Jökull - 01.01.2012, Síða 59

Holocene marine tephrochronology on the Iceland shelf
sea surface temperatures due to intensified influence
of Arctic waters of the East Icelandic Current. These
climate changes were then compared with records in
e.g. Greenland ice cores, documentary records from
Iceland, as well as terrestrial evidence of sea-ice,
glaciation extent and vegetation changes. The com-
parison shows that the palaeoceanographic record on
the North Iceland shelf reflect regional rather than lo-
cal climate signals and they demonstrate that it is pos-
sible to study leads and lags in the atmosphere-ocean
interactions when the marine records can be reliably
dated with tephrochronology.
Ocean reservoir age variability
Marine tephra layers have been used to estimate
changes in the ocean reservoir age (e.g. Austin et al.,
1995; 2011; Haflidason et al., 2000; Eiríksson et al.,
2000, 2004, 2011; Jennings et al., 2004; Sejrup et
al., 2010, 2011; Thornalley et al., 2011). The ma-
rine reservoir age is the difference between a terres-
trially dated sample and material dated from the ma-
rine environment. This difference is on average about
400 years (Stuiver and Braziunas, 1993). This ap-
parent age difference is caused by delay in exchange
rates between atmospheric CO2 and dilution effect
caused by mixing of surface waters with upwelled
deep waters (Mangerud, 1975). By correlating terres-
trially 14C dated tephra markers to their counterpart
in marine environments the reservoir age at that time
can be evaluated. On the Iceland shelf tephra layers
erupted from Icelandic volcanoes have been used to
collect information on changes in reservoir ages of the
water masses around Iceland during the Late glacial
and the Holocene (Eiríksson et al., 2000, 2004, 2011;
Jennings et al., 2002; Thornalley et al., 2011). The
discrepancy between reservoir corrected radiocarbon
dates of marine material and tephrochronological age
models is reflected in deviation of tens to hundreds
of years as observed on the North Iceland shelf (Jen-
nings et al., 2002; Eiríksson et al., 2000, 2004, 2011;
Kristjánsdóttir, 2005). Determining the reservoir ages
of the ocean through the geological record is very im-
portant because reservoir age correction needs to be
applied to a conventional marine 14C age to correct
for growth in a non-atmospheric (i.e. marine) carbon
reservoir (Stuiver et al., 1986). As previously noted
correction of 400 years is used but recent research
has shown that an additional correction is needed for
certain time intervals of the geological record. Thus
to enable a better comparisons of marine and terres-
trial palaeoclimatic proxies knowledge on temporal
and spatial variations in the ocean reservoir age is im-
portant. Moreover changes in reservoir ages may give
information on palaeoceanographic changes through
time as demonstrated from studies on the North Ice-
land shelf were higher reservoir ages are interpreted to
represent influence of cold Artic water (East Icelandic
current) but lower reservoir ages warmer Atlantic wa-
ters (Irminger current) (Eiríksson et al., 2004, 2011).
Eruption history
Tephra layer frequency has been used in Iceland to
infer explosive eruption frequency and history of vol-
canic systems (e.g. Thorarinsson 1967; Larsen et al.,
1998; Larsen and Eiríksson 2008a,b, Óladóttir et
al., 2008, 2011a). Correlation of Late-glacial and
Holocene high-resolution marine tephra stratigraphy
from the North Iceland shelf to high-resolution terres-
trial tephra stratigraphy in Iceland reveals the same
trend in tephra layer frequency in both regimes in-
dicating that the marine tephra stratigraphy can in
fact be used to gather information on past explosive
volcanic activity in Iceland (Gudmundsdóttir et al.,
2012).
IDENTIFICATION, ORIGIN AND INTEGRITY
OF MARINE TEPHRA LAYERS
Various methods are used to identify tephra layers
in marine environments. The methods that are most
commonly used are visual inspection, X-ray pho-
tographs, magnetic susceptibility (MS), grain size
analyses and counting of glass grains (e.g. Lacasse
et al., 1996, Eiríksson et al., 2000; Jennings et al.,
2002; Austin et al., 2004; Kristjánsdóttir et al., 2007;
Brendryen et al., 2010; Gudmundsdóttir et al., 2011a,
2012; Davies et al., 2012). A majority of the marine
tephra layers are not visible to the naked eye and of-
ten referred to as cryptotephra (Lowe and Hunt, 2001;
Turney et al., 2004). The shelf sediments around Ice-
land have a dominant volcanogenic origin adding to
the complexity of locating and identifying tephra lay-
ers. Whole core methods such as MS and X-ray pho-
JÖKULL No. 62, 2012 57