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doi:10.2204/iodp.proc.320321.201.2012

Results and discussion

Revised composite depth scales

Revised composite depth scales for Expedition 320 Sites U1331–U1334

Site U1331

The shipboard splice for Site U1331 (see the “Site U1331” chapter [Expedition 320/321 Scientists, 2010c]) was extensively revised (Figs. F2, F3, F4; Tables T1, T2, T3). The weak magnetic susceptibility and GRA signals in the siliceous ooze dominated the Eocene section of Site U1331, and frequent turbidites hampered straightforward correlation. The exact position and extent of turbidites is given in Table T4. The most problematic section is located around 35–40 rmcd (m revised CCSF-A), where a turbidite occurs that shows different thicknesses in each cored hole. This could be due to coring disturbance at the top of the Hole U1331C or strong thickness variations of the turbidite itself. We improved the splice to 175 rmcd (m revised CCSF-A), resulting in a growth factor of 1.12 (Fig. F4). Detailed correlation showed that the shipboard declination record of Cores 320-U1331B-4H and 9H had to be flipped by 180˚.

Site U1332

The shipboard splice for Site U1332 (see the “Site U1332” chapter [Expedition 320/321 Scientists, 2010d]) had to be moderately revised (Figs. F5, F6, F7; Tables T5, T6, T7). Good VGP, magnetic susceptibility, and GRA data enable improvement of the shipboard splice to 141 m revised CCSF-A, resulting in a growth factor of 1.09 (Fig. F7). At around 83 rmcd (m revised CCSF-A), a small gap in the data (Fig. F5) marks an uncertain tie point in the splice. However, detailed correlation to Sites U1331 and 1220 suggests no major break at the base of Chron C15n in the composite record of Site U1332 (see Fig. F23, 50–100 corrected rmcd).

Site U1333

The splice at Site U1333 (see the “Site U1333” chapter [Expedition 320/321 Scientists, 2010e]) needed no change in the upper 48 rmcd (m revised CCSF-A) (Figs. F8, F9, F10; Tables T8, T9, T10). Pronounced cycles in magnetic susceptibility and GRA data in this interval allowed the construction of a robust shipboard splice. Correlation to Sites U1334 and 1218 revealed an incorrect splice interval in the shipboard splice at Site U1333 around 48 rmcd (m revised CCSF-A). Readjustments of the splice reveal a 2 m gap in the shipboard splice, which has been eliminated by the new revised splice. Although the magnetic susceptibility signal is low between 82 and 132 rmcd (m revised CCSF-A), small distinct peaks can be correlated and then verified by VGP data (Fig. F8, 50–100 m). A minor change in the shipboard splice is required at 126 rmcd (m revised CCSF-A). We then follow the tie points of the shipboard splice to 151 rmcd (m revised CCSF-A) and maintain the complete uninterrupted splice to 156.44 rmcd (m revised CCSF-A). Below this depth, the splice can only be appended because there is no clear overlap between cores from adjacent holes. Between 180 and 200 rmcd (m revised CCSF-A), a composite record could be established mainly based on the VGP and magnetic susceptibility data. The new splice has a growth factor of 1.14 (Fig. F10).

Site U1334

At Site U1334 (see the “Site U1334” chapter [Expedition 320/321 Scientists, 2010f]), the shipboard splice was verified to 210 rmcd (m revised CCSF-A) (Figs. F11, F12, F13; Tables T11, T12, T13). Because of geochemical alteration of the magnetic susceptibility record, splicing was uncertain between 150 and 270 rmcd (m revised CCSF-A). Through extensive usage of augmented magnetic susceptibility, GRA, VGP, core images, and especially postcruise XRF core scanning data (T. Westerhold et al., unpubl. data; D. Liebrand et al., unpubl. data), we secured a complete composite record across the geochemically altered interval to 271 rmcd (m revised CCSF-A). Below this depth, we follow the shipboard splice with a major change in the interval from 297 to 306 rmcd (m revised CCSF-A). This change is important because it covers the interval before the Eocene/Oligocene boundary, which is characterized by strong fluctuations in calcium carbonate content (see the “Expedition 320/321 summary” chapter [Expedition 320/321 Scientists, 2010a]). Splicing this interval was a challenge because extended core barrel drilling produced strong biscuiting of the sediment. A complete splice was assembled for Site U1334 to 341 rmcd (m revised CCSF-A) with a growth factor of 1.16 (Fig. F13).

Revised composite depth scales for Leg 199 Sites 1218–1220

Before we accomplished a site-to-site correlation of Expedition 320 and Leg 199 sites, it was necessary to recheck the revised splices of Sites 1218 and 1219 (Pälike et al., 2005) and the shipboard splice of Site 1220 (Shipboard Scientific Party, 2002c).

Site 1218

At Site 1218 (Figs. F14, F15, F16; Tables T14, T15, T16), the revised splice had to be corrected below 210 corrected rmcd. Most of these adjustments benefited from detailed comparison to Site U1334. Prior to Expedition 320, Site 1218 was the only stratigraphically expanded and complete site from the equatorial Pacific covering the late Eocene and early Oligocene. The revisions are mainly in intervals with very high calcium carbonate content and low magnetic susceptibility. A complete splice can be constructed to 287 corrected rmcd, adding a growth factor of 1.11 (Fig. F16).

Site 1219

Changes to the splice of Site 1219 (Figs. F17, F18; Tables T17, T18) are very small, and thus we suggest continuing to use the table by Pälike et al. (2005) to construct a composite record.

Site 1220

In contrast, the shipboard splice of Site 1220 had to be corrected below 71 rmcd (Figs. F19, F20, F21; Tables T19, T20, T21). Compared to the shipboard splice the changes are minor, a few decimeters at most. The new revised composite record reached 136 rmcd and provided a growth factor of 1.10 (Fig. F21). Please note that Site 1220 was not part of the Pälike et al. (2005) splice revision. Therefore, this study is the first revision of the shipboard splice of Site 1220 and is indicated by the depth scale nomenclature revised meters composite depth (rmcd).

Cleaned magnetic susceptibility, GRA, and VGP data sets

For reference, we provide cleaned magnetic susceptibility and GRA density data sets for every spliced composite section of Sites U1331 (Table T22), U1332 (Table T23), U1333 (Table T24), and U1334 (Table T25). Cleaned magnetic susceptibility, GRA density, and VGP latitude data are compiled for Sites 1218 (Table T26), 1219 (Table T27), and 1220 (Table T28) (data sets are also available in “Supplementary material”). To obtain cleaned data we removed outliers and data collected close to end caps and cut out disturbed intervals (e.g., core tops). These data sets have been used for the subsequent site-to-site correlation and squeezing and stretching of core sections outside the spliced records. The mapping pairs from the squeezing and stretching can be used to position samples taken outside the splice to be placed into the new revised composite depth scales.

Site-to-site correlation

More than 800 dated paleomagnetic reversals are available for all PEAT sites (Pälike et al., 2009) and thus provide the perfect framework for the detailed intercalibration of all major fossil groups and refinement of magnetic polarity chrons, particularly in the Eocene. However, the shipboard preliminary paleomagnetic data from Expedition 320 used here have to be considered incomplete. To improve the quality of the magnetostratigraphy, stepwise demagnetization of U-channel samples accompanied by rock magnetic studies are being done as part of the postcruise science. High-quality and high-resolution paleomagnetic records covering the late Eocene, Oligocene, and early Miocene are available from Leg 199 (Pälike et al., 2005; Lanci et al., 2004, 2005). The sites from both expeditions presented here are ideal for the establishment of a fully integrated calibrated bio-, chemo-, and magnetostratigraphy for the early Eocene–early Miocene time interval for the equatorial Pacific. A prerequisite for successful integration of the stratigraphic data and subsequent assembly of the proposed equatorial Pacific Cenozoic megasplice is the correlation of decimeter-scale features in the sedimentary record from the drilled sites from both Leg 199 and Expedition 320/321 (Pälike et al., 2005, 2009). We follow the successful approach of previous deep-sea drilling expeditions (Shackleton et al., 1995, 1999; Shackleton and Crowhurst, 1997; Pälike et al., 2005; Westerhold and Röhl, 2006; Westerhold et al., 2007, 2008) by using physical property data (magnetic susceptibility and GRA) and XRF core scanning data to correlate site to site. In doing so we can transfer, for example, the high-resolution biostratigraphic data from one site to intervals of another site where, due to poor preservation, datums are not well constrained. Furthermore, we can locate hiatuses and condensed intervals that otherwise would not have been identified. For correlation, we first identified a reference site that has the most complete record and high sedimentation rates compared to the other sites. Then we correlated the other sites to the reference site by selecting tie points. We applied a linear interpolation of depth between tie points. Tie points are listed in Tables T29 and T30.

Correlation between Sites 1218, 1219, U1333, and U1334

Physical property data at Sites 1218, 1219, U1333, and U1334 show a remarkable match (Fig. F22) even though the sites are between 375 and 1100 km apart. All sites have an excellent magnetostratigraphy, and thus comparison of the VGP data indicate the high quality of correlation. We have chosen Site 1218 to be the reference site because Site 1218 is the most complete down to the Eocene/Oligocene boundary and has no geochemically altered interval, as found in the mid-Oligocene of Site U1334. The integrated record spans the interval from Chron C1 (Pleistocene) back to Chron C20 (middle Eocene) covering >40 m.y. of equatorial Pacific history. We correlated Sites 1219, U1333, and U1334 to Site 1218 (Table T29), providing a coherent and integrated record for the equatorial Pacific.

The correlation shows full coverage of magnetostratigraphy back to the base of early Oligocene Chron C11n.2n using Sites 1218 and U1334 alone. All four sites cover the interval from the base of Chron C6n to the base of C10n.2n (~20 to ~28 Ma) with a complete magnetostratigraphy. In the time span older than Chron C12n (~30.8 Ma), the magnetostratigraphic boundary positions can be transferred from Site U1333 to Sites 1218, 1219, and U1334 when necessary. The complete magnetostratigraphic record reaches back to the top of middle Eocene Chron C19n (~41 Ma).

Sedimentation rates in the section from 0 to 20 Ma at all sites are highest at Site 1218 (a low 0.35 cm/k.y.). Sites 1219 and U1333 have even lower sedimentation rates in that interval and a hiatus between the Pliocene–Pleistocene and the lower Miocene (Pälike et al., 2009). All these sediments consist of clays deposited near or below the calcium carbonate compensation depth. In the upper 40 m of the integrated stratigraphy (Fig. F22), correlations are based on the VGP data because magnetic susceptibility and GRA data do not provide patterns that can be matched with certainty. Below that interval, matching of different records was straightforward. From 20 Ma to the Eocene/Oligocene boundary, Site U1334 has the highest sedimentation rate (1.6 cm/k.y.) of all the sites (Site 1218 = 1.3 cm/k.y., Site 1219 = 1.2 cm/k.y., and Site U1333 = 1.1 cm/k.y.). In the upper Eocene section, sedimentation rates are slightly lower because of the decreased carbonate content (see the “Expedition 320/321 summary” chapter [Expedition 320/321 Scientists, 2010a]). Two short condensed intervals were discovered: one at Site 1219 between 112 and 114 corrected rmcd and one at Site U1333 between 137 and 140 rmcd (m revised CCSF-A) (Fig. F22).

Correlation between Sites 1220, U1331, and U1332

Physical property data from Sites 1220, U1331, and U1332 show a remarkable match (Fig. F23), being only 120 to 270 km apart. Sites 1220 and U1332 have an excellent magnetostratigraphy from Chrons C6n to C20n (Table T31). Site U1331 sediment covers Chrons C11–C20n. We chose Site 1220 to be the reference site because it is the most complete for this interval. The correlation with Sites U1331 and U1332 (Table T30) provides a coherent and integrated record.

All three sites show rather low sedimentation rates (~0.5 cm/k.y.) compared to the shallower sites (1218, 1219, U1333, and U1334) (Fig. F24). The upper Eocene sediments are dominated by siliceous ooze and almost entirely lack carbonate sediment. The dominance of siliceous ooze leads to low variability in the GRA density; hence, correlation could only be achieved using magnetic susceptibility data. The comparison of the VGP data suggests a very good match of the three sites in the Eocene. The increased sedimentation rate at Site U1331 in the Eocene is an artifact of the frequent turbidites in the record. The correlation of the Oligocene and Miocene section is straightforward to 28 rmcd (Fig. F23). Above this, Site 1220 can only be matched to Site U1332 using VGP data.

Radiolarians in the tropical Pacific

Cenozoic radiolarian stratigraphy of the tropics was largely developed in sediments from the Pacific Ocean; however, it did not begin to reach its full potential until Leg 199 studies were completed by Nigrini et al. (2006). This work, combined with that of earlier studies (e.g., Moore, 1995), sought to tie radiolarian datums to a paleomagnetic timescale that could be tuned to orbital frequencies. These studies also greatly expanded the number of first and last occurrences of species that were recorded and calibrated. This effort took advantage of the many important taxonomic and stratigraphic papers that have appeared over the last 50 years, in particular those written by such authors as William Riedel, Annika Sanfilippo, Catherine Nigrini, David Johnson, and Jean Westberg, who focused much of their efforts on material collected in the tropical Pacific.

Expedition 320 was very successful in recovering Pacific Cenozoic sections deposited on or very close to the paleoequator. Two of these drilled sites (U1333 and U1334) recovered what appear to be complete sections across the Eocene/Oligocene boundary. Only one other section, from Site 1218, has been recovered in the tropical Pacific that clearly shows the “two-step” shift in lithology and geochemistry at this boundary that we believe marks a truly complete stratigraphic section (Coxall et al., 2005). Using the stratigraphic datums defined primarily in Nigrini et al. (2006), we were able to provide very detailed stratigraphic control on the sections recovered during Expedition 320 (Tables T32, T33, T34, T35, T36, T37, T38). While producing this detailed integrated stratigraphy of the equatorial Pacific, we have had to deal with some complicated stratigraphic problems that still need to be fully addressed.

Reworking and mixing of older specimens into younger sections

Finding reworked older radiolarian specimens in younger sediments plagued the development of a reliable radiolarian stratigraphy in its early days. Such reworking was commonly found in piston cores and gravity cores from the tropical Pacific (e.g., Riedel and Funnell, 1964), and it was not until the Deep Sea Drilling Project (DSDP) and ODP started to collect thick pelagic sections that we were able to begin to develop a reliable sequence of first and last appearances of species. In studying these sections, several important observations have been made: (1) the reworked older forms were never older than the age of the crust on which the sediment lay, (2) reworking of older forms is most common in the upper parts of recovered sections, and (3) reworking of older forms from the Eocene is commonly found around the Eocene/Oligocene boundary and is often associated with a hiatus at this boundary (Moore et al., 1978; Moore, 1995). Because many of the biostratigraphic datums near the uppermost part of the Eocene are last appearances, the dependability of such datums are highly suspect and their calibration to a timescale is still open to question.

Taxonomic definition

Nigrini et al. (2006) described 12 new species, several of which are important in defining the Eocene/Oligocene boundary and in refining the stratigraphy of the Oligocene. These new species require the test of time and usage to make sure their definitions adequately encompass the characteristics and variability of their form. Similarly, other species may need modification of their descriptions in order to more consistently define biostratigraphic datums. Only a small percentage of the total number of radiolarian species present at any given time has been identified as being stratigraphically useful (Riedel and Sanfilippo, 1978). Further work in this area will continue to expand the resolution possible using radiolarian stratigraphy.

Preservation

Radiolarians are generally well preserved in the tropical Pacific; however, they are subject to dissolution, particularly just above basement and at levels of chert formation. Aside from these two problems, Eocene radiolarians are particularly robust (Moore, 1969; Lazarus et al., 2009), and, with their very diverse fauna, usually provide good stratigraphic control. Preservation in the Oligocene of the sites studied, however, is often only moderate and sometimes quite poor. It has yet to be determined if this variation in Oligocene preservation is site specific or time specific.

The radiolarian stratigraphic data presented herein represent a work in progress. Additional samples are being studied and the detailed site-to-site correlation that has been developed by the work presented here will be used to further refine the positions of individual biostratigraphic datums. Some of this more detailed work is shown in the radiolarian data tables (denoted by “Revised” in the column labeled “Source”). There remain many apparent small discrepancies in the levels of individual datums at different sites. It is yet to be determined whether these discrepancies are a result of reworking of radiolarians above or below the true level of the datum, a failure to recognize the presence of a rare species near its first or last appearance, a true diachrony of the datum, or a minor miscorrelation of the lithologic records themselves. Until these discrepancies can be studied further, we use the age assigned each of the datums as published by Nigrini et al. (2006).

Calcareous nannofossils

Shipboard calcareous nannofossil biostratigraphy provided critical age control during Leg 199 and Expedition 320, allowing for the identification of paleomagnetic reversals and the development of composite sections, especially within the successions of carbonate-rich Oligocene–Miocene nannofossil oozes. The new correlations presented here enable more refined assessments of the timing and controls on the expression of calcareous nannofossil datums in the equatorial Pacific. The presented tables of nannofossil datums (Tables T39, T40, T41, T42, T43, T44, T45) are a compilation of data from both shipboard and postcruise biostratigraphy from Expedition 320 and Leg 199 (Shipboard Scientific Party, 2002a, 2002b, 2002c; Pälike et al., 2006; see also “Biostratigraphy” in each site chapter [Expedition 320/321 Scientists, 2010c, 2010d, 2010e, 2010f]). Calibration ages for calcareous nannofossil datums from the Leg 199 timescale were made consistent with those of Expedition 320 (bottom [B] Sphenolithus ciperoensis at 27.1 Ma rather than 28.1 Ma; B Sphenolithus distentus at 30.0 Ma rather than 30.4 Ma; top [T] Reticulofenestra umbilicus at 32.0 Ma rather than 31.7 Ma), as were taxonomic concepts (use of Coccolithus formosus rather than Ericsonia formosa). These changes do not imply that the datum ages used during Expedition 320 are better calibrated than those used during Leg 199; revisions were undertaken prior to Expedition 320 partly based on postcruise work from Leg 199 material (e.g., Blaj et al., 2009). For example, during Expedition 320 it became clear that the Leg 199 biostratigraphic datum age of 28.1 Ma for B S. ciperoensis produced a better fit within the integrated stratigraphy than the revised age provided by Blaj et al. (2009) used during Expedition 320/321 of 27.1 Ma. These discrepancies are likely due to differences in taxonomic concept and boundaries within the intergrading Oligocene sphenolith lineage Sphenolithus predistentus-distentus-ciperoensis. Ongoing postcruise taxonomic and biostratigraphic work will address these issues.

Placing the existing calcareous nannofossil biostratigraphy of Leg 199 and Expedition 320 within the framework of these new stratigraphic correlations clearly shows that the accurate placement of calcareous nannofossil events is compromised by the occurrence of intervals with low or no carbonate deposition during the middle to late Eocene. A clear example of this is the placement of the latest Eocene event T Discoaster saipanensis (see Tables T42, T43, T44). This event is well constrained at Site 1218 at 244.52 ± 0.06 corrected rmcd and at Site U1334 at 301.33 ± 0.53 rmcd (m revised CCSF-A) (equal to 243.29 ± 0.45 corrected rmcd [Site 1218]). But this event is poorly constrained at Site 1219 within the interval 190.06 ± 13.83 rmcd (equal to 254.66 ± 12.03 corrected rmcd [Site 1218]), although the identified range is fully consistent with the stratigraphy of Sites 1218 and U1334. Where there is continuous carbonate sedimentation and reasonable nannofossil preservation, most of the nannofossil datums correlate among these equatorial Pacific sites within the accuracy of the current sampling resolution (e.g., top of R. umbilicus placed at 221.42, 222.97, 224.44, and 220.99 corrected rmcd [Site 1218] at Sites 1218, 1219, U1333, and U1334, respectively). Notable exceptions to this are the placement of the base and top of S. ciperoensis and the top of S. distentus. The base of S. ciperoensis is relatively consistent between Sites 1218, 1219, and U1333 at ~144 corrected rmcd (Site 1218), but S. ciperoensis is first noted at low abundance ~20 m lower at Site U1334 at ~164 corrected rmcd (Site 1218). This suggests the initial evolutionary appearance of S. ciperoensis is followed by a period of low abundance in the equatorial Pacific and then a marked abundance increase that is picked as the “B S. ciperoensis” in the majority of these study sites. The top of S. ciperoensis is also depressed by ~20 m at Site U1333 (~130 corrected rmcd [Site 1218]) with respect to the other sites (~111 corrected rmcd [Site 1218] at Sites 1218, 1219, and U1334); again, this may be due to low abundances at the top of this species’ range. The top of S. distentus is placed ~10 m higher in the biostratigraphy of Leg 199 than that of Expedition 320 (~130 corrected rmcd [Site 1218] at Sites 1218 and 1219 versus ~140 corrected rmcd [Site 1218] at Sites U1333 and U1334). This most likely reflects slightly different taxonomic concepts applied by different workers or simply the difficulty in applying consistent taxonomy in a complex species plexus undergoing gradual change, as we observe within this lineage of Oligocene sphenoliths. Improving the taxonomic definition of these sphenolith lineages and determining the abundance patterns and timing of their origin and extinction will be the focus of ongoing detailed biostratigraphic studies.

Reworked calcareous nannofossils were identified in limited intervals of the Oligocene at the top of Site U1331, associated with suspected gravity flow deposits. These intervals were easily identified during shipboard biostratigraphy, and reworking of older nannofossils into younger strata is not thought to have affected the placement of nannofossil datums. The stratigraphic framework presented here is an excellent basis for ongoing detailed assessments of late Eocene–Oligocene nannofossil bioevents, with a particular focus on the improved age resolution and the identification of genuine diachrony across the eastern equatorial Pacific. The integration of both radiolarian and calcareous nannofossil biostratigraphy proved essential for shipboard operations during both Leg 199 and Expedition 320, which both spanned the major lithologic transition from Eocene radiolarian oozes to Oligocene–Miocene calcareous nannofossil oozes. Continued biostratigraphic work on material recovered during these two expeditions and Expedition 321 should produce a greatly improved integrated tropical Pacific radiolarian-nannofossil biostratigraphy of the last ~50 m.y.

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