Publication Date:
2024-02-05
Description:
Holocene peat bog deposits in Northem Germany have been studied by physical, chemical, and petrological methods for the presence of (microscopic) tephra layers. Aim of this study was the detection, chemical-petrological characterization and conelation of isochronous tephra layers in the peat sequences, and the identification of their volcanic sources, and contribution to a tephrostratigraphic dating framework of Northem Europe through the definition and coITelation of regionally widespread tephrostratigraphic marker horizons. The raised peat bogs studied (Jardelunder Moor, Dosenmoor/Neumünster, Grambower Moor) are located along a northwest-southeast transsect, extending from Denmark via Schleswig-Holstein to Mecklenburg-Vorpommern. Their Holocene peat deposits comprise a time span ranging from Preboreal to present. A total of four cores was analyzed, three of which represent the central ranges of the peat bogs. In order to verify the local reproducibility/persistance of tephra layers, an additional core was studied from the marginal facies of the Dosenmoor, the bog showing the highest peat accumulation rate among the sections studied. None of the cores contained visible ash layers. Moreover, volcanic ash horizons could not be detected by non-destructive physical detection methods, including measurements of magnetic suszeptibility, saturation isothermal remanent magnetization (SIRM), and detection of natural gamma and alpha ray emmissions. By contiguous sampling in 4-6 cm intervals of the up to 750 cm long cores and enrichment of inorganic components by buming and chemical digestion techniques, 37 tephra horizons were found within the mineral residue at different levels of the cores. Glass shards occur in well-defined horizons with concentrations ranging from 〈1 to 〉30 shards/cm3 sample. Single glass shards are up to 115 μm in diameter, the median of the glass shards being generelly 〈40 μm. Most glass particles are colorless, highly vesicular pumice and bubble wall shards. Brownish glass shards are rare ( 〈〈1 Vol.-% ). Individual tephra layers cannot clearly be distinguished from each other on the basis of glass shard morphology alone, except for one layer: Tephra layer DOST-6, found in all four cores, contains abundant glass shards with microlites. The chemical composition of the glass shards was determined by adjusted Electron Microprobe techniques. Glass shards analyzed are rhyolitic to rhyodacitic (Si02 〉65 wt%), and rarely andesitic (Si02 〈62 wt%) in composition. Compositionally homogeneous and petrologically consistent glass shard populations, clast morphologies, and grain size distributions indicate that the tephra horizons in the bog sediments represent primary fallout layers from specific emptive events. Based on characteristic major and minor element compositions (i.e. Si02, Ti02, FeO, MgO, CaO, Na20 and K20) of their glass shards, tephra layers from eleven explosive ernptions have been identified and conelated between cores (Dosenmoor: DOST-2 to -9; Jardelunder Moor: JART-1 and -2; Grambower Moor: GRAT-1). Nine of the tephra markers occur in at least two of the drill cores studied.The approximate ages of the tephra layers were inferred from 14C-dated organic sediments bracketing the tephra layers in the Dosenmoor core. All tephra layers identified were deposited during the past 7000 years. Explosive ernptions from Holocene volcanic fields in Europe have been evaluated as potential volcanic sources for ash layers. Most of the volcanic areas can be ruled out as the source for the tephra layers based on the comparison of the glass shards. Potassium-poor to potassium-rich subalkalic glass shard compositions con-elate with magma compositions of explosive ernptive events on Iceland, and in one case possibly of an eruption on the Eolian Islands 1400 BP (DOST-3). Four of the tephra layers found in Northern Germany were unambiguously identified as the ultra-distal deposits of explosive, compositionally zoned eruptions of Hekla volcano, which produced at least 5 major plinian fallout ash layers during the past 7000 years (Hekla 1, Hekla 3, Hekla Selsund, Hekla 4, and Hekla 5; Larsen & Thorarinsson 1977). Tephra layer JART-2 has been identified as ash from Hekla 5 eruption (ca. 6600 BP). Tephra DOST-9 has been identified as a tephra layer from the Hekla 4 eruption (ca. 4000 BP). Tephra DOST-8 is interpreteted as the distal deposit of the Hekla Selsund eruption (3500 BP). Tephra layer DOST-7 represents the Hekla 3 eruption (ca. 2900 BP). Systematic variations of glass shard compositions (Si02 = 65 to 75 wt%; K20 = 3.0 to 2.0 wt%; Na20 = 5.0 to 3.0. wt%) reflect the compositional zonation of the erupted magmas. An unambiguous correlation of tephra layers JART-1, DOST-6, DOST-5, DOST-4, DOST-3 and DOST-2 (6600 BP to 1000 BP) with specific eruptions, however, is difficult. Even though all these tephra layers show magma compositions characteristic of the Icelandic volcanic centers Torfajökull, Krafla and Katla, only one explosive Plinian eruption is known from tephrostratigraphic studies on Iceland in the appropriate time interval (Krafla 2600 BP; Volcanic Explosivity Index VEI = 4). This indicates either that at least five major Torfajökull-, Kraflaor Katla-Eruptions (VEI≥4) are not recorded by near-vent tephra deposits as yet, or that even ash material from minor Icelandic eruptions (VEI≤3) was transported to Northern Germany. Tephra horizons from Northem German peat bogs can be correlated with Late Pleistocene to Holocene ash layers in bog and lake sediments in Ireland, Scotland, Northern England and Scandinavia, and marine sedimentary records in the North Atlantic. Correlation with tephra layers from the British Isles, based on the chemical composition of glass shards, show that all tephra layers identified in older deposits (〉 1000 years) were also identified in Northern German sections (Hekla 5: JART-2/LairgA; JART-1/LairgB/Hoy; Hekla 4: DOST-9; DOST-5/Glen Garry; DOST-2/Sluggan B; oblique = local British tephra names). Two of the most prominent tephra layers found in Northern German peat sections, however, Hekla-3 ash and DOST-6 ("Microlite-tephra"), have only been identified in Northern Germany as yet. Glass particles occuring in the younger sections (〈1000 years) are equally interpreted as traces of fallout ash from explosive eruptions. Concentrations and sizes of glass shards, however, did not permit an unambiguous chemical characterization by electron microprobe analysis, and correlation with Late Holocene tephra markers in Ireland and Scotland (Öraefajökull, Hekla 1, Loch Portain A+B). Judging from ist glass particle composition, the youngest tephra studied in Northern Germany (GRAT-1) probably represents the eruption of Askja 1875 AD, but has only been detected in the easternmost Grambower Moor section. Based on the volcanic ash layers in the Dosenmoor, Jardelunder Moor and Grambower Moor, a detailed Holocene stratigraphic dating framework. has been established for Northern Germany, and combined to a Holocene tephrochronological framework of Northern Europe based on widespread, correlated tephra marker horizons on the British Isles, Iceland and Scandinavia. Individual eruptions and tephra layers form „absolute" time markers, precisely dated by AMS radiocarbon methods (i.e. Hekla-4, Hekla-5; Pilcher et. al 1995), and/or represent important marker horizons in the vegetational and cultural history. The Hekla-3 tephra, for example, marks the transition from Subboreal to Subatlantic climate in the Northern German paleoclimate record. The identification and correlation of volcanic ash layers in the Northern German peat sections extends the application of tephrochronology beyond the limits of visible tephra layers, and enables the comparison and sychronization of paleoclimate records, vegetational evolution and settlement history from the North Atlantic region to continental Europe. Aerial distribution, transport direction and tephra volume of the Icelandic ernptions were reconstructed based on the occurrence, thickness and mass of tephra deposits in Northern Europe. Distal grain-size and isomass distributions do not indicate a transport in the stratosphere or lower troposphere. Strongly diverging fallout transport direction in the near-vent facies, which is dominated by frequently changing wind directions and velocities in the troposphere (Thorarinsson 1967), contrast with regular NW-SE and N-S transport directions of Icelandic ash clouds in the distal to ultradistal (〉 2000 km) facies, at least during the last 7 000 years. Transport directions and dispersal pattem of Holocene Icelandic tephra fall out fans petfectly agree with present-day wind directions and velocities in the Northern hemispheric tropopause. Nanow, highly elongated fallout fans (Hekla-3) reflect the control of high jetstream wind velocities on the ash dispersal. A relatively low tropopause height above Iceland (8-11 km), and frequent establishment of a polar front jetstream over Northern Europe during much of the Holocene, are considered to represent the cause of the unusually wide ash dispersal. Entry into the tropopause and transport by high-speed polar front jetstreams resulted in aerial and grain-size distributions even of minor (VEI≤3) Icelandic eruptions, which are otherwise characteristic of major explosive events with orders of magnitude larger tephra and magma eruption volumes.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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