Jón Eiríksson

The tephrochronology of Iceland and the North Atlantic region during the Middle and Late Quaternary: a review

The tephrochronology of Iceland and the North Atlantic region is reviewed in order to construct a unified framework for the last 400 kyr BP. Nearly all of the tephra layers described are also characterised geochemically. A number of new tephra layers are analysed for the first time for their geochemical signature and a number of pre-Holocene tephra layers have been given an informal denotation. The tephrostratigraphy of Ash Zone II is highlighted. Where possible the rhyolitic tephra layers found outside Iceland have been correlated to known Icelandic tephra layers or to the volcanic source area. The application of tephra fallout in various depositional environments is described and discussed. Copyright

Evidence for solar forcing of sea-surface temperature on the North Icelandic shelf during the late Holocene

Diatom proxies from the modern position of the oceanographic Polar Front north of Iceland record variability in sea-surface temperatures (SSTs) during the past 2 k.y. The sedimentary record is dated with tephrochronology, alleviating marine 14C reservoir age uncertainties. Comparison of changes in SSTs on the North Icelandic Shelf with variations in the atmospheric circulation above Greenland, North American Atlantic coastal SSTs, and mean temperature anomalies for the Northern Hemisphere suggests synchronous North Atlantic–wide fluctuations, which would seem to imply a common forcing factor. A positive and significant correlation between our SST record from the North Icelandic Shelf and reconstructed solar irradiance, together with modeling results, supports the hypothesis that solar forcing is an important constituent of natural climate variability in the northern North Atlantic region.

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.

Correlation of late Holocene terrestrial and marine tephra markers, north Iceland: implications for reservoir age changes

The tephrochronology of the last 3000 years has been investigated in soil sections in north Iceland and in a marine sediment core from the north Icelandic shelf, 50 km offshore. Tephra markers, identified with major element geochemical analysis of volcanic glass shards, serve to correlate the marine and terrestrial records. Hekla 3, the largest Holocene tephra marker from the volcano Hekla, in south Iceland, dated to 2980 years BP, is used as the basal unit in the tephra stratigraphy. AMS 14C dating of molluscs in the sediment core shows variable deviation from the tephrochronological age model, indicating that the reservoir age of the seawater mass at the coring site has varied with time. A standard marine reservoir correction of 400 14C years appears to be reasonable at the present day in the coastal and shelf waters around Iceland, which are dominated by the Irminger Current. However, values over 500 years are observed during the last 3000 years. We suggest that the intervals with increased and variable marine reservoir correction reflect incursions of Arctic water masses derived from the East Greenland Current to the area north of Iceland.

Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables

Canonical correspondence analysis of diatoms from surface sediment samples and oceanographic environmental variables shows that summer and winter sea-surface temperatures, water depth and winter sea-surface salinity are the main environmental factors affecting diatom distribution around Iceland. Of these, summer sea-surface temperature is the most important. Five diatom assemblages are distinguished and the distribution of these assemblages is clearly correlated with oceanic current patterns in the region. The sea-ice diatom assemblage is limited to the area where the East Greenland Current (Polar Water) has its strongest influence, and the cold diatom assemblage is basically controlled by the less cold East Icelandic Current (Modified Polar Water). The mixing diatom assemblage results from the interaction between the cold East Greenland and East Icelandic Currents and the warm Irminger Current. The warm diatom assemblage is located in the area dominated by the Irminger Current and may be used as an indicator of warm-water masses (Atlantic Water). The coastal diatom assemblage is the only one strongly influenced by both water depth and summer water temperatures.

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.

MODERN DISTRIBUTION OF BENTHIC FORAMINIFERA ON THE NORTH ICELANDIC SHELF AND SLOPE

Forty-six surface samples from the north Icelandic shelf and slope were analyzed with respect to both living (stained) and total (living/stained + dead/unstained) benthic foraminiferal faunas. Near-coastal samples are strongly dominated by species indicating a high-energy environment, among these various species of the genus Cibicides. Nonionellina labradorica, which has a strong affinity to areas of high surface primary production, is constrained to oceanic boundaries on the outer shelf. Faunistically, the area is further divided into eastern and western parts, the submarine Kolbeinsey Ridge forming a barrier. Calcareous species, particularly Melonis barleeanus, prevail in the western part, while the assemblages in the eastern part are strongly dominated by agglutinated foraminifera. Principal Component Analyses (PCA) help distinguish four main components of both the total (living + dead) assemblage (TA) and the total living assemblage (TLA), as well as of the total (living + dead) calcareous assemblage (CA) and the living calcareous assemblage (CLA). The components of the TA and TLA are similar, the major differences being controlled by species-dependent variables such as adaptability to changes in food supply and, in the eastern part of the area, depth of microhabitat and possibly a limited primary production. The PCA analyses of the calcareous species alone (CA and CLA) define components with significant differences in the foraminiferal population. The calcareous distribution in particular is important in application of modern distribution patterns to paleoceanographical and paleoclimatological reconstructions in areas where post-mortem disintegration of the agglutinated fauna may have occurred.

Application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in Holocene and Late Glacial shelf sediments off North Iceland

Abstract The north Icelandic shelf is partly distinguished by the Tjornes Fracture Zone featuring numerous active basins in a mud-dominated shelf environment. Late Glacial and Holocene high-resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area. Data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores. The correlation is extended with lithological logs and with magnetic susceptibility records. A detailed multidisciplinary study (including biostratigraphy and tephrochronology) of a late Holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal. yr. Of special interest is the exact timing of a marked drop in sea-surface temperature in the area, indicated by ice rafting debris concentration, to about 50 years before the Hekla 3 eruption, which occurred at 2980 cal. yr BP. This appears to predate most records of a general cooling event in NW Europe by a couple of centuries. Two different possible age models, one based on 14C dates combined with tephra markers and one based on tephra markers alone, are discussed in context with the problem of different marine reservoir ages of the water masses in the area.

Solar forcing of Holocene summer sea-surface temperatures in the northern North Atlantic

Mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes. However, the mechanisms for sun-climate links remain a topic of debate. Here we present a high-resolution summer sea-surface temperature (SST) record covering the past 9300 yr from a site located at the present-day boundary between polar and Atlantic surface-water masses. The record is age constrained via the identification of 15 independently dated tephra markers from terrestrial archives, circumventing marine reservoir age variability problems. Our results indicate a close link between solar activity and SSTs in the northern North Atlantic during the past 4000 yr; they suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation. Furthermore, the high-resolution SST record indicates that climate in the North Atlantic regions follows solar activity variations on multidecadal to centennial time scales.

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.