reduction in carbonate content (from ~80 to ~12 wt%) associated with the benthic extinction and reduced preservation of foraminifers makes it difficult to precisely define the level of the extinction. However, a sharp drop in carbonate content is observed at the same level as the carbon isotope excursion in other deep-sea sites. Therefore, it is reasonable to conclude that the base of the multicolored beds at Leg 199 sites will ultimately prove to contain the initiation of the carbon isotope anomaly. Data from Site 1220 make it quite clear that the "excursion fauna" (composed of the planktonic foraminifers, Acarinina africana, A. sibaiyaensis, and Morzovella allisonensis) predates the extinction of Paleocene benthic foraminifers. Hence, Leg 199 results strongly suggest that the "excursion Fauna" evolved before the P/E boundary. These species also range above the top of the multicolored beds and may survive termination of the carbon isotope anomaly. Counts of the abundance of the calcareous nannofossils at both ODP Sites 1220 and 1221 show that Fasciculithus is replaced by Rhomboaster well above the multicolored beds believed to represent the start of the Paleocene-Eocene thermal maximum. We have also documented the appearance of Thoracosphaera cysts in the boundary interval. Thoracosphaerid blooms are frequently associated with sediments immediately above the Cretaceous/Paleogene mass extinction horizon and have been interpreted to represent an opportunistic "disaster" flora (Brinkhuis and Biffi, 1993).

Geochemical Profiles of the P/E Boundary Interval

The shipboard inorganic geochemistry protocol for Leg 199 incorporated a program of bulk sediment analysis, including a detailed analysis of the P/E boundary interval at Sites 1220 and 1221 (Fig. F33). Results for the two sites are very similar although changes in elemental concentrations are somewhat sharper at Site 1220 than at Site 1221. Both sites have a significant enrichment in Mn concentration associated with the black layer, which probably contains Mn oxides. The dark, carbonate-poor portion of the P/E boundary at Sites 1220 and 1221 is associated with high levels of Si, Al, Ti, Fe, and Mg. This is consistent with a higher proportion of silicate minerals such as clays compared to the more carbonate-rich portions of the sediment column and in keeping with the interpretation that the boundary lithology represents an interval of pronounced shoaling of the CCD.

At both Sites 1220 and 1221, the P/E boundary is associated with a double peak in levels of Ba and P across the interval of major color change. Superimposed on the center of these broad increases in Ba and P is an interval of very low Ba and P, which corresponds to the black Mn-rich layer and the red layer. This decrease is also present in the Sr profiles. The Ba and P profiles may be explained in several ways. One possibility is that the double peak seen in the profiles for these elements reflects real changes in the rate of Ba and P delivery to seafloor during P/E boundary time. Alternatively, it is possible that the rates of Ba and P delivery to seafloor were high throughout P/E boundary time and that the double peak is a dilution artifact arising from a shorter interval of rapidly accumulating black and red sediments, possibly as a result of nearby hydrothermal activity. A third possibility is that the double peak in Ba and P arises from diagenetic remobilization of primary Ba and P signals. Regardless, the levels of Ba and P recorded across the P/E boundary at Sites 1220 and 1221 are high in comparison to those measured in other Leg 199 sediments. This observation, together with the congruency of behavior between the two elements, raises the intriguing possibility that the shipboard data record an increase in surface ocean productivity across this important paleoceanographic interval (Thomas et al., 2000; Bains et al., 2000) rather than an increase in barite saturation arising from the injection of Ba into the global ocean from the marine gas hydrate reservoir (Dickens, 2001).

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