Continuous high-resolution mass accumulation rates (MAR) and X-ray fluorescence (XRF) measurements from marine sediment records in the Bay of Biscay (NE Atlantic) have allowed the determination of ...the timing and the amplitude of the ‘Fleuve Manche’ (Channel River) discharges during glacial stages MIS 10, MIS 8, MIS 6 and MIS 4–2. These results have yielded detailed insight into the Middle and Late Pleistocene glaciations in Europe and the drainage network of the western and central European rivers over the last 350
kyr. This study provides clear evidence that the ‘Fleuve Manche’ connected the southern North Sea basin with the Bay of Biscay during each glacial period and reveals that ‘Fleuve Manche’ activity during the glaciations MIS 10 and MIS 8 was significantly less than during MIS 6 and MIS 2. We correlate the significant ‘Fleuve Manche’ activity, detected during MIS 6 and MIS 2, with the extensive Saalian (Drenthe Substage) and the Weichselian glaciations, respectively, confirming that the major Elsterian glaciation precedes the glacial MIS 10. In detail, massive ‘Fleuve Manche’ discharges occurred at ca 155
ka (mid-MIS 6) and during Termination I, while no significant discharges are found during Termination II. It is assumed that a substantial retreat of the European ice sheet at ca 155
kyr, followed by the formation of ice-free conditions between the British Isles and Scandinavia until Termination II, allowed meltwater to flow northwards through the North Sea basin during the second part of the MIS 6. We assume that this glacial pattern corresponds to the Warthe Substage glacial maximum, therefore indicating that the data presented here equates to the Drenthe and the Warthe glacial advances at ca 175–160
ka and ca 150–140
ka, respectively. Finally, the correlation of our records with ODP site 980 reveals that massive ‘Fleuve Manche’ discharges, related to partial or complete melting of the European ice masses, were synchronous with strong decreases in both the rate of deep-water formation and the strength of the Atlantic thermohaline circulation. ‘Fleuve Manche’ discharges over the last 350
kyr probably participated, with other meltwater sources, in the collapse of the thermohaline circulation by freshening the northern Atlantic surface water.
The direct sea–land correlation applied to core MD99-2331 retrieved from the north-western Iberian margin shows a two-phase pattern within Heinrich events 4, 2 and 1 in the ocean and in the adjacent ...landmasses. Changes between wet/cold and dry/cool conditions in the Iberian Peninsula detected during these extreme events cannot be explained by a simple oceanographic mechanism related to changes in the strength of the Atlantic Meridional Overturning Circulation. Here we propose an additional atmospheric mechanism able to produce this scenario based on the comparison between the MD99-2331 record and other available palaeoclimate sequences from the North Atlantic region (18–75°N and 0–75°W). The climatic asymmetry observed between mid- and subtropical eastern North Atlantic latitudes (wet/dry) and the Blake Outer Ridge (dry/wet) during H4, H2 and H1 can be explained by changes in the position of the Atlantic jet-stream. During the first phase of H4, H2 and H1 the Atlantic jet-stream was located further south following the southward displacement of the oceanic thermal front as far south as 35°–37°N. On the contrary, during the second phase of H4, H2 and H1 the jet-stream was located further north following the northward displacement of this thermal front as far north as 42°N. From the atmospheric point of view, these two phases are reminiscent of the present-day negative and positive prevailing modes of the North Atlantic Oscillation (NAO), respectively, but high-resolution studies of additional North Atlantic key sites and climate simulations are needed to confirm the hypothesis of a NAO-like mechanism operating on millennial timescales.
We present an eco-physiological model reproducing the growth of eight foraminifer species (Neogloboquadrina pachyderma, Neogloboquadrina incompta, Neogloboquadrina dutertrei, Globigerina bulloides, ...Globigerinoides ruber, Globigerinoides sacculifer, Globigerinella siphonifera and Orbulina universa). By using the main physiological rates of foraminifers (nutrition, respiration, symbiotic photosynthesis), this model estimates their growth as a function of temperature, light availability, and food concentration. Model parameters are directly derived or calibrated from experimental observations and only the influence of food concentration (estimated via Chlorophyll-a concentration) was calibrated against field observations. Growth rates estimated from the model show positive correlation with observed abundance from plankton net data suggesting close coupling between individual growth and population abundance. This observation was used to directly estimate potential abundance from the model-derived growth. Using satellite data, the model simulate the dominant foraminifer species with a 70.5% efficiency when compared to a data set of 576 field observations worldwide. Using outputs of a biogeochemical model of the global ocean (PISCES) instead of satellite images as forcing variables gives also good results, but with lower efficiency (58.9%). Compared to core tops observations, the model also correctly reproduces the relative worldwide abundance and the diversity of the eight species when using either satellite data either PISCES results. This model allows prediction of the season and water depth at which each species has its maximum abundance potential. This offers promising perspectives for both an improved quantification of paleoceanographic reconstructions and for a better understanding of the foraminiferal role in the marine carbon cycle.
The comparison between modern terrestrial and marine pollen signals in and off western Iberia shows that marine pollen assemblages give an integrated image of the regional vegetation colonising the ...adjacent continent. Present-day Mediterranean and Atlantic forest communities of Iberia are well discriminated by south and north marine pollen spectra, respectively. Results from Total Pollen Concentration together with recognized conceptual models of fine particle dynamics in the Iberian margin have allowed us to establish the present-day pattern of pollen dispersion in this region.
The 25
000 year-long record of continental (pollen) and marine (
δ
18O of
Globigerina bulloides, Ice-rafted detritus—IRD and
Neogloboquadrina pachyderma s.) proxies, from the Galician margin composite core (MD99-2331 and MD03-2697), show that vegetation cover in north-western Iberia has responded contemporaneously to the climate variability of the North-Atlantic. The vegetation response to the well known North Atlantic Heinrich events 2 and 1 (H2 and H1) is however complex and characterised by two vegetation phases at low and mid-altitudes of north-western Iberia. The beginning of each Heinrich event is marked on land by an important pine forest reduction and the expansion of heathers which are synchronous with the heaviest planktic
δ
18O values and the maxima of
N. pachyderma (s.) suggesting that these first phases were cold and wet.
Pinus forest expansion characterising the second phase of each Heinrich event indicates a less cold episode associated, during H1, with an increase of dryness as suggested by the development of semi-desert associations. The comparison of our Galician margin multi-proxy record with several pollen sequences from in and off Iberia allows us to demonstrate that H1 event is the marine equivalent of the Oldest Dryas on the continent.
The occurrence of temperate trees during the last glacial maximum (LGM) and the rapid expansion of deciduous
Quercus during the Bölling-Allerød period in our Galician margin composite sequence show that not only the southern but also north-western Iberia was a refugium zone for deciduous trees during the last glacial period, especially at low and mid-altitude zones. Furthermore, the comparison between southern and northern marine and terrestrial sequences allows us to confirm that vegetation responded to the Bölling-Allerød warming, the Younger Dryas cold event and the Holocene more quickly in low and mid-altitudes of north-western Iberia and in the south than in the high altitude northern region most likely as the result of the higher density of refugia for temperate trees in these zones during the LGM.
Heinrich events-abrupt climate cooling events due to ice-sheet instability that occurred during the last glacial period-are recorded in sediment cores throughout the North Atlantic Ocean. Modelling ...studies have described likely physical mechanisms for these events, but the quantitative characteristics of Heinrich events are less well known. Here we use a climate model of intermediate complexity that explicitly calculates the distribution of oxygen isotopes in the oceans to simulate Heinrich event 4 at about 40,000 yr ago. We compare an ensemble of scenarios for this Heinrich event with oxygen isotope data measured in foraminiferal calcite of a comprehensive set of sediment cores. From this comparison, we obtain a duration of 250 ± 150 yr and an ice release of 2 ± 1 m sea-level equivalent for Heinrich event 4, significantly reducing the uncertainties in both values compared to earlier estimates of up to 2,000 yr and 15 m of sea-level equivalent ice release, respectively. Our results indicate that the consequences of Heinrich events may have been less severe than previously assumed, at least with respect to Greenland climate and sea level.
The correlation of continental sedimentary records with the marine isotope stratigraphy is a challenge of central importance in Quaternary stratigraphy, particularly in Western Europe where long ...records of glaciation on land areas are particularly rare. Here we demonstrate for the first time the interrelationship of events during the last 1.2
Ma in an ocean-sediment core from the Bay of Biscay (NE Atlantic), SW of the Channel. The identification of discharge variations from tributary river systems to the ‘Fleuve Manche’ palaeoriver during glacio-eustatic sea-level lowstands demonstrates the correlation of the marine sediment stratigraphy to the expansion and recession of the European ice-sheets. The amplitude and chronology of European ice-sheet oscillations since the late Early Pleistocene is discussed and our results demonstrate that the first coalescence of the Fennoscandian and British ice-sheets in the North Sea basin ca 450
ka ago caused a profound change in lowstand European drainage alignment. This change caused a rerouting of Fennoscandian and eastern British ice-sheets-derived meltwaters from northwards into the Nordic Seas to southwards into the eastern North Atlantic thereafter. Besides allowing a thorough synchronisation of the European ice-sheet palaeogeography with the well-dated records of palaeoceanographical changes, our results improve the stratigraphy of the English Channel palaeovalleys and will provide important constraints on paleoclimatic scenarios considering the impact that such rerouting and meltwater surges might have on the stability of the oceanic conveyor belt.
High-resolution clay mineralogical analyses were performed on sediment deposited during the last 50,000 yr in the Alboran sea (ODP Site 976). The clay mineral record is compared with pollen ...assemblages and with annual precipitation (Pann) and mean temperatures of the coldest month (MTCO) reconstructed with the modern analog technique (MAT). Enhanced contribution of palygorskite, a typical wind-blown clay mineral, characterizes the North Atlantic cold climatic events. Coeval development of the semi-arid vegetation (Artemisia rich) associated with a drastic fall of reconstructed precipitations and temperatures, suggest cold and arid continental conditions in the West Mediterranean area during North Atlantic cold events. The clay mineral association, especially the palygorskite content and the illite-to- kaolinite ratio, indicate western Morocco as one of the major source of the clay-size fraction during the North Atlantic cold events. The maximum abundance of Artemisia associated with the presence of Argania pollen both indicate Morocco as the main origin for pollen during these cold periods. The comparison of these pollen and clay mineral-specific features allows us to pinpoint western Morocco as the dominant source of wind-blown particles during North Atlantic cold events. These specific mineralogical composition and palynological assemblages reveal enhanced aridity over North Africa and intensification of winds favouring dust erosion and transport from North Africa toward the Alboran Sea during the North Atlantic cold events. According to atmospheric models, such a meridian transport (1) likely results from the development of strong and stable anticyclonic conditions over the tropical Atlantic and North Africa, similar to today's summer meteorological configuration and (2) implies a northward position of the westerly winds during North Atlantic cold events. Finally the synoptic situation over the West Mediterranean during the North Atlantic cold events is compared with the North Atlantic Oscillation (NAO), suggesting that during the cold Atlantic events, weather regimes over Europe and North Africa may have been systematically shifted towards a positive NAO situation.
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