A. de Vernal

Absence of deep-water formation in the Labrador Sea during the last interglacial period

The two main constituent water masses of the deep North Atlantic Ocean—North Atlantic Deep Water at the bottom and Labrador Sea Water at an intermediate level—are currently formed in the Nordic seas and the Labrador Sea, respectively. The rate of formation of these two water masses tightly governs the strength of the global ocean circulation and the associated heat transport across the North Atlantic Ocean. Numerical simulations have suggested a possible shut-down of Labrador Sea Water formation as a consequence of global warming. Here we use micropalaeontological data and stable isotope measurements in both planktonic and benthic foraminifera from deep Labrador Sea cores to investigate the density structure of the water column during the last interglacial period, which was thought to be about 2 °C warmer than present. Our results indicate that todays stratification between Labrador Sea Water and North Atlantic Deep Water never developed during the last interglacial period. Instead, a buoyant surface layer was present above a single water mass originating from the Nordic seas. Thus the present situation, with an active site of intermediate-water formation in the Labrador Sea, which settled some 7,000 years ago, has no analogue throughout the last climate cycle.

Quantitative assessment of carbonate dissolution in marine sediments from foraminifer linings vs. shell ratios: Davis Strait, northwest North Atlantic

A new method to evaluate quantitatively the degree of calcium carbonate preservation and dissolution in high-latitude marine sediments is proposed on the basis of relative abundance of CaCO 3 shells and organic linings of benthic foraminifers. This method was applied to a late Quaternary sequence from Davis Strait in the northwest North Atlantic and shows that CaCO 3 dissolution in sediments has increased since the last glacial maximum (ca. 18,000 B.P.) and peaked when subpolar interglacial conditions were established in surface waters. The dissolution in Davis Strait sediments appears to be closely related to organic biogenic production in surface waters, with a regional pattern of bottom-water formation and circulation.

EARLY DIAGENETIC PROCESSES IN RECENT SEDIMENTS OF THE GULF OF ST-LAWRENCE:PHOSPHORUS, CARBON AND IRON BURIAL RATES

Abstract Selective extraction procedures were used to quantify different forms of solid-phase phosphorus, carbon and iron in marine sediments, and to evaluate the impact of authigenic formation of mineral forms such as carbonate fluorapatite (CFA), calcium carbonate (CaCO3) and pyrite (FeS2) on major elemental cycles during early diagenesis. Detrital P and Fe phases were successfully used as indicators of the constancy or variability of detrital inputs to several sedimentary environments from the deep channels of the Gulf of St-Lawrence. In cores characterized by near steady state influx rates, solid-phase P, C and Fe data and sediment burial rates indicate that CFA, CaCO3 and probably FeS2 are currently forming in the sediments of the Gulf. However, high concentrations and/or formation of CaCO3 in marine sediments appear to inhibit the formation of authigenic CFA. On the other hand, the formation of FeS2 does not influence authigenic CFA precipitation. In the deep troughs of the Gulf of St-Lawrence, total P burial rates range from ≈ 50 to 500 mgP/m2/yr. Truly authigenic precipitation of CFA, when observed, may represent up to ≈ 25% of the total burial rate of P. Bioturbation of sub-surface sediments reduces the potential for authigenic precipitation of CFA and CaCO3, thus affecting immobilization reactions that have a strong impact on the global oceanic cycles of C and P. The spatial heterogeneity of diagenetic reactions precludes the establishment of an accurate quantification of P removal on the scale of a continental shelf such as the Gulf of St-Lawrence.

Palynostratigraphy and chronostratigraphy of Baffin Bay deep sea cores: Climatostratigraphic implications

Abstract Palynostratigraphic considerations and 14C dates (AMS; accelerator mass spectrometry) on foraminifera samples from piston cores suggest sedimentation rates as high as ca. 8–11 cm/ka in deep Baffin Bay during the Late Pleistocene. Throughout this interval Baffin Bay experiences rigorous conditions and probably the extension of a dense sea-ice cover unfavourable to phytoplanktonic productivity: the dinocyst microflora indicates only brief episodes of higher productivity related to Arctic conditions and to moderately low salinites (ca. 30–31ℵ). The abundance of reworked pre-Quaternary palynomorphs in the deposits is interpreted as a response to glacial and glacio-marine activity in the northernmost areas surrounding Baffin Bay. The dinocyst stratigraphy spanning approximately over the last ice age (ca. 100 ka) reveals that Baffin Bay constituted a confined basin, isolated from the North Atlantic Ocean, prior to the present “Interglacial”.

Ocean-atmosphere responses to climatic change in the Labrador Sea: Pleistocene plankton and pollen records

Abstract Pleistocene climate change is examined using proxy-climatic records from oxygen isotope data, calcareous and organic-walled marine microfossils, pollen and terrestrial spores deposited during the past 0.9 Ma at ODP Site 646 in the Labrador Sea, 500 km north of the present polar front. Paleotransfer functions applied to planktonic foraminiferal assemblages show relative increases in interglacial summer (3–7°) and winter (3–5°) temperatures and in winter salinity (0.5–2%‰) for the past 0.4 Ma, but only two earlier intervals (early stage 11 and stage 17) have changes of comparable magnitude. Coccolith and dinoflagellate cyst accumulation rates show that primary productivity is generally correlated with temperature and salinity changes at the start of interglacials. These productivity peaks lag the ice volume changes by 2–4 ka. Dinocyst blooms seem to precede coccolith peaks, reflecting the tolerance of opportunistic species for large variations in temperature and salinity. Peaks in pollen and spore abundance are strongly correlated with ice volume which controls the position and stability of the polar jet stream in addition to the northern extent of forest vegetation. Time series analysis was made of 7 oceanographic variables (SST summer and winter, surface salinity, coccoliths, dinocysts, planktonic and benthic foraminifera) and 5 other variables (δ18, pollen-spores, percent sand, foraminiferal test fragmentation and reworked palynomorphs). Most variables showed significant power peaks at ≈ 100 Ka, and/or at ≈ 41 ka. However, sea surface responses showed minor peaks at 26 and 16 ka, and microfossil productivity also showed significant peaks at 68 Ka. The high latitude Labrador Sea records thus display large non-linear regional responses to climate changes in addition to the effects of orbital insolation forcing at ≈ 41 and ≈ 23 ka.

Report of 1st discussion group: The last interglacial in high latitudes of the Northern Hemisphere: Terrestrial and marine evidence

Abstract The paleoclimatic informations preserved at some selected sites from high northern latitudes correlative with the last interglacial maximum of Isotopic Substage 5e have been compiled on a hemisphere scale. The mapped compilation reveals much warmer climate conditions on average than present in both terrestrial and marine environments, and indicates a northward shift of subpolar and boreal bioclimatic zones. The data also suggest that temperature and precipitation gradients were significantly different than those of the Holocene, with particularly efficient latitudinal and eastward transport of warm air masses. The paleoenvironmental and paleoclimate trends through the last interglacial sensu lato (Isotopic Stage 5) are more difficult to assess because the chronostratigraphical framework is poorly constrained. Nevertheless, the data suggest important regionalism in the climatostratigraphical trends. In particular, the large amplitude climatic cyclicity that characterizes the last interglacial of Europe is barely observed elsewhere. Moreover, by the end of the last interglacial (Isotopic Substage 5a), conditions slightly cooler than present are recorded in western Europe, while relatively warm conditions, similar to today, apparently prevailed in circumpolar regions adjacent to the North Atlantic despite the onset of ice growth. The last interglacial provides paleosynoptic situations, unlike those of the present, that may contribute to understanding interactions within the environmental system on a hemispheric scale. There is, however, an imperative need for uniformization of data sets and for more accurate chronological control on regional scales.

Palynostratigraphy and Th/U ages of upper Pleistocene interglacial and interstadial deposits on Cape Breton Island, eastern Canada

On Cape Breton Island (Nova Scotia), near the margin of the Wisconsinan ice sheet, karst depressions in Mississippian gypsum-bearing rocks contain interglacial and interstadial organic-rich deposits. Three palynostratigraphic units have been observed and tentatively dated by Th/U measurements on embedded fossil wood. The first, dated at ca. 125 ka, may be assigned to the oceanic /sup 18/O substage 5e; thermophilous forests (Quercus, Ostrya, pinus strobus) developed in response to a climate warmer than the present. The second unit, rich in Abies balsamea pollen and dated at ca. 87 ka, may relate to the /sup 18/O substage 5a; it reflects a cool and wet climate not unlike that of today. The third unit probably spans part of the mid-Wisconsinan (/sup 18/O stage 3); it shows alternating boreal forest-tundra forest assemblages indicative of climatic oscillations during a generally cold interval.

New record shows pronounced changes in Arctic Ocean circulation and climate

Does the Arctic Ocean surface circulation north of Alaska oscillate to and fro like a slow washing machine on millennial timescales? New evidence from the sediment record over the last 10,000 years suggests that it does and that in the recent past, the western Arctic Ocean was much warmer than it is today. Similar Holocene climatic fluctuations are seen in many records worldwide, yet their origin remains enigmatic. Modeling and observational studies suggest that the Arctic may play an important role in these climate fluctuations through changes in surface albedo, modifications of oceanic thermohaline circulation, and changes in biogeochemical cycling of nutrients and radiatively important gases [PARCS, 1999].

Marine palynology and its use for studying nearshore environments

Palynology is the study of microfossils composed of highly resistant organic matter called palynomorphs. In the sediments of neritic environments, palynomorphs may include cysts of dinoflagellates, phycoma of prasinophytes, organic linings of benthic foraminifers and thecamoebians, in addition to inputs from the terrestrial vegetation (pollen grains and spores) or the freshwater biota (chlorococcales). Marine palynology is thus used for characterizing the type of sedimentary environment, identifying the source of organic matter in the sediment, and weighting the relative importance of fluvial and pelagic inputs. Among marine palynomorphs, dinoflagellate cysts or dinocysts usually dominate the assemblages. Dinocysts comprise phototrophic and heterotrophic taxa and occur in almost all aquatic environments. Along the continental margins, assemblages are usually characterized by high species diversity and cyst concentrations reaching up to 10 5 cysts cm -3 . The distribution of dinocyst assemblages in sediments shows latitudinal patterns in addition to onshore to offshore gradients. Multivariate analyses illustrate close relationships between dinocyst assemblages and sea-surface parameters such as sea-ice cover, salinity, temperature, seasonality and productivity. Transfer functions developed from dinocysts permit the reconstruction of sea- surface temperature and salinity and the evaluation of past productivity, with applications dealing with climate changes and eutrophication. Dinocysts are also used for the study of harmful algal blooms since a few taxa relate to toxic species.

PALEOCEANOGRAPHY, BIOLOGICAL PROXIES | Dinoflagellates

Dinoflagellates occur in most aquatic environments and constitute an important part of primary productivity. During their life cycle, some dinoflagellates produce organic-walled cysts (or dinocysts) that fossilize in sediment and are prepared for microscopic observation following palynological procedures. The dinoflagellates producing fossilizable cysts include autotrophic and heterotrophic species that belong either to the phytoplankton or to the microzooplankton. The study of dinocysts on the sea floor has revealed relatively diverse assemblages from the equator to polar environments, with particularly high fluxes in marine environments of the continental margins. The geographical distribution of dinocyst assemblages suggests relationships with sea-surface temperature, salinity, sea-ice cover, and productivity. Qualitative and quantitative approaches permit the reconstruction of past sea-surface conditions.