91探花

Introduction

The tephrostratigraphy group in MuHS investigates the occurrence, characteristics and ages of tephra horizons in geological profiles on land and from the seafloor. Highly explosive magmatic or phreatomagmatic eruptions can generate huge ash clouds directly from vent or as co-ignimbrite clouds which disperse huge volumes of mostly ash-grade tephra over large areas across land, lakes and the ocean. The resulting ash layers thus can extent across different depositional environments and are practically instantaneously emplaced, which makes them ideal chronostratigraphic marker horizons across land-sea and sediment facies boundaries, particularly because they can mostly be identified by their unique compositional characteristics. Moreover, these ash beds can be relatively precisely dated either by direct radiometric dating or by other geological methods. These often thin and fine-grained ash beds are best preserved in large submarine regions that are unaffected by erosion and carry relatively little fauna producing bioturbation, whereas distal ashes on land are easily washed off.

Distal marine ash layers are an important tool in volcanology where they help to improve determinations of magnitude, intensity and volatile budgets of past eruptions initially derived from proximal deposits, to identify and date volcanic events not recognized on land, and to reveal the temporal succession of physical and geochemical changes of magmatic systems. Marine ash layers are also important to reconstruct non-volcanic geologic processes, such as determination of laterally and temporally variable sedimentation rates, correlation of geologic events across environmental and facies boundaries, or determining the ages and rates of geological processes (e.g.basin evolution) and events (e.g. landslides) and paleo-environmental changes.

One focus of the tephrostratigraphy group lies on the temporal correlations between ash layer successions and paleoclimate and environmental proxies. Such correlations reveal that it is not only volcanic emissions (SO₂, CO₂, halogens) which can influence global climate and atmosphere chemistry, but that global climate changes can also exert some control on the frequency of volcanic eruptions.

Offshore tephrostratigraphy

We sample marine tephras by using various coring techniques, from shallow gravity cores extending into approximately 10⁵ years old sediment, to the advanced drilling techniques in the continental (ICDP) and deep sea (IODP) drilling programs, which reach sediments tens of millions of years old. Cores can contain tens to hundreds of tephra layers and each needs to be investigated in order to distinguish primary tephras, emplaced directly by volcanic eruptions and thus perfect isochrons, from layers of re-sedimented tephra which would yield false isochrons. Since one core provides only a 1-D record of a tephra succession which may also be incomplete due to local erosion, tephra horizons are best correlated between a number of spatially distributed cores in order to obtain a 3-D tephrostratigrafic framework. Such correlations are mostly done by comparing glass-shard geochemical compositions. Mostly we use major-element geochemical compositions measured by electron-microprobe (EMP) spot analyses, but where these are not distinctive enough, we apply advanced trace-element spot analyses by laser-ablation inductively coupled mass spectrometry (La-ICPMS). Erupted tephra does not always occur as a distinct layer, but also as ash grains dispersed in marine sediment over a limited depth interval. Thus, we also analyze cored sediment for their tephra contents. Moreover, since the same approaches used with marine tephra layers also apply to continental lakes, we also investigate the tephra record of lacustrine sediment core which offer the comparison with high-resolution and high-sensitivity paleoclimate and paleoenvironmental records.

Onshore tephrostratigraphy

The characteristics of highly explosive eruptions, such as ejected tephra volume, eruption intensity, eruptive mechanisms and eruption column heights, cannot be determined from distal (marine) tephra alone, but require the volcanological study of the more complex and much thicker proximal deposits on land close to the volcanoes. This reveals the processes and dynamics of an eruption and thus characterizes those eruption phases that were responsible for the emplacement of the distal marine tephra layers. Moreover, the much coarser (lapilli and blocks) juvenile clasts which contain magmatic textures in context, and lithic clasts which record the volcanic basement and magma reservoir environment, are needed to investigate the petrogenetic processes of magma evolution that ultimately led to the highly explosive eruptions. The complete characterization of compositional zonation of the proximal deposits also facilitates the correlation of distal marine tephra with the eruptive history. Therefore, we log, map and sample volcanic deposits on land in presumed source regions of marine tephras.