Jökull - 01.01.2009, Blaðsíða 55

Holocene sediment- and paleo-magnetic characteristics from the Iceland and E-Greenland margins
Because of the numerous adjustments that have to be
made to derive a single declination record (Stoner et
al., 2007)we use only the inclination data. In this case
the assumption that is universally made in marine pa-
leomagnetic studies is that the core was truly vertical
when collected. Usually there is no instrumentation
on the corer to test this assertion, but in most instances
it is a reasonable assumption (Stoner and St-Onge,
2007). In our study the results from JM96-1216 may
indicate that the core penetrated the sediment at an an-
gle. During the Holocene the gradual compaction of
the sediment (Andrews et al., 2002a) might lead to a
decrease in inclination, however, no such trend was
noticed.
RESULTS
1. Magnetic susceptibility
The three measures of magnetic susceptibility are
whole-core (WCMS, 10"5 SI), u-channel (10"5 SI),
and mass (dry sediment) magnetic susceptibility
(massMS 10"7 m3 kg"1). In JM96-1232 (NW Ice-
land) the correlation between the whole-core and u-
channel median values is only r = 0.02 whereas the
WCMS versus the massMS has r = 0.96. An error
in the u-channel measurements is considered the cul-
prit. Spatially the massMS data show a distinct differ-
ence between the two margins (Figure 2A). The Ice-
land data has a range in median values between 20
and!55"10"7 m3 kg"1. Values within the Vestfirðir
main fjord and trough system (Djúpáll) are higher
than data to the north (cores #317, #321, and #330), or
south (#347). The median East Greenland values are
much higher with a range between 65 and 82"10"7
m3 kg"1.
The median wt% of magnetite, determined by
XRD, varies linearly (r = 0.82) with median massMS
for the 5 Iceland sites with an increase of 22.5"10"7
m3 kg"1 per unit wt% (not shown). However, the
three East Greenland sites showed the opposite trend
(r= -0.98). Some of the differences in magnetic sus-
ceptibility are probably associated with dilution by
diamagnetic minerals such as quartz, carbonate and
carbon, and the amount of volcanic glass in the sed-
iment (Appendix 2). Quartz has much higher wt%
values off East Greenland, whereas carbonate and car-
bon are significantly higher around Iceland (Andrews
et al., 2002a) (Appendix 2). The Saksunarvatn tephra
can be detected in most cores by a pronounced re-
duction in massMS (Andrews et al., 2002b). Our
data (Appendix 2) indicates a strong negative associa-
tion (r2 = 0.74, n=8) between wt% volcanic glass and
massMS; 95% of the variance in the massMS is ex-
plained by the combined effects of carbonate, quartz,
and glass.
The CV% of massMS data clearly indicates a
low level of variability at East Greenland sites (CV%
<10), close to the major glacial sediment inputs (An-
drews et al., 1994). On the Iceland margin the CV%
values are generally >25% except for one site off
North Iceland (#317) and one inner fjord site from
Vestfirðir (#342).
2. ARM susceptibility
There is a strong correlation between the median
massMS and kARM with r2=0.83 (Figure 3A) indicat-
ing that the dominantmagneticminerals are magnetite
of a consistent grain size. For individual cores the cor-
relation between these two parameters is often r2 >0.8
but in three cores there is little association. The spa-
tial distribution of the medianmass data (Figure 2B) is
similar to bothWCMS andmassMS.MedianmassMS
and kARM are strongly correlated (Figure 3A). The
scatter plot of massMS versus kARM indicates that the
two regions plot along a common regression line, but
are distinct in terms of their values, with East Green-
land sites having higher values. Part of this might be
associated with the higher sediment densities off East
Greenland versus Iceland (Andrews et al., 2002a).
3. Magnetic ratios
The progressive demagnetization of samples in the
AF field allows a variety of ratios to be obtained
(Maher and Thompson, 1999), which represent dif-
ferent responses associated with coercivity, magnetic
grain-size, and mineralogy (Heider et al., 2001). For
each core, least squares regression equations were
calculated for each ARM demagnetization step, e.g.
ARM(J10) versus ARM(J0). The slope of the rela-
tionship expresses the proportional reduction in ARM
intensity after each demagnetization step. The re-
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