Ash Zone Research Articles

Late glacial sedimentology, foraminifera and stable isotope stratigraphy of the Hebridean Continental Shelf, northwest Scotland

Abstract The detailed stratigraphies of two shallow marine cores from the Hebridean shelf of northwest Scotland are presented. The results provide evidence on the timing of deglaciation from about 15.2 ka BP , with evidence of cold, low salinity water persisting on the shelf until at least 13.5 ka BP . After 13.5 ka BP , probably shortly before 13.0 ka BP , warm shallow shelf seas were established and persisted throughout much of the interstadial. There is some evidence for climatic instability during the interstadial and for a cooling trend from after 11.6 ka BP . Glacial isostatic rebound of the shelf seems to have kept pace with eustatic sea level rise until about 11 ka BP . The fully developed conditions of the Younger Dryas stadial are not observed until 11 ka BP . For the first time from the marine record the constituent tephra peaks of North Atlantic Ash Zone 1 are resolved into a sequence of eruptive events and demonstrate that extensive sea ice transgressed the western UK continental shelf throughout the Younger Dryas. The climatic conditions of the Younger Dryas persist until an initial, precursor warming at 10.2 ka BP , followed by a marked warming of about 8°C and fully established early Holocene temperatures before 10 ka BP . Many of the changes recorded through nearly 5 m of sediments accumulated during the Younger Dryas can be related to changing relative sea levels and source supply.

Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments

Four previously known ash layers (Ash Zones I and II, Saksunarvatn and the Settlement layer) all originating in Iceland, have been identified in the Central Greenland ice core GRIP. This correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash. This establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately. A precise connection with dates obtained by 14C beyond the range of dendrochronology is established which provides an excellent confirmation of 230Th- 234U dates from corals. Four additional Icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits.

Source of Ash Zone 1 in the North Atlantic

Geochemical evidence shows that the silicic component of the widespread Ash Zone 1 in the North Atlantic is derived from a major ignimbrite-forming eruption which occurred at the Katla caldera in southern Iceland during the transition from glacial to interglacial conditions in Younger Dryas time. Both trace and major element evidence of the rhyolitic products excludes the Oraefajokull volcano as a source. The high-Ti basaltic component in the marine ash zone can also be attributed to contemporaneous eruption in the Katla volcanic complex. Dispersal of tephra from this event is primarily attributed to the generation of co-ignimbrite ash columns in the atmosphere, with ash fallout on both sea ice and on the ocean floor north and east of Iceland. Owing to the changing ocean circulation characteristics of the glacial regime, including suppression of the Irminger Current and a stronger North Atlantic Current, tephra was rafted on sea ice south into the central North Atlantic and deposited as dispersed Ash Zone 1. Sediments south of Iceland also show evidence of the formation of ash turbidites, generated either by the entrance of pyroclastic flows into the sea, or during discharge of jokulhlaups or glacier bursts from this subglacial eruption.

Source of Ash Zone 1 in the North Atlantic

Source of Ash Zone 1 in the North Atlantic

Lake Torfadalsvatn: a high resolution record of the North Atlantic ash zone I and the last glacial‐interglacial environmental changes in Iceland

High resolution environmental records with a refined chronology are essential to understand, reconstruct and model the climate dynamics of the last glacial‐interglacial transition. Sediments from Lake Torfadalsvatn in northern Iceland contain at least four primary volcanic tephras that belong to ash zone I in the North Atlantic deep‐sea cores. We chemically define these basaltic/rhyolitic tephras and the high resolution allows us to date them to about 10,800, 10,600, 9300 and 8900 BP. This detailed tephrostratigraphy will act as a refined dating and correlation tool in the North Atlantic region and enable calibration between different absolute chronologies. The pollen stratigraphy of the sediments suggests that by 10,400 14C years BP plant colonization of coastal north Iceland had begun. The pollen stratigraphy shows a succession of pioneer plants, from open tundra vegetation towards birch‐juniper woodland, which probably also reflects a transition from a cool climate at 10,400 BP to conditions similar to today's sub‐polar oceanic climate around 9200 BP. Diatom data largely concur with the climatic information from pollen, indicating gradually increasing productivity in the lake.

The Skógar Tephra, a Younger Dryas marker in North Iceland

A composite stratigraphical sequence, the Fnjóskadalur Sequence, reveals ten cycles of glacier advances and formation of ice‐dammed lakes in Fnjóskadalur in central North Iceland. Chemical analyses of the Skógar Tephra, with its type locality in this valley, have enabled a correlation with Ash zone I in deep sea sediments of the North Atlantic and with the Vedde Ash Bed on land in western Norway, where it is dated to 10,600 BP. The Skógar Tephra is composed of two layers, a basaltic tephra (STP‐1) and a rhyolitic tephra (STP‐2) erupted almost simultaneously from two different Icelandic volcanoes. The STP‐1 tephra originates from the Katla volcano in South Iceland, and the öræfajökull volcano in Southeast Iceland is considered a plausible source of the STP‐2 tephra. This new dating of the Skógar Tephra puts the three youngest glacier advances of the Fnjóskadalur Sequence within a 1000 year period between 10,600 and 9650 BP. The redated Late Weichselian glacial history now extracted from the Fnjóskadalur Sequence shows that glaciers in North Iceland were more extended in Younger Dryas and Preboreal times than previously assumed. This fits with the revised deglaciation pattern which has evolved in recent years.

Quaternary volcanic ash zones on the Iceland Plateau, southern Norwegian Sea

AbstractFour Quaternary volcanic ash zones in the southern Norwegian Sea have been investigated in core P57‐7 from the Iceland Plateau. Both the geochemical composition and morphological variation of each ash layer have been studied. The four volcanic ash zones appear in the light oxygen isotope stages 1, 5, 7 and 11. The ash zones are composed of transparent platy grains, light brown transparent grains, brown blocky and black blocky grains and white/transparent pumice, and each zone shows a distinct stratigraphic evolution. The geochemical results show a mixture of basaltic and silicic grains in each ash zone, and that each zone contains grains from more than one eruption. The geochemical investigations strongly suggest that all the ashes are derived from Iceland. The youngest ash zone includes two layers, which based on their geochemical composition and stratigraphic position are correlated with the 14C‐dated Vedde Ash (10 600 yr BP) and Saksunarvatn Ash (9 100 yr BP). Possible sources on Iceland for these layers are discussed.

Organic petrology of a self-burning coal wastepile from Coleman, Alberta, Canada

The present study examines the reflectance and petrology of a self-burning coal wastepile from Coleman, Alberta, Canada. Unaltered coal, carbonization, combustion and ash zones have been identified in the section studied. Due to the presence of uncompacted coal fragments, air can circulate easily through open fractures acting as conduits for convecting gases. Heat generated by the combustion of coal has altered the coal to produce solid (semicoke/coke), liquid (tar) and volatile matter. Several types of tar have been recognized penetrating cracks in the residues, briquetting coal macerals or reacting with coal particles. It has been estimated that the coal was carbonized at a temperature of 500–550°C at a rate of heating of 10°C/min and subsequently combusted at a higher temperature (550–600°C). When comparing the optical and petrological profile of the Coleman self-burning coal wastepile with that of other burning coal seams in Western Canada, certain similarities and differences become apparent.

Quaternary tephrachronology on the Iceland Plateau, north of Iceland

AbstractFour major ash zones recorded in piston cores raised from the Iceland Plateau north of Iceland are shown to be coincident with the last four interglacial isotopic stages. Their geochemical composition links the ashes to volcanic events on Iceland. The occurrence of these ash layers, which record events orders of magnitude larger than the ‘normal’ Holocene volcanic eruptions, can not be explained by changes in sea ice cover and atmospheric circulation alone. It is suggested that these events are related to pressure releases in the magma chambers resulting from major deglaciations of the Icelandic Ice Cap.

Late Quaternary stratigraphy of the northern Rockall trough and Faeroe‐Shetland channel, northeast Atlantic Ocean

AbstractA late Quaternary deep‐water stratigraphic framework has been established for the deep‐water areas (>450m) of the northern Rockall Trough and Faeroe‐Shetland Channel. Four stratigraphic units (1–4) are identified; these are primarily biostratigraphic units based on dinoflagellate cyst evidence. Unit 1 represents the late Weichselian glacial (pre‐13 000 yr BP); unit 2 the Late Glacial Interstadial (11 000‐13 000 yr BP); unit 3 is of Younger Dryas age (10 000‐11 000 yr BP); and unit 4 represents the Holocene interglacial (post‐10 000 yr BP). This stratigraphy is supported by the discovery of the mixed Vedde Ash (10 600 yr BP) and North Atlantic Ash zone 1, and the Saksunarvatn Ash (9000–9100 yr BP), concentrated in units 3 and 4 respectively. The sedimentology indicates that the oceanographic regime underwent a major change between the glacial and interglacial stages. This is marked by the onset of strong bottom current activity, allied to the restoration of overflow of the Norwegian Sea Deep Water into the North Atlantic, towards the end of the Younger Dryas Stadial. Despite intense bioturbation and bottom‐current reworking the basic stratigraphic framework is maintained. Recognition of two volcanic ash markers enables correlation with established onshore and offshore sequences of marine and non‐marine environments.

An ash fall within the Loch Lomond stadial

AbstractCores recovered from the Witch Ground Basin (central North Sea) and the northern Rockall Trough, near the Wyville‐Thomson Ridge have been found to contain volcanic glass shards. These have been correlated with the Vedde Ash Bed of western Norway, which has an age of 10600± 60yr BP, and with North Atlantic ash zone 1. This is the first time that this important chronostratigraphic marker has been identified on the UK continental shelf and it is suggested that it might also be present in northern Scotland. If so, it would be a useful tool in the correlation of terrestrial, lacustrine and marine sequences of the Loch Lomond Stadial.

A 9000-year-old ash bed on the Faroe islands

Radiocarbon datings of the Saksunarvatn ash bed on the Faroe Islands indicate an age of 9000–9100 yr B.P. The Saksunarvatn ash bed differs geochemically from both ash zone 1 in the North Atlantic and the Vedde Ash Bed of Norway and the Norwegian Sea. All mentioned ashes are assumed to originate from Iceland. The Vedde Ash has been dated at 10,600 ± 60 yr B.P. Consequently, the Saksunarvatn and Vedde ash beds provide an opportunity for precise dating of events around the Pleistocene/Holocene boundary in marine cores, especially from the region where the two plumes overlap.

Micropaleontology, Textural and Isotopic Characteristics of Turbidites on Continental Slope and Rise off Newfoundland: ABSTRACT

Graded turbidite deposits observed in x-radiographs of gravity and piston cores collected from the lower slope and rise (2,300-3,210 m) occur below a well-documented 9,300 y.B.P. ash zone. The largest turbidite observed on the upper rise is 35-cm thick. C14 dates of foraminifera tests from intervals above and below this deposit average about 13,000 y.B.P. suggesting deposition during late glacial time. The base of this deposit is marked by a textural change to comparatively coarse sediments. It is also marked by an anomalous concentration of foraminiferal species (up to 50%) belonging to the miliolid family, that constitutes 2-4% of modern assemblages. A pronounced increase in the percentage of Valvulinaria artica in the turbidite suggests a middle-slope source The thinner gravity flows (10-15 cm thick) that occur in the deeper sections of the upper-rise cores have extrapolated C14 ages of less than 22,000 y.B.P. Their foraminiferal assemblages suggest outer-shelf to upper-slope sediment sources. A comparatively thick (150 cm), graded turbidite sequence of unknown source occurs on the lower slope, just above ash zone 2 (approximate age is 64,000 y.B.P.). The related turbidity-current event apparently took place during the latter part of isotope stage 4. Texturally, this sequence is similar to the 35-cm thick upper-rise turbidite, but it lacks the anomalous concentrations of foraminiferal species in its basal interval. Oxygen isotope values (sinistral N. pachyderma) are relatively homogeneous throughout the sequence at a value of + 3.75 ppt compared to + 2.75 ppt for the upper Holocene section of the core. End_of_Article - Last_Page 305------------

A Younger Dryas Ash Bed in Western Norway, and Its Possible Correlations with Tephra in Cores from the Norwegian Sea and the North Atlantic

A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic composition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.

Sedimentology of the Minoan deep-sea tephra layer in the Aegean and Eastern Mediterranean

The Minoan eruption of Santorini resulted in deposition of pyroclastic material over a large area of the Aegean Sea and Eastern Mediterranean. The eruptive activity commenced with a plinian phase, which was followed by a phreatomagmatic phase, and ended with numerous pyroclastic flows. We present a sedimentological study of the Minoan ash deposits found in cores taken throughout the region. The cores are divided into three groups: (1) those from the sub-marine slopes immediately surrounding Santorini; (2) those from the Aegean Sea where the topography consists of many steep-sided basins; and (3) those from the Eastern Mediterranean where the topography is relatively flat and smooth. This last group is designated the Distal Ash Zone. The deposits from the Aegean Sea have been remobilised as turbidity currents. Grain-size distribution analysis reveals that these deposits are bimodal, the Md ø of the coarse population increasing away from source, whilst that of the fine population remains uniform irrespective of distance from source. The bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co-ignimbrite deposits within the turbidity current. An unusual feature found in two of the cores is a coarse, density sorted layer beneath the Bouma “a” division of several turbidite units. This is interpreted as an example of a flow head deposit as described by Sparks and Wilson (1983). The deposits from the Distal Ash Fall Zone are fine grained and unimodal, containing phreatomagmatic and co-ignimbrite material. They show only very minor remobilisation of the original air-fall ash, due to the flat topography of the area. The Md ø of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal Aegean deposits. This is interpreted as the result of aggregation of fines in the phreatomagmatic and co-ignimbrite ash clouds. The deposits in both areas have been extensively bioturbated. A study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0.62 cm thick will be completely dispersed in the surrounding sediment, whilst those thicker than 2.5 cm will be preserved as discrete layers, the base being unaffected by bioturbation. The deposits on the submarine slopes of Santorini were not originally deposited as air-fall ash, but are derived from the erosion of coastal exposures of non-welded ignimbrite. They form storm-sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island-arc volcano.

A computer model of succession and fire response of the high-altitude Eucalyptus forest of the Brindabella Range, Australian Capital Territory

The BRIND model, a computer model of the high altitude forests in the Brindabella Range near Canberra (Australian Capital Territory), is documented and the results of a series of tests on the model are provided. The BRIND model simulates a 1/12 ha forest stand by computing the growth of each individual tree in the stand. It considers establishment and death of trees on a tree-by-tree basis using stochastic functions. The model also simulates the effects of prescribed fire and wildfire on the forests. The model presently is restricted to southeasterly facing slopes (moist, sheltered situations) above 850 m in altitude. The BRIND model is tested in four ways: (1) A single example simulation (for 500years) is inspected for agreement with stand dynamics in wet sclerophyll forests. (2) By varying wildfire frequency, the model is used to develop a succession diagram for forests in the alpine ash (E. delegatensis) zone of the Brindabella Range. This diagram is considered in terms of the successional patterns described for this ecological zone. (3) By subjecting the model to different climatic conditions and wildfire frequencies, a simulated altitude zonation is developed. This simulated pattern of forest types is compared with the extant forest types in the Brindabella Range. (4) The model is tested on its ability to duplicate basal area, stocking density, and average diameters for different age stands found in an independent data set. The model was found to simulate patterns of vegetation that resemble those of the forests of the Brindabella Range in both space and time. The successional pattern was found to be complex and to differ from classic theories of succession originating with Clements. Potential model applications are discussed

Labrador Sea bio-, tephro-, oxygen isotopic stratigraphy and Late Quaternary paleoceanographic trends

A stratigraphic framework for eastern Labrador Sea cores has been developed for the interval 0–90 000 years BP through analysis of oxygen isotopes, volcanic ash, benthonic foraminifera, and the radiolarian Diplocyclas davisiana. Benthonic and planktonic foraminiferal isotope stratigraphy and the time scale of Shackleton and Opdyke provide a basis for the approximate dating of a series of marker events which include ash zones at ca. 59 000 and ≤ 21 000 years BP; benthonic foraminiferal abundance maxima at ca. 83 000, 75 000, 60 000, 19 000, and 3000 years BP; and D. davisiana percentage maxima at ca. 90 000, 73 000, 64 000, 54 000, 45 000 – 32 000, and 10 000 years BP. Incursions of subpolar planktonic foraminifera into the area during parts of isotopic stage 2 (between about 13 000 and 25 000 years BP but probably excluding the 15 000–18 000 years BP glacial maximum interval) and during the isotopic stage 4/5a transition (around 75 000 years BP) suggest that the eastern Labrador Sea was free of sea ice, at least in summer during periods of rapid continental ice sheet growth which lead to the isotopic stage 4 and stage 2 glacial maxima. A larger than normal stage 1/stage 2 difference in the isotopic composition of benthonic foraminifera (1.8‰) implies that this open water and attendant surface cooling was a potential source for colder than modern deep water. In contrast the Norwegian Sea was a reservoir of warmer than modern deep water during the last glacial.

Late Pleistocene Biostratigraphy and Paleoclimate of Gulf of Mexico Deep-Sea Cores: ABSTRACT

Studies carried out on 25 of 55 piston cores collected during Kane surveys in the Gulf of Mexico show that detailed paleoclimatic curves can be determined for the late Pleistocene based on change in frequency of planktonic Foraminifera. Three carbonate-rich cores from the southwestern Gulf of Mexico have been examined in detail and show that most of the 28 species or forms distinguished in these cores show quantitative trends in response to paleoclimatic change. During warm intervals the Globorotalia menardii complex, Pulleniatina obliquiloculata, and Globorotaloides hexagona (interglacial) are characteristic whereas Globorotalia inflata and Globigerina falconensis indicate cool intervals. A total of 3 major warmings and 2 major coolings are recorded; these probably correlate with zones Z to V of Ericson and Wollin. In addition faunal changes of less magnitude reflect secondary temperature oscillations superimposed on the more marked (glacial-interglacial) climatic oscillations. The climatic oscillations are remarkably uniform in some cores if fairly constant sedimentation rates are assumed. Volcanic ash zones in 2 cores approximately correspond to the lowermost and uppermost boundaries of the last interglacial period. In one core a large increase in Orbulina universa coinciding with these ash zones may be due to temporary environmental changes associated with extensive ash deposition. End_of_Article - Last_Page 345------------

Correlation of Pleistocene Deposits of the Central Great Plains with the Glacial Section

Integration of Pleistocene chronologies of the central Great Plains and the glaciated area is a major problem of late Cenozoic stratigraphy in North America. Lenticular deposits of volcanic ash associated with fossil mollusks occur in both regions and furnish a widespread datum for interregional correlations. The ash lentils, collectively called Pearlette, can be differentiated petrographically from other late Cenozoic ash deposits of the Plains region and have been studied at localities extending from southeastern South Dakota to northwestern Texas. The associated molluscan fauna possesses an unforeseen degree of uniformity and stratigraphic significance. The Pearlette ash and faunal zone occurs above Kansas till and below Loveland loess and Iowa till in the Missouri Valley region and is judged to be early Yarmouthian in age. A modification of stratigraphic names for Kansas contributes to uniformity of terminology in the Plains region.