Jens Matthiessen

Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum

The Palaeocene/Eocene thermal maximum, ∼55 million years ago, was a brief period of widespread, extreme climatic warming, that was associated with massive atmospheric greenhouse gas input. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition. We show that sea surface temperatures near the North Pole increased from ∼18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the oceans bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations, but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms—perhaps polar stratospheric clouds or hurricane-induced ocean mixing—to amplify early Palaeogene polar temperatures.

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

Episodic fresh surface waters in the Eocene Arctic Ocean

It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions that characterized the early Palaeogene period (55–45 Myr ago) probably induced an intensified hydrological cycle with precipitation exceeding evaporation at high latitudes. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (∼50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an ∼800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from ∼10 °C to 13 °C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas.

A multiproxy reconstruction of the evolution of deep and surface waters in the subarctic Nordic seas over the last 30,000yr

On the basis of various lithological, mircopaleontological and isotopic proxy records covering the last 30,000 calendar years (cal kyr) the paleoenvironmental evolution of the deep and surface water circulation in the subarctic Nordic seas was reconstructed for a climate interval characterized by intensive ice-sheet growth and subsequent decay on the surrounding land masses. The data reveal considerable temporal changes in the type of thermohaline circulation. Open-water convection prevailed in the early record, providing moisture for the Fennoscandian-Barents ice sheets to grow until they reached the shelf break at ∼26 cal. kyr and started to deliver high amounts of ice-rafted debris (IRD) into the ocean via melting icebergs. Low epibenthic δ18O values and small-sized subpolar foraminifera observed after 26 cal. kyr may implicate that advection of Atlantic water into the Nordic seas occurred at the subsurface until 15 cal. kyr. Although modern-like surface and deep-water conditions first developed at ∼13.5 cal. kyr, thermohaline circulation remained unstable, switching between a subsurface and surface advection of Atlantic water until 10 cal. kyr when IRD deposition and major input of meltwater ceased. During this time, two depletions in epibenthic δ13C are recognized just before and after the Younger Dryas indicating a notable reduction in convectional processes. Despite an intermittent cooling at ∼8 cal. kyr, warmest surface conditions existed in the central Nordic seas between 10 and 6 cal. kyr. However, already after 7 cal. kyr the present day situation gradually evolved, verified by a strong water mass exchange with the Arctic Ocean and an intensifying deep convection as well as surface temperature decrease in the central Nordic seas. This process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal. kyr and, eventually, to todays steep east–west surface temperature gradient. The time discrepancy between intensive vertical convection after 5 cal. kyr but warmest surface temperatures already between 10 and 6 cal. kyr strongly implicates that widespread postglacial surface warming in the Nordic seas was not directly linked to the rates in deep-water formation.

Organic-walled dinoflagellate cysts: palynological tracers of sea-surface conditions in middle to high latitude marine environments

Abstract A reference data base of dinoflagellate cyst assemblages was developed from the palynological analysesof surface sediment samples collected at ∼450 sites from middle to high latitudes of the North Atlantic and adjacent basins. The data set is representative of a wide range of sea-surface conditions with respect to sea-ice cover (0–12 months/year), temperature (up to 25°C), salinity (20–36 psu), and seasonality. The analyses reveal relatively high dinoflagellate cyst concentrations especially along the continental margins (up to 105 cysts/cm3). The distribution of species and assemblages show close relationships with the salinity, the temperature of the warmest and coldest months, and the seasonal duration of the sea-ice cover. On these grounds, transfer functions using the best analogue method were developed in order to reconstruct the following sea-surface parameters with a reasonable degree of accuracy, i.e. within the range of interannual variations: temperature in February (± 1.2°C) and August (± 16°C), salinity (± 0.7 psu) and sea-ice cover extent (± 1.1 months/year). The transfer functions based on dinoflagellate cysts are original and yield paleoceanographic and paleoclimatic data complementary to those produced currently on the basis of transfer function using planktonic foraminifera.

Plankton in the Norwegian-Greenland Sea: from living communities to sediment assemblages —an actualistic approach

A synoptic study is carried out to reconstruct the development of the plankton community in the late Quaternary in the Norwegian-Greenland Sea. It comprises quantitative analyses of coccolithophores, dinoflagellate cysts, diatoms and radiolarians. An actualistic approach is applied to evaluate the different fossil records of these plankton groups. The preliminary results of the current investigation are reported here.The composition and distribution of living communities of coccolithophores are presented as an example. A close relationship between the distribution of regional groups and surface water masses is observed. Seasonal vertical fluxes of coccolithophores and radiolarians through the water column show similar patterns within different years. However, diatoms are highly variable, both in absolute fluxes and species composition. The differentiation of sporadic and periodic processes is evident only after several years of observation. During settling and sedimentation biotic and abiotic processes such as grazing, dissolution and lateral transport alter the assemblages.Investigation of death assemblages in surface sediments reveals that in spite of these alteration processes the abundance and species distribution are related to surface water masses. Higher abundances and diversities are usually found in sediments underlying the warm Norwegian Current. Concentrations decrease to the north-west towards the cold polar water masses.The sediment assemblages of all groups are strongly altered relicts of former living communities. They are characterized by distinct changes in species composition and absolute abundances related to palaeo-oceanographic development. Their variation through the sedimentary record is used to distinguish four ecostratigraphic units during the late Weichselian and Holocene.

Marine ice-rafted debris records constrain maximum extent of Saalian and Weichselian ice-sheets along the northern Eurasian margin

Ž. Ž . Ž . Ice-rafted debris IRD ) 2 mm , input in eight sediment cores along the Eurasian continental margin Arctic Ocean , have been studied over the last two glacialrinterglacial cycles. Together with the revised chronologies and new Ž. Ž . micropaleontological data of two cores from the northern Barents Sea PS2138 and northeastern Kara Sea PS2741 Ž. spanning Marine Isotope Stages MIS 6 to 1, the IRD data give new insights into the glacial history of northern Eurasian ice-sheets over the last 150 ka. The chronologies of the cores are based on stable isotope records, AMS 14 C datings, paleomagnetic and biostratigraphic data. Extensive episodes of northern Barents Sea ice-sheet growth, probably to the shelf edge, occurred during the late Ž. Ž . Weichselian MIS 2 and the Saalian MIS 6 . Major IRD discharge at the MIS 4r3-transition hints to another severe glaciation, probably onto the outer shelf, during MIS 4. IRD-based instabilities of the marine-based ice margin along the northern Barents Sea between MIS 4 and 2 are similar in timing with North Atlantic Heinrich events and Nordic Seas IRD events, suggesting similar atmospheric cooling over a broad region or linkage of ice-sheet fluctuations through small sea-level events. In the relatively low-precipitation areas of eastern Eurasia, IRD peak values during Termination II and MIS 4r3-transi- tion suggest a Kara Sea ice-sheet advance onto the outer shelf, probably to the shelf edge, during glacial MIS 6 and 4. This suggests that during the initial cooling following the interglacials MIS 5, and possibly MIS 7, the combined effect of sustained inflow of Atlantic water into the Arctic Ocean and penetration of moisture-bearing cyclones into easterly direction Ž. Ž . supported major ice build-up during Saalian MIS 6 and Mid-Weichselian MIS 4 glaciation. IRD peak values in MIS 5 indicate at least two advances of the Severnaya Semlya ice-sheet to the coast line during the Early Weichselian. In contrast, Ž.

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.

Variations in surface water mass conditions in the Norwegian Sea: Evidence from Holocene coccolith and dinoflagellate cyst assemblages

Coccolith and dinoflagellate cyst assemblages have been investigated in five sediment cores from the Norwegian Sea and Fram Strait. Both fossil groups are characterized by similar patterns of composition. The assemblages contain high proportions of single species. The coccolith flora is of low diversity and consists almost entirely of Coccolithus pelagicus and Emiliania huxleyi. The dinoflagellate cysts are generally dominated by Operculodinium centrocarpum and Nematosphaeropsis labyrinthus. Other species, especially Bitectatodinium tepikiense, Peridinium faeroense and Impagidinium pallidum, sometimes contribute considerably to the assemblages. Based on the abundance of the assemblages and the ratio change between the dominating species it has been possible to establish three intervals of distinct major changes in surface water mass conditions. Sparse occurrences of coccoliths and dinoflagellate cysts have been observed before 10,000 yrs. B.P., indicating harsh environmental conditions with a distinct influence of meltwater and temporarily very slight inflow of Atlantic water. The modern surface-water circulation pattern was reinitiated during Termination IB. The assemblages suggest slightly cooler and probably less saline surface water conditions than are present today until 7500 yrs B.P. Solar insolation may have caused a first temperature peak which is responsible for the early Holocene productivity maximum. A considerable change in the composition of dinocyst and coccolith assemblages occurs corresponding approximately to the onset of the Holocene climatic optimum. This change was most probably linked to an almost synchronous reorganization of the hydrographic properties in the entire North Atlantic realm after the ice sheets had vanished. Since 6000 yrs B.P. the Norwegian Current with its modern oceanographic and ecological properties has been fully established.

Late Quaternary dinoflagellate cyst stratigraphy at the Eurasian continental margin, Arctic Ocean: indications for Atlantic water inflow in the past 150,000 years

Four sediment cores located at the Eurasian continental margin underlying the Atlantic layer have been studied for their dinoflagellate cyst content. Concentrations of distinct dinoflagellate cyst taxa display fluctuations in the late Quaternary, which are linked to changes in the inflow of relatively warm Atlantic surface and near-surface waters, resulting in increased local production of cysts in certain time intervals. Based on the assumption that marked changes in strength of inflow occurred synchronously at the Eurasian continental margin, concentration maxima can be used to correlate sediment cores. A dinoflagellate cyst record from the northern Barents Sea continental margin has been related to the stable oxygen isotope and paleomagnetic records to provide direct chronological information. The combination of these methods permits definition of stratigraphic sections equivalent to oxygen isotope stages in carbonate-poor sequences from the Eurasian continental margin. Previous age models of sediment cores are revised, based on dinoflagellate cyst abundance peaks and species distribution, but a firm chronostratigraphy of sedimentary sequences at the eastern Laptev Sea continental margin cannot be established because of the weak signal at the sites furthest from Fram Strait. Ž. In the past 150,000 years, the influence of Atlantic sub- surface waters generally decreased from west to east along the Eurasian continental margin, in particular during the glacials. Pronounced concentration maxima of cosmopolitan and temperate–subpolar dinoflagellate cysts indicate the inflow of Atlantic waters and seasonally increased production of cysts in the Holocene and Eemian. The Holocene is well-marked at the entire Eurasian continental margin but it is more difficult Ž.