Howie D. Scher

Timing and Climatic Consequences of the Opening of Drake Passage

Age estimates for the opening of Drake Passage range from 49 to 17 million years ago (Ma), complicating interpretations of the relationship between ocean circulation and global cooling. Secular variations of neodymium isotope ratios at Agulhas Ridge (Southern Ocean, Atlantic sector) suggest an influx of shallow Pacific seawater approximately 41 Ma. The timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals. Circulation/productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide. These results also indicate that Drake Passage opened before the Tasmanian Gateway, implying the late Eocene establishment of a complete circum-Antarctic pathway.

A Cenozoic record of the equatorial Pacific carbonate compensation depth

Atmospheric carbon dioxide concentrations and climate are regulated on geological timescales by the balance between carbon input from volcanic and metamorphic outgassing and its removal by weathering feedbacks; these feedbacks involve the erosion of silicate rocks and organic-carbon-bearing rocks. The integrated effect of these processes is reflected in the calcium carbonate compensation depth, which is the oceanic depth at which calcium carbonate is dissolved. Here we present a carbonate accumulation record that covers the past 53 million years from a depth transect in the equatorial Pacific Ocean. The carbonate compensation depth tracks long-term ocean cooling, deepening from 3.0–3.5 kilometres during the early Cenozoic (approximately 55 million years ago) to 4.6 kilometres at present, consistent with an overall Cenozoic increase in weathering. We find large superimposed fluctuations in carbonate compensation depth during the middle and late Eocene. Using Earth system models, we identify changes in weathering and the mode of organic-carbon delivery as two key processes to explain these large-scale Eocene fluctuations of the carbonate compensation depth.

Preservation of seawater Sr and Nd isotopes in fossil fish teeth: bad news and good news ☆

Abstract We analyzed 87Sr/86Sr ratios in foraminifera, pore fluids, and fish teeth for samples ranging in age from Eocene to Pleistocene from four Ocean Drilling Program sites distributed around the globe: Site 1090 in the Cape Basin of the Southern Ocean, Site 757 on the Ninetyeast Ridge in the Indian Ocean, Site 807 on the Ontong-Java Plateau in the western equatorial Pacific, and Site 689 on the Maud Rise in the Southern Ocean. Sr isotopic ratios for dated foraminifera consistently plot on the global seawater Sr isotope curve. For Sites 1090, 757, and 807 Sr isotopic values of the pore fluids are generally less radiogenic than contemporaneous seawater values, as are values for fossil fish teeth. In contrast, pore fluid 87Sr/86Sr values at Site 689 are more radiogenic than contemporaneous seawater, and the corresponding fish teeth also record more radiogenic values. Thus, Sr isotopic values preserved in fossil fish teeth are consistently altered in the direction of the pore fluid values; furthermore, there is a correlation between the magnitude of the offset between the pore fluids and the seawater curve, and the associated offset between the fish teeth and the seawater curve. These data suggest that the hydroxyfluorapatite of the fossil fish teeth continues to recrystallize and exchange Sr with its surroundings during burial and diagenesis. Therefore, Sr chemostratigraphy can be used to determine rough ages for fossil fish teeth in these cores, but cannot be used to fine-tune age models. In contrast to the Sr isotopic system, our Nd concentration data, combined with published isotopic and rare earth element data, suggest that fish teeth acquire Nd during early diagenesis while they are still in direct contact with seawater. The concentrations of Nd acquired at this stage are extremely high relative to the concentrations in surrounding pore fluids. As a result, Nd isotopes are not altered during burial and later diagenesis. Therefore, fossil fish teeth from a variety of marine environments preserve a reliable and robust record of deep seawater Nd isotopic compositions from the time of deposition.

Eocene to Miocene magnetostratigraphy, biostratigraphy, and chemostratigraphy at ODP Site 1090 (sub-Antarctic South Atlantic)

At Ocean Drilling Program (ODP) Site 1090 (lat 42854.89S, long 8854.09E) located in a water depth of 3702 m on the Agulhas Ridge in the sub-Antarctic South Atlantic, ;300 m of middle Eocene to middle Miocene sediments were recovered with the advanced piston corer (APC) and the extended core barrel (XCB). U-channel samples from the 70‐230 meters composite depth (mcd) interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole-core and discrete sample data. The magnetostratigraphy can be interpreted by the fit of the polarity-zone pattern to the geomagnetic polarity time scale (GPTS) augmented by isotope data and bioevents with documented correlation to the GPTS. Three normal-polarity subchrons (C5Dr.1n, C7Ar.1n, and C13r.1n), not included in the standard GPTS, are recorded at Site 1090. The base of the sampled section is correlated to C19n (middle Eocene), although the interpretation is unclear beyond C17r. The top of the sampled section is correlated to C5Cn (late early Miocene), although, in the uppermost 10 m

Two-stepping into the icehouse: East Antarctic weathering during progressive ice-sheet expansion at the Eocene-Oligocene transition

In conjunction with increasing benthic foraminiferal δ 18 O values at the Eocene–Oligocene transition (EOT; ca. 34 Ma), coarse-grained ice-rafted debris (IRD; >425 μm) appears abruptly alongside fossil fish teeth with continentally derived neodymium (Nd) isotope ratios (e Nd ) in Kerguelen Plateau (Southern Ocean) sediments. Increased Antarctic weathering flux, as inferred from two steps to less radiogenic e Nd values, coincides with two steps in benthic foraminiferal δ 18 O values. These results indicate that two distinct surges of weathering were generated by East Antarctic ice growth during the EOT. Weathering by ice sheets during a precursor glaciation at 33.9 Ma did not produce significant IRD accumulation during the first e Nd shift. Glacial weathering was sustained during a terrace interval between the two steps, probably by small high-elevation ice sheets. A large increase in weathering signals the rapid coalescence of small ice sheets into an ice sheet of continental proportions ca. 33.7 Ma. Rapid ice sheet expansion resulted in a suppression of weathering due to less exposed area and colder conditions. Parallel changes in Antarctic weathering flux and deep-sea carbonate accumulation suggest that ice-sheet expansion during the EOT had a direct impact on the global carbon cycle; possible mechanisms include associated changes in silicate weathering on the East Antarctic craton and enhanced fertilization of Southern Ocean waters, both of which warrant further investigation.

Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies

Earth’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 system, 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.

A Nd isotopic study of southern sourced waters and Indonesian Throughflow at intermediate depths in the Cenozoic Indian Ocean

We present Nd isotopic data for fossil fish teeth recovered from the past 40 m.y. at ODP Site 757, currently located at 1650 m water depth on the Ninetyeast Ridge in the Indian Ocean. Although Site 757 sits in a region strongly influenced by weathering inputs from the Himalayas and volcanic inputs from the Indonesian arc, the pattern of Nd isotopic variations does not appear to respond to these potential sources of Nd. Instead, secular variations correlate to changes in the composition of intermediate to deep water masses bathing the site and circulation patterns in the Indian Ocean. From ∼40 to 10 Ma, ɛNd values and the pattern of change at Site 757 closely match those of ODP Site 1090, a deep water site in the Atlantic sector of the Southern Ocean. Comparison to data from several Fe-Mn crusts in the Indian Ocean suggests that intermediate to deep water flow paths were similar to the modern distribution of Circumpolar Deep Water. At approximately 10 Ma, Nd isotopic values increase in a step function by 2 ɛNd units, suggesting that plate motions had carried Site 757 into a region influenced by Indonesian Throughflow. Estimates of the vertical and horizontal position of this site at 10 Ma imply that Indonesian Throughflow extended as far south as ∼20°S and to a depth of ∼1500 m. From 10 to 0 Ma, Nd isotopic variations at Site 757 appear to record variations in Indonesian Throughflow. From 10 to 5.5 Ma, values at Site 757 overlap with those from crusts located in the southwest Pacific, indicating extensive flow through the Indonesian Seaway. From 5.5 to 3.4 Ma, ɛNd values become less radiogenic at Site 757 and more radiogenic in the southwest Pacific, suggesting increasing closure of the seaway and concomitant rerouting of equatorial Pacific waters. Beginning at 3.4 Ma, ɛNd values become more radiogenic again at Site 757, which may be attributed to enhanced opening of the seaway or to a change in the source of Throughflow waters from a southern to a northern Pacific region.

Accuracy and precision of 88Sr/86Sr and 87Sr/86Sr measurements by MC‐ICPMS compromised by high barium concentrations

Barite (BaSO4) is a widely distributed mineral that incorporates strontium (Sr) during formation. Mass-dependent fractionation of Sr isotopes occurs during abiotic precipitation of barite and formation of barite associated with biological processes (e.g., bacterial sulfide oxidation). Sr isotopes in barite can provide provenance information as well as potentially reconstruct sample formation conditions (e.g., saturation state, temperature, biotic versus abiotic). Incomplete separation of Ba from Sr has complicated measurements of Sr isotopes by MC-ICPMS. In this study, we tested the effects of Ba in Sr sample solutions and modified extraction chromatography of Sr using Eichrom Sr Spec (Eichrom Technologies LLC, USA) resin to enable rapid, accurate, and precise measurements of 88Sr/86Sr and 87Sr/86Sr ratios from Ba-rich matrices. Sr isotope ratios of sample solutions doped with Ba were statistically indistinguishable from Ba-free sample solutions below 1 ppm Ba. Deviations in both 87Sr/86Sr and δ88/86Sr occurred above 1 ppm Ba. An updated extraction chromatography method tested with barite and Ba-doped seawater produces Sr sample solutions containing 10–100 ppb levels of Ba. The practice of Zr spiking for external mass-discrimination correction of 88Sr/86Sr ratios was also evaluated, and it was confirmed that variable Zr levels do not have adverse effects on the accuracy and precision of 87Sr/86Sr ratios in the Zr concentration range required to produce accurate δ88/86Sr values.

Isotopic interrogation of a suspected late Eocene glaciation

Ephemeral 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.

Expanded oxygen minimum zones during the late Paleocene‐early Eocene: Hints from multiproxy comparison and ocean modeling

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (< ~35 µmol/kg), while higher O2 values (~60–100 µmol/kg) were obtained in an Earth system model (Grid ENabled Integrated Earth system model). Multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.