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Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean — a stratigraphic tool for carbonate-poor Quaternary sediments
Marine Geology 175 (2001) 167±181
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Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean Ð a stratigraphic tool for carbonate-poor Quaternary sediments U. Brathauer a,*, A. Abelmann a, R. Gersonde a, H.-S. Niebler b, D.K. FuÈtterer a a
Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, D-27515 Bremerhaven, Germany b UniversitaÈt Bremen, FB Geowissenschaften, Klagenfurter Straûe, D-28359 Bremen, Germany Received 25 February 2000; accepted 2 February 2001
Abstract We calibrated the Cycladophora davisiana abundances versus oxygen isotope stratigraphy back to 220 ka for the subantarctic Atlantic Ocean. The relative abundances of C. davisiana and d 18O measurements of benthic and planktic foraminifera have been determined in two sediment cores. Oxygen isotope stratigraphy has been used to date the C. davisiana records and to assign SPECMAP ages to the C. davisiana events. Comparisons with an existing calibration from the subantarctic Indian Ocean show, that the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' occur synchronous within the errors of the oxygen isotope stratigraphy in the Indian and the Atlantic sectors of the Southern Ocean. Larger deviations occur only for events `b1' and `g'. Furthermore, the long-term ¯uctuations in C. davisiana abundances have been studied in a sediment core covering the last 700 kyr. Based on biostratigraphic extinction levels, ages for early Brunhes C. davisiana events have been estimated. Major C. davisiana abundance maxima occur approximately every 100 ka in conjunction with glacial/interglacial cycles over the entire record. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Stratigraphy; Southern Ocean; Radiolaria; Paleoceanography
1. Introduction Broad areas of the Southern Ocean sea ¯oor are covered with carbonate poor or carbonate barren siliceous oozes. The lack or absence of carbonate shells in these sediments precludes the application of `standard dating methods' such as oxygen isotope stratigraphy or 14C dating. Hays et al. (1976a,b) developed a * Corresponding author. Present address: GeoForschungsZentrum Potsdam, PB 3.3 Ð Sedimente und Beckenbildung, Telegrafenberg, D-14473 Potsdam, Germany. Tel.: 149-331-288-1379; fax: 149-331-288-1302. E-mail address: [email protected] (U. Brathauer).
stratigraphy based on ¯uctuations in the abundances of the radiolarian species Cycladophora davisiana Ehrenberg relative to the entire radiolarian assemblage for carbonate barren Southern Ocean sediments. Hays et al. (1976a,b) recognized the general correlation of high C. davisiana abundances with glacial times and low C. davisiana abundances with interglacial times in late Quaternary sediments. The late Quaternary maxima and minima in the C. davisiana abundances have been named with lower case letters `a±o' and calibrated versus oxygen isotope stages, back to oxygen isotope stage 13 for the subantarctic Indian Ocean and back to oxygen isotope stage 2 for the subantarctic Atlantic Ocean. The correlation of
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00141-4
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oxygen isotope stage 2 and C. davisiana abundance peak `b' allowed the inclusion of carbonate barren sediment samples from the Southern Ocean into the Last Glacial Maximum study of the CLIMAP project (CLIMAP, 1981). Based on correlations between an oxygen isotope dated C. davisiana record from the subantarctic Indian Ocean and an oxygen isotope dated C. davisiana record from the North Atlantic Ocean, Morley and Hays (1979) were able to show, that the abundance ¯uctuations of C. davisiana occur synchronous in the high latitudes of both hemispheres. An explanation for the glacial/interglacial abundance variations of C. davisiana has been searched in the modern ecology of the species. C. davisiana is a cosmopolitan and deep-dwelling radiolaria (Petrushevskaya, 1967, 1972; Morley and Hays, 1979). Plankton data from the North Atlantic show, that C. davisiana prefers depths below 500 m in the open ocean (Bjùrklund and Ciesielski, 1994). In similar depths (400±1000 m) occurrences of living C. davisiana have been reported from the Southern Ocean (Abelmann and Gowing, 1997). In modern Southern Ocean surface sediment samples C. davisiana rarely exceeds 5% (Hays et al., 1976b; Morley, 1989). There is no modern analog for the high glacial abundances (about 20±30%) of C. davisiana within the Southern Ocean (Hays et al., 1976b). The abundances in the Southern Ocean surface sediment samples exhibit no correlation with longitude, latitude or temperature gradients (Hays et al., 1976b). Obviously C. davisiana is not controlled by sea surface temperatures (Hays et al., 1976b) and has been excluded from all Southern Ocean radiolarian based transfer functions (Lozano and Hays, 1976; Hays et al., 1976a; Morley, 1979; Morley, 1989; Abelmann et al., 1999). In the modern World Ocean, sediments containing high C. davisiana abundances (up to 49%) have only been found in the Sea of Okhotsk, a marginal sea in the northwest Paci®c (Morley and Hays, 1983). Thus the oceanographic conditions of the Sea of Okhotsk, with a low salinity surface layer accompanied by a strong temperature minimum near its base, have been proposed as an analog for the glacial Southern Ocean (Morley and Hays, 1983; Bjùrklund and Ciesielski, 1994). Since the early works of Hays et al. (1976a,b) and Morley and Hays (1979) C. davisiana has been established as a valuable stratigraphic tool not only in the
Southern Ocean (CLIMAP, 1981; Cooke and Hays, 1982; Labeyrie et al., 1986; Abelmann and Gersonde, 1988; Howard and Prell, 1992; Pudsey and Howe, 1998), but also in the Northwest Paci®c Ocean (Morley et al., 1982, 1995), and the North Atlantic Ocean (Ciesielski and Bjùrklund, 1995). Despite its importance for correlating and dating carbonate poor sediments, there are very few records in which C. davisiana events have been cross-correlated with oxygen isotope stages. In this work we present a ®rst calibration of C. davisiana events versus oxygen isotope stages over the last 220 kyr for the subAntarctic Atlantic Ocean. Our results show, that C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' occur synchronous in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean within the errors of the oxygen isotope stratigraphy. Larger deviations occur only for the C. davisiana maximum `b1' and the broad minimum `g'. Furthermore, we present a continuous C. davisiana record covering the last 700 kyr, which exhibits the long-term ¯uctuations in the C. davisiana abundances. It is well known from late Brunhes sediments, that major C. davisiana abundance maxima appear approximately every 100 ka. Our record shows, that this trend can also be observed in early Brunhes sediments. 2. Material and methods Sediment cores PS2082-1, PS2498-1, PS1752-1 and PS1778-5, recovered in the subantarctic zone and the polar frontal zone of the Atlantic sector of the Southern Ocean, have been examined in this study (Fig. 1, Table 1). Cores PS2082-1, PS2498-1 and PS1752-1 have been sampled every 10 cm and core PS1778-5 every 10±20 cm for radiolarian faunal analysis. Slide preparation followed the Alfred Wegener Institute standard method (Abelmann, 1988; Abelmann et al., 1999). On average 400 individuals have been counted on each slide. The abundances of C. davisiana Ehrenberg include all three subspecies C. davisiana (Ehr.) var. davisiana Petrushevskaya, C. davisiana (Ehr.) var. cornutoides Petrushevskaya and C. davisiana (Ehr.) var. semeloides Petrushevskaya. Stable oxygen isotope measurements have been carried out on the planktic foraminifer Globigerina bulloides for sediment core PS2498-1. The stable
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Fig. 1. Locations of investigated sediment cores. Frontal system according to Peterson and Stramma (1991).
isotopic composition was determined with a Finnigan MAT 251 isotope ratio gas mass spectrometer directly coupled to an automatic carbonate preparation device (Kiel I) and calibrated via NBS 19 to the PDB scale. The number of G. bulloides (200±250 mm) analyzed at a single measurement varied between six and ten. All values are given in d-notation versus Vienna Pee Dee Belemnite (VPDB). The overall precision of the measurements based on repeated analyses of a laboratory
standard over a one-year-period was better than 0.08½. Additionally the benthic foraminiferal (Cibicidoides spp.) oxygen isotope measurements for core PS2498-1 from Mackensen et al. (2001) and the planktic (G. bulloides) and benthic foraminiferal (Cibicidoides spp.) oxygen isotope measurements for core PS2082-1 from Mackensen et al. (1994) are used in this study.
Table 1 Locations of sediment cores investigated in this study Core
Latitude
Longitude
Water depth (m)
Sedimentation rate (cm/kyr)
PS2082-1 PS2498-1 PS1752-1 PS1778-5
43813.2 0 S 44809.2 0 S 45837.3 0 S 49800.7 0 S
11844.3 0 E 14813.7 0 W 9835.8 0 E 12841.8 0 W
4610 3783 4519 3380
4,3 9,1 1,2 6,3
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3. Results and discussion 3.1. Calibration of the C. davisiana stratigraphy in the subantarctic Atlantic Ocean versus oxygen isotope stratigraphy over the last 220 kyr The late Quaternary abundance ¯uctuations of C. davisiana have been determined in three sediment cores PS2498-1, PS2082-1, and PS1778-5. Following the nomenclature of Hays et al. (1976a,b) and Morley and Hays (1979) C. davisiana events `a±i2' have been identi®ed (Fig. 2). In order to calibrate the C. davisiana stratigraphy for the subantarctic Atlantic Ocean, sediment cores PS2082-1 and PS2498-1 have been dated with oxygen isotope stratigraphy. The calcium carbonate content in sediment core PS1778-5 is to low for establishing an oxygen isotope stratigraphy. To be consistent with an earlier calibration from the Indian sector of the Southern Ocean (Hays et al., 1976a,b; Howard and Prell, 1992) the oxygen isotope records of sediment cores PS2082-1 and PS2498-1 have been converted to SPECMAP (Imbrie et al., 1984) ages. The age model for core PS2082-1 has been taken from Mackensen et al. (1994). For core PS2498-1 the benthic foraminiferal oxygen isotopes from Mackensen et al. (2001) have been converted to SPECMAP ages (Figs. 3 and 4). The average sedimentation rates are 4 cm/1000 years for core PS2082-1 and 9 cm/1000 years for core PS2498-1 (Fig. 3, Table 1). The resulting mean sample distance is 2.3 kyr for core PS2082-1 and 1.1 kyr for core PS2498-1. The C. davisiana minima `a' coincides with low oxygen isotope values as characteristic for the Holocene (oxygen isotope stage 1) in sediment cores PS2498-1 and PS2082-1 (Figs. 4 and 5). Maximum percentages of C. davisiana are around 5%, which is typical for Holocene sediments (Hays et al., 1976b; Morley and Hays, 1983). Below this low abundance zone strong maxima of C. davisiana with abundances about 20±25% appear. These maxima fall into oxygen isotope stage 2. Despite this general correlation between high C. davisiana values and high oxygen isotope values during oxygen isotope stage 2, the largest maximum of C. davisiana, the abundance peak `b2' is not synchronous with the maximum in oxygen isotopes (oxygen isotope event 2.2) of planktonic foraminifera G. bulloides and benthic foraminifera
Cibicidoides spp. (Figs. 4 and 5). Based on the SPECMAP ages of the oxygen isotope records, the C. davisiana abundance maximum `b2' is 23 ka in core PS2498-1, and 20 ka in core PS2082-1 (Table 2). Thus the maximum in C. davisiana (event `b2') is 1±4 kyr older than isotopic event 2.2 (19 ka) in the SPECMAP time scale. In sediment core PS2082-1 the maximum in oxygen isotopes (event 2.2) coincides with the most recent but lower abundance peak `b1'. The SPECMAP age of peak `b1' in core PS2082-1 is 18 ka. In core PS2498-1 the abundance peak `b1' is not well expressed, and occurs slightly before the maximum in benthic foraminiferal d 18O (event 2.2) at 20 ka. During oxygen isotope stage 3 the benthic and planktic oxygen isotope records of cores PS2498-1 and PS2082-1 do not exhibit prominent excursions. In contrast the C. davisiana abundance ¯uctuations can clearly be subdivided in three distinct intervals. The minima `c1' and `c2' as well as an unnamed prominent maximum between `c1' and `c2' (Figs. 4 and 5). These variations are extremely well documented in sediment core PS2498-1, which shows highest sedimentation rates during oxygen isotope stage 3 (Fig. 3). Abundance minima `c2' occurred at 58 ka (PS2498-1) and 56 ka (PS2082-1) in the subantarctic Atlantic Ocean (Figs. 4 and 5). Minimum `c2' is followed by increased abundances of C. davisiana. In core PS2498-1 values stay high for an interval of 13,000 years (from 56 to 43 kyr). The transition to the following abundance minima `c1' is sharp and rapid. C. davisiana abundance minima `c1' has an age about 38 ka in core PS2498-1 and 36 ka in core PS2082-1. None of the benthic (Cibicidoides spp.) or planktic foraminiferal (G. bulloides) d 18O records measured on sediment cores PS2498-1 and PS2082-1 show similar substantial shifts during oxygen isotope stage 3, which points to C. davisiana as an important stratigraphic marker for oxygen isotope stage 3. Oxygen isotope stage 4 exhibits typical glacial conditions with high C. davisiana abundances in conjunction with maxima in oxygen isotope values (Figs. 4 and 5). In cores PS2498-1 and PS2082-1 C. davisiana event `d' is developed as a double-peak. For the age determination of peak `d' the midpoint of the double-peak has been taken (Figs. 4 and 5). This leads to ages of about 64 ka for core PS2498-1 and 66 ka for core PS2082-1.
Fig. 2. Relative abundances of C. davisiana in sediment cores PS1778-5, PS2498-1, PS2082-1 from the subpolar and subantarctic Atlantic Ocean and core RC11-120 from the subantarctic Indian Ocean. C. davisiana abundance data for core RC11-120 have been taken from Martinson et al. (1987), the oxygen isotope stage boundaries are from Howard and Prell (1992).
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Fig. 3. Age/depth relationship for sediment cores PS2498-1, PS2082-1 (data from Mackensen et al., 1994) and RC11-120 (data from Howard and Prell, 1992). For core PS2498-1 the benthic foraminiferal oxygen isotopes from Mackensen et al. (2001) have been converted to SPECMAP ages.
The oxygen isotope events 5.1, 5.3, and 5.5 have been clearly correlated to C. davisiana events `e1', `e2', and `e3' in the Indian sector of the Southern Ocean (Fig. 6) by Hays et al. (1976a,b) and in the North Atlantic Ocean by Morley and Hays (1979). In principle the same correlation can be observed in the subantarctic Atlantic Ocean cores PS2498-1 (Fig. 4) and PS2082-1 (Fig. 5). C. davisiana events `e1', `e2', and `e3' correspond to oxygen isotope events 5.1, 5.3, and 5.5. However, in the high resolution record PS2498-1 the C. davisiana event `e2' is markedly shifted with respect to the oxygen isotope record. C. davisiana event `e2' is leading oxygen isotope event 5.3 by 6 kyr (Fig. 4, Table 2). That this shift is only documented in core PS2498-1 might be a consequence of the high sedimentation rates in this core in comparison to the other cores. Oxygen isotope stage 6 is indicated by two
pronounced C. davisiana maxima `f' and `h', and between them the minima `g' in the C. davisiana records (Hays et al., 1976a,b; Morley and Hays, 1979). In core PS2082-1 the abundance maxima `f' (136 ka) and `h' (180 ka) are accompanied by high oxygen isotope values, but the abundance minima `g' does not ®nd a counterpart in the oxygen isotope records (Fig. 5). For the broad minimum `g' a ®x-point for the age determination is dif®cult to de®ne. The lowest value occurs about 146 ka in core PS2082-1. The C. davisiana events `i1' and `i2' correspond to middle to late oxygen isotope stage 7 (Fig. 5) (Hays et al., 1976a,b; Morley and Hays, 1979). In core PS2082-1 the age for C. davisiana event `i1' is 197 ka and for event `i2' 224 ka. During the last glacial/interglacial cycle the age differences between core PS2082-1 and core PS2498-1 are 2±3 kyr for most of the C. davisiana events (Table 2). Only events `e1' and `e2' (oxygen
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Fig. 4. Oxygen isotope measurements of planktic (G. bulloides) and benthic (Cibicidoides spp.) foraminifera as well as relative abundances of the radiolarian C. davisiana for sediment core PS2498-1. Benthic oxygen isotope data are from Mackensen et al. (2001).
isotope events 5.1 and 5.3) show larger deviations, with age differences of 5 and 10 kyr, respectively. 3.2. Comparison of subantarctic Atlantic and subantarctic Indian Ocean C. davisiana records The subantarctic Atlantic C. davisiana records from cores PS2498-1 and PS2082-1 have been compared with the standard C. davisiana record from the subantarctic Indian Ocean core RC11-120. For core RC11-120 the C. davisiana abundance data have been taken from Martinson et al. (1987) and the age model given by Howard and Prell (1992) has been chosen. Thus all three records are dated with oxygen isotope stratigraphy and represent SPECMAP ages.
This approach allows a direct comparison between the ages of the C. davisiana events from the subantarctic Atlantic Ocean and the ages of the C. davisiana events from the subantarctic Indian Ocean, assuming synchronism between the oxygen isotope records within the Southern Ocean. The C. davisiana records from the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are almost identical (Figs. 2, 4, 5, and 6). Especially the C. davisiana signals of sediment cores PS2082-1 and RC11-120, which show comparable sedimentation rates, are quiet similar (Figs. 2, 5, and 6). The different CaCO3-content levels in these cores exhibit, that different dissolution or production rates of CaCO3 do not in¯uence the C. davisiana signals.
Fig. 5. Oxygen isotope measurements of planktic (G. bulloides) and benthic (Cibicidoides spp.) foraminifera, relative abundances of the radiolarian C. davisiana and CaCO3content for sediment core PS2082-1. Oxygen isotope data and CaCO3 data are from Mackensen et al. (1994).
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Table 2 Oxygen isotope dated ages for C. davisiana events C. davisiana event
b1 b2 c1 c2 d e1 e2 e3 f g h i1 i2 a b c
Depth (cm)
Age (ka) based on oxygen isotope stratigraphy (SPECMAP time scale) a
RC11-120 b
PS2082-1
PS2498-1
RC11-120 c
PS2082-1
PS2498-1
mean
60 80 145 210 225 300 355 415 480 520 605 645 700
78 138 288 358 403 458 498 558 608 678 848 918 958
140 190 450 690 755 910 1021 1100
14 20 40 58 65 83 100 120 139 154 184 197 222
18 20 36 56 66 79 95 124 136 146 180 197 224
20 23 38 58 64 84 105 122
17 21 38 57 65 82 100 122 138 150 182 197 223
SPECMAP time scale (Imbrie et al., 1984). Data are from Martinson et al. (1987). Ages are from Howard and Prell (1992).
In the Indian Ocean core RC11-120 the C. davisiana abundance peaks `b1' and `b2' fall close together due to low sedimentation rates. Maximum `b1' occurs slightly after the maximum in planktic and benthic foraminiferal d 18O (oxygen isotope event 2.2) (Fig. 6) and has an age about 14 ka in this core (Howard and Prell, 1992). Thus C. davisiana abundance peak `b1' is 4±6 kyr younger in the Indian Ocean record than in the Atlantic Ocean records. The age of abundance maximum `b2' in the Indian Ocean core RC11-120 is 20 ka (Howard and Prell, 1992), which is identical with the age in Atlantic Ocean record PS2082-1 and which is 3 kyr younger than in Atlantic Ocean core PS2498-1. Generally the C. davisiana event `b2', which represents maximum abundances during oxygen isotope stage 2, is easier to recognise and the calculated ages based on oxygen isotopes from the Indian Ocean and the Atlantic Ocean records are more consistent than for abundance peak `b1'. For the C. davisiana events occurring during oxygen isotope stages 3 and 4 the age differences between the Atlantic and Indian Ocean records are small. The ages for C. davisiana event `c1' differ about 2±4 kyr. The ages for events `c2' and `d' match extremely well, the differences lie between 0±2 kyr. C. davisiana maxima `f' and `h' (oxygen
isotope stage 6) are 3±4 kyr older in the Indian Ocean record RC11-120 (Howard and Prell, 1992) compared to the ages in the Atlantic Ocean record PS2082-1 (Table 2). In both records a ®x-point for the age determination of the broad minimum `g' is dif®cult to de®ne (Figs. 5 and 6). The lowest abundance value in Indian Ocean core RC11-120 has an age of 154 ka (Howard and Prell, 1992). In core PS2082-1 the lowest value occurs about 146 ka, which results in an age difference of 8 kyr to core RC11-120 (Table 2). For the C. davisiana abundance minima `i1' and `i2' (oxygen isotope stage 7) both, the Atlantic Ocean and the Indian Ocean record give almost identical ages. The age for C. davisiana event `i1' is 197 ka in both records and the age for C. davisiana event `i2' is 224 ka in the subantarctic Atlantic Ocean (PS2082-1) and 222 ka in the subantarctic Indian Ocean (RC11-120; Howard and Prell, 1992) (Table 2). The age difference of the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are 0±5 kyr (Table 2). The error ranges for oxygen isotope ages dated with the SPECMAP time scale is 3±5 kyr (Imbrie et al., 1984). Thus the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' are
Fig. 6. Oxygen isotope measurements of planktic (G. bulloides) and benthic foraminifera, relative abundances of the radiolarian C. davisiana and CaCO3-content for sediment core RC11-120. Data are from Martinson et al. (1987), oxygen isotope stage boundaries after Howard and Prell (1992).
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synchronous within the errors of oxygen isotope chronology. Exceptions during the last 220 kyr are only event `b1', which is 4±6 ka younger in the Indian Ocean record compared to the Atlantic Ocean records, and event `g', which shows an age difference about 8 kyr. The SPECMAP calibrated ages of the C. davisiana events occurring during the last 220 kyr in the Indian Ocean record RC11-120 (Howard and Prell, 1992), and the Atlantic Ocean records PS2082-1 and PS2498-1 have been used to calculate mean SPECMAP ages for the C. davisiana events `b1 ±i2', which can be used to tie non-carbonate sediment cores from the subantarctic Indian and subantarctic Atlantic Ocean via C. davisiana stratigraphy to the SPECMAP time scale (Table 2). The deviation from the mean for the individual ages of the C. davisiana events in the three sediment cores lies between 0 and 5 kyr (Table 2). 3.3. Long-term variations in the C. davisiana abundances Ð the last 700 kyr Following the nomenclature of Hays et al. (1976a,b) and Morley et al. (1995) the C. davisiana abundance peaks `b±ff' have been determined in sediment core PS1752-1 from the subantarctic Atlantic Ocean (Fig. 7). C. davisiana events `gg' and `hh' were not de®ned in these works. Here the label `gg' is used to denote the minimum preceding C. davisiana abundance peak `ff', and the label `hh' is used to denote the maximum preceding C. davisiana abundance peak `ff' (Fig. 7). The carbonate content of core PS1752-1 is too low for measuring oxygen isotopes and establishing an oxygen isotope stratigraphy. Thus the age model is based on a combination of C. davisiana events, as far as they are calibrated to oxygen isotopes, and biostratigraphic extinction levels. For the last 400 kyr SPECMAP ages have been transferred to core PS1752-1 using C. davisiana events `b±x', which are correlated to oxygen isotope stratigraphy (Hays et al., 1976a,b; Morley and Hays, 1979) (Figs. 4±6). Further age control points are biostratigraphic extinction levels. The last-abundantappearance datum (LAAD) of the diatom Hemidiscus karstenii was recognised in 215 cm depth (BaÂrcena, 1994). Based on oxygen isotope stratigraphy this event has an age of 195 ka (Burckle et al., 1978). In 480 cm depth the extinction of the radiolarian Stylatractus universus has been observed. This extinction
177
has been shown to be globally synchronous (Hays and Shackleton, 1976; Morley and Shackleton, 1978) and it was calibrated versus oxygen isotope stratigraphy as well. The last appearance datum (LAD) of S. universus coincides with the oxygen isotope stage boundary 12/11 and has an age of about 425 ka (Hays and Shackleton, 1976; Morley and Shackleton, 1978). Furthermore, the LAD of the diatom Actinocyclus ingens has been identi®ed in 760 cm depth (BaÂrcena, 1994). This event has been calibrated versus magnetostratigraphy and has an age of about 650 ka (Gersonde and BaÂrcena, 1998). The resulting age/depth relationship for sediment core PS1752-1 displays a nearly constant sedimentation rate (Fig. 8). The average sedimentation rate is 1.2 cm/1000 years. It is well known, that C. davisiana is an indicator for glacial time periods during the last 450 kyr (Hays et al., 1976a,b). Major abundance peaks of C. davisiana appear approximately every 100 ka in analogy to major glacial stages, e.g. C. davisiana abundance peaks `b, f, h, n, r, and x' correspond to oxygen isotope stages 2, 6, 8, 10, and 12 (Figs. 4±6; Hays et al., 1976a,b; Morley and Hays, 1979). The C. davisiana events `aa±hh', which occur in the lower part of core PS1752-1, have not been calibrated versus oxygen isotope stratigraphy yet. Assuming linear sedimentation rates between the LAD of S. universus and the LAD of A. ingens, ages for C. davisiana events `aa±hh' can be estimated. The C. davisiana maximum `ff' falls exactly together with the LAD of A. ingens. Thus the C. davisiana maximum `ff' occurs about 650 kyr during oxygen isotope stage 16. Based on linear interpolation between the age control points the C. davisiana maximum `bb' has an estimated age about 570 ka (Fig. 8). Although this age does not exactly match the age range of oxygen isotope stage 14 (525±565 kyr; Imbrie et al., 1984), it is reasonable to assume, that the C. davisiana maximum `bb' is the counterpart to oxygen isotope stage 14 (Fig. 9), because it is the only prominent maximum between the C. davisiana maximum `x', which is correlated to oxygen isotope stage 12 and the C. davisiana maximum `ff' which occurs during oxygen isotope stage 16. The strongest and broadest maximum during the last 700 kyr is C. davisiana event `x', which occurs during oxygen isotope stage 12. The next comparable strong
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Fig. 7. Relative abundances of the radiolarian C. davisiana in sediment core PS1752-1. Arrows mark the last-abundant-appearance datum (LAAD) of the diatom H. karstenii (BaÂrcena, 1994), the last appearance datum (LAD) of the radiolarian S. universus and the LAD of the diatom A. ingens (BaÂrcena, 1994).
maximum appears approximately 400 kyr later, the C. davisiana event `b', during oxygen isotope stage 2. This might be a hint that the 400 kyr component of orbital forcing is recorded in the C. davisiana signals. 4. Summary and conclusions C. davisiana abundance ¯uctuations have been
shown to be an important stratigraphic tool for carbonate poor sediments. For two sediment cores from the subantarctic Atlantic Ocean the late Quaternary C. davisiana abundance ¯uctuations have been determined and measurements of benthic and planktic foraminiferal d 18O have been used to date the C. davisiana records independently. Thus it was possible to extent the C. davisiana abundance stratigraphy versus oxygen isotope stages back to 220 ka for the subantarctic
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Fig. 8. Age/depth relationship for sediment core PS1752-1. Arrows mark the last-abundant-appearance datum (LAAD) of the diatom H. karstenii (BaÂrcena, 1994), the last appearance datum (LAD) of the radiolarian S. universus and the LAD of the diatom A. ingens (BaÂrcena, 1994). Lower case letters mark C. davisiana events.
Atlantic Ocean. Comparisons with an existing calibration for the subantarctic Indian Ocean show, that the age difference of the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are 0±5 kyr. Thus they are synchronous within the errors of the oxygen isotope stratigraphy, which is 3±5 kyr (Imbrie et al., 1984). Based on the oxygen isotope dated C. davisiana records mean SPECMAP ages have been calculated for the C. davisiana events occurring during the last 220 kyr. With
regard to time slice studies of the Last Glacial Maximum it is important to note, that the maximum in C. davisiana abundances during oxygen isotope stage 2 leads the maximum in oxygen isotopes (event 2.2) by 2 kyr on average. The Long-term variations in C. davisiana abundances in the subantarctic Atlantic Ocean have been examined in a sediment core covering the last 700 kyr. The carbonate content of this record is too low for establishing an oxygen isotope chronology.
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ages for the C. davisiana events occurring in early Brunhes sediments. It is well known from late Brunhes sediments, that major C. davisiana abundance maxima appear approximately every 100 ka in conjunction with glacial/interglacial cycles. Our results show, that this trend is also visible in early Brunhes sediments. Acknowledgements We are grateful to Andreas Mackensen for discussions and suggestions linked to this work. We thank Gerhard Schmiedl, Herve Chamley and an anonymous reviewer for their helpful comments and suggestions. Presented C. davisiana data will be stored in the Pangaea database (www.pangaea.de). This is contribution No. 330 of the Sonderforschungsbereich 261 at Bremen University and the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven. References
Fig. 9. Comparison of C. davisiana abundance ¯uctuations (black line) with major shifts of the oxygen isotope record over the last 700 kyr (grey line; 0±620 ka SPECMAP stack from Imbrie et al. (1984), prior to 620 kyr ODP Site 677 from Shackleton et al. (1990)). Lower case letters mark C. davisiana events. Numbers mark oxygen isotope stages.
Thus the age model was based on C. davisiana events as far as they are calibrated to oxygen isotope stages (stage 12) and on biostratigraphic extinction levels of certain diatoms and radiolaria. Based on these extinction levels it became possible to estimate
Abelmann, A., 1988. Freeze-drying simpli®es the preparation of microfossils. Micropaleontology 34, 361. Abelmann, A., Gersonde, R., 1988. Cycladophora davisiana stratigraphy in Plio-Pleistocene cores from the Antarctic Ocean (Atlantic sector). Micropaleontology 34, 268±276. Abelmann, A., Gowing, M.M., 1997. Spatial distribution pattern of living polycystine radiolarian taxa-Baseline study for paleoenvironmental reconstructions in the Southern Ocean (Atlantic sector). Mar. Micropaleontol. 30, 3±28. Abelmann, A., Brathauer, U., Gersonde, R., Sieger, R., Zielinski, U., 1999. Radiolarian-based transfer function for the estimation of sea-surface temperatures in the Southern Ocean (Atlantic sector). Paleoceanography 14, 410±421. BaÂrcena, M.A., 1994. Diatomeas del Cuatenario superior del OceÂano AntaÂrtico Sector AtlaÂntico: Aportaciones reconstruccioÂn paleocenogra®ca. Thesis Doctoral, Universidad de Salamanca, 378 pp. Bjùrklund, K.R., Ciesielski, P.F., 1994. Ecology, morphology, stratigraphy, and the paleoceanographic signi®cance of Cycladophora davisiana davisiana. Part I: Ecology and morphology. Mar. Micropaleontol. 24, 71±88. Burckle, L.H., Clarke, D.B., Shackleton, N.J., 1978. Isochronous last-abundant-appearance datum (LAAD) of the diatom Hemidiscus karstenii in the sub-Antarctic. Geology 6, 243±246. Ciesielski, P.F., Bjùrklund, K.R., 1995. Ecology, morphology, stratigraphy, and the paleoceanographic signi®cance of Cycladophora davisiana davisiana. Part II: Stratigraphy in the North Atlantic (DSDP Site 609) and Labrador Sea (ODP Site 646B). Mar. Micropaleontol. 25, 67±86. CLIMAP, 1981. Seasonal reconstructions of the Earth's surface at
U. Brathauer et al. / Marine Geology 175 (2001) 167±181 the last glacial maximum. Geological Society of America, Map and chart series, Vol. C36, Boulder, Colorado, 18 pp. Cooke, D.W., Hays, J.D., 1982. Estimates of Antarctic Ocean seasonal sea-ice cover during glacial intervals. In: Craddock, C. (Ed.), Antarctic Geosciences. University of Wisconsin Press, Madison, pp. 1017±1025. Gersonde, R., BaÂrcena, M.A., 1998. Revision of the upper Pliocene±Pleistocene diatom biostratigraphy for the northern belt of the Southern Ocean. Micropaleontology 44, 84±98. Hays, J.D., Shackleton, N.J., 1976. Globally synchronous extinction of the radiolarian Stylatractus universus. Geology 4, 649±652. Hays, J.D., Imbrie, J., Shackleton, N.J., 1976a. Variations in the Earth's orbit: pacemaker of the ice ages. Science 194, 1121± 1132. Hays, J.D., Lozano, J.A., Shackleton, N.J., Irving, G., 1976b. Reconstruction of the Atlantic and western Indian Ocean sectors of the 18,000 B.P. Antarctic Ocean. Geol. Soc. Am. Mem. 145, 337±372. Howard, W.R., Prell, W.L., 1992. Late Quaternary surface circulation of the southern Indian Ocean and its relationship to orbital variations. Paleoceanography 7, 79±117. Imbrie, J., Hays, J.D., Martinson, D.G., McIntyre, A., Mix, A.C., Morley, J.J., Pisias, N.G., Prell, W.L., Shackleton, N.J., 1984. The orbital theory of Pleistocene climate: support from a revised chronology of the marine d 18O record. In: Berger, A., Imbrie, J., Hays, J., Kukla, G., Saltzman, B. (Eds.), Milankovitch and Climate Part 1, Volume C 126: NATO ASI Series, pp. 269±305. Labeyrie, L.D., Pichon, J.J., Labracherie, M., Ippolito, P., Duprat, J., Duplessy, J.C., 1986. Melting history of Antarctica during the past 60,000 years. Nature 322, 701±706. Mackensen, A., Grobe, H., Hubberten, H.-W., Kuhn, G., 1994. Benthic foraminiferal assemblages and the d 13C-signal in the Atlantic sector of the Southern Ocean: Glacial-to-interglacial contrasts. In: Zahn, R (Ed.), Carbon Cycling in the Glacial Ocean: Constraints on the Ocean's Role in Global Change. NATO ASI, Vol. I 17. Springer, New York, pp. 104±144. Mackensen, A., Rudolph, M., Kuhn, G., 2001. Late Pleistocene deep-water circulation and paleoproductivity in the subantarctic eastern Atlantic. Global and Planetary Change, in press. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C., Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: Development of a high-resolution 0±300,000-year chronostratigraphy. Quat. Res. 27, 1±29.
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Morley, J.J., 1979. A transfer function for estimating paleoceanographic conditions based on deep-sea surface sediment distribution of radiolarian assemblages in the South Atlantic. Quat. Res. 12, 381±395. Morley, J.J., 1989. Radiolarian-based transfer functions for estimating paleooceanographic conditions in the South Indian Ocean. Mar. Micropaleontol. 13, 293±307. Morley, J.J., Hays, J.D., 1979. Cycladophora davisiana: A stratigraphic tool for Pleistocene North Atlantic and interhemispheric correlation. Earth Planet. Sci. Lett. 44, 383±389. Morley, J.J., Hays, J.D., 1983. Oceanographic conditions associated with high abundances of the radiolarian Cycladophora davisiana. Earth Planet. Sci. Lett. 66, 63±72. Morley, J.J., Shackleton, N.J., 1978. Extension of the radiolarian Stylatractus universus as a biostratigraphic datum to the Atlantic Ocean. Geology 6, 309±311. Morley, J.J., Hays, J.D., Robertson, J.H., 1982. Stratigraphic framework for the late Pleistocene in the northwest Paci®c Ocean. Deep-Sea Res., Part A 29, 1485±1499. Morley, J.J., Victoria, L.T., Ashby, M.M., Kashgarian, M., 1995. A high-resolution stratigraphy for Pleistocene sediments from North Paci®c sites 881, 883, and 887 based on abundance variations of the radiolarian Cycladophora davisiana. In: Rea, D.K., Basov, I.A., Scholl, D.W., Allan, J.F. (Eds.), Proceedings of the Ocean Drilling Program, Scienti®c Results, 145, pp. 133±140. Peterson, R.G., Stramma, L., 1991. Upper-level circulation in the South Atlantic Ocean. Prog. Oceanogr. 26, 1±73. Petrushevskaya, M.G., 1967. Radiolarians of orders Spumellaria and Nassellaria of the Antarctic region. In: Andriyashev, A.P., Ushakov, P.V. (Eds.), Biological reports of the Soviet Antarctic expedition (1955±1958), 3, pp. 2±186 (in Russian, English edition 1968). Petrushevskaya, M.G., 1972. Some aspects of paleogeography, according to radiolarian analysis of deep-water bottom deposits. Oceanology 12, 534±545. Pudsey, C.J., Howe, J.A., 1998. Quaternary history of the Antarctic Circumpolar Current; evidence from the Scotia Sea. Mar. Geol. 148, 83±112. Shackleton, N.J., Berger, A., Peltier, W.R., 1990. An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Trans. R. Soc. Edinburg: Earth Sci. 81, 251±261.
www.elsevier.nl/locate/margeo
Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean Ð a stratigraphic tool for carbonate-poor Quaternary sediments U. Brathauer a,*, A. Abelmann a, R. Gersonde a, H.-S. Niebler b, D.K. FuÈtterer a a
Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, D-27515 Bremerhaven, Germany b UniversitaÈt Bremen, FB Geowissenschaften, Klagenfurter Straûe, D-28359 Bremen, Germany Received 25 February 2000; accepted 2 February 2001
Abstract We calibrated the Cycladophora davisiana abundances versus oxygen isotope stratigraphy back to 220 ka for the subantarctic Atlantic Ocean. The relative abundances of C. davisiana and d 18O measurements of benthic and planktic foraminifera have been determined in two sediment cores. Oxygen isotope stratigraphy has been used to date the C. davisiana records and to assign SPECMAP ages to the C. davisiana events. Comparisons with an existing calibration from the subantarctic Indian Ocean show, that the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' occur synchronous within the errors of the oxygen isotope stratigraphy in the Indian and the Atlantic sectors of the Southern Ocean. Larger deviations occur only for events `b1' and `g'. Furthermore, the long-term ¯uctuations in C. davisiana abundances have been studied in a sediment core covering the last 700 kyr. Based on biostratigraphic extinction levels, ages for early Brunhes C. davisiana events have been estimated. Major C. davisiana abundance maxima occur approximately every 100 ka in conjunction with glacial/interglacial cycles over the entire record. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Stratigraphy; Southern Ocean; Radiolaria; Paleoceanography
1. Introduction Broad areas of the Southern Ocean sea ¯oor are covered with carbonate poor or carbonate barren siliceous oozes. The lack or absence of carbonate shells in these sediments precludes the application of `standard dating methods' such as oxygen isotope stratigraphy or 14C dating. Hays et al. (1976a,b) developed a * Corresponding author. Present address: GeoForschungsZentrum Potsdam, PB 3.3 Ð Sedimente und Beckenbildung, Telegrafenberg, D-14473 Potsdam, Germany. Tel.: 149-331-288-1379; fax: 149-331-288-1302. E-mail address: [email protected] (U. Brathauer).
stratigraphy based on ¯uctuations in the abundances of the radiolarian species Cycladophora davisiana Ehrenberg relative to the entire radiolarian assemblage for carbonate barren Southern Ocean sediments. Hays et al. (1976a,b) recognized the general correlation of high C. davisiana abundances with glacial times and low C. davisiana abundances with interglacial times in late Quaternary sediments. The late Quaternary maxima and minima in the C. davisiana abundances have been named with lower case letters `a±o' and calibrated versus oxygen isotope stages, back to oxygen isotope stage 13 for the subantarctic Indian Ocean and back to oxygen isotope stage 2 for the subantarctic Atlantic Ocean. The correlation of
0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(01)00141-4
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oxygen isotope stage 2 and C. davisiana abundance peak `b' allowed the inclusion of carbonate barren sediment samples from the Southern Ocean into the Last Glacial Maximum study of the CLIMAP project (CLIMAP, 1981). Based on correlations between an oxygen isotope dated C. davisiana record from the subantarctic Indian Ocean and an oxygen isotope dated C. davisiana record from the North Atlantic Ocean, Morley and Hays (1979) were able to show, that the abundance ¯uctuations of C. davisiana occur synchronous in the high latitudes of both hemispheres. An explanation for the glacial/interglacial abundance variations of C. davisiana has been searched in the modern ecology of the species. C. davisiana is a cosmopolitan and deep-dwelling radiolaria (Petrushevskaya, 1967, 1972; Morley and Hays, 1979). Plankton data from the North Atlantic show, that C. davisiana prefers depths below 500 m in the open ocean (Bjùrklund and Ciesielski, 1994). In similar depths (400±1000 m) occurrences of living C. davisiana have been reported from the Southern Ocean (Abelmann and Gowing, 1997). In modern Southern Ocean surface sediment samples C. davisiana rarely exceeds 5% (Hays et al., 1976b; Morley, 1989). There is no modern analog for the high glacial abundances (about 20±30%) of C. davisiana within the Southern Ocean (Hays et al., 1976b). The abundances in the Southern Ocean surface sediment samples exhibit no correlation with longitude, latitude or temperature gradients (Hays et al., 1976b). Obviously C. davisiana is not controlled by sea surface temperatures (Hays et al., 1976b) and has been excluded from all Southern Ocean radiolarian based transfer functions (Lozano and Hays, 1976; Hays et al., 1976a; Morley, 1979; Morley, 1989; Abelmann et al., 1999). In the modern World Ocean, sediments containing high C. davisiana abundances (up to 49%) have only been found in the Sea of Okhotsk, a marginal sea in the northwest Paci®c (Morley and Hays, 1983). Thus the oceanographic conditions of the Sea of Okhotsk, with a low salinity surface layer accompanied by a strong temperature minimum near its base, have been proposed as an analog for the glacial Southern Ocean (Morley and Hays, 1983; Bjùrklund and Ciesielski, 1994). Since the early works of Hays et al. (1976a,b) and Morley and Hays (1979) C. davisiana has been established as a valuable stratigraphic tool not only in the
Southern Ocean (CLIMAP, 1981; Cooke and Hays, 1982; Labeyrie et al., 1986; Abelmann and Gersonde, 1988; Howard and Prell, 1992; Pudsey and Howe, 1998), but also in the Northwest Paci®c Ocean (Morley et al., 1982, 1995), and the North Atlantic Ocean (Ciesielski and Bjùrklund, 1995). Despite its importance for correlating and dating carbonate poor sediments, there are very few records in which C. davisiana events have been cross-correlated with oxygen isotope stages. In this work we present a ®rst calibration of C. davisiana events versus oxygen isotope stages over the last 220 kyr for the subAntarctic Atlantic Ocean. Our results show, that C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' occur synchronous in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean within the errors of the oxygen isotope stratigraphy. Larger deviations occur only for the C. davisiana maximum `b1' and the broad minimum `g'. Furthermore, we present a continuous C. davisiana record covering the last 700 kyr, which exhibits the long-term ¯uctuations in the C. davisiana abundances. It is well known from late Brunhes sediments, that major C. davisiana abundance maxima appear approximately every 100 ka. Our record shows, that this trend can also be observed in early Brunhes sediments. 2. Material and methods Sediment cores PS2082-1, PS2498-1, PS1752-1 and PS1778-5, recovered in the subantarctic zone and the polar frontal zone of the Atlantic sector of the Southern Ocean, have been examined in this study (Fig. 1, Table 1). Cores PS2082-1, PS2498-1 and PS1752-1 have been sampled every 10 cm and core PS1778-5 every 10±20 cm for radiolarian faunal analysis. Slide preparation followed the Alfred Wegener Institute standard method (Abelmann, 1988; Abelmann et al., 1999). On average 400 individuals have been counted on each slide. The abundances of C. davisiana Ehrenberg include all three subspecies C. davisiana (Ehr.) var. davisiana Petrushevskaya, C. davisiana (Ehr.) var. cornutoides Petrushevskaya and C. davisiana (Ehr.) var. semeloides Petrushevskaya. Stable oxygen isotope measurements have been carried out on the planktic foraminifer Globigerina bulloides for sediment core PS2498-1. The stable
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169
Fig. 1. Locations of investigated sediment cores. Frontal system according to Peterson and Stramma (1991).
isotopic composition was determined with a Finnigan MAT 251 isotope ratio gas mass spectrometer directly coupled to an automatic carbonate preparation device (Kiel I) and calibrated via NBS 19 to the PDB scale. The number of G. bulloides (200±250 mm) analyzed at a single measurement varied between six and ten. All values are given in d-notation versus Vienna Pee Dee Belemnite (VPDB). The overall precision of the measurements based on repeated analyses of a laboratory
standard over a one-year-period was better than 0.08½. Additionally the benthic foraminiferal (Cibicidoides spp.) oxygen isotope measurements for core PS2498-1 from Mackensen et al. (2001) and the planktic (G. bulloides) and benthic foraminiferal (Cibicidoides spp.) oxygen isotope measurements for core PS2082-1 from Mackensen et al. (1994) are used in this study.
Table 1 Locations of sediment cores investigated in this study Core
Latitude
Longitude
Water depth (m)
Sedimentation rate (cm/kyr)
PS2082-1 PS2498-1 PS1752-1 PS1778-5
43813.2 0 S 44809.2 0 S 45837.3 0 S 49800.7 0 S
11844.3 0 E 14813.7 0 W 9835.8 0 E 12841.8 0 W
4610 3783 4519 3380
4,3 9,1 1,2 6,3
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3. Results and discussion 3.1. Calibration of the C. davisiana stratigraphy in the subantarctic Atlantic Ocean versus oxygen isotope stratigraphy over the last 220 kyr The late Quaternary abundance ¯uctuations of C. davisiana have been determined in three sediment cores PS2498-1, PS2082-1, and PS1778-5. Following the nomenclature of Hays et al. (1976a,b) and Morley and Hays (1979) C. davisiana events `a±i2' have been identi®ed (Fig. 2). In order to calibrate the C. davisiana stratigraphy for the subantarctic Atlantic Ocean, sediment cores PS2082-1 and PS2498-1 have been dated with oxygen isotope stratigraphy. The calcium carbonate content in sediment core PS1778-5 is to low for establishing an oxygen isotope stratigraphy. To be consistent with an earlier calibration from the Indian sector of the Southern Ocean (Hays et al., 1976a,b; Howard and Prell, 1992) the oxygen isotope records of sediment cores PS2082-1 and PS2498-1 have been converted to SPECMAP (Imbrie et al., 1984) ages. The age model for core PS2082-1 has been taken from Mackensen et al. (1994). For core PS2498-1 the benthic foraminiferal oxygen isotopes from Mackensen et al. (2001) have been converted to SPECMAP ages (Figs. 3 and 4). The average sedimentation rates are 4 cm/1000 years for core PS2082-1 and 9 cm/1000 years for core PS2498-1 (Fig. 3, Table 1). The resulting mean sample distance is 2.3 kyr for core PS2082-1 and 1.1 kyr for core PS2498-1. The C. davisiana minima `a' coincides with low oxygen isotope values as characteristic for the Holocene (oxygen isotope stage 1) in sediment cores PS2498-1 and PS2082-1 (Figs. 4 and 5). Maximum percentages of C. davisiana are around 5%, which is typical for Holocene sediments (Hays et al., 1976b; Morley and Hays, 1983). Below this low abundance zone strong maxima of C. davisiana with abundances about 20±25% appear. These maxima fall into oxygen isotope stage 2. Despite this general correlation between high C. davisiana values and high oxygen isotope values during oxygen isotope stage 2, the largest maximum of C. davisiana, the abundance peak `b2' is not synchronous with the maximum in oxygen isotopes (oxygen isotope event 2.2) of planktonic foraminifera G. bulloides and benthic foraminifera
Cibicidoides spp. (Figs. 4 and 5). Based on the SPECMAP ages of the oxygen isotope records, the C. davisiana abundance maximum `b2' is 23 ka in core PS2498-1, and 20 ka in core PS2082-1 (Table 2). Thus the maximum in C. davisiana (event `b2') is 1±4 kyr older than isotopic event 2.2 (19 ka) in the SPECMAP time scale. In sediment core PS2082-1 the maximum in oxygen isotopes (event 2.2) coincides with the most recent but lower abundance peak `b1'. The SPECMAP age of peak `b1' in core PS2082-1 is 18 ka. In core PS2498-1 the abundance peak `b1' is not well expressed, and occurs slightly before the maximum in benthic foraminiferal d 18O (event 2.2) at 20 ka. During oxygen isotope stage 3 the benthic and planktic oxygen isotope records of cores PS2498-1 and PS2082-1 do not exhibit prominent excursions. In contrast the C. davisiana abundance ¯uctuations can clearly be subdivided in three distinct intervals. The minima `c1' and `c2' as well as an unnamed prominent maximum between `c1' and `c2' (Figs. 4 and 5). These variations are extremely well documented in sediment core PS2498-1, which shows highest sedimentation rates during oxygen isotope stage 3 (Fig. 3). Abundance minima `c2' occurred at 58 ka (PS2498-1) and 56 ka (PS2082-1) in the subantarctic Atlantic Ocean (Figs. 4 and 5). Minimum `c2' is followed by increased abundances of C. davisiana. In core PS2498-1 values stay high for an interval of 13,000 years (from 56 to 43 kyr). The transition to the following abundance minima `c1' is sharp and rapid. C. davisiana abundance minima `c1' has an age about 38 ka in core PS2498-1 and 36 ka in core PS2082-1. None of the benthic (Cibicidoides spp.) or planktic foraminiferal (G. bulloides) d 18O records measured on sediment cores PS2498-1 and PS2082-1 show similar substantial shifts during oxygen isotope stage 3, which points to C. davisiana as an important stratigraphic marker for oxygen isotope stage 3. Oxygen isotope stage 4 exhibits typical glacial conditions with high C. davisiana abundances in conjunction with maxima in oxygen isotope values (Figs. 4 and 5). In cores PS2498-1 and PS2082-1 C. davisiana event `d' is developed as a double-peak. For the age determination of peak `d' the midpoint of the double-peak has been taken (Figs. 4 and 5). This leads to ages of about 64 ka for core PS2498-1 and 66 ka for core PS2082-1.
Fig. 2. Relative abundances of C. davisiana in sediment cores PS1778-5, PS2498-1, PS2082-1 from the subpolar and subantarctic Atlantic Ocean and core RC11-120 from the subantarctic Indian Ocean. C. davisiana abundance data for core RC11-120 have been taken from Martinson et al. (1987), the oxygen isotope stage boundaries are from Howard and Prell (1992).
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Fig. 3. Age/depth relationship for sediment cores PS2498-1, PS2082-1 (data from Mackensen et al., 1994) and RC11-120 (data from Howard and Prell, 1992). For core PS2498-1 the benthic foraminiferal oxygen isotopes from Mackensen et al. (2001) have been converted to SPECMAP ages.
The oxygen isotope events 5.1, 5.3, and 5.5 have been clearly correlated to C. davisiana events `e1', `e2', and `e3' in the Indian sector of the Southern Ocean (Fig. 6) by Hays et al. (1976a,b) and in the North Atlantic Ocean by Morley and Hays (1979). In principle the same correlation can be observed in the subantarctic Atlantic Ocean cores PS2498-1 (Fig. 4) and PS2082-1 (Fig. 5). C. davisiana events `e1', `e2', and `e3' correspond to oxygen isotope events 5.1, 5.3, and 5.5. However, in the high resolution record PS2498-1 the C. davisiana event `e2' is markedly shifted with respect to the oxygen isotope record. C. davisiana event `e2' is leading oxygen isotope event 5.3 by 6 kyr (Fig. 4, Table 2). That this shift is only documented in core PS2498-1 might be a consequence of the high sedimentation rates in this core in comparison to the other cores. Oxygen isotope stage 6 is indicated by two
pronounced C. davisiana maxima `f' and `h', and between them the minima `g' in the C. davisiana records (Hays et al., 1976a,b; Morley and Hays, 1979). In core PS2082-1 the abundance maxima `f' (136 ka) and `h' (180 ka) are accompanied by high oxygen isotope values, but the abundance minima `g' does not ®nd a counterpart in the oxygen isotope records (Fig. 5). For the broad minimum `g' a ®x-point for the age determination is dif®cult to de®ne. The lowest value occurs about 146 ka in core PS2082-1. The C. davisiana events `i1' and `i2' correspond to middle to late oxygen isotope stage 7 (Fig. 5) (Hays et al., 1976a,b; Morley and Hays, 1979). In core PS2082-1 the age for C. davisiana event `i1' is 197 ka and for event `i2' 224 ka. During the last glacial/interglacial cycle the age differences between core PS2082-1 and core PS2498-1 are 2±3 kyr for most of the C. davisiana events (Table 2). Only events `e1' and `e2' (oxygen
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173
Fig. 4. Oxygen isotope measurements of planktic (G. bulloides) and benthic (Cibicidoides spp.) foraminifera as well as relative abundances of the radiolarian C. davisiana for sediment core PS2498-1. Benthic oxygen isotope data are from Mackensen et al. (2001).
isotope events 5.1 and 5.3) show larger deviations, with age differences of 5 and 10 kyr, respectively. 3.2. Comparison of subantarctic Atlantic and subantarctic Indian Ocean C. davisiana records The subantarctic Atlantic C. davisiana records from cores PS2498-1 and PS2082-1 have been compared with the standard C. davisiana record from the subantarctic Indian Ocean core RC11-120. For core RC11-120 the C. davisiana abundance data have been taken from Martinson et al. (1987) and the age model given by Howard and Prell (1992) has been chosen. Thus all three records are dated with oxygen isotope stratigraphy and represent SPECMAP ages.
This approach allows a direct comparison between the ages of the C. davisiana events from the subantarctic Atlantic Ocean and the ages of the C. davisiana events from the subantarctic Indian Ocean, assuming synchronism between the oxygen isotope records within the Southern Ocean. The C. davisiana records from the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are almost identical (Figs. 2, 4, 5, and 6). Especially the C. davisiana signals of sediment cores PS2082-1 and RC11-120, which show comparable sedimentation rates, are quiet similar (Figs. 2, 5, and 6). The different CaCO3-content levels in these cores exhibit, that different dissolution or production rates of CaCO3 do not in¯uence the C. davisiana signals.
Fig. 5. Oxygen isotope measurements of planktic (G. bulloides) and benthic (Cibicidoides spp.) foraminifera, relative abundances of the radiolarian C. davisiana and CaCO3content for sediment core PS2082-1. Oxygen isotope data and CaCO3 data are from Mackensen et al. (1994).
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Table 2 Oxygen isotope dated ages for C. davisiana events C. davisiana event
b1 b2 c1 c2 d e1 e2 e3 f g h i1 i2 a b c
Depth (cm)
Age (ka) based on oxygen isotope stratigraphy (SPECMAP time scale) a
RC11-120 b
PS2082-1
PS2498-1
RC11-120 c
PS2082-1
PS2498-1
mean
60 80 145 210 225 300 355 415 480 520 605 645 700
78 138 288 358 403 458 498 558 608 678 848 918 958
140 190 450 690 755 910 1021 1100
14 20 40 58 65 83 100 120 139 154 184 197 222
18 20 36 56 66 79 95 124 136 146 180 197 224
20 23 38 58 64 84 105 122
17 21 38 57 65 82 100 122 138 150 182 197 223
SPECMAP time scale (Imbrie et al., 1984). Data are from Martinson et al. (1987). Ages are from Howard and Prell (1992).
In the Indian Ocean core RC11-120 the C. davisiana abundance peaks `b1' and `b2' fall close together due to low sedimentation rates. Maximum `b1' occurs slightly after the maximum in planktic and benthic foraminiferal d 18O (oxygen isotope event 2.2) (Fig. 6) and has an age about 14 ka in this core (Howard and Prell, 1992). Thus C. davisiana abundance peak `b1' is 4±6 kyr younger in the Indian Ocean record than in the Atlantic Ocean records. The age of abundance maximum `b2' in the Indian Ocean core RC11-120 is 20 ka (Howard and Prell, 1992), which is identical with the age in Atlantic Ocean record PS2082-1 and which is 3 kyr younger than in Atlantic Ocean core PS2498-1. Generally the C. davisiana event `b2', which represents maximum abundances during oxygen isotope stage 2, is easier to recognise and the calculated ages based on oxygen isotopes from the Indian Ocean and the Atlantic Ocean records are more consistent than for abundance peak `b1'. For the C. davisiana events occurring during oxygen isotope stages 3 and 4 the age differences between the Atlantic and Indian Ocean records are small. The ages for C. davisiana event `c1' differ about 2±4 kyr. The ages for events `c2' and `d' match extremely well, the differences lie between 0±2 kyr. C. davisiana maxima `f' and `h' (oxygen
isotope stage 6) are 3±4 kyr older in the Indian Ocean record RC11-120 (Howard and Prell, 1992) compared to the ages in the Atlantic Ocean record PS2082-1 (Table 2). In both records a ®x-point for the age determination of the broad minimum `g' is dif®cult to de®ne (Figs. 5 and 6). The lowest abundance value in Indian Ocean core RC11-120 has an age of 154 ka (Howard and Prell, 1992). In core PS2082-1 the lowest value occurs about 146 ka, which results in an age difference of 8 kyr to core RC11-120 (Table 2). For the C. davisiana abundance minima `i1' and `i2' (oxygen isotope stage 7) both, the Atlantic Ocean and the Indian Ocean record give almost identical ages. The age for C. davisiana event `i1' is 197 ka in both records and the age for C. davisiana event `i2' is 224 ka in the subantarctic Atlantic Ocean (PS2082-1) and 222 ka in the subantarctic Indian Ocean (RC11-120; Howard and Prell, 1992) (Table 2). The age difference of the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are 0±5 kyr (Table 2). The error ranges for oxygen isotope ages dated with the SPECMAP time scale is 3±5 kyr (Imbrie et al., 1984). Thus the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' are
Fig. 6. Oxygen isotope measurements of planktic (G. bulloides) and benthic foraminifera, relative abundances of the radiolarian C. davisiana and CaCO3-content for sediment core RC11-120. Data are from Martinson et al. (1987), oxygen isotope stage boundaries after Howard and Prell (1992).
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synchronous within the errors of oxygen isotope chronology. Exceptions during the last 220 kyr are only event `b1', which is 4±6 ka younger in the Indian Ocean record compared to the Atlantic Ocean records, and event `g', which shows an age difference about 8 kyr. The SPECMAP calibrated ages of the C. davisiana events occurring during the last 220 kyr in the Indian Ocean record RC11-120 (Howard and Prell, 1992), and the Atlantic Ocean records PS2082-1 and PS2498-1 have been used to calculate mean SPECMAP ages for the C. davisiana events `b1 ±i2', which can be used to tie non-carbonate sediment cores from the subantarctic Indian and subantarctic Atlantic Ocean via C. davisiana stratigraphy to the SPECMAP time scale (Table 2). The deviation from the mean for the individual ages of the C. davisiana events in the three sediment cores lies between 0 and 5 kyr (Table 2). 3.3. Long-term variations in the C. davisiana abundances Ð the last 700 kyr Following the nomenclature of Hays et al. (1976a,b) and Morley et al. (1995) the C. davisiana abundance peaks `b±ff' have been determined in sediment core PS1752-1 from the subantarctic Atlantic Ocean (Fig. 7). C. davisiana events `gg' and `hh' were not de®ned in these works. Here the label `gg' is used to denote the minimum preceding C. davisiana abundance peak `ff', and the label `hh' is used to denote the maximum preceding C. davisiana abundance peak `ff' (Fig. 7). The carbonate content of core PS1752-1 is too low for measuring oxygen isotopes and establishing an oxygen isotope stratigraphy. Thus the age model is based on a combination of C. davisiana events, as far as they are calibrated to oxygen isotopes, and biostratigraphic extinction levels. For the last 400 kyr SPECMAP ages have been transferred to core PS1752-1 using C. davisiana events `b±x', which are correlated to oxygen isotope stratigraphy (Hays et al., 1976a,b; Morley and Hays, 1979) (Figs. 4±6). Further age control points are biostratigraphic extinction levels. The last-abundantappearance datum (LAAD) of the diatom Hemidiscus karstenii was recognised in 215 cm depth (BaÂrcena, 1994). Based on oxygen isotope stratigraphy this event has an age of 195 ka (Burckle et al., 1978). In 480 cm depth the extinction of the radiolarian Stylatractus universus has been observed. This extinction
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has been shown to be globally synchronous (Hays and Shackleton, 1976; Morley and Shackleton, 1978) and it was calibrated versus oxygen isotope stratigraphy as well. The last appearance datum (LAD) of S. universus coincides with the oxygen isotope stage boundary 12/11 and has an age of about 425 ka (Hays and Shackleton, 1976; Morley and Shackleton, 1978). Furthermore, the LAD of the diatom Actinocyclus ingens has been identi®ed in 760 cm depth (BaÂrcena, 1994). This event has been calibrated versus magnetostratigraphy and has an age of about 650 ka (Gersonde and BaÂrcena, 1998). The resulting age/depth relationship for sediment core PS1752-1 displays a nearly constant sedimentation rate (Fig. 8). The average sedimentation rate is 1.2 cm/1000 years. It is well known, that C. davisiana is an indicator for glacial time periods during the last 450 kyr (Hays et al., 1976a,b). Major abundance peaks of C. davisiana appear approximately every 100 ka in analogy to major glacial stages, e.g. C. davisiana abundance peaks `b, f, h, n, r, and x' correspond to oxygen isotope stages 2, 6, 8, 10, and 12 (Figs. 4±6; Hays et al., 1976a,b; Morley and Hays, 1979). The C. davisiana events `aa±hh', which occur in the lower part of core PS1752-1, have not been calibrated versus oxygen isotope stratigraphy yet. Assuming linear sedimentation rates between the LAD of S. universus and the LAD of A. ingens, ages for C. davisiana events `aa±hh' can be estimated. The C. davisiana maximum `ff' falls exactly together with the LAD of A. ingens. Thus the C. davisiana maximum `ff' occurs about 650 kyr during oxygen isotope stage 16. Based on linear interpolation between the age control points the C. davisiana maximum `bb' has an estimated age about 570 ka (Fig. 8). Although this age does not exactly match the age range of oxygen isotope stage 14 (525±565 kyr; Imbrie et al., 1984), it is reasonable to assume, that the C. davisiana maximum `bb' is the counterpart to oxygen isotope stage 14 (Fig. 9), because it is the only prominent maximum between the C. davisiana maximum `x', which is correlated to oxygen isotope stage 12 and the C. davisiana maximum `ff' which occurs during oxygen isotope stage 16. The strongest and broadest maximum during the last 700 kyr is C. davisiana event `x', which occurs during oxygen isotope stage 12. The next comparable strong
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Fig. 7. Relative abundances of the radiolarian C. davisiana in sediment core PS1752-1. Arrows mark the last-abundant-appearance datum (LAAD) of the diatom H. karstenii (BaÂrcena, 1994), the last appearance datum (LAD) of the radiolarian S. universus and the LAD of the diatom A. ingens (BaÂrcena, 1994).
maximum appears approximately 400 kyr later, the C. davisiana event `b', during oxygen isotope stage 2. This might be a hint that the 400 kyr component of orbital forcing is recorded in the C. davisiana signals. 4. Summary and conclusions C. davisiana abundance ¯uctuations have been
shown to be an important stratigraphic tool for carbonate poor sediments. For two sediment cores from the subantarctic Atlantic Ocean the late Quaternary C. davisiana abundance ¯uctuations have been determined and measurements of benthic and planktic foraminiferal d 18O have been used to date the C. davisiana records independently. Thus it was possible to extent the C. davisiana abundance stratigraphy versus oxygen isotope stages back to 220 ka for the subantarctic
U. Brathauer et al. / Marine Geology 175 (2001) 167±181
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Fig. 8. Age/depth relationship for sediment core PS1752-1. Arrows mark the last-abundant-appearance datum (LAAD) of the diatom H. karstenii (BaÂrcena, 1994), the last appearance datum (LAD) of the radiolarian S. universus and the LAD of the diatom A. ingens (BaÂrcena, 1994). Lower case letters mark C. davisiana events.
Atlantic Ocean. Comparisons with an existing calibration for the subantarctic Indian Ocean show, that the age difference of the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2' in the subantarctic Atlantic Ocean and the subantarctic Indian Ocean are 0±5 kyr. Thus they are synchronous within the errors of the oxygen isotope stratigraphy, which is 3±5 kyr (Imbrie et al., 1984). Based on the oxygen isotope dated C. davisiana records mean SPECMAP ages have been calculated for the C. davisiana events occurring during the last 220 kyr. With
regard to time slice studies of the Last Glacial Maximum it is important to note, that the maximum in C. davisiana abundances during oxygen isotope stage 2 leads the maximum in oxygen isotopes (event 2.2) by 2 kyr on average. The Long-term variations in C. davisiana abundances in the subantarctic Atlantic Ocean have been examined in a sediment core covering the last 700 kyr. The carbonate content of this record is too low for establishing an oxygen isotope chronology.
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ages for the C. davisiana events occurring in early Brunhes sediments. It is well known from late Brunhes sediments, that major C. davisiana abundance maxima appear approximately every 100 ka in conjunction with glacial/interglacial cycles. Our results show, that this trend is also visible in early Brunhes sediments. Acknowledgements We are grateful to Andreas Mackensen for discussions and suggestions linked to this work. We thank Gerhard Schmiedl, Herve Chamley and an anonymous reviewer for their helpful comments and suggestions. Presented C. davisiana data will be stored in the Pangaea database (www.pangaea.de). This is contribution No. 330 of the Sonderforschungsbereich 261 at Bremen University and the Alfred Wegener Institute for Polar and Marine Research, Bremerhaven. References
Fig. 9. Comparison of C. davisiana abundance ¯uctuations (black line) with major shifts of the oxygen isotope record over the last 700 kyr (grey line; 0±620 ka SPECMAP stack from Imbrie et al. (1984), prior to 620 kyr ODP Site 677 from Shackleton et al. (1990)). Lower case letters mark C. davisiana events. Numbers mark oxygen isotope stages.
Thus the age model was based on C. davisiana events as far as they are calibrated to oxygen isotope stages (stage 12) and on biostratigraphic extinction levels of certain diatoms and radiolaria. Based on these extinction levels it became possible to estimate
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