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Geochronology and paleothermometry of Neogene sediments from the Vøring Plateau using Sr, C and O isotopes
Earth and Planetary' Science Letters, 78 (1986) 368 378 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
368
[31
Geochronology and paleothermometry of Neogene sediments from the Voring Plateau using Sr, C and O isotopes P.C. Smalley 1, A. N o r d a a
2,.
a n d A. R h h e i m 1
J Institute for Energy Technology, P.O. Box 40, 2007 Kjeller (Norway) : Department of Geology', University of Oslo, Oslo (Norway) Received October 17, 1985; revised version accepted April 7, 1986 The Neogene sediments from DSDP site 341 on the Voring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show a 6180 shift of + 1%o during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawater 81SO. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb-Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 + 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of - 15 m / M a . The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate ( - 0.2 m / M a ) . The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, - 45 m / M a . This is the first application of C, O and Sr isotopic stratigraphy combined with Rb-Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.
1. Introduction
The Neogene was a time of change, for it was during this period that great thicknesses of ice accumulated at the polar ice caps [1-3], with corresponding global changes in sedimentology, oceanography and climate. The early Miocene saw the sinking of the Greenland-Scotland Ridge to a depth greater than 500 m below sea level [4], allowing cold bottom water from the Arctic to gain access to the North Atlantic via the Norwegian-Greenland Sea [5]. This affected the subsequent evolution of the Norwegian-Greenland Sea in terms of palaeoceanography and depositional palaeoenvironment. This paper represents part of a detailed study of the effects of these major oceanographic-climatic events on sedimentation in the Norwegian-Greenland Sea, based upon samples from Deep Sea Drilling Project site 341, situated on the Voring Plateau near the Norwegian continental margin. Initial work on * Present address: Fina Exploration Norway, Skogstostraen 37, P.O. Box 4055 Tasta, 4001 Stavanger, Norway. 0012-821X/86/$03.50
~ 1986 Elsevier Science Publishers B.V.
this site [6] has recently been supplemented by a detailed lithological and palaeontological study [7]. Three new wells from this area have also recently been completed for Ocean Drilling Project Leg 104 [8]. The present work concentrates on C, O and Sr isotopic analyses and the climatic and stratigraphic constraints that can be derived therefrom. The Voring Plateau extends seawards from the Norwegian continental margin at an average depth of 1300 m, bounded by the Norwegian continental shelf to the east, and deep basins to the west (Fig. 1). It is divided in two by the Voring Plateau Escarpment (VPE; Fig. 1), probably representing the boundary between thinned continental crustal basement to the east and thickened Icelandic type oceanic crust to the west [6,9]. The basement west of the VPE is covered by - 1700 m of sediments. East of the VPE, subsidence has allowed sediments to accumulate to thicknesses of 4 - 8 km [11]. DSDP Leg 38 involved the drilling of 5 boreholes on the Voring Plateau. Site 341 was situated east of the VPE, in the thick sediment pile
369 (Fig. 1). The main lithologies and biological events represented in the Leg 38 cores, were summarized by Schrader et al. [12]. Although they reported dominantly siliceous sediments of the Miocene giving way to glacial Pliocene/Pleistocene sediments, the Miocene/Pliocene transition is poorly documented because of poor core coverage and because sections of the late Miocene are represented by condensed glauconitic or encrusted zeolitic sequences. It is this interval that we have studied in detail at site 341.
2. Lithological and biostratigraphical summary
2.1. Lithology 114 samples from D S D P site 341, were studied from cores 20-34 (nomenclature of DSDP [6]) covering the interval 237.5-456.0 m below sea bed [7]. The sediments at site 341 can be divided into three main lithological units (Fig. 2). The lower part of unit 3, subunit 3C ( > 456-399 m), consists of consolidated undeformed diatomaceous radiolaria-rich mudstones with high clay and very low
sand content. Subunit 3B (399-352 m) has a higher content of sand and siliceous and calcareous pelagic organisms, and lower clay content. In subunit 3A (352-328 m) there is a marked change to mudstones with higher clay contents, and the content of calcareous organisms decreases drastically. Glauconitic pellets ( = glaucony of [13]) are locally very numerous. This subunit apparently represents a period of very slow deposition and low biological activity. Unit 2 (328-232 m) is partly disturbed by slumping and turbidity currents, causing redeposition of material (e.g. ostracods, bivalves) from shallower regions [6,14]. Quartz and feldspar contents and feldspar/quartz ratios are high relative to unit 3, indicating an increasing supply of terrigenous detritus. Unit 1 (232-0 m) is a pure glacial marine sequence which has not been included in the present study. Secondary gypsum is present in some samples above core 25, where it occurs as fibrous-dendritic masses. In core 25 and below, gypsum is much more common, frequently forming > 2 m m long euhedral prisms which have grown in situ.
69 °
68
67
6~
65
64 0°
5~
10 °
15 °
Fig. 1. Location and bathymetry of the Voring Plateau, showing position of DSDP site 341. Modified after [9].
370
UNIT D E S C R I P T I O N
Mud and calcareous mud with p e b b l e s SUBUNIT 1A - 0 - 3 8 . 0
.y C E PERCENTAGE COMPOSITION OF SAMPLES
LEGEND
38.0 SUSUNIT 1B - 3 8 . 0 - 6 3 . 0 Siliceous o o z e
~s
~,o .o ;o .,o
SITE
341
LAT.
67 ° 20. lO'N
LONG. 0 6 ° 0 6 . 6 4 ' E
63.0
W,D,
~
1439.0m
Diatom-Rador Silicious O o z e
SUBUNIT
1C - 6 3 . 0 - 2 3 2 . 0
S t r u c t u r e l e s s dark gray caicareot mud with pebbles
C a l c a r e o u s Ooze
~
NannofossilChalk Mud/Mudstone
232.0
~
DiatomOoze C l a y Minerals
UNIT 2 - L a y e r e d m u d , s a n d y m u d
and calcareous ooze, with pebble,,
Other Terrigenous Components including Glauconite
~
328.0
Volcanic Glass
Mud and muddy o o z e
Pelagic Siliceous Components
352.3 SUBUNIT 3 B - 3 5 2 . 3 - 3 9 8 , 8
Pelagic Calcareous Components
SUBUNIT 3 A - 3 2 8 . 0 - 3 5 2 . 3
Calcareous diatomite and d i a t o m a c e o u s mudstone
~
398.8 SUBUNIT 3C - 3 9 8 . 8 - 4 5 6 . 0
ncored Interval
Core Recovered
D i a t o m a c e o u s mudstone T O T A L PENETRATION: 4 5 6 . O m
Fig. 2. Lithologicalcolumn for DSDP site 341. Modified from [8].
2.2. Biostratigraphy The biostratigraphical zonal scheme previously built up for the Norwegian-Greenland Sea [12] is shown in Fig. 3. The most important biostratigraphical boundaries with respect to the present study are the middle M i o c e n e / l a t e Miocene (10.4 Ma [15]) and late Miocene/Pliocene (5.3 Ma [15]) boundaries. Using planktonic foraminifera, the m i d d l e / l a t e Miocene boundary at site 341 would be placed at the transition between Globorotalia mayeri and Neogloboquadrina acostaensis, which occurs in core 28 (Fig. 2). The Miocene/Pliocene boundary, as delineated by an arbitrary transition from N. atlantica (d)/N. atlantica (s) ratio > 1 to < 1 would be placed in core 26. Cores 25 to 23 are virtually barren of planktonic foraminifera, but
from core 22 upwards, unit 2 is dominated by N. pachyderma, typical of the Pliocene/Pleistocene (Fig. 3). The zonation scheme for benthic foraminifera (Fig. 3) is not suited to site 341, as Spirosigmoilinella sp. and Martinottiella communis coexist throughout subunits 3C and 3B, and both disappear abruptly in the lower part of core 25 (subunit 3A). Assemblages dominated by the Pliocene Cibicides teretis are not reached until core 20. The early to middle Miocene diatom Denticula hyalina occurs up to core 30, and Coscinodiscus plicatus occurs up to the middle of core 29, indicating that the sediments below core 29 are middle Miocene or older [16]. In the overlying rocks the key late Miocene and early Pliocene zonal diatoms (Fig. 3) are absent, Nitzschia sp. 8 dominating up to core 25. Diatoms are absent in the interval we
371
BIOSTRATIGRAPHIC ZONES FOR LEG 38 SEDIMENTS FORAMINIFERA AGE
Planktonic N.pochydormo
PLIOCENE-
Benthic
Silicoflagellates
Tho/ossios/ro oeslrup/'i Cib/c/des wuallorsforf/
Radiolaria Cyclodophoro dovl$iono
D Speculum
RhizosolenJo borboi
PLEISTOCENE N.ollonf/co S.
Diatoms
Tholoss/osl¢o kryophilo
Cl~lCl'de$ l e r e l l $ D bol/vienxis
Coscinodiscus morgi#otus
Anlorcf/sso while/'
Denliculo hu$ledfii
N. otlOntl'CO O.
Cymofosiro hihorenst$
Late Ooniolhoc/um tenue
N ocosfoensis
UNZONED
M cirCulos Rhizosolenio mioconico Mor/inolt/e/lo COlnln/nl$ Tholosslo$/~o grow'do tu
Middle
Lifhomeh'sso s t i g i
Co$c/nodiscus plicotus
0 :E
A c f i n o m m o hol/edoMi
C. l f i a c o n t o
mGt. m o y e f i "
Denticulo hyolino ~f/chocofy$ bicon/lco
Turrilino
Early
Spirosi~mo i l i n e l l a sp.
al$otico N. noviculo
A ngulogerino groc/7/'$
N Ioto
Cyr/o¢opsello oldholmi Coscinodiscus vigilons
6onMwanorio jopomco
Fig. 3. Biostratigraphicalzonal scheme for DSDP Leg 38 based on benthic and planktonic foraminifera,silicoflagellates, diatoms and radiolaria. After Schrader et el. [12].
have studied in unit 2, but Schrader and Fenner [16] reported Nitzschia sp. 8 as far up as core 4 in lithological unit 1. If this species indicates a late Miocene age [16], then the M i o c e n e / P l i o c e n e boundary would occur within core 29 (unit 3C). This does not agree with the boundary placing by foraminifera. The middle Miocene radiolarian Lithomelissa stigi occurs up to the boundary between cores 31 and 32, and this could thus be interpreted as the m i d d l e / l a t e Miocene boundary [12,17]. The overlying sediments up to core 28 are unzoned by radiolaria, but Antarctissa whitei, interpreted as Pliocene by Bjorklund [17] occurs in cores 28 to 25. Based on radiolaria, the M i o c e n e / P l i o c e n e boundary would thus be located in core 28 or below. There is thus little agreement in the palaeontological evidence for the positioning of the biostratigraphical boundaries at site 341 with the present zonal scheme (Fig. 3). The most important lithological change occurs in subunit 3A (core 2), rep-
resenting the transition from quiet pelagicdominated sedimentation to terrigenous glacial marine sedimentation with slumping and turbidity current activity. The palaeontological data suggest that unit 3A could be anything from Pliocene to late Miocene. This lithological transition is now examined in more detail using C, O and Sr isotopes. 3. Analytical techniques
3.1. Carbon and oxygen isotopes This study utilized benthic foraminifera; planktonic foraminifera were not present in sufficient amounts. The foraminiferal tests were concentrated in the 63-125 lam and 1 2 5 - 2 5 0 / a m size fractions, purified by hand-picking and examined by SEM and optical microscopy to check for diagenetic alteration. N o evidence for recrystallization or new growth of calcite was found. The samples were lightly crushed, cleaned in an ultra-
372 sonic bath to remove adhering micritic material, and heated at 400°C for 30 minutes in a vacuum oven to remove any organic matter. C and O isotopic analyses of CO 2 derived by phosphoric acid dissolution of the foraminiferal tests were carried out at the Geological Institute, University of Bergen, using a Finnigan M A T 251 spectrometer and procedures modified after Shackleton [18]. All 613C and 8180 values are quoted as variations per mil (%o) relative to the PDB standard. As foraminifera do not usually deposit tests in oxygen isotopic equilibrium with seawater, empirical corrections to 61So were made to take account of this "vital effect": +0.64%o for Cibicides spp. and + 0 . 4 0 ~ for Melonis barleeanum [18,19]. Uvigerina is an exception which apparently requires no correction [19].
machine, were normalized to S6Sr/SSSr = 0.1194. Repeated analyses of SrCO 3 standard NBS 987 during the period Of study yielded 0.71028 _+ 0.00004 (o). Age calculations were made using York's method [20] and XSVRb = 1.42 × 10 i1. All age and intercept errors are quoted at the 2o level.
3.2. Strontium isotopes
where 8c = 61So of CO 2 prepared from the calcite and 6w of CO 2 equilibrated with the water, and assuming 6w = 0 (relative to SMOW), the analyses yield temperature estimates in the range 4 - 6 ° C . In unit 2 the 6180 values increase to a mean of +3.9%0 for Cibicides spp. and +4.5%c for M. barleeanum (2 samples). These results correspond to temperatures of - 0 ° C if 8w = 0. The oxygen isotopic data thus suggest that the change in sedimentation from unit 3 to 2 could be parallelled by a drop in marine bottom water temperature of some 5°C. The shift in foraminifer 8180 could also be explained by a change in water composition from its pre-glacial value (61SOsMow= -1.2%o) to its present-day composition (-0%o), but as this parallels the change from dextral (warm water) to sinistral (cold water) forms of N. atlantica this implies that at least a component of the 6ago shift could be the result of a drop in temperature. A bottom temperature of 0°C, whilst not impossible, is slightly low compared to temperatures that might be expected, even at this latitude. The present-day bottom water is 2 - 3 ° C . This could imply that a component of the foraminifera 61SO shift is related to a rise in the 8180 of seawater due, for example, to ice-cap formation; changes in both temperature and seawater composition seem to have been important. The j u m p towards lower 8180 values in Cibicides spp. samples from cores 22 and 21 (unit 2; Fig. 4), and the spread in values from core 20 which exceeds the limit of analytical uncertainty,
The Rb-Sr isotopic study utilized glauconitic pellets, foraminifera and secondary gypsum. The sample preparation and chemical separation of Rb and Sr were performed at the Institute for Energy Technology, Kjeller, and the isotopic analysis at the Geological Museum, Oslo. The same size fractions were used as for the C and O analysis. Glauconitic pellets were separated electromagnetically and, where sufficient sample was available, grouped into more and less paramagnetic fractions, then purified to 100% by hand-picking. The resulting fractions had characteristic colours (Table 2, Fig. 5). The grains were cleaned by leaching for 1 minute in 1.55N HC1 to remove any carbonate and by ultrasonification to remove any other adherent matter. The cleaned grains were then treated for 24 hours in 1 N ammonium acetate solution to remove exchangeable Rb and Sr followed by repeated washing in acetone and water. The foraminifera and gypsum were also concentrated by handpicking, followed by cleaning in an ultrasonic bath. The gypsum was leached for 1 minute in cold 1.55 N HC1 to remove any adhering carbonate. The samples were spiked with a mixed SVRb84Sr spike where appropriate. Rb and Sr were concentrated by standard ion-exchange techniques, using miniature 1.5 ml resin beds. Total blanks were negligible ( < 0.27 ng Sr, 0.1 ng Rb). 87Sr/S6Sr ratios, measured on a VG Micromass 30
4. Oxygen isotope palaeothermometry Oxygen isotopic results for three benthic foraminifera, Melonis barleeanum, Cibicides spp. and Uvigerina pygmea are reported in Table 1. Variations in 813C and 8a80 with sample depth are shown in Fig. 4. Corrected 8180 values for all three species remain constant in all of unit 3, clustering around + 3%o. Using the equation [21]: t ( ° C ) = 1 6 . 9 - 4.38 ( 8 ~ - 8w) + 0.10 ( 8 ~ - 8w) 2
373
TABLE 1 Carbon site
and
341.
oxygen
Values
isotope
are
Sample
for
foraminifera
from
DSDP
813Cpc, B
~lSOpD B
+0.355 +0.633 + 0.396 + 0.463 +0.314 + 0.592 + 0.679 -0.050 + 0.123 + 0.350 + 0.583 + 0.654 +0.268 - 2.411 - 0.099 +0.329 - 0.223 + 0.214 - 0.777 - 0.178 +0.273 - 0.045 + 0.092 - 0.402 + 0.452 - 0.245 - 0.276 - 0.065 + 0.580
+3.632 +3.159 + 3.806 + 3.942 +3.917 + 3.355 + 4.149 +3.234 + 3.769 + 3.960 + 2.920 + 2.592 +3.988 + 3.206 + 2.631 +2.544 + 2.946 + 2.780 + 2.702 + 2.096 +2.532 + 2.320 + 2.416 + 2.658 + 2.781 + 2.698 + 2.754 + 2.401 + 2.599
258.77 308.58 310.83 353.15 355.33 356.03 357.66 358.25 359.50 405.12 406.60
- 1.373 - 1.992 - 1.842 -2.166 - 1.641 - 2.043 - 1.771 - 1.839 - 2.994 - 1.952 - 1.540
+ 4.980 + 4.246 + 3.937 +2.534 + 2.833 + 3.410 + 2.677 + 2.808 + 3.055 + 2.641 + 2.782
373.66 406.60
- 1.503 -1.737
+ 2.770 +2.233
Depth
Cibicides
results
uncorrected
(m)
spp.
20-2. 2 - 4 20-2. 7 8 - 7 8 20-3. 2 - 6 20-3.75-78 20-4. 2 - 5 20-4. 7 6 - 7 6 20-5. 2 - 6 20-5.74-78 20-6 2-7 20-6. 7 3 - 7 9 2l-5 14-19 22-1 4-9 23-2 82-86 25-2 57-50 26-1.69-74 26-3 81-86 26-4 2-6 26-5 14-19 26-6 3-7 27-3 15-17 27-7 12-17 28-1 3-8 29-2 7-12 29-5 10-15 29-6 6-12 30-1 9 - 1 4 30-3 1 2 - 1 7 30-6 7 - 1 2 31-1 3 1 - 3 6
239.04 239.77 240.54 241.27 242.04 242.76 243.54 244.26 245.05 245.76 262.67 275.57 306.34 334.49 352.21 355.33 356.03 357.66 359.50 373.66 378.14 389.57 400.06 405.12 406.60 408.63 411.66 416.10 418.32
Melonis barleeanum 21-2 23-4 23-5 26-2 26-3 26-4 26-5 26-6 26-6 29-5 29-6,
74-79 3-10 81-86 12-16 81-86 2-6 14-19 72-77 3-7 10-15 6-12
Uoigerina pygmea 27-3, 1 5 - 1 7 29-6, 6 - 1 2
could be due to rapid fluctuations in bottom water temperature a n d / o r 8180 of the water. Alternatively, the data could be the result of contamination by resedimented middle Miocene or older
foraminifera caused by the observed slumping and turbidity current activity. These possibilities will be constrained further in the light of the Sr and C data. 5. Geochronology 5.1. Rb-Sr dating of glauconitic pellets
The Rb-Sr results are presented in Table 2. Five fractions of glauconitic pellets were analysed from core 25, subunit 3A. The most reliable material for geochronological work is claimed to be highly evolved glauconitic pellets with K > 6% [22]. Based on X R D analysis and SEM studies of surface structures, all of our samples fulfill this criterion. Nevertheless, this in itself does not ensure that the Rb-Sr age for a particular sample will be correct, and we emphasize that another criterion must be met: fractions of glauconitic pellets from within one sample must lie on an isochron with contemporary seawater (carbonate). Rb-Sr model ages for the glauconitic pellet fractions (Fig. 5), calculated assuming an initial 87Sr/S6Sr ratio equal to that of contemporary seawater as defined by the stratigraphically closest foraminiferal test analysis (0.70905 _+ 0.00006, sample 25-2, 4 7 - 5 0 cm), range between 11.5 and 16.3 Ma. Two fractions from sample 25-3, 1 - 4 cm, the stratigraphically oldest pellets analysed, lie on an isochron with contemporary carbonate, defining a date of 11.6 + 0.2 Ma (MSWD = 1.28) (Fig. 5). This indicates that, in this sample, isotopic equilibrium existed during deposition between glauconitic pellets at different stages of evolution and seawater, and this is thus likely to be a true sedimentary age for this particular stratigraphic horizon. If so, the older model ages from the overlying stratigraphically younger rocks cannot also be representing sedimentary ages. The two glauconitic pellet fractions from sample 25-2, 4 7 - 5 0 cm, show no linear relationship with carbonate foraminiferal tests from within the same sample. These pellets must have been in disequilibrium with seawater at the time of deposition of their host sedimentary layers, containing a relative excess of radiogenic 87Sr and hence yielding model ages which are too old. There are two possible explanations for this: either these pellets originally formed at an earlier time and were resedimented,
374 -
z OXYGEN, CARBON AND STRONTIUM ISOTOPIC RECORD FOR DSDP SITE 341 l- o e.o uJ o
tu ¢ O
a=
+5
+4
%,/%,
+d 3
sd 8
u
=
+3
*2
20
-1
-2
-3 0.7086 i
'
.7090 I
! AGE RESULTSi
AGE (Ma)
SE
-~ 5~5? (C)
250 21 2
22
30023 24
3A
25
.~6(Sr
h.6±o.:
350-
(Rb-Sr •~14 (C
26
O :E
3B
27
~15(C
28 400-
,--17(Sr
29
~c
3O 31 32
3C"
~17(C
+18(c
450
t
34
~
w
i '33
" Me~oresborleeonum I@ethic/des spp.
L
×Uvigerino pygmeo ®Combined foraminifero ~Secondory
gypsurn
Fig. 4. 6tSO, 613C (expressed as %~ PDB) and 875r/86Sr isotopic variations in foraminiferal tests from DSDP site 341. Absolute age estimates based on Rb-Sr glauconite dating (Rb-Sr) and comparison with known marine 87Sr/S6Sr (Sr) and ~]3C (C) curves. Oxygen values are corrected for "vital effect" fractionation (see text).
or they contain relics of Sr from a substrate material with a radiogenic Sr isotopic signature which did not equilibrate with seawater during the transformation to glauconitic minerals. The former possibility is unlikely in view of the general quiescence during the deposition of the glauconitic sediments, the low sedimentation rate and lack of evidence for sediment reworking at this time. Ad-
ditionally, the glauconitic pellets with the anomalously old model ages (samples 25-1, 87-92 cm and 25-2, 47-50 cm) contain possible relics of micaceous substrate material and the old dates yielded by these pellets can thus be attributed to inheritance of radiogenic Sr from a detrital precursor. The difference in Sr isotope systematics be-
TABLE 2 Rb-Sr isotope data for glauconitic pellet samples from DSDP site 341. " T y p e " denotes colour of groupings during hand picking. Rb and Sr values by isotope dilution Sample: Type:
25-1, 87-92 cm very dark green
25-2, 4 % 5 0 cm dark green
25-2, 47-50 cm dark green
25-3, 1 - 4 cm dark green
25-3, 1 - 4 cm light green
Rb (ppm) Sr (ppm) Rb/Sr 87Sr/86Sr 87Sr/86Sr ±2 SE×10
209.1 5.55 37.65 109.2 0.73315 ± 68
227.1 6.38 35.59 103.2 0.73287 ± 32
213.1 4.36 48.84 141.7 0.73693 ± 22
214.9 6.21 34.61 100.3 0.72538 ± 50
220.2 22.43 9.41 28.41 0.71380 + 10
5
375
%r/8%r
DSDP SITE 341,CORE 25 KEY
0.75 DGlauconitefractions (2o-error boxes) o F o r a m i n i f e r a , sample 2 5 - 2 , 4 7 - 5 0 c m Rb-Sr model ages shown for ISr =
0.70905±0.00006 All e r r o r s at 2o-level 25-2,47-50cm(dark green) 13.9-'0.2 Ma
0.74
25-2,47-50cm(very dark) 2 5 - 1 , 8 7 - 9 2 c m 16.3+-0.4 Ma 1 5 - 5 + 0 - 6 Ma 0.73
0.72
25_3 l _ 4 c m ( d a r k
gre:n) ~ s /M
K:~
~~111,6+-0.2 Ma ] a . Ii IS M r ~/01~7092C: x 0 " 0 0 0 0 6 I
.
.~5 - 3 , 1 - 4 c m(light green)
0.71 ,
,
i 50
,
,
,
,
I 100
,
,
,
,
I 150
8tnh/"6Sr-z^
Fig. 5. Rb-Sr isochron diagram showing5 glauconite fractions and one carbonate (foraminifera) sample from core 25, subunit 3A. All glauconites were hand-picked: characterization based on colour in reflected light is shown.
tween the various glauconitic samples is not merely a result of degree of evolution of the pellets, for those containing an inherited component of Sr are apparently just as evolved in terms of crystallinity, colour (i.e. darkness), surface nanno-structure and K-content. Rather, the composition of the precursor has probably played an important role. For example, the glauconitic pellets containing inherited Sr may have evolved from detrital substrate grains whereas the ones yielding the sedimentary age evolved from marine carbonate or phosphate-rich material. If this were the case then the precursors to the latter group of pellets would have been in equilibrium with seawater, and there could not have been any inheritance of radiogenic Sr. The nature of the glauconitic pellet precursor material may thus have a very important influence over whether or not a sedimentary age is obtained using the Rb-Sr method, and this should be borne in mind in future studies. For DSDP site 341, the most important conclusion to be drawn from the glauconitic pellet study is that the sediments corresponding to sample 25-3, 1-4 cm, in lithological unit 3A, were deposited 11.6 _+ 0.2 Ma ago, i.e. they correspond to the latest part of the middle Miocene [15].
5.2. Strontium seawater stratigraphy
The Sr isotopic composition of four samples of foraminifera1 tests were measured (Table 3). The R b / S r ratios of the calcite tests are so low as to effectively stop any increase in 87Sr/S6Srby in situ decay of SVRb, and they consequently record the 8VSr/86Sr of the seawater from which they were precipitated. As this ratio in seawater varied rapidly during the Neogene along a path which is now well documented [23-27], the Sr data can be used to place further constraints on the chronology of site 341. The deepest sample, 28-5, 8-13 cm, corresponds to a seawater value at - 1 7 Ma (cf. [24-27]), whereas sample 25-1, 47-50 cm, from unit 3A just a few metres above the glauconitic pellets that yielded the 11.6_+ 0.2 Ma isochron, gives a - 6 Ma age. This lends support to the hypothesis that unit 3A is a condensed sequence representing all or most of the late Miocene. It is significant that at DSDP site 346 the period from - 12 Ma to - 5 Ma has also been reported to be partially or completely replaced by a glauconitic section [12]. Some authors [24,25] have suggested a slight
376 TABLE 3 Sr isotope data for foraminifera and secondary gypsum from DSDP site 341. Sr values by isotope dilution Sample: Type:
20-2,2-4cm foraminifera
22-1,4-9cm foraminifera
25-2,47-50cm foraminifera
28-5,8 13cm foraminifera
25-1,87-92cm gypsum
25-5,81-84cm gypsum
Sr (ppm) 87Sr/S6Sr 0.70875 _+12 +_2 S E x 105
0.70893 _+12
1036.8 0.70905 + 6
0.70882 _+8
379.9 494.1 0.70912 + 10- 5 0.70911 _+8
32-2,20-25cm gypsum 0.70906 _+12
- = not determined.
decrease in s e a w a t e r 8 7 S r / 8 6 S r in late M i o c e n e / Pliocene times. The two samples we have analysed from cores 22-1 and 20-2 have lower values compared to those from cores 25 and 28. This could be reflecting a brief drop in s e a w a t e r 87Sr/86Sr in the Pliocene, although this would have to be much greater than that previously reported [24,25]. Nevertheless, these samples are from the sequence of rocks which show evidence of slumping and turbidity current activity, and it is thus possible that the anomalous Sr ratio are the product of contamination by older resedimented fossils with low 87Sr/86Sr ratios. This could also explain the anomalously low 8180 values for foraminifera from these samples (Fig. 4).
5.3. Carbon isotope stratigraphy The 613C of H C O 3 in the oceans, as reflected by benthic foraminiferal tests, has varied through time as a result of changing fluxes of reduced (13C-depleted) and oxidized (13C-rich) carbon to and from the oceans and oxidation-reduction of organic matter within the oceans. For example, high oceanic HCO~- 813C values can relate to marine transgressions, where large amounts of 13C-depleted organic carbon are deposited and preserved on continental shelves. Conversely, regressions can lead to a drop in 613C of oceanic H C O 3. Contemporaneous shifts in oceanic 613C can be correlated on a global scale [28-30]. The 613C of benthic foraminiferal tests is affected by non-equilibrium precipitation from seawater. Correction factors are not sufficiently well defined to be able to use numerical 613C values as a means of correlation in the same way as with Sr. The most promising method is correlation of identifiable 613C "events" [30] by comparison of curve shape. This involves some subjectivity and caution must thus be observed particularly, as in our case,
where the stratigraphic 613C coverage is not dense. The results for tests of Cibicides spp. and M. Barleeanum follow parallel patterns throughout the studied stratigraphic interval (Fig. 4), but with the latter species having 613C values consistently 1.5%o lower, caused by "vital effect" fractionation. The shapes of the patterns are very similar, and compare well with the previously published curves [28-30]. Several "events" can be correlated below subunit 3A: The M-shaped configuration in core 26 (Fig. 4) with Miocene carbon events [30] MC 10-13 ( - 1 4 Ma), the high in core 27 with MC 14 ( - 1 5 Ma), and the two sharp highs in cores 29 and 30 with MC 15 and MC 17 ( - 17 Ma and - 18 Ma). These results tie in with both the Rb-Sr data from the glauconitic pellets and the Sr results for the foraminifera. For the samples above the glauconitic zone, 613C values show a gradual increase of about 0.6%o reaching highest values in core 22, followed by a rapid decrease in 613C values at the base of core 20. This could correlate with MC 1 ( - 5 Ma) although the sparse sample coverage in cores 21-24 make such a correlation tentative. Most of the palaeontological data imply that a Pliocene ( < 5.3 Ma) age is reached before core 20.
6. Origin of secondary gypsum Three samples of the euhedrai secondary gypsum crystals were analysed for Sr isotopes (Table 3; Fig. 4). Samples 25-1, 87-92 cm and 25-5, 81-84 cm, from just above and below foraminifera sample 25-2, 47-50 cm, yielded 8 7 8 r / 86Sr ratios identical to that of the foraminifera (Table 3). Gypsum sample 32-2, 20-25 cm, from some 100 m deeper in the sequence, also yielded a similar value, significantly higher than the expected 8 7 S r / 8 6 S r ratio of seawater at the time of deposition of the sediments at that depth ( <
377
0.7086, see Fig. 4). The euhedral gypsum apparently has the s a m e 87Sr/86Sr ratio throughout the studied section, similar to that of seawater at - 6 Ma. The 87Sr/86 Sr of the gypsum reflects that of the pore water from which it precipitated. As the original 875r/86Sr r a t i o of pore water in the sediments from core 32-2 would have been that of contemporary seawater ( < 0.7086), the composition of the pore water must have become more radiogenic by the time of gypsum formation. We suggest that this relates of a period of deep penetration of seawater into the sediments at - 6 Ma, i.e. contemporaneously with the deposition of part of the glauconitic horizon. This correlates with the period of very slow deposition and, interestingly, occurred during the Messinian, a period known to have had significant rapid changes in sea level which could provide a mechanism for seawater penetration.
of the Norwegian Sea of up to - 5 ° C a n d / o r change in seawater composition, as documented by the oxygen isotope data, can now be dated to have occurred in the late Miocene. This can be interpreted as either parallelling a global trend of falling T [1], or being the result of a purely oceanographic process, such as the sinking of the Scotland-Greenland Ridge to a sufficient depth to allow cold bottom waters into the area. The possible rise in seawater 6180 could be related to polar ice-cap growth at this time. The present geochronological data illustrate the usefulness of combined radiogenic and stable isotope methods in sediment dating in situations where palaeontological criteria are lacking or problematic. With the aid of the isotopic data, a temporally more accurate zonation scheme can be constructed for the Voring Plateau, which will be described in a separate paper.
Acknowledgements 7. Discussion The combined C, O and Sr data set allow a detailed geochronological scheme to be constructed for DSDP site 341 (Fig. 4). From these data the best placing for the early/middle Miocene boundary would be in core 28, approximately at the boundary between lithological subunits 3C and 3B. Subunit 3B is middle Miocene in age, the middle/late Miocene boundary occurring in the lower part of core 25 in subunit 3A, which probably contains all of the late Miocene. The Miocene/Pliocene boundary most likely occurs at the top of subunit 3A (core 25) or not very far above it. Consideration of present thickness, without making allowances for compaction, water saturation, etc., shows that during the early and middle Miocene the minimum mean rate of deposition was relatively high, about 15 m / M a . The rate ~'ecreased greatly in the late Miocene (subunit 3A) tt, about 20 c m / M a . It is likely that deposition of the glauconitic pellet-bearing subunit 3A was interrupted by periods of non-deposition. With the change in type of sedimentation from open pelagic marine to glacial marine in unit 2, the mean rate of deposition increased dramatically to - 4 5 m/Ma. The possible drop in bottom water temperature
We thank the NAVF-supported National Laboratories for mass spectrometry at the Department of Geology, University of Bergen, and the Geological Museum, Oslo, for the provision of analytical facilities. P.C.S. acknowledges receipt of a Postdoctoral Fellowship from the Royal Norwegian Council for Scientific and Industrial Research. Strontium work at the Institute for Energy Technology was supported by Conoco Norway. Fina Exploration Norway kindly provided draughting facilities. We thank O.P. Hansen, C. Needham, G. Qvale, J. Thiede and the Journal's anonymous reviewers for their constructive comments. E.-B. Jorgensen typed the manuscript.
References 1 S. Savin, R.G. Douglas and F.G. Stehli, Tertiary marine paleotemperatures, Geol. Soc. Am. Bull. 86, 1499-1510, 1975. 2 R.C. Thunell, Cenozoic paleotemperature changes and planktonic foraminiferal speciation, Nature 289, 670 672, 1981. 3 J.W. Valentine, Neogene marine climatic trends: implications for biogeography and evolution of the Shallow-sea biota, Geology 12, 647-650, 1984. 4 K.G. Miller and B.E. Tucholke, Development of Cenozoic abyssal circulation south of the Greenland-Scotland Ridge, in: Structure and Development of the Greenland-Scotland Ridge--New Methods and Concepts, H.B. Bott, S. Saxou,
378
5
6
7
8 9
10
11
12
13 14
15
16
M. Talwani and J. Thiede, eds., pp. 549 590, Nato Conference Series, Series IV: Marine Sciences, Plenum Press, New York and London. 1981. J. Thiede, Paleoceanography, margin stratigraphy and paleophysiography of the Tertiary North Atlantic and Norwegian-Greenland Seas, Philos. Trans. R. Soc. London, Ser. A. 293, 177-185, 1977. M. Talwani G. Udintsev et al., Initial Reports of the Deep Sea Drilling Project, Vol. 38, 1256 pp., U.S. Government Printing Office, Washington, D.C., 1976. A. Nordaa, Neogen stratigrafi og sedimentasjonsmiljo fra DSDP hull 341, Voringplathet, Norskehavet, Hovedfagsoppgave i geologi, University of Oslo, unpublished, 1984. Ocean Drilling Project Leg 104 Scientific Party, Formation of the Norwegian Sea, Nature 319, 360-361, 1986. M. Talwani and O. Eldholm, The continental margin off Norway. A geophysical study, Geol. Soc. Am. Bull. 83, 3575-3608, 1972. T. Hagevang, Voring Plateau Escarpment; geophysical signature and tectonic significance, summarized in R.H. Gabrielsen and A.J. Koestler, First Meeting of the Tectonics and Structural Geology Studies Group of N G F on "Facult Tectonics of the Norwegian mainland, Svalbard and continental shelf", Norsk Geol. Tidsskr., pp. 217-220, 1985. K. Hinz, The seismic crustal structure of the Norwegian continental margin in the Voring Plateau, in the Norwegian deep sea and on the eastern flank of the Jan Mayen Ridge, between 66 ° and 68 ° , Proc. 24th Int. Geol. Congr., Montreal, pp. 28-36, 1972. H.J. Schrader, K. Bjorklund, S. Manum, E. Martini and J.E. van Hinte, Cenozoic biostratigraphy, physical stratigraphy and paleoceanography in the Norwegian-Greenland Sea, DSDP Leg 38: paleontological synthesis, in: Initial Reports of the Deep Sea Drilling Project, 38, pp. 1197-1211, U.S. Government Printing Office, Washington, D.C., 1976. G.S. Odin and A. Matter, De glauconiarum origine, Sedimentology 28, 611-641, 1981. M. Talwani, G. Udintsev et al., Initial Reports of the Deep Sea Drilling Project, Vol. 38, Supplement to XXVIII, XXXIX, X L XLI, 488 pp., U.S. Government Printing Office, Washington, D.C., 1978. W.A. Berggren, D.V. Kent, J.J. Flynn and J.A. van Couvering, Cenozoic geochronology, Geol. Soc. Am. Bull. 96, 1407-1418, 1985. H.J. Schrader and J. Fenner, Norwegian Sea Cenozoic
diatom biostratigraphy and taxonomy, in: Initial Reports of the Deep Sea Drilling Project, 38, pp. 921 1099, U.S. Government Printing Office, Washington, D.C., 1976. 17 K. Bjorklund, Radiolaria from the Norwegian Sea, Leg 38 of the Deep Sea Drilling Project, in: Initial Reports of the Deep Sea Drilling Project, 38, pp. 1101 1168, U.S. Government Printing Office, Washington, D.C., 1976. 18 N.J. Shackleton, Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uc'igerina: isotopic changes in the ocean during the last glacial, C.N.R.S. Colloq. 219, 203 209, 1974. 29 S.S. Streeter and N.J. Shackleton, Paleocirculation of the deep North Atlantic: 150,000 year record of benthic foraminifera and oxygen-18, Science 203, 168 171, 1979. 20 D. York, Lead squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5, 320 324, 1969. 21 N.J. Shackleton, Oxygen isotope analysis and palaeotemperatures reassessed, Nature 215, 15-17, 1967. 22 G.S. Odin, How to measure glaucony ages, in: Numerical Dating in Stratigraphy, G.S. Odin, ed., pp. 387-403, John Wiley and Sons, 1982. 23 W.H. Burke, R.E. Denison, E.A. Hetherington, R.B. Koepnick, H.F. Nelson and J.B. Otto, Variation of seawater 87Sr/86Sr throughout Phanerozoic time, Geology 10, 516-519, 1982. 24 D.J. DePaolo and B.L. Ingrain, High-resolution stratigraphy with strontium isotopes, Science 277, 938-941, 1985. 25 M.R. Palmer and H. Elderfield, Sr isotope composition of sea water over the past 75 Myr, Nature 314, 526-528, 1985. 26 J. Hess, M . L Bender and J.G. Schilling, Seawater 87Sr/86Sr evolution from Cretaceous to Present--applications to paleoceanography, submitted to Science. 27 D.J. DePaolo, Correlation of strontium and carbon isotope variations in Miocene marine carbonates, submitted to Science. 28 F. Woodruff and S.M. Savin, 613C values of Miocene Pacific benthic foraminifera: correlations with sea level and biological productivity, Geology 13, 119-122, 1985. 29 F. Woodruff, S.M. Savin and R.G. Douglas, Miocene stable isotope record: A detailed deep Pacific Ocean study and its paleoclimatic implications, Science 212, 665 668, 1981. 30 T.S. Loutit, N.G. Pisias and J.P. Kennett, Pacific Miocene carbon isotope stratigraphy using benthic foraminifera, Earth Planet. Sci. Lett. 66, 48 62, 1983.
368
[31
Geochronology and paleothermometry of Neogene sediments from the Voring Plateau using Sr, C and O isotopes P.C. Smalley 1, A. N o r d a a
2,.
a n d A. R h h e i m 1
J Institute for Energy Technology, P.O. Box 40, 2007 Kjeller (Norway) : Department of Geology', University of Oslo, Oslo (Norway) Received October 17, 1985; revised version accepted April 7, 1986 The Neogene sediments from DSDP site 341 on the Voring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show a 6180 shift of + 1%o during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawater 81SO. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb-Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 + 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of - 15 m / M a . The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate ( - 0.2 m / M a ) . The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, - 45 m / M a . This is the first application of C, O and Sr isotopic stratigraphy combined with Rb-Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.
1. Introduction
The Neogene was a time of change, for it was during this period that great thicknesses of ice accumulated at the polar ice caps [1-3], with corresponding global changes in sedimentology, oceanography and climate. The early Miocene saw the sinking of the Greenland-Scotland Ridge to a depth greater than 500 m below sea level [4], allowing cold bottom water from the Arctic to gain access to the North Atlantic via the Norwegian-Greenland Sea [5]. This affected the subsequent evolution of the Norwegian-Greenland Sea in terms of palaeoceanography and depositional palaeoenvironment. This paper represents part of a detailed study of the effects of these major oceanographic-climatic events on sedimentation in the Norwegian-Greenland Sea, based upon samples from Deep Sea Drilling Project site 341, situated on the Voring Plateau near the Norwegian continental margin. Initial work on * Present address: Fina Exploration Norway, Skogstostraen 37, P.O. Box 4055 Tasta, 4001 Stavanger, Norway. 0012-821X/86/$03.50
~ 1986 Elsevier Science Publishers B.V.
this site [6] has recently been supplemented by a detailed lithological and palaeontological study [7]. Three new wells from this area have also recently been completed for Ocean Drilling Project Leg 104 [8]. The present work concentrates on C, O and Sr isotopic analyses and the climatic and stratigraphic constraints that can be derived therefrom. The Voring Plateau extends seawards from the Norwegian continental margin at an average depth of 1300 m, bounded by the Norwegian continental shelf to the east, and deep basins to the west (Fig. 1). It is divided in two by the Voring Plateau Escarpment (VPE; Fig. 1), probably representing the boundary between thinned continental crustal basement to the east and thickened Icelandic type oceanic crust to the west [6,9]. The basement west of the VPE is covered by - 1700 m of sediments. East of the VPE, subsidence has allowed sediments to accumulate to thicknesses of 4 - 8 km [11]. DSDP Leg 38 involved the drilling of 5 boreholes on the Voring Plateau. Site 341 was situated east of the VPE, in the thick sediment pile
369 (Fig. 1). The main lithologies and biological events represented in the Leg 38 cores, were summarized by Schrader et al. [12]. Although they reported dominantly siliceous sediments of the Miocene giving way to glacial Pliocene/Pleistocene sediments, the Miocene/Pliocene transition is poorly documented because of poor core coverage and because sections of the late Miocene are represented by condensed glauconitic or encrusted zeolitic sequences. It is this interval that we have studied in detail at site 341.
2. Lithological and biostratigraphical summary
2.1. Lithology 114 samples from D S D P site 341, were studied from cores 20-34 (nomenclature of DSDP [6]) covering the interval 237.5-456.0 m below sea bed [7]. The sediments at site 341 can be divided into three main lithological units (Fig. 2). The lower part of unit 3, subunit 3C ( > 456-399 m), consists of consolidated undeformed diatomaceous radiolaria-rich mudstones with high clay and very low
sand content. Subunit 3B (399-352 m) has a higher content of sand and siliceous and calcareous pelagic organisms, and lower clay content. In subunit 3A (352-328 m) there is a marked change to mudstones with higher clay contents, and the content of calcareous organisms decreases drastically. Glauconitic pellets ( = glaucony of [13]) are locally very numerous. This subunit apparently represents a period of very slow deposition and low biological activity. Unit 2 (328-232 m) is partly disturbed by slumping and turbidity currents, causing redeposition of material (e.g. ostracods, bivalves) from shallower regions [6,14]. Quartz and feldspar contents and feldspar/quartz ratios are high relative to unit 3, indicating an increasing supply of terrigenous detritus. Unit 1 (232-0 m) is a pure glacial marine sequence which has not been included in the present study. Secondary gypsum is present in some samples above core 25, where it occurs as fibrous-dendritic masses. In core 25 and below, gypsum is much more common, frequently forming > 2 m m long euhedral prisms which have grown in situ.
69 °
68
67
6~
65
64 0°
5~
10 °
15 °
Fig. 1. Location and bathymetry of the Voring Plateau, showing position of DSDP site 341. Modified after [9].
370
UNIT D E S C R I P T I O N
Mud and calcareous mud with p e b b l e s SUBUNIT 1A - 0 - 3 8 . 0
.y C E PERCENTAGE COMPOSITION OF SAMPLES
LEGEND
38.0 SUSUNIT 1B - 3 8 . 0 - 6 3 . 0 Siliceous o o z e
~s
~,o .o ;o .,o
SITE
341
LAT.
67 ° 20. lO'N
LONG. 0 6 ° 0 6 . 6 4 ' E
63.0
W,D,
~
1439.0m
Diatom-Rador Silicious O o z e
SUBUNIT
1C - 6 3 . 0 - 2 3 2 . 0
S t r u c t u r e l e s s dark gray caicareot mud with pebbles
C a l c a r e o u s Ooze
~
NannofossilChalk Mud/Mudstone
232.0
~
DiatomOoze C l a y Minerals
UNIT 2 - L a y e r e d m u d , s a n d y m u d
and calcareous ooze, with pebble,,
Other Terrigenous Components including Glauconite
~
328.0
Volcanic Glass
Mud and muddy o o z e
Pelagic Siliceous Components
352.3 SUBUNIT 3 B - 3 5 2 . 3 - 3 9 8 , 8
Pelagic Calcareous Components
SUBUNIT 3 A - 3 2 8 . 0 - 3 5 2 . 3
Calcareous diatomite and d i a t o m a c e o u s mudstone
~
398.8 SUBUNIT 3C - 3 9 8 . 8 - 4 5 6 . 0
ncored Interval
Core Recovered
D i a t o m a c e o u s mudstone T O T A L PENETRATION: 4 5 6 . O m
Fig. 2. Lithologicalcolumn for DSDP site 341. Modified from [8].
2.2. Biostratigraphy The biostratigraphical zonal scheme previously built up for the Norwegian-Greenland Sea [12] is shown in Fig. 3. The most important biostratigraphical boundaries with respect to the present study are the middle M i o c e n e / l a t e Miocene (10.4 Ma [15]) and late Miocene/Pliocene (5.3 Ma [15]) boundaries. Using planktonic foraminifera, the m i d d l e / l a t e Miocene boundary at site 341 would be placed at the transition between Globorotalia mayeri and Neogloboquadrina acostaensis, which occurs in core 28 (Fig. 2). The Miocene/Pliocene boundary, as delineated by an arbitrary transition from N. atlantica (d)/N. atlantica (s) ratio > 1 to < 1 would be placed in core 26. Cores 25 to 23 are virtually barren of planktonic foraminifera, but
from core 22 upwards, unit 2 is dominated by N. pachyderma, typical of the Pliocene/Pleistocene (Fig. 3). The zonation scheme for benthic foraminifera (Fig. 3) is not suited to site 341, as Spirosigmoilinella sp. and Martinottiella communis coexist throughout subunits 3C and 3B, and both disappear abruptly in the lower part of core 25 (subunit 3A). Assemblages dominated by the Pliocene Cibicides teretis are not reached until core 20. The early to middle Miocene diatom Denticula hyalina occurs up to core 30, and Coscinodiscus plicatus occurs up to the middle of core 29, indicating that the sediments below core 29 are middle Miocene or older [16]. In the overlying rocks the key late Miocene and early Pliocene zonal diatoms (Fig. 3) are absent, Nitzschia sp. 8 dominating up to core 25. Diatoms are absent in the interval we
371
BIOSTRATIGRAPHIC ZONES FOR LEG 38 SEDIMENTS FORAMINIFERA AGE
Planktonic N.pochydormo
PLIOCENE-
Benthic
Silicoflagellates
Tho/ossios/ro oeslrup/'i Cib/c/des wuallorsforf/
Radiolaria Cyclodophoro dovl$iono
D Speculum
RhizosolenJo borboi
PLEISTOCENE N.ollonf/co S.
Diatoms
Tholoss/osl¢o kryophilo
Cl~lCl'de$ l e r e l l $ D bol/vienxis
Coscinodiscus morgi#otus
Anlorcf/sso while/'
Denliculo hu$ledfii
N. otlOntl'CO O.
Cymofosiro hihorenst$
Late Ooniolhoc/um tenue
N ocosfoensis
UNZONED
M cirCulos Rhizosolenio mioconico Mor/inolt/e/lo COlnln/nl$ Tholosslo$/~o grow'do tu
Middle
Lifhomeh'sso s t i g i
Co$c/nodiscus plicotus
0 :E
A c f i n o m m o hol/edoMi
C. l f i a c o n t o
mGt. m o y e f i "
Denticulo hyolino ~f/chocofy$ bicon/lco
Turrilino
Early
Spirosi~mo i l i n e l l a sp.
al$otico N. noviculo
A ngulogerino groc/7/'$
N Ioto
Cyr/o¢opsello oldholmi Coscinodiscus vigilons
6onMwanorio jopomco
Fig. 3. Biostratigraphicalzonal scheme for DSDP Leg 38 based on benthic and planktonic foraminifera,silicoflagellates, diatoms and radiolaria. After Schrader et el. [12].
have studied in unit 2, but Schrader and Fenner [16] reported Nitzschia sp. 8 as far up as core 4 in lithological unit 1. If this species indicates a late Miocene age [16], then the M i o c e n e / P l i o c e n e boundary would occur within core 29 (unit 3C). This does not agree with the boundary placing by foraminifera. The middle Miocene radiolarian Lithomelissa stigi occurs up to the boundary between cores 31 and 32, and this could thus be interpreted as the m i d d l e / l a t e Miocene boundary [12,17]. The overlying sediments up to core 28 are unzoned by radiolaria, but Antarctissa whitei, interpreted as Pliocene by Bjorklund [17] occurs in cores 28 to 25. Based on radiolaria, the M i o c e n e / P l i o c e n e boundary would thus be located in core 28 or below. There is thus little agreement in the palaeontological evidence for the positioning of the biostratigraphical boundaries at site 341 with the present zonal scheme (Fig. 3). The most important lithological change occurs in subunit 3A (core 2), rep-
resenting the transition from quiet pelagicdominated sedimentation to terrigenous glacial marine sedimentation with slumping and turbidity current activity. The palaeontological data suggest that unit 3A could be anything from Pliocene to late Miocene. This lithological transition is now examined in more detail using C, O and Sr isotopes. 3. Analytical techniques
3.1. Carbon and oxygen isotopes This study utilized benthic foraminifera; planktonic foraminifera were not present in sufficient amounts. The foraminiferal tests were concentrated in the 63-125 lam and 1 2 5 - 2 5 0 / a m size fractions, purified by hand-picking and examined by SEM and optical microscopy to check for diagenetic alteration. N o evidence for recrystallization or new growth of calcite was found. The samples were lightly crushed, cleaned in an ultra-
372 sonic bath to remove adhering micritic material, and heated at 400°C for 30 minutes in a vacuum oven to remove any organic matter. C and O isotopic analyses of CO 2 derived by phosphoric acid dissolution of the foraminiferal tests were carried out at the Geological Institute, University of Bergen, using a Finnigan M A T 251 spectrometer and procedures modified after Shackleton [18]. All 613C and 8180 values are quoted as variations per mil (%o) relative to the PDB standard. As foraminifera do not usually deposit tests in oxygen isotopic equilibrium with seawater, empirical corrections to 61So were made to take account of this "vital effect": +0.64%o for Cibicides spp. and + 0 . 4 0 ~ for Melonis barleeanum [18,19]. Uvigerina is an exception which apparently requires no correction [19].
machine, were normalized to S6Sr/SSSr = 0.1194. Repeated analyses of SrCO 3 standard NBS 987 during the period Of study yielded 0.71028 _+ 0.00004 (o). Age calculations were made using York's method [20] and XSVRb = 1.42 × 10 i1. All age and intercept errors are quoted at the 2o level.
3.2. Strontium isotopes
where 8c = 61So of CO 2 prepared from the calcite and 6w of CO 2 equilibrated with the water, and assuming 6w = 0 (relative to SMOW), the analyses yield temperature estimates in the range 4 - 6 ° C . In unit 2 the 6180 values increase to a mean of +3.9%0 for Cibicides spp. and +4.5%c for M. barleeanum (2 samples). These results correspond to temperatures of - 0 ° C if 8w = 0. The oxygen isotopic data thus suggest that the change in sedimentation from unit 3 to 2 could be parallelled by a drop in marine bottom water temperature of some 5°C. The shift in foraminifer 8180 could also be explained by a change in water composition from its pre-glacial value (61SOsMow= -1.2%o) to its present-day composition (-0%o), but as this parallels the change from dextral (warm water) to sinistral (cold water) forms of N. atlantica this implies that at least a component of the 6ago shift could be the result of a drop in temperature. A bottom temperature of 0°C, whilst not impossible, is slightly low compared to temperatures that might be expected, even at this latitude. The present-day bottom water is 2 - 3 ° C . This could imply that a component of the foraminifera 61SO shift is related to a rise in the 8180 of seawater due, for example, to ice-cap formation; changes in both temperature and seawater composition seem to have been important. The j u m p towards lower 8180 values in Cibicides spp. samples from cores 22 and 21 (unit 2; Fig. 4), and the spread in values from core 20 which exceeds the limit of analytical uncertainty,
The Rb-Sr isotopic study utilized glauconitic pellets, foraminifera and secondary gypsum. The sample preparation and chemical separation of Rb and Sr were performed at the Institute for Energy Technology, Kjeller, and the isotopic analysis at the Geological Museum, Oslo. The same size fractions were used as for the C and O analysis. Glauconitic pellets were separated electromagnetically and, where sufficient sample was available, grouped into more and less paramagnetic fractions, then purified to 100% by hand-picking. The resulting fractions had characteristic colours (Table 2, Fig. 5). The grains were cleaned by leaching for 1 minute in 1.55N HC1 to remove any carbonate and by ultrasonification to remove any other adherent matter. The cleaned grains were then treated for 24 hours in 1 N ammonium acetate solution to remove exchangeable Rb and Sr followed by repeated washing in acetone and water. The foraminifera and gypsum were also concentrated by handpicking, followed by cleaning in an ultrasonic bath. The gypsum was leached for 1 minute in cold 1.55 N HC1 to remove any adhering carbonate. The samples were spiked with a mixed SVRb84Sr spike where appropriate. Rb and Sr were concentrated by standard ion-exchange techniques, using miniature 1.5 ml resin beds. Total blanks were negligible ( < 0.27 ng Sr, 0.1 ng Rb). 87Sr/S6Sr ratios, measured on a VG Micromass 30
4. Oxygen isotope palaeothermometry Oxygen isotopic results for three benthic foraminifera, Melonis barleeanum, Cibicides spp. and Uvigerina pygmea are reported in Table 1. Variations in 813C and 8a80 with sample depth are shown in Fig. 4. Corrected 8180 values for all three species remain constant in all of unit 3, clustering around + 3%o. Using the equation [21]: t ( ° C ) = 1 6 . 9 - 4.38 ( 8 ~ - 8w) + 0.10 ( 8 ~ - 8w) 2
373
TABLE 1 Carbon site
and
341.
oxygen
Values
isotope
are
Sample
for
foraminifera
from
DSDP
813Cpc, B
~lSOpD B
+0.355 +0.633 + 0.396 + 0.463 +0.314 + 0.592 + 0.679 -0.050 + 0.123 + 0.350 + 0.583 + 0.654 +0.268 - 2.411 - 0.099 +0.329 - 0.223 + 0.214 - 0.777 - 0.178 +0.273 - 0.045 + 0.092 - 0.402 + 0.452 - 0.245 - 0.276 - 0.065 + 0.580
+3.632 +3.159 + 3.806 + 3.942 +3.917 + 3.355 + 4.149 +3.234 + 3.769 + 3.960 + 2.920 + 2.592 +3.988 + 3.206 + 2.631 +2.544 + 2.946 + 2.780 + 2.702 + 2.096 +2.532 + 2.320 + 2.416 + 2.658 + 2.781 + 2.698 + 2.754 + 2.401 + 2.599
258.77 308.58 310.83 353.15 355.33 356.03 357.66 358.25 359.50 405.12 406.60
- 1.373 - 1.992 - 1.842 -2.166 - 1.641 - 2.043 - 1.771 - 1.839 - 2.994 - 1.952 - 1.540
+ 4.980 + 4.246 + 3.937 +2.534 + 2.833 + 3.410 + 2.677 + 2.808 + 3.055 + 2.641 + 2.782
373.66 406.60
- 1.503 -1.737
+ 2.770 +2.233
Depth
Cibicides
results
uncorrected
(m)
spp.
20-2. 2 - 4 20-2. 7 8 - 7 8 20-3. 2 - 6 20-3.75-78 20-4. 2 - 5 20-4. 7 6 - 7 6 20-5. 2 - 6 20-5.74-78 20-6 2-7 20-6. 7 3 - 7 9 2l-5 14-19 22-1 4-9 23-2 82-86 25-2 57-50 26-1.69-74 26-3 81-86 26-4 2-6 26-5 14-19 26-6 3-7 27-3 15-17 27-7 12-17 28-1 3-8 29-2 7-12 29-5 10-15 29-6 6-12 30-1 9 - 1 4 30-3 1 2 - 1 7 30-6 7 - 1 2 31-1 3 1 - 3 6
239.04 239.77 240.54 241.27 242.04 242.76 243.54 244.26 245.05 245.76 262.67 275.57 306.34 334.49 352.21 355.33 356.03 357.66 359.50 373.66 378.14 389.57 400.06 405.12 406.60 408.63 411.66 416.10 418.32
Melonis barleeanum 21-2 23-4 23-5 26-2 26-3 26-4 26-5 26-6 26-6 29-5 29-6,
74-79 3-10 81-86 12-16 81-86 2-6 14-19 72-77 3-7 10-15 6-12
Uoigerina pygmea 27-3, 1 5 - 1 7 29-6, 6 - 1 2
could be due to rapid fluctuations in bottom water temperature a n d / o r 8180 of the water. Alternatively, the data could be the result of contamination by resedimented middle Miocene or older
foraminifera caused by the observed slumping and turbidity current activity. These possibilities will be constrained further in the light of the Sr and C data. 5. Geochronology 5.1. Rb-Sr dating of glauconitic pellets
The Rb-Sr results are presented in Table 2. Five fractions of glauconitic pellets were analysed from core 25, subunit 3A. The most reliable material for geochronological work is claimed to be highly evolved glauconitic pellets with K > 6% [22]. Based on X R D analysis and SEM studies of surface structures, all of our samples fulfill this criterion. Nevertheless, this in itself does not ensure that the Rb-Sr age for a particular sample will be correct, and we emphasize that another criterion must be met: fractions of glauconitic pellets from within one sample must lie on an isochron with contemporary seawater (carbonate). Rb-Sr model ages for the glauconitic pellet fractions (Fig. 5), calculated assuming an initial 87Sr/S6Sr ratio equal to that of contemporary seawater as defined by the stratigraphically closest foraminiferal test analysis (0.70905 _+ 0.00006, sample 25-2, 4 7 - 5 0 cm), range between 11.5 and 16.3 Ma. Two fractions from sample 25-3, 1 - 4 cm, the stratigraphically oldest pellets analysed, lie on an isochron with contemporary carbonate, defining a date of 11.6 + 0.2 Ma (MSWD = 1.28) (Fig. 5). This indicates that, in this sample, isotopic equilibrium existed during deposition between glauconitic pellets at different stages of evolution and seawater, and this is thus likely to be a true sedimentary age for this particular stratigraphic horizon. If so, the older model ages from the overlying stratigraphically younger rocks cannot also be representing sedimentary ages. The two glauconitic pellet fractions from sample 25-2, 4 7 - 5 0 cm, show no linear relationship with carbonate foraminiferal tests from within the same sample. These pellets must have been in disequilibrium with seawater at the time of deposition of their host sedimentary layers, containing a relative excess of radiogenic 87Sr and hence yielding model ages which are too old. There are two possible explanations for this: either these pellets originally formed at an earlier time and were resedimented,
374 -
z OXYGEN, CARBON AND STRONTIUM ISOTOPIC RECORD FOR DSDP SITE 341 l- o e.o uJ o
tu ¢ O
a=
+5
+4
%,/%,
+d 3
sd 8
u
=
+3
*2
20
-1
-2
-3 0.7086 i
'
.7090 I
! AGE RESULTSi
AGE (Ma)
SE
-~ 5~5? (C)
250 21 2
22
30023 24
3A
25
.~6(Sr
h.6±o.:
350-
(Rb-Sr •~14 (C
26
O :E
3B
27
~15(C
28 400-
,--17(Sr
29
~c
3O 31 32
3C"
~17(C
+18(c
450
t
34
~
w
i '33
" Me~oresborleeonum I@ethic/des spp.
L
×Uvigerino pygmeo ®Combined foraminifero ~Secondory
gypsurn
Fig. 4. 6tSO, 613C (expressed as %~ PDB) and 875r/86Sr isotopic variations in foraminiferal tests from DSDP site 341. Absolute age estimates based on Rb-Sr glauconite dating (Rb-Sr) and comparison with known marine 87Sr/S6Sr (Sr) and ~]3C (C) curves. Oxygen values are corrected for "vital effect" fractionation (see text).
or they contain relics of Sr from a substrate material with a radiogenic Sr isotopic signature which did not equilibrate with seawater during the transformation to glauconitic minerals. The former possibility is unlikely in view of the general quiescence during the deposition of the glauconitic sediments, the low sedimentation rate and lack of evidence for sediment reworking at this time. Ad-
ditionally, the glauconitic pellets with the anomalously old model ages (samples 25-1, 87-92 cm and 25-2, 47-50 cm) contain possible relics of micaceous substrate material and the old dates yielded by these pellets can thus be attributed to inheritance of radiogenic Sr from a detrital precursor. The difference in Sr isotope systematics be-
TABLE 2 Rb-Sr isotope data for glauconitic pellet samples from DSDP site 341. " T y p e " denotes colour of groupings during hand picking. Rb and Sr values by isotope dilution Sample: Type:
25-1, 87-92 cm very dark green
25-2, 4 % 5 0 cm dark green
25-2, 47-50 cm dark green
25-3, 1 - 4 cm dark green
25-3, 1 - 4 cm light green
Rb (ppm) Sr (ppm) Rb/Sr 87Sr/86Sr 87Sr/86Sr ±2 SE×10
209.1 5.55 37.65 109.2 0.73315 ± 68
227.1 6.38 35.59 103.2 0.73287 ± 32
213.1 4.36 48.84 141.7 0.73693 ± 22
214.9 6.21 34.61 100.3 0.72538 ± 50
220.2 22.43 9.41 28.41 0.71380 + 10
5
375
%r/8%r
DSDP SITE 341,CORE 25 KEY
0.75 DGlauconitefractions (2o-error boxes) o F o r a m i n i f e r a , sample 2 5 - 2 , 4 7 - 5 0 c m Rb-Sr model ages shown for ISr =
0.70905±0.00006 All e r r o r s at 2o-level 25-2,47-50cm(dark green) 13.9-'0.2 Ma
0.74
25-2,47-50cm(very dark) 2 5 - 1 , 8 7 - 9 2 c m 16.3+-0.4 Ma 1 5 - 5 + 0 - 6 Ma 0.73
0.72
25_3 l _ 4 c m ( d a r k
gre:n) ~ s /M
K:~
~~111,6+-0.2 Ma ] a . Ii IS M r ~/01~7092C: x 0 " 0 0 0 0 6 I
.
.~5 - 3 , 1 - 4 c m(light green)
0.71 ,
,
i 50
,
,
,
,
I 100
,
,
,
,
I 150
8tnh/"6Sr-z^
Fig. 5. Rb-Sr isochron diagram showing5 glauconite fractions and one carbonate (foraminifera) sample from core 25, subunit 3A. All glauconites were hand-picked: characterization based on colour in reflected light is shown.
tween the various glauconitic samples is not merely a result of degree of evolution of the pellets, for those containing an inherited component of Sr are apparently just as evolved in terms of crystallinity, colour (i.e. darkness), surface nanno-structure and K-content. Rather, the composition of the precursor has probably played an important role. For example, the glauconitic pellets containing inherited Sr may have evolved from detrital substrate grains whereas the ones yielding the sedimentary age evolved from marine carbonate or phosphate-rich material. If this were the case then the precursors to the latter group of pellets would have been in equilibrium with seawater, and there could not have been any inheritance of radiogenic Sr. The nature of the glauconitic pellet precursor material may thus have a very important influence over whether or not a sedimentary age is obtained using the Rb-Sr method, and this should be borne in mind in future studies. For DSDP site 341, the most important conclusion to be drawn from the glauconitic pellet study is that the sediments corresponding to sample 25-3, 1-4 cm, in lithological unit 3A, were deposited 11.6 _+ 0.2 Ma ago, i.e. they correspond to the latest part of the middle Miocene [15].
5.2. Strontium seawater stratigraphy
The Sr isotopic composition of four samples of foraminifera1 tests were measured (Table 3). The R b / S r ratios of the calcite tests are so low as to effectively stop any increase in 87Sr/S6Srby in situ decay of SVRb, and they consequently record the 8VSr/86Sr of the seawater from which they were precipitated. As this ratio in seawater varied rapidly during the Neogene along a path which is now well documented [23-27], the Sr data can be used to place further constraints on the chronology of site 341. The deepest sample, 28-5, 8-13 cm, corresponds to a seawater value at - 1 7 Ma (cf. [24-27]), whereas sample 25-1, 47-50 cm, from unit 3A just a few metres above the glauconitic pellets that yielded the 11.6_+ 0.2 Ma isochron, gives a - 6 Ma age. This lends support to the hypothesis that unit 3A is a condensed sequence representing all or most of the late Miocene. It is significant that at DSDP site 346 the period from - 12 Ma to - 5 Ma has also been reported to be partially or completely replaced by a glauconitic section [12]. Some authors [24,25] have suggested a slight
376 TABLE 3 Sr isotope data for foraminifera and secondary gypsum from DSDP site 341. Sr values by isotope dilution Sample: Type:
20-2,2-4cm foraminifera
22-1,4-9cm foraminifera
25-2,47-50cm foraminifera
28-5,8 13cm foraminifera
25-1,87-92cm gypsum
25-5,81-84cm gypsum
Sr (ppm) 87Sr/S6Sr 0.70875 _+12 +_2 S E x 105
0.70893 _+12
1036.8 0.70905 + 6
0.70882 _+8
379.9 494.1 0.70912 + 10- 5 0.70911 _+8
32-2,20-25cm gypsum 0.70906 _+12
- = not determined.
decrease in s e a w a t e r 8 7 S r / 8 6 S r in late M i o c e n e / Pliocene times. The two samples we have analysed from cores 22-1 and 20-2 have lower values compared to those from cores 25 and 28. This could be reflecting a brief drop in s e a w a t e r 87Sr/86Sr in the Pliocene, although this would have to be much greater than that previously reported [24,25]. Nevertheless, these samples are from the sequence of rocks which show evidence of slumping and turbidity current activity, and it is thus possible that the anomalous Sr ratio are the product of contamination by older resedimented fossils with low 87Sr/86Sr ratios. This could also explain the anomalously low 8180 values for foraminifera from these samples (Fig. 4).
5.3. Carbon isotope stratigraphy The 613C of H C O 3 in the oceans, as reflected by benthic foraminiferal tests, has varied through time as a result of changing fluxes of reduced (13C-depleted) and oxidized (13C-rich) carbon to and from the oceans and oxidation-reduction of organic matter within the oceans. For example, high oceanic HCO~- 813C values can relate to marine transgressions, where large amounts of 13C-depleted organic carbon are deposited and preserved on continental shelves. Conversely, regressions can lead to a drop in 613C of oceanic H C O 3. Contemporaneous shifts in oceanic 613C can be correlated on a global scale [28-30]. The 613C of benthic foraminiferal tests is affected by non-equilibrium precipitation from seawater. Correction factors are not sufficiently well defined to be able to use numerical 613C values as a means of correlation in the same way as with Sr. The most promising method is correlation of identifiable 613C "events" [30] by comparison of curve shape. This involves some subjectivity and caution must thus be observed particularly, as in our case,
where the stratigraphic 613C coverage is not dense. The results for tests of Cibicides spp. and M. Barleeanum follow parallel patterns throughout the studied stratigraphic interval (Fig. 4), but with the latter species having 613C values consistently 1.5%o lower, caused by "vital effect" fractionation. The shapes of the patterns are very similar, and compare well with the previously published curves [28-30]. Several "events" can be correlated below subunit 3A: The M-shaped configuration in core 26 (Fig. 4) with Miocene carbon events [30] MC 10-13 ( - 1 4 Ma), the high in core 27 with MC 14 ( - 1 5 Ma), and the two sharp highs in cores 29 and 30 with MC 15 and MC 17 ( - 17 Ma and - 18 Ma). These results tie in with both the Rb-Sr data from the glauconitic pellets and the Sr results for the foraminifera. For the samples above the glauconitic zone, 613C values show a gradual increase of about 0.6%o reaching highest values in core 22, followed by a rapid decrease in 613C values at the base of core 20. This could correlate with MC 1 ( - 5 Ma) although the sparse sample coverage in cores 21-24 make such a correlation tentative. Most of the palaeontological data imply that a Pliocene ( < 5.3 Ma) age is reached before core 20.
6. Origin of secondary gypsum Three samples of the euhedrai secondary gypsum crystals were analysed for Sr isotopes (Table 3; Fig. 4). Samples 25-1, 87-92 cm and 25-5, 81-84 cm, from just above and below foraminifera sample 25-2, 47-50 cm, yielded 8 7 8 r / 86Sr ratios identical to that of the foraminifera (Table 3). Gypsum sample 32-2, 20-25 cm, from some 100 m deeper in the sequence, also yielded a similar value, significantly higher than the expected 8 7 S r / 8 6 S r ratio of seawater at the time of deposition of the sediments at that depth ( <
377
0.7086, see Fig. 4). The euhedral gypsum apparently has the s a m e 87Sr/86Sr ratio throughout the studied section, similar to that of seawater at - 6 Ma. The 87Sr/86 Sr of the gypsum reflects that of the pore water from which it precipitated. As the original 875r/86Sr r a t i o of pore water in the sediments from core 32-2 would have been that of contemporary seawater ( < 0.7086), the composition of the pore water must have become more radiogenic by the time of gypsum formation. We suggest that this relates of a period of deep penetration of seawater into the sediments at - 6 Ma, i.e. contemporaneously with the deposition of part of the glauconitic horizon. This correlates with the period of very slow deposition and, interestingly, occurred during the Messinian, a period known to have had significant rapid changes in sea level which could provide a mechanism for seawater penetration.
of the Norwegian Sea of up to - 5 ° C a n d / o r change in seawater composition, as documented by the oxygen isotope data, can now be dated to have occurred in the late Miocene. This can be interpreted as either parallelling a global trend of falling T [1], or being the result of a purely oceanographic process, such as the sinking of the Scotland-Greenland Ridge to a sufficient depth to allow cold bottom waters into the area. The possible rise in seawater 6180 could be related to polar ice-cap growth at this time. The present geochronological data illustrate the usefulness of combined radiogenic and stable isotope methods in sediment dating in situations where palaeontological criteria are lacking or problematic. With the aid of the isotopic data, a temporally more accurate zonation scheme can be constructed for the Voring Plateau, which will be described in a separate paper.
Acknowledgements 7. Discussion The combined C, O and Sr data set allow a detailed geochronological scheme to be constructed for DSDP site 341 (Fig. 4). From these data the best placing for the early/middle Miocene boundary would be in core 28, approximately at the boundary between lithological subunits 3C and 3B. Subunit 3B is middle Miocene in age, the middle/late Miocene boundary occurring in the lower part of core 25 in subunit 3A, which probably contains all of the late Miocene. The Miocene/Pliocene boundary most likely occurs at the top of subunit 3A (core 25) or not very far above it. Consideration of present thickness, without making allowances for compaction, water saturation, etc., shows that during the early and middle Miocene the minimum mean rate of deposition was relatively high, about 15 m / M a . The rate ~'ecreased greatly in the late Miocene (subunit 3A) tt, about 20 c m / M a . It is likely that deposition of the glauconitic pellet-bearing subunit 3A was interrupted by periods of non-deposition. With the change in type of sedimentation from open pelagic marine to glacial marine in unit 2, the mean rate of deposition increased dramatically to - 4 5 m/Ma. The possible drop in bottom water temperature
We thank the NAVF-supported National Laboratories for mass spectrometry at the Department of Geology, University of Bergen, and the Geological Museum, Oslo, for the provision of analytical facilities. P.C.S. acknowledges receipt of a Postdoctoral Fellowship from the Royal Norwegian Council for Scientific and Industrial Research. Strontium work at the Institute for Energy Technology was supported by Conoco Norway. Fina Exploration Norway kindly provided draughting facilities. We thank O.P. Hansen, C. Needham, G. Qvale, J. Thiede and the Journal's anonymous reviewers for their constructive comments. E.-B. Jorgensen typed the manuscript.
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