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Correlation of six cores from the equatorial Atlantic and the Caribbean
Deep-Sea ~
,
19M, VoL 8, pp. 104 to I U .
Peqrumxz Prem Ltd., London.
Correlation of six cores from the equatorial Atlantic and the Caribbean DAVID B. ERICSON and GOESTA WOLLIN
(Received25 August 1955) Albstmct--Curves of late Pleistoceneclimatic variation based on vertical distribution of planktonk Foraminifera in six cores from the Equatorial Atlantic and Caribbean have been satisfactorily correlated. Variation in percentage of material coarser than 74 microns and variation in coifing direction of Globorotalia truncatu/inoides have also been used in correlation. Such correlation is construed as evidence that the sediment sections in these cores have accumulated slowly, and without interruption by slumping or turbidity current deposition. INTRODUCTION UNTIL fairly recently it was supposed that a core taken almost anywhere in deep water would contain an unbroken record of Pleistocene climatic changes. From study of hundreds of cores taken in the North Atlantic (Fig. 1) by Lamont Geological Observatory, Columbia University, it is now evident that slow uninterrupted particle by particle deposition in the deep oceans is the exception and not the rule. The interfering processes are, particularly, current scour, removal or emplacement of sediment by slumping and mud flow, and deposition by turbidity currents according to SI-mPARD (1948), KUENEN (1950), ERICSON, et al. (1952). The six cores discussed in this paper were chosen for temperature determinations and radio-carbon dating because of good evidence that the contained sediment sections represent slow, uninterrupted particle by particle deposition. This evidence is briefly reviewed in the following paper. In addition, curves of climatic variation based on vertical distribution of planktonic Foraminifera are presented. THE CORE APPARATUS AND THE CONDITION OF THE CORES
Description of the Core Apparatus. The cores were obtained with a piston core apparatus which is mainly a modification of the apparatus designed by K U L ~ G (1947). A drawing of the apparatus is shown in Fig. 2. The weight which supplies the force that drives the coring tube into the ocean bottom sediment is usually 1,000 to 2,000 lbs, and is made of lead. Instead of the lead discs as described in Fig. 2 the weight has lately been streamlined, and consists of one solid piece of lead as illustrated in Fig. 3. The weight is attached to the trawl wire by a trigger mechanism which has an arm that extends about 4 ft from the wire. A trigger-weight is attached to the arm by a cord. The length of the cord is chosen so that the trigger-weight hangs 10 to 15 ft below the lower end of the coring tube. The trigger-weight trips the release when it hits the ocean bottom, and allows the apparatus to fall freely into the sediment. 104
Correlation of six cores from the equatorial Atlantic and the Caribbean
105
The standard coring tube is seamless steel tube obtained in nominal 20 ft lengths with 2½ in. inside diameter and a wall thickness o f 18 in. W h e n more than 60 ft o f tubing is used the top section has a wall thickness o f ] in. and the section next to the top section a wall thickness o f ½ in. The 20 ft tube lengths are coupled together and attached to the main weight by couplings which have a clearance o f about •003 in. There are 18 holes in 3 rings o f 6 for the screws at each connection. After the screws have been screwed in the couplings are wrapped with friction tape as an extra safety measure to stop water from entering the tube. 6~
L[GEND :• • • o
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CORES WITH GRADED SAND LAYERS CORES WITH SAND LAYERS NOT GRADED CORES WITH SILT LAYERS CORES WITHOUT SAND OR SILT LAYERS
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IO~ 1 80° 70 ° bOO 500 aO ° 20° Fig. I. Locations and distribution of the sediments in the majority of about 550 deep-sea cores which have bccn obtained from the Atlantic and the Caribbean. About 230 of the col~s contain sand or silt layers. Many of the cores which on the chart arc shown to contain graded sand layers also contain nongraded sand layers and silt layers; cores with non-graded sand layers also contain silt layers, and many of the silt layers arc graded. The sand and silt layers are almost always found below a variable thickness of abyssal forarniniferal lutite or red clay. It is believed that the sand and silt have been transported and deposited by turbidity currents. Only about 15 of the cores without sand or silt layers consists completely of sediments that have bccn accumulated by normal precis of deposition, that is, uninterrupted particle by particle deposition. In the other cores without sand and silt layers the sequence has been broken by slumping or mud flow. The cutting edge is attached to the lower end o f the coring tube by six screws similar to those used at the couplings. The core catcher consists o f a piece o f coring tube, as in. long, and a piece o f spring bronze which is spot-welded to the inside o f the piece o f coring tube. The piece o f bronze is cut so that it has 15 equally spaced fingers which bend out against the wall o f the tube when the core is entering.
106
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Fig. 2. The piston core apparatus: about 1,000 cores, ranging in length from less than a metre to more than 13 metres, have been obtained by Lamont Geological Observatory, Columbia University, with this type of apparatus. The great majority of the cores were raised from depths of more than 2,000 metres, two from depths greater than 8,000 metres. The cores have been taken during 38 expeditions, and therefore most of the cores were obtained with full benefit of the knowledge of the relationship between submarine topography'and sedimentation acquired on earlier expeditions.
Fig. 3. The last operation before the piston core apparatus is lowered into the ocean: the safety pin is removed from the release mechanism. The main weight, below the tail-fin,, is streamlined, cast of lead, and weighs 1,100 lb. The coring tube is assembled and taken apart while the apparatus is resting on supports which are attached to the hull.
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Fig. 4. To the left the top section of a piston core and to the right a trigger-weight core. The cores were obtained simultaneously at the same station. The split halves of the cores were cut out from the same photo, so the scale is the same. The difference in the width of the cores is due to the fact that the diameter, 6.3 cm, of the piston core is twice as big as that of the trigger-weight core. The piston core section is considered to be a correct representation of the sediment column in situ. Correlation of the sediment layers show that the trigger-weight core has been shortened about 50 per cent. The shortening occurred because the core was taken with an apparatus without piston. The trigger-weight cores are obtained in order to be certain that the topmost sediments of the piston cores are present-day deposits. The tops of the trigger-weight cores are considered to represent present-day deposits because, taken short in plastic liners, they can be kept upright until sampled in the laboratory. Comparrson of the core tops in the photo show that the piston core top is undisturbed.
Correlation of six cores from the equatorial Atlantic and the Caribbean
107
If the core should start to slip out when the apparatus is hauled up, the core itself will force the fingers to bend in and prevent the core from slipping out of the tube. A fiege fitting is attached to the outboard end of the trawl wire, and by its use a piston can be bolted to the end of the wire. The piston has three leather cups of the type used as piston rings in conventional water well pumps. The piston is placed at the bottom of the coring tube in contact with the catcher. The trawl wire from the piston extends through the coring tube, the axial tubular opening, the main weight, the release mechanism, the A-frame, to the winch. There is a constriction in diameter to 2 in. at a point near the bottom of the main weight, so that a bumper attached to the fiege fitting will not pass the constriction w h e n the apparatus is hauled up. Because this constriction impedes the flow of water during rapid ascent of the piston in the pipe, there are ports just below it to permit s o m e water to escape. A clamp, known as a come-along, which is self-tightening, is used to attach the release mechanism to the trawl wire. The advantage of this device is that it can, after being hauled up and made fast to the A-frame, be loosened and thus allow the wire to pass through it while the main weight with the tube is hauled up. The come-along also makes it possible to hoist the main weight and tube into the cradle and supports, which are attached to the hull of the vessel, by means of a trawl winch; that is, without using auxiliary hoisting equipment. The trigger weight is usually cast of lead, and ranges from 50 to 80 lb., depending upon the weight of the main weight. Attached to the bottom of the trigger-weight is a sampling tube, 1¼ in. in diameter and about 1 ft long, with cutting edge, catcher, and a plastic liner. The Core's Representation of the Sediment Column in situ. When a core is taken with an apparatus which has no piston, shortening occurs and the sediment column is not correctly represented by the core. The shortening of the core occurs because part of the sediment is squeezed aside as the friction between the sediment and the inner wall of the tube builds up. This problem has been studied and discussed by PRATJE (t934, 1939), EMERY and DIETZ (1941), PIGGOT (1941), and others. To illustrate the shortening a photograph of the trigger-weight core and the top section of the piston core A179-5 (19°30'N, 76°26'W; 4,390 m) is shown in Fig. 4. The cores were obtained at the same time. The top section of the piston core and the trigger weight core are cut out from the same photograph so the scale is the same. It can clearly be seen that the core obtained by a corer without piston, the trigger-weight core, is shortened about 50 per cent. Core A179-5 was selected for the illustration because in this core the shortening is more obvious than in the cores which are described in this paper. That a core which is obtained by a piston core apparatus is a correct representation of the sediment column in situ is discussed by KULLENBERG(1947). Good evidence, we believe, that the cores here described were not shortened by squeezing is provided by burrows of circular cross-section which can be found even at the bottom of the cores. There are also some unfilled burrows in the cores, but even these retain an undistorted cross-section. Therefore it is assumed that, when correlatable zones differ in thickness from core to core, it is because of a real difference in rate of accumulation, and not because of changes in the section due to the coring process.
108
DAVID B. ERICSONand Gov.sr^ WOLUN
The Tops of Piston Cores and Present-day Deposits. To be certain that the t o p m o s t layers o f the piston cores are truly present-day deposits, trigger-weight cores are obtained at the same time. Because these cores are short, and taken in plastic liners which can be kept upright until sampled in the laboratory, the uppermost millimetre o f sediment remains undisturbed and should be really representative o f present-day deposits. Comparison o f piston core tops with corresponding trigger-weight core tops shows that the piston cores have either undisturbed tops, or that only a b o u t 2 em are missing at the tops o f the piston cores. Other evidence for the relatively undisturbed tops o f the cores are the following radiocarbon dates by SuEss: 3,700 -t- 200 years for a sample taken between 0 and 10 cm from core A179-4; 2,960 ± 200 years for a sample taken between 0-8 cm from core A180-73. LOCATIONS OF THE CORES AND TOPOGRAPHY The locations o f the cores are shown by Fig. 5. depths o f water are given in Table I.
The geographical positions and
Table 1. Locations, depths and lengths of cores Core
Latitude
Longitude
Depth in metres
Length in centimetres
A172-6 A179-4 A180-72 A180-73 A 180-74 A 180-76
14°59'N 16°36'N 00°35.5'N 00°10'N 00°0YS 00°46'S
68°51'W 74°48'W 21°47'W 23o00'W 24°10'W 26°02'W
4160 2965 3840 3750 3330 3510
935 690 472 490 480 425
,AI79"4
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so° 70° eoo 50° 4o° jO e 20 ° iOo Fig. 5. Locations of the cores, which were selected from about 550 cores from the Atlantic and the Caribbean for the investigations reported in this paper. The cores were chosen because preliminary investigations showed that the cores contained an unbroken record of climatic and biological changes. The distance between core A180-72 and A179-4 is about 6,0~)Okm, and between AI80-72 and AI80-76 about 480 kin. The cores were raised from gently sloping areas. A172-6 was taken on the crest o f an eastern extension o f the Beata Ridge. A179-4 was taken on a gently sloping b o t t o m southeast of the Albatross Bank to the east o f Jamaica. The two stations are about 600 km apart and are separated by the Beata Ridge.
Correlation of six cores from the equatorial Atlantic and the Caribbean
109
The four equatorial cores are about 6,000 km distant from A179-4. These latter were taken on the gently sloping flanks of the Mid-Atlantic Ridge, 72 and 73 on the NE slope, 74 near the crest, and 76 on the SW slope. 0 ~ om
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500 5O0 Fig. 6. Correlation by lithology and size-fraction analyses of the equatorial cores. The lines within each core in the top half o f the drawing !ndieate boundaries where there are changes from darkbrown or dark-gray to light-brown or hght-gray forammifcral lutite. Corresponding numbers indicate correlating layers. Some numbers are missing in cores A180-74 and A180-76, because the layers are not clearly identified. The line of x's marks a thin layer which has an abundance of diatoms. The lower half of the drawing shows correlation of size fractions greater than 74 microns. The correlations also show that the rate of accumulation was about the same at stations A180-72 and 180-73, and that it was faster at A!80-74 and AI80-76. LITHOLOGICAL DESCRIPTIONS
Core A172-6. The core consists of remarkably uniform brown foraminiferal lutite. Although there is some change in the shade of brown, there is nowhere in the core a section made up of cyclical gray layers, though it must include the time
110
DAVID B. ERICSON and GOESTAWOLLIN
interval represented by the gray layers. Presumably the brown terrigenous sediment in this core was derived from the South American continent. Apparently, Wiirm climate in this part of the world was not so different as to lead to any marked change in colour of the fine terrigenous sediment supplied by South American rivers to the Caribbean. It would seem, therefore, that the gray lutite of the equatorial areas must have had its origin either within the glaciated regions as rock flour or in the region peripheral to the continental glaciers. Probably there was some contribution from both sources. Core ,4179-4. The core is brown foraminiferal lutite. As in the case of A172-6, though there are variations in shade of brown there are no cyclical gray layers. On the whole this core is very similar to A172-6 in lithology, and consequently needs no special comment. Core A 180-73. Essentially the core is made up of a series of layers of foraminiferai lutite of various shades of tan and gray. From zero to 18 cm the colour is gray, gradually changing downward to tan which remains fairly uniform to 36 cm. From 36"to 225 cm the colour is gray only. From 225 to 376 cm the colour is dominantly tan, but with two layers having a total thickness of 40 cm; thus about ] of the section is tan. From 376 to 490 cm the colour is dominantly gray, except for one tan layer only 14 cm thick. Core ,4180-72, A180-74, and A180-76. These three cores of the equatorial suite are essentially similar in sequence of layers, differing principally in thicknesses of the individual layers as shown in Fig. 6. The only notable difference is that in the two westerly cores the layers numbered from 11 to 14 are so poorly defined that no attempt has been made to differentiate them. The association of gray lutite with the zone of cold-water Foraminifera suggests that the source of terrigenous sediment was in some way influenced by the glacial climate. However, it is far from clear how the change in sediment coiour was brought about. A possible explanation may be that the glacial climate led to greater productivity of organic matter in this region, with consequent reduction of iron in the accumulating sediments. Another puzzling characteristic of the cold-water section is its "cyclical "layering. It is made up of nine distinct layers, each distinguished by abrupt change from light gray to dark gray at the base, with gradual change upward into dark gray lutite again. The boundaries of each layer are well defined except to the extent that they are somewhat blurred by burrowing animals. Again, rather sudden minor climatic changes within the Wiirm glacial stage might conceivably have led to marked changes in organic productivity, and consequently to more or less pigmentation or reduction of the resulting sediments. However, until more is known about the chemical and organic content of these peculiar layers it is hardly profitable to speculate on their origin. SIZE FRACTIONS GREATER THAN 74 MICRONS Samples for paleontological examination are prepared by washing the sample on a 200-mesh sieve having 74 micron spaces between wires.
Correlation of six cores from the equatorial Atlantic and the Caribbean
!I!
It has for some times been routine practice to weigh the original sample and the coarse material retained on the sieve from which the percentage of coarse material is then taken. In these cores taken at stations where the bottom has not been subject to bottom scour, or deposition by turbidity currents, the tests o f foraminifera and the smallest terrigenous particles settled essentially vertically downwards, and having reached the bottom remained undisturbed. Since coarse mineral particles and shells of benthic Foraminifera occur in negligible amounts in" these deep-sea sediments, and AI7g-4
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Fig. 7. Comparison of the size fraction analyses of the Caribbean cores, AI72-6 and AI79-4, and a core from the equatorial Atlantic. The curves indicate percentage of size fraction greater than 74 microns. Correlation between AI80-73 and A!79-4 is evident. since essentially all of the tests o f planktonic Foraminifera are larger than 74 micron diameter, it is evident that the percentage o f material coarser than 74 microns in any sample is a rough measure o f the productivity o f planktonic Foraminifera at the time o f deposition. In the long run this productivity should be related to large scale climatic variations. Such climatic changes would probably affect large areas o f the North Atlantic simultaneously. A masking effect should result from variations
112
DAVID B. ERIcsoN and GOtS'rA WOLLIN
in productivity of the Coccolithoforidae, particularly if their productivity increases concurrently with that of the Foraminifera. However, curves of variation in percentage of the seventy-four micron size fraction in the four equatorial cores show that masking effect of fine organically precipitated calcium carbonate has not destroyed the usefulness of this simple method. The results of the size-fraction analyses are shown in Figs. 6 and 7. CHEMICAL ANALYSES The results of carbonate analyses of samples from two of the cores are shown in Table 2. The alkalimeter method was used. This gives a direct measure of the weight Table 2.
Calcium carbonate analyses
Core A 179-4
Core A 180-73
Sample position in cm from top
% CaC03 77.0 67.0 50.7 43.7 49.6 53'6 48.8 63.3 46.4 61"4 59-7 55'0
Top 21 35 152 262 390 440 490 540 590 640 690
Table 3.
Sample position in cm from top
% CaC03
Top 61 91 98 177 192 291 336
66.1 58"0 76.0 68.0 53.0 61.0 62.0 73.5
Spectrochemical analyses o f top samples
SiO2 AI203 TiO2 Fe203 MgO CaO Na20 K20
SrO MnO CuO V2Os BaO Cr203 B203 PbO
A172-6 %
A 179-4 %
19"0 I I'0 0"32 3"3 3"0 36"0
17"0 9"0 0"28 3"1 3"4 37"0 2"5 1"1 0"12 0-18 @008 0"018 0-024 0"018 0.06
3.0 1.5
0-092 0.15 0.004 0.020 0.022 @018 0"07 0.OO8
@008
of carbon dioxide lost by treatment of the sediment with hydrochloric acid. In order to make our values roughly comparable with other published data, we have assumed the carbon dioxide to have been present in the sediment as calcium carbonate. Thus the values given are percentages by weight of " c a l c i u m carbonate '" in dry
Correlation of six cores from the equatorial Atlantic and the Caribbean
Table 4. Sample in c m from top Top
Distribution o.f planktonic foraminifera in Core A172-6 '~ .~
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A A A V V V R
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140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490
X V V V V X X X X X X X X X X X 90% X X X F X X X X X X X X X F A V A V V
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X X X R R X R X R X X X X X X X
R R R X R R F F C X R X X F R X
V V V V V V A V A V V V V V V V
V V V V V V V V V V V V V V V V
V V V V V V V V V V V
V V V V V V C V A V A A A A A V
V V V V V V V A V V V V V A C C
V A A C F X X X X C A C A A V V
X C R R R X X X X X X X X F C R
5 F 3 A 2 F 3 F 3 V 4 V 0 V 4 V I V 3 V I F 4 V 3 F 8 X 2 X 3 C 9 F 8 V 4 V 6 V 4 V 3 R 7 R 8 X
X C F F V V A A A X X X R R C A A X C C A R R R
X X X X X X X X X X X X X X X X X X X X X X X X
X X F F X R R R X X X X R X X R C A A R R R R R
X C V V C X X R A F V V V V V V A A R A A R A A
V V V V V V V V V V V V V V V V V V V V V V V V
V!V V ViA A V i v ~ V V v V V IV A V ~V V V V V V V V V V V V V V :V V V IV V V iV V V iV V V V V V V V V V V V A A V A V V C V V , C R
V V
F X
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13 11 10 3 I 4 I l I I 2 I 9 I 2
2 26 25
V V V V
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114
DAVID B. Emcsos and GOESTAWOLLIN
Table 4 (cont.).
Distribution of planktonic foraminifera in Core A172-6
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v v
620 630 64O 650 66O 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 870 880 890 900 910 920 935
V A X X X 73 108 V V A X X X 100 5 V V A X X X !100 5 V V A X X X 100 2 V V A X X A 100 ! V V A X X A 100 4 V 80 ~ br( ,ken 80% broken 85 % broken A 43 3 R R I X X F X IX X,F V 38 5 R X X X X V 22 15 V X X X !X A 7 34 V X X X XlA 4 57 V X X X X A 26 50 V X X X IX A 19 65 V X X X :x A 53 46 V R: X X x A 100 l0 V 90% broken 60% broken 70% broken X [ X X X C 68 14 R X X X X A 36 47 X X X X X A 50 31 R X X X X A 6 25 C X X X X A 54 5 V A X X X A 105 2 V V X X ~X A 100 2 V V X X 'X C I00 4 V V X X X C 25 i V V~X X X F 100 2 V 80% broken
,v
"
~
~
~
~
~
V X X X X C
X X R X X X
A A X A A A
A A A A A A
v
v
A
v
x x
V V V V V V
V A V V V V
V V V V V V
V V V V V A
X X X X X X
X X X X X F
V V
X X
A X
A A
V V
V V
F A
V V
V V
A A
V V V F V V A
X X X X X X X
X X X X X X X
A A X X X X X
V V V V V V V
V V V V V V V
C C C A A A A
V V A V V V V
V V V V V V A
A °C F F C F C
A×A
A
~
X
A
V
V
V
V
V
V
R X X
X X X
X X X
A A A
V V V
V V V
V V A
V V V
V A A
V A A
X
X
X
A
V
V
V
V
V
A
X A C A V
A X R X X
X A A C A
A A A C C
V V V V V
V V V V V
V V A A F
V A A V V
X X X V X
X X R F F
I
unwashed sediment. F r o m the nature o f the analyses any m ag n esi u m c a r b o n a t e will be included in t h e " c a l c i u m c a r b o n a t e , " however, the error in total c a r b o n a t e f r o m this source is p r o b a b l y s o m e w h a t less than that inherent in the alkalimeter m e t h o d itself, in w h i c h the e r r o r is in the o r d er o f one or two per cent. Th e point to be emphasized is that.the values given are a p p r o x i m a t e measures o f total c a r b o n a t e present, an d not percentages o f calcium c a r b o n a t e alone• A d m i t t e d l y the m e t h o d is crude. H o w e v e r , since the v a r ia ti o n in c a r b o n a t e co n t en t from sample to sample is so m u c h greater than any e r r o r due to the m e t h o d , we feel that the data are o f sufficient significance to b e published. T h e results o f spectrochemical analyses o f the top samples from C o r e A 172-6 and A179-4 are s h o w n in T a b l e 3. T h e analyses were made by Dr. R. G. SmALLEY o f C a l i f o r n i a Research C o r p o r a t i o n . M ICROPALAEONTOLOGICAL ANALYSIS In this l a b o r a t o r y it is c u s t o m a r y to take samples a b o u t ! cm thick at intervals o f a p p r o x i m a t e l y 10 cm. T h e weights o f such samples vary a b o u t an average o f 5 gin. Af t er drying a n d weighing, the samples are washed on a sieve. It has been
Correlation o f six cores from the equatorial Atlantic and the Caribbean
115
Table 5. Distribution of planktonic foramin Cerain Core A 179-4 Sample ~ position ~ in cm from top ~ 6 Top 10 20 4O 50
V V A
~
d.~ ~A~
~ 6
~ 6
A
X
V
X
XxgX R R
X
X
170 180 190 200 210 220 230 240 250 260 265 270 275 280 290 300 310 320 330 340 350 360
X X R R F X R R R R C V V V V A V V V V R F R V A V A A A C A V
X X X X X X R X X R X X V A A C X X X X R C A A V V V A V V V
X X X X X X X X X R A A V V V V
380
A
6o
70 80 90 100 I10 120 130 140 150
x x x
160
v
X X X X X A V A A A V C V V
•
~a~ ~,
~
~ ~" ~, "~ ~ 6 6 6 6 58 !1 68 73 100 100 100 100 100 95 84 100! I00 i 1001 iOOi 97 100 35 51 I00 27 100 I00 14 18 100 100 100 100 100i
21 5 1 3 2 4 22 14 31 40 7 21 2l 81 7 39 60 17 17 4 l0 6 4 3 11 2 20 4 4
390
A V V
4OO
V
420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570
V V V V V V V V V V V V V V V V
100 2 100 1 100 II 100 0 100 5 100 14 80 6 5 2 4 1 66 84 83 43 100 7 100 5 100 5 491 7 100! 4
v
v
100
59
V C C A A A V V V F F F X X F
V X F V V V V C F A F F X X R
~
.~ ~
~ 6
~ "" 6 6
C C A A V A V V V V V A V V V V A V V V V A V V A V V V V V
X X C V V C F V V V R V V A A C C X X C X X X X C X V V V A
X X X F X X X R R C A A A C R R X X X C C R C C C C C X V' V
.R X X X X X X X X X X X X C C
A V V V A A V V V A V V V V V V V V V C A V V V V V V A V
A X X X X X X X X X V X V C X X A X X X X R A R X F X X X
X R X X X X X X X R C C C F C C C X X X R X X X X X X C F
C C C C C X X X X R A A F X F C C X X R X X R F C C A X X
6 X X X X X X X R R X X R R X
I
~
.~ -~ G ~ ~ ~ ~ 6 6 6 6 'V V V V V W V V V V V V V V vvv W
V V V V V V V V V V
..: ..~ "~ ~ ..z 6 ~
F 'A C V C V A V A V A V A V A V A A V v v V V
~ .~
C C A C C V X V X X X X X X X X X X R X R X V X V,X A X C R A X R X A C A A A A A R A X A R V C A A A R V A X X V X X X C X A X A R V R A X V A V R V C V C V C V X V R V X V R F X A R A R A F F C R F A A A A R F R R R R X X R X X X R C R A
X VvVVV,iV,IVVv V
v
v
A
v V V
v
V V V V
W
v x W
v v
R V A V
V
W
v
V
v v
V
via
v
v v
V
6 3 100 2 100 2 100 18 100 8 100 6 100 3 1001 2. 100 2 100 3 100 0
A
,.:
v
V V V V V
v v
:
V V A V
v
V V V V V
v
v
! V V v V v
V V v V v
vl
v
v V
v V
V v V v V
V v V v V
A v
v
c
v
V v V
V v V
V V V V I
V V A A
~ 2 AA A
v v
v v
116
DAVID B. ERICSONand GOIkSTA WOLLIN
Table 5 (cont.).
Distribution of planktonic foraminifera in Core A179-4
n
Sample position in c m
from top 580 590 600 610 620 630 640 650 660 670 680 690
6
6
6
6
6
6
6
6
6
C
R
R R
X X
X X X X C C C X X
R R R X X X X
8
V
X
291 69: 70 100 34 92 69 25 54 100 53
2 5 2 31 30 45
R
A A V A A A A A A A A A
V V V
A C A C C A A F C
X X X X X F X
17 10 12 31
V V V V
X A V A A V F X
100
48
V
C C X X
X X X X
V V
A
6
R
X X F
6
6
A F A
A A V
R
A
F A A A A F
A A A V V A
6
6
v
vV
V V V V V V V V V V
V V V V V V V V V V
8 6
6
V A F F V A A C C C A A
V V A V V V A V V V V V
A A V A A A V A A A A A
V V V C R F V A A A A A
J found that a 200 mesh (74 micron openings) sieve is most satisfactory for this purpose• As a rule samples of this size yield at least enough material to cover a tray of 50 cm 2 area. The tray having been covered, the species of planktonic Foraminifera are noted and the abundance of each is indicated on the faunal chart by one of the following symbols• (These symbols are also used in Tables 4, 5 and 6.) X R F C A V
Absent Rare Frequent Common Abundant Very abundant
I-5 specimens per tray 6-10 11-25 26-100 more than 100
At the same time right and left coiling shells of Globorotalia truncatufinoides are counted. If the species is very abundant, the count is continued until 100 shells of one or the other coiling direction have been counted. This has been found to give the ratio with sufficient accuracy without too great expenditure of time. (In Tables 4, 5 and 6 there are columns for the count of right- and left,coiling shells of Globorotalia truncatulinoides.) Normally benthic species of Foraminifera make up less than one per cent of the material on the tray. Thus change in abundance of a planktonic species is normally not due to dilution of the sample by mineral particles or benthic species. It is due rather to a change in the proportion of individuals of the particular species in the total population of planktonic Foraminifera• Admittedly this method of analysis is crude, but it has the great merit of rapidity, and this has made it possible to examine suites of samples from hundreds of cores taken in the North Atlantic• The resulting abundance of data has, we feel, given a statistical validity to our findings which could not otherwise have been obtained.
Correlation of six cores from the equatorial Atlantic and the Caribbean
Table 6.
Distribution of planktonic foraminifera in Core A180-73
Sample ~ ~. ~
position
from t o p Top 10 20 30 40 50 60 70 80 90 100 ll0 120 130 140 150 160 170 177 193 200 210 220 230 240 250 260 270 280 290 300 310 320 330 3~5 340 350 360 370 380 400 440 460 490
117
.~
E
~
~
<
<
~
~.
6
6
6
6
6
~
6
"-
V V R X X X X X X X X X X X X X X X X X X R F A V V V V A A A V V V V V V A C X R A V V
A V R R R X X X X X X X X X X X X X X X X X X F V V V V V V A V V V V F F X X X X A X V
X X X X X X X X X X X X X X X X X X X X X X R F A V V V V V A V V V V F X X R X X X F V
X R R X X X X F X X R R X X X X R X X X R X R X X X R X X X F R R X X X X X X X F X X R
C C V V V V V V A V A A A A A A A A A V C C V V V A C F A A C C C C V V A A A A V V C A
21 0 A 47 0 A 101 2 V 100 3 F 100 3 X 100 ! F 100 6 R 100 5 R 100 4 F 100 3 R 100 3 R 100 3 F 100 3 R 100 l A 100 2 F 100 2 F 100 2 C 100 7 F 100 I R 100 4 X 100 45 R 54 100 C 100 8 C 100 2 C 100 5 V 100 4 X 140 3 R 130 3 F 100 4 C 100 2 R 100 5 X 100 6 X 100 6 X 100 12 R 46 100! C 44 17 C 76 3 F 100 3 R 100 2 R 100 3 R 100 2: R 100 31 R 36 2 C 59 3 F
6
~-
.d
6
6
6
6
6
6
6
6
~.
X X X V V V A V V V V V A A A F A C C F F C A A F C A A A R X X X R R R R A V C V A V A
X X X X X X X X A F X X X X A X X X R X X R X X X R X X F R X X X X X X X X F V C X X R
X X X X X A V C C C R A A A A C A A V V X R X A R F X X R R X X X X K X X A V A X V C R
C A A V V V V V V V V V V V V V V V V V V V V V V V A V V V V A A V V A V A A V V V V V
V V V V V A V V V V V V V V V V V V V A A A V V V V V V V V V V V V V V V V V V V V V V
V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V R V V V V V V V
X X X R X R A F F A g R X X X R R R A A R R R R C A C C A V C R C A A C C F R R F F F C
R A A X R X A C C A X F F R R X A C C R R R R V F A X R V V R R R F F C V F C C C R C X
V V V X R X X X X X X F A V V V V V V V V V V V V V V V V V V V V V V V V V V X X X F V
R C X X X X X X X X X X X X X X X X X X X R R R R X X R X X X X R R C C C R X X X X X X
T h e f o l l o w i n g list i n c l u d e s t h e s p e c i e s o f p l a n k t o n i c F o r a m i n i f e r a w h i c h h a v e been noted: A b b r e v i a t i o n s u s e d in T a b l e s
Globorotalia Gioborotalia Globorotalia Globorotalia Globorotalia Globorotalia
menardii menardii ( d ' O r b i g n y ) menardii tumida (H. B. B r a d y ) menardii flexuosa ( K o c h ) punctulata hirsuta ( d ' O r b i g n y ) punctulata punctulata ( d ' O r b i g n y ) truncatulinoides ( d ' O r b i g n y )
G. G. G. G. G. G.
m. men. m. turn. re.flex. p. hirs. p. punct. trc. ( R a n d L i n d i c a t e R i g h t
a n d Left c o i l i n g shells)
118
DAVID B. EmCSONand GOmTA WOtUN
Globorotalia scitula (H. B. Brady) Globigerina inflata d'Orbigny Globigerina bulloides d'Orbigny Globigerina pachyderma (Ehrenberg) Globigerina eggeri (R.humbler) Globigerinoides ruber (d'Orbigny) Globigerinoides sacculifer (H. B. Brady) Globigerinoides conglobatus (H. B. Brady) Orbulina universa d'Orbigny Pulleniatina obliquiloculata (Parker and Jones) Sphaeroidinella dehiscens (Parker and Jones)
G. bull. G.. pach. G. egg. G. rub. G. sac. G. cong. O. uni. Pul. obl. Sph. deh.
These species have been excellently figured and discussed in papers by CUSHMAN and HENBEST(1940), and by PI-ILEGER, PARKER and PIERSON(1953). Some modification of the usual classification has seemed admissible in dealing with variation in G. menardii, (s.l.) and G. punctulata, (s.L) We have found that in some samples and at some stations there is intergradation between G. menardii (s.s.) and G. turnida (Brady). Thus the interrelationship is below the species level. Evidently H. B. BRADYappreciated this, for he described G. tumida as Pulvinulina menardii var. tumida. Since G. menardii has priority we are making G. m. menardii the nominate subspecies, and are reducing G. tumida to the rank of subspecies, as G. menardii tumida. G. m. flexuosa was first described by KOCH (1923) from the late Cenozoic of Java as Pulvinulina tumida var. flexuosa. It occurs in abundance below the zone of cool climate deposition, frequently in association with G. m. menardii. We are making KOCH'S variety a sub-species of G. menardii because, as with G. m. tumida, there is complete intergradation between it and G. m. menardii. Apparently this subsepcies is extinct in the North Atlantic. The same relationship has been found to exist between G. hirsuta (d'Orbigny) and G. punctulata (d'Orbigny). Since G. punctulata has priority over G. hirsuta, we are taking G. p. punctulata to be the nominate subspecies. The cores considered in, this paper were taken in areas within which G. m. menardii is presently abundant and dominant over G. m. tumida. Lower in the section the relationship is reversed, G. m. tumida being dominant over G. m. mendardii. EMILIANI (1954) has found, by means of oxygen isotope analyses, that G. m. tumida registers a lower temperature than G. m. menardii. If then G. m. tumida is tolerant of cooler water, it seems reasonable to interpret dominance of G. m. tumida, over G. m. menardii as an indication of cooler climate than that of the present. The next lower well-defined faunal zone is marked by reduction of G. menardii (s.l.) to rare or absent, and by an abundance of G. inflata. Cores taken by the Meteor in the equatorial Atlantic reached this same zone of rare or no G. menardii (s.l.) The top of this zone was considered by W. SCHOT'r (1935) to represent the end of the last glacial stage. We believe that recent findings confirm his and OvEy's (1950) conclusions. Age determinations by radiocarbon (Fig. 8) of samples from the boundary layer agree satisfactorily with the date of the close of the Wtirm (Wisconsin) glacial stage as obtained by BRAMLETTEand BRADLEY(1940), and WISEMAN(1954). The evidence from our much longer cores shows that this was indeed a major
Correlation of six cores from the equatorial Atlantic and the Caribbean
119
climatic change w h i c h lasted for m a n y tens o f thousands o f years. By this a s s u m p t i o n we gain a valuable criterion by w h i c h to distinguish glacial f r o m interglacial stages, even in the equatorial region where, all else being equal, the climatic AI79- 4
AI72-b w
C
W
C
AI80-73 W C
O
2960 ± 200 15~OO+ 300
!,
2 7 0 0 0 +- ISOO
el ~ ' |
I00
2OC
/
f \
300
q
1
400
I
/ 5OO
6oo
:
700
"~%
800 I gO0
Fig. 8. Correlation of faunas in the Caribbean cores, AI72-6 and AI79-4, and a core from the equatorial Atlantic. The curves are based on the proportion of the number of warm-water to cold-water planktonic Foraminifera. W indicates relatively warm climate and C relatively cold climate. The blacked-out zones indicate the parts of the cores which were sampled for radiocarbon age determinations. The numbers to the right of the blacked-out zones are radiocarbon dates in years by HAr~S E. SUESS. The broken lines connect faunal and climatic changes which are considered to have taken place at the same time, the upper one considered to be the end of the Wisconsin (Wiirm). The radiocarbon dates give about the same rate of accumulation, 2.2 cm per 1,000 years, during post-Wisconsin time for Core AI79-4 and A!80-73, but from about 27,000 years to the end of Wisconsin the rate was 3 cm in Ai79-4 and 4-3 cm in AI80-73 per 1,000 years. Correlation of the curves show the same relationship between these two cores. In core AI72-6 the radiocarbon date gives a rate of 3.2 cm per !,000 years for the top 56 cm. The fauna also indicates a greater rate of accumulation during post-Wisconsin time for this core. Further down in core AI72-6, however, the changes in fauna and climate indicate a slower rate for this core compared with the other two. change was p r o b a b l y s o m e w h a t less drastic than in higher latitudes. If c o m p a r a b l e extremes o f climate o b t a i n e d during the earliest stages o f the Pleistocene, they s h o u l d be recorded by c o m p a r a b l e faunal changes.
120
DAVID B. EmcsoN a n d Gom'r^ W o t ~ s
The base of this zone is rather sharply defined by a layer contaitfin8 G. m.flexuosa in abundance. Since this subspecies is extinct, we have no direct evidence on its temperature tolerance. Because it is close morphologically to G. m. menardii, we are tentatively giving it the same weight as G. m. menardii as an indicator of warm climate.
ell
&180-72 w___<
AI80"73 ......
cm
.
.
.
AIIIO.,illt w ~
.
q
ioo
tr t
200
300-
I
o
! t
. I
""-:.: / ;;;; / t
zoo
3oo
l t I
! !
500
Ccm
AI80-72 Ioo%L IO0%R
AI80-73 zoo%L io¢ %g
AI80-74 Joo%L !.(x %R
ioo
ioc-
zoc
AI80-76 ioo%L too%Ro
~
zoo
g)o
i 4O0
v' l j j ~ /
',,, %% %%
40o
30 Fig. 9. Correlation of faunas and coiling direction of Globorotalia truncatalinoides in the equatorial cores. The curves in the top half of the drawing are based on the proportion of the number of warm-water to cold-water planktonic Foraminifera. W indicates relatively warm climate and C relatively cold climate. The curves in the lower half of the drawing are based on the ratio in percentnge between right and left coiling shells of Globorotalia truncatulinoides. L indicates left and R right. Comparison of the broken lines which indicate the correlation of changes in fauna and climate with the broken lines which indicate the corelation of changes in the coiling direction of the shells show a relationship. There is some indication that when there was a distinct change in climate there was also a distinct change in the coiling direction of the shells; for example, there is a change from cold to warm at 220 cm, and from warm to cold at 340cm in core Ai80-72, and at the same points there are changes in the coiling direction. The correlations also show that the rate of accumulation was about the same at stations A180-72 and 73, and that it was faster at A!80-74 and 76.
Correlation of six cores from the equatorial Atlantic a n d the Caribbean
121
The writers consider the following species to be most significant as climatic indicators. G. m. menardii, G. m. flexuosa, and G. tumida are particularly characteristic of warm water. G. p. punctulata, G. scitula, and particularly Globigerina inflata, in abundance, are indicative of relatively cold water. Globigerina bulloides and G. pachyderrna occur in great abundance at northern stations in the Atlantic, but in At?2-b
AI72-b
oS
W
¢
,,,,
40°
30° 20 o
\
t 1
I00
.I
¢. 2CXl d" / O0
31Xl AiTq-4 O:
w
AITq- 4
¢
40 ° 30o 20°
m! l
.)
/
%
1t
,/
AI80-73
o- W
AI00-73
C
3S° 25°
IS°
IOO
I(~-
1•~"
)
=
I
i
<
200
: 300
Fig. I0. Comparison of climate determinations by planktonic Foraminifera with temperatures by the oxygen isotopa n~thod. The curves with W and C at the top are based on the proportion of the number of walTn-water to cold-water planktonic Foraminifera, W indicates relatively warm climate and C relatively cold climate. The other curves aep based on isotopic temperature data by CESARE E~aLXANI; degrees in Celsius. The climate determinations obtained by the different methods correlate well except from part of Core A180-73. It is assumed that the species of the top sample are adapted to the present climate. This present assemblage is taken as a standard to which assemblages lower in the core are referred. Present climate is plotted on the mid-line, and inferred past climates are plotted with respect to it.
122
DAVID B. EalCSON and GOE$1"AWOLLIN
the equatorial and Caribbean cores they are not, we believe, sufficiently abundant at any level in the cores to be of much use as indicators of cold climate. Figs. 8 and 9 show the climatic curves. Pulleniatina obliquiloculata and Sphaeroidinella dehiscens are of somewhat doubtful significance. In the equatorial cores Pulleniatina obliquiloculata is very abundant in the lower half of the zone of cool climate. This we interpreted as indicating a climate slightly less cold than that of the upper half, in which the species is absent. On the whole this interpretation is supported by the oxygen isotope data, which show progressive cooling during this time (Fig. 10). The other species, for example, Globigerinoides ruber, G. saccul(fer, and Orbulina universe are either erratic in distribution or are so tolerant of temperature variation that they are useless as indicators. However, for the sake of completeness they have been included in the faunal distribution tables. CORRELATION The four equatorial cores are a particularly good example of correlation. When these cores were first inspected, it was evident from the colours of the various sediment layers that there was at least a gross lithological correlation (Fig. 6). Weighing and sieving of samples for palaeontological study has shown that the percentage of material coarser than 74 microns varies correspondingly from core to core (Fig. 6). This in itself is good evidence of slow particle by particle deposition, undisturbed by turbidity currents or any process such as slumping or current scour. It is hardly conceivable that these agents coiald have worked simultaneously and uniformly at four stations widely separated in depth and horizontal distance. Microscopic examination of the washed material has yielded evidence confirming the identification of layer 8 (Fig. 6) in the four cores. It was found to contain frustules of a large diatom, Ethmodiscus sp., in abundance. Nowhere else in the cores does this form occur, except rarely. Two more layers are identifiable in the four cores by an independent criterion, that is, the coiling direction of G. truncatulinoides (BOLLI, 1950; 1951, ERICSON, et el., 1954). Thanks to these independent checks by means of lithology, particle size fraction, and coiling direction of G. truncatulinoides, we are confident that the gross faunal variations from which we have inferred climatic changes are real, and afford a reliable record of regional climatic variation during the late Pleistocene. Correlation of the inferred climatic curves in these four cores is shown by Fig. 9. Can these correlations be extended 6,000 km and into the Caribbean ? The diatom layer we can eliminate at once. It is quite absent from the Caribbean cores. Fig. 1 ! shows comparison of curves of coiling direction of G. truncatulinoides in the Caribbean cores and one of the equatorial cores. Evidently no close correlation exists between the equatorial and Caribbean, or even between the two Caribbean cores. The well defined point at 170 cm in A179-4 is absent from A172-6. This might have been predicted from recent distribution of coiling in the Caribbean. There is much variation there from place to place. On the other hand, in the equatorial region the coiling is over 90 per cent right throughout. Presumably conditions influencing coiling in the equatorial region are and have been fairly uniform.
Correlation of six cores from the equatorial Atlantic and the Caribbean
123
The 74 micron fraction gives a rather strikingly good correlation between A180-73, the equatorial core, and A179-4, the most remote of the two Caribbean cores. Correlation with A172-6 is less satisfactory, but there is at least some correspondence between the curves. AI72-6
o-
ioo%L
AI79-4
ioo%R
A180-73
ioo%R
\
i
IO0
ioo%L
/
~oo%L
ioo% ,4 O
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I 200-
lO0
!
i
I
300 ~-
40C1 -
i
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t
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t
S
500
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700
.+t,, oo
Fig. 11. Comparison of the coiling direction of Globorotalia truncatulinoides in the Caribbean cores, AI72-6 and A179--4, and a core from the equatorial Atlantic. The curves are based on the ratio in
percentage between right and left coiling shells. L indicates left and R right. correlation can be seen.
Evidence of some
Fig. 8 shows correlation between climatic curves. Here the correspondence between the Caribbean cores is excellent, and that between them and the equatorial core is good. CONCLUSIONS
Correlation by variation in coiling direction of G. truncatulinoides is reliable in the equatorial region over distances in the order of at least 500 kin. However, correlation between the equatorial region and the Caribbean by this means is not satisfactory. Correlation by variation in percentage of coarse material (74 microns) is an
124
DAVID B. ERICSONand Gor.s'rA WOLUN
effective method of correlation in the equatorial region, and may even be extended 6,000 km into the Caribbean. The most reliable method of correlation between the equatorial region and the Caribbean is by climatic zones inferred from variations in numbers of certain temperature-sensitive species of planktonic foraminifera. Climatic zones based on study of the planktonic foraminifera are in good agreement with water temperatures obtained by the oxygen isotope method. Radiocarbon age determinations show that the rate of sediment accumulation in the equatorial region varies from 2-2 cm per thousand years to 4.3 cm per thousand years. It is at least suggestive that the rapid rate of accumulation took place during the latter part. of Wisconsin time. A similar relationship, but in less degree, is found in one core (A179-4) from the Caribbean. Acknowledgements---The authors acknowledge with gratitude the co-operation of C E s ~ EMILIAm and HANs E. SUESS in the study of the cores. We are greatly indebted to MAumcF EWING, Director of the Lament Geological Observatory, Columbia University, for the privilege of working on the cores which he has obtained, and for his generous support of the work. We are greatful to BRUCE C. HEEZ~,~, who was responsible for taking the A180 series of cores, and who kindly advised us regarding the topography. To JA~,~T WOLUN we are thankful for valuable work in the laboratory, to MARIE THARP and JOHN GORSLINEfor drafting of the illustrations, and to R. E. HvmNCnAM and T. HALL for photographic work. The cores were obtained by expeditions which were supported under Contract Nobsr 43355 with the Bureau of Ships, and N6 onr 271 task orders 24 and 13 with Office of Naval Research, both of the Department of the Navy. The study of the cores was financed in part by Contract Nonr 266 (01) with the Office of Naval Research, and in part by the National Science Foundation Research Grant NSF-G763. Lament Geological Observatory, Columbia University. Contribution No. 167. REFERENCES BOLLI,H. (1950), The direction of coiling in the Evolution of some Globorotaliidae. Cushman Found. Foram. Research Contr., 1, 82-89. BOLt,I, H. (1951), Notes on the Direction of Coiling of Rotalid Foraminifera. Cushman Found. Foram. Research Contr., 2, 139-143. BRAML~, M. N. and BRADLEY,W. H. (1940), Geology and Biology of North Atlantic deep-sea cores, It. 1, Lithology and geologic interpretations. U.$. Geol. Surv., Prof. Paper 196-A, 1-34. CUSHMAN, J. A. and HENBEST,L. G. (1940), Geology and biology of North Atlantic deepsea cores, Pt. 2. Foraminifera. U.S. Geol. Sure., Prof. Paper 196-A, 35-50. EMERY, K. O. and D~'rz, R. S. (1941), Gravity coring instrument and mechanics of sediment coring. Geol. Soc. Amer. Bull., [52, 1685. E~nL~NI, C. (1954), Depth habits of some species of pelagic Foraminifera as indicated by oxygen isotope ratios. Amer. J. Sci., 252, 149-158. ERICSON, D. B., Ewn~G, M. and H~ZEEN, B. C. (1952), Turbidity currents and sediments in the North Atlantic. Amer. Assoc. Petrol. Geol. Bull., 36, 489-511. EmCSON, D. B., WOLLIN, G. and WOLLrN, J. (1954), Coiling direction of Globorotalia truncatulinoides in deep-sea cores. Deep-sea Res., 2, 152-158. KOCH, R. (1923), Die jungtertiare Foraminiferenfauna yon Kabn. Eclogae Geol. Helv., Lausanne, 18, (2), 357. K~, PH. H. (1950), Marine Geology. John Wiley & Sons, Inc., New York, 568 pp. KULLEI,~ERG, B. (1947), The piston core sampler. Svenska Hydr.-Biol. [Comm. Skr., Tr. Hydr. 1, (2), 1-46.
Correlation of six cores from the equatorial Atlantic and the Caribbean
125
l~tt,EO~R, F. B., PARKER, F. L. and PIERSON, J. F. (1953), North Atlantic Foraminifera. Swedish Deep-Sea Expedition Repts., 7, (1), 122 pp. [ht:~ffr, C. S. (1941), Factors involved in submarine core sampling. Geol. Soc. Amer. Bull., 52, 1513-1523. PaATaE, O. (1934), Sind die Bodenprofile aus den R6hrenloten ohne Unterbrechungen ? Versuche tiber die Arbeitsweise der R6hrenlote. Ann. Hydr. Mar. Met., 62, 137-144. PRATaE, O. (1939), Die Sedimentation in der stidlichen Ostsee. A'nn. Hydr. Mar. Met., 67, 209-221. SC8OTT, W. (1935), Die Foraminiferen in den aquatorialen Teil des Atlantischen Ozeans. Deutsche Atlantische Exped. Meteor 1925-1927, 3, (3), Lief IB, 43-134. SHEPARD, F. P. (1948), Submarine Geology. Harper and Bros., N.Y. 348 pp. WmEMAN, J. D..H. (1954), The determination and significance of past temperature changes in the upper layer of the equatorial Atlantic Ocean. Proc. Roy. Soc., A 222, 296-323.
,
19M, VoL 8, pp. 104 to I U .
Peqrumxz Prem Ltd., London.
Correlation of six cores from the equatorial Atlantic and the Caribbean DAVID B. ERICSON and GOESTA WOLLIN
(Received25 August 1955) Albstmct--Curves of late Pleistoceneclimatic variation based on vertical distribution of planktonk Foraminifera in six cores from the Equatorial Atlantic and Caribbean have been satisfactorily correlated. Variation in percentage of material coarser than 74 microns and variation in coifing direction of Globorotalia truncatu/inoides have also been used in correlation. Such correlation is construed as evidence that the sediment sections in these cores have accumulated slowly, and without interruption by slumping or turbidity current deposition. INTRODUCTION UNTIL fairly recently it was supposed that a core taken almost anywhere in deep water would contain an unbroken record of Pleistocene climatic changes. From study of hundreds of cores taken in the North Atlantic (Fig. 1) by Lamont Geological Observatory, Columbia University, it is now evident that slow uninterrupted particle by particle deposition in the deep oceans is the exception and not the rule. The interfering processes are, particularly, current scour, removal or emplacement of sediment by slumping and mud flow, and deposition by turbidity currents according to SI-mPARD (1948), KUENEN (1950), ERICSON, et al. (1952). The six cores discussed in this paper were chosen for temperature determinations and radio-carbon dating because of good evidence that the contained sediment sections represent slow, uninterrupted particle by particle deposition. This evidence is briefly reviewed in the following paper. In addition, curves of climatic variation based on vertical distribution of planktonic Foraminifera are presented. THE CORE APPARATUS AND THE CONDITION OF THE CORES
Description of the Core Apparatus. The cores were obtained with a piston core apparatus which is mainly a modification of the apparatus designed by K U L ~ G (1947). A drawing of the apparatus is shown in Fig. 2. The weight which supplies the force that drives the coring tube into the ocean bottom sediment is usually 1,000 to 2,000 lbs, and is made of lead. Instead of the lead discs as described in Fig. 2 the weight has lately been streamlined, and consists of one solid piece of lead as illustrated in Fig. 3. The weight is attached to the trawl wire by a trigger mechanism which has an arm that extends about 4 ft from the wire. A trigger-weight is attached to the arm by a cord. The length of the cord is chosen so that the trigger-weight hangs 10 to 15 ft below the lower end of the coring tube. The trigger-weight trips the release when it hits the ocean bottom, and allows the apparatus to fall freely into the sediment. 104
Correlation of six cores from the equatorial Atlantic and the Caribbean
105
The standard coring tube is seamless steel tube obtained in nominal 20 ft lengths with 2½ in. inside diameter and a wall thickness o f 18 in. W h e n more than 60 ft o f tubing is used the top section has a wall thickness o f ] in. and the section next to the top section a wall thickness o f ½ in. The 20 ft tube lengths are coupled together and attached to the main weight by couplings which have a clearance o f about •003 in. There are 18 holes in 3 rings o f 6 for the screws at each connection. After the screws have been screwed in the couplings are wrapped with friction tape as an extra safety measure to stop water from entering the tube. 6~
L[GEND :• • • o
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IO~ 1 80° 70 ° bOO 500 aO ° 20° Fig. I. Locations and distribution of the sediments in the majority of about 550 deep-sea cores which have bccn obtained from the Atlantic and the Caribbean. About 230 of the col~s contain sand or silt layers. Many of the cores which on the chart arc shown to contain graded sand layers also contain nongraded sand layers and silt layers; cores with non-graded sand layers also contain silt layers, and many of the silt layers arc graded. The sand and silt layers are almost always found below a variable thickness of abyssal forarniniferal lutite or red clay. It is believed that the sand and silt have been transported and deposited by turbidity currents. Only about 15 of the cores without sand or silt layers consists completely of sediments that have bccn accumulated by normal precis of deposition, that is, uninterrupted particle by particle deposition. In the other cores without sand and silt layers the sequence has been broken by slumping or mud flow. The cutting edge is attached to the lower end o f the coring tube by six screws similar to those used at the couplings. The core catcher consists o f a piece o f coring tube, as in. long, and a piece o f spring bronze which is spot-welded to the inside o f the piece o f coring tube. The piece o f bronze is cut so that it has 15 equally spaced fingers which bend out against the wall o f the tube when the core is entering.
106
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Fig. 2. The piston core apparatus: about 1,000 cores, ranging in length from less than a metre to more than 13 metres, have been obtained by Lamont Geological Observatory, Columbia University, with this type of apparatus. The great majority of the cores were raised from depths of more than 2,000 metres, two from depths greater than 8,000 metres. The cores have been taken during 38 expeditions, and therefore most of the cores were obtained with full benefit of the knowledge of the relationship between submarine topography'and sedimentation acquired on earlier expeditions.
Fig. 3. The last operation before the piston core apparatus is lowered into the ocean: the safety pin is removed from the release mechanism. The main weight, below the tail-fin,, is streamlined, cast of lead, and weighs 1,100 lb. The coring tube is assembled and taken apart while the apparatus is resting on supports which are attached to the hull.
/
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Fig. 4. To the left the top section of a piston core and to the right a trigger-weight core. The cores were obtained simultaneously at the same station. The split halves of the cores were cut out from the same photo, so the scale is the same. The difference in the width of the cores is due to the fact that the diameter, 6.3 cm, of the piston core is twice as big as that of the trigger-weight core. The piston core section is considered to be a correct representation of the sediment column in situ. Correlation of the sediment layers show that the trigger-weight core has been shortened about 50 per cent. The shortening occurred because the core was taken with an apparatus without piston. The trigger-weight cores are obtained in order to be certain that the topmost sediments of the piston cores are present-day deposits. The tops of the trigger-weight cores are considered to represent present-day deposits because, taken short in plastic liners, they can be kept upright until sampled in the laboratory. Comparrson of the core tops in the photo show that the piston core top is undisturbed.
Correlation of six cores from the equatorial Atlantic and the Caribbean
107
If the core should start to slip out when the apparatus is hauled up, the core itself will force the fingers to bend in and prevent the core from slipping out of the tube. A fiege fitting is attached to the outboard end of the trawl wire, and by its use a piston can be bolted to the end of the wire. The piston has three leather cups of the type used as piston rings in conventional water well pumps. The piston is placed at the bottom of the coring tube in contact with the catcher. The trawl wire from the piston extends through the coring tube, the axial tubular opening, the main weight, the release mechanism, the A-frame, to the winch. There is a constriction in diameter to 2 in. at a point near the bottom of the main weight, so that a bumper attached to the fiege fitting will not pass the constriction w h e n the apparatus is hauled up. Because this constriction impedes the flow of water during rapid ascent of the piston in the pipe, there are ports just below it to permit s o m e water to escape. A clamp, known as a come-along, which is self-tightening, is used to attach the release mechanism to the trawl wire. The advantage of this device is that it can, after being hauled up and made fast to the A-frame, be loosened and thus allow the wire to pass through it while the main weight with the tube is hauled up. The come-along also makes it possible to hoist the main weight and tube into the cradle and supports, which are attached to the hull of the vessel, by means of a trawl winch; that is, without using auxiliary hoisting equipment. The trigger weight is usually cast of lead, and ranges from 50 to 80 lb., depending upon the weight of the main weight. Attached to the bottom of the trigger-weight is a sampling tube, 1¼ in. in diameter and about 1 ft long, with cutting edge, catcher, and a plastic liner. The Core's Representation of the Sediment Column in situ. When a core is taken with an apparatus which has no piston, shortening occurs and the sediment column is not correctly represented by the core. The shortening of the core occurs because part of the sediment is squeezed aside as the friction between the sediment and the inner wall of the tube builds up. This problem has been studied and discussed by PRATJE (t934, 1939), EMERY and DIETZ (1941), PIGGOT (1941), and others. To illustrate the shortening a photograph of the trigger-weight core and the top section of the piston core A179-5 (19°30'N, 76°26'W; 4,390 m) is shown in Fig. 4. The cores were obtained at the same time. The top section of the piston core and the trigger weight core are cut out from the same photograph so the scale is the same. It can clearly be seen that the core obtained by a corer without piston, the trigger-weight core, is shortened about 50 per cent. Core A179-5 was selected for the illustration because in this core the shortening is more obvious than in the cores which are described in this paper. That a core which is obtained by a piston core apparatus is a correct representation of the sediment column in situ is discussed by KULLENBERG(1947). Good evidence, we believe, that the cores here described were not shortened by squeezing is provided by burrows of circular cross-section which can be found even at the bottom of the cores. There are also some unfilled burrows in the cores, but even these retain an undistorted cross-section. Therefore it is assumed that, when correlatable zones differ in thickness from core to core, it is because of a real difference in rate of accumulation, and not because of changes in the section due to the coring process.
108
DAVID B. ERICSONand Gov.sr^ WOLUN
The Tops of Piston Cores and Present-day Deposits. To be certain that the t o p m o s t layers o f the piston cores are truly present-day deposits, trigger-weight cores are obtained at the same time. Because these cores are short, and taken in plastic liners which can be kept upright until sampled in the laboratory, the uppermost millimetre o f sediment remains undisturbed and should be really representative o f present-day deposits. Comparison o f piston core tops with corresponding trigger-weight core tops shows that the piston cores have either undisturbed tops, or that only a b o u t 2 em are missing at the tops o f the piston cores. Other evidence for the relatively undisturbed tops o f the cores are the following radiocarbon dates by SuEss: 3,700 -t- 200 years for a sample taken between 0 and 10 cm from core A179-4; 2,960 ± 200 years for a sample taken between 0-8 cm from core A180-73. LOCATIONS OF THE CORES AND TOPOGRAPHY The locations o f the cores are shown by Fig. 5. depths o f water are given in Table I.
The geographical positions and
Table 1. Locations, depths and lengths of cores Core
Latitude
Longitude
Depth in metres
Length in centimetres
A172-6 A179-4 A180-72 A180-73 A 180-74 A 180-76
14°59'N 16°36'N 00°35.5'N 00°10'N 00°0YS 00°46'S
68°51'W 74°48'W 21°47'W 23o00'W 24°10'W 26°02'W
4160 2965 3840 3750 3330 3510
935 690 472 490 480 425
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so° 70° eoo 50° 4o° jO e 20 ° iOo Fig. 5. Locations of the cores, which were selected from about 550 cores from the Atlantic and the Caribbean for the investigations reported in this paper. The cores were chosen because preliminary investigations showed that the cores contained an unbroken record of climatic and biological changes. The distance between core A180-72 and A179-4 is about 6,0~)Okm, and between AI80-72 and AI80-76 about 480 kin. The cores were raised from gently sloping areas. A172-6 was taken on the crest o f an eastern extension o f the Beata Ridge. A179-4 was taken on a gently sloping b o t t o m southeast of the Albatross Bank to the east o f Jamaica. The two stations are about 600 km apart and are separated by the Beata Ridge.
Correlation of six cores from the equatorial Atlantic and the Caribbean
109
The four equatorial cores are about 6,000 km distant from A179-4. These latter were taken on the gently sloping flanks of the Mid-Atlantic Ridge, 72 and 73 on the NE slope, 74 near the crest, and 76 on the SW slope. 0 ~ om
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500 5O0 Fig. 6. Correlation by lithology and size-fraction analyses of the equatorial cores. The lines within each core in the top half o f the drawing !ndieate boundaries where there are changes from darkbrown or dark-gray to light-brown or hght-gray forammifcral lutite. Corresponding numbers indicate correlating layers. Some numbers are missing in cores A180-74 and A180-76, because the layers are not clearly identified. The line of x's marks a thin layer which has an abundance of diatoms. The lower half of the drawing shows correlation of size fractions greater than 74 microns. The correlations also show that the rate of accumulation was about the same at stations A180-72 and 180-73, and that it was faster at A!80-74 and AI80-76. LITHOLOGICAL DESCRIPTIONS
Core A172-6. The core consists of remarkably uniform brown foraminiferal lutite. Although there is some change in the shade of brown, there is nowhere in the core a section made up of cyclical gray layers, though it must include the time
110
DAVID B. ERICSON and GOESTAWOLLIN
interval represented by the gray layers. Presumably the brown terrigenous sediment in this core was derived from the South American continent. Apparently, Wiirm climate in this part of the world was not so different as to lead to any marked change in colour of the fine terrigenous sediment supplied by South American rivers to the Caribbean. It would seem, therefore, that the gray lutite of the equatorial areas must have had its origin either within the glaciated regions as rock flour or in the region peripheral to the continental glaciers. Probably there was some contribution from both sources. Core ,4179-4. The core is brown foraminiferal lutite. As in the case of A172-6, though there are variations in shade of brown there are no cyclical gray layers. On the whole this core is very similar to A172-6 in lithology, and consequently needs no special comment. Core A 180-73. Essentially the core is made up of a series of layers of foraminiferai lutite of various shades of tan and gray. From zero to 18 cm the colour is gray, gradually changing downward to tan which remains fairly uniform to 36 cm. From 36"to 225 cm the colour is gray only. From 225 to 376 cm the colour is dominantly tan, but with two layers having a total thickness of 40 cm; thus about ] of the section is tan. From 376 to 490 cm the colour is dominantly gray, except for one tan layer only 14 cm thick. Core ,4180-72, A180-74, and A180-76. These three cores of the equatorial suite are essentially similar in sequence of layers, differing principally in thicknesses of the individual layers as shown in Fig. 6. The only notable difference is that in the two westerly cores the layers numbered from 11 to 14 are so poorly defined that no attempt has been made to differentiate them. The association of gray lutite with the zone of cold-water Foraminifera suggests that the source of terrigenous sediment was in some way influenced by the glacial climate. However, it is far from clear how the change in sediment coiour was brought about. A possible explanation may be that the glacial climate led to greater productivity of organic matter in this region, with consequent reduction of iron in the accumulating sediments. Another puzzling characteristic of the cold-water section is its "cyclical "layering. It is made up of nine distinct layers, each distinguished by abrupt change from light gray to dark gray at the base, with gradual change upward into dark gray lutite again. The boundaries of each layer are well defined except to the extent that they are somewhat blurred by burrowing animals. Again, rather sudden minor climatic changes within the Wiirm glacial stage might conceivably have led to marked changes in organic productivity, and consequently to more or less pigmentation or reduction of the resulting sediments. However, until more is known about the chemical and organic content of these peculiar layers it is hardly profitable to speculate on their origin. SIZE FRACTIONS GREATER THAN 74 MICRONS Samples for paleontological examination are prepared by washing the sample on a 200-mesh sieve having 74 micron spaces between wires.
Correlation of six cores from the equatorial Atlantic and the Caribbean
!I!
It has for some times been routine practice to weigh the original sample and the coarse material retained on the sieve from which the percentage of coarse material is then taken. In these cores taken at stations where the bottom has not been subject to bottom scour, or deposition by turbidity currents, the tests o f foraminifera and the smallest terrigenous particles settled essentially vertically downwards, and having reached the bottom remained undisturbed. Since coarse mineral particles and shells of benthic Foraminifera occur in negligible amounts in" these deep-sea sediments, and AI7g-4
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112
DAVID B. ERIcsoN and GOtS'rA WOLLIN
in productivity of the Coccolithoforidae, particularly if their productivity increases concurrently with that of the Foraminifera. However, curves of variation in percentage of the seventy-four micron size fraction in the four equatorial cores show that masking effect of fine organically precipitated calcium carbonate has not destroyed the usefulness of this simple method. The results of the size-fraction analyses are shown in Figs. 6 and 7. CHEMICAL ANALYSES The results of carbonate analyses of samples from two of the cores are shown in Table 2. The alkalimeter method was used. This gives a direct measure of the weight Table 2.
Calcium carbonate analyses
Core A 179-4
Core A 180-73
Sample position in cm from top
% CaC03 77.0 67.0 50.7 43.7 49.6 53'6 48.8 63.3 46.4 61"4 59-7 55'0
Top 21 35 152 262 390 440 490 540 590 640 690
Table 3.
Sample position in cm from top
% CaC03
Top 61 91 98 177 192 291 336
66.1 58"0 76.0 68.0 53.0 61.0 62.0 73.5
Spectrochemical analyses o f top samples
SiO2 AI203 TiO2 Fe203 MgO CaO Na20 K20
SrO MnO CuO V2Os BaO Cr203 B203 PbO
A172-6 %
A 179-4 %
19"0 I I'0 0"32 3"3 3"0 36"0
17"0 9"0 0"28 3"1 3"4 37"0 2"5 1"1 0"12 0-18 @008 0"018 0-024 0"018 0.06
3.0 1.5
0-092 0.15 0.004 0.020 0.022 @018 0"07 0.OO8
@008
of carbon dioxide lost by treatment of the sediment with hydrochloric acid. In order to make our values roughly comparable with other published data, we have assumed the carbon dioxide to have been present in the sediment as calcium carbonate. Thus the values given are percentages by weight of " c a l c i u m carbonate '" in dry
Correlation of six cores from the equatorial Atlantic and the Caribbean
Table 4. Sample in c m from top Top
Distribution o.f planktonic foraminifera in Core A172-6 '~ .~
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.~
6
"~ I < ~5 6
"I~
X
X
X
C
A
A
F
A
C
C
, × 5 A 8A 15 A ,7A
X X R X × F × R × g ×
~ A A V V V , V V g A V × × V
~ A ~ A A A R ~ C A C V C A C V A A ~
× × X
60 70 80 90 100 110 120
X R X X X X X
X X X X X X X
X X X X X X X
X X X X X X X
2 A 1 R 4 C I R 25 A 10 A 26 V
A A A V V V R
X X X X X X X
X X X R X X R
V V V V A V V
V V V V V V V
V V V V V V V
A A C A A F C
V V V V V V V
X X X X X X X
X X X X X X X
,30
×
×
×
7~
140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400 410 420 430 440 450 460 470 480 490
X V V V V X X X X X X X X X X X 90% X X X F X X X X X X X X X F A V A V V
X F F R R X X X C V V
500 5lO
v
A A A A C A X
82 69 66 72 100 100 100
×
F
,00
X A V V V X X X C C V V V V V V V V V V broken V V V V V V V V V X V X V X V C V C V F V C V C V C V C V R V A A! A V V X X xi c x A v W
X X X X X X X X X X X
C V V A V R R F F A A
X X X X
C A A A
100 69 97 95 58 90 61 100 100 100 100 100 100 100 100 100
v.v ~
~
C
100 100 100 100 100 100 100 100 100 82 100 100 100 100 100 100 100 100 100 100 100 100 100 100
V V[ C
2 32 12
R X X , F F X F R C C A R A X A X F X F X F X R X F X R X R X F X F X R X X X R x A x A
520 530
v V F
540
×
560 570 580 590 600
70% broken 80% b r o k e n R A v ij X A V A X A V A,X
I i
|
~
VA x ~ , A C x X x ~ g 18 X A 98 ~ C X X C 156 R RX,X ~,00
io ~o 30 40 50
X
~
~
×
~
v
v
v
A
v
~
×
V C V V V V C A V V V V V V X X
V F C F V V A A X R A R X X X X
X X X R R X R X R X X X X X X X
R R R X R R F F C X R X X F R X
V V V V V V A V A V V V V V V V
V V V V V V V V V V V V V V V V
V V V V V V V V V V V
V V V V V V C V A V A A A A A V
V V V V V V V A V V V V V A C C
V A A C F X X X X C A C A A V V
X C R R R X X X X X X X X F C R
5 F 3 A 2 F 3 F 3 V 4 V 0 V 4 V I V 3 V I F 4 V 3 F 8 X 2 X 3 C 9 F 8 V 4 V 6 V 4 V 3 R 7 R 8 X
X C F F V V A A A X X X R R C A A X C C A R R R
X X X X X X X X X X X X X X X X X X X X X X X X
X X F F X R R R X X X X R X X R C A A R R R R R
X C V V C X X R A F V V V V V V A A R A A R A A
V V V V V V V V V V V V V V V V V V V V V V V V
V!V V ViA A V i v ~ V V v V V IV A V ~V V V V V V V V V V V V V V :V V V IV V V iV V V iV V V V V V V V V V V V A A V A V V C V V , C R
V V
F X
R V A
X X X
R A A
A A A
V V V
V V V
13 11 10 3 I 4 I l I I 2 I 9 I 2
2 26 25
V V V V
F
V V
F X
V V A A
X R R R
V A A V R X X X X X X X
X R R A R X X X X R R F
A A R
A F R
~v~A~ V R
V V A
I
C V V
114
DAVID B. Emcsos and GOESTAWOLLIN
Table 4 (cont.).
Distribution of planktonic foraminifera in Core A172-6
mi,io,, in
.
cm
Iro,,,top
..
~
~
~
~
~
A
x
R,oo
-.
~
~
~
610
v v
620 630 64O 650 66O 670 680 690 700 710 720 730 740 750 760 770 780 790 800 810 820 830 840 850 860 870 880 890 900 910 920 935
V A X X X 73 108 V V A X X X 100 5 V V A X X X !100 5 V V A X X X 100 2 V V A X X A 100 ! V V A X X A 100 4 V 80 ~ br( ,ken 80% broken 85 % broken A 43 3 R R I X X F X IX X,F V 38 5 R X X X X V 22 15 V X X X !X A 7 34 V X X X XlA 4 57 V X X X X A 26 50 V X X X IX A 19 65 V X X X :x A 53 46 V R: X X x A 100 l0 V 90% broken 60% broken 70% broken X [ X X X C 68 14 R X X X X A 36 47 X X X X X A 50 31 R X X X X A 6 25 C X X X X A 54 5 V A X X X A 105 2 V V X X ~X A 100 2 V V X X 'X C I00 4 V V X X X C 25 i V V~X X X F 100 2 V 80% broken
,v
"
~
~
~
~
~
V X X X X C
X X R X X X
A A X A A A
A A A A A A
v
v
A
v
x x
V V V V V V
V A V V V V
V V V V V V
V V V V V A
X X X X X X
X X X X X F
V V
X X
A X
A A
V V
V V
F A
V V
V V
A A
V V V F V V A
X X X X X X X
X X X X X X X
A A X X X X X
V V V V V V V
V V V V V V V
C C C A A A A
V V A V V V V
V V V V V V A
A °C F F C F C
A×A
A
~
X
A
V
V
V
V
V
V
R X X
X X X
X X X
A A A
V V V
V V V
V V A
V V V
V A A
V A A
X
X
X
A
V
V
V
V
V
A
X A C A V
A X R X X
X A A C A
A A A C C
V V V V V
V V V V V
V V A A F
V A A V V
X X X V X
X X R F F
I
unwashed sediment. F r o m the nature o f the analyses any m ag n esi u m c a r b o n a t e will be included in t h e " c a l c i u m c a r b o n a t e , " however, the error in total c a r b o n a t e f r o m this source is p r o b a b l y s o m e w h a t less than that inherent in the alkalimeter m e t h o d itself, in w h i c h the e r r o r is in the o r d er o f one or two per cent. Th e point to be emphasized is that.the values given are a p p r o x i m a t e measures o f total c a r b o n a t e present, an d not percentages o f calcium c a r b o n a t e alone• A d m i t t e d l y the m e t h o d is crude. H o w e v e r , since the v a r ia ti o n in c a r b o n a t e co n t en t from sample to sample is so m u c h greater than any e r r o r due to the m e t h o d , we feel that the data are o f sufficient significance to b e published. T h e results o f spectrochemical analyses o f the top samples from C o r e A 172-6 and A179-4 are s h o w n in T a b l e 3. T h e analyses were made by Dr. R. G. SmALLEY o f C a l i f o r n i a Research C o r p o r a t i o n . M ICROPALAEONTOLOGICAL ANALYSIS In this l a b o r a t o r y it is c u s t o m a r y to take samples a b o u t ! cm thick at intervals o f a p p r o x i m a t e l y 10 cm. T h e weights o f such samples vary a b o u t an average o f 5 gin. Af t er drying a n d weighing, the samples are washed on a sieve. It has been
Correlation o f six cores from the equatorial Atlantic and the Caribbean
115
Table 5. Distribution of planktonic foramin Cerain Core A 179-4 Sample ~ position ~ in cm from top ~ 6 Top 10 20 4O 50
V V A
~
d.~ ~A~
~ 6
~ 6
A
X
V
X
XxgX R R
X
X
170 180 190 200 210 220 230 240 250 260 265 270 275 280 290 300 310 320 330 340 350 360
X X R R F X R R R R C V V V V A V V V V R F R V A V A A A C A V
X X X X X X R X X R X X V A A C X X X X R C A A V V V A V V V
X X X X X X X X X R A A V V V V
380
A
6o
70 80 90 100 I10 120 130 140 150
x x x
160
v
X X X X X A V A A A V C V V
•
~a~ ~,
~
~ ~" ~, "~ ~ 6 6 6 6 58 !1 68 73 100 100 100 100 100 95 84 100! I00 i 1001 iOOi 97 100 35 51 I00 27 100 I00 14 18 100 100 100 100 100i
21 5 1 3 2 4 22 14 31 40 7 21 2l 81 7 39 60 17 17 4 l0 6 4 3 11 2 20 4 4
390
A V V
4OO
V
420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570
V V V V V V V V V V V V V V V V
100 2 100 1 100 II 100 0 100 5 100 14 80 6 5 2 4 1 66 84 83 43 100 7 100 5 100 5 491 7 100! 4
v
v
100
59
V C C A A A V V V F F F X X F
V X F V V V V C F A F F X X R
~
.~ ~
~ 6
~ "" 6 6
C C A A V A V V V V V A V V V V A V V V V A V V A V V V V V
X X C V V C F V V V R V V A A C C X X C X X X X C X V V V A
X X X F X X X R R C A A A C R R X X X C C R C C C C C X V' V
.R X X X X X X X X X X X X C C
A V V V A A V V V A V V V V V V V V V C A V V V V V V A V
A X X X X X X X X X V X V C X X A X X X X R A R X F X X X
X R X X X X X X X R C C C F C C C X X X R X X X X X X C F
C C C C C X X X X R A A F X F C C X X R X X R F C C A X X
6 X X X X X X X R R X X R R X
I
~
.~ -~ G ~ ~ ~ ~ 6 6 6 6 'V V V V V W V V V V V V V V vvv W
V V V V V V V V V V
..: ..~ "~ ~ ..z 6 ~
F 'A C V C V A V A V A V A V A V A A V v v V V
~ .~
C C A C C V X V X X X X X X X X X X R X R X V X V,X A X C R A X R X A C A A A A A R A X A R V C A A A R V A X X V X X X C X A X A R V R A X V A V R V C V C V C V X V R V X V R F X A R A R A F F C R F A A A A R F R R R R X X R X X X R C R A
X VvVVV,iV,IVVv V
v
v
A
v V V
v
V V V V
W
v x W
v v
R V A V
V
W
v
V
v v
V
via
v
v v
V
6 3 100 2 100 2 100 18 100 8 100 6 100 3 1001 2. 100 2 100 3 100 0
A
,.:
v
V V V V V
v v
:
V V A V
v
V V V V V
v
v
! V V v V v
V V v V v
vl
v
v V
v V
V v V v V
V v V v V
A v
v
c
v
V v V
V v V
V V V V I
V V A A
~ 2 AA A
v v
v v
116
DAVID B. ERICSONand GOIkSTA WOLLIN
Table 5 (cont.).
Distribution of planktonic foraminifera in Core A179-4
n
Sample position in c m
from top 580 590 600 610 620 630 640 650 660 670 680 690
6
6
6
6
6
6
6
6
6
C
R
R R
X X
X X X X C C C X X
R R R X X X X
8
V
X
291 69: 70 100 34 92 69 25 54 100 53
2 5 2 31 30 45
R
A A V A A A A A A A A A
V V V
A C A C C A A F C
X X X X X F X
17 10 12 31
V V V V
X A V A A V F X
100
48
V
C C X X
X X X X
V V
A
6
R
X X F
6
6
A F A
A A V
R
A
F A A A A F
A A A V V A
6
6
v
vV
V V V V V V V V V V
V V V V V V V V V V
8 6
6
V A F F V A A C C C A A
V V A V V V A V V V V V
A A V A A A V A A A A A
V V V C R F V A A A A A
J found that a 200 mesh (74 micron openings) sieve is most satisfactory for this purpose• As a rule samples of this size yield at least enough material to cover a tray of 50 cm 2 area. The tray having been covered, the species of planktonic Foraminifera are noted and the abundance of each is indicated on the faunal chart by one of the following symbols• (These symbols are also used in Tables 4, 5 and 6.) X R F C A V
Absent Rare Frequent Common Abundant Very abundant
I-5 specimens per tray 6-10 11-25 26-100 more than 100
At the same time right and left coiling shells of Globorotalia truncatufinoides are counted. If the species is very abundant, the count is continued until 100 shells of one or the other coiling direction have been counted. This has been found to give the ratio with sufficient accuracy without too great expenditure of time. (In Tables 4, 5 and 6 there are columns for the count of right- and left,coiling shells of Globorotalia truncatulinoides.) Normally benthic species of Foraminifera make up less than one per cent of the material on the tray. Thus change in abundance of a planktonic species is normally not due to dilution of the sample by mineral particles or benthic species. It is due rather to a change in the proportion of individuals of the particular species in the total population of planktonic Foraminifera• Admittedly this method of analysis is crude, but it has the great merit of rapidity, and this has made it possible to examine suites of samples from hundreds of cores taken in the North Atlantic• The resulting abundance of data has, we feel, given a statistical validity to our findings which could not otherwise have been obtained.
Correlation of six cores from the equatorial Atlantic and the Caribbean
Table 6.
Distribution of planktonic foraminifera in Core A180-73
Sample ~ ~. ~
position
from t o p Top 10 20 30 40 50 60 70 80 90 100 ll0 120 130 140 150 160 170 177 193 200 210 220 230 240 250 260 270 280 290 300 310 320 330 3~5 340 350 360 370 380 400 440 460 490
117
.~
E
~
~
<
<
~
~.
6
6
6
6
6
~
6
"-
V V R X X X X X X X X X X X X X X X X X X R F A V V V V A A A V V V V V V A C X R A V V
A V R R R X X X X X X X X X X X X X X X X X X F V V V V V V A V V V V F F X X X X A X V
X X X X X X X X X X X X X X X X X X X X X X R F A V V V V V A V V V V F X X R X X X F V
X R R X X X X F X X R R X X X X R X X X R X R X X X R X X X F R R X X X X X X X F X X R
C C V V V V V V A V A A A A A A A A A V C C V V V A C F A A C C C C V V A A A A V V C A
21 0 A 47 0 A 101 2 V 100 3 F 100 3 X 100 ! F 100 6 R 100 5 R 100 4 F 100 3 R 100 3 R 100 3 F 100 3 R 100 l A 100 2 F 100 2 F 100 2 C 100 7 F 100 I R 100 4 X 100 45 R 54 100 C 100 8 C 100 2 C 100 5 V 100 4 X 140 3 R 130 3 F 100 4 C 100 2 R 100 5 X 100 6 X 100 6 X 100 12 R 46 100! C 44 17 C 76 3 F 100 3 R 100 2 R 100 3 R 100 2: R 100 31 R 36 2 C 59 3 F
6
~-
.d
6
6
6
6
6
6
6
6
~.
X X X V V V A V V V V V A A A F A C C F F C A A F C A A A R X X X R R R R A V C V A V A
X X X X X X X X A F X X X X A X X X R X X R X X X R X X F R X X X X X X X X F V C X X R
X X X X X A V C C C R A A A A C A A V V X R X A R F X X R R X X X X K X X A V A X V C R
C A A V V V V V V V V V V V V V V V V V V V V V V V A V V V V A A V V A V A A V V V V V
V V V V V A V V V V V V V V V V V V V A A A V V V V V V V V V V V V V V V V V V V V V V
V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V V R V V V V V V V
X X X R X R A F F A g R X X X R R R A A R R R R C A C C A V C R C A A C C F R R F F F C
R A A X R X A C C A X F F R R X A C C R R R R V F A X R V V R R R F F C V F C C C R C X
V V V X R X X X X X X F A V V V V V V V V V V V V V V V V V V V V V V V V V V X X X F V
R C X X X X X X X X X X X X X X X X X X X R R R R X X R X X X X R R C C C R X X X X X X
T h e f o l l o w i n g list i n c l u d e s t h e s p e c i e s o f p l a n k t o n i c F o r a m i n i f e r a w h i c h h a v e been noted: A b b r e v i a t i o n s u s e d in T a b l e s
Globorotalia Gioborotalia Globorotalia Globorotalia Globorotalia Globorotalia
menardii menardii ( d ' O r b i g n y ) menardii tumida (H. B. B r a d y ) menardii flexuosa ( K o c h ) punctulata hirsuta ( d ' O r b i g n y ) punctulata punctulata ( d ' O r b i g n y ) truncatulinoides ( d ' O r b i g n y )
G. G. G. G. G. G.
m. men. m. turn. re.flex. p. hirs. p. punct. trc. ( R a n d L i n d i c a t e R i g h t
a n d Left c o i l i n g shells)
118
DAVID B. EmCSONand GOmTA WOtUN
Globorotalia scitula (H. B. Brady) Globigerina inflata d'Orbigny Globigerina bulloides d'Orbigny Globigerina pachyderma (Ehrenberg) Globigerina eggeri (R.humbler) Globigerinoides ruber (d'Orbigny) Globigerinoides sacculifer (H. B. Brady) Globigerinoides conglobatus (H. B. Brady) Orbulina universa d'Orbigny Pulleniatina obliquiloculata (Parker and Jones) Sphaeroidinella dehiscens (Parker and Jones)
G. bull. G.. pach. G. egg. G. rub. G. sac. G. cong. O. uni. Pul. obl. Sph. deh.
These species have been excellently figured and discussed in papers by CUSHMAN and HENBEST(1940), and by PI-ILEGER, PARKER and PIERSON(1953). Some modification of the usual classification has seemed admissible in dealing with variation in G. menardii, (s.l.) and G. punctulata, (s.L) We have found that in some samples and at some stations there is intergradation between G. menardii (s.s.) and G. turnida (Brady). Thus the interrelationship is below the species level. Evidently H. B. BRADYappreciated this, for he described G. tumida as Pulvinulina menardii var. tumida. Since G. menardii has priority we are making G. m. menardii the nominate subspecies, and are reducing G. tumida to the rank of subspecies, as G. menardii tumida. G. m. flexuosa was first described by KOCH (1923) from the late Cenozoic of Java as Pulvinulina tumida var. flexuosa. It occurs in abundance below the zone of cool climate deposition, frequently in association with G. m. menardii. We are making KOCH'S variety a sub-species of G. menardii because, as with G. m. tumida, there is complete intergradation between it and G. m. menardii. Apparently this subsepcies is extinct in the North Atlantic. The same relationship has been found to exist between G. hirsuta (d'Orbigny) and G. punctulata (d'Orbigny). Since G. punctulata has priority over G. hirsuta, we are taking G. p. punctulata to be the nominate subspecies. The cores considered in, this paper were taken in areas within which G. m. menardii is presently abundant and dominant over G. m. tumida. Lower in the section the relationship is reversed, G. m. tumida being dominant over G. m. mendardii. EMILIANI (1954) has found, by means of oxygen isotope analyses, that G. m. tumida registers a lower temperature than G. m. menardii. If then G. m. tumida is tolerant of cooler water, it seems reasonable to interpret dominance of G. m. tumida, over G. m. menardii as an indication of cooler climate than that of the present. The next lower well-defined faunal zone is marked by reduction of G. menardii (s.l.) to rare or absent, and by an abundance of G. inflata. Cores taken by the Meteor in the equatorial Atlantic reached this same zone of rare or no G. menardii (s.l.) The top of this zone was considered by W. SCHOT'r (1935) to represent the end of the last glacial stage. We believe that recent findings confirm his and OvEy's (1950) conclusions. Age determinations by radiocarbon (Fig. 8) of samples from the boundary layer agree satisfactorily with the date of the close of the Wtirm (Wisconsin) glacial stage as obtained by BRAMLETTEand BRADLEY(1940), and WISEMAN(1954). The evidence from our much longer cores shows that this was indeed a major
Correlation of six cores from the equatorial Atlantic and the Caribbean
119
climatic change w h i c h lasted for m a n y tens o f thousands o f years. By this a s s u m p t i o n we gain a valuable criterion by w h i c h to distinguish glacial f r o m interglacial stages, even in the equatorial region where, all else being equal, the climatic AI79- 4
AI72-b w
C
W
C
AI80-73 W C
O
2960 ± 200 15~OO+ 300
!,
2 7 0 0 0 +- ISOO
el ~ ' |
I00
2OC
/
f \
300
q
1
400
I
/ 5OO
6oo
:
700
"~%
800 I gO0
Fig. 8. Correlation of faunas in the Caribbean cores, AI72-6 and AI79-4, and a core from the equatorial Atlantic. The curves are based on the proportion of the number of warm-water to cold-water planktonic Foraminifera. W indicates relatively warm climate and C relatively cold climate. The blacked-out zones indicate the parts of the cores which were sampled for radiocarbon age determinations. The numbers to the right of the blacked-out zones are radiocarbon dates in years by HAr~S E. SUESS. The broken lines connect faunal and climatic changes which are considered to have taken place at the same time, the upper one considered to be the end of the Wisconsin (Wiirm). The radiocarbon dates give about the same rate of accumulation, 2.2 cm per 1,000 years, during post-Wisconsin time for Core AI79-4 and A!80-73, but from about 27,000 years to the end of Wisconsin the rate was 3 cm in Ai79-4 and 4-3 cm in AI80-73 per 1,000 years. Correlation of the curves show the same relationship between these two cores. In core AI72-6 the radiocarbon date gives a rate of 3.2 cm per !,000 years for the top 56 cm. The fauna also indicates a greater rate of accumulation during post-Wisconsin time for this core. Further down in core AI72-6, however, the changes in fauna and climate indicate a slower rate for this core compared with the other two. change was p r o b a b l y s o m e w h a t less drastic than in higher latitudes. If c o m p a r a b l e extremes o f climate o b t a i n e d during the earliest stages o f the Pleistocene, they s h o u l d be recorded by c o m p a r a b l e faunal changes.
120
DAVID B. EmcsoN a n d Gom'r^ W o t ~ s
The base of this zone is rather sharply defined by a layer contaitfin8 G. m.flexuosa in abundance. Since this subspecies is extinct, we have no direct evidence on its temperature tolerance. Because it is close morphologically to G. m. menardii, we are tentatively giving it the same weight as G. m. menardii as an indicator of warm climate.
ell
&180-72 w___<
AI80"73 ......
cm
.
.
.
AIIIO.,illt w ~
.
q
ioo
tr t
200
300-
I
o
! t
. I
""-:.: / ;;;; / t
zoo
3oo
l t I
! !
500
Ccm
AI80-72 Ioo%L IO0%R
AI80-73 zoo%L io¢ %g
AI80-74 Joo%L !.(x %R
ioo
ioc-
zoc
AI80-76 ioo%L too%Ro
~
zoo
g)o
i 4O0
v' l j j ~ /
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30 Fig. 9. Correlation of faunas and coiling direction of Globorotalia truncatalinoides in the equatorial cores. The curves in the top half of the drawing are based on the proportion of the number of warm-water to cold-water planktonic Foraminifera. W indicates relatively warm climate and C relatively cold climate. The curves in the lower half of the drawing are based on the ratio in percentnge between right and left coiling shells of Globorotalia truncatulinoides. L indicates left and R right. Comparison of the broken lines which indicate the correlation of changes in fauna and climate with the broken lines which indicate the corelation of changes in the coiling direction of the shells show a relationship. There is some indication that when there was a distinct change in climate there was also a distinct change in the coiling direction of the shells; for example, there is a change from cold to warm at 220 cm, and from warm to cold at 340cm in core Ai80-72, and at the same points there are changes in the coiling direction. The correlations also show that the rate of accumulation was about the same at stations A180-72 and 73, and that it was faster at A!80-74 and 76.
Correlation of six cores from the equatorial Atlantic a n d the Caribbean
121
The writers consider the following species to be most significant as climatic indicators. G. m. menardii, G. m. flexuosa, and G. tumida are particularly characteristic of warm water. G. p. punctulata, G. scitula, and particularly Globigerina inflata, in abundance, are indicative of relatively cold water. Globigerina bulloides and G. pachyderrna occur in great abundance at northern stations in the Atlantic, but in At?2-b
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Fig. I0. Comparison of climate determinations by planktonic Foraminifera with temperatures by the oxygen isotopa n~thod. The curves with W and C at the top are based on the proportion of the number of walTn-water to cold-water planktonic Foraminifera, W indicates relatively warm climate and C relatively cold climate. The other curves aep based on isotopic temperature data by CESARE E~aLXANI; degrees in Celsius. The climate determinations obtained by the different methods correlate well except from part of Core A180-73. It is assumed that the species of the top sample are adapted to the present climate. This present assemblage is taken as a standard to which assemblages lower in the core are referred. Present climate is plotted on the mid-line, and inferred past climates are plotted with respect to it.
122
DAVID B. EalCSON and GOE$1"AWOLLIN
the equatorial and Caribbean cores they are not, we believe, sufficiently abundant at any level in the cores to be of much use as indicators of cold climate. Figs. 8 and 9 show the climatic curves. Pulleniatina obliquiloculata and Sphaeroidinella dehiscens are of somewhat doubtful significance. In the equatorial cores Pulleniatina obliquiloculata is very abundant in the lower half of the zone of cool climate. This we interpreted as indicating a climate slightly less cold than that of the upper half, in which the species is absent. On the whole this interpretation is supported by the oxygen isotope data, which show progressive cooling during this time (Fig. 10). The other species, for example, Globigerinoides ruber, G. saccul(fer, and Orbulina universe are either erratic in distribution or are so tolerant of temperature variation that they are useless as indicators. However, for the sake of completeness they have been included in the faunal distribution tables. CORRELATION The four equatorial cores are a particularly good example of correlation. When these cores were first inspected, it was evident from the colours of the various sediment layers that there was at least a gross lithological correlation (Fig. 6). Weighing and sieving of samples for palaeontological study has shown that the percentage of material coarser than 74 microns varies correspondingly from core to core (Fig. 6). This in itself is good evidence of slow particle by particle deposition, undisturbed by turbidity currents or any process such as slumping or current scour. It is hardly conceivable that these agents coiald have worked simultaneously and uniformly at four stations widely separated in depth and horizontal distance. Microscopic examination of the washed material has yielded evidence confirming the identification of layer 8 (Fig. 6) in the four cores. It was found to contain frustules of a large diatom, Ethmodiscus sp., in abundance. Nowhere else in the cores does this form occur, except rarely. Two more layers are identifiable in the four cores by an independent criterion, that is, the coiling direction of G. truncatulinoides (BOLLI, 1950; 1951, ERICSON, et el., 1954). Thanks to these independent checks by means of lithology, particle size fraction, and coiling direction of G. truncatulinoides, we are confident that the gross faunal variations from which we have inferred climatic changes are real, and afford a reliable record of regional climatic variation during the late Pleistocene. Correlation of the inferred climatic curves in these four cores is shown by Fig. 9. Can these correlations be extended 6,000 km and into the Caribbean ? The diatom layer we can eliminate at once. It is quite absent from the Caribbean cores. Fig. 1 ! shows comparison of curves of coiling direction of G. truncatulinoides in the Caribbean cores and one of the equatorial cores. Evidently no close correlation exists between the equatorial and Caribbean, or even between the two Caribbean cores. The well defined point at 170 cm in A179-4 is absent from A172-6. This might have been predicted from recent distribution of coiling in the Caribbean. There is much variation there from place to place. On the other hand, in the equatorial region the coiling is over 90 per cent right throughout. Presumably conditions influencing coiling in the equatorial region are and have been fairly uniform.
Correlation of six cores from the equatorial Atlantic and the Caribbean
123
The 74 micron fraction gives a rather strikingly good correlation between A180-73, the equatorial core, and A179-4, the most remote of the two Caribbean cores. Correlation with A172-6 is less satisfactory, but there is at least some correspondence between the curves. AI72-6
o-
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Fig. 11. Comparison of the coiling direction of Globorotalia truncatulinoides in the Caribbean cores, AI72-6 and A179--4, and a core from the equatorial Atlantic. The curves are based on the ratio in
percentage between right and left coiling shells. L indicates left and R right. correlation can be seen.
Evidence of some
Fig. 8 shows correlation between climatic curves. Here the correspondence between the Caribbean cores is excellent, and that between them and the equatorial core is good. CONCLUSIONS
Correlation by variation in coiling direction of G. truncatulinoides is reliable in the equatorial region over distances in the order of at least 500 kin. However, correlation between the equatorial region and the Caribbean by this means is not satisfactory. Correlation by variation in percentage of coarse material (74 microns) is an
124
DAVID B. ERICSONand Gor.s'rA WOLUN
effective method of correlation in the equatorial region, and may even be extended 6,000 km into the Caribbean. The most reliable method of correlation between the equatorial region and the Caribbean is by climatic zones inferred from variations in numbers of certain temperature-sensitive species of planktonic foraminifera. Climatic zones based on study of the planktonic foraminifera are in good agreement with water temperatures obtained by the oxygen isotope method. Radiocarbon age determinations show that the rate of sediment accumulation in the equatorial region varies from 2-2 cm per thousand years to 4.3 cm per thousand years. It is at least suggestive that the rapid rate of accumulation took place during the latter part. of Wisconsin time. A similar relationship, but in less degree, is found in one core (A179-4) from the Caribbean. Acknowledgements---The authors acknowledge with gratitude the co-operation of C E s ~ EMILIAm and HANs E. SUESS in the study of the cores. We are greatly indebted to MAumcF EWING, Director of the Lament Geological Observatory, Columbia University, for the privilege of working on the cores which he has obtained, and for his generous support of the work. We are greatful to BRUCE C. HEEZ~,~, who was responsible for taking the A180 series of cores, and who kindly advised us regarding the topography. To JA~,~T WOLUN we are thankful for valuable work in the laboratory, to MARIE THARP and JOHN GORSLINEfor drafting of the illustrations, and to R. E. HvmNCnAM and T. HALL for photographic work. The cores were obtained by expeditions which were supported under Contract Nobsr 43355 with the Bureau of Ships, and N6 onr 271 task orders 24 and 13 with Office of Naval Research, both of the Department of the Navy. The study of the cores was financed in part by Contract Nonr 266 (01) with the Office of Naval Research, and in part by the National Science Foundation Research Grant NSF-G763. Lament Geological Observatory, Columbia University. Contribution No. 167. REFERENCES BOLLI,H. (1950), The direction of coiling in the Evolution of some Globorotaliidae. Cushman Found. Foram. Research Contr., 1, 82-89. BOLt,I, H. (1951), Notes on the Direction of Coiling of Rotalid Foraminifera. Cushman Found. Foram. Research Contr., 2, 139-143. BRAML~, M. N. and BRADLEY,W. H. (1940), Geology and Biology of North Atlantic deep-sea cores, It. 1, Lithology and geologic interpretations. U.$. Geol. Surv., Prof. Paper 196-A, 1-34. CUSHMAN, J. A. and HENBEST,L. G. (1940), Geology and biology of North Atlantic deepsea cores, Pt. 2. Foraminifera. U.S. Geol. Sure., Prof. Paper 196-A, 35-50. EMERY, K. O. and D~'rz, R. S. (1941), Gravity coring instrument and mechanics of sediment coring. Geol. Soc. Amer. Bull., [52, 1685. E~nL~NI, C. (1954), Depth habits of some species of pelagic Foraminifera as indicated by oxygen isotope ratios. Amer. J. Sci., 252, 149-158. ERICSON, D. B., Ewn~G, M. and H~ZEEN, B. C. (1952), Turbidity currents and sediments in the North Atlantic. Amer. Assoc. Petrol. Geol. Bull., 36, 489-511. EmCSON, D. B., WOLLIN, G. and WOLLrN, J. (1954), Coiling direction of Globorotalia truncatulinoides in deep-sea cores. Deep-sea Res., 2, 152-158. KOCH, R. (1923), Die jungtertiare Foraminiferenfauna yon Kabn. Eclogae Geol. Helv., Lausanne, 18, (2), 357. K~, PH. H. (1950), Marine Geology. John Wiley & Sons, Inc., New York, 568 pp. KULLEI,~ERG, B. (1947), The piston core sampler. Svenska Hydr.-Biol. [Comm. Skr., Tr. Hydr. 1, (2), 1-46.
Correlation of six cores from the equatorial Atlantic and the Caribbean
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l~tt,EO~R, F. B., PARKER, F. L. and PIERSON, J. F. (1953), North Atlantic Foraminifera. Swedish Deep-Sea Expedition Repts., 7, (1), 122 pp. [ht:~ffr, C. S. (1941), Factors involved in submarine core sampling. Geol. Soc. Amer. Bull., 52, 1513-1523. PaATaE, O. (1934), Sind die Bodenprofile aus den R6hrenloten ohne Unterbrechungen ? Versuche tiber die Arbeitsweise der R6hrenlote. Ann. Hydr. Mar. Met., 62, 137-144. PRATaE, O. (1939), Die Sedimentation in der stidlichen Ostsee. A'nn. Hydr. Mar. Met., 67, 209-221. SC8OTT, W. (1935), Die Foraminiferen in den aquatorialen Teil des Atlantischen Ozeans. Deutsche Atlantische Exped. Meteor 1925-1927, 3, (3), Lief IB, 43-134. SHEPARD, F. P. (1948), Submarine Geology. Harper and Bros., N.Y. 348 pp. WmEMAN, J. D..H. (1954), The determination and significance of past temperature changes in the upper layer of the equatorial Atlantic Ocean. Proc. Roy. Soc., A 222, 296-323.