Jökull - 01.01.2012, Side 61

Holocene marine tephrochronology on the Iceland shelf
et al., 1993; Ruddiman and Glover, 1972; Lacksche-
witz and Wallrabe-Adams, 1997; Austin et al., 2004;
Brendryen et al., 2010). Other processes known to in-
fluence tephra deposition in marine environments in-
cluded reworking by currents, bioturbation and mass
movements (Brendryen et al., 2010). Sea ice or ice-
berg transported tephra may contain ice rafted detritus
(IRD) as additional components of grains of differ-
ent provenance (Lackschewitz and Wallrabe-Adams,
1997) and poorly sorted tephra grain size distribu-
tion (Austin et al., 2004). Signs of bioturbation
are indistinct boundaries of the tephra layers, espe-
cially the lower boundary (Ruddiman and Glover,
1972). Reworking by currents and mass movements
are reflected in grain size distribution and most likely
grain morphology where remobilized grains display
rounded form. Results from morphological measure-
ments on tephra layers in marine sediments from the
North Iceland shelf show that fresh glass grains are
more rugged and/or elon- gated than the background
material. This demon- strates the usefulness of grain
morphology measure- ments in determining whether
a tephra horizon is a primary deposit or a layer of re-
worked tephra (Gudmundsdóttir et al., 2011a).
Defining cryptotephras in marine environments,
i.e. their boundaries and the isochrone position, is de-
pendent on grain counting or shard concentration and
grain size analyses (e.g. Lacasse et al., 1996; Hafliða-
son et al., 2000; Jennings et al., 2005; Austin et al.,
2004; Kristjánsdóttir et al., 2007; Gudmundsdóttir et
al., 2011a; Davies et al., 2012). In a volcanogenic en-
vironment, where the majority of the inorganic mate-
rial is volcanic glass as on the Iceland shelf, additional
methods such as grain morphology and geochemistry
are needed to define the boundaries and isochrone po-
sition (Gudmundsdóttir et al., 2011a). Determining
the exact stratigraphical position of the time signal the
tephra layers provide is a prerequisite for using them
as chronostratigraphic horizons or isochrones. In gen-
eral the peak concentration or first abundance peak is
considered to reflect the exact timing of the volcanic
eruption (Ruddiman and Glover, 1972; Jennings et al.,
2002; Davies et al., 2012). A multi-parameter study
of tephra layers from marine sediments on the Ice-
land shelf suggest that the exact timing is were the
rate of change in these parameters, grain concentra-
tion, grain size, grain morphology and chemical com-
position, is greatest from background levels below the
tephra (Gudmundsdóttir et al., 2011a). In some cases
vertical shard concentration profiles are complicated
if there is no distinct peak or there are more than
one (e.g. Davies et al., 2007; Pyne-O’Donnell et al.,
2008). Therefore, detailed grain counting, grain size,
grain morphology and chemical analyses in addition
to careful consideration of sedimentation processes
are needed when working with marine tephra layers.
TEPHRA LAYERS ON THE ICELAND
SHELF
The number of tephra layers identified on the shelf
around Iceland so far, is about 130 spanning the Late
glacial and Holocene where the oldest tephra layer is
about 15,000 cal. BP and the youngest from AD 1947
(Kristjánsdóttir, 1999, 2007; Eiríksson et al., 2000,
2004, 2011; Jónsdóttir, 2001; Andrews et al., 2002,
2003; Geirsdóttir et al., 2002; Søndergaard, 2005;
Knudsen et al., 2008, 2011; Thornalley et al., 2011;
Gudmundsdóttir et al., 2011a,b, 2012). However it
can be expected that sediments on the shelf of Iceland
extend back to the Miocene time period (Verhoeven et
al., 2012) and thus older tephra layers can be antici-
pated within the shelf sediments.
Tephra layers in the shelf sediments around Ice-
land have been identified in 30 marine cores (Figure 1,
Tables 1 and 2) obtained in five research cruises; the
BIOICE cruise in 1995 (HM107-) with R/V Håkon
Mosby (Eiríksson et al., 2000), in 1997 with R/V
Bjarni Sæmundsson (B997-) (Helgadóttir, 1997), in
1999 on MD114 IMAGES V cruise (MD9922-) with
the R/V Marion Dufresne (Labeyrie, 2003) and in
2004 on RAPiD cruise (RAPiD-) with RSS Charles
Darwin (McCave, 2005). The IMAGES cores MD99-
2271, -2272, -2273 and -2275 were supplemented
with multicores and box cores (B05-2006) during the
Millennium cruise with the R/V Bjarni Sæmunds-
son in 2006 (Eiríksson and Bartels-Jónsdóttir, 2006).
Tephra layers have been identified in multicores B05-
2006-MC03, at the location of core MD99-2273,
and B05-2006-MC04 at core site MD99-2275. The
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