Search

Pronounced seasonal variability, particularly in the surface ocean heat flux, imparts an important control that drives thermal stratification of the tide-dominated middle- and high-latitude shelf seas. Bottom water temperature and salinity data, resolved on a grid 20' latitude by 30' longitude, were combined with a regional synthesis of the salinity:δ18O relationship in order to generate a spatial and temporal understanding of oxygen isotopes in seawater around the shelf seas of NW Europe. The data are expressed according to equilibrium calcite (δ18OEq.calcite) and, in the shallow mixed water column, exhibit large seasonal changes that are primarily driven by bottom water temperature. Annual bottom water temperature varies from < 3°C to > 17°C in the southern North Sea, generating a seasonal δ18OEq.calcite signal of up to 3.2‰. The amplitude of the seasonal δ18OEq.calcite signal is significantly damped (0.1-0.2%) in deeper, thermally stratified shelf waters. Maps of the monthly distribution δ18OEq.calcite provide the first systematic overview of the spatial and temporal changes on the NW European shelf and highlight the importance of understanding seasonal growth on the incorporation of geochemical signatures into marine organisms. [ABSTRACT FROM AUTHOR]

The underlying aim of the HOLSMEER project has been to improve our understanding of natural climate variability through the search for, interpretation and quantification of, climatic variability in very high-resolution shallow marine records from Atlantic Europe covering the last 2000 years. This has been achieved through detailed analyses of a series of coastal and shallow marine sites spanning the Atlantic seaboard from Iberia to western Norway, and extending across to Iceland. HOLSMEER partners have documented pronounced instability in the thermohaline circulation (THC) during the period immediately prior to the recent significant anthropogenic impact on the environment. For the first time we have been able to document that these changes in the coastal ocean are correlated with significant changes in terrestrial palaeoclimate proxies, notably during the last 1000 years. The notable changes are the significance of warm sea surface temperatures (SST) associated with active THC between AD 700 and 1000, a transition phase to much colder SST and reduced THC between AD 1000 and AD 1300, colder SST through to AD 1900 followed by an active re-establishment of warm surface water circulation during the twentieth century. These switch-like reorganizations of the climate system have influenced the entire seaboard from western Iberia to western Norway, and have forced changes in ocean productivity, iceberg frequency and sea ice coverage. These changes have also directly influenced sea level through steric effects. The project has also resulted in significant advances in the establishment of new palaeoclimate proxies, including transfer functions related to benthic foraminifera, diatoms and dinoflagellate cysts. Annual growth band series from fossil specimens of the long-lived bivalve mollusc Arctica islandica from the northern North Sea have been successfully cross-matched, and independently verified by radiocarbon dating, to provide the longest Arctica chronology, and the first floating chronology constructed entirely from marine fossils. [ABSTRACT FROM AUTHOR]

Crucial periods of change in the latest glacial to early-Holocene palaeogeographic and palaeoceanographic evolution in the Skagerrak region are portrayed in four time-slice maps on a calibrated age scale at 14.0 kyr, 11.2 kyr, 10.2 kyr and 8.1 kyr. The course of palaeoenvironmental events is set in a high-resolution chronological framework using IMAGES core MD99-2286 and a grid of chirp sonar profiles. These maps visualize the complex, rapid palaeogeographic and palaeoceanographic changes during the glacial-interglacial transition, which is characterized by major changes in circulation, sediment sources, and depositional processes. [ABSTRACT FROM AUTHOR]

Integrated understanding of phasings within the climate system over the last glacial cycle, and at higher frequencies, is inhibited because no absolute timescale for the marine environment currently exists. This precludes identification of forcings and feedbacks, accurate temporal calibration of the marine radiocarbon reservoir effect, and the application of radiocarbon as a proxy of short-timescale ocean ventilation. This has prompted a search for annually banded marine proxies in the hope of establishing an accurate marine chronometer. We present annual growth band series from dead-collected specimens of the long-lived bivalve mollusc Arctica islandica from the northern North Sea and demonstrate their successful cross-matching, with the general timescale context independently verified by radiocarbon dating. Though at present limited to only a few statistically cross-matched series, this has already generated the longest Arctica chronology, and the first `floating' chronology constructed entirely from marine fossils. The record covers the period from c. AO 1000 to 1400 and integrates a 267-yr series from the longest-lived Arctica specimen yet recorded from the North Sea. This breakthrough in cross-matching demonstrates that Arctica islandica can fulfill its potential as the 'tree of the sea' to provide an absolute timescale for the marine environment. [ABSTRACT FROM AUTHOR]

Marine localities on the west European shelf have been studied to reconstruct the nearshore palaeoceanography of the last two millennia. The sites form a transect from the Iberian margin northeastward via Scotland to western Norway and Iceland. Proxies used for palaeoclimatic reconstructions include stable isotopes, benthic and planktonic foraminfera, diatoms, dinoflagellates, as well as geochemical and sedimentological parameters. Major changes as well as long-term trends in oceanographic conditions are observed in the records, including a general cooling trend through much of the last millennium. There is a clear linkage between the atmospheric processes and the oceanic circulation, and the ocean temperature variability in the records can be correlated with the so-called 'Mediaeval Warm Period' and 'Little Ice Age'. These oscillations are, however, by no means unique within the last two millennia. As an example, sea surface temperatures to the north of Iceland and on the Iberian margin were higher in the Roman Warm Period than at any time during the 'Mediaeval Warm Period'. However, the palaeoceanographic record generally supports a distinct cooling at the transition between the 'Mediaeval Warm Period' and the 'Little Ice Age'. While a number of records indicate a warming of coastal and shelf waters during the last 200 years, the twentieth century does not appear to be unusual when the proxy records spanning the last two millennia are examined. [ABSTRACT FROM AUTHOR]

The Tagus Prodelta (W Portugal) and the Muros Ría (NW Spain) are areas of high deposition rates registering high-resolution palaeoclimatic records for western Iberia. We compare the climatic conditions of the two areas over the last two millennia based on proxies of temperature (sea surface temperatures and oxygen isotopes), continental input (grain size, iron and magnetic susceptibility) and productivity (inorganic and organic carbon, carbon isotopes, benthic foraminifera and diatoms). Biogeochemical changes in the Tagus Prodelta reflect widely recognized North Atlantic climatic periods encompassing the Roman Period (AD 0-350), the Dark Ages (AD 400-700), the 'Mediaeval Warm Period' (MWP; AD 800-1200) and the 'Little Ice Age' (LIA; AD 1300-1750). The atmospheric North Atlantic Oscillation (NAO) drives the Tagus Prodelta multidecadal, long-term variability in precipitation-river input during cold periods (negative NAO) and marine upwelling during warmer periods (positive NAO), a scheme that is reversed in the Galician region. The Muros Ría shows only local hydrodynamics until AD 1150, including a 'suboxic' event in the inner Ría around AD 500-700. Since AD 1150 Atlantic warm upwelled waters have ventilated the outer Ría but only reach the inner Ría at AD 1750. The twentieth- century records are also interpreted as a reflex of the inverse NAO mode in both areas, resulting in amplification of the LIA biogeochemical water conditions. Centennial-scale solar activity appears to be another important forcing mechanism (or the only one, if solar activity drives the NAO and 'Bond-cycles') behind changes in the hydrography of the Tagus Prodelta, and primary production, bottom ventilation and organic carbon degradation in the Muros Ría. [ABSTRACT FROM AUTHOR]

We present a high-resolution palaeoenvironmental reconstruction covering the late Holocene from the Skagerrak and other sites in the North Sea area. The data, which are based on the analyses of marine sediment cores, reveal a marked environmental shift that took place between AD 700 and AD 1100, with the most pronounced changes occurring at AD 900. Both surface and bottom waters in the Skagerrak were subject to major circulation and productivity changes at this time due to an enhanced advection of Atlantic waters to the North Sea marking the beginning of the 'Mediaeval Warm Period' (MWP). The observed increase in bottom current strength is especially remarkable as there is hardly any comparable signal in the older part of the record going back to 1000 BC. At the transition to the 'Little Ice Age' (LIA) the bottom current strength remains at a high level, now probably forced by atmospheric circulation. Thus, despite opposite temperature forcing, these two consecutive climate scenarios are apparently able to generate distinctly stronger bottom currents in the Skagerrak than observed in the preceding 2000 years, and demonstrate the significance of climatic forcing in shaping the marine environment. Indeed, both the MWP and the LIA are reported as strong climatic signals in northwest Europe, being the warmest (except the late twentieth century) and coldest periods, respectively, during at least the last 2000 years. [ABSTRACT FROM AUTHOR]

A high-resolution late-Holocene sea-level record is produced from salt-marsh deposits at Viõarhólmi in Snæfellsnes, western Iceland. The stratigraphy of Viõarhólmi saltmarsh is documented using detailed descriptions of ten exposed sections and numerous hand-drilled cores. Fossil foraminifera are used as proxy sea-level indicators in an exposed section of salt-marsh peat. The agglutinated foraminifera Jadammina macrescens and Paratrochammina (Lepidoparatrochammina) haynesi are most useful as sea-level indicators because of their narrow vertical extent on the marsh surface and their good preservation in the peaty marsh deposits. We collected compaction-free sea-level index points from salt-marsh peat directly overlying the bedrock surface to establish the pre-industrial millennial-scale trend of sea-level rise and evaluate effects of autocompaction on the stratigraphy. The chronology of the sea-level reconstruction is based on tephra stratigraphy, AMS 14C, 137Cs, Pb and palaeomagnetic analyses. The main tephra layer visible in the stratigraphy of Viõarhólmi salt marsh is the Landnám (settlement) layer, previously dated to AD 875 ±6. A sea-transported pumice layer was correlated to the 'Mediaeval Layer' of AD 1226/27. Our reconstruction indicates that relative sea level along the coast of western Iceland has risen by about 1.3 m since c. AD 100. The detrended sea-level record shows a slow rise between AD 100 and 500, followed by a slow downward trend reaching a lowstand in the first half of the nineteenth century. This falling trend is consistent with a steric change estimated from reconstructions of sea-surface and sea-bottom temperatures from shelf sediments off Northern Iceland. The sea-level record shows a marked recent rise of about 0.4 m that commenced AD 1820 ± 20 as dated by palaeomagnetism and Pb produced by European coal burning. This rapid sea-level rise is interpreted to be related to global temperature rise. The rise has continued up to the present day and has also been measured, since 1957, by the Reykjavik tide gauge. [ABSTRACT FROM AUTHOR]