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  • 1
    Publication Date: 2022-12-07
    Description: Future precipitation levels remain uncertain because climate models have struggled to reproduce observed variations in temperature‐precipitation correlations. Our analyses of Holocene proxy‐based temperature‐precipitation correlations and hydrological sensitivities from 2,237 Northern Hemisphere extratropical pollen records reveal a significant latitudinal dependence and temporal variations among the early, middle, and late Holocene. These proxy‐based variations are largely consistent with patterns obtained from transient climate simulations (TraCE21k). While high latitudes and subtropical monsoon areas show mainly stable positive correlations throughout the Holocene, the mid‐latitude pattern is temporally and spatially more variable. In particular, we identified a reversal from positive to negative temperature‐precipitation correlations in the eastern North American and European mid‐latitudes from the early to mid‐Holocene that mainly related to slowed down westerlies and a switch to moisture‐limited convection under a warm climate. Our palaeoevidence of past temperature‐precipitation correlation shifts identifies those regions where simulating past and future precipitation levels might be particularly challenging.
    Description: Plain Language Summary: Predicting future precipitation levels reliably is more challenging than predicting temperature change. Accordingly, we need to understand the relationship between temperature and precipitation and its changes in space and time. We used climate proxy‐data derived from 2,237 pollen records from lake sediments and peats from the Northern Hemisphere extratropics for the early, middle, and late Holocene (i.e., 12,000–8,000, 8,000–4,000, 4,000–0 years before present, respectively). Our results reveal a significant latitudinal dependence and temporal variation of the temperature‐precipitation relationship. These proxy‐based variations are largely consistent with patterns obtained from simulations using climate models. While high latitudes and subtropical monsoon areas show mainly stable positive correlations throughout the Holocene (i.e., warm conditions co‐occur with wet conditions), the mid‐latitude pattern is temporally and spatially more variable. In particular, we identified a reversal to negative temperature‐precipitation correlations in the eastern North American and European mid‐latitudes from the early to middle Holocene. We hypothesize that weak westerly circulation, warm climate, and climate‐soil feedbacks limited evaporation and as such reduced convection during the middle Holocene which led to a negative relationship between temperature and precipitation. Our analysis of past temperature‐precipitation correlation shifts identifies regions where past changes in the temperature‐precipitation relationships are variable and thus where predicting precipitation might be particularly challenging in a warming climate.
    Description: Key Points: We analyzed Holocene temperature‐precipitation correlations and hydrological sensitivities using climate proxy (pollen) and model data from Northern Hemisphere extratropics. We found reversals to negative temperature‐precipitation correlations from the cold early Holocene to the warm mid‐Holocene likely related to moisture‐limited convection. Correlations and hydrological sensitivities were mostly stable positive in polar and extratropical monsoon‐areas.
    Description: EC European Research Council http://dx.doi.org/10.13039/501100000781
    Description: PALMOD
    Description: China Scholarship Council http://dx.doi.org/10.13039/501100004543
    Description: https://doi.pangaea.de/10.1594/PANGAEA.930512
    Description: https://doi.org/10.5281/zenodo.5910989
    Description: https://zenodo.org/record/7038402%23.YxBL1uzP3V8
    Keywords: ddc:551 ; ddc:561 ; Holocene ; pollen ; Northern Hemisphere ; temperature-precipation correlations
    Language: English
    Type: doc-type:article
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  • 2
    ISSN: 1573-0417
    Keywords: carbon and oxygen isotopes ; Melanoides tuberculata ; Ethiopian Rift Valley ; Holocene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract Carbon and oxygen isotope ratios in the shells of the freshwater snail Melanoides tuberculata yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the snails' life span. Melanoides is particularly important because it is widespread in Quaternary deposits throughout Africa and Asia and is ubiquitous in both fresh and highly evaporated lakes. Whole-shell and incremental growth data were collected from modern and fossil shells from two lakes in the Ethiopian Rift Valley. δ18O values in the modern shells from Lake Awassa are in equilibrium with modern waters, while δ18O values in subfossil shells from the margins of Lake Tilo indicate high rainfall during the early Holocene. Sequential analysis along the growth spiral of the shell provides information on seasonal or shorter-term variability of lake water during the lifetime of the organism.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1573-0417
    Keywords: Africa ; climate change ; conductivity ; diatoms ; Ethiopia ; Holocene ; lake levels ; palaeolimnology ; Rift Valley
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract A 6,500-year diatom stratigraphy has been used to infer hydrochemical changes in Lake Awassa, a topographically closed oligosaline lake in the Ethiopian Rift Valley. Conductivity was high from ~6400-6200 BP, and from 5200-4000 BP, with two brief episodes of lower conductivity during the latter period. Although the timing of the conductivity changes is similar to the timing of lake-level change in the nearby Zwai-Shalla basin, their directions are the reverse of that expected from a climatic cause. Dissolution of the tephras which precede both phases of high conductivity cannot explain the increases in salinity, because rhyolitic tephras are only sparingly soluble. Instead, the pulsed input of groundwater made saline by the reaction of silicate minerals and volcanic glass with carbonic acid, formed from the solution of carbon dioxide degassed from magma under the Awassa Caldera, is suggested as a plausible mechanism for the observed change in lake chemistry. Diatom-inferred hydrochemistry cannot therefore be used to reconstruct climate change in Lake Awassa.
    Type of Medium: Electronic Resource
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  • 4
    Publication Date: 2020-02-06
    Description: Pollen-based quantitative reconstructions of past climate variables is a standard palaeoclimatic approach. Despite knowing that the spatial extent of the calibration-set affects the reconstruction result, guidance is lacking as to how to determine a suitable spatial extent of the pollen-climate calibration-set. In this study, past mean annual precipitation (Pann) during the Holocene (since 11.5 cal ka BP) is reconstructed repeatedly for pollen records from Qinghai Lake (36.7°N, 100.5°E; north-east Tibetan Plateau), Gonghai Lake (38.9°N, 112.2°E; north China) and Sihailongwan Lake (42.3°N, 126.6°E; north-east China) using calibration-sets of varying spatial extents extracted from the modern pollen dataset of China and Mongolia (2559 sampling sites and 168 pollen taxa in total). Results indicate that the spatial extent of the calibration-set has a strong impact on model performance, analogue quality and reconstruction diagnostics (absolute value, range, trend, optimum). Generally, these effects are stronger with the modern analogue technique (MAT) than with weighted averaging partial least squares (WA-PLS). With respect to fossil spectra from northern China, the spatial extent of calibration-sets should be restricted to radii between ca. 1000 and 1500 km because small-scale calibration-sets (〈800 km radius) will likely fail to include enough spatial variation in the modern pollen assemblages to reflect the temporal range shifts during the Holocene, while too broad a scale calibration-set (〉1500 km radius) will include taxa with very different pollen-climate relationships.
    Type: Article , PeerReviewed
    Format: text
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  • 5
    Publication Date: 2022-01-31
    Description: Proxy-based reconstructions and modeling of Holocene spatiotemporal precipitation patterns for China and Mongolia have hitherto yielded contradictory results indicating that the basic mechanisms behind the East Asian Summer Monsoon and its interaction with the westerly jet stream remain poorly understood. We present quantitative reconstructions of Holocene precipitation derived from 101 fossil pollen records and analyse them with the help of a minimal empirical model. We show that the westerly jet-stream axis shifted gradually southward and became less tilted since the middle Holocene. This was tracked by the summer monsoon rain band resulting in an early-Holocene precipitation maximum over most of western China, a mid-Holocene maximum in north-central and northeastern China, and a late-Holocene maximum in southeastern China. Our results suggest that a correct simulation of the orientation and position of the westerly jet stream is crucial to the reliable prediction of precipitation patterns in China and Mongolia.
    Type: Article , PeerReviewed
    Format: text
    Format: archive
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  • 6
    Publication Date: 2024-02-07
    Description: Future precipitation levels remain uncertain because climate models have struggled to reproduce observed variations in temperature-precipitation correlations. Our analyses of Holocene proxy-based temperature-precipitation correlations and hydrological sensitivities from 2237 Northern Hemisphere extratropical pollen records reveal a significant latitudinal dependence and temporal variations among the early, middle, and late Holocene. These proxy-based variations are largely consistent with patterns obtained from transient climate simulations (TraCE21k). While high latitudes and subtropical monsoon areas show mainly stable positive correlations throughout the Holocene, the mid-latitude pattern is temporally and spatially more variable. In particular, we identified a reversal from positive to negative temperature-precipitation correlations in the eastern North American and European mid-latitudes from the early to mid-Holocene that mainly related to slowed down westerlies and a switch to moisture-limited convection under a warm climate. Our palaeoevidence of past temperature-precipitation correlation shifts identifies those regions where simulating past and future precipitation levels might be particularly challenging.
    Type: Article , PeerReviewed
    Format: text
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  • 7
    Publication Date: 2011-09-01
    Description: An understanding of the interaction between ice sheet dynamics and forcing mechanisms, such as oceanic and atmospheric circulation, is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea-level change. Here we report new benthic foraminiferal data from Disko Bugt, West Greenland, showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier Jakobshavn Isbrae over the past 100 yr. In particular, our faunal data show that warm ocean currents entered a bay, Disko Bugt, during the retreat phases of Jakobshavn Isbrae from A.D. 1920 to 1950 and since 1998. We also show a link between West Greenland ocean temperature and the Atlantic Multidecadal Oscillation, a key climate indicator in the North Atlantic Ocean. The close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea-level change.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 8
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    PANGAEA
    In:  Supplement to: Tegzes, Andrea D; Jansen, Eystein; Telford, Richard J (2015): Which is the better proxy for paleo-current strength: Sortable-silt mean size (SS) or sortable-silt mean grain diameter (dSS)? A case study from the Nordic Seas. Geochemistry, Geophysics, Geosystems, 16(10), 3456-3471, https://doi.org/10.1002/2014GC005655
    Publication Date: 2023-05-12
    Description: The coarseness of the 10-63 µm terrigenous silt (i.e., sortable-silt) fraction tends to vary independently of sediment supply in current-sorted muds in the world's oceans, with coarser sediments representing relatively greater near-bottom flow speeds. Traditionally, the coarseness of this size fraction is described using an index called sortable-silt mean size (SS), which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10-63 µm terrigenous silt fraction, where the relative weights of the individual size bins become increasingly disproportionate, with respect to the actual number of grains within those size bins, toward the coarse end of the size range. This not only increases the absolute value of the apparent "mean size" within the 10-63 µm terrigenous silt fraction, but it may also affect the apparent pattern of relative changes in the coarseness of the sortable-silt fraction along the core. In addition, it makes SS more prone to biases due to, for example, analytical errors. Here we present a detailed analysis of grain-size distributions over three selected Holocene time intervals from two complementary sediment cores (JM97-948/2A and MD95-2011), extracted from the center of a high-accumulation area along the flow path of the main branch of the Atlantic Inflow into the Nordic Seas and show that differential-number-based statistics, which likely better describes variations in the actual coarseness of the sortable-silt fraction, may provide a more robust alternative to SS.
    Type: Dataset
    Format: application/zip, 23 datasets
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  • 9
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    PANGAEA
    In:  Bjerknes Centre for Climate Research | Supplement to: Andersson, Carin; Pausata, Francesco S R; Jansen, Eystein; Risebrobakken, Bjørg; Telford, Richard J (2010): Holocene trends in the foraminifer record from the Norwegian Sea and the North Atlantic Ocean. Climate of the Past, 6(2), 179-193, https://doi.org/10.5194/cp-6-179-2010
    Publication Date: 2023-01-13
    Description: The early to mid-Holocene thermal optimum is a well-known feature in a wide variety of paleoclimate archives from the Northern Hemisphere. Reconstructed summer temperature anomalies from across northern Europe show a clear maximum around 6000 years before present (6 ka). For the marine realm, Holocene trends in sea-surface temperature reconstructions for the North Atlantic and Norwegian Sea do not exhibit a consistent pattern of early to mid- Holocene warmth. Sea-surface temperature records based on alkenones and diatoms generally show the existence of a warm early to mid-Holocene optimum. In contrast, several foraminifer and radiolarian based temperature records from the North Atlantic and Norwegian Sea show a cool mid- Holocene anomaly and a trend towards warmer temperatures in the late Holocene. In this paper, we revisit the foraminifer record from the Vøring Plateau in the Norwegian Sea. We also compare this record with published foraminifer based temperature reconstructions from the North Atlantic and with modelled (CCSM3) upper ocean temperatures. Model results indicate that while the seasonal summer warming of the seasurface was stronger during the mid-Holocene, sub-surface depths experienced a cooling. This hydrographic setting can explain the discrepancies between the Holocene trends exhibited by phytoplankton and zooplankton based temperature proxy records.
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 10
    Publication Date: 2023-02-07
    Keywords: AGE; DEPTH, sediment/rock; Giant box corer; GKG; JM97-948/2A; Sea surface temperature, June-August; Transfer function, maximum liklihood method
    Type: Dataset
    Format: text/tab-separated-values, 60 data points
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