| 1. |
- McMichael, C. N. H., et al.
(författare)
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30,000 years of landscape and vegetation dynamics in a mid-elevation Andean valley
- 2021
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Ingår i: Quaternary Science Reviews. - : Elsevier. - 0277-3791 .- 1873-457X. ; 258
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Tidskriftsartikel (refereegranskat)abstract
- The mid-elevation settings of the Andes are important biodiversity hotspots, yet little is known of their long-term ecology or environmental change. Here, we assess 30,000 years of landscape and vegetation dynamics on an alluvial terrace located in a mid-elevation valley of the Ecuadorian Andes (Campo Libre). We used loss-on-ignition and particle size analysis to reconstruct past river dynamics, charcoal analysis to reconstruct past fire regimes, and phytolith analysis to reconstruct vegetation change through time. Our results show that Campo Libre was a part of the active floodplain system of the Quijos River until 18,000 cal yr BP. The biggest vegetation change in vegetation at Campo Libre occurred ca. 13,000 cal yr BP, when the site warmed and dried, transforming the swampy alluvial terrace into a palm forest. As Ho-locene precipitation increased, the site transformed back into a swamp around 7500 cal yr BP, and it remained that way until maize agriculture began around 4600 cal yr BP. Local and regional fires were absent from the system until regional fires were detected ca. 3300 cal yr BP. By ca. 2700 cal yr BP, maize cultivation became frequent and regular. Climate, tectonic activity, and the human history have shaped the modern vegetation around Campo Libre, although during different periods of the Holocene. Our results demonstrate the ability of phytoliths to reconstruct vegetation change through time, and show that the mid-elevation Andean valley systems were highly dynamic over the last 30,000 years.
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| 2. |
- Missiaen, L., et al.
(författare)
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Radiocarbon Dating of Small-Sized Foraminifer Samples : Insights into Marine sediment Mixing
- 2020
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Ingår i: Radiocarbon. - : UNIV ARIZONA DEPT GEOSCIENCES. - 0033-8222 .- 1945-5755. ; 62:2, s. 313-333
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Tidskriftsartikel (refereegranskat)abstract
- Radiocarbon (C-14) can be used to build absolute chronologies and reconstruct ocean ventilation over the last 40 ka. Sample size requirements have restricted C-14 measurements in marine cores with low foraminifer content, impeding C-14-based studies focused on abrupt climate events. Recent developments have demonstrated that small-sized foraminifer samples can now be dated using a gas introduction system at the cost of a small decrease in precision. We explore the potential of gas measurements on benthic and planktonic foraminifers from core SU90-08 (43 degrees 03'1 '' N, 30 degrees 02'5 '' W, 3080 m). Gas measurements are accurate, reproducible within 2 sigma uncertainty and comparable to graphite measurements. Both techniques yield negative C-14 benthic-planktonic (B-P) age-offsets after Heinrich event 1. We argue that negative B-P ages result from bioturbation and changes in foraminifer abundances, with the chance of negative B-P especially increased when the C-14 age gradient between the deep and surface waters is decreased. Small-sized C-14 measurements seem to capture the variance of the foraminifera age distribution, revealing the active mixing in those archives. Sediment deposition and mixing effects possibly pose a greater obstacle for past C-14-based dating and ocean ventilation reconstructions than the measurement precision itself, particularly in relatively low sedimentation rate settings.
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| 3. |
- Baykal, Yunus
(författare)
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Source and age of late Quaternary loess deposits in Europe
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Atmospheric mineral dust is a fundamental component of the Earth’s climate system, with dust both responding to and driving climate change. This close link between dust and climate is recorded in archives of past dust activity, which show that abrupt 101-3 yr shifts in temperature during the last glacial period were accompanied with fluctuations in dust activity. However, the precise mechanism behind this close coupling of dust and climate and the specific role dust plays in modulating rapid climate change remains unclear. Terrestrial wind-blown dust deposits (loess) in Europe serve as source proximal archives of past dust activity. Loess formation chronologies across this region generally indicate greatly enhanced dust deposition during the last glacial cold phases of MIS 4 and, most notably, MIS 2. However, currently chronological precision is not sufficient to constrain more abrupt changes in dust activity and their potential links to climate change. More fundamentally, uncertainties over the sources of loess in Europe limit understanding of the causes of this last glacial dust deposition variability. The four chapters that comprise this thesis address these uncertainties through detailed analysis of the age and sources of loess in the Northern European Plain and English Channel region. Overall, the results demonstrate that abrupt changes in dust deposition during the late last glacial were a function of changes in ice sheet driven sediment supply. Eurasian and Alpine Ice Sheet derived meltwater pulses periodically greatly enhanced sediment availability and dust emission along their drainage routes, as reflected by abrupt dust deposition variability recorded in European loess deposits. Upon discharge into the North Atlantic, these meltwater pulses are also believed to have interacted with ocean circulation, potentially driving abrupt climate fluctuations during the last glacial. This provides a mechanism linking changes in dust, climate and ocean circulation on millennial timescales via ice sheet dynamics and provides the first coherent explanation of the close coupling of millennial scale variation in climate and dust during the Quaternary. Moreover, these findings suggest that meltwater pulses not only affected last glacial climate by changing ocean circulation but also through their impact on the high latitude dust cycle.
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| 4. |
- Henderiks, Jorijntje, et al.
(författare)
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Shifts in Phytoplankton Composition and Stepwise Climate Change During the Middle Miocene
- 2020
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Ingår i: Paleoceanography and Paleoclimatology. - : American Geophysical Union (AGU). - 2572-4517 .- 2572-4525. ; 35:8
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Tidskriftsartikel (refereegranskat)abstract
- The abundance and composition of modern phytoplankton are primarily related to equator‐to‐pole temperature gradients and global ocean circulation, which in turn determine the availability of nutrients in the photic zone. The nutricline is found at greater depths in warm, tropical waters, whereas more vigorous surface mixing in higher latitudes (seasonally) enhances nutrient availability and primary productivity. Ocean temperatures were ~7°C higher during the middle Miocene Climatic Optimum (MCO; ~16.9–14.7 million years ago, Ma), which was followed by Antarctic glaciation and global cooling during the middle Miocene Climate transition (MMCT; 14.7–13.8 Ma). Four decades ago, Haq (1980, https://doi.org.10.2307/1485353) already related migration patterns of low‐latitude versus high‐latitude calcareous nannoplankton in the Atlantic Ocean to major climatic fluctuations during the Miocene. Here, we detail and discuss the macroevolutionary patterns and processes across the middle Miocene (~16.5–11 Ma) at five deep sea sites on a north‐south transect in the Atlantic Ocean (57°N to 28°S). We show that the major cooling step toward the modern “icehouse” world impacted coccolithophore communities at all latitudes. Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the MMCT, we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids. Global cooling and related increased meridional overturning circulation are implicated as likely forcings for this macroevolutionary step toward establishing modern coccolithophore communities that are dominated by eurythermal and eurytrophic species such as Emiliania huxleyi.Plain Language SummaryHow will marine plankton communities respond to a much warmer world than today? How fast or slow would such changes be? We can learn valuable lessons from the fossil record of coccolithophores that represent a prominent phytoplankton group in both past and modern oceans. Changes in the composition of fossil assemblages show that species tracked past climate change on submillion year scales in the Atlantic Ocean. When ocean temperatures were ~7°C warmer during the middle Miocene (~16 million years ago), tropical species reached into the midlatitudes (42°N) of the North Atlantic. When the Antarctic ice sheet grew larger and the oceans cooled after ~14.7 million years ago, a group known as the reticulofenestrids diversified and became the most common everywhere, also in the tropics. Apparently, this group maintained high fitness in an “icehouse” world with greater latitudinal temperature contrasts, more vigorous ocean circulation, and higher nutrient availability in the photic zone. Indeed, their cosmopolitan distribution implies broad temperature and other environmental tolerances, similar to the group's latest descendant, the modern species Emiliania huxleyi that also displays a wide genetic and ecophenotypic diversity.
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| 5. |
- Karatsolis, Boris-Theofanis, et al.
(författare)
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Abrupt conclusion of the late Miocene-early Pliocene biogenic bloom at 4.6-4.4 Ma
- 2022
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Ingår i: Nature Communications. - : NATURE PORTFOLIO. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- The late Miocene-early Pliocene biogenic bloom was an extended time interval characterised by elevated ocean export productivity at numerous locations. As primary productivity is nutrient-limited at low-to-mid latitudes, this bloom has been attributed to an increase or a redistribution of available nutrients, potentially involving ocean-gateway or monsoon-related mechanisms. While the exact causal feedbacks remain debated, there is even less consensus on what caused the end of the biogenic bloom. Here, we compile Mio-Pliocene paleoproductivity proxy data from all major ocean basins to evaluate the timing and pacing of this termination. This systematic analysis reveals an abrupt and sustained reduction in low-latitude ocean productivity at 4.6–4.4 Ma. The decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity, an astronomical configuration linked to reduced East Asian Monsoon intensity and decreased riverine nutrient supply.
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| 6. |
- Lougheed, Bryan C., 1982-
(författare)
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Orbital, the Box : an Interactive Educational Tool for In-depth Understanding of Astronomical Climate Forcing
- 2022
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Ingår i: Open Quaternary. - : Ubiquity Press. - 2055-298X. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- “Orbital, the Box” provides an interactive tool with graphical user interface (GUI) for stimulating active, visual learning for understanding of astronomical climate forcing. This cross-platform tool can be run locally on a personal computer using a standard web browser environment with no need for plugins, thus maximising accessibility for students and teachers alike. The tool facilitates in the development of a holistic and quantitative understanding of astronomical climate forcing by allowing students to independently vary orbital parameters, after which they can instantaneously see the resulting effect upon the seasonal and latitudinal distribution of solar irradiance arriving at the top of the Earth’s atmosphere. Such an approach follows a classic controlled experimental design whereby one parameter can be changed while all others are kept constant. This experimental tool can be deployed as a virtual laboratory, including within a flipped classroom setting, to promote active learning of traditionally challenging concepts such as the roles of eccentricity and precession in astronomical climate forcing, and in particular their interaction with Kepler’s second law and the subsequent consequences for season length.
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| 7. |
- Lougheed, Bryan C., 1982-, et al.
(författare)
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Re-evaluating 14C dating accuracy in deep-sea sediment archives
- 2020
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Ingår i: Geochronology. - : Copernicus GmbH. - 2628-3719. ; 2:1, s. 17-31
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Tidskriftsartikel (refereegranskat)abstract
- The current geochronological state of the art for applying the radiocarbon (14C) method to deep-sea sediment archives lacks key information on sediment bioturbation. Here, we apply a sediment accumulation model that simulates the sedimentation and bioturbation of millions of foraminifera, whereby realistic 14C activities (i.e. from a 14C calibration curve) are assigned to each single foraminifera based on its simulation time step. We find that the normal distribution of 14C age typically used to represent discrete-depth sediment intervals (based on the reported laboratory 14C age and measurement error) is unlikely to be a faithful reflection of the actual 14C age distribution for a specific depth interval. We also find that this deviation from the actual 14C age distribution is greatly amplified during the calibration process. Specifically, we find a systematic underestimation of total geochronological error in many cases (by up to thousands of years), as well as the generation of age–depth artefacts in downcore calibrated median age. Even in the case of “perfect” simulated sediment archive scenarios, whereby sediment accumulation rate (SAR), bioturbation depth, reservoir age and species abundance are all kept constant, the 14C measurement and calibration processes generate temporally dynamic median age–depth artefacts on the order of hundreds of years – whereby even high SAR scenarios (40 and 60 cm kyr−1) are susceptible. Such age–depth artefacts can be especially pronounced during periods corresponding to dynamic changes in the Earth's Δ14C history, when single foraminifera of varying 14C activity can be incorporated into single discrete-depth sediment intervals. For certain lower-SAR scenarios, we find that downcore discrete-depth true median age can systematically fall outside the calibrated age range predicted by the 14C measurement and calibration processes, thus leading to systematically inaccurate age estimations. In short, our findings suggest the possibility of 14C-derived age–depth artefacts in the literature. Furthermore, since such age–depth artefacts are likely to coincide with large-scale changes in global Δ14C, which themselves can coincide with large-scale changes in global climate (such as the last deglaciation), 14C-derived age–depth artefacts may have been previously incorrectly attributed to changes in SAR coinciding with global climate. Our study highlights the need for the development of improved deep-sea sediment 14C calibration techniques that include an a priori representation of bioturbation for multi-specimen samples.
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| 8. |
- Lougheed, Bryan C., 1982-
(författare)
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SEAMUS (v1.20) : a Δ14C-enabled, single-specimen sediment accumulation simulator
- 2020
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Ingår i: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 13:1, s. 155-168
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Tidskriftsartikel (refereegranskat)abstract
- The systematic bioturbation of single particles (such as foraminifera) within deep-sea sediment archives leads to the apparent smoothing of any temporal signal as recorded by the downcore, discrete-depth mean signal. This smoothing is the result of the systematic mixing of particles from a wide range of depositional ages into the same discrete-depth interval. Previous sediment models that simulate bioturbation have specifically produced an output in the form of a downcore, discrete-depth mean signal. However, palaeoceanographers analysing the distribution of single foraminifera specimens from sediment core intervals would be assisted by a model that specifically evaluates the effect of bioturbation upon single specimens. Taking advantage of advances in computer memory, the single-specimen SEdiment AccuMUlation Simulator (SEAMUS) was created for MATLAB and Octave, allowing for the simulation of large arrays of single specimens. This model allows researchers to analyse the post-bioturbation age heterogeneity of single specimens contained within discrete-depth sediment core intervals and how this heterogeneity is influenced by changes in sediment accumulation rate (SAR), bioturbation depth (BD) and species abundance. The simulation also assigns a realistic 14C activity to each specimen, by considering the dynamic Δ14C history of the Earth and temporal changes in reservoir age. This approach allows for the quantification of possible significant artefacts arising when 14C-dating multi-specimen samples with heterogeneous 14C activity. Users may also assign additional desired carrier signals to single specimens (stable isotopes, trace elements, temperature, etc.) and consider a second species with an independent abundance. Finally, the model can simulate a virtual palaeoceanographer by randomly picking whole specimens (whereby the user can set the percentage of older, “broken” specimens) of a prescribed sample size from discrete depths, after which virtual laboratory 14C dating and 14C calibration is carried out within the model. The SEAMUS bioturbation model can ultimately be combined with other models (proxy and ecological models) to produce a full climate-to-sediment model workflow, thus shedding light on the total uncertainty involved in palaeoclimate reconstructions based on sediment archives.
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| 9. |
- Lougheed, Bryan C., 1982-, et al.
(författare)
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Testing the effect of bioturbation and species abundance upon discrete-depth individual foraminifera analysis
- 2022
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Ingår i: Biogeosciences. - : Copernicus Publications. - 1726-4170 .- 1726-4189. ; 19:4, s. 1195-1209
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Tidskriftsartikel (refereegranskat)abstract
- We used a single foraminifera enabled, holistic hydroclimate-to-sediment transient modelling approach to fundamentally evaluate the efficacy of discrete-depth individual foraminifera analysis (IFA) for reconstructing past sea surface temperature (SST) variability from deep-sea sediment archives, a method that has been used, amongst other applications, for reconstructing El Nino-Southern Oscillation (ENSO). The computer model environment allows us to strictly control for variables such as SST, foraminifera species abundance response to SST, as well as depositional processes such as sediment accumulation rate (SAR) and bioturbation depth (BD) and subsequent laboratory processes such as sample size and machine error. Examining a number of best-case scenarios, we find that IFA-derived reconstructions of past SST variability are sensitive to all of the aforementioned variables. Running 100 ensembles for each scenario, we find that the influence of bioturbation upon IFA-derived SST reconstructions, combined with typical samples sizes employed in the field, produces noisy SST reconstructions with poor correlation to the original SST distribution in the water. This noise is especially apparent for values near the tails of the SST distribution, which is the distribution region of particular interest in the case of, e.g. ENSO. The noise is further increased in the case of increasing machine error, decreasing SAR and decreasing sample size. We also find poor agreement between ensembles, underscoring the need for replication studies in the field to confirm findings at particular sites and time periods. Furthermore, we show that a species abundance response to SST could in theory bias IFA-derived SST reconstructions, which can have consequences when comparing IFA-derived SST distributions from markedly different mean climate states. We provide a number of idealised simulations spanning a number of SAR, sample size, machine error and species abundance scenarios, which can help assist researchers in the field to determine under which conditions they could expect to retrieve significant results.
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| 10. |
- Lougheed, Bryan C., 1982-
(författare)
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USING SEDIMENTOLOGICAL PRIORS TO IMPROVE C-14 CALIBRATION OF BIOTURBATED SEDIMENT ARCHIVES
- 2022
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Ingår i: Radiocarbon. - : Cambridge University Press. - 0033-8222 .- 1945-5755. ; 64:1, s. 135-151
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Tidskriftsartikel (refereegranskat)abstract
- Radiocarbon (C-14) dating is often carried out upon multi-specimen samples sourced from bioturbated sediment archives, such as deep-sea sediment. These samples are inherently heterogeneous in age, but existing C-14 calibration techniques were originally developed for age homogeneous material, such as archaeological artifacts or individual tree rings. A lack of information about age heterogeneity leads to a systematic underestimation of a sample's true age range, as well as the possible generation of significant age-depth artifacts during periods of the Earth's history coinciding with highly dynamic atmospheric Delta C-14. Here, a new calibration protocol is described that allows for the application of sedimentological priors describing sediment accumulation rate, bioturbation depth and temporally dynamic species abundance. This Bayesian approach produces a credible calibrated age distribution associated with a particular laboratory C-14 determination and its associated sedimentological priors, resulting in an improved calibration, especially in the case of low sediment accumulation rates typical of deep-sea sediment. A time-optimized computer script (biocal) for the new calibration protocol is also presented, thus allowing for rapid and automated application of the new calibration protocol. This new calibration protocol could be applied within existing age-depth modeling software packages to produce more accurate geochronologies for bioturbated sediment archives.
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