Anne de Vernal

Reconstructed changes in Arctic sea ice over the past 1,450 years

Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean and traditional lifestyles in the Arctic. Although observations show a more or less continuous decline for the past four or five decades, there are few long-term records with which to assess natural sea ice variability. Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable. Here we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that—although extensive uncertainties remain, especially before the sixteenth century—both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.

Natural variability of Greenland climate, vegetation, and ice volume during the past million years.

The response of the Greenland ice sheet to global warming is a source of concern notably because of its potential contribution to changes in the sea level. We demonstrated the natural vulnerability of the ice sheet by using pollen records from marine sediment off southwest Greenland that indicate important changes of the vegetation in Greenland over the past million years. The vegetation that developed over southern Greenland during the last interglacial period is consistent with model experiments, suggesting a reduced volume of the Greenland ice sheet. Abundant spruce pollen indicates that boreal coniferous forest developed some 400,000 years ago during the “warm” interval of marine isotope stage 11, providing a time frame for the development and decline of boreal ecosystems over a nearly ice-free Greenland.

Dinocyst distribution in surface sediments from the northeastern Pacific margin (40–60°N) in relation to hydrographic conditions, productivity and upwelling

Seventy-six surface marine sediment samples from the northwest margin of North America, between 40°N and 60°N, were analysed for their palynological content in order to document the regional distribution of dinocyst assemblages and their relationships with environmental conditions (sea-surface temperature and salinity, productivity and upwelling). The results illustrate a high concentration of dinocysts, notably in the neritic area (up to 34 000 cysts cm−3) and a relatively high species diversity with 32 taxa identified. The assemblages include cysts of both autotrophic and heterotrophic species. Brigantedinium spp. accompanied by other heterotrophic taxa such as Votadinium spp., Quinquecuspis concreta, Trinovantedinium variabile and Lejeunecysta spp. dominate in the nearshore areas influenced by seasonal upwelling. The offshore sites are dominated by autotrophic taxa represented mainly by Operculodinium centrocarpum, Pyxidinopsis reticulata, Nematosphaeropsis labyrinthus and Impagidinium aculeatum in the south, and by O. centrocarpum, Pentapharsodinium dalei, Spiniferites ramosus and Spiniferites elongatus in the Gulf of Alaska. Principal component analysis demonstrates that the regional distribution of dinocyst assemblages is controlled by the primary productivity and upwelling, and by the winter temperature gradient.

Dinoflagellate cyst distribution in surface sediments of the southern Indian Ocean

We analysed 70 surface sediment samples collected in the southern Indian Ocean in order to document the distribution of dinoflagellate cyst assemblages. These organic-walled micro-organisms have a greater potential for preservation than carbonate or silicate microfossils. A total of 53 dinoflagellate cyst taxa were identified, and two new endemic species are described: Selenopemphix antarctica sp. nov. and Impagidinium variaseptum sp. nov. Dominant taxa allowed the recognition of assemblages which show a latitudinal distribution. The circum-Antarctic domain is characterized by assemblages dominated by S. Antarctica sp. nov. and accompanied by I. pallidum. The Subantarctic domain is marked by the dominance of Brigantedinium spp. accompanied by Nematosphaeropsis labyrinthus. The Subtropical domain shows high species diversity, taxa dominance varying along onshore to offshore gradient: the neritic assemblage is dominated by Brigantedinium spp., and Spiniferites spp., the outer neritic is characterized by Operculodinium centrocarpum and the oceanic assemblage is dominated by N. labyrinthus. Principal component analysis illustrates that the distribution of dinoflagellate cyst assemblages is controlled by temperature and salinity. Transfer functions based on the best analogues method are developed to reconstruct past sea-surface conditions.

Modem organic-walled dinoflagellate cysts in arctic marine environments and their (paleo-) environmental significance

The Arctic Ocean is one of the least known marine regions of the world. Because of its major influence on global climate and its hostile environmental conditions it is a fascinating area for paleoecological, paleoclimatic and paleoceanographic research. The composition of planktic microfossil assemblages, and both the trace-element and stable isotope compositions of hard parts, provide us with valuable information about the physical and biochemical parameters of surface waters in the high northern latitudes. Calcareous and biosiliceous microfossils that are traditionally used in Quaternary paleoenvironmental studies are of limited value in the Arctic Ocean because of their low abundances, low diversity and/or low preservation potential. The past several decades have seen considerable progress in our knowledge of the ecology and biogeography of dinoflagellates and their organic-walled cysts in the high northern latitudes, and these dinoflagellate cysts are now important proxies for reconstructing surface water conditions in the Quaternary. This arcticle gives an overview of the ecology of dinoflagellates and their cysts, the processes that transform the living communities into sediment communities, and the environmental gradients that may be reconstructed from fossil dinoflagellate cysts assemblages in the high northern latitudes.KurzfassungDer Arktische Ozean ist eine der am wenigsten untersuchten marinen Regionen des Weltozeans. Durch seinen Einfluss auf das globale Klima und die lebensfeindlichen Umweltbedingungen ist es eines der faszinierendsten Gebiete für paläoökologische, paläoklimatische und paläoozeanographische Forschung. Die Zusammensetzung der planktischen Mikrofossilvergesellschaftungen, sowie die Spurenelement- und die stabile Isotopenzusammensetzung der Hartteile, liefern wertvolle Informationen über physikalische und biochemische Parameter der Oberflächenwas-sermassen in den hohen nördlichen Breiten. Kalkige und kieselige Mikrofossilien, die traditionell in Paläoumwelt-studien des Quartärs benutzt werden, sind aufgrund von geringen Häufigkeiten, geringer Diversität und/oder geringem Fossilisationspotenzial von eingeschränkter Bedeutung. Beträchtlicher Fortschritt wurde in unseren Kenntnissen der Ökologie und Biogeographie der Dinoflagellaten und ihrer Zysten in den polaren und subpolaren Gebiete der hohen nördlichen Breiten in den vergangenen Jahrzehnten gemacht, und diese organisch-wandigen Mikrofossilien sind deshalb wichtige Proxies für die Rekonstruktion der Eigenschaften der oberflächennahen Wassermassen im Quartär. Diese Arbeit gibt einen Überblick über die Ökologie der Dinoflagellaten und ihrer Zysten, der Prozesse, die die Lebendgemeinschaften in Sedimentgemeinschaften umwandeln, und den Umweltgradienten, die mit den fossilen Dinoflagellaten-Zysten Vergesellschaftungen in den hohen nördlichen Breiten rekonstruiert werden können.

Organic-walled microfossils and geochemical tracers: sedimentary indicators of productivity changes in the North Water and northern Baffin Bay during the last centuries

Analyses performed on 26 surface sediment samples collected with a box corer at 17 stations throughout the North Water and northern Baffin Bay (75–791N; 68–801W) revealed abundantorganic-walled microfossils, most ly dinoflagellate cysts (10 3 –10 4 cysts g � 1 ) and organic linings of benthic foraminifers (10 2 –10 3 OL g � 1 ), as well as high organic carbon concentrations (0.87–2.81% dry weight). These data indicate high productivity in both the pelagic and benthic environments of the North Water and slightly lower productivity in northern Baffin Bay. The data also showed calcium carbonate and biogenic silica dissolution throughout the study area. The dinocyst assemblages were relatively uniform in the North Water and dominated by heterotrophic taxa (Algidasphaeridium? minutum and Brigantedinium spp.), whereas northern Baffin Bay assemblages were dominated by autotrophic taxa, notably Operculodinium centrocarpum and Spiniferites elongatus. The difference between these two assemblages may be related to higher diatomaceous primary production in the North Water than in northern Baffin Bay, since diatoms constitute the principal food source of heterotrophic dinoflagellates. The biogeographical boundary between the North Water and northern Baffin Bay has been maintained for at least the last few centuries as shown by analyses of microfossils and geochemical tracers in two sediment cores, one taken in the southeastern part of the North Water (76117 0 N, 72102 0 W) and the other in northeastern Baffin Bay (75135 0 N, 70148 0 W). The analyses of the North Water core revealed relatively uniform microfossil assemblages and organic carbon fluxes ranging from 1.1 to 1.5 mg Corg cm � 2 yr � 1 for the last few centuries, which corresponded to 4–6% of the present annual primary production in the euphotic zone. These data suggest high productivity and relatively stable conditions in the polynya on a decadal time scale. In the northeastern Baffin Bay core, the analyses indicated generally lower organic carbon fluxes, ranging from 0.3 to 0.6 mg Corg cm � 2 yr � 1 , and (from the microfossil data) significant variations in sea-surface conditions at this lower latitude over the last centuries. r 2002 Elsevier Science Ltd. All rights reserved.

Micropaleontology and palynology of core PAR87A‐10: A 23,000 year record of paleoenvironmental changes in the Gulf of Alaska, northeast North Pacific

Micropaleontological data of core PAR87A-10 reveal that the last glacial interval, prior to 13 ka, was marked by low biogenic fluxes and poor CaCO3 preservation. Quantitative estimates of sea-surface conditions based on dinocyst assemblages suggest that cold temperatures and freezing winter conditions existed during this period. The glacial to interglacial transition, i.e., the 13–8 ka interval, was characterized by an increase in fluxes of microfossils indicating enhanced productivity in surface waters. A higher biogenic carbonate production probably resulted in better preservation of CaCO3. This interval was marked by relatively low salinity and by sea-surface temperatures increasing toward modern values. Relatively high pollen flux during the transition suggests nutrient inputs through atmospheric and/or fluvial transport from the adjacent North American continent. After 8 ka, diminished fluxes of plankton, concomitant with a decline in pollen input, are associated with decreasing nutrient supply as predominantly eastward winds became established over the North Pacific.

Chapter Thirteen Transfer Functions: Methods for Quantitative Paleoceanography Based on Microfossils

Publisher Summary Transfer functions, in a broad sense, are powerful methods that allow complex biological assemblages analyzed by micropaleontologists to be translated into simple abiotic parameters for the use of the entire scientific community working in the fields of paleoceanography and paleoclimatology. They are extremely useful, but they imply a high degree of simplification of multiple biotic variables that are themselves dependent upon complex abiotic and biotic parameters. Various methods are developed based either on calibrations between assemblages and climate parameters or on the degree of similarity between assemblages. Among these methods, those yielding the most accurate reconstructions of the modern conditions are the weighting average — WA-PLS method, the ANN, the MATs and RAM. The RAM provides the possibility of improving the reference data sets in some parts of their range, but there must be caution not to create virtual samples that are too distant from the actual climatic space. It is probably one of the most appropriate and reliable methods currently used in paleoceanography. A multi-method approach has been proposed to obtain more robust reconstructions and to identify the assemblages for which the performance of the transfer function is low or problematic. The best candidates for a multi-method approach are MAT, RAM, ANN and WA-PLS. The respective performances of the transfer function methods can be evaluated from the cross-validation of modern data sets.

Pliocene paleoclimatic reconstruction using dinoflagellate cysts: Comparison of methods

The application of quantitative and semiquantitative methods to assemblage data from dinoflagellate cysts shows potential for interpreting past environments, both in terms of paleotemperature estimates and in recognizing water masses and circulation patterns. Estimates of winter sea-surface temperature (WSST) were produced by using the Impagidinium Index (II) method, and by applying a winter-temperature transfer function (TFw). Estimates of summer sea-surface temperature (SSST) were produced by using a summer-temperature transfer function (TFs), two methods based on a temperature-distribution chart (ACT and ACTpo), and a method based on the ratio of gonyaulacoid:protoperidinioid specimens (G:P). WSST estimates from the II and TFw methods are in close agreement except where Impagidinium species are sparse. SSST estimates from TFs are more variable. The value of the G:P ratio for the Pliocene data in this paper is limited by the apparent sparsity of protoperidinioids, which results in monotonous SSST estimates of 14–26°C. The ACT methods show two biases for the Pliocene data set: taxonomic substitution may force ‘matches’ yielding incorrect temperature estimates, and the method is highly sensitive to the end-points of species distributions. Dinocyst assemblage data were applied to reconstruct Pliocene sea-surface temperatures between 3.5−2.5 Ma from DSDP Hole 552A, and ODP Holes 646B and 642B, which are presently located beneath cold and cool-temperate waters north of 56°N. Our initial results suggest that at 3.0 Ma, WSSTs were a few degrees C warmer than the present and that there was a somewhat reduced north-south temperature gradient. For all three sites, it is likely that SSSTs were also warmer, but by an unknown, perhaps large, amount. Past oceanic circulation in the North Atlantic was probably different from the present.

THE CYST OF THE CALCAREOUS DINOFLAGELLATE SCRIPPSIELLA TRIFIDA: RESOLVING THE FOSSIL RECORD OF ITS ORGANIC WALL WITH THAT OF ALEXANDRIUM TAMARENSE

Abstract Scrippsiella trifida Lewis, 1991 ex Head, 1996 is a nontoxic marine calciodinelloidean dinoflagellate whose resting cyst has a distinctive wall containing large, erect, trifurcate, recurving calcareous processes that separate two organic layers. We show that the organic wall layers of living Scrippsiella trifida cysts are resistant to acetolysis and can therefore potentially fossilize, and we report on abundant Scrippsiella trifida cysts from latest Pleistocene and early Holocene marine sediments off eastern Canada, representing the first confirmed fossil discovery of this species in the North Atlantic. A reappraisal of late Quaternary palynological records now shows that the organic remains of Scrippsiella trifida cysts have been widely misidentified as cysts of Alexandrium tamarense (Lebour, 1925) Balech, 1985, a goniodomacean (and hence noncalcareous) dinoflagellate and major cause of paralytic shellfish poisoning in humans. The morphology of these two cyst types is contrasted, and the modern and fossil distribution of Scrippsiella trifida cysts in sediments of the North Atlantic and adjacent areas is now clarified. It is apparent from this distribution that Scrippsiella trifida favors neritic environments characterized by cool winters and relatively warm (14°–25°C) summers. Extremely high fluxes of S. trifida cysts in nearshore areas off Nova Scotia and southern Greenland during deglaciation and early postglacial time (14–7 ka) have no modern analog but may signal a reduction in salinity caused by meltwater discharge. In general, the organic walls of calcareous dinoflagellate cysts are more common components of palynological assemblages than hitherto realized.