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Modern temperate-water and warm-water shelf carbonate sediments contrasted
Marine Geology Elsevier PubliShing Company, Amsterdam - Printed in The Netherlands
Letter Section Modern temperate-water
and warm-water shelf carbonate sediments contrasted
ALAN LEES and ANTONY T. BULLER
Institut GJologique, Universit~ de Louvain, Louyain {Belgium) Geology Department, The University, Dundee {Great BritainJ (Accepted for publication September 19, 1972)
ABSTRACT Lees, A. and Bullet, A.T., 1972. Modern temperate-water and warm-water shelf carbonate sediments contrasted. Mar. Geol., 13: M67-M73. Information on grain types ptesem in modern, marine, carbonate sands has been collected from published descriptions of 78 shelf areas between the equator and latitudes 60° S and 60° N. Two major associations of skeletal grains are recognised: one (chlorozoan) is almost entirely restricted to warm, tropical waters, the other (foramol) is characteristic of temperate waters but also extends well into the tropics. Non-skeletal grains are largely restricted to warm-water shelf environments with the chlorozoan association, but pellets can also occur with the foramol association in areas near to the chlorozoan/foramol boundary. Annual minimum and mean water temperatures appear to be major factors limiting the development of the chlorozoan association, but the distributions of the skeletal associations cannot be explained in terms of temperature alone.
INTRODUCTION TradRionally, present-day shelf carbonate sedimentation is regarded as being a low-latitude ( 3 0 ° S - 3 0 ° N ) phenomenon (Rodgers, 1957). However, information assembled b y Chave (1967), together with subsequent reports o f highly calcareous sediments forming well outside the tropics show that there is no such restriction. The non-tropical deposits have sometimes been labelled, rather apologetically, carbonates o f "non-carbonate" areas. Certainly, when measured in terms o f percentage areal coverage o f the continental shelves, these carbonates are often xehtively unimportant. They can, nevertheless, extend over considerable areas. Perhaps the most extensive are the virtually pure carbonates which occupy many thousands o f square kilometres on the shelf off southern Australia between latitudes 32 ° and 40+ ° S (Conolly and Von der Borch, 1967; g a s s et al., 1970). But, even in higher latitudes, as on the northwest European shelf, carbonates m a y dominate over hundreds or even thousands o f square kilometres o f the sea floor (e.g., Boiliot, 1965; Boiliot et al., 1971 ; Hommeril, 1971). Enough information now exists to permit a preliminary assessment o f the common
M68
LETTER SECTION
characteristics of these mid-latitude or temperate-water carbonates. As such deposits could form significant limestone bodies, if preserved, it is also pertinent to consider whether they would possess features which would distinguish them from their warm-water relatives. SOURCES OF INFORMATION As a first step, attention has been directed towards the sand, and coarser fractions of the sediments. We have collected and tabulated information on carbonate grain types and major environmental parameters from 92 publications* referring to sediments** in 78 areas of the continental shelves*** between the equator and latitudes 60°N and S. Thirty of the areas lie between latitudes 30°-60°N and 30°-60°S. Although efforts were made to collect data representative of all the oceans and seas, there is an inevitable bias towards those areas, such as the North Atlantic, which have been relatively thoroughly investigated. With the exception of Australia, the southern hemisphere is poorly represented. Relict sediments, where recognisable, were excluded from the tabulation. DISTRIBUTIONOF GRAIN TYPES
Skeletal grains On tabulating the skeletal grains in terms of major taxonomic groups it becomes clear that there are systematic differences between those carbonates forming well outside the tropics and those of "classic" warm-water type. The main groups of organisms represented in the "temperate-water" association are ~. (1) animals: molluscs, foraminiferams (benthonic), echinoderms, bryozoans, barnacles, ostracods, sponges (calcareous spicules), worms (tubes), and ahermatypic corals; (2) plants: calcareous red algae (e.g., Lithothamnium). Not all of these constituents are present in each of the areas studied: molluscs and forams are ubiquitous; echinoderms and bryozoans are often present; the others occur more sporadically. Ahermatypic corals are only very occasionally recorded, and even then they are a minor constituent. Depending on local conditions (depth, substrate, etc.) the remains of molluscs, forams, bryozoans, barnacles, or calcareous red algae may constitute the dominant grain type. The "warm-water" association may include most of the above-mentioned components but differs in that: (1) it always contains, in addition, significant contributions from corals (presumably mainly hermatypic) and/or calcareous green algae (such as Halimeda); (2) barnacles apparently never contribute measurably to the sediments; and (3) bryozoans * To save space these publications are not quoted in the References, but the list will be published later (lees, in preparation). ** krespective of the percentage of CaCOj in the total sediment. *** Depth range mainly 0-50 fathoms: in deeper waters pelagic and relict components tend to increase in importance.
LETTER SECTION
M69
are rarely more than minor constituents. It is well-known that hermatypic corals and calcareous green algae live only in warm seas (e.g., Teichert, 1958; Johnson, 1961), so it is not surprising to find that their debris is similarly distributed. Barnacles and bryozoans, on the other hand, live in both warm and cool seas. The scarcity or absence of their debris from the "warm-water" association presumably reflects a relatively feeble productivity perhaps in part due to their ecological replacement by other organisms (e.g., corals replacing barnacles). These two associations of skeletal grains appear to be the only ones with global significance. As such, they deserve more formal recognition than they have received hitherto. Terminologies based on water temperature ("warm", "temperate") or on latitude are not always appropriate, as it will be shown later. We propose, therefore, the term chlorozoan (Chlorophyta + Zoantharia) for the "warm-water" group, and foramol for the other association (in which forams and molluscs are nearly always present and may be dominant). In terms of areal distribution, the zone of transition from one association to the other is generally narrow. Naturally, if one were to plot the distribution of the skeletal components in terms of the lower taxonomic divisions (e.g., genera), then the change would appear less abrupt since it would be heralded by other changes. We have, however, deliberately avoided such taxonomic detail, partly because it is not available for many of the sediments concerned, but mainly because it would have little value in any attempt to interpret ancient limestones in the light of present-day distributions.
Non-skeletal grains For tabulation the non-skeletal grains were classified as pellets, ooliths, and aggregates (e.g., grapestone). The majority of pellet records are from areas occupied by the chlorozoan association but they also occur with the foramol association in areas bordering the chlorozoan/foramol boundary. The other non-skeletal grains (ooliths and aggregates) are largely restricted to areas with chlorozoan sediments*. Naturally occurring combinations of skeIetal and non-skeletal associations are shown schematically in Fig. 1.
DISTRIBUTIONOF SKELETALASSOCIATIONSWITHRESPECTTO LATITUDE Because the chlorozoan association is the "normal" carbonate sediment in the 30°S-30°N latitude belt it is easy to be misled into thinking that a//shelf carbonates in that belt are of this type. Our tabulation shows that, whereas only 1 out of 34 examples of chlorozoan deposit occurs outside 30 ° latitude, 15 out of 44 foramol examples lie * The main accumulations of aragonite (needle) mud seem to be similarlyconfined.
M70
LETTER SECTION
OOLITH
skeletal/
AGGREGATES PELLET enE~ I CALCAREOUS j ALGAE
RED
CORAL (hermotypic) MOLLUSC FORAM (benthonic) BRYOZOA
Animol
BARNACLE OTHER : echinoderm, ostracod, sponge spicule, worm lube, ohermatypic
FORAMOL
CHLOROZOAN l
ABSENT
PELLET OOLITH,AGGREGATES : PELLET ONLY AND PELLET POSSIBLEIIi ONLY
I
coral
GRAIN ASSOC IATIONS
Fig. 1. The principal carbonate grain types arranged to show the various combinations (associations) in which they occur in shelf sediments, For discussion of factors controlling the distribution, see text. Thickness of bars has no significance. Shading in bar indicates that the grain type concerned may be an important or dominant 'component. Distribution of coral/algal reefs is similar to that of the chlorozoan association. between the equator and 30 °. These low-latitude foramol sediments are not thus classified simply because they lack the chlorozoan " k e y " grains; they m a y contain significant barnacle and bryozoan contributions as do those of higher latitudes (e.g., Masse, 1970).
FACTORS CONTROLLING THE DISTRIBUTION OF SKELETAL ASSOCIATIONS
The composition of shelf carbonates is known to be influenced b y many environmental factors, and this is no place for a detailed discussion o f the topic. It is nevertheless useful to consider briefly the conditions controlling the boundary between the foramol and chlorozoan associations. Within the 0--50 fathom range considered, depth appears to have no influence on
LETTER SECTION
M71
the association developed. However, the Persian Gulf could be an exception, with chlorozoan sediments in the shallows being replaced at depth by foramol. The Gulf is also abnormal because calcareous green algae are everywhere absent. As one would imagine, water temperature is a significant factor, but the relationship is not a simple one. The threshold for development of the chiorozoan association appears to be a minimum surface (or near-surface) water temperature of about 1 4 - 1 5 ° C (Fig.2B). But, there are many areas (e.g., the West African shelf; Masse, 1970; McMaster et al., 1971) where the foramol association persists although the minimum temperatures exceed 15~C (Fig.2A). Some o f these distribution anomalies can be removed if the annual mean temperature is also taken into account. Most areas with chlorozoan sediments have mean temperatures of at least 23°C (Fig.2). By plotting, on a map of the winter and summer isotherms of , Mean
o
23"C t
•
Mean
"'+:~
. ,,,,..
o • o o
18eC
" +'....o ,
t+. . . . .... .+'Mr " ~ ,,;++
Minimum
14"0
~
"'" ~++I:+ ,,,-"
5O
o
o.]O.°__°
o~
~';'..o o
20
.........: ............
_+
/ i t .......... "
; l,r,
.......
+'!7:,+:: . . . . . . [...: ......... '-.. / i
......................................
do
......,
++'.+..
....
B 0
I0
20
T.°C. M A X .
30
40
2'o
3o
o 4o
T.°C. M A X .
Fig.2. Distribution of skeletal grain associations with respect to minimum and maximum near-surface water temperatures. A+ Foramol association; B. Chlorozoan association (points plotted as triangles refer to the Shark Bay area - see text). Individual points represent particular areas of shelf where temperature conditions can be charactezized by single maxima and minima. Areas where temperature extremes vary from place to place are represented by points joined by dashed lines, the points indicating the extremes of the range. Data from various sources, see footnote p.M68.
oceanic waters, those points having minima of 14--15°C and means of 23°C, one can trace this foramol/chlorozoan boundary and see that it matches, broadly, that shown by the sediments themselves. In the belt from the equator to 30 ° latitude, water temperatures tend to be higher on the western than on the eastern sides of the oceans. As a result, the foramol association extends well into the tropics on the shelves bordering the oceans on the east. This relationship is shown schematically on Fig.3 which is based on Emery's (1968, fig.2 and 3) concept of an ideal ocean. It is significant, however, that alfltough these temperature thresholds broadly account for the known distribution of the skeletal associations, important anomalies remain. Foramol sediments are found in a number of areas where, on the basis of water temperatures,
M72
LETTER SECTION
pole
pole
60*
..~,l.~,~. ,~,,7 60 °
3~O*
|
Oo SKELETAL
~
ASSOCIATIONS
Fo. . . .
I
Chlorozoon
O* NON-SKELETAL
ASSOCIATIONS
~
Pellet
~
Oolith/oggregote
Fig.3. Distributions of the main carbonate grain associations on the continental shelves of an ideal ocean. Only northern hemisphere shown. Width of shelves exaggerated. Unshaded area in centre of each diagram represents the ocean basin. The oolith/aggregate association is shown as being restricted to part of the western side of the ocean to correspond with the known present-day distributionthe chlorozoan association would be expected (Flg.2A), and in one area (Shark Bay, e.g., Davies, 1970) calcareous green algae, although uncommon, appear as sediment components at mean temperatures as low as 19°C (Fig.2B). Thus, in terms of temperature, there is considerable overlap of the ranges of the foramol and chlorozoan associations. It may be argued that it is unrealistic to expect a perfect correlation between the sediment components, which may have been moved from the site of their formation, and an environmental factor which can only be effective during the life of the organisms from which the grains were formed. This is true, of course, but it can only explain relatively small displacements. In those anomalous situations which do not obey the temperature "laws" the displacement is much too great to be explained by any known transport mechanism. Furthermore, the skeletal grain components can always be checked against the organisms living in the area. One is thus quite justified in asking why the anomalies exist. There are many factors which, together with temperature could be responsible but, as will be shown elsewhere (Lees, in preparation) salinity is probably one of the most important. ACKNOWLEDGEMENTS We would like to thank Dr. R.G.C. Bathurst of Liverpool University for his helpful comments on a draft of this article.
LETTER SECTION
M73
REFERENCES Boillot, G., 1965. Organogenic gradients in the study of neritic deposits of biological origin: the example of the western English Channel Mar. GeoL, 3: 359-367. Boillot, G., Bouysse, P. and Lamboy, M., 1971. Morphology, sediments and Quaternary history of the continental shelf between the Straits of Dover and Cape Finisterre. In: F.M. Deiany (Editor), The Geology of the East Atlantic Continental Margin, 3. Europe. Rept. No. 70/15, N.E.R.C, Institute of Geological Sciences, London. pp. 79-90. Chave, K.E., 1967. Recent carbonate sediments - an unconventional view. A.G.L Counc. Educ. GeoL ScL, Short Rev., 7: 200-204. Conolly, J.R. and Von der Botch, C.C., 1967. Sedimentation and physiography of the sea floor south of Australia.'Sediment. Geol,, 1: 181-220. Davies, G.R., 1970. Carbonate bank sedimentation, eastern Shark Bay, Western Australia. In: B.W. Logan, G.R. Davies, J.F. Read and D.E. Cebulski (Editors), Carbonate Sedimentation and Environments, Shark Bay, Western Australia - Am- Assoc. Pet. Geol. Mere,, 13: 85-168. Emery, K.O., 1968. Relict sediments on continental shelves of the world. Bull, Am. Assoc. Pet. GeoL, 52: 445-464. Hommeril, P., 1971. Dynamique du transport des s~diments calcaires darts la pattie nord du golfe normand-breton. Bull So~ G~oL France, 7 (XII): 31-41. Johnson, J.H., 1961. Limestone-BuildingAlgae andAlgal Limestones. Colo. School Mines, Boulder, Colo., 297 pp. Lees, A., (in preparation). The influence of salinity and temperature on grain types and associations in modern shelf carbonate sediments. Masse, J.P., 1970. Contribution ~ l'~tude de la cartographic s&timentaire du plateau continental s~n~galais. Rapp. Proc. Verb., Conseil. lnt.Explor. Mer, 159: 12-14. McMaster, R.L., Miliiman, J.D. and Ashraf, A., 1971. Continental shelf and upper slope sediments off Portuguese Guinea, Guinea, and Sierra Leone, West Africa. J. Sediment. PetroL, 41 : 150-158. Rodgers, J., 1957. The distribution of marine carbonate sediments' a review. In: R.J. Leblanc and J.G. Breeding (Editors), Regional Aspects of Carbonate Deposition - Soc. Econ. Paleontol. Mineral., Spec PubL, 5: 2-14. Teichert, C., 1958. Cold- and deep-water coral banks. Bull Am. Assoc. Pet. Geol., 42: 1064-1082. Wass, tLE., ConoUy, J.R. and Macintyre, R.J., 1970. Bryozoan carbonate sand continuous along southern Australia. Marine GeoL, 9: 63-73.
Letter Section Modern temperate-water
and warm-water shelf carbonate sediments contrasted
ALAN LEES and ANTONY T. BULLER
Institut GJologique, Universit~ de Louvain, Louyain {Belgium) Geology Department, The University, Dundee {Great BritainJ (Accepted for publication September 19, 1972)
ABSTRACT Lees, A. and Bullet, A.T., 1972. Modern temperate-water and warm-water shelf carbonate sediments contrasted. Mar. Geol., 13: M67-M73. Information on grain types ptesem in modern, marine, carbonate sands has been collected from published descriptions of 78 shelf areas between the equator and latitudes 60° S and 60° N. Two major associations of skeletal grains are recognised: one (chlorozoan) is almost entirely restricted to warm, tropical waters, the other (foramol) is characteristic of temperate waters but also extends well into the tropics. Non-skeletal grains are largely restricted to warm-water shelf environments with the chlorozoan association, but pellets can also occur with the foramol association in areas near to the chlorozoan/foramol boundary. Annual minimum and mean water temperatures appear to be major factors limiting the development of the chlorozoan association, but the distributions of the skeletal associations cannot be explained in terms of temperature alone.
INTRODUCTION TradRionally, present-day shelf carbonate sedimentation is regarded as being a low-latitude ( 3 0 ° S - 3 0 ° N ) phenomenon (Rodgers, 1957). However, information assembled b y Chave (1967), together with subsequent reports o f highly calcareous sediments forming well outside the tropics show that there is no such restriction. The non-tropical deposits have sometimes been labelled, rather apologetically, carbonates o f "non-carbonate" areas. Certainly, when measured in terms o f percentage areal coverage o f the continental shelves, these carbonates are often xehtively unimportant. They can, nevertheless, extend over considerable areas. Perhaps the most extensive are the virtually pure carbonates which occupy many thousands o f square kilometres on the shelf off southern Australia between latitudes 32 ° and 40+ ° S (Conolly and Von der Borch, 1967; g a s s et al., 1970). But, even in higher latitudes, as on the northwest European shelf, carbonates m a y dominate over hundreds or even thousands o f square kilometres o f the sea floor (e.g., Boiliot, 1965; Boiliot et al., 1971 ; Hommeril, 1971). Enough information now exists to permit a preliminary assessment o f the common
M68
LETTER SECTION
characteristics of these mid-latitude or temperate-water carbonates. As such deposits could form significant limestone bodies, if preserved, it is also pertinent to consider whether they would possess features which would distinguish them from their warm-water relatives. SOURCES OF INFORMATION As a first step, attention has been directed towards the sand, and coarser fractions of the sediments. We have collected and tabulated information on carbonate grain types and major environmental parameters from 92 publications* referring to sediments** in 78 areas of the continental shelves*** between the equator and latitudes 60°N and S. Thirty of the areas lie between latitudes 30°-60°N and 30°-60°S. Although efforts were made to collect data representative of all the oceans and seas, there is an inevitable bias towards those areas, such as the North Atlantic, which have been relatively thoroughly investigated. With the exception of Australia, the southern hemisphere is poorly represented. Relict sediments, where recognisable, were excluded from the tabulation. DISTRIBUTIONOF GRAIN TYPES
Skeletal grains On tabulating the skeletal grains in terms of major taxonomic groups it becomes clear that there are systematic differences between those carbonates forming well outside the tropics and those of "classic" warm-water type. The main groups of organisms represented in the "temperate-water" association are ~. (1) animals: molluscs, foraminiferams (benthonic), echinoderms, bryozoans, barnacles, ostracods, sponges (calcareous spicules), worms (tubes), and ahermatypic corals; (2) plants: calcareous red algae (e.g., Lithothamnium). Not all of these constituents are present in each of the areas studied: molluscs and forams are ubiquitous; echinoderms and bryozoans are often present; the others occur more sporadically. Ahermatypic corals are only very occasionally recorded, and even then they are a minor constituent. Depending on local conditions (depth, substrate, etc.) the remains of molluscs, forams, bryozoans, barnacles, or calcareous red algae may constitute the dominant grain type. The "warm-water" association may include most of the above-mentioned components but differs in that: (1) it always contains, in addition, significant contributions from corals (presumably mainly hermatypic) and/or calcareous green algae (such as Halimeda); (2) barnacles apparently never contribute measurably to the sediments; and (3) bryozoans * To save space these publications are not quoted in the References, but the list will be published later (lees, in preparation). ** krespective of the percentage of CaCOj in the total sediment. *** Depth range mainly 0-50 fathoms: in deeper waters pelagic and relict components tend to increase in importance.
LETTER SECTION
M69
are rarely more than minor constituents. It is well-known that hermatypic corals and calcareous green algae live only in warm seas (e.g., Teichert, 1958; Johnson, 1961), so it is not surprising to find that their debris is similarly distributed. Barnacles and bryozoans, on the other hand, live in both warm and cool seas. The scarcity or absence of their debris from the "warm-water" association presumably reflects a relatively feeble productivity perhaps in part due to their ecological replacement by other organisms (e.g., corals replacing barnacles). These two associations of skeletal grains appear to be the only ones with global significance. As such, they deserve more formal recognition than they have received hitherto. Terminologies based on water temperature ("warm", "temperate") or on latitude are not always appropriate, as it will be shown later. We propose, therefore, the term chlorozoan (Chlorophyta + Zoantharia) for the "warm-water" group, and foramol for the other association (in which forams and molluscs are nearly always present and may be dominant). In terms of areal distribution, the zone of transition from one association to the other is generally narrow. Naturally, if one were to plot the distribution of the skeletal components in terms of the lower taxonomic divisions (e.g., genera), then the change would appear less abrupt since it would be heralded by other changes. We have, however, deliberately avoided such taxonomic detail, partly because it is not available for many of the sediments concerned, but mainly because it would have little value in any attempt to interpret ancient limestones in the light of present-day distributions.
Non-skeletal grains For tabulation the non-skeletal grains were classified as pellets, ooliths, and aggregates (e.g., grapestone). The majority of pellet records are from areas occupied by the chlorozoan association but they also occur with the foramol association in areas bordering the chlorozoan/foramol boundary. The other non-skeletal grains (ooliths and aggregates) are largely restricted to areas with chlorozoan sediments*. Naturally occurring combinations of skeIetal and non-skeletal associations are shown schematically in Fig. 1.
DISTRIBUTIONOF SKELETALASSOCIATIONSWITHRESPECTTO LATITUDE Because the chlorozoan association is the "normal" carbonate sediment in the 30°S-30°N latitude belt it is easy to be misled into thinking that a//shelf carbonates in that belt are of this type. Our tabulation shows that, whereas only 1 out of 34 examples of chlorozoan deposit occurs outside 30 ° latitude, 15 out of 44 foramol examples lie * The main accumulations of aragonite (needle) mud seem to be similarlyconfined.
M70
LETTER SECTION
OOLITH
skeletal/
AGGREGATES PELLET enE~ I CALCAREOUS j ALGAE
RED
CORAL (hermotypic) MOLLUSC FORAM (benthonic) BRYOZOA
Animol
BARNACLE OTHER : echinoderm, ostracod, sponge spicule, worm lube, ohermatypic
FORAMOL
CHLOROZOAN l
ABSENT
PELLET OOLITH,AGGREGATES : PELLET ONLY AND PELLET POSSIBLEIIi ONLY
I
coral
GRAIN ASSOC IATIONS
Fig. 1. The principal carbonate grain types arranged to show the various combinations (associations) in which they occur in shelf sediments, For discussion of factors controlling the distribution, see text. Thickness of bars has no significance. Shading in bar indicates that the grain type concerned may be an important or dominant 'component. Distribution of coral/algal reefs is similar to that of the chlorozoan association. between the equator and 30 °. These low-latitude foramol sediments are not thus classified simply because they lack the chlorozoan " k e y " grains; they m a y contain significant barnacle and bryozoan contributions as do those of higher latitudes (e.g., Masse, 1970).
FACTORS CONTROLLING THE DISTRIBUTION OF SKELETAL ASSOCIATIONS
The composition of shelf carbonates is known to be influenced b y many environmental factors, and this is no place for a detailed discussion o f the topic. It is nevertheless useful to consider briefly the conditions controlling the boundary between the foramol and chlorozoan associations. Within the 0--50 fathom range considered, depth appears to have no influence on
LETTER SECTION
M71
the association developed. However, the Persian Gulf could be an exception, with chlorozoan sediments in the shallows being replaced at depth by foramol. The Gulf is also abnormal because calcareous green algae are everywhere absent. As one would imagine, water temperature is a significant factor, but the relationship is not a simple one. The threshold for development of the chiorozoan association appears to be a minimum surface (or near-surface) water temperature of about 1 4 - 1 5 ° C (Fig.2B). But, there are many areas (e.g., the West African shelf; Masse, 1970; McMaster et al., 1971) where the foramol association persists although the minimum temperatures exceed 15~C (Fig.2A). Some o f these distribution anomalies can be removed if the annual mean temperature is also taken into account. Most areas with chlorozoan sediments have mean temperatures of at least 23°C (Fig.2). By plotting, on a map of the winter and summer isotherms of , Mean
o
23"C t
•
Mean
"'+:~
. ,,,,..
o • o o
18eC
" +'....o ,
t+. . . . .... .+'Mr " ~ ,,;++
Minimum
14"0
~
"'" ~++I:+ ,,,-"
5O
o
o.]O.°__°
o~
~';'..o o
20
.........: ............
_+
/ i t .......... "
; l,r,
.......
+'!7:,+:: . . . . . . [...: ......... '-.. / i
......................................
do
......,
++'.+..
....
B 0
I0
20
T.°C. M A X .
30
40
2'o
3o
o 4o
T.°C. M A X .
Fig.2. Distribution of skeletal grain associations with respect to minimum and maximum near-surface water temperatures. A+ Foramol association; B. Chlorozoan association (points plotted as triangles refer to the Shark Bay area - see text). Individual points represent particular areas of shelf where temperature conditions can be charactezized by single maxima and minima. Areas where temperature extremes vary from place to place are represented by points joined by dashed lines, the points indicating the extremes of the range. Data from various sources, see footnote p.M68.
oceanic waters, those points having minima of 14--15°C and means of 23°C, one can trace this foramol/chlorozoan boundary and see that it matches, broadly, that shown by the sediments themselves. In the belt from the equator to 30 ° latitude, water temperatures tend to be higher on the western than on the eastern sides of the oceans. As a result, the foramol association extends well into the tropics on the shelves bordering the oceans on the east. This relationship is shown schematically on Fig.3 which is based on Emery's (1968, fig.2 and 3) concept of an ideal ocean. It is significant, however, that alfltough these temperature thresholds broadly account for the known distribution of the skeletal associations, important anomalies remain. Foramol sediments are found in a number of areas where, on the basis of water temperatures,
M72
LETTER SECTION
pole
pole
60*
..~,l.~,~. ,~,,7 60 °
3~O*
|
Oo SKELETAL
~
ASSOCIATIONS
Fo. . . .
I
Chlorozoon
O* NON-SKELETAL
ASSOCIATIONS
~
Pellet
~
Oolith/oggregote
Fig.3. Distributions of the main carbonate grain associations on the continental shelves of an ideal ocean. Only northern hemisphere shown. Width of shelves exaggerated. Unshaded area in centre of each diagram represents the ocean basin. The oolith/aggregate association is shown as being restricted to part of the western side of the ocean to correspond with the known present-day distributionthe chlorozoan association would be expected (Flg.2A), and in one area (Shark Bay, e.g., Davies, 1970) calcareous green algae, although uncommon, appear as sediment components at mean temperatures as low as 19°C (Fig.2B). Thus, in terms of temperature, there is considerable overlap of the ranges of the foramol and chlorozoan associations. It may be argued that it is unrealistic to expect a perfect correlation between the sediment components, which may have been moved from the site of their formation, and an environmental factor which can only be effective during the life of the organisms from which the grains were formed. This is true, of course, but it can only explain relatively small displacements. In those anomalous situations which do not obey the temperature "laws" the displacement is much too great to be explained by any known transport mechanism. Furthermore, the skeletal grain components can always be checked against the organisms living in the area. One is thus quite justified in asking why the anomalies exist. There are many factors which, together with temperature could be responsible but, as will be shown elsewhere (Lees, in preparation) salinity is probably one of the most important. ACKNOWLEDGEMENTS We would like to thank Dr. R.G.C. Bathurst of Liverpool University for his helpful comments on a draft of this article.
LETTER SECTION
M73
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