Cibicidoides wuellerstorfi

Transcription

1 212 Cibicidoides wuellerstorfi

2 CHAPTER Synthesis and Perspectives W. Feldmeijer 213

3 Chapter.1 Synthesis and perspectives Stable isotopes, foremost δ 1 O, of benthic and planktonic foraminifera have been analysed for some years now [Emiliani, 1a, 1]. These data have provided mankind with information on past climate variability that is unsurpassed. Whether it is short scale temporal variability [e.g. Bartels-Jónsdóttir et al., ; Chappell and Shackleton, 1; demenocal et al., ] or variability covering millions of years [e.g. Douglas and Savin, 1; Lear et al., ; Zachos et al., 1, ] these δ 1 O curves capture specific conditions over particular periods of time and provide information on changes in Earths climatic state within a temporal framework. For all practical purposes, the resolution with which these fluctuations can be detected depends on the sedimentation rate, i.e. how much sedimentary matter and foraminifera rains down and accumulates on the sea floor within a certain period of time. Localities characterized by a low sedimentation rate [e.g. Ravelo et al., ; Thomas and Shackleton, 1; Westerhold et al., 11], as opposed to sites typified by a high sedimentation rate [Channell et al., 12; Genovesi et al., 11; Xiang et al., ], will provide less information on shorter time scales but for a longer period of time. For the same core length, localities with a high sedimentation rate provide less information in deep time but will offer a more detailed view on short scale climate variability. Very few localities, however, have sedimentation rates sufficiently high, continuous and undisturbed to resolve seasonal variability in sediment cores through climate change. In order to unravel seasonal variability I explored Single Shell Analysis (SSA) of planktonic foraminifera. As argued in this thesis, SSA provides a glimpse at the internal variability and the range found in stable isotopes that relates to the seasonal amplitude in e.g. temperature and depth habitat of species. Within the project Sensing Seasonality I tried to quantify seasonality and the seasonal extremes recorded in planktonic foraminifera from oceanic sediments. Planktonic foraminifera are passively moving, single-celled organisms which calcify within the upper waters of all oceans and record ambient environmental parameters (temperature (i.e. insolation thus also light availability), salinity and [CO 3 2- ]) in their shell chemistry and isotopic composition. In addition the geochemical variability of certain species is influenced by vital effects caused by e.g. respiration of the host symbionts or photosynthesis of the symbiotic algae [e.g. Zeebe et al., 3]. Depending on their ecologic niche they calcify at different depths and in different seasons, while their life cycles vary from several weeks to about a year. Furthermore, foraminifera cannot actively move through the water column and therefore are at the mercy of the wind-driven ocean currents. All these factors need to be considered to understand the ecology and geochemistry of these organisms. It is therefore pivotal to have a thorough understanding of what a measured Mg/Ca or stable isotope signal from a foraminifer represents. Some questions I have tried to answer within my research are: How can past natural variability within these geochemical proxies be explained? How can we deconvolve the seasonal signal from fossil foraminifera? 21

4 Synthesis & Perspectives In order to answer these questions I first investigated to what extent some chemical additives used commonly in sample processing or mechanical errors introduced during sample preparation may compromise later geochemical analysis. Additives like e.g. hydrogen peroxide ( to %) have no significant influence on the Mg/Ca ratio and stable isotope composition of planktonic foraminifera (Chapter 2). Furthermore, measuring individuals per sample for Single Shell Analysis (SSA), as is done in most of the studies within this thesis, is shown to yield an acceptable (i.e. within machine error) standard error on the mean (Chapter 3). Studies that involve the stable isotope composition of planktonic foraminifera require some careful planning before one measures either too many or too few specimens than needed of a species that may not be suited for the aims. Earlier studies required the measurement of many specimens for sufficient analytical mass. Nowadays single specimens, even single chambers, can be analysed which brings up a whole new range of questions, possibilities but also limitations. If one is strictly interested in core stratigraphy it is preferable to measure a species with a small range in oxygen isotopes as it requires few individuals for a good estimate of the mean. For that purpose, N. dutertrei is a suitable candidate for core stratigraphy in the Arabian Sea and Timor Sea (Chapter 3) and has been used as such previously [Jung et al., 2; Kiefer et al., ; Prins and Postma, ; Rostek et al., 1; Steens et al., 11, 12]. Furthermore, isotopic variability appears smaller in coarser size fractions for Globorotalia inflata and Globorotalia truncatulinoides from the North Atlantic, which leads to a smaller standard deviation and subsequently less specimens are needed to obtain a small standard error on the mean (Chapter ). Therefore, it is advisable to pick specimens from the >2 μm size fraction but preferably larger as this decreases the necessary number of specimens per measurement due to increased weight. As studies widely vary in their paleoceanographic objectives, so do the species used to reach these. For example the SSA study by Ganssen et al. (11) portrayed seasonal sea surface temperature (SST) ranges off Somalia using Globigerina bulloides and G. ruber (Figure.1). As these species calcify during different seasons, i.e. the cold and warm western Arabian Sea monsoon, they allow for the determination of the annual range in SST. Both these species, especially G. bulloides, show a relatively large range in δ 1 O (Chapter 3) making them less suited for stratigraphic purposes, yet fitting for studies aimed at deducing seasonality. So once the aim of a study is clear, most important in species selection is first determining what species are sufficiently abundant for analysis and the quantity of analyses that need to be performed. But what does this geochemical information on a planktonic foraminiferal shell tell us? And why analyse individuals (and increase the number of analyses - fold) and not perform conventional analysis of pooled shells? R & A 21

5 Chapter δ 1 O shell (, V-PDB) temperature [ C] age [kyr] cm cm 1. C 1.3 C C. C kyr 1. kyr cm 1 cm 1 cm 1. C 1. C 1. C C 2. C 2. C 3. kyr. kyr. kyr cm 2 cm 1. C 1.1 C C 2.1 C.1 kyr. kyr 2 cm 1. C C. kyr 3 cm 1.3 C C 11.2 kyr 3 cm cm 1. C 1. C C 2. C 13.2 kyr 1.2 kyr cm cm 1. C 1. C C 2. C 1.1 kyr. kyr δ 1 O shell (, V-PDB) temperature [ C] 21

6 Synthesis & Perspectives For foraminiferal calcite, the Mg/Ca ratio provides an estimator of calcification temperature, independent of δ 1 O [Ganssen et al., 11; Nürnberg et al., 1; Chapter ]. Whereas the most studies use bulk Mg/Ca ratios, it is also possible to analyse single shells. Coupling of the single shell geochemistry can be carried out by applying Laser Ablation Induced Coupled Plasma Mass Spectrometry (LA-ICP-MS) which only requires minute amounts of foraminiferal calcite for analysis leaving sufficient material for stable isotope analysis of the same specimen. LA-ICP-MS, therefore, is a good alternative to try and resolve the enclosed range and can show individual chamber to chamber variability, even layer by layer variability within a chamber wall. Conventional Mg/Ca analysis has yielded many species specific, even site specific, temperature calibration curves all based on pooled analysis of multiple individuals [Anand et al., 3; Chiessi et al., ; Cléroux et al., ; Dekens et al., 2; Elderfield and Ganssen, ; Groeneveld and Chiessi, 11; Lea et al., ; Mashiotta et al., 1; McConnell and Thunell, ; McKenna and Prell, ; Regenberg et al., ]. These calibration curves have become more reliable over time, also because of the addition of calibration curves from culture experiments, and can be used for a good temperature estimate within a sample. There is quite some scatter around these calibration curves though, probably due to strong specimen to specimen variability within a sample [e.g. Bolton et al., 11; Eggins et al., 3; Marr et al., 11]. Single specimen Mg/Ca and stable isotope analysis was performed on G. bulloides, G. ruber and Neogloboquadrina dutertrei from the western and eastern Arabian Sea (Chapter ). As both the Mg/Ca ratio and δ 1 O of foraminiferal calcite are used as proxy for SST one would expect that the results for these proxies from the same foraminiferal shell would strongly correlate. However, this appeared not to be the case (Figure.2) as even temperatures calculated using average Mg/ Ca ratios and δ 1 O (ice volume corrected) per sample did not correlate. Possibly such measured offsets between the Mg/Ca ratio and δ 1 O are due to differences in salinity induced by river run off and/or precipitation in this monsoon influenced area, which do affect δ 1 O but not Mg/Ca. In the various chapters in this thesis relatively large variation is seen in the foraminiferal stable isotope SSA results between but also within species. Especially time intervals marked by large scale climate transitions (i.e. Glacial- Interglacial transitions) reveal ranges up to ~3. as opposed to more stable Figure.1 (opposite page). Single shell oxygen isotope data for piston core NIOPP G. ruber (in red) and G. bulloides (in blue) plotted in.2 wide bins in % (left panel). Inferred calcification temperature ranges for both species (right panel). Open dots are the mean temperatures derived from the Mg/Ca analysis for both species of the respective samples, the lines indicate the total range of calcification temperatures obtained for each species with the most extreme values indicated by triangles. Here the stable oxygen isotope values are anchored by the Mg/Ca-based temperature estimates. Using this approach, the spread in stable oxygen isotope values reflects variability in seasonal sea surface temperatures. From Ganssen et al. [11, Figure ]. R & A 21

7 Chapter G. bulloides (IS) G. bulloides (HE) G. ruber (IS) G. ruber (HE) N. dutertrei (IS) N. dutertrei (HE) Mg/Ca (mmol/mol) δ 1 O ( VPDB) Figure.2. Cross-plot of single shell δ 1 O and final chamber Mg/Ca ratio from core NIOPP for G. bulloides (dark and light blue), G. ruber (red and orange) and N. dutertrei (dark and light green). IS is interstadial and HE is Heinrich event. Gunshot pattern reveals the lack of a strong correlation between these two temperature proxies. periods which show ranges as low as ~1. But what does this mean and how can it inform on climate change effects, if at all? Foraminiferal proxy data is gathered to get a better understanding of past large and small scale climate perturbations which subsequently can be applied in climate modelling studies to predict future, potentially threatening, climate fluctuations. Modelling efforts have all contributed to a better understanding of current and past climate [e.g. Cox et al.,, 13; Gillett et al., 11; Pagani et al., 11; Shakun et al., 12; Solomon et al., ] and are the scientific basis for future climate predictions governed by the Intergovernmental Panel on Climate Change [IPCC; Stocker et al., 13]. These climate predictions are used for global regulations on CO 2 emissions and thus affect the global economy. As models depend on data input, among others some models implement foraminiferal proxy data or use it as validation [Schmidt, 1], the better proxy data are, the more accurate predictions will become. What we see, and can measure today, should hold for measurements from fossil material (e.g. foraminiferal SSA results to deconvolve past seasonality) which will allow more accurate predictions of future climate. Although this seems straightforward, resolving seasonality from the foraminiferal fossil record requires the consideration of some other factors. 21

8 Synthesis & Perspectives These pitfalls but also perspectives on how to further improve the measurement and interpretation of the foraminiferal proxy record will be discussed below..2 Pitfalls Whereas foraminiferal SSA appears as a promising method to get a grip on past seasonal SST ranges there are certain issues that arise. Bioturbation, i.e. the up and down mixing of the sediment by benthic organisms, may cause unduly large ranges in foraminiferal proxy values when (pale)oceanographic and/or climatic gradients are steep, or sedimentation rates are low [Barker et al., ; DeMaster and Cochran, 12; Groß, 2; Keigwin and Guilderson, ; Löwemark et al., ; Wit et al., 13]. This mixed signal is smoothed out when applying pooled analysis but poses a significant threat to SSA as it can introduce e.g. bimodality in oxygen isotopes when glacial and interglacial specimens are mixed. Measuring multiple species could offer a solution if not all show bimodality (Chapter ). Detailed stratigraphic analysis prior to sampling and SSA helps to identify hiatuses and intervals that are too heavily mixed with respect to the questions posed, e.g. by non-destructive core scanning techniques using colour imaging, XRF chemostratigraphy and magnetic susceptibility. Depending on the location where sampling is performed expatriation can introduce a spatial bias [Berger, 1b]. Water masses can move foraminifera laterally which then can be found in the sediment at higher or lower latitudes [Ganssen and Kroon, 11]. Whereas expatriation poses a relatively minor problem for most locations, the biggest issue with the stable isotopic composition of foraminiferal calcite is the contribution by so-called vital effects on paleotemperature estimates. The vital effect reflects metabolic processes and rates during ontogeny, due to respiration or the hosting of photosynthetic symbionts. Extensive research has been done to unravel this vital effect but there is no full understanding of the planktonic foraminiferal calcification process. Some species are known to calcify in isotope equilibrium [Deuser and Ross, 1; Erez and Honjo, 11; Fairbanks et al., 12; Lončarić et al., ; Mortyn and Charles, 3; Peeters et al., 2; Sagawa et al., 13; Sautter and Thunell, 11; Wilke et al., ; Williams et al., 11] yet most species calcify out of equilibrium [Curry and Matthews, 11; Kahn, 1; Pados et al., 1; Sagawa et al., 13; Shackleton et al., 13; Vergnaud Grazzini, 1] caused by e.g. photosynthesizing symbiotic algae [Spero and DeNiro, 1; Spero and Williams, 1; Zeebe et al., 3]. This paragraph also raises the question as to what extent do cryptic, genetically different but morphological similar, species have a different vital effect. It is shown for different sizes of both coiling varieties of G. truncatulinoides, considered different genetic species adapted to different hydrographic conditions [de Vargas et al., 1; Renaud and Schmidt, 3], that oxygen and carbon isotopes are similar (Chapter ). This could mean that the vital effect can be neglected when studying species with similar ontogeny. Not only the stable isotope composition but also the Mg/Ca ratio of R & A 21

9 Chapter planktonic foraminifera is subject to the aforementioned biases albeit that the increase in measurement resolution (i.e. measuring separate chambers) in this proxy poses additional issues. Previous studies have shown the potential of LA- ICP-MS and, for example, a relatively large range of ~2 to mmol/mol is found in G. bulloides, G. ruber and N. dutertrei (Chapter ). Some species- specific temperature calibration curves have been developed on plankton material using LA-ICP-MS [Bolton et al., 11; Marr et al., 11] though it has proven difficult to apply these to fossil specimens [Marr et al., 13b] due to the +/- 1 mmol/ mol scatter on the calibration curves themselves. Deducing temperatures from these ranges is difficult foremost because these are logarithmic calibration curves which do not allow the interpretation of a measured range as opposed to a mean. The mean of these single shell measurements or bulk Mg/Ca analysis can still be helpful and a method previously employed by Ganssen et al. [11] (Figure.1)..3 Perspectives Charney and Eliassen [1] were the first to develop a quantitative climate model, an effort followed by many [e.g. Manabe and Stouffer, 1; North, 1; Sellers, 1] and technical (computing) improvements have led to more intricate climate models of intermediate complexity [e.g. Ganopolski et al., 1; Goosse et al., ; Petoukhov et al., ; Roche et al., ] and even complex climate models [e.g. Ferreira et al., 11; Knutti and Sedláček, 13; Koster et al., ]. As these models become more and more complex, so becomes the demand for more accurate and high resolution proxy data to validate model output. Although the aforementioned pitfalls might in first instance paint a negative picture of the potential of foraminiferal proxy work we are getting closer and closer to a full understanding of a foraminifers life cycle and thus how, when and where the proxies are generated. Culturing experiments have allowed for a view at the diurnal banding within a foraminifers shell by ever increasing technical abilities and precision [e.g. Eggins et al., ; Sadekov et al., ; Spero et al., 12; Vetter et al., 1]. LA-ICP-MS data on G. bulloides, G. ruber and N. dutertrei show differences in Mg/Ca ratio in the subsequent chambers of an individual (Figure.3) which could be indicative of calcification at different depths. Potentially this technique can be employed to further constrain the movement of G. truncatulinoides through the water column of both coiling varieties and subsequently get a better grip on past variation in the depth of the permanent thermocline (Chapter ). All in all I think that the geochemical analysis of single shells of planktonic foraminifera yields a great deal of information that, even though at the moment not completely understood, can prove vital in the understanding of past climate and thus in predicting future climate. Given the knowledge I have gathered over the past four years on planktonic foraminifera and the information their spatial distribution and geochemistry yields (and given the possibility to do so) I would extend the research described in Chapter. The bimodal distribution seen in the δ 1 O of N. pachyderma captures 2

10 Synthesis & Perspectives.. G.ruber G.bulloides N. dutertrei 1 Mg/Ca (mmol/mol) F F-1 F-2 F F-1 F-2 F F-1 F-2 Figure.3. Mg/Ca ratio of final (F), penultimate (F-1) and antepenultimate (F-2) chamber for G. bulloides (blue), G. ruber (red) and N. dutertrei (green) from core NIOPP. Arrows indicate general direction of change which can amount to a difference between the antepenultimate and final chamber of ~3 mmol/mol. the northward movement of the ice sheet during the last deglaciation. It would be of great interest to analyse N. pachyderma over the last deglaciation from a northsouth transect of cores from the North Atlantic. This should reveal a more exact timing of rates of progressive movement of the ice sheet. Potentially looking at the coiling ratio of N. pachyderma/n. incompta can reveal an associated progression in SST [Darling et al., ]. Locations south of core T-3P ( N; Chapter ) will likely lose the bimodal distribution sooner due to the poleward rise in SST whereas cores further north will retain this bimodality longer. Subsequently, the lateral movement of the ~ C SST boundary, considered as the maximum calcification temperature for N. pachyderma [Darling et al., ], can be traced through time up to its current location in the northern North Atlantic. A further interesting avenue would be performing SSA on N. pachyderma from the <2 μm size fraction. Smaller specimens potentially live at shallower depths [Mortyn and Charles, 3] recording relatively higher temperatures which could show an even stronger bimodal distribution than the >2 μm size fraction I studied over Termination I as seasonal temperature differences are largest in the surface waters. R & A 221

11 Chapter GISP δ 1 O ( ) -2 - Byrd δ 1 O ( ) Age (kyr) Figure.. Oxygen isotope data for Greenland ice core GISP2 [Grootes et al., 13] and Antarctic ice core Byrd [Johnsen et al., 12] showing millennial-scale oscillations being out of phase, i.e. the bipolar seesaw. Furthermore, a comparable north-south transect, following the mid- Atlantic Ridge, could be analysed similarly (i.e. SSA, abundance counts and coiling ratio N. pachyderma/n. incompta) from the Antarctic region to determine whether the response of N. pachyderma also shows a movement of the Antarctic ice sheet that was similar to that of the Arctic. This will give an indication whether, and to what extent, the ice sheets on both hemispheres migrate in or out of phase as the response to millennial-scale climate variability is known to be out of phase [Blunier and Brook, 1], i.e. the bipolar seesaw, from ice cores (Figure.). Besides a north-south transect, an east-west transect on both hemispheres including upwelling areas could also hold valuable information as, e.g. the Antarctic response could be offset by input from the Benguela upwelling cell off SW Africa. High abundances of N. pachyderma and N. incompta are found within cores from this area [Farmer et al., ; Giraudeau, 13; Giraudeau and Rogers, 1; Little et al., 1] yet as upwelling leaves a strong imprint on seasonal Figure.. (opposite page) Shell weight data plotted against atmospheric CO 2 concentration (pco 2 ) from Vostok Ice core [Petit et al., 1]. Subsample (unfilled circles, n = ) and sample average (colored line, n ) of shell weight of G. bulloides (top row), G. inflata (middle row) and G. truncatulinoides dextral (bottom row) for 3- μm, -3 μm, 2- μm and μm size fractions (left to right) in comparison with pco 2 [Ruddiman and Raymo, 3], grey line against age (kyr) before present. Note that the primary y-axis varies depending upon the size fraction. Both G. bulloides and G. inflata appear in anti-phase with CO 2 as opposed to G. truncatulinoides. 222

12 Synthesis & Perspectives Atmospheric Carbon Dioxide (pco 2 ) Globigerina bulloides Shell weight Globorotalia inflata Shell weight 22 Shell weight (µg) µm 2- µm -3 µm 3- µm Globorotalia truncatulinoides (d) Shell weight Age (kyr) 2 3 R & A 223

13 Chapter temperature fluctuations in this area it is to be expected that the foraminiferal isotopic signal shows a different response during the last deglaciation. Afore mentioned scientific endeavours all focus on N. pachyderma given the potential elaborated in Chapter but other species, e.g. G. truncatulinoides (Chapter ), also have great potential in unravelling past climate fluctuations. As depicted in Figure., G. truncatulinoides reveals a surprising response in shell weight to atmospheric CO 2 concentration (pco 2 ). Variation in shell weight between the responses of surface, intermediate and deep dwelling planktonic foraminifera to pco 2 might reflect differences in the life cycle (i.e. longevity), wall structure (potential evolutionary characteristic), species optima (i.e. abundance or biogeographic zonation) or preferred depth habitat. For instance species with longer longevity, i.e. G. truncatulinoides [Anderson et al., 1], could allow for greater adaptability to changes in seawater carbonate chemistry. Results from Termination III show that shell weight of the surface and intermediate species G. bulloides and G. inflata tracks pco 2 yet the shell weight of the deeper dweller, G. truncatulinoides, does not (Figure.). For species that reside permanently at, or near, the surface with a narrow depth interval are more likely to have been effected with changes in atmospheric pco 2 than those that reside at depth. Species that have a larger depth habitat, i.e. G. inflata, may potentially homogenate this signal. Even though this provides an explanation for the offset seen between species, this pco 2 signal is mixed down the water column, and thus into the depth habitat of G. truncatulinoides, within a relatively short time span and it is therefore surprising that this deep dweller responds differently. Further research into the response of different species to pco 2, possibly by culturing, has been a topic on the rise in recent years due to the apparent consequences of ocean acidification [Alexandersson, 1; Fabry et al., ; Orr, 11; Orr et al., ], due to burning of fossils fuels and subsequent elevated pco 2. There is still a lot of information to be unlocked in the world s ocean sediments and fossil (planktonic) foraminifera therein, from sediment cores already retrieved during countless marine expeditions, and every small piece of the puzzle adds information to the growing body of palaeoceanographic data. Combining this fossil data with recent flux, plankton tow and culturing data will all add to the understanding of past and current (seasonal) climatic developments. 22

14 Synthesis & Perspectives R & A 22