Ranking of selective dissolution of modern cool-water planktonic foraminifera… · 2011-05-06 ·...
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G. quinqueloba -1.5 -10 -0.5 00 0.5 1.0 1.5 2.0 G. truncatulinoides 9 -0.5-: 0.0 05 1.0 1.5 20 a, 0 U U) C-) U) x a, C C 0 0 U) U) a G. glutinata 0 1 2 3 4 5 G. uvula 0 5 10 15 20 25 N. pachyderma dextral 0 1 2 3 4 5 6 G. inflata 0. 5 10 15 20 25 -1 -1 -o C 0 N. pachyderina sinistral 30 40 50 60 70 -0.5 0.0 0.5 1.0 1.5 2.0 G. bulloides 0 5 10 15 20 Relative abundance, % ff Ranking of selective dissolution of modern cool-water planktonic foraminifera, South Atlantic B. A. MALMGREN Department of Geology Stockholm University 5-106 91 Stockholm, Sweden Because of their abundance in most oceanic environments and rapidity of evolution, planktonic foraminifera are amongst the most important indicators of past oceanographic changes. Their calcite test makes them susceptible to dissolution in the water column or on the ocean floor. Dissolution is a selective process; some species are more susceptible to dissolution, whereas others are more tolerant. Knowledge of differential dissolution susceptibility of planktonic foraminiferal species is essential for valid interpretations of faunal changes through time. Several studies have dealt with ranking of dissolution in modern planktonic foraminiferal species (Ruddiman and Heezen 1967; Berger 1968, 1970; Parker and Berger 1971; Adel- seck 1977; Thunell and Honjo 1981), but none has dealt ex- clusively with cool-water species that are deposited in areas less favorable for calcium carbonate preservation. I here summarize the results of a study of dissolution effects on modern cool- water planktonic foraminifera from the South Atlantic Ocean. The study has permitted the integration of these forms within previous dissolution ranking schemes. I have analyzed 32 surface-sediment samples from the Maurice Ewing Bank (eastern Falkland Plateau), South Atlantic Ocean. The Maurice Ewing Bank is an ideal area for studies of dissolution in cool waters, because it permits sampling of carbo- nate-rich sediments from a wide range of water depths. Further- more, there are no major biogeographic differences along this narrow latitudinal belt, so differences in faunal compositions should be due to dissolution. Samples were taken at water depths of between 1,200 meters and 3,500 meters. The calcium carbonate compensation depth in this area is at about 3,500 meters (Berger and Winterer 1974). Biostratigraphic analyses indicate that all samples are of a Holocene age. Relative abundances of eight species or varieties of planktonic foraminifera were determined in each sample, using the greater than 90-micrometers fraction: Globorotalia inflata, G. trun- catulinoides, Globigerina bulloides, G. quinqueloba, Neoglobo- quadrina pachyderma sinistral form, N. pachyderma dextral form, Globigerinita glutinata, and G. uvala. The same fraction was used to quantify the relative abundances of the following sedimen- tologic particles: mineral grains (quartz and feldspar), radi- olarians, benthonic foraminifera, and planktonic foraminiferal fragments (unidentifiable remains). In addition, I determined the calcium carbonate contents and also ratios of a dissolution tolerant species of planktonic foraminifera (G. inflata) in relation to a more susceptible species (G. bulloides). These variables are, taken separately, potential dissolution indices, but to determine a single index, they were subjected to a principal components analysis. The rationale was to isolate the portions of their vari- ability that result from dissolution. The first principal compo- nent displays calcium carbonate contents in an inverse rela- tionship with relative abundances of mineral grains, radi- olarians, planktonic foraminiferal fragments, and G. inflata to G. bulloides ratios and is interpreted as a dissolution index. This is supported by a general direct relationship observed between component scores and water depths. Several species (G. inflata, G. truncatulinoides, and N. pachyder- ma sinistral form) increase in abundance with increasing dis- solution, whereas others (G. quinqueloba, N. pachyderma dextral form, and G. uvula) decrease in abundance (see figure). Two species (G. bulloides and G. glutinata) do not show any obvious trends. Changes in relative abundances of the various species with increasing dissolution (higher scores on the dissolution index). 98 ANTARCTIC JOURNAL
Transcript of Ranking of selective dissolution of modern cool-water planktonic foraminifera… · 2011-05-06 ·...
G. quinqueloba
-1.5
-10
-0.5
00
0.5
1.0
1.5
2.0
G. truncatulinoides9
-0.5-:
0.0
05
1.0
1.5
20
a,0UU)
C-)
U)
xa,
C
C0
0U)U)
a
G. glutinata012345
G. uvula0510152025
N. pachyderma dextral0123456
G. inflata0.510152025
-1
-1
-oC
0
N. pachyderina sinistral3040506070
-0.5
0.0
0.5
1.0
1.5
2.0
G. bulloides05101520
Relative abundance, %
ff
Ranking of selective dissolution ofmodern cool-water planktonicforaminifera, South Atlantic
B. A. MALMGREN
Department of GeologyStockholm University
5-106 91 Stockholm, Sweden
Because of their abundance in most oceanic environmentsand rapidity of evolution, planktonic foraminifera are amongstthe most important indicators of past oceanographic changes.Their calcite test makes them susceptible to dissolution in thewater column or on the ocean floor. Dissolution is a selectiveprocess; some species are more susceptible to dissolution,whereas others are more tolerant. Knowledge of differentialdissolution susceptibility of planktonic foraminiferal species isessential for valid interpretations of faunal changes throughtime. Several studies have dealt with ranking of dissolution inmodern planktonic foraminiferal species (Ruddiman andHeezen 1967; Berger 1968, 1970; Parker and Berger 1971; Adel-seck 1977; Thunell and Honjo 1981), but none has dealt ex-clusively with cool-water species that are deposited in areas lessfavorable for calcium carbonate preservation. I here summarizethe results of a study of dissolution effects on modern cool-water planktonic foraminifera from the South Atlantic Ocean.The study has permitted the integration of these forms withinprevious dissolution ranking schemes.
I have analyzed 32 surface-sediment samples from theMaurice Ewing Bank (eastern Falkland Plateau), South AtlanticOcean. The Maurice Ewing Bank is an ideal area for studies ofdissolution in cool waters, because it permits sampling of carbo-
nate-rich sediments from a wide range of water depths. Further-more, there are no major biogeographic differences along thisnarrow latitudinal belt, so differences in faunal compositionsshould be due to dissolution. Samples were taken at waterdepths of between 1,200 meters and 3,500 meters. The calciumcarbonate compensation depth in this area is at about 3,500meters (Berger and Winterer 1974). Biostratigraphic analysesindicate that all samples are of a Holocene age.
Relative abundances of eight species or varieties of planktonicforaminifera were determined in each sample, using the greaterthan 90-micrometers fraction: Globorotalia inflata, G. trun-catulinoides, Globigerina bulloides, G. quinqueloba, Neoglobo-quadrina pachyderma sinistral form, N. pachyderma dextral form,Globigerinita glutinata, and G. uvala. The same fraction was usedto quantify the relative abundances of the following sedimen-tologic particles: mineral grains (quartz and feldspar), radi-olarians, benthonic foraminifera, and planktonic foraminiferalfragments (unidentifiable remains). In addition, I determinedthe calcium carbonate contents and also ratios of a dissolutiontolerant species of planktonic foraminifera (G. inflata) in relationto a more susceptible species (G. bulloides). These variables are,taken separately, potential dissolution indices, but to determinea single index, they were subjected to a principal componentsanalysis. The rationale was to isolate the portions of their vari-ability that result from dissolution. The first principal compo-nent displays calcium carbonate contents in an inverse rela-tionship with relative abundances of mineral grains, radi-olarians, planktonic foraminiferal fragments, and G. inflata toG. bulloides ratios and is interpreted as a dissolution index. Thisis supported by a general direct relationship observed betweencomponent scores and water depths.
Several species (G. inflata, G. truncatulinoides, and N. pachyder-ma sinistral form) increase in abundance with increasing dis-solution, whereas others (G. quinqueloba, N. pachyderma dextralform, and G. uvula) decrease in abundance (see figure). Twospecies (G. bulloides and G. glutinata) do not show any obvioustrends.
Changes in relative abundances of the various species with increasing dissolution (higher scores on the dissolution index).
98 ANTARCTIC JOURNAL
To rank the order of susceptibility, I divided the samples intotwo groups, the one composed of samples showing moderatesigns of dissolution (negative scores on the dissolution index;see figure) and the other of more strongly dissolved samples(positive scores). The differences in relative abundances be-tween these dissolution regimes were quantified, using gener-alized distance analysis; these distances form the basis for rank-ing the species. The order of tolerance to dissolution was foundto be the following: N. pachyderma sinistral form, G. inflata, G.truncatulinoides, G. bulloides and G. glutinata, N. pachydernia dex-tral form, G. uvula, and G. quinqueloba. This order generallyagrees well with other ranking schemes (see table). The onlymajor difference is the relatively low rank given to G. bulloides byBerger (1970). This might be explained by a difference in dis-solution between warm- and cool-water morphotypes of thisspecies.
Sediment samples for this study were obtained from corestaken during USNS Etanin and ARA Islas Orcadas cruises. Thesamples were provided by the Florida State University antarcticmarine geology and core facility, which is supported by theNational Science Foundation.
References
Adelseck, C.G., Jr. 1977. Dissolution of deep-sea carbonate: Preliminarycalibration of preservational and morphologic aspects. Deep-Sea Re-search, 24, 1167-1185.
Berger, W.H. 1968. Planktonic foraminifera: Selective solution and pa-leoclimatic interpretation. Deep-Sea Research, 15, 31-43.
Berger, W.H. 1970. Planktonic foraminifera: Selective solution and thelysocline. Marine Geology, 8, 111-138.
Berger, W.H., and E.L. Winterer. 1974. Plate stratigraphy and the fluc-
Comparisons of rankings of dissolution tolerances deducedhere and in some other studies. Rank 1 is the most
tolerant species. Only those of the species that wereincluded in this study are listed.
This studyBergerBerger (1968);Thunell and(1970)Parker andHonjo (1981)
Berger (1971)
N. pachydermasinistral
(1) _aG. inflata 2 1
2
G. truncatulinoides3 2G. bulloides 6 3
3
G. glutinata 4 -N. pachyderma
dextral
(1) -G. uvulaG. quinqueloba5 -
a "" denotes data not analyzed in previous studies.
tuating carbonate line. In K.J. Hsü and H.C. Jenkyns (Eds.), PelagicSediments: On Land and Under the Sea. (Special publication.) Oxford,England: International Association of Sedimentologists.
Parker, FL., and W.H. Berger. 1971. Faunal and solution patterns ofplanktonic foraminifera in surface-sediments of the South Pacific.Deep-Sea Research, 18, 73-107.
Ruddiman, W.F., and B.C. Heezen. 1967. Differential solution ofplanktonic foraminifera. Deep-Sea Research, 14, 801-808.
Thunell, R. C., and S. Honjo. 1981. Calcite dissolution and the modifica-tion of planktonic foraminiferal assemblages. Marine Micropaleon-tology, 6, 169-182.
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