Another primary objective of Leg 199 science is to asses the level of productivity over the Paloegene by using biogenic MARs in concert with other geochemical and micropaleontological data. With this goal in mind we designed a shipboard program to monitor the composition of pore waters in the sediments and measure the bulk chemical composition of recovered sediments.Organic Carbon Diagenesis and Interstitial Waters
In the context of productivity and biogenic sediment flux considerations it is notable that none of the Leg 199 sediments contained significant organic carbon (Corg). As measured on board the ship, Corg levels in Leg 199 sediments were consistently low (~0.10.2 wt%), essentially the detection limit for the analysis at sea. Chemical gradients in Leg 199 interstitial waters primarily reflect the relatively limited organic matter diagenesis, as well as dissolution of biogenic silica and varying amounts of diffusive influence of reactions in the underlying basalt (Fig. F15). Sulfate concentrations are high (>25 mM) throughout the transect, indicating little oxidation of labile organic matter (Fig. F15). These high sulfate levels mean that barite in the sediments will be well preserved and therefore will potentially be useful for paleoproductivity studies. This is especially important because it appears that any organic matter that may have been delivered to these sites has long since been degraded, if in fact it ever was deposited at all. Ammonium, another by-product of organic matter degradation, is only present in extremely low levels at all sites.
Dissolved silica concentrations generally increase with depth at all sites (Fig. F15). These high interstitial water silica values are consistent with the presence and dissolution of biogenic silica throughout the sediment. Strontium concentrations are generally constant at about seawater value (87 mM) for several of the sites containing little carbonate (Sites 12151217, 1220, and 1221), but strontium increases with depth at Sites 1218 and 1219, reflecting the presence and dissolution of biogenic carbonate (Fig. F15).
Calcium, magnesium, and potassium concentrations at most Leg 199 sites show little evidence for exchange with basalt and subsequent diffusion (Fig. F15). However, the increase in calcium concentration and decrease in magnesium, potassium, and lithium concentrations decreases with depth at Site 1219 are significantly greater than that seen at other Leg 199 sites. These patterns are consistent with alteration of basement rocks and with the recovery of highly altered basalt at Site 1219, unlike the other sites in the Leg 199 transect. The high levels of lithium at Site 1215 could be linked to the volcanic ash layers recovered at that site.Bulk Sediment Analyses
The Leg 199 shipboard geochemical program differed from typical ODP protocol by incorporating relatively detailed downcore profiles of bulk sediment composition. The resulting profiles primarily reflect the shifts in lithology between sediments dominated by silica to those dominated by carbonate (Fig. F16). When these data are combined with sedimentation rates (Fig. F17) and with bulk density data, it is possible to examine burial fluxes through the Paleogene. MAR calculations are a means to distinguish rates of elemental deposition even under conditions of significant dilution by other sedimentary phases. This is best shown by comparing Si weight percent profiles at Sites 1218 and 1219 to the Si MAR profiles (Fig. F16). Low Si contents between 20 and 30 Ma are primarily caused by dilution of Si by CaCO3. The MAR profile indicates that Si fluxes decreased much less dramatically than percentage data might seem to indicate.
Changes in Si MARs should reflect biogenic opal production because the detrital Si contribution by aluminosilicates is relatively low and constant. Al analyses reflect the detrital aluminosilicate contribution (Fig. F18) and are relatively low with respect to the high Si MAR in the Eocene and Oligocene. A rough indicator of baseline aluminosilicate contribution to Si MARs is given by the Si MAR in the period younger than 15 Ma, when clays were the principal sediment accumulating at all Leg 199 drill sites. By comparison to this baseline (15 Ma to present value) it is clear that biogenic Si deposition has decreased since a peak in the middle Eocene between ~38 and 45 Ma.
Virtually all calcium MAR results from the deposition of CaCO3 (Fig. F16). High Ca contents and high Ca MARs occur in the high-CaCO3 intervals of the Oligocene and lower Miocene. Smaller events can be seen in the middle Eocene, particularly ~40 Ma. The lower Eocene Ca MAR is approximately one-half to two-thirds of the high rates of deposition in the Oligocene. Phosphorus has a primary source term driven by primary productivity, but its burial in sediments can be affected by early diagenesis (Fig. F18). The phosphorus MAR peaks in the early Oligocene and also in the period ~4045 Ma in the middle Eocene, suggesting that this part of the middle Eocene had elevated paleoproductivity relative to the late Eocene.
Ratios to an element assumed to be constantly delivered (or nearly so) is another quick way to assess changes in deposition. Figure F19 illustrates this approach by showing ratios of Si, Ba, and Al to Ti. We assume that titanium is bound to aluminosilicates and has relatively constant deposition. The plots of Si/Ti and Ba/Ti suggest that there was relatively high burial of Ba and Si during the Eocene. The relatively constant Al/Ti is an indication that clay minerals are a primary contributor of the Ti flux.Latitudinal MAR Transects
Observing MARs in time slices along latitudinal transects is another way to assess how sedimentation in the Eocene differs from that in the Neogene and Oligocene. We have chosen three time slices for shipboard comparison: (1) an early Eocene time slice (5055 Ma), when all parts of the 56-Ma transect were above the CCD; (2) a middle Eocene time slice (3845 Ma), during the period of highest radiolarian ooze deposition; and (3) an Oligocene time slice (2534 Ma) when high CaCO3 sedimentation had been established.
The Si MAR is highest in the middle Eocene (Fig. F20), but the latitudinal gradient of the Si MAR does not peak in the equatorial region as it does in the Neogene. Instead, the Si MAR increases southward. A high Si MAR in the middle Eocene may be in part because Eocene radiolarians are heavily silicified (Moore, 1969). An individual middle Eocene radiolarian test has an average weight about four or five times that of an average Pleistocene radiolarian test and is much less susceptible to dissolution at the seafloor. The high Si MAR may represent low dissolution prior to burial, as well as relatively high Si flux to the sediments.
In the early Eocene, we observe low Si MARs at all latitudessufficiently low that a large contribution to Si MARs must be from clay minerals. Only the Oligocene has a MAR pattern that resembles that of the Neogene, with the highest MAR peaking in the equatorial region. The flux of Si at the Oligocene equator is roughly equivalent to the modern MAR at 110°W (Lyle, 1992). Lyle (1992) expressed the modern flux as opal (SiO2), rather than biogenic Si. The opal fluxes in the Holocene equatorial region are equivalent, however, to ~50 mg/cm2/k.y. biogenic Si.
Ca MARs in the early Eocene time slice (Fig. F21) are actually equivalent to or higher than modern fluxes of Ca from CaCO3 along a 110°W transect, except in the equatorial region (Lyle, 1992). There is no well-developed equatorial maximum in CaCO3 deposition. In fact, the equatorial region may bury only half as much CaCO3 as the subtropical flank of the transect. In the middle Eocene, essentially no CaCO3 is deposited anywhere except the southernmost part of the transect, reflecting the shallow middle Eocene CCD. Only the Oligocene latitudinal transect resembles a Neogene equatorial profile. CaCO3 deposition at the equator is as high as Holocene deposition.
It is clear from our shipboard examination that sedimentation patterns of the Eocene are significantly different from modern equatorial sedimentation. Postcruise analyses utilizing other paleoproductivity indicators will be needed to understand the development of primary productivity through the Eocene. Nevertheless, there are few indicators that productivity was high in the Eocene central tropical Pacific Ocean.
One important problem to be approached by postcruise studies is whether Corg levels are low in Eocene sediments because of a long period of exposure to oxidants diffusing into the sediment from seawater, or whether Corg levels were never high in the first place. Shipboard paleomagnetic studies provide one further clue to the riddle of low Corg. Normally, paleomagnetic studies are limited by diagenesis. Fe3+-Fe2+ reduction dissolves magnetite and eventually destroys the capability of the sediments to retain a magnetic signal. Along the Leg 138 transect, the Neogene equivalent of the Leg 199 transect, sites at the equator or under regions of relatively high Corg deposition have short paleomagnetic records (Mayer, Pisias, Janecek, et al., 1992), whereas sites under regions of low primary productivity have long paleomagnetic records. The long paleomagnetic records obtained on the Leg 199 transect suggest that there never were high levels of Corg deposition in the Eocene.
A major success of Leg 199 is the recovery of continuous sedimentary records with uninterrupted sets of distinct Cenozoic geomagnetic polarity zones from the paleoequatorial Pacific Ocean (Fig. F22). The sedimentation rates of the recovered and complete composite sections provide acceptable resolution for meaningful magnetobiochronologic calibrations for the early middle Eoceneearly Miocene time interval. Biogenic silica is ubiquitously present and will permit, for the first time, the establishment of a precise Cenozoic biochronology of radiolarians, diatoms, and silicoflagellates from a tropical Pacific Ocean setting.
Biogenic carbonate shows variable states of preservation, from moderate to complete dissolution. The upper Paleocenelower Eocene and Oligocenelower Miocene intervals show best preservation, thus offering huge potential for establishing a tropical Pacific Ocean biochronology of calcareous nannofossils and planktonic foraminifers tied to magnetostratigraphy. Nannofossils can also be calibrated through much of the Eocene. As most existing age estimates for these groups have been derived from the Atlantic Ocean region, the establishment of an accurate biochronology based on Leg 199 sediments will permit assessments of the degree of interbasin synchrony among the calcareous plankton.
It follows that the sediments recovered during Leg 199 from the paleoequatorial Pacific Ocean will be an exceptionally valuable reference material for years to come for paleoceanography, as well as, studies of the evolution, biochronology, and intergroup and interbasin correlations among siliceous and calcareous micro- and nannofossils.
One of the enduring legacies of Leg 199 will undoubtedly be the revision of tropical radiolarian biostratigraphy. The opportunity to tie radiolarian biostratigraphic events directly to unambiguous magnetochronology will provide some of the first direct ties between the tropical siliceous microfossil record and the absolute timescale of the Paleogene. Furthermore, the repeated recovery of lower Miocene, Oligocene, and middle and upper Eocene radiolarian-rich sediments in four sites makes it possible to check the reliability of radiolarian events.
Most radiolarian bioevents have been calibrated indirectly to the absolute timescale by correlation to calcareous nannofossil biostratigraphy. On Leg 199, radiolarian bioevents were accompanied by high-resolution magnetostratigraphy, allowing the ages of biozone boundaries to be determined directly from the Cande and Kent (1995) timescale. Figure F23 shows a comparison between the newly determined zonal boundary ages, calibrated using the reversal boundary stratigraphy from Site 12181220, with those estimated by Sanfilippo and Nigrini (1998) (SN98). The age estimates of SN98 are based on an unpublished catalog and chart constructed from a reexamination of all Paleogene low- and middle-latitude DSDP/ODP sites from Leg 1 through 135 in which there is a recognizable radiolarian fauna. The published information was reevaluated using current, uniform species concepts and integrated, where possible, with published nannofossil and paleomagnetic data. Sanfilippo and Nigrini (1998) cautioned that their chronology of Paleogene radiolarian zonal boundary events is at best a good approximation.
However, it is apparent that most radiolarian events are synchronous (or nearly so) among the Leg 199 sites. The Leg 199 age estimates tend to be older than the published estimates for 11 of 17 events whereas the others agree to within 100 k.y. or less of previously published ages. Discrepancies in published vs. Leg 199 ages may reflect, for example, uncertainties in the age estimates of SN98 ("at best a good approximation"), paleobiogeographic diachroneity of the events, and difficulties in consistently recognizing evolutionary transitional events. The current radiolarian tropical biostratigraphic zonation includes numerous evolutionary transitions that tend to be subject to interpretation more than distinct first or last occurrence datums.
Postcruise refinements of the biostratigraphy of each site will improve the overall estimate of the ages for both zone marker events and recognition of new bioevents. A cycle stratigraphy based on physical properties measurements in each core will help refine the age estimates of biostratigraphic datums within magnetochrons. The ability to compare the timing of the same events at different Leg 199 sites can identify biostratigraphic datums that may be locally unreliable owing to reworking or latitudinal differences in the timing of evolutionary first or last appearances of species. The abundant, well preserved radiolarian assemblages will permit the recognition of other, easily identified datum levels with a consequent increase in the resolution of siliceous biostratigraphy.
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