Younger Dryas and Allerød summer temperatures at Gerzensee (Switzerland) inferred from fossil pollen and cladoceran assemblages

Linear- and unimodal-based inference models for mean summer temperatures (partial least squares, weighted averaging, and weighted averaging partial least squares models) were applied to a high-resolution pollen and cladoceran stratigraphy from Gerzensee, Switzerland. The time-window of investigation included the Allerød, the Younger Dryas, and the Preboreal. Characteristic major and minor oscillations in the oxygen-isotope stratigraphy, such as the Gerzensee oscillation, the onset and end of the Younger Dryas stadial, and the Preboreal oscillation, were identified by isotope analysis of bulk-sediment carbonates of the same core and were used as independent indicators for hemispheric or global scale climatic change. In general, the pollen-inferred mean summer temperature reconstruction using all three inference models follows the oxygen-isotope curve more closely than the cladoceran curve. The cladoceran-inferred reconstruction suggests generally warmer summers than the pollen-based reconstructions, which may be an effect of terrestrial vegetation not being in equilibrium with climate due to migrational lags during the Late Glacial and early Holocene. Allerød summer temperatures range between 11 and 12°C based on pollen, whereas the cladoceran-inferred temperatures lie between 11 and 13°C. Pollen and cladocera-inferred reconstructions both suggest a drop to 9–10°C at the beginning of the Younger Dryas. Although the Allerød–Younger Dryas transition lasted 150–160 years in the oxygen-isotope stratigraphy, the pollen-inferred cooling took 180–190 years and the cladoceran-inferred cooling lasted 250–260 years. The pollen-inferred summer temperature rise to 11.5–12°C at the transition from the Younger Dryas to the Preboreal preceded the oxygen-isotope signal by several decades, whereas the cladoceran-inferred warming lagged. Major discrepancies between the pollen- and cladoceran-inference models are observed for the Preboreal, where the cladoceran-inference model suggests mean summer temperatures of up to 14–15°C. Both pollen- and cladoceran-inferred reconstructions suggest a cooling that may be related to the Gerzensee oscillation, but there is no evidence for a cooling synchronous with the Preboreal oscillation as recorded in the oxygen-isotope record. For the Gerzensee oscillation the inferred cooling was ca. 1 and 0.5°C based on pollen and cladocera, respectively, which lies well within the inherent prediction errors of the inference models.

Floodplain environmental change during the Younger Dryas and Holocene in Northwest Europe: Insights from the lower Kennet Valley, south central England

Many lowland rivers across northwest Europe exhibit broadly similar behavioural responses to glacial-interglacial transitions and landscape development. Difficulties exist in assessing these, largely because the evidence from many rivers remains limited and fragmentary. Here we address this issue in the context of the river Kennet, a tributary of the Thames, since c. 13,000 cal BP. Some similarities with other rivers are present, suggesting that regional climatic shifts are important controls. The Kennet differs from the regional pattern in a number of ways. The rate of response to sudden climatic change, particularly at the start of the Holocene and also mid-Holocene forest clearance, appears very high. This may reflect abrupt shifts between two catchment scale hydrological states arising from contemporary climates, land use change and geology. Stadial hydrology is dominated by nival regimes, with limited winter infiltration and high spring and summer runoff. Under an interglacial climate, infiltration is more significant. The probable absence of permafrost in the catchment means that a lag between the two states due to its gradual decay is unlikely. Palaeoecology, supported by radiocarbon dates, suggests that, at the very start of the Holocene, a dramatic episode of fine sediment deposition across most of the valley floor occurred, lasting 500-1000 years. A phase of peat accumulation followed as mineral sediment supply declined. A further shift led to tufa deposition, initially in small pools, then across the whole floodplain area, with the river flowing through channels cut in tufa and experiencing repeated avulsion. Major floods, leaving large gravel bars that still form positive relief features on the floodplain, followed mid-Holocene floodplain stability. Prehistoric deforestation is likely to be the cause of this flooding, inducing a major environmental shift with significantly increased surface runoff. Since the Bronze Age, predominantly fine sediments were deposited along the valley with apparently stable channels and vertical floodplain accretion associated with soil erosion and less catastrophic flooding. The Kennet demonstrates that, while a general pattern of river behaviour over time, within a region, may be identifiable, individual rivers are likely to diverge from this. Consequently, it is essential to understand catchment controls, particularly the relative significance of surface and subsurface hydrology. (c) 2005 Elsevier B.V. All rights reserved.

The Younger Dryas and millenial-scale oceanographic variability in the Sulu Sea, tropical western Pacific

EXTRACT (SEE PDF FOR FULL ABSTRACT): A high resolution, AMS carbon-14-dated sediment record from the Sulu Sea clearly indicates the Younger Dryas climatic event affected the western equatorial Pacific. Presence of the Younger Dryas in the tropical western Pacific indicates this climatic event is not restricted to the North Atlantic nor to high latitudes, but is global in extent.

Synchronous records of pCO2 and Δ14C suggest rapid, ocean-derived pCO2 fluctuations at the onset of Younger Dryas

Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 records have shown a sharp increase in atmospheric CO2 concentration (pCO2) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO2 record from southern Sweden, with the IntCal13 record of radiocarbon (Δ14C). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing Δ14C just before the YD onset when pCO2 peaks, consistent with a source of “old” CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi-decadal timescale fluctuations in pCO2 through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO2 may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.

AGCM experiments on the Younger Dryas Climate

In this study three experiments were carried out with the ECHAM3 model at T42 resolution. The objectives of these experiments were firstly to analyze the effect of a North Atlantic Ocean cooled to Younger Dryas temperatures on the atmosphere under present conditions and secondly to simulate a Younger Dryas climate to improve the insight into mechanisms responsible for cooling during the Younger Dryas and moreover to find out if the Younger Dryas was a global or a regional event.

An oceanic origin for the increase of atmospheric radiocarbon during the Younger Dryas

Variations in carbon-14 to carbon-12 ratio in the atmosphere (Δ14Catm) provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change. The (Atlantic) marine archive suggests a rapid Δ14Catm increase of 50‰ at the onset of the Younger Dryas (YD) cold reversal (12.9–11.7 kyr BP), which has not yet been satisfactorily explained in terms of magnitude or causal mechanism, as either a change in ocean ventilation or production rate. Using Earth-system model simulations and comparison of marine-based radiocarbon records from different ocean basins, we demonstrate that the YD Δ14Catm increase is smaller than suggested by the marine archive. This is due to changes in reservoir age, predominantly caused by reduced ocean ventilation.