Planktonic foraminiferal dissolution at high latitudes of
rakes, L. A. 1975. Paleoclimatic significance of some sedimentary components at site 274. In: Initial reports of the Deep Sea Drilling Project, 28 (D. E. Hayes et al., eds.). U.S. Government Printing Office, Washington, D.C. pp. 785-789. Gombos, A. M. 1977. Paleogene and Neogene diatoms from the Falkland Plateau and Malvinas Outer Basin: leg 36, Deep Sea Drilling Project. In: Initial reports of the Deep Sea Drilling Project, 36 (P. F. Barker et al., eds.). U.S. Government Printing Office, Washington, D.C. pp. 575-688. Hayes. D. E., and L. A. Frakes. 1975. General synthesis, Deep Sea Drilling Project, leg 28. In: Initial reports of the Deep Sea Drilling Project, 28 (D. E. Hayes et al., eds.). U.S. Government Printing Office, Washington, D.C. pp. 919-942. Kennett,J. P., and N. D. Watkins. 1974. Late Miocene-early Pliocene paleomagnetic stratigraphy, paleoclimatology, and biostratigraphy in New Zealand. Geological Society of America Bulletin, 85: 1385-1398. Peck, Douglas, T. M. Missimer, and S. W. Wise. 1976. Glacialeustatic control of an upper Miocene transgressive-regressive sequence in a low latitude (Southeastern U.S.) coastal plain.
Planktonic foraminiferal dissolution at high latitudes of the southwestern Atlantic BJ5RN A. MALMGREN and HANS G. CR0NBLAD Department of Geology Stockholm University Box 6801 11386 Stockholm, Sweden
As part of a study of Recent planktonic foraminifera from the South Atlantic sector of the southern ocean, we have analyzed 53 core-top samples (Eltanin and Islas Orcadas cruises) from the southwestern Atlantic (Scotia and Weddell Seas; figure 1). In the eastern South Atlantic (east of the MidAtlantic Ridge), the calcium-carbonate compensation depth (CCD) under antarctic waters is 3,700 meters (Lozano and Hays, 1976). In the western regime, none of the core tops in areas south of the Antarctic Convergence (Polar Front) have calcium carbonate in excess of 10 percent (figure 2). CaCO3 was analyzed by the Hiilsemann (1966) gasometric method. If the definition of CCD as the depth at which the amount of CaCO 3 decreases to 10 percent is followed (Lisitzin, 1972), this indicates a CCD of less than 2,000 meters in the western basin, which is much shallower than in the eastern basin. Using the same criteria, the CCD in the Polar Front Zone is between 2,100 and 3,000 meters (figure 1). We could not determine CCD north of the Polar Front because we used samples only from depths shallower than 4,000 meters. However, Lozano and Hays (1976) have reported a CCD of 4,900 meters under subtropical and subantarctic waters in this area. This shows that the CCD rapidly shallows southward across the Polar Front. October 1978
Abstracts with Programs, (Geological Society of America), 8: 1044. (Abstract) Tucholke, B. E., and G. B. Carpenter. 1977. Sediment distribution and Cenozoic sedimentation patterns on the Agulhas Plateau. Geological Society of America Bulletin, 88: 1337-1346. Van Couvering,J. A., W. A. Berggren, R E. Drake, E. Aguirre, and G. H. Curtis. 1976. The terminal Miocene event. Marine Micropaleontology, 1: 263-286. Weaver, F. M. 1976. Late Miocene and Pliocene radiolarian paleobiogeography and biostratigraphy of the Southern Ocean. Unpublished doctoral dissertation, Department of Geology, Florida State University. Wise, S. W., P. F. Ciesielski, D. T. MacKenzie, F. H. Wind, K. E. Busen, A. M. Gombos, B. U. Haq, G. P. Lohmann, R C. Tjalsma, W. K. Harris, R. W. Hedlund, D. N. Beju, D. L. Jones, G. Plafker, and W. V. Sliter. In press. Paleontologic synthesis for the Southwest Atlantic Ocean Basin based on Jurassic to recent faunas and floras from the Falkland Plateau. Paper presented at SCAR Symposium on Antarctic Geology and Geophysics, Madison, Wisconsin, 22-27 August 1977.
Differential -dissolution effects were analyzed using three dissolution -sensitive parameters (figure 3): percentages of fragments of planktonic foraminiferal tests (in relation to fragments plus whole tests), percentages of benthonic foraminifera (in relation to benthonic plus planktonic foraminifera), and proportions between the frequencies of the planktonic foraminifera Globorotalia inflata and Globigerina bulloides. Fragmentation is relatively constant down to about 3,500 meters, after which it increases considerably, indicating an increase in dissolution below this depth. An increase in benthonic foraminiferal percentages occurs at about the same depth. High percentages of benthonic foraminifera at depths shallower than 2,000 meters probably are related not to dissolution, but rather to greater relative abundances of benthonic foraminifera in shallower depths of the Maurice Ewing Bank (eastern Falkland Plateau; figure 1). The ratio between the dissolution-resistant G. inflata and the more susceptible G. bulloides (Berger, 1970) appears to be particularly sensitive to dissolution. The samples are from a restricted latitudinal range, and hence the ratio between the two species is not considered to represent biogeographic changes between the two species. G. bulloides is dominant at depths of less than 2,000 meters while G. inflata is dominant below about 3,400 meters. The inflection point between 2,000 and 2,500 meters may represent the lysocline in the subantarctic. The strong depth dependence of the G. inflataiG. bulbides ratios indicates severe selective dissolution in this region. Great care must thus be exercised when interpreting distribution charts of relative species frequencies in this area. The number of planktonic foraminiferal tests (in the more than 150 micrometer fraction) per gram of raw sediment is strongly related to percent of CaCO 3 (r= 0.87). Twenty-five core tops have planktonic foraminifera, including 15 of the 16 samples taken north of the Antarctic Boundary of the Polar Front (figure 1). The number varies between about 3,000 and 45,000 tests in subantarctic sediments, and bctween about 100 and 23,000 within the Polar Front zone (figure 1). Ten of the antarctic samples contained planktonic foraminifera, with 107
45'iii__
i E22-10
SOUTc7 AMERICA
177-43 10-47 10-40
o
FALKLAND IS. 1.7- 551
1.7-1 E9-4
--
E22- 11 ?. . ..E2212
1.7-53
•
1.7-37 E22-13 E9-21 '\-1 E 1: E22-7 / o SOUTH I 0 / , 3 •E7-2 GEORGIA IS, ° E9-20 0.1 E22-14 " )) ----- E9-19 - - E8-8 E22-27 E22 -26 E7-3 _- - E6-5 E22-32 'E22-31 0.01 I. 32 E8-6 I 0 -- ° E22- sI • E2 1 I s-ii E1228. ----, 0 0 1-1-6 E9-15 •E8-29 E22-4 1E7-7 E22-16 E12-19 •E8-21 I ) ° 0.004 E12-24E22-20 60° _22-21
9
I 1E1225
• 9 I
0 E22-17 I
-1
017 E22-19
E12-1O• o 0.5 E 10
ALMER PENINSULA sp
E22-22 E8-23 0 POLAR FRONT ZONE 0.01 SUBANTARCTIC BOUNDARY -- ANTARCTIC BOUNDARY POSTULATED POSITION CORE NUMBER
A
I
65^*L ^
I I
W 5 60° 55' 50° 45 40°
NUMBER OF PLANKTONIC FORAMINIFERA (10)
I I
350 30' 25 20' 15 10'
Figure 1. Locations of Eltanin and Islas Orcadas core tops from the southwestern Atlantic used and absolute planktonic foraminiferal abundances determined as numbers of tests in the >150 micrometer fraction per gram of raw sediment. (Numbers below circles: full circle, 0 specimens and empty circle, 50,000 specimens. The position of the Polar Front Is after Gordon and others, 1977.)
1000-
S
£
1500
E 2000-i I.-
CL
Cr w I-
£
U
£ U A
5.
2500 S•
A A
5
3000S S
3500-1
I.
•
£
A NORTH OF POLAR FRONT U POLAR FRONT ZONE ' SOUTH OF POLAR FRONT
4000]' 0 10 20 30 40 50 60 CaCO 3 ,%
Figure 2. Relationship between calcium-carbonate content and water depth In the samples from figure 1. 108
frequencies generally decreasing poleward. The number of tests ranges between 1 and about 5,000. Seven species of planktonic foraminifera are recognized in this region: Neogloboquadrina pachyderma (sinistral and dextral), Globorotalia inflata, Globigerina bulloides, Globorotalia truncatulinoides (sinistral and dextral), Globigerinita glutinata, Globigerina quinqueloba, and Globigerinita uvula. A late Quaternary age of all core tops containing planktonic foraminifera was confirmed by identification of the planktonic foraminiferal G. truncatulinojdes zone (less than 200,000 years in subantarctic waters; Kennett, 1970); the coccolith Emiliania huxleyi zone (less than 270,000 years; Thierstein et al., 1977), or by the absence of the radiolarian Stylatractus universus (Hays and Shackleton, 1976). The distribution of planktonic foraminifera in our material largely reflects the distribution in the water column as reported by Be (1969) and is similar to those of Kennett (1969) in the South Pacific and Herb (1968) in the Drake Passage. The sinistral form of N. pachyderma is dominant (greater than 95 percent in the Antarctic and greater than 60 percent north of the Antarctic Boundary of the Polar Front), and it penetrates farthest to the south (down to 63'S. in our samples). The frequency of the dextral form of N. pachyderma is less than 6 , percent. It persists southward to the same latitude as the sinistral form in our samples. G. inflata and G. bulloides are distributed a few degrees of latitude south of the Polar Front as in the South Pacific (Kennett, 1969). The frequency of G. bulloides approaches 20 percent in relatively undissolved subantarctic samples, but are ANTARCTIC JOURNAL
1000
1000
1500 A
1500
1500
2000
2000
2000
2500
2500
3000
3000
3500
3500
4000
4000
1000
E
£
•a ± • I— Q_ w 2500 0
cr w
3000-
•
r 0.47
(1%
Planktonic foraminiferal dissolution at high latitudes of the southwestern Atlantic BJ5RN A. MALMGREN and HANS G. CR0NBLAD Department of Geology Stockholm University Box 6801 11386 Stockholm, Sweden
As part of a study of Recent planktonic foraminifera from the South Atlantic sector of the southern ocean, we have analyzed 53 core-top samples (Eltanin and Islas Orcadas cruises) from the southwestern Atlantic (Scotia and Weddell Seas; figure 1). In the eastern South Atlantic (east of the MidAtlantic Ridge), the calcium-carbonate compensation depth (CCD) under antarctic waters is 3,700 meters (Lozano and Hays, 1976). In the western regime, none of the core tops in areas south of the Antarctic Convergence (Polar Front) have calcium carbonate in excess of 10 percent (figure 2). CaCO3 was analyzed by the Hiilsemann (1966) gasometric method. If the definition of CCD as the depth at which the amount of CaCO 3 decreases to 10 percent is followed (Lisitzin, 1972), this indicates a CCD of less than 2,000 meters in the western basin, which is much shallower than in the eastern basin. Using the same criteria, the CCD in the Polar Front Zone is between 2,100 and 3,000 meters (figure 1). We could not determine CCD north of the Polar Front because we used samples only from depths shallower than 4,000 meters. However, Lozano and Hays (1976) have reported a CCD of 4,900 meters under subtropical and subantarctic waters in this area. This shows that the CCD rapidly shallows southward across the Polar Front. October 1978
Abstracts with Programs, (Geological Society of America), 8: 1044. (Abstract) Tucholke, B. E., and G. B. Carpenter. 1977. Sediment distribution and Cenozoic sedimentation patterns on the Agulhas Plateau. Geological Society of America Bulletin, 88: 1337-1346. Van Couvering,J. A., W. A. Berggren, R E. Drake, E. Aguirre, and G. H. Curtis. 1976. The terminal Miocene event. Marine Micropaleontology, 1: 263-286. Weaver, F. M. 1976. Late Miocene and Pliocene radiolarian paleobiogeography and biostratigraphy of the Southern Ocean. Unpublished doctoral dissertation, Department of Geology, Florida State University. Wise, S. W., P. F. Ciesielski, D. T. MacKenzie, F. H. Wind, K. E. Busen, A. M. Gombos, B. U. Haq, G. P. Lohmann, R C. Tjalsma, W. K. Harris, R. W. Hedlund, D. N. Beju, D. L. Jones, G. Plafker, and W. V. Sliter. In press. Paleontologic synthesis for the Southwest Atlantic Ocean Basin based on Jurassic to recent faunas and floras from the Falkland Plateau. Paper presented at SCAR Symposium on Antarctic Geology and Geophysics, Madison, Wisconsin, 22-27 August 1977.
Differential -dissolution effects were analyzed using three dissolution -sensitive parameters (figure 3): percentages of fragments of planktonic foraminiferal tests (in relation to fragments plus whole tests), percentages of benthonic foraminifera (in relation to benthonic plus planktonic foraminifera), and proportions between the frequencies of the planktonic foraminifera Globorotalia inflata and Globigerina bulloides. Fragmentation is relatively constant down to about 3,500 meters, after which it increases considerably, indicating an increase in dissolution below this depth. An increase in benthonic foraminiferal percentages occurs at about the same depth. High percentages of benthonic foraminifera at depths shallower than 2,000 meters probably are related not to dissolution, but rather to greater relative abundances of benthonic foraminifera in shallower depths of the Maurice Ewing Bank (eastern Falkland Plateau; figure 1). The ratio between the dissolution-resistant G. inflata and the more susceptible G. bulloides (Berger, 1970) appears to be particularly sensitive to dissolution. The samples are from a restricted latitudinal range, and hence the ratio between the two species is not considered to represent biogeographic changes between the two species. G. bulloides is dominant at depths of less than 2,000 meters while G. inflata is dominant below about 3,400 meters. The inflection point between 2,000 and 2,500 meters may represent the lysocline in the subantarctic. The strong depth dependence of the G. inflataiG. bulbides ratios indicates severe selective dissolution in this region. Great care must thus be exercised when interpreting distribution charts of relative species frequencies in this area. The number of planktonic foraminiferal tests (in the more than 150 micrometer fraction) per gram of raw sediment is strongly related to percent of CaCO 3 (r= 0.87). Twenty-five core tops have planktonic foraminifera, including 15 of the 16 samples taken north of the Antarctic Boundary of the Polar Front (figure 1). The number varies between about 3,000 and 45,000 tests in subantarctic sediments, and bctween about 100 and 23,000 within the Polar Front zone (figure 1). Ten of the antarctic samples contained planktonic foraminifera, with 107
45'iii__
i E22-10
SOUTc7 AMERICA
177-43 10-47 10-40
o
FALKLAND IS. 1.7- 551
1.7-1 E9-4
--
E22- 11 ?. . ..E2212
1.7-53
•
1.7-37 E22-13 E9-21 '\-1 E 1: E22-7 / o SOUTH I 0 / , 3 •E7-2 GEORGIA IS, ° E9-20 0.1 E22-14 " )) ----- E9-19 - - E8-8 E22-27 E22 -26 E7-3 _- - E6-5 E22-32 'E22-31 0.01 I. 32 E8-6 I 0 -- ° E22- sI • E2 1 I s-ii E1228. ----, 0 0 1-1-6 E9-15 •E8-29 E22-4 1E7-7 E22-16 E12-19 •E8-21 I ) ° 0.004 E12-24E22-20 60° _22-21
9
I 1E1225
• 9 I
0 E22-17 I
-1
017 E22-19
E12-1O• o 0.5 E 10
ALMER PENINSULA sp
E22-22 E8-23 0 POLAR FRONT ZONE 0.01 SUBANTARCTIC BOUNDARY -- ANTARCTIC BOUNDARY POSTULATED POSITION CORE NUMBER
A
I
65^*L ^
I I
W 5 60° 55' 50° 45 40°
NUMBER OF PLANKTONIC FORAMINIFERA (10)
I I
350 30' 25 20' 15 10'
Figure 1. Locations of Eltanin and Islas Orcadas core tops from the southwestern Atlantic used and absolute planktonic foraminiferal abundances determined as numbers of tests in the >150 micrometer fraction per gram of raw sediment. (Numbers below circles: full circle, 0 specimens and empty circle, 50,000 specimens. The position of the Polar Front Is after Gordon and others, 1977.)
1000-
S
£
1500
E 2000-i I.-
CL
Cr w I-
£
U
£ U A
5.
2500 S•
A A
5
3000S S
3500-1
I.
•
£
A NORTH OF POLAR FRONT U POLAR FRONT ZONE ' SOUTH OF POLAR FRONT
4000]' 0 10 20 30 40 50 60 CaCO 3 ,%
Figure 2. Relationship between calcium-carbonate content and water depth In the samples from figure 1. 108
frequencies generally decreasing poleward. The number of tests ranges between 1 and about 5,000. Seven species of planktonic foraminifera are recognized in this region: Neogloboquadrina pachyderma (sinistral and dextral), Globorotalia inflata, Globigerina bulloides, Globorotalia truncatulinoides (sinistral and dextral), Globigerinita glutinata, Globigerina quinqueloba, and Globigerinita uvula. A late Quaternary age of all core tops containing planktonic foraminifera was confirmed by identification of the planktonic foraminiferal G. truncatulinojdes zone (less than 200,000 years in subantarctic waters; Kennett, 1970); the coccolith Emiliania huxleyi zone (less than 270,000 years; Thierstein et al., 1977), or by the absence of the radiolarian Stylatractus universus (Hays and Shackleton, 1976). The distribution of planktonic foraminifera in our material largely reflects the distribution in the water column as reported by Be (1969) and is similar to those of Kennett (1969) in the South Pacific and Herb (1968) in the Drake Passage. The sinistral form of N. pachyderma is dominant (greater than 95 percent in the Antarctic and greater than 60 percent north of the Antarctic Boundary of the Polar Front), and it penetrates farthest to the south (down to 63'S. in our samples). The frequency of the dextral form of N. pachyderma is less than 6 , percent. It persists southward to the same latitude as the sinistral form in our samples. G. inflata and G. bulloides are distributed a few degrees of latitude south of the Polar Front as in the South Pacific (Kennett, 1969). The frequency of G. bulloides approaches 20 percent in relatively undissolved subantarctic samples, but are ANTARCTIC JOURNAL
1000
1000
1500 A
1500
1500
2000
2000
2000
2500
2500
3000
3000
3500
3500
4000
4000
1000
E
£
•a ± • I— Q_ w 2500 0
cr w
3000-
•
r 0.47
(1%
