n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

,2 I I I I I I I
150 roo SO o 50 IOD 150
WEST WEST M E EAST
OISIANCE FROM WEST EME OF COAST PLUTONlC COMPLEX. IN KILOMETERS
18 16
F$ure 8%-A Ofg 0 fractionatibn between quartz
and plagioclase (A 0 ) of analyzed samples versus
distance of sample lo@ly from the west edge of the
Coast plutonic complex. Triangles, samples analyzed
in this study; dots, samples analyzed by Magaritz and
Taylor (1976).
whether any major geotechical trends are paralleled
by faunal trends.
Initial sample gees and vertical spacing of
samples were limited by availability of material from
the cores; thus, all the ostraccde assemblages contain
small numbers of individuh. The ostracode fama
from each sample was tabuleted dd compared with
the distinct depth assemblages I have determined for
the modern ostracode faunas in this region.
The cores were taken in water depths of 82 to
156 m, which incorporates the mlddle neritlc (50-100
rn) and outer neritlc (100-200 m) depth zone. The
present environmental conditions for middle neritic
depths include moderate variation in annual bottom
temperatures (2°-80~) and slight variatfon of bottom
salinity (31-32 ppt) (Royer, 1975). In cantrest, the
outer neritic zone is slightly more stable, with
maximum annual bottom-temperature variatlona of 2'
to ~OC, though primarily from 2' to ~OC, and bottom
salinity of aWut 32 parts per thousand. Winter
downwelling and storms further cause an overturn of
low-temperature surface waters to the bottom.
The ostracde faunas from cores 'IOQC, 711, and
715 have been interpreted to represent two
components. The first component (A) consists of a
fauna that presently lives in this region In an
environment of deeper middle neritic to shdow outer
neritic conditions. In addition, a large number of inner
neritic ostracode specles wcur In this component;
some of these specles are probably Living at the edge
of their habitat, whereas others are being transported
in by downslope sediment movement. The
transportation espects are well Wustrated by selective
movement of the Ughter, smaller juveniIe specimens
(Brouwers, 1980). Component A comprhes the upper
parts of cores 709C, 715, and 7048 and all of core 711
(flg. 92).
The second component (131 consists of an
ostracode fauna that lives today primarily In outer
neritic depths, where the annual-bottom-temperatupe
variation is reduced and summer maximum
temperatures are lower. Aspects of the shallower
neritic depths are greatly reduced and may be largely
accounted for by offshore transport and by species
living at the edge of their habitats. Component B
occurs at the bottoms of cores 709C and 715. This
subtle change h fauna can be interpreted to represent
a different thermal regime than that which prevails
today; this difference could be due to either colder
temperatures or lesser variation in the annual range of
bottom temperature.
Wgh-resolution cllmatfc cycles (on the order of
the SO-year cycles determined by dendrochronology)
were not observed in the cores. I interpret thk result
as due to two factors. First, the sample dzes were too
small to make the desired lqer, more representative
oounts of the ostracode populations. Second, and more
importantly, the offshore environment is more
ameliorated In comparison with to the onshore
cycles. The mixing by current systems and winds and
the large water masses involved do not respond so
quickly to smaller scale changes in seasonal climates,
especially in depths of greater than 100 m. A better
area to look for these changes in the marine record Is
In shallow bays. However nearly all the bays In the
Gulf of Alaska have short (less than 100 years) records
because of the presence of more advanced glaciers and
consequent bottom scouring during the 20th century.
Core 7040 was subsampled because it was taken
In an area of nondeposition, and the likelihood of
occurrence of the Holocene/Pleistocene boundary in
this cote was large. The upper part of this core can be
correlated in terms of similar environment with
component A of the other three cores; below this
interval are several astraeode speaies that have not
been documented to date as living today in the Gulf of
Alaska, and are interpreted as being fossil (that is,
extinct in thk region) (component C). These fossil
species have been found in oolder environments, such
as the southern Bering Sea. In addition, a bwer
sedimentation rate Is fndicated by the greater number
of adult versus juvenlle epecimens, especially in
comparison with the other three cores. himarib on
the basis of tile presence of these fmll specfes and, to
a lesser extent, the appearance of a change of
preservation of the specimens, this lower interval of
the core suggests that older, probable Pleistocene
sediment Is present, dating from nn environrn ent with
colder water conditions.