Planktic foraminifera in surface sediments around the Cavalli Islands, northern New Zealand

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TANE
25, 1979
PLANKTIC
FORAMINIFERA
IN
SURFACE
SEDIMENTS
AROUND
THE
CAVALLI
ISLANDS,
NORTHERN
NEW
ZEALAND
by
Bruce W.
Hayward
New Zealand
Geological
Survey,
P.O. Box
30368,
Lower Hutt
SUMMARY
Planktics
comprise 0 to 10% of the
foraminiferal
faunas of 55 dredge
samples taken
from
surface sediments at
depths
of 0 to 41m around the
Cavalli
Islands. Samples
from
10m and shallower average 0.8% planktics,
whereas samples
from
15 to 41m average 2.7% planktics. The fauna
con-
tains 17 species, dominated by
Globorotalia
inflata and
Globigerina
falconensis. Two of the remaining species are most characteristic of the sub-
polar,
and eight of the subtropical, planktic
foraminiferal
provinces. The
overall
composition and diversity of the
Cavalli
Islands' fauna
would
place
it
in the warm part of the transitional province, between subpolar and sub-
tropical.
INTRODUCTION
Fifty-seven
dredge samples of surface sediment were taken at
depths
from
0
to 41m around the
Cavalli
Islands (Fig. 1), during the Offshore Island
Research
Group's trip there, over the New
Year
period, 1978-1979. The
Cavalli
Islands (longitude 174
°E,
latitude 35
°S)
lie some 2-3km off the
east
coast of Northland, between the Bay of Islands and Whangaroa Harbour
(Fig.
1).
The
dredge was made
from
a one
gallon
petrol tin,
similar
to
that
described
by Grace and Whitten (1974), and sampled to a sediment depth of
approximately
6cm. A
split
was taken
from
55 of the dredge samples for
laboratory
processing and
foraminiferal
studies and the remaining material
was analysed in a macrobenthos study (Grace and Grace in prep.). The
foraminiferal
splits were preserved in 10% ethanol and on return to the
laboratory,
they were thoroughly washed over a 200-mesh sieve (nominal
opening
0.075mm), stained
with
Rose
Bengal
(a stain for protoplasm, used
here
to
identify
benthic foraminifera
that
were
living
at the time of
collec-
tion),
thoroughly washed again to remove all excess stain, and
finally
dried.
Each
dried sample was
divided
up using a
microsplitter,
until
only
enough
sediment was left to contain approximately 300 benthic foraminifera. All
the foraminifera (both benthic and planktic) in the microsplits were
picked
and
mounted on slides for
identification
and census counts.
This
note
reports on the planktics; the much larger study on the benthics is to be
presented elsewhere.
149

150

RELATIVE
ABUNDANCE
OF
PLANKTIC
FORAMINIFERA
The
percentage of foraminifera in a sample
that
are planktic is often used
by
paleoecologists as a guide to the distance
from
shore and consequently
also
the depth of
fossil
samples at the time of accumulation
(Vella
1962).
This
is because the number of planktic foraminifera
living
in the surface
waters of the ocean generally increases
with
distance
from
shore
(Boltov-
skoy
and
Wright
1976).
The
percentage of planktic foraminifera in census counts of 300 benthic
foraminifera
in the 55 samples
from
around the
Cavalli
Islands ranges
from
0
to 10% (Fig. 1). There is some correlation between the number of
planktics
present and the distance
from
shore, but better correlation
with
depth. There appears to be
little
correlation
with
sediment grain size
(unpublished
data).
(a) Samples
from
10m depth or shallower: 75% of
these
have 1% or fewer
planktics;
all the remainder have between 1% and 3% planktics.
(b) Samples
from
15m to 41m: 30% of
these
have 1% or fewer planktics;
50%
have between 1% and 3% planktics; the remainder
(three
samples)
have 4%, 6.5% and 10% planktics,
from
depths of 41m, 41m, and 20m
respectively.
The
three
samples
with
more than 3% planktics all occur in the outer part
of
the
Cavalli
Passage and are undoubtedly more exposed to the open ocean
than most other stations.
Approximately
16 500 benthic and 206 planktic foraminifera were
obtained
from
the microsplits of the 55 samples.
Overall
therefore in the
sediments surrounding the
Cavalli
Islands at depths
from
0-40m, planktics
average 1.3% of the foraminifera. In samples
from
0 to 10m depth they
average 0.8% and in samples
from
15 to 41m depth they average 2.7%.
These low planktic percentages are
similar
to those reported in many
other parts of the
world
for shallow, near-shore
areas
(Boltovskoy
and
Wright
1976), but is in
bold
contrast to the much higher abundances of
planktics
(13-17%) found in intertidal samples on the west coast of
North-
land
(Hayward 1979). The study by
Hulme
(1964)
would
tend to support the
observation
that
planktic foraminifera occur in far greater abundances in
near-shore sediments on the west coast of
Northland
than on the
east
coast.
The
reasons for this are probably related to the regional mean
wind
pat-
terns: onshore on the west coast, offshore on the
east.
TAXONOMIC
COMPOSITION
(Table 1)
The
most common species present around the
Cavalli
Islands are
Globorotalia
inflata
(Fig.
2h) and
Globigerina
falconensis
(Fig.
2c).
Co-occurring
species in order of decreasing abundance are
Globigerina
bulloides (Fig.
2a), G. quinqueloba (Fig. 2i),
Globorotalia
truncatulinoides (Fig. 2k),
151

Table 1.
Percentage
of
each
species
of
planktic
foraminifer
in the
total
count
of
planktics
(206
specimens)
obtained
from the
microsplits
of 55
samples
from
around
the Cavalli
Islands.
Globigerina
bulloides
d'Orbigny
Globigerina
cf.
calida
Parker
Globigerina
falconensis
Blow
Globigerina
juvenilis
Bolli
Globigerina
quinqueloba
Natland
Globigerinita
sp.
Globigerinoides
ruber
(d'Orbigny)
Globigerinoides
trilobus
(Reuss)
Globorotalia
dutertrei
(d'Orbigny)
Globorotalia
hirsuta
(d'Orbigny)
Globorotalia
inflata
(d'Orbigny)
Globorotalia
menardii
(Parker,
Jones
& Brady)
Globorotalia
scitula
(Brady)
Globorotalia
truncatulinoides
(d'Orbigny)
Hastigerina
siphonifera
(d'Orbigny)
Orbulina
universa
(d'Orbigny)
Pulleniatina
obliqueloculata
(Parker
&
Jones)
8.5
32.5
x
6.5
3.5
2.5
1.5
x
32.5
x
2
4.5
2.5
x
x
Globoigerinoides
ruber
(Fig. 2d), G. trilobus (Fig. 2e), Hastigerina
siphonifera, Globorotalia scitula (Fig. 2j), G.
dutertrei
(Fig. 2f),
Globigerina
cf. calida
(Fig.
2b) and six other species
represented
by only one
specimen each. Most specimens are juveniles or small adults; large adult
specimens are uncommon.
The overall composition and diversity of the planktic foraminiferal fauna
around the
Cavallis
is remarkably similar to
that
recorded from sediments
at 2000m depth off the
east
coast of Northland (Kustanowich 1963, samples
A178,
A181),
and from plankton tows
made
100-200km
north-east
of
Northland
(Eade 1973). The only differences may be
that
G. inflata is
slightly
less
abundant
in
these
near-shore
samples than offshore, where it
commonly
comprises over 50% of the planktics; and the G. bulloides—G.
falconensis group and G. quinqueloba may be somewhat more common
inshore.
Five
planktic foraminiferal provinces or zones are generally recognised
throughout the world's oceans—equatorial-tropical, subtropical, transi-
tional,
subpolar, polar (Be 1977, Boltovskoy and Wright 1976). These pro-
vinces are roughly parallel latitudinal belts
that
are duplicated on either side
of
the equatorial-tropical province. The transitional faunal province,
characterised by
abundant
G. inflata and a diverse fauna of mixed
cool
and
warm
water species, is usually drawn as a narrow belt passing through most
of
New Zealand's waters, with the subtropical province impinging on
Northland
and the subpolar province immediately south of Stewart Island
BIOGEOGRAPHIC COMPARISONS
152

Fig.
2. Planktic foraminifera
from
sediments
around the
Cavalli
Islands, a. Globigerina
bulloides,
x 80, FP2908; b. Globigerina cf.
calida,
x 100, FP2909; c. Globigerina
falconensis.
x
135, FP2910; d.
Globigerinoides
ruber,
x 100, FP2911; e.
Globigerinoides
trilobus,
x 65,
FP2912; f. Globorotalia
dutertrei,
x 100, FP2913; g. Globorotalia
hirsuta.
x 80, FP2914; h.
Globorotalia
inflata,
x 80, FP2915; i. Globigerina
quinqueloba,
x 190, FP2916; j. Globorotalia
scitula,
x 135, FP2917, k. Globorotalia
truncatulinoides,
x 55. FP2918; 1. Pulleniatina
obliqueloculata,
x 80, FP2919. (Number prefixed by FP are N.Z. Geological Survey
catalogue
numbers.)
153

(Eade 1973, Be 1977).
The
Cavallis
Islands' planktic
foraminiferal
fauna must be considered to
lie
within
the transitional province because of the abundance of G. inflata,
G. falconensis, and the
mixed
warm and
cool
water elements in the rest of
the fauna.
Many
workers consider G. falconensis to be a warm water
form
of
G. bulloides and to occur in
greatest
abundance in the warmer parts of
the transitional province and cooler parts of the subtropical province (Kennett 1967, 1976; Be 1969, 1977).
Most
of the co-occurring species
from
around
the
Cavalli
Islands occur quite commonly in the transitional pro-
vince
elsewhere in the
world
(Be 1977,
Boltovskoy
and
Wright
1976),
especially
G. scitula, G. truncatulinoides and
Orbulina
universa. Species
that
are more characteristic of the subpolar province are G. bulloides and
G. quinqueloba, and those more
typical
of the subtropical province are G.
cf.
calida,
G. ruber, G. trilobus (possibly a variant of Globigerinoides
sacculifer),
Globorotalia
hirsuta (Fig. 2g), G. menardii, G. dutertrei, H.
siphonifera
and
Pulleniatina
obliqueloculata
(Fig.
21).
The
large number of subtropical species are probably carried down to the
east
coast of
Northland
in southward
moving
currents derived
from
the sub-
tropical
waters of the
east
flowing,
East
Australia
Current
gyral
(Eade
1973),
and together
with
the abundance of G. falconensis, tend to support
the placement of the
Cavalli
Islands' fauna in the warmer part of the
transitional
province.
ACKNOWLEDGEMENTS
I am
very grateful
for the
help
received
from
Roger
Grace in
collecting
the
dredge
samples.
I
also
wish
to
thank
Tony Edwards and
Norcott
Hornibrook for reading the
manuscript
and
suggesting
improvements.
The
script
was
typed
by Fay Tonks, and
SEM
photographs
taken
by
Barry
Burt.
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A.W.H.
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marine
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