| 1. |
- Barrientos, Natalia, et al.
(author)
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Arctic Ocean benthic foraminifera Mg/Ca ratios and global Mg/Ca-temperature calibrations: New constraints at low temperatures
- 2018
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In: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533. ; 236, s. 240-259
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Journal article (peer-reviewed)abstract
- We explore the use of Mg/Ca ratios in six Arctic Ocean benthic foraminifera species as bottom water palaeothermometers and expand published Mg/Ca-temperature calibrations to the coldest bottom temperatures (<1 °C). Foraminifera were analyzed in surface sediments at 27 sites in the Chukchi Sea, East Siberian Sea, Laptev Sea, Lomonosov Ridge and Petermann Fjord. The sites span water depths of 52–1157 m and bottom water temperatures (BWT) of −1.8 to +0.9 °C. Benthic foraminifera were alive at time of collection, determined from Rose Bengal (RB) staining. Three infaunal and three epifaunal species were abundant enough for Mg/Ca analysis. As predicted by theory and empirical evidence, cold water Arctic Ocean benthic species produce low Mg/Ca ratios, the exception being the porcelaneous species Quinqueloculina arctica. Our new data provide important constraints at the cold end (<1 °C) when added to existing global datasets. The refined calibrations based on the new and published global data appear best supported for the infaunal species Nonionella labradorica (Mg/Ca = 1.325 ± 0.01 × e^(0.065 ± 0.01 × BWT), r2 = 0.9), Cassidulina neoteretis (Mg/Ca = 1.009 ± 0.02 × e^(0.042 ± 0.01 × BWT), r2 = 0.6) and Elphidium clavatum (Mg/Ca = 0.816 ± 0.06 + 0.125 ± 0.05 × BWT, r2 = 0.4). The latter is based on the new Arctic data only. This suggests that Arctic Ocean infaunal taxa are suitable for capturing at least relative and probably semi-quantitative past changes in BWT. Arctic Oridorsalis tener Mg/Ca data are combined with existing O. umbonatus Mg/Ca data from well saturated core-tops from other regions to produce a temperature calibration with minimal influence of bottom water carbonate saturation state (Mg/Ca = 1.317 ± 0.03 × e^(0.102 ± 0.01 BWT), r2 = 0.7). The same approach for Cibicidoides wuellerstorfi yields Mg/Ca = 1.043 ± 0.03 × e^(0.118 ± 0.1 BWT), r2 = 0.4. Mg/Ca ratios of the porcelaneous epifaunal species Q. arctica show a clear positive relationship between Mg/Ca and Δ[CO32−] indicating that this species is not suitable for Mg/Ca-palaeothermometry at low temperatures, but may be useful in reconstructing carbonate system parameters through time.
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| 2. |
- Bradshaw, Catherine D., et al.
(author)
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Hydrological impact of Middle Miocene Antarctic ice-free areas coupled to deep ocean temperatures
- 2021
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In: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 14, s. 429-436
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Journal article (peer-reviewed)abstract
- Oxygen isotopes from ocean sediments (δ18O) used to reconstruct past continental ice volumes additionally record deep water temperatures (DWTs). Traditionally, these are assumed to be coupled (ice-volume changes cause DWT changes). However, δ18O records during peak Middle Miocene warmth (~16–15 million years ago) document large rapid fluctuations (~1–1.5‰) difficult to explain as huge Antarctic ice sheet (AIS) volume changes. Here, using climate modelling and data comparisons, we show DWTs are coupled to AIS spatial extent, not volume, because Antarctic albedo changes modify the hydrological cycle, affecting Antarctic deep water production regions. We suggest the Middle Miocene AIS had retreated substantially from previous Oligocene maxima. The residual ice sheet varied spatially more rapidly on orbital timescales than previously thought, enabling large DWT swings (up to 4 °C). When Middle Miocene warmth terminated (~13 million years ago) and a continent-scale AIS had stabilized, further ice-volume changes were predominantly in height rather than extent, with little impact on DWT. Our findings imply a shift in ocean sensitivity to ice-sheet changes occurs when AIS retreat exposes previously ice-covered land; associated feedbacks could reduce the Earth system’s ability to maintain a large AIS. This demonstrates ice-sheet changes should be characterized not only by ice volume but also by spatial extent.
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| 3. |
- Coxall, Helen K., et al.
(author)
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Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation
- 2018
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In: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 11:3, s. 190-196
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Journal article (peer-reviewed)abstract
- The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland-Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.
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| 4. |
- Edgar, Kirsty M., et al.
(author)
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New composite bio- and isotope stratigraphies spanning the Middle Eocene Climatic Optimum at tropical ODP Site 865 in the Pacific Ocean
- 2020
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In: Journal of Micropalaeontology. - : Copernicus GmbH. - 0262-821X .- 2041-4978. ; 39:2, s. 117-138
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Journal article (peer-reviewed)abstract
- The Middle Eocene Climatic Optimum (MECO) at ca. 40 Ma is one of the largest of the transient Eocene global warming events. However, it is relatively poorly known from tropical settings since few sites span the entirety of the MECO event and/or host calcareous microfossils, which are the dominant proxy carrier for palaeoceanographic reconstructions. Ocean Drilling Program (ODP) Pacific Ocean Site 865 in the low-latitude North Pacific (Allison Guyot) has the potential to provide a useful tropical MECO reference, but detailed stratigraphic and chronological constraints needed to evaluate its completeness were previously lacking. We have addressed this deficit by generating new high-resolution biostratigraphic, stable isotope, and X-ray fluorescence (XRF) records spanning the MECO interval ( similar to 38.0-43.0 Ma) in two holes drilled at Site 865. XRF-derived strontium / calcium (Sr/Ca) and barium / strontium (Ba/Sr) ratios and Fe count records allow correlation between holes and reveal pronounced rhythmicity, enabling us to develop the first composite section for Holes 865B and 865C and a preliminary cyclostratigraphy for the MECO. Using this new framework, the sedimentary record is interpreted to be continuous across the event, as identified by a pronounced transient benthic foraminiferal delta O-18 shift of similar to 0.8 parts per thousand. Calcareous microfossil biostratigraphic events from widely used zonation schemes are recognized, with generally good agreement between the two holes, highlighting the robustness of the new composite section and allowing us to identify planktic foraminiferal Zones E10-E15 and calcareous nannofossil Zones NP15-18. However, discrepancies in the relative position and ordering of several primary and secondary bioevents with respect to published schemes are noted. Specifically, the stratigraphic highest occurrences of planktic foraminifera, Acarinina bullbrooki, Guembelitrioides nuttalli, and Morozovella aragonensis, and calcareous nannofossils, Chiasmolithus solitus and Sphenolithus furcatolithoides, and the lowest occurrence of Reticulofenestra reticulata all appear higher in the section than would be predicted relative to other bioevents. We also note conspicuous reworking of older microfossils (from planktic foraminiferal Zones E5-E9 and E13) into younger sediments (planktic foraminiferal Zones E14-15) within our study interval consistent with reworking above the MECO interval. Regardless of reworking, the high-quality XRF records enable decimetre-scale correlation between holes and highlight the potential of Site 865 for constraining tropical environmental and biotic changes, not just across the MECO but also throughout the Palaeocene and early-to-middle Eocene interval.
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| 5. |
- Hutchinson, David K., et al.
(author)
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The Eocene-Oligocene transition : a review of marine and terrestrial proxy data, models and model data comparisons
- 2021
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In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 17:1, s. 269-315
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Research review (peer-reviewed)abstract
- The Eocene-Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring similar to 34 million years ago (Ma) and lasting similar to 790 kyr. The change is marked by a global shift in deep-sea delta O-18 representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climateadapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (similar to 325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data-model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be undersensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.
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| 6. |
- Hutchinson, David K., et al.
(author)
-
The Eocene-Oligocene transition: a review of marine and terrestrial proxy data, models and model-data comparisons
- 2021
-
In: Climate of the Past. - : European Geosciences Union (EGU). - 1814-9324 .- 1814-9332. ; 17:1, s. 269-315
-
Journal article (peer-reviewed)abstract
- The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼ 34 million years ago (Ma) and lasting ∼ 790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean tempera- ture indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate- adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively com- pare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼ 325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes play- ing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be consid- ered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not in- clude dynamic ice sheets and in some cases may be under- sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.
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| 7. |
- Lear, Caroline H., et al.
(author)
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Neogene ice volume and ocean temperatures : Insights from infaunal foraminiferal Mg/Ca paleothermometry
- 2015
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In: Paleoceanography. - 0883-8305 .- 1944-9186. ; 30:11, s. 1437-1454
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Journal article (peer-reviewed)abstract
- Antarctic continental-scale glaciation is generally assumed to have initiated at the Eocene-Oligocene Transition, yet its subsequent evolution is poorly constrained. We reconstruct changes in bottom water temperature and global ice volume from 0 to 17 Ma using delta O-18 in conjunction with Mg/Ca records of the infaunal benthic foraminifer, O. umbonatus from Ocean Drilling Program (ODP) Site 806 (equatorial Pacific; similar to 2500m). Considering uncertainties in core top calibrations and sensitivity to seawater Mg/Ca (Mg/Ca)(sw), we produce a range of Mg/Ca-temperature-Mg/Ca-sw calibrations. Our favored exponential temperature calibration is Mg/Ca = 0.66 +/- 0.08 x Mg/Ca-sw(0.27 +/- 0.06) x e((0.114 +/- 0.02 x BWT)) and our favored linear temperature calibration is Mg/Ca = (1.21 +/- 0.04 + 0.12 +/- 0.004 x BWT (bottom water temperature)) x (Mg/Ca-sw -0.003 +/- 0.02) (stated errors are 2 s.e.). The equations are obtained by comparing O. umbonatus Mg/Ca for a Paleocene-Eocene section from Ocean Drilling Program (ODP) Site 690 (Weddell Sea) to delta O-18 temperatures, calculated assuming ice-free conditions during this peak warmth period of the Cenozoic. This procedure suggests negligible effect of Mg/Ca-sw on the Mg distribution coefficient (D-Mg). Application of the new equations to the Site 806 record leads to the suggestion that global ice volume was greater than today after the Middle Miocene Climate Transition (similar to 14Ma). ODP Site 806 bottom waters cooled and freshened as the Pacific zonal sea surface temperature gradient increased, and climate cooled through the Pliocene, prior to the Plio-Pleistocene glaciation of the Northern Hemisphere. The records indicate a decoupling of deep water temperatures and global ice volume, demonstrating the importance of thresholds in the evolution of the Antarctic ice sheet.
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| 8. |
- Lunt, Daniel J., et al.
(author)
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DeepMIP : model intercomparison of early Eocene climatic optimum (EECO) large-scale climate features and comparison with proxy data
- 2021
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In: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 17:1, s. 203-227
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Journal article (peer-reviewed)abstract
- We present results from an ensemble of eight climate models, each of which has carried out simulations of the early Eocene climate optimum (EECO, similar to 50 million years ago). These simulations have been carried out in the framework of the Deep-Time Model Intercomparison Project (DeepMIP; http://www.deepmip.org , last access: 10 January 2021); thus, all models have been configured with the same paleogeographic and vegetation boundary conditions. The results indicate that these non-CO2 boundary conditions contribute between 3 and 5 degrees C to Eocene warmth. Compared with results from previous studies, the DeepMIP simulations generally show a reduced spread of the global mean surface temperature response across the ensemble for a given atmospheric CO2 concentration as well as an increased climate sensitivity on average. An energy balance analysis of the model ensemble indicates that global mean warming in the Eocene compared with the preindustrial period mostly arises from decreases in emissivity due to the elevated CO2 concentration (and associated water vapour and long-wave cloud feedbacks), whereas the reduction in the Eocene in terms of the meridional temperature gradient is primarily due to emissivity and albedo changes owing to the non-CO2 boundary conditions (i.e. the removal of the Antarctic ice sheet and changes in vegetation). Three of the models (the Community Earth System Model, CESM; the Geophysical Fluid Dynamics Laboratory, GFDL, model; and the Norwegian Earth System Model, NorESM) show results that are consistent with the proxies in terms of the global mean temperature, meridional SST gradient, and CO2, without prescribing changes to model parameters. In addition, many of the models agree well with the first-order spatial patterns in the SST proxies. However, at a more regional scale, the models lack skill. In particular, the modelled anomalies are substantially lower than those indicated by the proxies in the southwest Pacific; here, modelled continental surface air temperature anomalies are more consistent with surface air temperature proxies, implying a possible inconsistency between marine and terrestrial temperatures in either the proxies or models in this region. Our aim is that the documentation of the large-scale features and model-data comparison presented herein will pave the way to further studies that explore aspects of the model simulations in more detail, for example the ocean circulation, hydrological cycle, and modes of variability, and encourage sensitivity studies to aspects such as paleogeography, orbital configuration, and aerosols.
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| 9. |
- Lunt, Daniel J., et al.
(author)
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The DeepMIP contribution to PMIP4 : experimental design for model simulations of the EECO, PETM, and pre-PETM (version 1.0)
- 2017
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In: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 10:2, s. 889-901
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Journal article (peer-reviewed)abstract
- Past warm periods provide an opportunity to evaluate climate models under extreme forcing scenarios, in particular high (>800 ppmv) atmospheric CO2 concentrations. Although a post hoc intercomparison of Eocene (similar to 50 Ma) climate model simulations and geological data has been carried out previously, models of past high-CO2 periods have never been evaluated in a consistent framework. Here, we present an experimental design for climate model simulations of three warm periods within the early Eocene and the latest Paleocene (the EECO, PETM, and pre-PETM). Together with the CMIP6 pre-industrial control and abrupt 4 x CO2 simulations, and additional sensitivity studies, these form the first phase of DeepMIP - the Deep-time Model Intercomparison Project, itself a group within the wider Paleo-climate Modelling Intercomparison Project (PMIP). The experimental design specifies and provides guidance on boundary conditions associated with palaeogeography, greenhouse gases, astronomical configuration, solar constant, land surface processes, and aerosols. Initial conditions, simulation length, and output variables are also specified. Finally, we explain how the geological data sets, which will be used to evaluate the simulations, will be developed.
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| 10. |
- Viganò, Allyson, et al.
(author)
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Calcareous nannofossils across the Eocene-Oligocene transition at Site 756 (Ninetyeast Ridge, Indian Ocean) : implications for biostratigraphy and paleoceanographic clues
- 2023
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In: Newsletters on stratigraphy. - : Schweizerbart. - 0078-0421. ; 56:2, s. 187-223
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Journal article (peer-reviewed)abstract
- The timing and modalities of calcareous phytoplankton community and evolutionary responses to the Eocene-Oligocene transition (EOT, ~34 Ma) are still under-investigated. In order to better constrain the dynamics of these pelagic primary producers during the climate transition, we conducted high resolution assemblage analysis on calcareous nannofossils across a ~19 m-thick interval of nannofossil ooze at Ocean Drilling Program (ODP) Site 756 (Ninetyeast Ridge, Indian Ocean; Peirce et al. 1989) (paleolatitude ~43° S; Zachos et al. 1992). We explored the diversity patterns against a new integrated planktonic foraminifera and calcareous nannofossil biostratigraphy produced for the site, as well as new benthic foraminifera and bulk sediment stable isotope (C, O) records, which documents ocean-climate changes, and provides independent chemostratigraphy. The study section spans nannofossil Zones NP20-NP23 (equivalent to CNE20-CNO4) and lasts 5.5 Myr. The results show that the hankeninid extinction falls within the ~4.5 m-thick EOT isotopic interval (0.67 m below the base of the second positive δ18O shift – EOIS), which is consistent with previous studies, making Hole 756C one of a few sites globally boasting both the familiar stepped δ18O and δ13C structure of the EOT and the primary biostratigraphic marker defining the base of the Oligocene. A series of potentially useful new calcareous nannofossil bioevents were identified that could help improve dating and correlation of this crucial interval. In this context, changes in calcareous nannofossil assemblages observed across EOT are interpreted in terms of modifications of paleoecological parameters that typically control the abundance and distribution of different taxa. Variations in sea surface temperature and nutrient availability are considered to be the most likely triggers for the calcareous phytoplankton changes observed across EOT. Specifically, our data suggest that increased nutrients in the mixed layer played a key role in shaping the late Eocene – early Oligocene calcareous nannofossil assemblages.
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