| 1. |
- Auer, Gerald, et al.
(författare)
-
Timing and Pacing of Indonesian Throughflow Restriction and Its Connection to Late Pliocene Climate Shifts
- 2019
-
Ingår i: Paleoceanography and Paleoclimatology. - : AMER GEOPHYSICAL UNION. - 2572-4517 .- 2572-4525. ; 34:4, s. 635-657
-
Tidskriftsartikel (refereegranskat)abstract
- The Pliocene was characterized by a gradual shift of global climate toward cooler and drier conditions. This shift fundamentally reorganized Earth's climate from the Miocene state toward conditions similar to the present. During the Pliocene, the progressive restriction of the Indonesian Throughflow (ITF) is suggested to have enhanced this shift toward stronger meridional thermal gradients. Reduced ITF, caused by the northward movement of Australia and uplift of Indonesia, impeded global thermohaline circulation, also contributing to late Pliocene Northern Hemisphere cooling via atmospheric and oceanographic teleconnections. Here we present an orbitally tuned high-resolution sediment geochemistry, calcareous nannofossil, and X-ray fluorescence record between 3.65 and 2.97 Ma from the northwest shelf of Australia within the Leeuwin Current. International Ocean Discovery Program Site U1463 provides a record of local surface water conditions and Australian climate in relation to changing ITF connectivity. Modern analogue-based interpretations of nannofossil assemblages indicate that ITF configuration culminated similar to 3.54 Ma. A decrease in warm, oligotrophic taxa such as Umbilicosphaera sibogae, with a shift from Gephyrocapsa sp. to Reticulofenestra sp., and an increase of mesotrophic taxa (e.g., Umbilicosphaera jafari and Helicosphaera spp.) suggest that tropical Pacific ITF sources were replaced by cooler, fresher, northern Pacific waters. This initial tectonic reorganization enhanced the Indian Oceans sensitivity to orbitally forced cooling in the southern high latitudes culminating in the M2 glacial event (similar to 3.3 Ma). After 3.3 Ma the restructured ITF established the boundary conditions for the inception of the Sahul-Indian Ocean Bjerknes mechanism and increased the response to glacio-eustatic variability.
|
|
| 2. |
- Cappelli, C., et al.
(författare)
-
The Early to Middle Eocene Transition : An Integrated Calcareous Nannofossil and Stable Isotope Record From the Northwest Atlantic Ocean (Integrated Ocean Drilling Program Site U1410)
- 2019
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 34:12, s. 1913-1930
-
Tidskriftsartikel (refereegranskat)abstract
- The early to middle Eocene is marked by prominent changes in calcareous nannofossil assemblages coinciding both with long-term climate changes and modification of the North Atlantic deep ocean circulation. In order to assess the impact of Eocene climate change on surface water environmental conditions of the Northwest Atlantic, we developed calcareous nannoplankton assemblage data and bulk stable isotope records (delta O-18 and delta C-13) across an early to middle Eocene interval (~52-43 Ma) at Integrated Ocean Drilling Program Site U1410 (Southeast Newfoundland Ridge, ~41 degrees N). At this site, early Eocene sediments are pelagic nannofossil chalk, whereas middle Eocene deposits occur as clay-rich drift sediments reflecting the progressive influence of northern-sourced deep currents. Between the end of Early Eocene Climatic Optimum (EECO) and the Ypresian/Lutetian boundary, calcareous nannofossils switched from an assemblage mainly composed of warm-water and oligotrophic taxa (Zygrhablithus, Discoaster, Sphenolithus, Coccolithus) to one dominated by the more temperate and eutrophic reticulofenestrids. The most prominent period of accelerated assemblage change occurred during a ~2-Myr phase of relatively high bulk delta O-18 values possibly related to the post-EECO cooling. Although the dominance of reticulofenestrids persisted unvaried throughout the middle Eocene interval, early Lutetian (~47.4 to 47 Ma) stable isotope records indicate a reversal in the paleoenvironmetal trends suggesting a potential restoration of warmer conditions. Importantly, our data indicate that the ~2-Myr interval immediately following the EECO was crucial in establishing the modern calcareous nannofossil assemblage structure and also reveal that the establishment of Reticulofenestra-dominated assemblage occurred prior to the onset of persistent deep current system in the Northwest Atlantic.
|
|
| 3. |
- Cronin, Thomas M., et al.
(författare)
-
Interglacial Paleoclimate in the Arctic
- 2019
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 34:12, s. 1959-1979
-
Tidskriftsartikel (refereegranskat)abstract
- Marine Isotope Stage 11 from similar to 424 to 374 ka experienced peak interglacial warmth and highest global sea level similar to 410-400 ka. MIS 11 has received extensive study on the causes of its long duration and warmer than Holocene climate, which is anomalous in the last half million years. However, a major geographic gap in MIS 11 proxy records exists in the Arctic Ocean where fragmentary evidence exists for a seasonally sea ice-free summers and high sea-surface temperatures (SST; similar to 8-10 degrees C near the Mendeleev Ridge). We investigated MIS 11 in the western and central Arctic Ocean using 12 piston cores and several shorter cores using proxies for surface productivity (microfossil density), bottom water temperature (magnesium/calcium ratios), the proportion of Arctic Ocean Deep Water versus Arctic Intermediate Water (key ostracode species), sea ice (epipelagic sea ice dwelling ostracode abundance), and SST (planktic foraminifers). We produced a new benthic foraminiferal delta O-18 curve, which signifies changes in global ice volume, Arctic Ocean bottom temperature, and perhaps local oceanographic changes. Results indicate that peak warmth occurred in the Amerasian Basin during the middle of MIS 11 roughly from 410 to 400 ka. SST were as high as 8-10 degrees C for peak interglacial warmth, and sea ice was absent in summers. Evidence also exists for abrupt suborbital events punctuating the MIS 12-MIS 11-MIS 10 interval. These fluctuations in productivity, bottom water temperature, and deep and intermediate water masses (Arctic Ocean Deep Water and Arctic Intermediate Water) may represent Heinrich-like events possibly involving extensive ice shelves extending off Laurentide and Fennoscandian Ice Sheets bordering the Arctic.
|
|
| 4. |
- Lougheed, Bryan C., et al.
(författare)
-
A Rapid, Deterministic Age-Depth Modeling Routine for Geological Sequences With Inherent Depth Uncertainty
- 2019
-
Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517 .- 2572-4525. ; 34:1, s. 122-133
-
Tidskriftsartikel (refereegranskat)abstract
- Traditional approaches to age-depth modeling typically assume no uncertainty for the depth value of dated intervals. However, such an assumption may not be fully valid in the case of poor coring recovery or significant sediment deformation, as well as in the case of a large subsampling interval. In consideration of these issues, we present a new age-depth modeling routine, Undatable, which includes uncertainty in both age and depth. Undatable uses Bayesian radiocarbon (C-14) calibration software (MatCal) and a deterministic approach with a positive sediment accumulation rate assumed a priori which, combined with efficient programming practices, allows for the rapid production (in a matter of seconds in many cases) of age-depth models for multiple types of geological archives. Undatable has so far been successfully applied to coral archives, as well as sediment archives from estuarine, lacustrine, and deep-sea environments. Through the inclusion of a bootstrapping option, the software performs particularly well in the case of a large scatter in age-depth constraints by expanding the uncertainty envelope of the age-depth model. Unlike other deterministic models, increasing the density of age-depth constraints results in increased precision in Undatable, even at centennial scale, thus emulating the results of probabilistic models. In addition to the code itself, we also provide an interactive graphical user interface (GUI) that allows users to experiment with multiple age-depth model settings to investigate the sensitivity of a given data set to multiple parameters.
|
|
| 5. |
- Lupi, C., et al.
(författare)
-
Calcareous Nannofossil Response to Climate Variability During the Middle Pleistocene Transition in the Northwest Pacific Ocean (Ocean Drilling Program Leg 198 Site 1209)
- 2019
-
Ingår i: Paleoceanography and Paleoclimatology. - : AMER GEOPHYSICAL UNION. - 2572-4517 .- 2572-4525. ; 34:4, s. 600-615
-
Tidskriftsartikel (refereegranskat)abstract
- The Middle Pleistocene Transition represents an important reorganization of climate that switched from a 41-kyr periodicity to a quasiperiodic 100-kyr cycle. Here, we have investigated this time interval using calcareous nannofossils from a midlatitude well-preserved sedimentary record from the Ocean Drilling Program Site 1209 in the North West Pacific Ocean. We studied samples from 1.45 to 0.45 Ma, looking for significant changes in the nannofossil assemblages. Micropaleontological data were processed using an information theoretic model comparison that deconvolutes variations in nannofossil assemblages using different predictors. The major findings are that calcareous nannofossils permit the detection of discrete events during the Middle Pleistocene Transition. They display three different associations belonging, respectively, to the 41-kyr world, the transition, and the 100-kyr world. The statistical approach demonstrated that nannofossils responded abruptly at the boundaries of the transition developing in two rapid steps superimposed on a more gradual process. Moreover, a strong decrease in the genus Reticulofenestra and the beginning of the Gephyrocapsa caribbeanica acme permitted us to pinpoint climate events within the Middle Pleistocene Transition. Finally, through the nannofossil paleoecology, we documented an anomalous long-term warming trend that could be related to an intensification of the Walker circulation and the stabilization of long-term conditions like La Nina at the midlatitudes of the NW Pacific during the Middle Pleistocene Transition.
|
|
