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Pleistocene changes in the pore concentration of a planktonic foraminiferal species from the Pacific Ocean
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION OF A PLANKTONIC F O R A M I N I F E R A L SPECIES FROM THE PACIFIC OCEAN* WILLIAM W. WILES
Department of Geology, Newark College of Rutgers University Abstract. A study of the pores on the test wall surfaces of the planktonic foraminiferal species Globigerina eggeri shows marked changes through time in pore concentration per unit area. In Atlantic and Caribbean core samples, the changes in pore concentration correlate with the Pleistocene climatic variations, as established both by faunal analyses and by oxygen- 18 studies. High counts per unit area occur during interglacial ages, and low counts per unit area are typical of glacial ages. Pore concentration counts of Globigerina eggeri sampled from a Lamont core (V 19--47) taken in the Pacific Ocean north of Easter Island show similar changes through time, and presumably these changes relate to Pleistocene ages, as in the Atlantic and Caribbean. However there are no marked faunal variations in the core, suggesting that the Pacific Ocean remained warm throughout the Pleistocene. Evidently pore concentration in Globigerina eggeri is a response to a Pleistocene environmental change other than temperature, and may provide a key for further studies of Pleistocene chronology and rates of deposition in the Pacific Ocean. INTRODUCTION
Correlation of deep-sea sediments has been chiefly accomplished, with varying degrees of confidence, by geochemical datings, paleotemperature determinations, sedimentary changes, and biological variations as recorded in the microfossils. The purpose of this paper is to suggest that, among the several kinds of biological variations that have been noticed and used, changes in the concentrations per unit area of pores in the test wall of common planktonic foraminiferal species may represent another criterion for correlation. Furthermore, the pore count method can be used in the Pacific Ocean, where other methods, useful in the Atlantic Ocean and elsewhere, have not been conspicuously successful. * Lamont Geological Observatory Contr. No. 976. 153
154
WILLIAM W. WILES
The author gratefully acknowledges the aid and advice given him by David B. Ericson, Allan W. H. B6, and many others at Columbia University's Lamont Geological Observatory, and the Rutgers University Research Council. This research was partially supported by National Science Foundation Contract NSF-G4376. METHOD
OF STUDY
The planktonic foraminiferal species selected for this study is Globigerina eggeri, a name which will be used here because of its use in many of the cited references. PARKER (1962) places Globigerina eggeri, in synonymy with Globoquadrina dutertrei. Nearly all of the common planktonic species show some variation of pore concentration through time, but G. eggeri seems to exhibit a more prominent amount of variation, and is also fairly abundant and easily identified. To determine the pore concentration of any particular species of planktonic Foraminifera, twenty specimen tests are chosen at random from a washed and sieved core sample, the core having been sampled at 10 centimeter intervals. The number twenty was established empirically as being small enough for convenience yet large enough for reliability. Evidently the pore concentration of any particular species of planktonic Foraminifera at any moment in time is a rather constant character, as HOFICER (1950; 1951a, b) has pointed out, and twenty tests are adequate. The twenty tests are then placed on a standard glass microscope slide, a drop of liquid mounting medium is placed on each test, and a small square cover glass is rested on top of each drop. Pressure on top of each cover glass, gently applied by a probe, crushes the specimen beneath, and the test fragments are spread by the lateral flow of the liquid mounting medium as the drop is flattened by pressure. The larger test fragments tend to orient themselves parallel to the surface of the glass slide. Two days allow the mounting medium to harden. A petrographic microscope is used for counting the pore concentrations on the test fragments. A magnification of × 270 seems appropriate, though other magnifications might serve equally well if used consistently. The unit area for pore concentration counts is provided by an ocular grid, the square unit area measuring 45/~ on each side. Several rules are followed in obtaining pore counts. All pores that touch the margins of the unit area are counted along with those that fall wholly within the unit area. Fragments that include an aperture or chamber margin are avoided. At least three fragments of a specimen are counted; and each fragment is counted at least three times, each time moving the slide slightly so that the fragment obtains a different orientation beneath the ocular grid. The pore concentrations are counted only on the gently concave inner surface of the test
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
155
fragments. This last rule eliminates the problem caused by secondary thickening of the test wall. Calcite crystal growth occurring on the convex outer surface causes a spreading apart of the pores on the outer surface. Thus the outer surface of a test fragment would have a lower pore concentration than the inner surface, depending upon the amount of secondary thickening. This possible discrepancy is avoided by counting only inner surfaces. The pore concentration count for an individual test is expressed and recorded as a range which includes all of the counts actually made on that test. The average of the ranges of all twenty specimens on the slide represents the typical pore concentration of the particular species chosen for measurement from the core sample. This average can be plotted against a scale of relative time expressed by the level or position of the sample in the core. RESULTS
Following the above procedure, average pore concentration counts of Globigerina eggeri have been obtained for Caribbean Core A179-4 from the Lamont Geological Observatory core collections. The choice of this particular core was dictated by several reasons. It has been intensely sampled and investigated for various purposes, and the results of these investigations are published in papers by EMILIANI (1955); ERICSON and WOLLIN (1956a, b); ERICSON, BROECKER, KULP a n d WOLLIN (1956); ERICSON, EWING, WOLLIN a n d HEEZEN (196 I); ERICSON, EWING and WOLLIN(1964). These papers establish a standard against which the pore count results can be compared. Also, Core A179-4 evidently represents slow but continuous pelagic deposition, without interruption by submarine erosion or contamination by rapid turbidite deposition. Core A179-4 was taken on a gentle slope southeast of the Albatross Bank, east of Jamaica and southwest of Hispaniola. Its coordinates are 16 ° 36' N, 74 ° 48' W. The core is 6.9 meters in length, and was obtained from a depth of 2963 meters. ERICSON and WOLLIN (1956a, b) describe Core A179-4 as foraminiferal lutite, generally uniform in texture and lacking in bedding, except between 250 and 400 cm below the top where some bedding is faintly defined. Evidence of burrowing occurs throughout the core. Carbon 14 age determinations of Core A179-4, reported by ERICSON, BROECKER, KtrLP and WOLLIN (1956), are as follows (in years before the present): A - - 3 9 5 0 ± 250 B - - 1 1 , 8 0 0 4 - 300 C --15,700 4- 400 D - - 2 1 , 3 0 0 ± 800 E --27,600 ± 1000 PO--M
156
WILLIAM W. WILES
The locations in Core A179-4 of these radiocarbon dates are indicated at the left margin of Fig. 1. The left-hand curve in Fig. I shows the plot of the pore count averages of G. eggeri from samples taken at ten centimeter intervals in Core A179--4. The middle curve in Fig. 1 is from EMILIANI (1955) and shows his oxygen 18 A 179-4
19 14 CI4
PORE COUNT
16
20
25
30
O.
I.
2.
RATIO
FIG. 1. Caribbean Core A179-4. Comparison of pore concentration counts of G. eggeri, oxygen-18 temperatures, and G. menardii ratios. See text for carbon-14 dates.
isotope paleotemperature determinations for the same core. In the right-hand curve ERICSONand WOLLIN (1956a, b) show the ratio of Globorotalia menardii abundance to the weight of the greater-than-74 p fraction of the sediment in each sample. G. menardii is a warm water habitant, and the ratio of G. menardii to coarse sediment weight thus reflects climate variations. ERICSONand WOLLIN (1956b) place the Wisconsin-Recent boundary at 30 cm, the Wisconsin interstadial from 190 to 250 cm, the Sangamon-Wisconsin boundary at 280 cm, and the Illinoian-Sangamon boundary at 570 cm.
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
157
Comparison of the three curves in Fig. 1 shows good agreement. Since the G. menardii curve and the oxygen 18 curve both reflect climate variations, the G. eggeripore concentration curve evidently also responds to climate changes, with low pore concentration counts during cold moments in time, and high pore concentration counts during warm. In effect, pore concentration counts of G. eggeri serve as an independent method of determination of Pleistocene chronology as based upon climatic changes, agreeing with and supplementing other methods. This has been confirmed by pore concentration counts in other Caribbean and Atlantic cores. The pore concentration study is illustrated for the Pacific Ocean with Lamont Core V19-47. This core, 4.5 m long, was taken from a depth of 3422 m at 17 ° 00' S, 111 ° 12' W, north of Easter Island. It is composed of foraminiferal lutite, and generally gives the appearance of slow uninterrupted deposition. The coarser-than-74/~ fraction of the sediment ranges from 10 to 30 percent. The core lacks bedding and shows some signs of burrowing. A larger than average amount of broken foraminiferal test fragments occurs in the interval between 110 and 140 cm, at 220 cm, at 310 cm, and at 450 cm below the top. While it is possible that this fragmentation resulted from sampling or sieving procedures, the greater likelihood is that it is somehow diagenetic. Figure 2 shows the average G. eggeri pore concentration counts for Core V19-47. As in the case of Core A179--4, the pore count is shown to vary through time, and by about the same range of variation. Other analyses, such as G. menardii ratios, have not yet been done on this core. Vl9-47 12 |
.
.
13
.
.
14
,5 io
~PORE ~-~ COUNT
r-
,-r
-I
3'
4 i
I
i
i
I
i
i
Fxo. 2. Pacific Core V 1 9 q 7 . Pore concentration c o u n t s o f G. eggeri.
158
WILLIAM W. WILES DISCUSSION
Two observations may be restated from the foregoing: (1) pore concentration counts in the Caribbean core correlate markedly well with other methods of investigation used to recognize Pleistocene climate fluctuations in that core; (2) the average pore concentration counts for G. eggeri show similar degrees of variation through time in both the Caribbean and Pacific cores. It would then seem to follow that the pore count curve in Pacific Core V19-47 also reflects or responds to Pleistocene climate fluctuations. What makes this particularly interesting is the fact that there is no obvious foraminiferal variation in Core V 19--47. G. menardiiis present throughout. There are no obvious shifts between warm water planktonic foraminiferal assemblages and cold water planktonic foraminiferal assemblages. Furthermore EMILIANI(1955; and EMILIANIand FLINT, 1963) states that the oxygen 18 temperature determinations from another eastern equatorial Pacific core (Core 58 of the Swedish Deep-Sea Expedition, 1947-1948) show negligible oscillations of temperature. It would appear that the Pacific Ocean, or at least the eastern equatorial portion of it, did not experience any marked lowering of temperature during the Pleistocene glacial ages. And yet the G. eggeri pore concentration counts in Core V19--47, as shown in Fig. 2, indicate a considerable fluctuation through time. Perhaps the pore concentrations of planktonic Foraminifera in general, and of G. eggeri in particular, are not directly or exclusively related to the temperature conditions of the environment. To find out what environmental factor or combination of factors the pore concentrations of planktonic Foraminifera are responsive to, it would be useful to know what purpose or function the pores serve for the organism. Unfortunately the function of pores is not known, a circumstance that can largely be attributed to the very limited success of attempts to propagate planktonic Foraminifera in a laboratory and thus bring the physiological aspects of the living animal under controlled conditions and scrutiny. One can only hypothesize about many of their life processes and the relationships, if any, between these life processes and such morphological features of the tests as pores. One of the hypothetical functions of pores, suggested by BOLLI, LOEBLICH, and TAPt'AN (1957), is that of buoyancy. Tests having a greater number of and/or larger pores would be lighter, and thus would aid the animal in maintaining its floating mode of life. The density of sea water would determine the degree to which such buoyancy factors as pores might be utilized. Heavier water would require fewer pores. Colder waters are heavier, and low pore concentrations in G. eggeri are found during Pleistocene glacial ages. But in the eastern equatorial Pacific the water temperatures apparently remained relatively unchanged during the Pleistocene. Saltier waters also are heavier. The eustatic lowering of sea level brought about
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
159
by the growth of the continental ice sheets must have resulted in a concentration of salts in the remaining sea waters, and this increased salinity would be distributed through all oceans, barring the usual local exceptions. Perhaps, therefore, the pore concentration counts of G. eggeri in Core V19-47 reflect changes in the salinity of the eastern equatorial Pacific, which relate in turn to glacial changes in sea level. All of this reasoning, of course, should be regarded as a tentative possibility, as it is based upon the uncertain hypothesis that buoyancy is a major function of pores. ARRHE~US (1952) has published climatic curves from some cores in the eastern equatorial Pacific that are based on calcium carbonate content of the sediments. He equates high calcium carbonate content with glacial ages. He reasons that changes in atmospheric circulation during glacial ages caused an increase in the rate of ocean circulation, especially vertical stirring. The upwelling of nutrient-rich waters caused an increase in biological productivity which, in turn, resulted in a higher rate of calcium carbonate secretion and deposition. Figure 3 compares the G. eggeri pore concentration counts of RATE OF CIRCULATION
STAGES
V 19 - - 4 7 PORE COUNT
0
r~ I00
Oo 200 0 -<
~ 300 400 500
FiG. 3. C o m p a r i s o n o f pore concentration c o u n t s o f G. eggeri f r o m Core V 1 9 4 7 , on right, with rate o f circulation, f r o m ARRHENIUS (1952), on left. A tentative correlation between the two curves a n d Pleistocene stages is s h o w n by the dashed lines.
Core V19--47 with the rate of ocean circulation that Arrhenius inferred largely from the carbonate content of cores 58 through 62 of the Swedish Deep-Sea Expedition. The pore concentration count curve has been reversed from .Fig. 2 to make it congruent with Arrhenius's curve in showing cold to the right and warm to the left. The numbers in the center represent Pleistocene
160
WILLIAM W. WILES
stages inferred b y A r r h e n i u s . His ages, given at the left, are b a s e d u p o n the r a d i o c a r b o n - t i t a n i u m m e t h o d o f dating. I n spite o f the c o n s i d e r a b l e distance b e t w e e n C o r e V19-47 a n d A r r h e n i u s ' s line o f cores, there is a g o o d general a g r e e m e n t between the two curves, as s h o w n b y the d a s h e d lines, suggesting a basis for correlation. I f valid, the c o r r e l a t i o n indicates a r a t h e r slow rate o f d e p o s i t i o n for Core V 1 9 - 4 7 : 4 m p e r 500 t h o u s a n d years, if the dates c a n be relied u p o n . F u t u r e studies are p l a n n e d to see if this c o r r e l a t i o n can be confirmed. T o conclude, it a p p e a r s t h a t even t h o u g h the Pacific O c e a n might n o t have b e c o m e a p p r e c i a b l y c o l d e r d u r i n g glacial ages, there p r o b a b l y were o t h e r e n v i r o n m e n t a l changes, such as a quickening o f circulation or an increase in salinity, to m a k e it possible to establish Pleistocene c h r o n o l o g y a n d d e t e r m i n e average rates o f sedimentation. P o r e c o n c e n t r a t i o n studies o f p l a n k t o n i c f o r a m i n i f e r a l species m a y p r o v e to be a worthwhile a p p r o a c h to the p r o b l e m .
REFERENCES ARRHENIUS, G. O. S. (1952) Sediment cores from the east Pacific. Swedish Deep-Sea Exped., Repts., 5, 227 pp. BOLLI, H. M., LOEBLICH, A. R., Jr. and TAPPAN, H. (1957) Planktonic foraminiferal families Hantkeninidae, Orbilinidae, Globorotaliidae and Globotruncanidae. U.S. Nat. Mus. Bull., 215, 3-50. EMILIANI,C. (1955) Pleistocene temperatures. J. Geol., 63, (6), 538-578. EMILIANI, C. and FtaNT, R. F. (1963) The Pleistocene record. In: The Sea, M. N. Hill editor, Wiley, New York, 3, 888-927. ERICSON, D. B. and WOLLIN,G. (1956a) Micropaleontological and isotopic determinations of Pleistocene climates. Micropaleontology, 2, (3), 257-270. ERICSON, D. B. and WOLLIN, G. (1956b) Correlation of six cores from the equatorial Atlantic and the Caribbean. Deep-Sea Res., 3, (2), 104-125. ERICSON, D. B., BROECKER, W. S., KULP, J. L. and WOLLIN, G. (1956) Late Pleistocene climates and deep-sea sediments. Science, 124, (3218), 385-389. ERICSON, D. B., EWING, M., WOLLIN, G. and HEEZEN, B. C. (1961) Atlantic deep-sea sediment cores. Geol. Soc. Amer., Bull., 72, 193-286. ERICSON, D. B., EWINC, M. and WOLLIN, G. (1964) The Pleistocene Epoch in deep-sea sediments. Science, 146, (3645), 723-732. HOFKER, J. (1950) Is more concerted effort possible in establishing the regional significance of planktonic Foraminifera as indices of geologic age ?, The Micropaleontologist, 4, (2). HOFKER, J. (1951a) The Foraminifera of the Siboga Expedition IlL Siboga-Exped., IVb, 1-513. HOFKER, J. (1951b) Pores of Foraminifera. The Micropaleontologist, 5, (4). PARKER, F. L. (1962) Planktonic foraminiferal species in Pacific sediments. Micropaleontology, 8, (2), 219-254.
Department of Geology, Newark College of Rutgers University Abstract. A study of the pores on the test wall surfaces of the planktonic foraminiferal species Globigerina eggeri shows marked changes through time in pore concentration per unit area. In Atlantic and Caribbean core samples, the changes in pore concentration correlate with the Pleistocene climatic variations, as established both by faunal analyses and by oxygen- 18 studies. High counts per unit area occur during interglacial ages, and low counts per unit area are typical of glacial ages. Pore concentration counts of Globigerina eggeri sampled from a Lamont core (V 19--47) taken in the Pacific Ocean north of Easter Island show similar changes through time, and presumably these changes relate to Pleistocene ages, as in the Atlantic and Caribbean. However there are no marked faunal variations in the core, suggesting that the Pacific Ocean remained warm throughout the Pleistocene. Evidently pore concentration in Globigerina eggeri is a response to a Pleistocene environmental change other than temperature, and may provide a key for further studies of Pleistocene chronology and rates of deposition in the Pacific Ocean. INTRODUCTION
Correlation of deep-sea sediments has been chiefly accomplished, with varying degrees of confidence, by geochemical datings, paleotemperature determinations, sedimentary changes, and biological variations as recorded in the microfossils. The purpose of this paper is to suggest that, among the several kinds of biological variations that have been noticed and used, changes in the concentrations per unit area of pores in the test wall of common planktonic foraminiferal species may represent another criterion for correlation. Furthermore, the pore count method can be used in the Pacific Ocean, where other methods, useful in the Atlantic Ocean and elsewhere, have not been conspicuously successful. * Lamont Geological Observatory Contr. No. 976. 153
154
WILLIAM W. WILES
The author gratefully acknowledges the aid and advice given him by David B. Ericson, Allan W. H. B6, and many others at Columbia University's Lamont Geological Observatory, and the Rutgers University Research Council. This research was partially supported by National Science Foundation Contract NSF-G4376. METHOD
OF STUDY
The planktonic foraminiferal species selected for this study is Globigerina eggeri, a name which will be used here because of its use in many of the cited references. PARKER (1962) places Globigerina eggeri, in synonymy with Globoquadrina dutertrei. Nearly all of the common planktonic species show some variation of pore concentration through time, but G. eggeri seems to exhibit a more prominent amount of variation, and is also fairly abundant and easily identified. To determine the pore concentration of any particular species of planktonic Foraminifera, twenty specimen tests are chosen at random from a washed and sieved core sample, the core having been sampled at 10 centimeter intervals. The number twenty was established empirically as being small enough for convenience yet large enough for reliability. Evidently the pore concentration of any particular species of planktonic Foraminifera at any moment in time is a rather constant character, as HOFICER (1950; 1951a, b) has pointed out, and twenty tests are adequate. The twenty tests are then placed on a standard glass microscope slide, a drop of liquid mounting medium is placed on each test, and a small square cover glass is rested on top of each drop. Pressure on top of each cover glass, gently applied by a probe, crushes the specimen beneath, and the test fragments are spread by the lateral flow of the liquid mounting medium as the drop is flattened by pressure. The larger test fragments tend to orient themselves parallel to the surface of the glass slide. Two days allow the mounting medium to harden. A petrographic microscope is used for counting the pore concentrations on the test fragments. A magnification of × 270 seems appropriate, though other magnifications might serve equally well if used consistently. The unit area for pore concentration counts is provided by an ocular grid, the square unit area measuring 45/~ on each side. Several rules are followed in obtaining pore counts. All pores that touch the margins of the unit area are counted along with those that fall wholly within the unit area. Fragments that include an aperture or chamber margin are avoided. At least three fragments of a specimen are counted; and each fragment is counted at least three times, each time moving the slide slightly so that the fragment obtains a different orientation beneath the ocular grid. The pore concentrations are counted only on the gently concave inner surface of the test
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
155
fragments. This last rule eliminates the problem caused by secondary thickening of the test wall. Calcite crystal growth occurring on the convex outer surface causes a spreading apart of the pores on the outer surface. Thus the outer surface of a test fragment would have a lower pore concentration than the inner surface, depending upon the amount of secondary thickening. This possible discrepancy is avoided by counting only inner surfaces. The pore concentration count for an individual test is expressed and recorded as a range which includes all of the counts actually made on that test. The average of the ranges of all twenty specimens on the slide represents the typical pore concentration of the particular species chosen for measurement from the core sample. This average can be plotted against a scale of relative time expressed by the level or position of the sample in the core. RESULTS
Following the above procedure, average pore concentration counts of Globigerina eggeri have been obtained for Caribbean Core A179-4 from the Lamont Geological Observatory core collections. The choice of this particular core was dictated by several reasons. It has been intensely sampled and investigated for various purposes, and the results of these investigations are published in papers by EMILIANI (1955); ERICSON and WOLLIN (1956a, b); ERICSON, BROECKER, KULP a n d WOLLIN (1956); ERICSON, EWING, WOLLIN a n d HEEZEN (196 I); ERICSON, EWING and WOLLIN(1964). These papers establish a standard against which the pore count results can be compared. Also, Core A179-4 evidently represents slow but continuous pelagic deposition, without interruption by submarine erosion or contamination by rapid turbidite deposition. Core A179-4 was taken on a gentle slope southeast of the Albatross Bank, east of Jamaica and southwest of Hispaniola. Its coordinates are 16 ° 36' N, 74 ° 48' W. The core is 6.9 meters in length, and was obtained from a depth of 2963 meters. ERICSON and WOLLIN (1956a, b) describe Core A179-4 as foraminiferal lutite, generally uniform in texture and lacking in bedding, except between 250 and 400 cm below the top where some bedding is faintly defined. Evidence of burrowing occurs throughout the core. Carbon 14 age determinations of Core A179-4, reported by ERICSON, BROECKER, KtrLP and WOLLIN (1956), are as follows (in years before the present): A - - 3 9 5 0 ± 250 B - - 1 1 , 8 0 0 4 - 300 C --15,700 4- 400 D - - 2 1 , 3 0 0 ± 800 E --27,600 ± 1000 PO--M
156
WILLIAM W. WILES
The locations in Core A179-4 of these radiocarbon dates are indicated at the left margin of Fig. 1. The left-hand curve in Fig. I shows the plot of the pore count averages of G. eggeri from samples taken at ten centimeter intervals in Core A179--4. The middle curve in Fig. 1 is from EMILIANI (1955) and shows his oxygen 18 A 179-4
19 14 CI4
PORE COUNT
16
20
25
30
O.
I.
2.
RATIO
FIG. 1. Caribbean Core A179-4. Comparison of pore concentration counts of G. eggeri, oxygen-18 temperatures, and G. menardii ratios. See text for carbon-14 dates.
isotope paleotemperature determinations for the same core. In the right-hand curve ERICSONand WOLLIN (1956a, b) show the ratio of Globorotalia menardii abundance to the weight of the greater-than-74 p fraction of the sediment in each sample. G. menardii is a warm water habitant, and the ratio of G. menardii to coarse sediment weight thus reflects climate variations. ERICSONand WOLLIN (1956b) place the Wisconsin-Recent boundary at 30 cm, the Wisconsin interstadial from 190 to 250 cm, the Sangamon-Wisconsin boundary at 280 cm, and the Illinoian-Sangamon boundary at 570 cm.
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
157
Comparison of the three curves in Fig. 1 shows good agreement. Since the G. menardii curve and the oxygen 18 curve both reflect climate variations, the G. eggeripore concentration curve evidently also responds to climate changes, with low pore concentration counts during cold moments in time, and high pore concentration counts during warm. In effect, pore concentration counts of G. eggeri serve as an independent method of determination of Pleistocene chronology as based upon climatic changes, agreeing with and supplementing other methods. This has been confirmed by pore concentration counts in other Caribbean and Atlantic cores. The pore concentration study is illustrated for the Pacific Ocean with Lamont Core V19-47. This core, 4.5 m long, was taken from a depth of 3422 m at 17 ° 00' S, 111 ° 12' W, north of Easter Island. It is composed of foraminiferal lutite, and generally gives the appearance of slow uninterrupted deposition. The coarser-than-74/~ fraction of the sediment ranges from 10 to 30 percent. The core lacks bedding and shows some signs of burrowing. A larger than average amount of broken foraminiferal test fragments occurs in the interval between 110 and 140 cm, at 220 cm, at 310 cm, and at 450 cm below the top. While it is possible that this fragmentation resulted from sampling or sieving procedures, the greater likelihood is that it is somehow diagenetic. Figure 2 shows the average G. eggeri pore concentration counts for Core V19-47. As in the case of Core A179--4, the pore count is shown to vary through time, and by about the same range of variation. Other analyses, such as G. menardii ratios, have not yet been done on this core. Vl9-47 12 |
.
.
13
.
.
14
,5 io
~PORE ~-~ COUNT
r-
,-r
-I
3'
4 i
I
i
i
I
i
i
Fxo. 2. Pacific Core V 1 9 q 7 . Pore concentration c o u n t s o f G. eggeri.
158
WILLIAM W. WILES DISCUSSION
Two observations may be restated from the foregoing: (1) pore concentration counts in the Caribbean core correlate markedly well with other methods of investigation used to recognize Pleistocene climate fluctuations in that core; (2) the average pore concentration counts for G. eggeri show similar degrees of variation through time in both the Caribbean and Pacific cores. It would then seem to follow that the pore count curve in Pacific Core V19-47 also reflects or responds to Pleistocene climate fluctuations. What makes this particularly interesting is the fact that there is no obvious foraminiferal variation in Core V 19--47. G. menardiiis present throughout. There are no obvious shifts between warm water planktonic foraminiferal assemblages and cold water planktonic foraminiferal assemblages. Furthermore EMILIANI(1955; and EMILIANIand FLINT, 1963) states that the oxygen 18 temperature determinations from another eastern equatorial Pacific core (Core 58 of the Swedish Deep-Sea Expedition, 1947-1948) show negligible oscillations of temperature. It would appear that the Pacific Ocean, or at least the eastern equatorial portion of it, did not experience any marked lowering of temperature during the Pleistocene glacial ages. And yet the G. eggeri pore concentration counts in Core V19--47, as shown in Fig. 2, indicate a considerable fluctuation through time. Perhaps the pore concentrations of planktonic Foraminifera in general, and of G. eggeri in particular, are not directly or exclusively related to the temperature conditions of the environment. To find out what environmental factor or combination of factors the pore concentrations of planktonic Foraminifera are responsive to, it would be useful to know what purpose or function the pores serve for the organism. Unfortunately the function of pores is not known, a circumstance that can largely be attributed to the very limited success of attempts to propagate planktonic Foraminifera in a laboratory and thus bring the physiological aspects of the living animal under controlled conditions and scrutiny. One can only hypothesize about many of their life processes and the relationships, if any, between these life processes and such morphological features of the tests as pores. One of the hypothetical functions of pores, suggested by BOLLI, LOEBLICH, and TAPt'AN (1957), is that of buoyancy. Tests having a greater number of and/or larger pores would be lighter, and thus would aid the animal in maintaining its floating mode of life. The density of sea water would determine the degree to which such buoyancy factors as pores might be utilized. Heavier water would require fewer pores. Colder waters are heavier, and low pore concentrations in G. eggeri are found during Pleistocene glacial ages. But in the eastern equatorial Pacific the water temperatures apparently remained relatively unchanged during the Pleistocene. Saltier waters also are heavier. The eustatic lowering of sea level brought about
PLEISTOCENE CHANGES IN THE PORE CONCENTRATION
159
by the growth of the continental ice sheets must have resulted in a concentration of salts in the remaining sea waters, and this increased salinity would be distributed through all oceans, barring the usual local exceptions. Perhaps, therefore, the pore concentration counts of G. eggeri in Core V19-47 reflect changes in the salinity of the eastern equatorial Pacific, which relate in turn to glacial changes in sea level. All of this reasoning, of course, should be regarded as a tentative possibility, as it is based upon the uncertain hypothesis that buoyancy is a major function of pores. ARRHE~US (1952) has published climatic curves from some cores in the eastern equatorial Pacific that are based on calcium carbonate content of the sediments. He equates high calcium carbonate content with glacial ages. He reasons that changes in atmospheric circulation during glacial ages caused an increase in the rate of ocean circulation, especially vertical stirring. The upwelling of nutrient-rich waters caused an increase in biological productivity which, in turn, resulted in a higher rate of calcium carbonate secretion and deposition. Figure 3 compares the G. eggeri pore concentration counts of RATE OF CIRCULATION
STAGES
V 19 - - 4 7 PORE COUNT
0
r~ I00
Oo 200 0 -<
~ 300 400 500
FiG. 3. C o m p a r i s o n o f pore concentration c o u n t s o f G. eggeri f r o m Core V 1 9 4 7 , on right, with rate o f circulation, f r o m ARRHENIUS (1952), on left. A tentative correlation between the two curves a n d Pleistocene stages is s h o w n by the dashed lines.
Core V19--47 with the rate of ocean circulation that Arrhenius inferred largely from the carbonate content of cores 58 through 62 of the Swedish Deep-Sea Expedition. The pore concentration count curve has been reversed from .Fig. 2 to make it congruent with Arrhenius's curve in showing cold to the right and warm to the left. The numbers in the center represent Pleistocene
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WILLIAM W. WILES
stages inferred b y A r r h e n i u s . His ages, given at the left, are b a s e d u p o n the r a d i o c a r b o n - t i t a n i u m m e t h o d o f dating. I n spite o f the c o n s i d e r a b l e distance b e t w e e n C o r e V19-47 a n d A r r h e n i u s ' s line o f cores, there is a g o o d general a g r e e m e n t between the two curves, as s h o w n b y the d a s h e d lines, suggesting a basis for correlation. I f valid, the c o r r e l a t i o n indicates a r a t h e r slow rate o f d e p o s i t i o n for Core V 1 9 - 4 7 : 4 m p e r 500 t h o u s a n d years, if the dates c a n be relied u p o n . F u t u r e studies are p l a n n e d to see if this c o r r e l a t i o n can be confirmed. T o conclude, it a p p e a r s t h a t even t h o u g h the Pacific O c e a n might n o t have b e c o m e a p p r e c i a b l y c o l d e r d u r i n g glacial ages, there p r o b a b l y were o t h e r e n v i r o n m e n t a l changes, such as a quickening o f circulation or an increase in salinity, to m a k e it possible to establish Pleistocene c h r o n o l o g y a n d d e t e r m i n e average rates o f sedimentation. P o r e c o n c e n t r a t i o n studies o f p l a n k t o n i c f o r a m i n i f e r a l species m a y p r o v e to be a worthwhile a p p r o a c h to the p r o b l e m .
REFERENCES ARRHENIUS, G. O. S. (1952) Sediment cores from the east Pacific. Swedish Deep-Sea Exped., Repts., 5, 227 pp. BOLLI, H. M., LOEBLICH, A. R., Jr. and TAPPAN, H. (1957) Planktonic foraminiferal families Hantkeninidae, Orbilinidae, Globorotaliidae and Globotruncanidae. U.S. Nat. Mus. Bull., 215, 3-50. EMILIANI,C. (1955) Pleistocene temperatures. J. Geol., 63, (6), 538-578. EMILIANI, C. and FtaNT, R. F. (1963) The Pleistocene record. In: The Sea, M. N. Hill editor, Wiley, New York, 3, 888-927. ERICSON, D. B. and WOLLIN,G. (1956a) Micropaleontological and isotopic determinations of Pleistocene climates. Micropaleontology, 2, (3), 257-270. ERICSON, D. B. and WOLLIN, G. (1956b) Correlation of six cores from the equatorial Atlantic and the Caribbean. Deep-Sea Res., 3, (2), 104-125. ERICSON, D. B., BROECKER, W. S., KULP, J. L. and WOLLIN, G. (1956) Late Pleistocene climates and deep-sea sediments. Science, 124, (3218), 385-389. ERICSON, D. B., EWING, M., WOLLIN, G. and HEEZEN, B. C. (1961) Atlantic deep-sea sediment cores. Geol. Soc. Amer., Bull., 72, 193-286. ERICSON, D. B., EWINC, M. and WOLLIN, G. (1964) The Pleistocene Epoch in deep-sea sediments. Science, 146, (3645), 723-732. HOFKER, J. (1950) Is more concerted effort possible in establishing the regional significance of planktonic Foraminifera as indices of geologic age ?, The Micropaleontologist, 4, (2). HOFKER, J. (1951a) The Foraminifera of the Siboga Expedition IlL Siboga-Exped., IVb, 1-513. HOFKER, J. (1951b) Pores of Foraminifera. The Micropaleontologist, 5, (4). PARKER, F. L. (1962) Planktonic foraminiferal species in Pacific sediments. Micropaleontology, 8, (2), 219-254.