| dc.description.abstract | This Ph.D. research project develops new techniques for the detection of particulate matter and
aerosols in peat, a relatively underexplored atmospheric archive and carries out novel
investigations of atmospheric deposition at two Irish peat sites at historic and Holocene
timescales.
Multi-element (trace and minor) peat geochemistry is traditionally obtained through solution-
inductively coupled plasma (ICP)-mass spectrometry (MS) analysis. However, this technique can
be costly, time-consuming, and prone to volatile element loss. Therefore, the first sub-project
demonstrates method development for a novel, multi-element, bulk peat solid sampling technique,
using electrothermal vaporisation-inductively coupled plasma-optical emission spectrometry
(ETV-ICP-OES). Results show that sample masses are optimum for analyte detection between
0.6-2 mg. Pyrolysis (breaking down the organic matrix) at 600-650˚C is a key step in isolating
volatile elements such as Cd, Pb, S and Se and ensuring best signal detection. Evaporation at
50˚C is important in detecting loosely bound, volatile Hg. Validation using a high-C CRM
(NIST1632b), and two known mineral-rich peat samples shows successful development of this
method, with concentration (ppm) recoveries between 70-115% for Al, As, Ba, Be, Co, Cr, Cu, Hg,
Mn, Mo, Ni, Pb, S, Si, Sr, Ti, Tl, V, Zn and Zr.
Cryptotephra (distal microscopic volcanic ash) in peat and sediment samples are traditionally
separated for geochemical characterisation using chemical or density floatation techniques
following initial tephra identification and shard counting through analysis of inorganic residue via
light microscopy. However, these practices can be time consuming, subject to practitioner
experience and material type, with a potential for sample loss. Therefore, the second sub-project
presents an alternative, more automated approach to identify cryptotephra in peat and
sedimentary samples, whereby inorganic material is mounted directly in epoxy resin and analysed
through back scattered electron (BSE) imaging via scanning electron microscopy-energy
dispersive X-ray spectroscopy (SEM-EDX). Cryptotephra is identified by unsupervised
geochemical maps. This method proves successful and shows 96% recovery of a known
proportion of Icelandic tephra in peat. We apply the method to a minerotrophic peat from
Brackloon Wood, Mayo, Ireland, where the Laki AD 1783-84 cryptotephra is successfully
identified.
Records of volcanic heavy metal and sulphur deposition are notoriously restricted to the poles.
Therefore, in the third sub-project, bulk minor, and trace element geochemistry of Brackloon
Wood, the site of the Icelandic Laki AD 1783-84 cryptotephra deposition in Ireland is investigated
using the novel ETV-ICP-OES method and traditional Hg analysis using atomic absorption
spectrometry (AAS). Results show enrichment of heavy metals (e.g., Hg, Cu, Ni, Cd, Co, Se, Tl)
and S, coincident with the deposition of the Laki AD 1783-84 cryptotephra. A volcanic aerosol
origin is demonstrated for enrichments of Hg, Ni, Cu, S, Cd, Co, Se, Sb and Cr, with significant
loads from Hg, Cu and Ni. Correlation to Laki aerosols in Greenland ice is evident for Cu, Cd and
S. This study represents the first findings of deposited volcanic aerosols for this event outside of
the poles and further strengthens the use of peat as an archive for volcanic aerosol deposition,
whereby novel techniques such as ETV-ICP-OES may prove highly useful in future studies.
Northern Ireland is highly sensitive to atmospheric circulation in the North Atlantic region.
However, multi-element peat geochemistry in this region is wholly unexplored. In the fourth sub-
project, an analysis of geochemical enrichment is carried out at Fallahogy Bog, Co. Derry for the
first time, to chemically identify the change from minerotrophic to ombrotrophic conditions and
subsequently investigate atmospheric deposition in this region throughout the Holocene. Icelandic
Hekla AD 1947, Hekla AD 1104 and Mexican MOR-T4 cryptotephra layers are identified using the
novel back scattered electron (BSE) geochemical mapping technique and are used to reconstruct
the chronology of the site, combined with radiocarbon dating. Calcium/Mg molar ratios identify the
transition to ombrotrophic conditions at 6544 cal. yrs. BP and prove as a more precise indicator
of this transition compared to classic plant-stratigraphical approaches. Enrichment factor
calculations and principal component analysis reveal distinct zones of multi-element lithogenic
dust enrichment linked to humification analysis indications of wetter surface conditions between
~9000-2700 cal. yrs. BP. Analysis of enriched lithophile element ratios (REE, Zr, Th) show mixed
source between local and Saharan dust. Deposition of Saharan dust appears linked to storm-
related transport and dust scavenging via the North Atlantic Oscillation, especially during the Early
and Middle Holocene. Enrichments of more volatile heavy metals (Cd, Hg, As, Zn, Sb) indicate
deposition of volcanic aerosols coincident with the known age ranges for the Lairg A and Lairg B
cryptotephras from the Icelandic eruptions of Hekla and Torfajökull volcanoes respectively. This
sub-project highlights Northern Ireland as a key region for future high-resolution studies of
Saharan dust and volcanic aerosol deposition, to compliment polar and European records. | en |
