Stable isotope record of foraminifera from ODP Hole 114-704A

Isotopic and sedimentologic data from Ocean Drilling Program hole 704A suggest that isotopic stages 7, 9, and 11 were marked by unusually strong interglacial conditions in surface waters of the southern ocean. During interglacial stages 9 and 11, warm surface waters penetrated far poleward and may have led to destabilization of the West Antarctic Ice Sheet. In contrast, the strongest glacial conditions in surface waters of the subantarctic South Atlantic occurred during oxygen isotopic stage 12. Comparisons of benthic carbon isotopic gradients between sites located in the North Atlantic, southern ocean, and Pacific indicate that the production of upper North Atlantic Deep Water (uNADW) was strongest during stages 7,9, and 11 and weakest during stage 12, These results suggest a possible link between the flux of uNADW and paleoceanographic change in the southern ocean and support the traditional NADW-Antarctic connection whereby increased NADW leads to warming of the southern ocean.

Onset of long-term cooling of Greenland near the Eocene-Oligocene boundary as revealed by branched tetraether lipids

The Eocene-Oligocene (E-O) boundary interval is considered to be one of the major transitions in Earth's climate, witnessing the first major expansion of the East Antarctic Ice Sheet. However, the extent of the associated climatic cooling, especially for high northern latitude continental landmasses, is poorly constrained. In this study we reconstruct the first mean annual air temperature (MAAT) for the Greenland landmass during the late Eocene and early Oligocene by applying a new proxy based on the distribution of branched tetraether lipids derived from soil bacteria preserved in a marine sediment core from the Greenland Basin. The temperature estimates are compared with a composite continental temperature record based on bio-climatic analysis of pollen assemblages. Both proxies reveal comparable late Eocene MAATs of ~13-15 °C and a gradual long-term cooling of ~3-5 °C starting near the E-O boundary. These data are in agreement with other MAAT reconstructions from northern midlatitude continents and suggest a general cooling of the Northern Hemisphere during the E-O transition.

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.

Age data of ODP Site 181-1124

ODP Site 1124, located 600 km east of the North Island of New Zealand, records post-middle Oligocene variations in the Pacific Deep Western Boundary Current (DWBC) and New Zealand's climatic and tectonic evolution. Sediment parameters, such as terrigenous grain size, flux, magnetic fabric, and non-depositional episodes, are used to interpret DWBC intensity and Antarctic climate. Interpretations of DWBC velocities indicate that the Antarctic Circumpolar Current reached modern intensities at ~23 Ma, as the tectonic seaways expanded, completing the thermal isolation of Antarctica. Periods of more intense bottom water formation are suggested by the presence of hiatuses formed under the DWBC at 22.5-17.6, 16.5-15, and 14-11 Ma. The oldest interval of high current intensity occurs within a climatically warm period during which the intensity of thermohaline circulation around Antarctica increased as a result of recent opening of circum-Antarctic gateways. The younger hiatuses represent glacial periods on Antarctica and major fluctuations in the East Antarctic Ice Sheet, whereas intervals around the hiatuses represent times of relative warmth, but with continued current activity. The period between 11 to 9 Ma is characterized by conditions surrounding a high velocity DWBC around the time of the formation and stabilization of the West Antarctic Ice Sheet. The increased terrigenous input may result from either changing Antarctic conditions or more direct sediment transport from New Zealand. The Pacific DWBC did not exert a major influence on sedimentation at Site 1124 from 9 Ma to the present; the late Miocene to Pleistocene sequence is more influenced by the climatic and tectonic history of New Zealand. Despite the apparent potential for increased sediment supply to this site from changes in sediment channeling, increasing rates of mountain uplift, and volcanic activity, terrigenous fluxes remain low and constant throughout this younger period.

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.

Foraminiferal study of ODP Hole 189-1171

Sequence boundary ages determined in shallow-water sediments obtained from ODP (Ocean Drilling Program) Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, United States, and northwestern Europe) and d18O increases from deep-sea records, indicating that significant (>10 m) eustatic changes occurred during the early to middle Eocene (51-42 Ma). Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO3 content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO3 content, and an abrupt increase in glauconite above the surface. Foraminiferal biofacies and planktonic/benthic foraminiferal ratios were used to estimate water-depth changes. Ages of six sequence boundaries (50.9, 49.2, 48.5-47.8, 47.1, 44.5, and 42.6 Ma) from Site 1171 correlate well to the timings of d18O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and d18O increases indicates a global control and that glacioeustasy was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric pCO2 estimates showing that the first time pCO2 levels decreased below a threshold that would support the development of an Antarctic ice sheet occurred at ca. 51 Ma. Estimates of sea-level amplitudes range from ~20 m for the early Eocene (51-49 Ma) and ~25 m to ~45 m for the middle Eocene (48-42 Ma) using constraints established for Oligocene d18O records.

Neogene and Quaternary paleoenvironmental history at DSDP Leg 71 Holes

The Neogene and Quaternary sedimentary record of Leg 71 and previously drilled sequences from the Southern Ocean reveal evidence of a major late Miocene change of oceanic and glacial conditions in the southern high latitudes during paleomagnetic Chron 9. The characteristics of late Miocene sedimentation and in particular the study of erosional patterns and ice-rafted debris suggest the following conclusions. 1) In the late Miocene, the Polar Front first migrated to the northern latitudes of the Southern Ocean and surface water temperatures became similar to those of today. 2) Extensive ice shelves or ice tongues were not present along the Antarctic margin until late Chron 9 (about 9.0 Ma). 3) Before Chron 9, West Antarctica was occupied by an archipelago and the West Antarctic Sea. 4) Extensive ice shelves formed in the West Antarctic region, eventually coalescing and thickening to form the grounded West Antarctic ice sheet by Chron 9. 5) The newly formed West Antarctic ice sheet was probably unstable and frequently became an ungrounded ice shelf, thus accounting for the scarcity of late Miocene ice-rafted debris. 6) Extensive erosion or nondeposition of sediment was probably the result of increased Antarctic Bottom Water (AABW) formation in the West Antarctic region during the initial formation of extensive West Antarctic ice shelves and during periods when the West Antarctic ice sheet was ungrounded. 7) In the Southwest Atlantic, AABW velocity waned during the latest Miocene. During the late Gilbert Chron a major and permanent change occurred in the pattern of ice-rafting to the South Atlantic, and after 4.35 Ma the increased IRD accumulation rate and frequency of major episodes of IRD accumulation suggest increased stability of the West Antarctic ice sheet. In addition, radiolarian faunas of Hole 514 record at least eight migrations of the Polar Front to the north of the site during the past 4.07 m.y. An apparent increase in the frequency of Polar Front migrations occurred about 2.7-2.6 Ma, possibly in response to oceanic change induced by fluctuations in glacial conditions of the Northern Hemisphere.

Stable isotopes and calcium carbonate at DSDP Holes 85-574 and 85-574A

Detailed stable isotopic and calcium carbonate records (with a sampling resolution of 3000 yr.) from the middle Miocene section of hydraulic piston corer (HPC) Hole 574A provide a sequence that records the major shift in the oxygen isotopic composition of the world's oceans that occurred at about 14 Ma. The data suggest that this transition was rapid and spans about 30,000 yr. of sediment deposition. In intervals before and after the shift, the mean d18O values are characterized by a constant mean with a high degree of variability. The degree of variability in both the d18O and d13C records is comparable to that observed for the Pliocene and earliest Pleistocene and does not show a significant change before or after the major shift in the d18O record. Whereas the oxygen isotopic record is characterized by relatively stable mean values before and after the middle Miocene event, the d13C record shows a number of significant offsets in the mean value separated by intervals of high-frequency variations. Time and frequency domain analysis of all records from Hole 574A indicate that the frequency components shown to be related to orbital changes in the Pleistocene record are also present in the middle Miocene. The high variability observed in the Site 574 isotopic records places important constraints on models describing the role of formation of the Antarctic ice sheet during the middle Miocene climatic transitions. Thus, HPC Hole 574A provides a valuable sequence for detailed study of climatic variability during an important time in the Earth's history, although we cannot provide a definitive explanation of the major oxygen isotopic event of the middle Miocene.

Nd bulk isotopes and 40Ar/39Ar ages of individual minerals from Aurora and Wilkes sub-glacial basin sediments

The Wilkes and Aurora basins are large, low-lying sub-glacial basins that may cause areas of weakness in the overlying East Antarctic ice sheet. Previous work based on ice-rafted debris (IRD) provenance analyses found evidence for massive iceberg discharges from these areas during the late Miocene and Pliocene. Here we characterize the sediments shed from the inferred areas of weakness along this margin (94°E to 165°E) by measuring40Ar/39Ar ages of 292 individual detrital hornblende grains from eight marine sediment core locations off East Antarctica and Nd isotopic compositions of the bulk fine fraction from the same sediments. We further expand the toolbox for Antarctic IRD provenance analyses by exploring the application of 40Ar/39Ar ages of detrital biotites; biotite as an IRD tracer eliminates lithological biases imposed by only analyzing hornblendes and allows for characterization of samples with low IRD concentrations. Our data quadruples the number of detrital 40Ar/39Ar ages from this margin of East Antarctica and leads to the following conclusions: (1) Four main sectors between the Ross Sea and Prydz Bay, separated by ice drainage divides, are distinguishable based upon the combination of 40Ar/39Ar ages of detrital hornblende and biotite grains and the e-Nd of the bulk fine fraction; (2) 40Ar/39Ar biotite ages can be used as a robust provenance tracer for this part of East Antarctica; and (3) sediments shed from the coastal areas of the Aurora and Wilkes sub-glacial basins can be clearly distinguished from one another based upon their isotopic fingerprints.