James D. Scourse

Persistent Positive North Atlantic Oscillation Mode Dominated the Medieval Climate Anomaly

The Medieval Climate Anomaly (MCA) was the most recent pre-industrial era warm interval of European climate, yet its driving mechanisms remain uncertain. We present here a 947-year-long multidecadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. Supplementary reconstructions based on climate model results and proxy data indicate a clear shift to weaker NAO conditions into the Little Ice Age (LIA). Globally distributed proxy data suggest that this NAO shift is one aspect of a global MCA-LIA climate transition that probably was coupled to prevailing La Niña–like conditions amplified by an intensified Atlantic meridional overturning circulation during the MCA.

The origin of Heinrich layers: evidence from H2 for European precursor events

Recent well-dated isotopic (Sr–Nd) fingerprinting of Heinrich layer ice-rafted detritus (IRD) on the European margin indicates supply from European ice sheets as precursors to Laurentide Ice Sheet (LIS) supply [F.E. Grousset et al., Geology 28 (2000) 123–126, H. Snoeckx et al., Mar. Geol. 158 (1999) 197–208]. These precursorevents lead LIS input by up to 1.5 ka [F.E. Grousset et al., Geology 28 (2000) 123–126] and have been interpreted to indicate LIS collapse during Heinrichevents stimulated by events originating on the European side of the Atlantic [F.E. Grousset et al., Geology 28 (2000) 123–126]. Such phasing of IRD supply from different sources within Heinrich layers therefore has implications for the origin and mechanics of Heinrichevents. We present evidence here that the IRD comprising Heinrich layer 2 (H2; ∼20–21 14C ka BP) on the European continental margin contains detrital Campanian Upper Chalk deriving from bedrock sources eroded on the Celtic shelf by the British Ice Sheet (BIS) in addition to lithic material sourced from the LIS. High-resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of LIS-sourced detritus. The specificity of the chalk fingerprint to the BIS enables a 700–1000 yr lag between the BIS and LIS events to be identified. This phasing indicates a more rapid response of the outlet lobes draining the smaller BIS than those draining the LIS and implicates external climatic forcing of Heinrichevents. It is unlikely that this precursorevent represents IRD event 18, the recently identified 1–2 ka IRD cycle event which immediately precedes H2, because the lag between precursor and main event is here less than 1.5 ka and because such pervasive periodicity is not apparent in European continental margin IRD records. The later synchroneity between the BIS and LIS input in H2 identifies glacio–eustatic sea-level rise associated with LIS discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during Heinrichevents.

VERY LONG-LIVED MOLLUSKS CONFIRM 17TH CENTURY AD TEPHRA-BASED RADIOCARBON RESERVOIR AGES FOR NORTH ICELANDIC SHELF WATERS

Marine sediment records from the north Icelandic shelf, which rely on tephrochronological age models, reveal an average Δ R (regional deviation from the modeled global surface ocean reservoir age) of approximately 150 yr for the last millennium. These tephra-based age models have not hitherto been independently verified. Here, we provide data that corroborate Δ R values derived from these sediment archives. We sampled the youngest portion (ontogenetic age) of a bivalve shell, Arctica islandica (L.), for radiocarbon analysis, which was collected alive in 2006 from the north Icelandic shelf in ~80 m water depth. Annual band counting from the sectioned shell revealed that this clam lived for more than 405 yr, making it the longest-lived mollusk and possibly the oldest non-colonial animal yet documented. The 14C age derived from the umbo region of the shell is 951 ± 27 yr BP. Assuming that the bivalve settled onto the seabed at AD 1600, the corresponding local value of Δ R is found to be 237 ± 35 yr by comparison of the 14C age with the Marine04 calibration curve (Hughen et al. 2004) at this time. Furthermore, we cross-matched a 287-yr-old, dead-collected, A. islandica shell from AD 1601 to 1656 from the same site with the live-caught individual. 14C analysis from the ventral margin of this shell revealed a Δ R of 186 ± 50 yr at AD 1650. These values compare favorably with each other and with the tephra-based Δ R values during this period, illustrating that 14C from A. islandica can effectively record 14C reservoir changes in the shelf seas.

surface changes in the north Atlantic meridional overturning circulation during the last millennium

Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation dynamics likely amplified the relatively warm conditions during the Medieval Climate Anomaly and the relatively cool conditions during the Little Ice Age within the North Atlantic sector.

Control of modern dinoflagellate cyst distribution in the Irish and Celtic seas by seasonal stratification dynamics

Abstract Surface sediments from seven stations located in the seasonally stratified, frontal and mixed water regions in the Celtic and Irish seas have been analysed for their dinoflagellate cyst assemblages and dinosterol content. A total of 45 dinoflagellate cyst taxa have been identified and the assemblages related to surface and sediment conditions. Sediments from the mixed water region, at 30 m water depth, are characterised by a relatively low cyst concentration (∼2300 cysts/g dry weight) and high relative abundances of Lingulodinium machaerophorum accompanied by Spiniferites membranaceus , Brigantedinium spp. and Dubridinium caperatum . Assemblages from stratified and frontal water stations are dominated by Spiniferites ramosus associated with Operculodinium centrocarpum , Brigantedinium spp., cysts of Polykrikos schwartzii and Selenopemphix quanta . Ordination techniques performed on a restricted number of 35 taxa from the assemblages differentiated the stratified and frontal assemblages based on the abundance of the less abundant species Bitectatodinium tepikiense and Spiniferites elongatus . Among the environmental parameters (sea-surface temperature and salinity, stratification index, chlorophyll concentration and sediment grain-size classes), the seasonal stratification and sedimentological context, itself a function of tidal dynamics, explain most of the variance in the environmental conditions. These results indicate that dinoflagellate cyst analyses of shelf sediment records can be used to document the planktonic signal of seasonal stratification dynamics.

Sedimentology and micropalaeontology of glacimarine sediments from the Central and Southwestern Celtic Sea

Abstract Thin discontinuous glacigenic sediments occur at or close to the sea bed as far south as 49°N in the Celtic Sea. The northern samples (facies A) are clast-rich, overconsolidated lodgement tills or proximal glacimarine sediments containing sparse reworked microfaunas. The southern samples (facies B) are distal glacimarine plastic silty clays containing abundant cold water microfaunas. Both these facies are correlated with the Late Devensian Scilly Till, enabling a quantitative reconstruction of ice thicknesses, grounding line, sea-level and shoreline elevations in the Celtic Sea at 19 000 years BP. The ice advance terminated in marine waters towards the shelf edge break, and is likely to have constituted a thin lobate surge over deformable marine sediments.

Deglacial laminated facies on the NW European continental margin: The hydrographic significance of British‐Irish Ice Sheet deglaciation and Fleuve Manche paleoriver discharges

We have compiled results obtained from four high sedimentation rate hemipelagic sequences from the Celtic sector of the NW European margin (NE Atlantic) to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles. We focus on periods characteristic of deglacial transitions. We adopt a multiproxy sedimentological, geochemical, and micropaleontological approach, applying a sampling resolution down to ten microns for specific intervals. The investigation demonstrates the relationships between the Bay of Biscay hydrography and the glacial/deglacial history of both the proximal British-Irish Ice Sheet (BIIS) and the western European continent. We identify recurrent phases of laminae deposition concurrent with major BIIS deglacial episodes in all the studied cores. Evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale. We discuss their impact at a global scale considering the present and past key location of the Bay of Biscay versus the Atlantic Meridional Overturning Circulation (AMOC).

THE DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE CELTIC SEA: THE SIGNIFICANCE OF SEASONAL STRATIFICATION

Seasonal stratification is an important phenomenon in tidally-stirred shelf seas, influencing biological productivity, sedimentation rates, the organic content of shelf sediments, and the climate of surrounding landmasses. Previous micropaleontological and stable isotopic investigation investigation of a Holocene sequence from the Celtic Sea suggests that benthic foraminiferal distributions are linked to the physical and biological oceanographic characteristics associated with stratification. We have tested this hypothesis by analyzing the living and dead foraminiferal faunas from surface samples collected during across-frontal cruises during the summers of 1995 and 1996. Foraminiferal and environmental data for 56 samples are presented. Live and dead foraminiferal data were analyzed by factor analysis and, along with the environmental data, canonical correspondence analysis (CCA). Four distinct assemblages were identified from factor analysis of the live data: (1) a frontal assemblage characterized by Stainforthia fusiformis, (2) a mixed water assemblage characterized by Cibicides lobatulus, Textularia bockii, Spiroplectammina wrightii, Ammonia batavus and Quinqueloculina seminulum, (3) a stratified assemblage characterized by Bulimina marginata, Hyalinea balthica, Adercotryma wrighti and Nonionella turgida, and (4) an eastern assemblage dominated by Bulimina gibba, Elphidium excavatum and Eggerelloides scaber. Factor analysis of the dead data reproduces all groupings except the frontal assemblage. These data therefore support interpretations based on earlier stratigraphic data, and highlight the significance of benthic foraminifera as faunal indicators of paleostratification in shelf seas. The distributions also support predicted cross-frontal transfer of nutrients and the existence of surface converging circulation cells. Statistical analyses indicate the significance of unmeasured ecological variables which we speculate might be food supply, and oxygen concentration of bottom and sediment pore waters.

Distribution of dinoflagellate cyst assemblages in surface sediments from the northern and western shelf of Iceland

Abstract In order to provide calibration for palaeoceanographic investigations, 54 surface sediment samples from the northern and western margin of Iceland and Greenland have been analysed for their dinoflagellate cyst assemblages. Relatively high diversity was observed with a total of 28 taxa. Cysts of Pentapharsodinium dalei are dominant accompanied by Operculodinium centrocarpum and Nematosphaeropsis labyrinthus . Three groups of assemblages have been recognised based on multivariate statistical analyses and these are related to surface water masses and currents. The first association, Group I, located west of Iceland, is characterised by high abundance of O. centrocarpum and N. labyrinthus accompanied by significant occurrence of the heterotrophic taxa Brigantedinium spp., Selenopemphix quanta , cysts of Polykrikos schwartzii and cysts of Protoperidinium americanum . This region is under the influence of the Irminger Current (IC) and contains the most productive waters around Iceland. The second association, Group II, comprising high relative abundance of cysts of P. dalei accompanied by significant occurrence of O. centrocarpum and a very high concentration (up to 256 920 cysts/g), is situated in the northern Icelandic shelf, in the Polar Front realm, i.e. the marginal zone between the East Icelandic Current (EIC) and the northern branch of the IC. The third group (Group III) is in the region influenced by the East Greenland Current and the EIC and characterised by the co-dominance of cysts of P. dalei and O. centrocarpum , relatively high representation of Impagidinium pallidum and low concentrations (from 370 to 56 220 cysts/g). This investigation demonstrates that dinocyst distribution is clearly related to upper water mass physical factors. Advection of temperate species around Iceland suggests significant transport by surface currents, especially the IC; however, the apparent boundaries between the associations and decreasing percentages related to environmental gradients indicate that dispersal by surface transport is limited by the ecological requirements of the dinoflagellate cells. Bottom currents may play a role in the accumulation rate of cysts.

Foraminiferal isoleucine epimerization determinations from the Nar Valley Clay, Norfolk, UK; implications for Quaternary correlations in the southern North Sea Basin

Fully temperate freshwater, brackish and marine sediments overlying Anglian till and glacilacustrine sediments in the Nar Valley area of northwest Norfolk, UK, have been attributed to the Middle Pleistocene Hoxnian temperate stage on palynological grounds, and basal peats associated with this sequence have been recently correlated with oxygen isotope stage 9 on the basis of a series of 230 Th/ 238 U dates (mean 317±14 ka). At Tottenhill these sediments (Nar Valley Freshwater Beds, Nar Valley Clay) underlie a deltaic complex attributed to the Wolstonian ice margin. The lithostratigraphical relations between the major formations in the Nar Valley, and the pollen stratigraphy of the fully temperate sequence, are very similar to the Pleistocene sequence in the Inner Silver Pit area of the southern North Sea, and correlation has been proposed between the successions described from these two localities. However, the Inner Silver Pit sequence has yielded aminostratigraphic data consistent with isotopic stage 11. Benthic foraminiferal assemblages and foraminiferal amino-acid determinations have been investigated from the Nar Valley Clay in order to test further the palaeoenvironmental setting of the sequence and to help resolve the age of the sequence and correlation with the Inner Silver Pit interglacial. The foraminiferal assemblages support previous sedimentological and palaeontological evidence for a transgressive tendency within this sequence. Multiple isoleucine epimerization determinations on Ammonia beccarii and Aubignyna perlucida from five levels within the Nar Valley Clay give mean aIle/Ile ratios of 0.135 and 0.111, respectively. The A. beccarii ratios are much lower than mean aIle/Ile ratios on equivalent species from the interglacial sequence in the Inner Silver Pit (upper Sand Hole Formation), which are close to 0.2, and the two datasets fail to overlap at the 1σ level. The new aminostratigraphic ratios indicate correlation of the Nar Valley Clay with oxygen isotope stage 9, and therefore support the pre-existing 230 Th/ 238 U data. These results suggest that two temperate stages of Hoxnian palynological affinity are present in the Quaternary record of East Anglia and the southern North Sea basin, a conclusion consistent with independent new U-series data from other Hoxnian sites in East Anglia. An alternative model in which the amino-acid ratios are explained as a function of different post-depositional thermal histories, related to length of cover by ice and water, is discussed but considered unlikely. The conclusions have important implications for the timing and number of glacial events in and around the southern North Sea basin, and help to resolve discrepancies in relative sea-level histories and biogeography in temperate sequences hitherto accommodated within a single stage.