Table of contents Abstract 1 Introduction 2 Methods • Site description 3 • Age model 4 • Analytical methods
Stable isotope analysis S Trace metal analysis 6 Paleotempcratures 7 Seawater Mg/Ca ratios and carbonate saturation state 8
Rcsults
• Benthic foraminifera accumulation rates (BFAR) 11 • Carbonate mass accumulation rates (CaCOj M A R ) 11
• Interspecies caliliration 13 • Stable isotope and trace metal record
Cleaning efficiency 13 Mg/Ca species calibration 15
6 " C 15 Mg/Ca ratios and 5 " 0 17
Seawater Mg/Ca and Sr/Ca ratios ^ 19 Li/Ca and the carbonate saturation state (ACOj"") 20
Mg/Ca based paleotempcratures 21
8w and ice volume 22 Discussion . BFAR's 24 • Species calibration 24 • Stable isotopes 25 • Postdepositional alteration 26 • Trace metals
Seawater Mg/Ca ratios 27 Mg/Ca based paleotempcratures 28 Li/Ca record and influence on Mg/Ca ratios 29
Sw, ice volume and global sea level during the Neogene 31
Conclusions 33 Future work 34 Acknowledgements 35 References 35 Appendix A : Comparison of Site 1264 with composite records for
stable isotopes
Appendix B: Stereoscopic and (SEM) pictures of benthic foraminifera Appendix A : Comparison of Site 1264 with different records for
Mg/Ca ratios
A combmed study of stable isotope analysis and Mg/Ca ratios
in benthic foraminifera in order to obtain bottom water
temperatures and global ice volume for the last 30 Ma
B.D.A. (David) Naafs
Faculty o f Geosciences, department of Earth Sciences Utrecht University
Utrecht, The Netherlands
4
I n t r o d u c t i o n
Many different proxies Iiave been used to reconstruct tfie timing and magnitude of the different climate states of the late Cenozoic. The most widely used proxy is 5 ' ° 0 of deep-sea benthic foraminifera e.g. (Sbackleton and Kennett 1975; Miller et al. 1987; Zachos et al. 2001). The strength of this method is that the deep ocean is isolated and hence, is much less subjected to regional effects such as seasonal temperature changes. Changes in the deep ocean therefore can reflect gradual changes in the global climate to a greater extent than the surface ocean or terrestrial records, which are subject to large short t e r m variability (Lear et al. 2000b; Zachos et al. 2001). However, 5^^0 of benthic foraminifera is amongst others mainly function of temperature and the 5^°0 composition of seawater ( 5 w ) , of which the latter for the largest part depends on changes in global ice volume. These two cannot be separated without the use of an independent second proxy. This research fits in the recent trend of using benthic foraminiferal trace element and stable oxygen isotope records simultaneously to separate deep-ocean temperature and continental ice volume during the Cenozoic e.g. (Lear et al. 2000b; Billups and Schrag 2002; Martin et al. 2002; Barker et al. 2003; Billups and Schrag 2003; Lear et a l . 2004; Dutton et al. 2005). However, almost all of these studies did not involve the whole Neogene or are on a very coarse resolution. Secondly, with the increasing number of papers on Mg/Ca, more complications arise in the use of Mg/Ca as a paleothermometry for benthic foraminifera and species calibrations are continuously being modified [e.g. (Elderfield et al. 2006; Marchitto et al. 2 0 0 7 ) ] . I n this study I used different approaches to avoid the problems concerning inconsistencies in calibrations between calcite Mg/Ca ratios and temperature, problems due to the influence of the carbonate ion concentration of the surrounding seawater [e.g. (Elderfield et al. 2006)] and problems due to changing Mg/Ca ratios in the surrounding seawater [e.g. (Billups and Schrag 2 0 0 3 ) ] . This will involve the use of new species calibration f r o m (Elderfield et al. 2006), Mg/Ca f r o m echinoid spines (Ries 2004) and foraminiferal Sr/Ca ratios (Lear et al. 2004) as a proxy f o r sea water Mg/Ca ratios and Li/Ca ratios f r o m multiple species with different habitats to monitor the influence of the carbonate ion effect (Lear and Rosenthal 2006). Secondly, I will use over 200 samples with an average resolution of 150 kyr and multiple benthic foraminifera species, leading to a high resolution Mg/Ca record over the whole Neogene. The 5w record will be compared to the global sea level curve (Miller et al. 2005). Separating deep-ocean temperature and continental ice volume during the Neogene will help to understand the transition f r o m the late Paleogene with only small (Southern hemisphere) ice caps to the present world with large ice caps on both poles. This will also give more insight to which extend the 5'^0 record is biased by changes in the 5w, which will greatly improve the interpretation of f u t u r e and existing 5'^0 records. 5'^C, BFAR, CaCOs MAR and trace metal records will be used to monitor the influence of global events (e.g. late Miocene carbon shift) and local effects (e.g. silicate contamination, etc.) on the 5'^0 and Mg/Ca record
3
A b s t r a c t
In this research a low-resolution ( ~ 150,000 years) integrated study of stable oxygen isotopes (5*^0) and Mg/Ca ratios in different benthic foraminifera species was conducted. Samples f r o m the Walvis Ridge, in the south-eastern Atlantic Ocean (Site 1264A, ODP Leg 208) spanning the last 30 Myr, were used. The aim of this study was to combine the independent temperature proxy of Mg/Ca [e.g. (Elderfield et al. 2006)] with 5^^0 analyses to separate deep-ocean temperature and continental ice volume.
• Age model
During the cruise an age model was constructed based on the LO and FO of planktonic foraminifera and nannofossils and magnetostratigraphic boundaries. All ages are given according to the latest time scale by (Lourens et al. 2004). However, the preliminary age model constructed during the cruise contained some uncertainties. For example, in the early Miocene the preliminary age model indicates low sedimentation rates leading to a highly compressed interval. For the oldest part of the record (ages older then 16 Ma) I therefore used a recently revised age model, based on detailed correlation of the magneto susceptibility (MS) of Site 1264 with Site 1265 (Liebrand personal communication). Site 1265 contains good magneto stratigraphy, leading to a better age control. By using this correlation, the compressed interval in the early Miocene disappears. A linear sedimentation rate was assumed between each selected age control point to assign an age to all samples.
Ago - depth modol for silo 1264
Figure 2: Age-depth model for Site 1264. This age-depth model was constructed 1) with the help of ages assigned to the stratigraphic position of selected planktonic foraminifera, nanofossils and magneto-stratigraphic boundaries during the cruise for ages younger then 16 Ma and 2) for the older part of the record a comparison with the dated magneto susceptibility record of Site 1265 and the magneto susceptibility of Site 1264 was made to obtain a better age-depth model. Black squares indicate the three core gaps for Site 1264A
I I I I I I I I I I I I I I I ' I I I I I ' I < I I { I I I I
0 2 t a 9 to 12 10 ta 20 K M 20 ïa 30
^. (Mo)
Methods
• Site and core description
I n 2003, a complete carbonate-rich succession f o r the last 30 Myr was recovered f r o m the Walvis Ridge in the Southern Atlantic Ocean by the drilling of t w o holes at Site 1264 (Latitude: 2 8 ° 3 1 . 9 5 ' S , Longitude: 2050.73'E, Water depth: 2505m) during ODP Leg 208. The Walvis Ridge is a northeast-southwest-trending ridge that divides the eastern South Atlantic Ocean into two basins; the Angola Basin to the north and the Cape Basin t o the south (see figure 1 ) . The topography o f t h e ridge blocks Antarctic bottom water (AABW) f r o m flowing in the Angola basin directly. The Angola basin is filled to the bottom with North Atlantic deep water (NADW). I n the Cape Basin, the NADW flows on top of AABW which fills the basin below 4000 m . Site 1264 is located at the middle of the ridge at a current depth of 2505 m and is bathing in NADW [ e . g . (Moore et al. 1984; Schmiedl et al. 1997) and references t h e r e i n ] . Paleodepths f o r Site 1264 are poorly constrained; all but except the oldest part of the record is thought to be upper abyssal (Zachos et a l . 2004). Due to its intermediate depth. Site 1264 was situated well above the CCD during the upper Oligocene to Pleistocene [ e . g . (van Andel et al. 1 9 7 7 ) ] . The location in an oligotrophic part of the South Atlantic ocean (Schmiedl et al. 1997) makes sediments f r o m Site 1264 to contain almost no organic matter (values range between 0.0 and 0.3 w t % ) and mainly consist of a nannofossil- and foraminifera-bearing nannofossil ooze. Foraminifers are abundant and reworking of foraminiferal specimens is rare and mostly happens in the Oligocene part of the section f o r ages older then 25 Ma (Zachos et al. 2004). A composite record was constructed f r o m two holes, which resulted in a continuous succession f o r the entire Neogene and upper Oligocene. This research uses samples f r o m the A hole only ( f u r t h e r designated as Site 1264A). Do to the use of samples f r o m the A hole only, several gaps in the Oligocene part of the record exist. These gaps are located between ~23.5-24.6; 26.4-27.7; and 28.3-28.5 Ma. Secondly, the middle Miocene interval is condensed relative to other parts of the record. Total
8
spectrometer. Samples were individually reacted witii phosphoric acid at 74 °C under a pressure of ~ 50 pbar Calibration to the international standards (and in-house standards) NBS-18, NBS-19, NBS-20, BYM and B l revealed an analytical precession better than 0.03 %» and 0.04 %» f o r 5 " C and 5'^0 respectively. All isotope data are reported relative to VPDB after calibration with the National Institute of Standards and Technology carbonate isotope standard NBS-19. A total of 527 samples were analyzed f o r their isotope signature. For samples in which multiple species were analyzed, the average of all of the species after correction with the newly obtained correction factors was used to produce a continuous stable isotope record over the last 30 My.
o Trace metal analysis
For the trace metal analysis only samples with sufficient ( > 100 pg) material left of either of the two Cibiddoides species were used, meaning t h a t only samples containing multiple specimens were analyzed. An additional number of 30 samples containing the species O. umbonatus were analyzed to see whether discrepancies between the two records exist. Samples selected f o r trace metals were f u r t h e r cleaned in a clean bench using the procedure described by (Barker et a l . 2003). This procedure involves several cleaning steps to remove clay, organic matter, coarse-grained silicates and contaminants. The silicate removing step involved transferring each sample to new samples tubes only. Dissolution of the samples was carried out shortly before analysis in 5 ml 0.1 M ultra-pure HNO3 to avoid Mg contamination f r o m the sample tubes during storage (Shen et al. 2007). Trace metal ratios were measured at Utrecht University using a Thermo Electron X-series I I inductively coupled plasma mass spectrometer
(ICP-MS). To overcome any significant matrix effect of Ca on the Mg/Ca determination, all samples were first diluted to similar Ca concentrations of 4 ppm (Shen et al. 2 0 0 7 ) . To obtain the correct dilution factor f o r each sample, all samples were shortly measured on the ICP-MS f o r their Ca concentration only. Samples were then diluted gravimetrically to 4 ppm Ca with 0.1 M ultra-pure HNO3. Every 25 samples, a run of 5 standards was measured with changing element ratios. Two extra standards with higher Mg/Ca ratios were measured only during the analysis of the echinoid spines, which have Mg/Ca ratios up to 100 m m o l / m o l (Ries 2004; Ries 2 0 0 6 ) .
The nuclides of ^'^Mg and ""^Ca were selected to determine Mg/Ca ratios. ^^Mg was selected because ^''Mg has interference of "^^Ca^*, although the formation of double charged ions was continuously monitored and had to be below 2 % . ''^Ca was selected because of the low abundance of this nuclide, leading to an abundance of the same order of magnitude as magnesium. This means t h a t both nuclides could be measured by pulse counting, which has the advantage t h a t the error t h a t occurs due to a switch in detection mode f r o m pulse to analog counting disappears (Rosenthal et al. 1999). Secondly, intensive testing prior to analysis has shown t h a t the lowest standard deviation in the count ratios was achieved with these nuclides. As a last advantage, with measuring "^Ca instead of '*^Ca which is usually done, the problem with interference of Sr/Ca on Mg/Ca ratios is eliminated. On the
7
• Analytical methods
o Stable isotope analysis
From Site 1264A approximately 200 samples were taken over the entire ~310 meter composite depth with a resolution of 1.5m. Each sample represents approximately 1 kyr. The samples span a time interval f r o m the Pleistocene at ~ 0.6 Ma to the early Oligocene at ~ 30 Ma. Bulk sediment samples were oven dried, weighed and washed through three different sieves (38, 63 and 150 p m ) . Weight percent of the larger then 63 \jm fraction was calculated by weighing the samples a f t e r sieving. Carbonate mass accumulation rates f o r the larger than 63 pm fraction (CaCOs MAR > 63 pm) were calculated by multiplying the CaCOa MAR data f r o m (Zachos et al. 2004), which was corrected to the new age model, with the weight percent of the larger then 63 pm fraction. Due to the high relative abundance of planktonic foraminifera in the larger then 63 pm fraction of Site 1264A, CaCOs MAR > 63 p m mainly reflects surface productivity.
A commonly used species f o r combined stable isotope and trace metal ratios studies is Cibiddoides wuellerstorfi (Lear et al. 2000b; Billups and Schrag 2002; Lear et al. 2002; Billups and Schrag 2003). However this species cannot be used to obtain a stable isotope and trace metal record f o r the whole section, because this species has its first occurrence at Site 1264A around 14 Ma and is not present in all samples of younger age. For analysis of the oldest part of the section, I therefore used the species Cibiddoides mundulus, which is present in almost all samples. I n addition to the two
Cibiddoides species, I also analyzed the benthic foraminiferal species
Oridorsalis umbonatus and Globocassidulina subglobosa f o r their stable isotope signature. Both O. umbonatus and G. subglobosa can be found throughout the core, although their presence is not continuous. Between 3 and 114 benthic foraminifera belonging to the benthic foraminifera species
Cibiddoides wuellerstorfi, Cibiddoides mundulus, Oridorsalis umbonatus and
Globocassidulina subglobosa were hand picked f r o m the > 63 pm fraction. Benthic foraminiferal accumulation rates (BFAR), which are a proxy f o r paleo export productivity [e.g. (Van der Zwaan et al. 1999; Herguera 2000; Diester-Haass et al. 2 0 0 6 ) ] , were calculated using the following equation:
BFAR=[ " ' ' " h ^ o f Picl^^dfor^^ ilcaCO, MAR > 63 um fraction (g/an'/k.y.) (1)
LweightpickedsedimentCg)] j , k j
• Seawater Mg/Ca ratios and carbonate saturation state Besides the choice of calibration, |vig/Ca based temperatures are heavily influenced by the assumed ratio between past and modern seawater Mg/Ca ratios [ e . g . (Billups and Schrag 2 0 0 3 ) ] . Modern ratios are around 5.2 m o l / m o l (a factor 1000 higher then the ratios seen in benthic foraminifera), but at the end of the Paleogene are thought to be as low as 2.5 mol/mol [an overview of different studies is given in (Dickson 2 0 0 2 ) ] . My record involves samples f r o m the whole period in which the Mg/Ca ratios o f seawater are thought to increase f r o m 2.5 to 5.2 mol/mol and hence, precise knowledge of past seawater is necessary. However, the data used to reconstruct past seawater Mg/Ca ratios is rather limited. I n this study I tried to use echinoid spines to obtain past seawater Mg/Ca ratios. (Ries 2004) published a calibration relating seawater Mg/Ca ratios with temperature and the Mg/Ca in echinoid spines:
[Mg/Ca]sp,„„ =8(0.000837 x T + 0.0155)[Mg/Ca]™ (3) Where S is the species normalization factor. S can be calculated by solving
equation 3 f o r modern representatives of the fossil echinoid species, when the temperature and Mg/Ca ratio of the seawater f r o m which the specimen was taken are known. When equation 2 and 3 are combined, equation 4 can be solved f o r samples in which both echinoid spines and benthic foraminifera are measured f o r their Mg/Ca content.
u, , ^ 1 /0.00837
l n ( [ M g / c 4 J - l / ' ^ ^ ^ f ; ^ ° l - ' | H - 0 . 0 1 S 5 \ . ^ . [ M g / C . ] r W
To obtain a continuous record, echinoid spines were picked approximately every 1 My and cleaned using the same procedure as applied to the foraminifera (Barker et a l . 2003). Secondly, (Lear et al. 2004) argue t h a t " i f variations in seawater Mg/Ca are solely driven by changes in calcium concentration they should be similar to variations in seawater Sr/Ca". I n (Lear et a l . 2003) they give a partition coefficient relating benthic foraminiferal Sr/Ca and seawater Sr/Ca f o r different species. I measured benthic foraminiferal Sr/Ca ratios to see whether they fitted previously published Sr/Ca records f o r seawater and compared it with the newly obtained seawater Mg/Ca ratios f r o m echinoid spines to see if any correlation was f o u n d .
Another possible complication in the Mg/Ca paleothermometry method is the f a c t that Mg/Ca ratios seem to be sensitive to the seawater calcite saturation state (A[C03^"]) the so called: "carbonate ion effect" (Russell et al. 2004; Elderfield et al. 2006; Marchitto et a l . 2 0 0 7 ) . The idea behind the carbonate ion effect is that benthic foraminiferal calcite is a function of both temperature and carbonate saturation state, the latter in the modern ocean becoming important at lower temperatures (Elderfield et al. 2 0 0 6 ) . I tested whether the carbonate ion effect affected Mg/Ca ratios in this study by comparing the epifaunal Cibiddoides species (Lutze and Thiel 1989) with the
other hand, ''^Ti could cause interference f o r the "^Ca measurements. ""^Ti abundance was monitored to asses any interference of Ti on ""^Ca. ''^Ti/''^Ca ratios always were below 0.4 % , indicating negligible interference of Ti on '^Ca. The nuclides of " A l , ^^Mn and " F e were used to monitor cleaning efficacy. As a threshold f o r diagenetic overgrowths, Mn/Ca ratios of more then 100 pmol/mol were taken (Boyle 1983). Silicate contamination is identified by covariance between Al/Ca and Mg/Ca ratios and Fe/Mg ratios of more then 100 m m o l / m o l (Barker et al. 2 0 0 3 ) . Each measurement was repeated 8 times to obtain a higher precision. This required between 500 and 750 pl and took approximately 4.5 m i n . The standards with known trace metal ratios were used to obtain a linear relation between the counts ratio o f the different trace metal nuclides and their concentrations in m m o l / m o l . Analytical precision was calculated f r o m the relative standard deviation o f t h e repeatedly measured standards and f o r Mg/Ca was in the order of 0.35 % . Replicate analyses on basis of 6 sample splits f o r Mg/Ca lie within ~ 6 % .
• Paleotemperatures
To convert Mg/Ca ratios f r o m foraminiferal calcite to paleotemperature, different calibrations have been used. Most studies have assumed and exponential relation o f t h e f o r m :
[Mg/Ca] ^ , =
[M^cajr'
'"''^ '''^
Where the subscripts refer to seawater today (sw-0) and at time t {sw-t) and the constants m i and m^ differ between benthic foraminifera species and calibration (Rosenthal et al. 1997; Billups and Schrag 2002; Lear et a l . 2002; Martin et al. 2002; Rathmann et al. 2004; Elderfield et a l . 2006). From the two constants, m j determines the absolute temperature, while m2
determines the magnitude of the temperature change. Although the exponential f o r m is used in most studies and is predicted by thermodynamics, lately it has been questioned f o r Cibiddoides species (Marchitto et al. 2007). Different authors have also noticed that the data at low temperatures deviates f r o m the exponential f o r m . This lead several authors to correct the exponential f o r m into a steeper linear relation, resulting in colder bottom waters [e.g. (Billups and Schrag 2 0 0 2 ) . However, (Elderfield et al. 2006) state that " A t low temperatures it would be incorrect to use calibrations based on the steep slopes of empirical relationships between benthic Mg/Ca and water temperature because these slopes contain a significant Mg/Ca component associated with carbonate ion saturation state". They reconfirm the widely used exponential calibration f o r different
Cibiddoides species with m i and m ; of 0.9 and 0 . 1 , respectively and I will use this calibration to convert Mg/Ca ratios to paleotemperature. Since C.
mundulus is abundant through the whole core, all data f o r C. wuellerstorfi
c J : (0 <u 0 ) «1 (0 P ,t; i-, x: o
£ 5 >. 2
4Jc
" <U T J Q . O V.^ 1 S g
1
8 c 4-. 15 cJ 52 5 - Q _.-aB
V ~ to ^ c r ; ci- c ^ o ra cu CO .ro
S iS ~ s 1 ^ U _. O "^m .E ro .5 (U • -0 ) O E Bro
12 # Cibiddoides wuellerstorfi 0 10 20 30 40 50 # Cibiddoides mundulus 0 10 20 30 40 50 # Oridorsalis umbonatus 0 10 20 30 40 50# Globocassidulina subglobosa g/cmVk.y.
0 10 20 30 40 50 0,4 0,8 M M I 0 10 20 # C, wuellerstorfllcm'Iky M M M 0 10 20 # C, munduluslcrrftk.y. M M I M M M 0 10 20 0 10 20 # O, umbonatuslcmVk.y. # G. subglobosalcmyk.y.
della I S c delta ISq
Figure S: Paired Isotope analyses of Cibiddoides species and Oridorsalis umbonatus. The calibration line was calculated on basis of the data of all studies. The 95% confidence interval was calculated from the standard deviation of the difference between the raw data of the two species.
Figure 6: Paired isotope analyses of Cibiddoides species and Globocassidulina
subglobosa. The calibration line was calculated on basis of the data of all studies. The
95% confidence interval was calculated from the standard deviation of the difference between the raw data o f t h e two species.
R e s u l t s
• Benthic foraminifera accumulation rates (BFAR)
In almost all samples, all f o u r benthic foraminifera species were f o u n d .
Cibiddoides wuellerstorfi is only found in samples younger then ~ 14 Ma. The relative abundance, as well as the absolute number of G. subglobosa,
decreases with increasing depth. Because samples were picked until approximately 10 specimens of one of the Cibiddoides species were f o u n d , the absolute number of specimens picked remains more or less constant throughout the section. BFAR were calculated f o r the f o u r different species. Across the record BFAR's are low and range between 0 and 5 forams/cmVk.y. Only during a few short episodes BFAR reached values above 5 forams/cmVk.y. The highest BFARs are found in the early Pliocene when BFARs peak with values as high as 27 forams/cmVk.y.
• Carbonate mass accumulation rates (CaCOs MAR)
During the Oligocene and most of the early Miocene the CaCOa MAR > 63 pm are below 0.1 g/cmVk.y. At the end of the middle Miocene CaCOj MAR increase towards 0.5 g/cmVk.y., where they remain f o r a few million years. CaCOs MAR reach the highest value of 0.9 g/cmVk.y. during the early Pliocene, after which they return to values between 0.2 and 0.4 g/cmVk.y.
16
Figure 7: iv|g/Ca and Al/Ca ratios plotted against each other for samples containing the three benthic foraminifera species C.
wuellerstorfi, C. mundulus and O. umbonatus. The lack of a ciear
relation between Mg/Ca and Al/Ca supports my statement that silicate contamination on Mg/Ca ratios is only minor for Site 1264A. Error bars indicate uncertainty that arises from the calibration between count ratios and concentrations. Data for echinoid spines is not shown due to their high Mg/Ca content, but Al/Ca for echinoid spines never exceed values of 1 mmol/mol.
I n total, 262 samples were analyzed f o r their trace metal ratios, of which 29 samples were excluded f r o m the Mg/Ca record on the basis of: a too high standard deviation (12 samples), too high Mn/Ca ratios (14 samples) or strange Mg/Ca values t h a t were considered as outliers (3 samples). To correct f o r any d r i f t in the measurements, the linear relation used to convert count ratios to concentrations was based f o r each series of 25 samples on the 5 standards at the start and the 5 at the end of t h a t series. Coefficients of determination ( r ' ) f o r these calibrations were always above 0.97 and typical around 0.99 f o r all elements.
Figure 8: Comparison of the Mg/Ca ratio of the two benthic foraminifera species Q'bicldoides mundulus and
Cibiddoides wuellerstorfi. The red
line represents the average dtfference of 0.157 mmol/mol between the two species. The distribution plot shows the difference in Mg/Ca. In blue data from (Billups and Schrag 2003) is given for comparison. The data from (Billups and Schrag 2003) fits with my correction factor.
15
• Interspecies calibration
In order to be able to compare the isotope data of the different species (which all have their own offset f r o m calcite precipitated in equilibrium with seawater), I obtained new species correction factors. The correction factors used in this study were based on the data of all studies as plotted in the figures 4-6. I assumed t h a t the widely sited species offset f o r the Cibiddoides
species of 0 and 0.64 % o f r o m equilibrium f o r 5 " C and 5^^0, respectively, is correct [ e . g . (Graham et al. 1 9 8 1 ; Sbackleton and Hall 1 9 8 4 ) ] . Hence, isotope data f r o m O. umbonatus and G. subglobosa was f i r s t corrected to
Cibiddoides species with the newly obtained interspecies correction factors and then corrected to equilibrium with 0 and 0.64 % o f o r 5'^C and respectively. Correcting the two other species to Cibiddoides species allows me to compare the record with most other records, even if the offset between the Cibiddoides species and seawater appears to be incorrect. To obtain the new species correction factors, a linear trend line correlating the raw isotope data of the two species with the Cibiddoides species was calculated. For the rest of this study, all isotope data of the species
Oridorsalis umbonatus and Globocassidulina subglobosa is corrected to
Cibiddoides species values using theses equations.
Species Correction equation St. dev. 95%
confidence interval Oridorsalis umbonatus y = 0.7790X + 0.9764 0.803 0.34 %o ± 0.68 %o Oridorsalis umbonatus 5 " 0 y = 0.889SX - 0.1297 0.955 0.24 %o ± 0.48 %o Globocassidulina subglobosa y = 0.7436X + 0.5000 0.829 0.24 %o ± 0.47 %t> Globocassidulina subglobosa a"o y = 0.7473X + 0.1900 0.784 0.25 %o ± 0.49 %o Table 1 : Correction equations for O. umbonatus and G. subglobosa to Cibiddoides species, which then were corrected to equilibrium with 0 and 0.64 %o for 5 " C and 5^^0, respectively.
• Trace metal record o Cleaning efficiency
Mg/Ca ratios
O 2 4 6 8 10 12 1 4 1 6 1 8 20 22 24 26 28 30
O 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Ago (Ma)
Figure 10: A ) Stable oxygen isotope record of Site 1264A. Both the record of only the two
Cibiddoides species wueileistorfi and mundulus (in yellow) as the record from all four species Cin
blue) is shown. For samples in which multiple species were analyzed the average between the species was used. A 0.5 Myr moving average was calculated form the record of all four species (in red). Ail data is corrected for genus-specific isotope vital effects. I n the 5'°0 record the identifiable Mi-events shown as dashed arrows. From young to old: Mi7, MIS, Mi3, Mi3a, Mi2a, Mi2 and M i l . Ages of Mi7 (8.7 Ma), MiS (11.7 Ma), and Mi3 (13.8 Ma) are from (Westerhold et al. 2005). The age for M i l (22.95 Ma) is from (Paul et al. 2000; Lear et al. 2004). Ages for Mi3a (14.2 Ma), Mi2a (14.8 Ma) and Mi2 (16.1 Ma) are from (Miller et al. 1998). B) Mg/Ca ratios of Cibiddoides species
wuellerstorfi and mundulus and O. umbonatus. Data for C. wuellerstorfi was corrected to C. mundulus according to the correction factor given in figure 8. Error bars indicate uncertainty that
arises from the calibration between count ratios and concentrations.
• Mg/Ca species calibration
For a total of 17 samples, both the species C wuellerstorfi and C. mundulus
were measured f o r their Mg/Ca ratios (see figure 8 ) . On average C. mundulus contains higher Mg/Ca ratios by 0.157 m m o l / m o l . All C.
wuellerstorfi Mg/Ca data was corrected to C. mundulus with this correction factor of 0.157 mmol/mol to obtain a more continuous record. In samples where both species were measured, only the data f r o m the species C.
mundulus was used.
0 2 4 6 8 10 12 14 15 18 20 22 24 26 28 30 Age (Ma)
Figure 9: Stable carbon isotope record of Site 1264A. Both the record of only the two
Cibiddoides species wuellerstoiil and mundulus (in yellow) as the record from all four species
(in blue) is shown. For samples in which multiple species were analyzed the average between the species was used. A 0.5 Myr moving average was calculated form the record of all four species (in red). All data is corrected to equilibrium.
. 5 " C
20
B) Paleotemperature
- ClbtödDides spades - 1 Myr mt)\rtno average C/Wddo/tJes
- 0. umbonatus
- 1 Myr moving average 0. dmfiaintus
Pliocene I I 1 10 14 16 18 20 22 V;' piigpcehe,' 26 28 M M I 12 14 16 18 Age (Ma)
Figure 1 1 : A ) Mg/Ca ratios of Cibiddoides species wuellerstorfi and mundulus and O. umbonatus. B) Mg/Ca based temperature for the three benthic foraminifera species assuming moderns seawater Mg/Ca ratios. Cibiddoides temperatures were calculated using equation 5. For O.
umbonatus the calibration from (Lear et al. 2000a) was used.
19
• Mg/Ca ratios and 5'^0
5^*0 and Mg/Ca ratios f o r Site 1264 show an increase in the early Oligocene. A dip in the Mg/Ca ratios and increase in 5'^0 occur around 25.5 Ma, followed by a quick increase during which Mg/Ca ratios reach their peak before the minimum in the S'-^O record. Beginning around 24.5 and continuing across the O/M boundary, Mg/Ca ratios decrease and 5^°0 increase. Mg/Ca ratios continue to decrease to a distinct minimum around 20 Ma, while 5^^0 ratios fluctuate between 2.5 and 3 % o . The gradual trend of decreasing Mg/Ca ratios is interrupted by a minimum at 22.9 Ma during M i l and two distinct maxima at 20.3 and 20.8 Ma with the highest Mg/Ca ratios of the whole record. The M i l is a glaciation event of which during the Miocene and Oligocene multiple took place (Miller et al. 1987). I n my 5 ' ° 0 record, probably due to the coarse resolution, none of the Oi-events can be identified. 7 Mi-events can be identified (see figure 10). I n the Mg/Ca record only the M i l and Mi3 glaciation events are clearly be identified. From 19.4 onwards the 5^^0 record decreases, while Mg/Ca ratios increase to a maximum around 15 Ma during the early Miocene climatic o p t i m u m (EMCO). Following the EMCO, Mg/Ca ratios rapidly decrease and 5'^0 increase during the middle Miocene climatic transition (MMCT). The remainder o f t h e Miocene is characterized by fluctuating Mg/Ca ratios with a repetitive pattern of increasing and decreasing Mg/Ca ratios with a period of approximately 2.5 Myr. The 5^^0 record on the other hand continues to increase into the Pliocene. With the beginning o f the Pliocene, Cibiddoides Mg/Ca ratios start t o increase again. 5 " 0 values remain constant until 4 Ma and afterwards increase until the end of the record. Cibiddoides Mg/Ca ratios continue to increase to a maximum around 3 Ma. From 3 Ma onwards, Cibiddoides
Mg/Ca ratios decrease until the end of the record at 0.6 Ma. Overall Mg/Ca ratios f r o m O. umbonatus show a decrease over the whole period that they were measured (0-5 Ma). A sort maximum in Mg/Ca ratios f r o m O.
Sr/Ca ratio's in bentliic foraminifera can also be used to calculate past seawater Mg/Ca (Lear et al. 2 0 0 4 ) . Benthic foraminiferal Sr/Ca ratios were corrected to C. wuellerstorfi according to the interspecies offset given by (Lear et al. 2 0 0 3 ) . Sr/Ca ratios were then converted to seawater Sr/Ca ratios by using the constant partition coefficient of 0.165 between benthic foraminiferal and seawater Sr/Ca ratios.
Sr/Ca ratio of seawater
@ C. miDtloi^oifi • Cmmdulus ^ 0. timtanolus
I
1.4 • IT 1.3 •I
I ' I I ' I I i I I I 16 20 2 5 Ago (Ma) I M I I I { I I I I I I I 1 15 20 Ago (Ma)Figure 13; Sr/Ca ratios of all three different benthic foraminifera species and Sr/Ca ratios of seawater. All benthic foraminiferal Sr/Ca data was corrected to C.
wuellerstorfi according to
the interspecies offset given by (Lear et al. 2003). From this data seawater Sr/Ca ratios were calculated using a constant partition coefficient.
During the Oligocene, seawater Sr/Ca ratios show some large scatter, but appear to remain constant. During the eariy Miocene, seawater Sr/Ca ratios start to increase with a peak around 15 Ma. From 15 to 6 Ma seawater Sr/Ca ratios decrease to their lowest values. From 6 Ma onwards, seawater Sr/Ca ratios rapidly increase to the highest values o f t h e whole record.
• Li/Ca and the carbonate saturation state {ACO/')
Since benthic foraminiferal Mg/Ca ratios are influenced by high ACOs^" values (Elderfield et al. 2006), Li/Ca was measured to see under which AC03^' values the calcite was f o r m e d . However the standards used in my study exceed the Li/Ca concentrations in benthic foraminifera by an order of 10. By forcing the calibrations based on the standards with too high Li/Ca ratios through 0, Li/Ca ratios of 50-300 pmol/mol were f o u n d . Of course these ratios are much higher then Li/Ca ratios seen in other records (Lear and Rosenthal 2006). Therefore I assume t h a t the absolute values of Li/Ca are too high, however the general trend in my record can still be used. Li/Ca ratios of C. wuellerstorfi were corrected to C. mundulus by adding 0.03 m m o l / m o l , based on 5 samples in which both species were analyzed.
• Seawater Mg/Ca and Sr/Ca ratios
I n order to convert Mg/Ca ratios to seawater temperature, a correction had to be made f o r changing seawater Mg/Ca ratios. I n an effort to obtain seawater Mg/Ca ratios, echinoid spines were used. O f t h e total of 3 1 Echinoid spines t h a t were analyzed f o r their Mg/Ca content, 5 samples were rejected on the basis o f a too high standard deviation. The oldest spines all had Mn/Ca concentrations of more then 200 pmol/mol and also were rejected. The 17 remaining samples cover a period f r o m 2.6 - 28.4 Ma, which is the period in which the increase towards Mg/Ca ratios of 5.2 mol/mol is predicted (Dickson 2002). To obtain the species normalization factor (S) in equation 3, this equation had to be solved f o r a sample in which temperature and Mg/Ca seawater ratio was known. Because no modern representative o f t h e echinoid species was analyzed, I assumed t h a t the youngest sample (age o f 2.6 Ma) was f o r m e d under a modern Mg/Ca ratio of 5.2 mol/mol and that the temperature ( ~ 6.1 °C) obtained f r o m the Mg/Ca o f the benthic foraminifera species f o r this samples was not biased by a different Mg/Ca seawater ratio. By this way I could solve equation 3 f o r S, leading to a value of 0.84. Equation 4 was then solved f o r all 17 samples.
Mg/Ca ratio of seawater from echinoid spines
•B
002
-Figure 12: Mg/Ca ratios of the 17 samples containing Echinoid spines. 2 samples were rejected as being outliers. Errors due to a difference in calibration are negligible
" T T " T "
0 5 10 15 20 25 30 Age (Ma)
24
give temperatures of around 10.5 °C. During tfie EIMCO bottom temperatures increase by 2.5 °C to 9 "C, only to shortly drop again by 3 °C during the MMCT. For the remainder of the Miocene, bottom waters fluctuate around 7 "C. I n the Pliocene bottom water warm again by almost 1 "C. From 3 Ma onwards, bottom waters cool by 1.5 "C to values of 5.5 °C at the end of the record. The overall temperature decrease f o r m f r o m the Oligocene to the Pleistocene is some 3 °C. Temperatures f o r the last 5 Ma obtained f r o m O.
umbonatus are 1 °C warmer in the early Pliocene compared to Cibiddoides
spp., but cool more rapidly between 3.8 and 2 Ma to the same temperatures as obtained f r o m Cibiddoides species at the of the record.
• 5w and ice volume
Seawater 5'^0 (5w) was calculated by solving (Sbackleton 1974) his 5^^0 paleotemperature equation f o r samples in which both Mg/Ca ratios and stable isotopes were measured.
r = 1 6 . 9 - 4 - ( ^ ' ' 0 , - < 5 w ) (6) The obtained 5w record shows several episodes of continental ice sheet
g r o w t h ; the early Oligocene, the O/M boundary, the middle Miocene, the late Miocene and the Plio- and Pleistocene (see figure 15). During the Oligocene and early Miocene, 5w fluctuates around 0.3 % o . The MMCT marks the beginning o f m a j o r permanent glaciation with an increase in 5w of ~ 0.8 % o .
5w values do not return to values seen before the MMCT. The last episode of m a j o r glaciation began around 4 Ma with an increase in 5w of around 1 % o
towards the end of the record. The overall increase in 5w over the whole record ranges f r o m values of around 0 % o in the Oligocene to values as high as 2 % o in the Pleistocene. 23 U/Ca I I I I I I I I i I M ! I I I I I I M I I i I 1 ! i ! M I 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Age (Ma)
Figure 14; Li/Ca ratios of Cibiddoides wuellerstorfi and mundulus lor Site 1264A. Error bars indicate uncertainty that arises from the calibration between count ratios and concentrations. Li/Ca ratios of C. wuellerstorfi are corrected to C. mundulus based on 5 samples in which both species were analyzed.
At the beginning of the Li/Ca record, a short peak is f o u n d , followed by a minimum around 27 Ma. During the late Oligocene and early Miocene the Li/Ca record shows a trend of increasing Li/Ca ratios to a peak around 20 Ma with the highest Li/Ca ratios of the whole record. A t 15 Ma a minimum in Li/Ca ratios is f o u n d , after which Li/Ca ratios gradually increase until the Pliocene. A maximum in Li/Ca ratios is reached j u s t before 3 Ma. Beginning around 3 Ma and continuing into the late Pliocene and Pleistocene, Li/Ca ratios decrease again, although the youngest sample has one o f the highest Li/Ca ratios of the whole record.
• Mg/Ca based paleotemperatures
Substituting the values of m i and m2 f r o m (Elderfield et a l . 2006) into equation 2, and solving f o r T gives the following equation:
I
0-9-R )l (5), [Mg/Ca ] ^ _ , in which R =
[Mg/Ca L
D i s c u s s i o n • BFAR's
Tfie abundance of the f o u r different benthic foraminfera species at Site 1264A is in agreement with (Schmiedl et al. 1997), who find these species dominating present day assemblages at the Walvis ridge and resembles the oligothrophic character of the region. The FA of C. wuellerstorfi at Site 1264A is around 1 million year later compared to previously published biostratigraphic datum levels (Woodruff and Savin 1991), although this age was based on an older time scale which maybe the reason f o r the difference in t i m i n g . Since the abundance of C. wuellerstorfi is associated with streaming water conditions and highly oxygenated North Atlantic deep water (NADW) (Lutze and Thiel 1989; Woodruff and Savin 1989; Rathbum et a l . 1996), the FA at 14 Ma might indicate the initiation of northern component water (NCW) reaching Site 1264A. This is also found by (Kastanja and Henrich 2007) who argue that initiation of the young NCW (NADW) occurred around 14 at site 1265, which is located directly next to Site 1264A. Whether the FA of C. wuellerstorfi marks the onset of NCW reaching Site 1264A or not, it does follow the trend of changing benthic foraminiferal assemblages, which took place around 14 Ma, due to changing deep water circulation patterns (Kurihara and Kennett 1986) and bottom water cooling (this study) during the MMCT. The magnitude of the BFARs is lower compared to values seen in other records [ e . g . (Diester-Haass et al. 2 0 0 6 ) ] . However, my study only involved 4 selected benthic foraminifera species, which are usually used f o r combined stable isotope and trace metal records and not f o r determining and quantifying paleoproductivity. Hence, quantifying the paleoproductivity can not be done with this record. The peak in BFAR and CaCOj MAR in the Pliocene between 3 and 5 Ma is found in records all over the world and is named the biogenic bloom [ e . g . (Farrell et al. 1995; Dickens and Owen 1999; Diester-Haass et a l . 2002; Grant and Dickens 2002; Diester-Haass et al. 2005; Diester-Haass et al. 2 0 0 6 ) ] . At Site 1264A the biogenic bloom is separated f r o m the LMCS by more then 2 Myr.
• Species calibration
The general assumption that no fractionation exists between the two
Cibiddoides species f o r both 5 " C and 5*°0 is confirmed by my data, which give a small offset between C. wuellerstorfi and C. mundulus f r o m the theoretical y = x line. Secondly, the 9 5 % confidence interval of ± 0.28 % o
and ± 0.32 % o f o r 5'^0 and 5 " C , respectively, is of the same order as the difference between duplo measurements, again validating the assumption that no fractionation exists between the two Cibiddoides species. (Hodell et al. 2001) made the suggestion that C. mundulus may sometimes have a more infaunal habitat, leading to an offset between the two Cibiddoides
species at low 5 " C values ( < 0.75 % o ) . Although this study involved much less samples then the study of Hodell, I find a stable offset between the t w o
Cibiddoides species over the whole range of 5^^C values. For this study I therefore assumed that both Cibiddoides species record seawater 5^^C and 5^^0 identically. The newly obtained species calibrations between O. umbonatus and Globocassidulina subglobosa cover a wide range of values.
28
• Postdepositional alteration
The offset of 0.5 % o (equivalent to 2 °C) f o r Site 1264A compared to the composite record is not unique. Besides unpublished stable oxygen isotope records f o r Site 1264 (Liebrand, Hodell and Kroon, personal communication), several authors have indicated an offset in 5'^0 values f o r other (mostly intermediate) Sites [e.g. (Billups et al. 2002; Westerhold et al. 2005). Secondly, (Woodruff and Savin 1989) already showed that during the whole Miocene, S'^O values in the South Atlantic were higher compared to the North Atlantic at comparable depths. The reason f o r this offset is mostly explained as a difference in water masses. However, sites that are in the vicinity of Site 1264A, and should bathe In the same water mass, match with the composite curve. Salinity changes up to 1 % o are needed to achieve the offset of this magnitude in 5'^0, which is not realistic at depths were salinity changes are on the order of tens of promille. Due to its intermediate depth Site 1264A should record warmer temperatures instead of colder compared t o the deeper Sites in the composite record. Although the composite record of (Zachos et al. 2001) shows a shift in selected sites in the late Oligocene, this can not explain the offset with Site 1264 since sites f r o m different depths and ocean basin are continued to be used in the composite record. The mostly likely candidate is post depositional alteration because inorganic precipitated calcite has a lower S^'^O signal. Although the Shipboard data states that foraminifera preservation f o r Site 1264 is good (Zachos et al. 2004), detailed examination of all f o u r different species with a stereoscopic and scanning electron microscope (SEM) reveals the presence of micron-scale inorganic calcite overgrowths covering the surface and growing on the interior walls o f the foraminiferal test. Sites at intermediate depths are more vulnerable to diagenesis then deep ocean sites since shallower sites will contain more percentage metastable carbonate, which have a high diagenetic potential. One examined foraminfera {Cibiddoides wuellerstorfi) contained numerous nannofossils (see appendix B f o r stereoscopic and scanning electron microscope (SEM) pictures of all f o u r different species). SEM pictures were made on foraminifera that were not cleaned. The cleaning prior to isotope analysis will remove most of the nannofossils, but is unlikely to remove the inorganic calcite present on all examined foraminifera. (Sexton et al. 2006; Pearson et al. 2007) find that micron-scale inorganic calcite overgrowths influence the planktonic S'-^O records f r o m the Eocene significantly. Of course, the change f o r planktonic foraminifera is much larger then the influence of secondary calcite on benthic foraminifera. Even though, most likely a change of 0.5 % o in benthic foraminifera as found f o r Site 1264 is caused by the diagenetic alteration of calcite. Secondary calcite is also likely to influence Mg/Ca ratios since inorganic precipitated calcite has higher Mg/Ca ratios (Nurnberg et al. 1996), although the extend of bias has been questioned by (Sexton et al. 2006). Since the presence of inorganic calcite, as cleariy visible on the SEM pictures, causes both 5 " 0 and Mg/Ca ratios to increase, 5w will become unrealistically high f o r samples with inorganic calcite. 5w f o r Site 1264A therefore may be too high, as suggested by values up t o 2 % o in the Pleistocene. I therefore focused on the trends seen in the S'^^O, Mg/Ca and 5w record and not so much on absolute values.
27
with data f r o m the whole Neogene (and Paleogene f o r O. umbonatus),
different paleodepths and f r o m different ocean basins. Since the calibrations are valid f o r the whole range of values, the offset between species can be used throughout the Neogene (and Paleogene f o r O. umbonatus), regardless of depth, temperature or ocean basin. This is congruent with (Katz et al. 2003) who state that environmental parameters (temperature, 5*^0 water, and DIC 5 " C ) exerted the primary influence on these stable isotopic signatures through the Paleocene and Eocene. The newly obtained species correction equations differ f r o m previously published data [e.g. (Sbackleton and Hall 1984; Sbackleton et al. 1995; Sbackleton and Hall 1997; Katz et al. 2 0 0 3 ) ] , mainly because I used a linear correction equation with a slope different f r o m 1. Although several studies have indicated t h a t the 5 " C record of O. umbonatus can not be used in paleoceanographic reconstructions, because of its infaunal habitat [e.g. (Sbackleton and Hall 1 9 9 7 ) ] , I find a stable offset between O. umbonatus and the two Cibiddoides species, validating the use of the 5"C record of O. umbonatus in paleoceanographic reconstructions. The offset f o r Mg/Ca ratios between the two Cibiddoides
species agrees with previously published offset [e.g. (Billups and Schrag 2002; Elderfield et al. 2006)] and validates the correction factor I used to correct C. wuellerstorfi to C. mundulus.
• Stable isotopes
t h a t are confirmed by 5'^0 temperatures in an ice-free assumed world. (Lear et al. 2000a) even corrected their whole data set f o r the Cenozoic with this model and also obtained realistic bottom water temperatures. However, precise knowledge about the pattern of increase in the seawater Mg/Ca data is missing, which made it hard to correct paleotemperatures f o r changing seawater Mg/Ca ratios. For example, applying a linear increase in seawater Mg/Ca ratios artificially lead to a decreasing trend in bottom waters and increased 5w during the Oligocene and Miocene. Clearly we lack knowledge on the precise effect of seawater Mg/Ca on foraminiferal calcite. Possibly the seawater correction factor R in the calibrations f o r benthic foraminifera is incorrect since an increasing amount of evidence indicates that seawater Mg/Ca ratios have changed during the Neogene, but this does not f i t with temperatures obtained when changing the factor R. Before I can correct paleotemperatures f o r seawater ratios, more knowledge is needed on the sensitivity of the calibrations f o r changing seawater Mg/Ca ratios and the precise evolution of seawater Mg/Ca ratios. Possibly the method involving echinoid spines will fill this gap, but this method f i r s t needs more investigation on the precise habitat of the echinoids and a higher sample resolution. Therefore I did not correct bottom water temperatures f o r changing seawater Mg/Ca ratios and assume modern Mg/Ca seawater ratios f o r the whole record. This is an oversimplified method and will probably underestimate bottom temperatures and 5w f o r the Oligocene and early Miocene by 2-3 °C and 0.6 % o .
Age [ M g / C a ] „ [Mg/Ca],^ [ M g / C a ] „ [Mg/Ca],„ (Ma) Modem 4 mol/mol 3 mol/mol 2.5 mol/mol
T 5w T 5w T 5w T 5w 10 5.82 0.54 9.20 1.14 11.82 1.79 13.48 2.2 20 7.62 -0.02 10.00 0.58 12.62 1.23 14.28 1.85 30 8.74 0.36 11.13 0.96 13.74 1.61 15.40 2.02
Table 2 ; Mg/Ca based temperatures and Sw under changing Mg/Ca seawater ratios.
• Mg/Ca based paleotemperatures
Equation 5 was used to convert Mg/Ca ratios of benthic foraminifera to temperature with assuming modern seawater ratios f o r the whole record ( R = l ) . By this method, bottom waters fluctuate between 5 and 10 °C (see figure 11). The Mg/Ca based paleotemperatures show all the main features of Neogene's climate such as the EMCO, MMCT and the cooling in the Plio- and Pleistocene accompanied with the development of Northern Hemisphere glaciation. These temperatures are comparable to values seen f o r other Sites at intermediate depths (Billups and Schrag 2003), but colder then temperatures obtained f o r deep ocean Sites (Lear et al. 2000a; Lear et al. 2004). A comparison between the different sites f o r Mg/Ca is given in appendix C. Lear mainly used the benthic foraminifera species O. umbonatus
or corrected to this species, which has a different calibration then
Cibiddoides. When their calibration is applied to my O. umbonatus data, the temperatures f r o m the Cibiddoides species were confirmed with temperatures between 6 and 9 °C f o r the last 5 Ma. Possibly the diagenetic alteration caused bottom water temperatures at intermediate depth to be higher then values of around 4 °C as we see today f o r bottom waters at
• Trace metals
• Seawater Mg/Ca ratios
All exponential calibrations bear a correction factor f o r the Mg/Ca o f the seawater in which the foraminifera were growing (R in equation 5 ) . My newly obtained seawater Mg/Ca record is a first e f f o r t in truly quantifying seawater Mg/Ca ratios during the Neogene. A few assumptions had to be made in order to obtain the seawater Mg/Ca record. First I assumed that the benthic foraminifera were living in the same environment as the echinoids. Echinoids are known to live f r o m the shallow beach site to the deep ocean. However, there is no evidence to confirm the coexistence of the echinoid with the benthic foraminifera. Secondly, to obtain the species coefficient S, I made the rough assumption that the youngest samples with an age of 2.6 Ma formed under modern seawater Mg/Ca ratios. These assumptions have an important effect on the value f o r S but more over if the echinoid spines and benthic foraminifera were not living in the same environment the method o f combining equation 2 and 3 is not valid at all. As last, the echinoid spines give seawater Mg/Ca ratios of 1 mol/mol in the Oligocene. These are below the most conservative model f o r seawater Mg/Ca ratios, which gives values of 2.5 mol/mol as the lowest Mg/Ca seawater ratio f o r the end of the Paleogene (Hardie 1996). The increase f r o m 1 to 5 mol/mol in a period of 15 Myr is also unrealistic seen the residence time f o r Mg and Ca of 13 and 1 Myr, respectively (Broecker and Peng 1982).
When the seawater Sr/Ca record is compared to the seawater Mg/Ca record i t becomes clear that the t w o records are not identical (figure 12 and 13). This is partially due to the very coarse resolution of the Mg/Ca spine record, but i t seems t h a t the decrease seen in the Sr/Ca in the middle and late Miocene is not recorded in the seawater Mg/Ca record. This decrease in seawater Sr/Ca is also seen in the record of (Lear et al. 2003). Of course, Sr/Ca ratios are influenced by other factors then Mg/Ca ratios (such as river influx (Lear et al. 2 0 0 3 ) ) , but the discrepancy between slowly increasing Mg/Ca ratios and decreasing Sr/Ca seawater ratios during the middle and late Miocene at least ask f o r a higher resolution. Nonetheless, both the seawater Sr/Ca and Mg/Ca record show a rapid increase f r o m the Late Miocene onwards, suggesting t h a t during this period seawater to calcium ratios did change and this will have influenced trace metal incorporation.
To demonstrate the influence of changing seawater Mg/Ca ratios, in table 2 temperatures and 5w were corrected f o r seawater Mg/Ca ratios. Three scenarios are given with seawater Mg/Ca ratio covering literature values f o r the end of the Paleogene and Neogene (Wilkinson and Algeo 1989; Dickson 2002). When seawater Mg/Ca ratios in the Oligocene are considered to be as low as 2.5 m o l / m o l , temperatures are almost doubled to 15.4 °C and 5w will increase to around 2 % o . For bottom waters this is too warm combined with 5w values that resemble full Pleistocene glaciation. Secondly, these temperatures are also not confirmed by 5 ' ° 0 values of around 2.5 % o .
32
u/Ca from CA/aob^ sw. ™ ™ ™ = « » 1 Myr rrwing Momgs MgfCa
Pleistocene ^ | pliocene 1 Miocene r'! • '•' i ' Oligocene
I ' l i I I I i I I M I I I 1 I 1 M i 1 1 I ' 'I ' 1 I 1 1 I O 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Age (Ma)
Figure 16; U/Ca and Mg/Ca raüos of the Cibiddoides spp. together. A blow-up showing Mg/Ca ratios of cibiddoides spp. and 0. umbonatus together with the Li/Ca and 5 ^ 0 record of
Cibiddoides spp. of the last 7 Ma is shown in the right corner.
31
comparable depths (see above). I t is also interesting to see that the three Mg/Ca outliers (Billups and Schrag 2002) identified in their record are also f o u n d in my record. Especially the t w o high Mg/Ca ratios in the early Miocene at 20.8 and 20.3 Ma f o r my Site 1264A were also f o u n d in the record o f Site 747, although probably due to a difference in age model the timing is slightly earlier f o r Site 7 4 7 . These t w o maxima, at least f o r Site 1264A, coincide with the highest Li/Ca values (see next section).
• Li/Ca record and influence on Mg/Ca ratios
Although quantifying the Li/Ca ratios was not possible f o r my samples, the trend seen in my record is similar to the record o f (Lear and Rosenthal 2 0 0 6 ) . These authors also argue that Li/Ca ratios follow the pattern of CCD depth due to the linear relation between Li/Ca and carbonate saturation state. Comparing my Li/Ca record with the South Atlantic paleoCCD reconstructions f r o m (van Andel 1975; van Andel et al. 1977; Hsu and Wright 1985; Schellenberg et al. 2004) gives a good correlation. The records all show a CCD deepening during most of the Oligocene and f r o m the middle Miocene onwards, which matches increasing Li/Ca ratios in my record. The dip in Li/Ca ratios around 15 Ma matches with a short CCD shallowing during the middle Miocene. The consistency between CCD movement and Li/Ca ratios in my record suggest t h a t Li/Ca ratios are truly related to CCD and hence carbonate saturation state. I t ' s interesting t h a t during the period of increased Li/Ca ratios around 20 Ma the two extreme peaks in Mg/Ca ratios are f o u n d . Since increased Li/Ca ratios are t h o u g h t to be related to an increase in carbonate saturation state ACOB^" (Lear and Rosenthal 2 0 0 6 ) , which again influences Mg/Ca ratios (Elderfield et al. 2 0 0 6 ) , the elevated Mg/Ca ratios f o u n d in m y record and that of (Billups and Schrag 2002) around 20 Ma might reflect an increased influence of ACOa^' on Mg/Ca ratios and not so much increased temperatures. The period of increased Li/Ca ratios around 20 Ma coincides with a CCD deepening (Hsti and Wright 1985). As last, the t w o maxima in Mg/Ca ratios coincide with a minimum in the 5 " C record. Overall no covariance between the Li/Ca and Mg/Ca record is f o u n d (see figure 16), although when the last 5 Ma are examined in more detail it appears as if the Cibiddoides record follows the trend in the Li/Ca record. This would again indicate possible bias of the Mg/Ca record due to increased ACOa^". Even more interesting is the f a c t that the record f r o m the (transitional) infaunal living 0 . umbonatus, which might not be influenced by the carbonate saturation state as suggested by (Elderfield et al. 2006) due to its infaunal habitat, shows a much steeper decrease in Mg/Ca ratios during the Pliocene. So possibly the high AC03^", as suggested by the Li/Ca record, in the Pliocene lead to a lag in the drop in Mg/Ca ratios of the Cibiddoides
record, while Mg/Ca ratios of O. umbonatus were not effected due to its infaunal habitat. The increased Mg/Ca and Li/Ca ratios in the Cibiddoides
with (Hodell et al. 2001) who argue that the glaciation event preceded the deposition of evaporates in the Mediterranean. The fact that the 5w record shows a steady increase f r o m 4 Ma onwards, suggests t h a t the ice sheets by this time became a stable feature in global climate. No large decrease in 5w is seen that would suggest a collapse of m a j o r ice sheets during the Plio- and Pleistocene. I f the Mg/Ca based temperatures f o r Cibiddoides species in the Pliocene are biased by an increased carbonate saturation state and temperatures cool quickly and remain stable f r o m 2 Ma onwards as suggested by the record of O. umbonatus, the glaciation at the end of the record becomes less intense with 5w values between 1 and 1.5 % o during the late Pliocene and Pleistocene.
When the 5w record is compared to the global sealevel curve of (Miller et al. 2005) some inconsistencies can be f o u n d . The five major increases in 5w are not all recorded in the global sea level record, while some larger changes in the sea level record are not seen in the 5w record. This is probably partly due to the coarse resolution of my study, but the MMCT f o r example is not recorded in the sea level curve. This is the second largest glaciation event besides the Plio- and Pleistocene glaciation and must had an impact on global sea level. For the last 7 Ma (Miller et al. 2005) used benthic foraminiferal S'^O values. From my record I can conclude that this 5^^0 record partly reflects changes in bottom temperature. I t would be better to use the 5w record. This can be seen in the late Miocene during which the sea level curve shows no regression due to constant 5^'0 values, while the 5w record indicates a period of m a j o r glaciation.
• 5w, ice volume and global sea level during the Neogene Seawater S'-^O (5w) was calculated by solving equation 6. 5w is a function of continental ice growth and decay and local salinity. I t also depends on the 5^°0 signature of continental ice volume, which is likely to have changed over t i m e . However, to a f i r s t approximation equation 6 can be used to identify changes in 5w due to continental ice sheet growth and decay. The 5w record might be too high due to benthic foraminiferal S'^O with an offset of 0.5 % o
36
influenced by the increased carbonate saturation state. The ice volume increase during the MMCT consists of two parts with f i r s t bottom water cooling, followed by bottom water warming. The first part coincides with the last part of the Monterey excursion and a drop in temperatures due to low pCOz. Around 13 Ma Antarctica became thermally isolated and the east Antarctic ice sheet continued to grow, independent of global temperatures. The LMCS coincides with another glaciation event, supporting the original idea of (Vincent et al. 1980) that the shift in the S"C record is caused by the f l u x of organic carbon f r o m the exposed shelves to the open ocean. High 5w f r o m the MMCT onwards suggest that ice sheets became a stable feature in global climate. The steady Increase in Sw w i t h o u t the presence of a m a j o r melting event in the Plio- and Pleistocene verifies the suggestion that the Antarctic ice sheets by this time were stable. The increase in 5^^0 during the late Oligocene and early Miocene climatic optimum are mainly an increase in bottom water temperatures and not so much a reduction in ice volume. A comparison of the 5w record with global sea level curve reveals some discrepancies between the t w o , especially during the MMCT and the late Miocene during which major glaciation events are not mimicked by a global sea level fall.
F u t u r e w o r k
Future work will involve two methods to determine past seawater Mg/Ca ratios. First of all a study will be made into the habitat of the used echinoids. Core top samples will then be used to obtain a more accurate species specific calibration f o r echinoid spines. Secondly, a combination of both Cibiddoides ssp. and the Pyrgo species murrhina and depressa will be used to obtain another independent measurement of sea water Mg/Ca ratios during the Neogene using the offset in the calibration lines of the Pyrgo and
Cibiddoides species. By this method, I hope to finally resolve the issue on past seawater Mg/Ca ratios and see whether the correction f o r this factor as implemented in the calibrations is correct. Trace metal analysis of the species O. umbonatus f o r older samples will show whether more discrepancies between the infaunal and epifaunal benthic foraminifera exist. Especially interesting will be to see whether the the t w o maxima in Mg/Ca ratios that are seen in my record and the record of (Billups and Schrag 2002) around 20 Ma and who are possibly caused by an increased carbonate saturation state are also recorded in the Mg/Ca record of 0 . umbonatus.
35
C o n c l u s i o n s
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A c k n o w l e d g e m e n t s
I would like to thank Lucas Lourens and Gert-Jan Reichart f o r letting me participate in an international research project f o r which I spend 5 months in the United States. Of course m y stay in the US could not be possible without the help of Ellen Thomas and Dave Hodell. I would like to thank Dave Hodell and Jason Curtis f o r helping me with my isotope analysis at the University of Florida. I would especially like to thanks Ellen Thomas f o r all the time she spend on me during my stay at Yale University and introducing me into the taxonomy of benthic foraminifera. Last but not least I would like to thank Lucy Stap who was always willing to help me and with whom I spend numerous hours in the clean lab.
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