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The abundance and species composition of nannofossil assemblages in sediments from continental shelf to offshore basin, western Tasman Sea
Deep-Sea Research, 1975, Vol. 22, pp. 425 to 431. Pergamon Press. Printed in Great Britain.
The abundance and species composition of nannofossil assemblages in sediments from continental shelf to offshore basin, western Tasman Sea D. A. BURNS* (Received 12 October 1973; hz revi6ed form 22 October 1974; accepted 30 October 1974)
Abstract--Abundance and species composition of recent nannofossil assemblages are described from continental shelf, slope, rise and offshore basin sediments (36 to 37°S Lat.). Shelf and upper slope assemblages contain extremely few nannofossil specimens and species. Numbers of specimens and species diversity increases from slope to basin areas. The overall nannofossil content of the sediment is divided into three groups: all very small species; Gephyrocapaa oceanica; the remaining large species. It is the abundance of the large species which changes most noticeably between shelf, slope, rise and basin areas. Some individual species have specificdistribution patterns. Some decrease and others increase in proportion from shelf to basin. Comparing shelf, slope and rise assemblages with basin assemblages indicates that shelf and slope assemblages are not indicative of offshore oceanic hydrological regimes. These differences may be attributed to a variety of hydrological factors which distinguish coastal shelf waters from oceanic waters. Rise assemblages are the shallowest water assemblages which give an indication of offshore oceanic hydrological conditions. INTRODUCTION RECENT nannofossil assemblages from North Atlantic (MclNTVRE and B~, 1967) and southwest Pacific (BURNS, 1973) deep-water sediments have been correlated with present-day hydrological regimes. Such investigations form the basis for interpreting nannofossil assemblages of older sediments. Changes in nannofossil assemblages across the continental shelf, continental slope, rise and adjacent deep-sea basin environments of deposition have not previously been assessed. Nor have nannofossil assemblages of continental shelf and slope sediments been established as typical of the offshore hydrological conditions. The aims of the present work were therefore to investigate-I. the changes in nannofossil numbers and species diversity from continental shelf to offshore basin sediments; 2. to assess whether nannofossil assemblages of continental shelf, slope and rise sediments differ from those of oceanic basin sediments. SAMPLES Five groups of sediment samples (total 24) from the outer continental shelf, slope, inner and outer rise and adjacent sedimentary basin were analysed. The site of collection of these samples relative to the shelf-basin environment was accurately known from the available bathymetric records. The geographic and bathymetric positions of the samples are shown in Figs. 1 and 2. Samples were selected from within *New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, P.O. Box 8099, Wellington, New Zealand. 425
426
D . A . BURNS
. . . . . . . . . . . . . . . . . . . . . . . . . .
as narrow a latitudinal zone as possible, as assemblages vary with latitude in this region (BURNS, 1973). BATHYMETRIC
AND
SEDIMENTARY
FEATURES
OF T H E A R E A
The four distinct depth zones (continental shelf, continental slope, continental rise and offshore basin) sampled are indicated in the reconstruction of the bottom profiles of the sampling area (Figs. 3 and 4). A detailed analysis of the distribution and composition of the sediments in the sampling area has been given by McDOUGALL(1973) and BURNS(1974). TECHNIQUES
A standard slide preparation technique was devised so that the number of nannofossil specimens within a sediment could be compared between different samples. This technique consisted of suspending a weighed (l g) sample of the sediment in 50 ml of 0.2% sodium hexa-meta-phosphate solution. When the sediment was completely disaggregated and dispersed, a pipette subsample was taken and three drops of this subsample were used to make the slide. Three counts were made on each sample: 1. The average number of nannofossils within ten randomly selected microscope fields of view examined at a magnification of × 1200. 2. The relative abundance of all very small nannofossil species [such as Emiliania huxleyi (Lohmann)] to Gephyrocapsa oceanica Kamptner and other larger nannofossil species. 166°E
170=
t74 °
17~~
] 72 °
I 75 °
'\
; 74 °
~30
Z 124[ II~'x
'\
175 °
l Als~ Auckland
/
t
/
""Ao?eo
L.
S ......
t
-
j'/
~
L4~z
f'
< 39, ~
/
g
)
i I,'.E4~4
,Y
/
I
Figs. I and 2.
Geographic and bathymetric position of the sampling area.
427
The abundance and species composition of nannofossil assemblages ao Of hers
20
/
~30 40
• oc~
I
3
~
2o t I0 =
Others
1
I
Ge/~hyrocop*o oceonico
I
o
zo E
'°
40 .S
/ 40 ~o
EmlU
huxley/' and other small coccoliths
.~° ~ '~o ; _~ IZ0 o
t ~I o ~
Emihenio huxleyt ,
~t 5 0 ~ qo
~other ~ - Osmall - 4co¢colitht I 00 o
and
S oe
,
L0
J¢~ ~
0 o I le~'e,tm)~
-4 ~'~"~.. z
=
E4S5 E457
500 I000
E456
W
0
oth . . . . .
.
,1
and
. . . . . liths
li Io
E
,o~ ~s.
s o p e ~
~ z
~1¢1lev¢,4
AISG Z~241
.
o JlOto~ ® °
~o.~ ~ ] Inner r 5e I
Emtlion/o huxley/
0 ~ ~
zo
500 (m
000
A157
W
500
40
40 30 20
Of hers
20
01"hers
0I 0 ~ 40 o
•
~0
Gephyrocopse oceonico
ofher small coccoIIfhs
~
Emd(an/e huxley/ and of her small coccoli'ths
tO 7O 60
~o
~~
50 ~
ego
z~679j
Z1~83
Z~L~
5~ t~vel (m) 5O0 ~000 1500 2O0O
Rhobdosohoer# c/#viger
Rhobdosphoero cloviger
Uml~/licosphoer° mirobil[$
40
.
I0 ~
~,
Syrocosphoero pulchro
~
Gephyro¢opsa ocean/co
t!
pv~h,o
sy~,=,$p^o,,o
]~
Sy,o~o,pho, ro ~v,~^ro
'
s
Umbllicosphaera mirobih~ Helic~oontosp~oero gompfn~ri He/moponfosphoera k
o
<
Co¢¢ollthus peloglcu$ r Ri~t I ~tOP" ,
Rhabdosphoero tie v/get
am4
I ShaCf c
s
.
Oufer r~se
~ (4~e
=~N,fw IOO0 Isoo
?~o
a =
Coccol¢¢hv$ pe/ogicus
~
no ,
|
J ~nner rqse JSlope ]~helf
[ o
=
moo
[47g
E4$Z
Rhobd~ph . . . . . lay/get
iooo
E48~
moo a
!~
Syrucosph~r~ pulchro Umb#Jcosphoero mirobdis iz
P°4
Cyc/ococco/ifhu$ ~p~oporu$ Hell~/~oePospheero kompfnBri
Figs. 3 and 4.
s
~-
Distribution of nannofossils relative to the sample position in the continental shelf-slope rise and basin sequence of sediments.
428
D . A . BURNS
3. The relative abundance of the different larger nannofossil species. For each of the last two analyses a minimum of 300 specimens was counted. RESULTS
Nannofossii numbers in the different sedimentary areas Continental shelf and upper slope sediments contain few nannofossils (Fig. 3), but there is a progressive increase in the number of specimens present in mid-slope (A155), lower slope (A156, Fig. 3), and continental rise (E483, A157, E456, Fig. 3) sediments. Basin sediments, however, contain the highest proportions of nannofossils (Fig. 3).
Distribution of individual species within the different sediment areas The nannofossil assemblages in each sample can be subdivided into three different groups. 1. The largest group which consists of extremely small coceoliths such as Emiliania huxleyi. These small coccoliths account for 40 to 78% of all nannofossil specimens in all of the sedimentary areas analysed (Fig. 3). It should be stressed, however, that counts of these very small species include all small forms. Although these coccoliths are of sufficient size to be clearly seen and counted on the light microscope using polarized light and phase contrast, the resolution of the light microscope is not sufficient to give accurate species separation. For this reason all these small species have been counted together as one group, and are not considered in any further detail. 2. The single species Gephyroeapsa oceanica, is the second largest group (14 to 30%) present in the sediments. The species is present in fairly consistent proportions (Fig. 3) but is occasionally present in much higher proportions (C329, Fig. 3) in slope sediments in this area. 3. The third and smallest group consists of the remaining large nannofossil species such as Cocco6thus pelagicus (Wallich), Cycloeoceolithus leptoporus (Murray & Blackman) and the slightly smaller Umbilicosphaera mirabilis Lohmann. Shelf sediments contain low proportions of these large species (5 to 18 %, Fig. 3). Slope sediments contain higher proportions (20 to 30%) but continental rise and oceanic basin sediments contain the highest proportions (20 to 40°;,). INDIVIDUAL SPECIES DISTRIBUIION
Coccofithus pelagicus, Cycloeoccolithus leptoporus are consistently present in approximately the same proportion in all of the assemblages from the shelf to basin (Fig. 4). However, the other species show quite distinct differences in distribution. Umbilicosphaera mirabilis is present only in low proportions in shelf assemblages, but is in considerably higher proportions in slope, rise and basin sediments (Fig. 4). Conversely, Helicopontosphaera kamptneri Hay & Mohler is present in slope sediments in generally higher proportions than in basin sediments (Fig. 4). Two species in particular, Rhabdosphaera claviger Murray & Blackman and Syracosphaera pulchra Lohmann which are consistently present in basin sediments have considerable variation in their contribution to the assemblages of shallower water areas. Syracosphaera pulchra can be present in unusually higher numbers (E405, Fig. 4) or completely absent (C305) from shell"or slope assemblages. The same species, however,
The abundance and species composition of nannofossil assemblages
429
is consistently present in rise assemblages. Similarly, Rhabdosphaera claviger is not present in shelf assemblages, sporadically present in slope assemblages, but consistently present in rise and basin assemblages. Several specimens of transported tropical water species such as Pontosphaera .japonica (Takayama), Thoracosphaera heimi (Lohmann), Scyphosphaera apsetni Loh.mann and Discosphaera tubifer (Murray & Blackman) which are not found in subtropical water sediments of the latitude examined here (BURNS, 1973) were found in the basin assemblages. A few specimens of these species were also present in the rise assemblages, but none was present in the slope or shelf assemblages. DISCUSSION General trends of species distributions are most apparent when the shelf, slope and rise areas are compared with the basin areas. Nannofossil content (number and species diversity) increase considerably from shelf to basin sediments. There is also a marked increase in the content of large species from shelf to basin. It is only in the outer rise sediments that the content of these species approaches that found in basin sediments. This is of importance as it is these large species which indicate latitudinal positions in deep-sea sediments (BURNS, 1973). If the proportions of each species in adjacent samples is compared within the different sedimentary areas, it can be seen that there are marked differences. (i) Within several basin sediments sampled over a relatively large area the proportional contribution of each species to the assemblage varies little between adjacent samples (Fig. 4). Rise sediments are similar, but have a slightly higher variation between samples (Fig. 4). (ii) Within several slope or shelf sediments, sampled over the much smaller area of the slope or shelf, the proportional contribution of each species to the assemblage varies considerably between adjacent samples, both in the number of specimens present and presence or absence of a particular species (Fig. 4).
NANNOEOSSIL ASSEMBLAGES AND HYDROLOGICAL CONDITIONS The surface waters around the coast of New Zealand can be subdivided into two hydrological types: 1. Coastal shelf water, and 2. Oceanic water. These two water types differ by their different salinities. Coastal water has a lower salinity than oceanic water (GARNER, 1961; RIDGWAY and STANTON, 1969). There may also be a considerable gradient of reducing salinity from the water which overlies the outer shelf region to that which is closer to the shore (RIDGWAY and STANTON, 1969). The differences in the nannofossil content of shelf to basin sediment can be correlated with these different hydrological conditions. The basin assemblages, with their high species diversity, are similar to others (BURNS, 1973)forming beneath oceanic waters of this latitude. As such they must be mirroring the offshore oceanic hydrological regime of the area, and can be taken as the typical nannofossil assemblage for oceanic areas at this latitude. However, in comparison, the sediments of the outer shelf and upper slope which lack many of the species found in basin assemblages (Figs. 3 and 4) are clearly not being formed under hydrological or depositional controls
430
D.A. BroRr~s
similar to oceanic basin areas. These low numbers of nannofossils may therefore be correlated with the presence of the less saline shelf water. The depletion of the nannofossil assemblage may be due to coccolithophorids being unable to live in the lower salinity of the shelf waters. However, other factors, such as dilution of their presence by terrigenous material depositing on the shelf, or nannofossils being removed during or after sedimentation and redepositing along with other material over the slope or deeper water areas, may also be significant. It is clear, however, that the nannofossil assemblages of the continental shelf and upper slope represent a sediment formed in and below a vastly different hydrological regime to that of oceanic basins. CONCLUSIONS 1. Characteristic features of nannofossil assemblages can be used to categorize shelf, slope, rise and basin sediments. (i) Shelf sediments have an extremely low content of nannofossils mostly poorly preserved: assemblages have only a few species. (ii) Slope sediments have low numbers of nannofossil specimens: assemblages are more diverse than shelf assemblages and preservation of specimens is slightly better. (iii) Rise sediments have a moderate number of nannofossil specimens, mostly well preserved, but not in the abundance found in basin sediments. (iv) Basin sediments contain abundant, well preserved nannofossils and the assemblages are diverse. Basin sediments are virtually nannofossil oozes. 2. The nannofossil assemblages in each sample can be separated into three different sized groups: (a) The largest group consisting of all very small species such as Emiliania huxle),i; (b) The second largest group, formed by one species Gephyrocapsa ocean ica; (c) The remaining larger nannofossils. 3. There are considerable differences between the assemblages of several shelf sediment samples within a small area. Adjacent slope assemblages show a similar degree of variation. However, there is very little variation between assemblages of several rise or basin sediments sampled over a relatively large area. 4. The difference in nannofossil content of the different sedimentary areas can be correlated with the hydrological distinction between oceanic water and the less saline coastal shelf water. Acktwwledgements--The author gratefully acknowledges the help of Miss P. LAWRENCE of N.Z. Oceanographic Institute, D.S.I.R., Wellington, for draughting the diagrams.
REFERENCES BURNS D. A. (1973) The latitudinal distribution and significance of calcareous nannofossils in the bottom sediments of the southwest Pacific Ocean (Lats 15-55°S) around New Zealand. In: Oceanography of the South Pacific 1972, R. FRASER,compiler, N.Z. National Commission for UNESCO, Wellington, pp. 221-228. BURNS D. A. (1974) Changes in the carbonate component of recentsediments with depth: a guide to paleoenvironmental interpretation. Marine Geology, 16, M I3-MI9. GARNER D. M. (1961) Hydrology of New Zealand coastal waters, 1955. Memoirs. New Zealand Oceanographic Institute, 8. Bulletin of the New Zealand Department of Scientific and Industrial Research, 138, 9-84.
The abundance and species composition of nannofossil assemblages
43 l
McDoUGALL J. C. (1973) Carbonate variations in slope sediments, Kaipara, New Zealand. New Zealand Journal of Geology and Geophysics, 15(4), 558-571. MCINTYRE A. and A. W. H. B~ (1967) Modern Coccolithophorida of the Atlantic Ocean---l. Placoliths and Cyrtoliths. Deep-Sea Research, 14, 561-597. RIDGWAY N. M. and B. R. STANTON0969) Some hydrological features of Hawke Bay and nearby shelf waters. New Zealand Journal of Marine and Freshwater Research, 3, 545-559.
The abundance and species composition of nannofossil assemblages in sediments from continental shelf to offshore basin, western Tasman Sea D. A. BURNS* (Received 12 October 1973; hz revi6ed form 22 October 1974; accepted 30 October 1974)
Abstract--Abundance and species composition of recent nannofossil assemblages are described from continental shelf, slope, rise and offshore basin sediments (36 to 37°S Lat.). Shelf and upper slope assemblages contain extremely few nannofossil specimens and species. Numbers of specimens and species diversity increases from slope to basin areas. The overall nannofossil content of the sediment is divided into three groups: all very small species; Gephyrocapaa oceanica; the remaining large species. It is the abundance of the large species which changes most noticeably between shelf, slope, rise and basin areas. Some individual species have specificdistribution patterns. Some decrease and others increase in proportion from shelf to basin. Comparing shelf, slope and rise assemblages with basin assemblages indicates that shelf and slope assemblages are not indicative of offshore oceanic hydrological regimes. These differences may be attributed to a variety of hydrological factors which distinguish coastal shelf waters from oceanic waters. Rise assemblages are the shallowest water assemblages which give an indication of offshore oceanic hydrological conditions. INTRODUCTION RECENT nannofossil assemblages from North Atlantic (MclNTVRE and B~, 1967) and southwest Pacific (BURNS, 1973) deep-water sediments have been correlated with present-day hydrological regimes. Such investigations form the basis for interpreting nannofossil assemblages of older sediments. Changes in nannofossil assemblages across the continental shelf, continental slope, rise and adjacent deep-sea basin environments of deposition have not previously been assessed. Nor have nannofossil assemblages of continental shelf and slope sediments been established as typical of the offshore hydrological conditions. The aims of the present work were therefore to investigate-I. the changes in nannofossil numbers and species diversity from continental shelf to offshore basin sediments; 2. to assess whether nannofossil assemblages of continental shelf, slope and rise sediments differ from those of oceanic basin sediments. SAMPLES Five groups of sediment samples (total 24) from the outer continental shelf, slope, inner and outer rise and adjacent sedimentary basin were analysed. The site of collection of these samples relative to the shelf-basin environment was accurately known from the available bathymetric records. The geographic and bathymetric positions of the samples are shown in Figs. 1 and 2. Samples were selected from within *New Zealand Oceanographic Institute, Department of Scientific and Industrial Research, P.O. Box 8099, Wellington, New Zealand. 425
426
D . A . BURNS
. . . . . . . . . . . . . . . . . . . . . . . . . .
as narrow a latitudinal zone as possible, as assemblages vary with latitude in this region (BURNS, 1973). BATHYMETRIC
AND
SEDIMENTARY
FEATURES
OF T H E A R E A
The four distinct depth zones (continental shelf, continental slope, continental rise and offshore basin) sampled are indicated in the reconstruction of the bottom profiles of the sampling area (Figs. 3 and 4). A detailed analysis of the distribution and composition of the sediments in the sampling area has been given by McDOUGALL(1973) and BURNS(1974). TECHNIQUES
A standard slide preparation technique was devised so that the number of nannofossil specimens within a sediment could be compared between different samples. This technique consisted of suspending a weighed (l g) sample of the sediment in 50 ml of 0.2% sodium hexa-meta-phosphate solution. When the sediment was completely disaggregated and dispersed, a pipette subsample was taken and three drops of this subsample were used to make the slide. Three counts were made on each sample: 1. The average number of nannofossils within ten randomly selected microscope fields of view examined at a magnification of × 1200. 2. The relative abundance of all very small nannofossil species [such as Emiliania huxleyi (Lohmann)] to Gephyrocapsa oceanica Kamptner and other larger nannofossil species. 166°E
170=
t74 °
17~~
] 72 °
I 75 °
'\
; 74 °
~30
Z 124[ II~'x
'\
175 °
l Als~ Auckland
/
t
/
""Ao?eo
L.
S ......
t
-
j'/
~
L4~z
f'
< 39, ~
/
g
)
i I,'.E4~4
,Y
/
I
Figs. I and 2.
Geographic and bathymetric position of the sampling area.
427
The abundance and species composition of nannofossil assemblages ao Of hers
20
/
~30 40
• oc~
I
3
~
2o t I0 =
Others
1
I
Ge/~hyrocop*o oceonico
I
o
zo E
'°
40 .S
/ 40 ~o
EmlU
huxley/' and other small coccoliths
.~° ~ '~o ; _~ IZ0 o
t ~I o ~
Emihenio huxleyt ,
~t 5 0 ~ qo
~other ~ - Osmall - 4co¢colitht I 00 o
and
S oe
,
L0
J¢~ ~
0 o I le~'e,tm)~
-4 ~'~"~.. z
=
E4S5 E457
500 I000
E456
W
0
oth . . . . .
.
,1
and
. . . . . liths
li Io
E
,o~ ~s.
s o p e ~
~ z
~1¢1lev¢,4
AISG Z~241
.
o JlOto~ ® °
~o.~ ~ ] Inner r 5e I
Emtlion/o huxley/
0 ~ ~
zo
500 (m
000
A157
W
500
40
40 30 20
Of hers
20
01"hers
0I 0 ~ 40 o
•
~0
Gephyrocopse oceonico
ofher small coccoIIfhs
~
Emd(an/e huxley/ and of her small coccoli'ths
tO 7O 60
~o
~~
50 ~
ego
z~679j
Z1~83
Z~L~
5~ t~vel (m) 5O0 ~000 1500 2O0O
Rhobdosohoer# c/#viger
Rhobdosphoero cloviger
Uml~/licosphoer° mirobil[$
40
.
I0 ~
~,
Syrocosphoero pulchro
~
Gephyro¢opsa ocean/co
t!
pv~h,o
sy~,=,$p^o,,o
]~
Sy,o~o,pho, ro ~v,~^ro
'
s
Umbllicosphaera mirobih~ Helic~oontosp~oero gompfn~ri He/moponfosphoera k
o
<
Co¢¢ollthus peloglcu$ r Ri~t I ~tOP" ,
Rhabdosphoero tie v/get
am4
I ShaCf c
s
.
Oufer r~se
~ (4~e
=~N,fw IOO0 Isoo
?~o
a =
Coccol¢¢hv$ pe/ogicus
~
no ,
|
J ~nner rqse JSlope ]~helf
[ o
=
moo
[47g
E4$Z
Rhobd~ph . . . . . lay/get
iooo
E48~
moo a
!~
Syrucosph~r~ pulchro Umb#Jcosphoero mirobdis iz
P°4
Cyc/ococco/ifhu$ ~p~oporu$ Hell~/~oePospheero kompfnBri
Figs. 3 and 4.
s
~-
Distribution of nannofossils relative to the sample position in the continental shelf-slope rise and basin sequence of sediments.
428
D . A . BURNS
3. The relative abundance of the different larger nannofossil species. For each of the last two analyses a minimum of 300 specimens was counted. RESULTS
Nannofossii numbers in the different sedimentary areas Continental shelf and upper slope sediments contain few nannofossils (Fig. 3), but there is a progressive increase in the number of specimens present in mid-slope (A155), lower slope (A156, Fig. 3), and continental rise (E483, A157, E456, Fig. 3) sediments. Basin sediments, however, contain the highest proportions of nannofossils (Fig. 3).
Distribution of individual species within the different sediment areas The nannofossil assemblages in each sample can be subdivided into three different groups. 1. The largest group which consists of extremely small coceoliths such as Emiliania huxleyi. These small coccoliths account for 40 to 78% of all nannofossil specimens in all of the sedimentary areas analysed (Fig. 3). It should be stressed, however, that counts of these very small species include all small forms. Although these coccoliths are of sufficient size to be clearly seen and counted on the light microscope using polarized light and phase contrast, the resolution of the light microscope is not sufficient to give accurate species separation. For this reason all these small species have been counted together as one group, and are not considered in any further detail. 2. The single species Gephyroeapsa oceanica, is the second largest group (14 to 30%) present in the sediments. The species is present in fairly consistent proportions (Fig. 3) but is occasionally present in much higher proportions (C329, Fig. 3) in slope sediments in this area. 3. The third and smallest group consists of the remaining large nannofossil species such as Cocco6thus pelagicus (Wallich), Cycloeoceolithus leptoporus (Murray & Blackman) and the slightly smaller Umbilicosphaera mirabilis Lohmann. Shelf sediments contain low proportions of these large species (5 to 18 %, Fig. 3). Slope sediments contain higher proportions (20 to 30%) but continental rise and oceanic basin sediments contain the highest proportions (20 to 40°;,). INDIVIDUAL SPECIES DISTRIBUIION
Coccofithus pelagicus, Cycloeoccolithus leptoporus are consistently present in approximately the same proportion in all of the assemblages from the shelf to basin (Fig. 4). However, the other species show quite distinct differences in distribution. Umbilicosphaera mirabilis is present only in low proportions in shelf assemblages, but is in considerably higher proportions in slope, rise and basin sediments (Fig. 4). Conversely, Helicopontosphaera kamptneri Hay & Mohler is present in slope sediments in generally higher proportions than in basin sediments (Fig. 4). Two species in particular, Rhabdosphaera claviger Murray & Blackman and Syracosphaera pulchra Lohmann which are consistently present in basin sediments have considerable variation in their contribution to the assemblages of shallower water areas. Syracosphaera pulchra can be present in unusually higher numbers (E405, Fig. 4) or completely absent (C305) from shell"or slope assemblages. The same species, however,
The abundance and species composition of nannofossil assemblages
429
is consistently present in rise assemblages. Similarly, Rhabdosphaera claviger is not present in shelf assemblages, sporadically present in slope assemblages, but consistently present in rise and basin assemblages. Several specimens of transported tropical water species such as Pontosphaera .japonica (Takayama), Thoracosphaera heimi (Lohmann), Scyphosphaera apsetni Loh.mann and Discosphaera tubifer (Murray & Blackman) which are not found in subtropical water sediments of the latitude examined here (BURNS, 1973) were found in the basin assemblages. A few specimens of these species were also present in the rise assemblages, but none was present in the slope or shelf assemblages. DISCUSSION General trends of species distributions are most apparent when the shelf, slope and rise areas are compared with the basin areas. Nannofossil content (number and species diversity) increase considerably from shelf to basin sediments. There is also a marked increase in the content of large species from shelf to basin. It is only in the outer rise sediments that the content of these species approaches that found in basin sediments. This is of importance as it is these large species which indicate latitudinal positions in deep-sea sediments (BURNS, 1973). If the proportions of each species in adjacent samples is compared within the different sedimentary areas, it can be seen that there are marked differences. (i) Within several basin sediments sampled over a relatively large area the proportional contribution of each species to the assemblage varies little between adjacent samples (Fig. 4). Rise sediments are similar, but have a slightly higher variation between samples (Fig. 4). (ii) Within several slope or shelf sediments, sampled over the much smaller area of the slope or shelf, the proportional contribution of each species to the assemblage varies considerably between adjacent samples, both in the number of specimens present and presence or absence of a particular species (Fig. 4).
NANNOEOSSIL ASSEMBLAGES AND HYDROLOGICAL CONDITIONS The surface waters around the coast of New Zealand can be subdivided into two hydrological types: 1. Coastal shelf water, and 2. Oceanic water. These two water types differ by their different salinities. Coastal water has a lower salinity than oceanic water (GARNER, 1961; RIDGWAY and STANTON, 1969). There may also be a considerable gradient of reducing salinity from the water which overlies the outer shelf region to that which is closer to the shore (RIDGWAY and STANTON, 1969). The differences in the nannofossil content of shelf to basin sediment can be correlated with these different hydrological conditions. The basin assemblages, with their high species diversity, are similar to others (BURNS, 1973)forming beneath oceanic waters of this latitude. As such they must be mirroring the offshore oceanic hydrological regime of the area, and can be taken as the typical nannofossil assemblage for oceanic areas at this latitude. However, in comparison, the sediments of the outer shelf and upper slope which lack many of the species found in basin assemblages (Figs. 3 and 4) are clearly not being formed under hydrological or depositional controls
430
D.A. BroRr~s
similar to oceanic basin areas. These low numbers of nannofossils may therefore be correlated with the presence of the less saline shelf water. The depletion of the nannofossil assemblage may be due to coccolithophorids being unable to live in the lower salinity of the shelf waters. However, other factors, such as dilution of their presence by terrigenous material depositing on the shelf, or nannofossils being removed during or after sedimentation and redepositing along with other material over the slope or deeper water areas, may also be significant. It is clear, however, that the nannofossil assemblages of the continental shelf and upper slope represent a sediment formed in and below a vastly different hydrological regime to that of oceanic basins. CONCLUSIONS 1. Characteristic features of nannofossil assemblages can be used to categorize shelf, slope, rise and basin sediments. (i) Shelf sediments have an extremely low content of nannofossils mostly poorly preserved: assemblages have only a few species. (ii) Slope sediments have low numbers of nannofossil specimens: assemblages are more diverse than shelf assemblages and preservation of specimens is slightly better. (iii) Rise sediments have a moderate number of nannofossil specimens, mostly well preserved, but not in the abundance found in basin sediments. (iv) Basin sediments contain abundant, well preserved nannofossils and the assemblages are diverse. Basin sediments are virtually nannofossil oozes. 2. The nannofossil assemblages in each sample can be separated into three different sized groups: (a) The largest group consisting of all very small species such as Emiliania huxle),i; (b) The second largest group, formed by one species Gephyrocapsa ocean ica; (c) The remaining larger nannofossils. 3. There are considerable differences between the assemblages of several shelf sediment samples within a small area. Adjacent slope assemblages show a similar degree of variation. However, there is very little variation between assemblages of several rise or basin sediments sampled over a relatively large area. 4. The difference in nannofossil content of the different sedimentary areas can be correlated with the hydrological distinction between oceanic water and the less saline coastal shelf water. Acktwwledgements--The author gratefully acknowledges the help of Miss P. LAWRENCE of N.Z. Oceanographic Institute, D.S.I.R., Wellington, for draughting the diagrams.
REFERENCES BURNS D. A. (1973) The latitudinal distribution and significance of calcareous nannofossils in the bottom sediments of the southwest Pacific Ocean (Lats 15-55°S) around New Zealand. In: Oceanography of the South Pacific 1972, R. FRASER,compiler, N.Z. National Commission for UNESCO, Wellington, pp. 221-228. BURNS D. A. (1974) Changes in the carbonate component of recentsediments with depth: a guide to paleoenvironmental interpretation. Marine Geology, 16, M I3-MI9. GARNER D. M. (1961) Hydrology of New Zealand coastal waters, 1955. Memoirs. New Zealand Oceanographic Institute, 8. Bulletin of the New Zealand Department of Scientific and Industrial Research, 138, 9-84.
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McDoUGALL J. C. (1973) Carbonate variations in slope sediments, Kaipara, New Zealand. New Zealand Journal of Geology and Geophysics, 15(4), 558-571. MCINTYRE A. and A. W. H. B~ (1967) Modern Coccolithophorida of the Atlantic Ocean---l. Placoliths and Cyrtoliths. Deep-Sea Research, 14, 561-597. RIDGWAY N. M. and B. R. STANTON0969) Some hydrological features of Hawke Bay and nearby shelf waters. New Zealand Journal of Marine and Freshwater Research, 3, 545-559.