1,721,033 research outputs found
No change in Southern Ocean circulation in the Indian Ocean from the Eocene through Late Oligocene
No full textDeciphering the evolution of Southern Ocean circulation during the Eocene and Oligocene has important implications for understanding the development of the Antarctic Circumpolar Current and transition to Earth's "icehouse" climate. To better understand ocean circulation patterns in the Indian Ocean sector of the Southern Ocean, we generated a new fossil fish tooth neodymium isotope record (εNd) from the upper Eocene to upper Oligocene sections (36-23 Ma) of Ocean Drilling Program Sites 744 and 748 (Kerguelen Plateau, Indian Ocean). Reconstructed seawater εNd values from fossil fish teeth are used to trace changes in water masses across ocean basins. The records from Site 748 and Site 744 reveal a gradual shift from εNd values around -6.5 to -7.5 in the late Eocene to εNd values between -7.5 and -8.3 by the late Oligocene, consistent with a Circumpolar Deep Water (CDW) influence at the Kerguelen Plateau throughout the Oligocene. We interpret the shift to less radiogenic values to reflect the increased export of Northern Component Water to the Southern Ocean, likely into the proto-CDW. However, the records show no major change in water mass composition around the Kerguelen Plateau that would accompany an increase in Pacific throughflow related to the opening of Drake Passage and imply that Pacific throughflow via the Drake Passage occurred by the late Eocene. High-frequency variability in eNd values at Site 744 is interpreted as an imprint of Oligocene glacial activity, with a particularly pronounced excursion at 32.6 Ma roughly coinciding with other glacial weathering indicators around Antarctica.</p
Isotopic interrogation of a suspected late Eocene glaciation
No full textEphemeral polar glaciations during the middle-to-late Eocene (48–34?Ma) have been proposed based on far-field ice volume proxy records and near-field glacigenic sediments, although the scale, timing, and duration of these events are poorly constrained. Here we confirm the existence of a transient cool event within a new high-resolution benthic foraminiferal ?18O record at Ocean Drilling Program (ODP) Site 738 (Kerguelen Plateau; Southern Ocean). This event, named the Priabonian oxygen isotope maximum (PrOM) Event, lasted ~140?kyr and is tentatively placed within magnetochron C17n.1n (~37.3?Ma) based on the correlation to ODP Site 689 (Maud Rise, Southern Ocean). A contemporaneous change in the provenance of sediments delivered to the Kerguelen Plateau occurs at the study site, determined from the <63?µm fraction of decarbonated and reductively leached sediment samples. Changes in the mixture of bottom waters, based on fossil fish tooth ?Nd, were less pronounced and slower relative to the benthic ?18O and terrigenous ?Nd changes. Terrigenous sediment ?Nd values rapidly shifted to less radiogenic signatures at the onset of the PrOM Event, indicating an abrupt change in provenance favoring ancient sources such as the Paleoproterozoic East Antarctic craton. Bottom water ?Nd reached a minimum value during the PrOM Event, although the shift begins much earlier than the terrigenous ?Nd excursion. The origin of the abrupt change in terrigenous sediment provenance is compatible with a change in Antarctic terrigenous sediment flux and/or source as opposed to a reorganization of ocean currents. A change in terrigenous flux and/or source of Antarctic sediments during the oxygen isotope maximum suggests a combination of cooling and ice growth in East Antarctica during the early late Eocene
(Table S2) Nd isotopic record of fossil this teeth from ODP Site 177-1090
No full textSediment depth is given in mcd
Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies
No full textEarth’s mightiest ocean current, the Antarctic Circumpolar Current (ACC), regulates the exchange of heat and carbon between the ocean and the atmosphere, and influences vertical ocean structure, deep-water production and the global distribution of nutrients and chemical tracers. The eastward-flowing ACC occupies a unique circumglobal pathway in the Southern Ocean that was enabled by the tectonic opening of key oceanic gateways during the break-up of Gondwana (for example, by the opening of the Tasmanian Gateway, which connects the Indian and Pacific oceans). Although the ACC is a key component of Earth’s present and past climate system1, the timing of the appearance of diagnostic features of the ACC (for example, low zonal gradients in water-mass tracer fields) is poorly known and represents a fundamental gap in our understanding of Earth history. Here we show, using geophysically determined positions of continent–ocean boundaries, that the deep Tasmanian Gateway opened 33.5 ± 1.5 million years ago (the errors indicate uncertainty in the boundary positions). Following this opening, sediments from Indian and Pacific cores recorded Pacific-type neodymium isotope ratios, revealing deep westward flow equivalent to the present-day Antarctic Slope Current. We observe onset of the ACC at around 30 million years ago, when Southern Ocean neodymium isotopes record a permanent shift to modern Indian–Atlantic ratios. Our reconstructions of ocean circulation show that massive reorganization and homogenization of Southern Ocean water masses coincided with migration of the northern margin of the Tasmanian Gateway into the mid-latitude westerly wind band, which we reconstruct at 64° S, near to the northern margin. Onset of the ACC about 30 million years ago coincided with major changes in global ocean circulation and probably contributed to the lower atmospheric carbon dioxide levels that appear after this time
(Table 4a) Nd isotope ratios of fossil fish teeth samples of ODP Hole 119-738B
No full text(Table 4a) Nd isotope ratios of fossil fish teeth samples of ODP Hole 119-738
Clay-fraction strontium and neodymium isotopes, K/Al ratios, and bulk grain size from IODP Sites U1456 and U1457
No full textThis study presents data collected from International Ocean Discovery Program (IODP) Expedition 355 Arabian Sea Monsoon to investigate changes in sediment provenance as well as the effect of sediment transport processes on deep-sea sediment samples. From Site U1457, 83 sediment samples were analyzed, complemented by 44 samples from Site U1456. Samples were analyzed for bulk grain-size using a Malvern Mastersizer 2000. The clay fraction was separated, digested, and analyzed for K/Al ratios using inductively couple plasma optical emission spectroscopy (ICP-OES). Radiogenic strontium (87Sr/86Sr) was measured using a Neptune Plus multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) and a Triton Plus multicollector thermal ionization mass spectrometer (TIMS). A subset of 38 samples were analyzed for radiogenic neodymium composition (143Nd/144Nd) using TIMS
(Appendix A4) Strontium isotope ratios and concentrations of fish teeth of ODP Site 113-689B
No full textSr of foraminifera see Mead and Hodell (1995) data set: doi:10.1594/PANGAEA.55541. Sr data of fluid see Egebert (1990) data set: doi:10.1594/PANGAEA.70818
(Table S5) Nd isotope ratios of fossil fish teeth MECO samples of OPD Hole 119-738B
No full text(Table S5) Nd isotope ratios of fossil fish teeth MECO samples of OPD Hole 119-738
(Appendix A2) Strontium isotope ratios and concentrations of fish teeth, foraminifera and pore fluids of ODP Site 121-757B
No full textAge 1 based on nannofossil biostratigraphy (Peirce et al., 1989, doi:10.2973/odp.proc.ir.121.1989) and converted to Cande and Kent (1995, doi:10.1029/94JB03098) using Mead (1996, doi:10.1029/95JB03745). Age 2 based on Sr chemostratigraphy from 15 to 30 M
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