Sedimentology and Lead and Strontium isotope ratios of Miocene sediments from the Kerguelen Plateau

Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then.

Petrology and geochemistry of silicified upper Miocene chalk at DSDP Site 69-504

Chert, Porcellanite, and other silicified rocks formed in response to high heat flow in the lower 50 meters of 275 meters of sediments at Deep Sea Drilling Project Site 504, Costa Rica Rift. Chert and Porcellanite partly or completely replaced upper Miocene chalk and limestone. Silicified rock occurs as nodules, laminae, stringers, and casts of burrows, and consists of quartz and opal-CT in varying amounts, associated with secondary calcite. The secondary silica was derived from dissolution of opal-A (biogenic silica), mostly diatom frustules and radiolarian tests. Temperature data obtained at the site indicate that transformation of opal-A to opal-CT began at about 50°C, and transformation from opal-CT to quartz at about 55°C. Quartz is most abundant close to basement basalts. These silica transformations occurred over the past 1 m.y., and took place so rapidly that there was incomplete ordering of opal-CT before transformation to quartz; opal-CT formed initially with an uncommonly wide d spacing. Quartz shows poor crystallinity. Chemical data show that the extensively silicified rocks consist of over 96% SiO2; in these rocks, minor and trace elements decreased greatly, except for boron, which increased. Low Al2O3 and TiO2 contents in all studied rocks preclude the presence of significant volcanic or terrigenous detritus. Mn content increases with depth, perhaps reflecting contributions from basalts or hydrothermal solutions. Comparisons with cherts from oceanic plateaus in the central Pacific point to a more purely biogenic host sediment for the Costa Rica Rift cherts, more rapid precipitation of quartz, and formation nearer a spreading center. Despite being closer to continental sources of ash and terrigenous detritus, Costa Rica Rift cherts have lower Al2O3, Fe2O3, and Mn concentrations.

Revised geomagnetic polarity timescale for the late Oligocene to early Miocene of ODP Site 177-1090

At Ocean Drilling Program (ODP) Site 1090 (subantarctic South Atlantic), benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through early Miocene (~24-16 Ma) at a temporal resolution of ~5 ky. Over the same interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model, we use the newly derived age of the Oligocene/Miocene (O/M) boundary of 23.0 Ma of Shackleton et al. (2000, doi:10.1130/0091-7613(2000)28<447:ACAFTO>2.0.CO;2), revised to the new astronomical calculation (La2003) of Laskar et al (2004, doi:10.1016/j.icarus.2004.04.005) to recalculate the spline ages of Cande and Kent (1995, doi:10.1029/94JB03098). We then tune the Site 1090 dekta18O record to obliquity using La2003. In this manner, we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n through C6Bn from Shackleton et al. (2000) when rescaled to La2003. The results from Site 1090 provide independent evidence for the revised age of the Oligocene/Miocene boundary of 23.0 Ma. For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are generally within one obliquity cycle of those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma) and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1995).

Neogene benthic foraminifera in the Southwest Pacific

Early Miocene to Quaternary benthic foraminifers have been quantitatively studied (>63 ?m size fraction) in a southwest Pacific traverse of DSDP sites at depths from about 1300 to 3200 m down the Lord Howe Rise (Site 590,1299 m; Site 591, 2131 m; Site 206, 3196 m). Benthic foraminiferal species smaller than 150 µm are by far dominant in the samples, averaging from 78 to 89% of the total benthic foraminiferal assemblages in the three sites examined. Although about 150 benthic foraminiferal species or taxonomic groups have been identified, only a few species dominate the assemblages. These dominant species include Epistominella exigua, E. rotunda, and Globocassidulina subglobosa, which prevail in the three sites, and Oridorsalis umbonatus, E. umbonifera, and Cassidulina carinata, which occur usually in frequencies of between 10 and 30%. Faunal changes in Neogene benthic foraminiferal assemblages are not similar in each of the three sites, but faunal successions are most similar between the two shallowest sites. The deepest site differs in composition and distribution of dominant species. There are three intervals during which the most important changes occur in benthic foraminiferal assemblages: the early middle Miocene (14 Ma; the Orbulina suturalis Zone and the Globorotalia fohsi s.l. Zone); the late Miocene (6 Ma; the Globigerina nepenthes Zone) and near the Pliocene/Pleistocene boundary at about 2 Ma. A Q-mode factor analysis of the faunal data has assisted in recognizing assemblage changes during the Neogene at each of the sites. Early Miocene assemblages were dominated by Globocassidulina subglobosa at Site 590 (1299 m), by G. subglobosa and Oridorsalis umbonatus at Site 591 (2131 m), and by G. subglobosa, E. exigua, and Bolivina pusilla at Site 206 (3196 m). In the early middle Miocene at Sites 590 and 591, a marked increase occurred in the frequencies of E. exigua. Epistominella exigua reached maximum abundance in the early Miocene in the deeper Site 206, and in the middle and early late Miocene in the shallower Sites 590 and 591. In the late Miocene, a spike occurred in the frequencies of E. umbonifera in Site 206, whereas the dominant species changed from E. exigua to E. rotunda at Site 590. Latest Miocene to late Pliocene assemblages were dominated by E. rotunda at Site 590, by E. exigua at Site 591, and by G. subglobosa-E. exigua (early Pliocene) and E. rotunda-E. exigua (late Pliocene) at Site 206. At the Pliocene/Pleistocene boundary, E. exigua temporarily diminished in importance at Sites 591 and 206. Quaternary assemblages were dominated by E. rotunda and Cassidulina carinata at Site 590, by E. rotunda at Site 591, and by E. exigua at Site 206. These major faunal changes are all associated with known major paleoceanographic events-the middle Miocene development of the Antarctic ice sheet; the latest Miocene global cooling and increased polar glaciation; and the onset of quasiperiodic glaciation of the Northern Hemisphere. These major paleoceanographic events undoubtedly had a profound effect on the intermediate and deep water mass structure of the Tasman Sea as recorded by changes in benthic foraminiferal assemblages.

Late Miocene to Holocene planktonic foraminifers from ODP Leg 108 holes

Planktonic foraminifers were examined from 27 holes situated at 12 separate sites in the tropical Atlantic. The sites are located in various environments, including areas of upwelling, areas affected by cool currents, areas of strong dissolution, and areas that show little dissolution in warm-water settings. Paleomagnetic results were variable at these sites, but accumulation rate curves have been produced by combining the existing paleomagnetic data with the available microfaunal data. Determinations of the ages of the planktonic foraminifer datums from these accumulation rate curves show some species to be strongly diachronous, while others provide good stratigraphic markers. The warmest water sites with the least dissolution show the most complete ranges of species.

Oligocene-Miocene strontium isotopes of ODP Hole 120-747A

This study tests and improves on previously published early and middle Miocene 87Sr/86Sr marine correlations, presents Sr isotopic age correlations for this interval using the new timescale of Cande and Kent [1992 doi:10.1029/92JB01202], and evaluates Sr isotopic changes against an inferred glacioeustatic proxy. We generated a latest Oligocene to early late Miocene 87Sr/86Sr isotope record from Ocean Drilling Program (ODP) Hole 747A; this site provides an excellent magnetostratigraphic record during most of this interval for independent age estimates, very good foraminiferal preservation, and excellent core recovery. Comparisons of new 87Sr/86Sr data from Hole 747A with previously published data from Deep Sea Drilling Project (DSDP) Sites 608 [Miller et al., 1991 doi:10.1029/90PA01941] and 588 [Hodell et al., 1991 doi:10.1130/0091-7613(1991)019<0024:VITSIC>2.3.CO;2] yield the following results: (1) confirmation and refinement of the early Miocene Sr isotope changes, (2) improved definition of the timing of the changes in slope of 87Sr/86Sr near 15.4 Ma and 22.8 Ma, (3) improved Sr isotopic age resolution for the middle Miocene with resolution as good as +/- 0.7 m.y., and (4) identification of an inflection in the Sr isotope record at 28.0 Ma based on the combined records from DSDP Site 522 [Miller et al., 1988 doi:10.1029/PA003i002p00223] and ODP Hole 747A. We have been unable to determine the cause of middle Miocene offset between Site 588 and Hole 747A data, although we believe it may be attributed to problems in the age assignments for Hole 588A for the interval ~14-11 Ma and Site 747 for the interval 11-8 Ma. Because Hole 747A results provide a better chronology than Site 588 for most of the Miocene and a better middle Miocene Sr isotope record than Site 608, we propose that Hole 747A serves as the best reference section for Miocene 87Sr/86Sr variations from ca. 23 to 11 Ma. Using 87Sr/86Sr data from Sites 522, 608, and 747A, we relate late Eocene to early Miocene inflections in the 87Sr/86Sr isotope record to oxygen isotope increases and decreases inferred to represent glacioeustatic events. The decreases (deglaciations) observed in the ?18O record apparently lead the 87Sr/86Sr inflections by 1 to 1.5 m.y.

Miocene to Pleistocene planktonic foraminifer biostratigraphy of ODP Leg 135 sites

Diverse and well-preserved planktonic foraminifers were recovered from six sites (834-839) drilled in the Lau Basin. Planktonic faunas from the Tongan Platform sites varied from those of the Lau Basin sites by being less well preserved (Site 840) to being very poorly preserved and very sparse (Site 841); at Site 841 most samples were barren. All sites penetrated a volcaniclastic sequence in which thick ash beds were encountered; foraminifer populations within the ash beds were often very small, making it difficult to obtain biostratigraphic data. No hiatuses were encountered in the upper Miocene to Pleistocene sections of the Lau Basin, but a possible break occurs at Site 840 on the Tongan Platform. Site 834 penetrated through a Quaternary-Pliocene sequence overlying basaltic basement, and topmost Miocene (Zone N17B) sediments interbedded within the volcanic sequence. Site 835 penetrated into the lower Pliocene (Zones N19 to N19-20). Site 836 penetrated the shortest section, with Zone N22 {Globorotalia (Truncorotalia) crassaformis hessi Subzone) directly overlying basalts. Site 837 penetrated into the basal part of Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) overlying basalt. Site 838 failed to encounter basalts, with the oldest sediment being from Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone). Site 839, within the same basin as Site 838, located Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) sediments directly overlying igneous basement. Site 840 penetrated into the upper Miocene Zone N17A without encountering any major unconformity. Site 841, studied mainly from core-catcher samples, penetrated a Quaternary to questionable upper Miocene sequence that was in fault contact with middle Miocene (Zones N8 to N9) sediments. For the Lau Basin sites, reworking was encountered only in Sites 834 and 835. Site 834 was drilled adjacent to the Lau Ridge, on which are developed numerous reef al and shallow-water environments, where erosional conditions could have been expected during sea-level lowstands. Site 835 was drilled in a narrow basin that has been remote from these erosional influences; slumping and erosion of material from the adjacent basin slopes appears to have been the source of the reworking. For the Tongan Platform sites, reworking was observed only in the lower part of the upper Miocene section at Site 841, where late Eocene larger foraminifers are present in conglomerates and grits. The presence of Globorotalia (Globorotalia) multicamerata and small specimens of Sphaeroidinellopsis spp. in the Pleistocene of Site 840 may indicate reworking, but this is not clear. Unit I, which marks a reduction in volcanic activity in the Lau Basin, ranges in age from the lower part of Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) at Sites 834 and 835, to within Zone N22 (Globorotalia crassaformis hessi Subzone) at Sites 836 to 838, and within the upper part of Zone N22 (Bolliella praeadamsi Subzone) at Site 839. Units II and III are generally represented by thick to very thick ash beds, which generally contain low-diversity and often poorly preserved assemblages. Igneous sources seem to have remained important contributors of sediment up to the present day.

Late Eocene-earliest Miocene Sr isotopic reference section of DSDP Site 73-522

We present a revised calibration of Sr isotopes to the geomagnetic polarity timescale (GPTS) using closely spaced (~0.15 m.y. resolution) samples from the classic uppermost Eocene through lowermost Miocene section at Site 522, eastern South Atlantic. The Sr isotopic data are fit with two linear segments with a sharp change in slope at circa 27.5 Ma from 0.000038/m.y. (27.5 to 34.4 Ma) to 0.000051/m.y. (23.8 to 27.5 Ma). Regression analysis indicates that stratigraphic resolution ranges from ±1 m.y. (for one analysis) to ±0.6 m.y. (for three analyses) for the younger interval and ±1.2 m.y. (for one analysis) to ±0.7 m.y. (for three analyses) for the older interval, representing an increase in resolution from previous studies of ±1-2 m.y. The paleoceanographic significance of this change in slope is unclear. It occurs during an interval of intermittent Antarctic glaciation, between the Oi2a and Oi2b glaciations. The subsequent interval from circa 27 to 24 Ma appears to be an interval of minimal glaciation. Thus this observation does not support previous suggestions that increases in rates of Sr isotopic change are directly associated with the frequency of Antarctic glaciations. Rather, the increase in slope may be related to increased weathering associated with the "mid-Oligocene" glaciation.

Miocene benthic foraminifer oxygen and carbon isotope record of ODP Site 115-709

The benthic isotopic record of Miocene Cibicidoides from Site 709 provides a record of conditions in the Indian Ocean at a depth of about 3200 mbsf. As expected, the record qualitatively resembles those of other Deep Sea Drilling Project and Ocean Drilling Program sites. The data are consistent with the scenario for the evolution of thermohaline circulation in the Miocene Indian Ocean proposed by Woodruff and Savin (1989, doi:10.1029/PA004i001p00087). Further testing of that scenario, however, requires isotopic data for Cibicidoides from other Indian Ocean sites. There is a correlation between d13C values of Cibicidoides and planktonic:benthic (P:B)ratios of Site 709 sediments, implying a causal relationship between the corrosiveness of deep waters and concentration of CO2 derived from oxidation of organic matter.

Nannofossil assemblages and flux rates during the late Oligocene-early Miocene

This study investigates abundance variations in Noelaerhabdaceae assemblages during the late Oligocene-early Miocene at three subtropical sites in the Atlantic and Pacific oceans (DSDP Sites 516, 608 and 588). At these three sites, nannofossil assemblages were characterized by the successive high proportion of Cyclicargolithus, Dictyococcites and Reticulofenestra. Local paleoceanographic changes, such as the input of nutrient-poor water masses, might explain shifts in ecological prominence within the Noelaerhabdaceae at DSDP Site 516 (South Atlantic). But the similar timing of a decline in Cyclicargolithus at the three studied sites more likely corresponds to a global process. Here, we explore possible causes for this long-term taxonomic turnover. A global change in climate, associated with early Miocene glaciations, could have triggered a decline in fitness of the taxon Cyclicargolithus. The ecological niche made vacant because of the decrease in Cyclicargolithus could then have been exploited by Dictyococcites and Reticulofenestra that became prominent in the assemblages after 20.5 Ma. Alternatively, this global turnover might reflect a gradual evolutionary succession and be the result of other selection pressures, such as increased competition between Cyclicargolithus and Dictyococcites/Reticulofenestra. A diversification within Dictyococcites/Reticulofenestra, indicated by an expansion in the size variation within this group since ~ 20.5 Ma, may have contributed to the decreased fitness of Cyclicargolithus.