PP51B-1489
Effect of elevated nitrate concentration on calcification in Emiliania huxleyi
It is known that the percentage of aberrant coccoliths in cultures is higher than in natural samples, which makes extrapolating laboratory-based results of calcium carbonate production and coccolith morphology to those in the ocean difficult. The reason for the hampered morphogenesis of coccoliths in cultured specimens is still unknown. Nimer and Merrett revealed a decreased calcification rate under a nitrate concentration of 1000 micromol per liter in 14C uptake experiments with Emiliania huxleyi. Therefore, we studied growth rate, calcification rate, particulate organic carbon production, and the ratio of aberrant coccoliths to normal coccoliths in a culture of Emiliania huxleyi (PML92/11) grown in natural seawater under varying nitrate concentrations from 10 to 890 micromol per liter. None of these parameters showed a trend over the range of nitrate concentrations tested. We conclude that high nitrate concentrations do not affect morphogenesis of coccoliths and particulate carbon in Emiliania huxleyi (PML92/11).
PP51B-1490
Growth characteristics of 28 strains of white tide forming coccolithophorids and elementary analysis under different temperatures
With the increase of the CO2 concentration in the atmosphere, global warming and acidification of the oceans are troublesome, influencing the marine organisms and the ecosystem. Coccolithophorids are marine unicellular haptophytes producing minute calcareous scales (coccoliths). Because of the conversion of CO2 to CaCO3 and the sedimentation of coccoliths, coccolithophorids are one of the most important organisms participating to the global biogeochemical cycles. Among the coccolithophorids, Gephyrocapsa oceanica and Emiliania huxleyi distribute widely in both oceanic and coastal environments. They often form massive water bloom called white tide and are known to be the major producer of CaCO3 in the present ocean. Moreover, elementary analysis of coccoliths accumulated in sediments provides valuable information on the analysis of paleoenvironment. Although the growth characteristics and calcification properties are important information to estimate their contribution to the global carbon cycle and paleoenvironment, most of the studies have targeted only few strains. Here, we focused on the effect of the temperature on the growth and elementary component of the coccolith for a variety of strains of G. oceanica and E. huxleyi collected from different environments. This approach would clarify the temperature response to various populations and the importance of the use of coccolith as an environmental indicator. 28 strains (20 of G. oceanica, 8 of E. huxleyi) were originally established as cultures or obtained from culture collection. We measured their growth rate from the chlorophyll fluorescence under different temperatures (10, 15, 20, 25, 30°C). We also analysed the oxygen isotope ratio, Sr/Ca, U/Ca, Ba/Ca for 5 selected strains with ICP-MS and ICP-AES. In most strains, the optimal growth temperatures were obtained at 20 and 25°C, while the growth rates dropped with temperature decrease. No significant differences between the two species were observed. We noted that one Emiliania strain from Bering Sea kept a high growth rate at the lowest temperature (10°C) and one Gephyrocapsa strain from tropical sea showed the highest growth at 30°C. Locality differences provide more obvious physiological differences than the species differences, suggesting the existence of diverse ecotypes in those cosmopolitan species. The elementary analysis exhibited a high correlation of the cultivation temperatures with the oxygen isotope ratio (R2=0.97) suggesting a high reliability but less correlation of the temperatures with Sr/Ca, U/Ca and Ba/Ca.
PP51B-1491
ADAPTATION OF COCCOLITH CALCIFICATION TO SEA WATER CARBONATE CHEMISTRY
Coccolithophores are major calcifiers and through calcification cause feedbacks to atmospheric CO2 cycling. The formation of CaCO3 in seawater, in fact, causes a shift of the carbonate system towards CO2, which in turn affects atmosphere / ocean CO2 exchange. A change in marine calcification provides a concomitant feedback in organic carbon export and would lead to a change in the drawdown of atmospheric CO2. Coccolithophore culture experiments and field observations showed controversial results regarding the response of calcification to high CO2. The three strains of Emiliania huxleyi (the most abundant living coccolithophore species) tested so far show both increased and decreased calcification at high CO2 levels (lower pH). Living E. huxleyi is known to have a large variability in both size and carbonate content. The hypothesis that we want to test in this work is the importance of adaptation of calcification to the seawater carbonate chemistry where coccolithophores calcify. We selected 4 strains of E. huxleyi maintained at the Roscoff culture collection, collected from different oceanographic settings with different carbon speciation. The selected strains are collected from environments with very different water carbonate chemistry and they have different carbonate mass. They have been experimentally grown at different CO2 levels to test the strain calcification response to sea water carbonate chemistry. . With these experiments we test the importance of the calcification strain adaptation to carbonate chemistry. Size and possibly different responses to carbonate chemistry variations will also be discussed.
PP51B-1492
What Really Controls Isotopic Offsets Between Seawater and Coccolith Calcite: Vital Effect or Environmental Effect?
The geochemistry of calcareous microfossils offers a valuable methodology for reconstructing the thermal and trophic structure of the uppermost water column. However, culture studies of coccolithophores have highlighted the importance of isotopic disequilibria between seawater and coccolith calcite, assigned to be a 'vital effect'. In order to refine the geochemistry of coccoliths for paleoceanography, it is of primary importance to know if vital effects measured by nannoplankton culture experiments are transposable to 'natural' assemblages of modern pelagic sediments. We studied Indian ocean core-tops from which we separated the various carbonate producers: planktonic foraminifera by hand-picking, coccoliths and calcispheres by microfiltering steps (Minoletti et al., in press). We obtained monogenetic assemblages of Thoracosphaera sp. calcispheres and Calcidiscus leptoporus coccoliths that afford geochemical analyses at the species level and compared their carbon and oxygen isotopic ratios with seawater parameters found in ocean databases (http://data.giss.nasa.gov/o18data/ and www.nodc.noaa.gov). It is thus possible to meaningfully compare isotopic offsets between seawater and calcite with specific vital effects provided by nannoplankton culture studies. These isotopic offsets seem to agree broadly with culture studies results, although discrepancies exist. Further investigations should be performed on sites of different trophic regimes to test the influence of environmental settings on isotopic offsets between coccoliths and seawater. Coupled with this biogeochemical approach, the microseparation of key-taxa thriving at different depths of surface seawater, could contribute for further understanding how phytoplankton have fedback on Cenozoic climate.
PP51B-1493
Changes in C37 alkenones flux on the eastern continental shelf of the Bering Sea: the record of Emiliania huxleyi bloom over the past 100 years
Flourishes of coccolithophores can be detected by ocean color imagery with data from the satellite-borne Sea-viewing Wide Field-of-view sensor SeaWiFs that was launched in 1997. Thus, temporally and spatially large-scale blooms of Emiliania huxleyi (E. huxleyi) have been distinguished annually in the eastern continental shelf of the Bering Sea since 1997. In 1997, a combination of atmospheric mechanisms produced summer weather anomalies such as calm winds, clear skies, and warm air temperature over the Bering Sea and the weather anomalies caused depletion of the subpycnocline nutrient reservoir (Napp and Hunt, 2001). After depletion of nitrate and silicate, a sustained (more than 4-month-long) bloom of E. huxleyi was observed (Stockwell et al., 2001). Because of the speed and magnitude with which parts of the Bering Sea ecosystem responded to changes in atmospheric factors (Napp and Hunt, 2001) and because a bloom of the coccolithophorid, Coccolithus pelagicus has also been detected in the northeastern Atlantic Ocean off Iceland every year since 1997 (Ostermann, 2001), the appearance of an E. huxleyi bloom in the Bering Sea could be related to atmospherically forced decadal oscillations or global factors. We have investigated spatial expansion and temporal development of E. huxleyi bloom on the continental shelf in the Bering Sea by using a biomarker of E. huxleyi, C37 alkenones flux recorded in the sediments during the past 100 years. As a result, the E. huxleyi bloom had been prominent since 1970fs at latest during the last 100 years. In this presentation, we will discuss the relationship between E. huxleyi bloom and activity of Aleutian low, and also changes in diatom assemblages. References Napp and Hunt, 2001, Fish Oceanogr., 10, 61-68. Ostermann, 2001, WHOI annual report, pp.17-18. Stockwell et al., 2001, Fish Oceanogr., 10, 99-116.
PP51B-1494
Linking Ocean Variabilty to Phytoplankton Changes Over the Last 2000 Years off N. Iceland
Understanding ocean variability at decadal to century time-scale is a key issue of climate research, yet it remains poorly described partly because high resolution marine records are lacking. Recent observations also suggested that climate-driven oceanic changes may also affect marine productivity reflecting adjustments of the pelagic ecosystem. To explore these issues we investigated sediment cores (a Calypso and a box core) off North Iceland at unprecedented high temporal resolution (2 to 5 years). The area lies in a sensitive boundary region with strong climatic gradients between the Arctic and the North Atlantic realms. Surface hydrology is affected by warm, high salinity Atlantic water (Irminger current-IC) and cold, low salinity water of the East Icelandic Current (EIC), as well as by sea ice and drifting ice exported from the Arctic Ocean and East Greenland. Using biomarker proxies we reconstructed, over the past two millennia, high frequency surface ocean properties, such as Sea Surface Temperature (SST) and patterns of major phytoplankton groups. Records from a box core covering the last 80 years and comparison with instrumental data allowed to improve constrain of the proxies used. The SST curve reveals 4 major oscillations of about 20-25 years, depicting also the Great Salinity Anomaly (GSA). It reflects ocean circulation changes with varying impact of Polar and Atlantic waters (EIC, IC). Similarities with the NAO (North Atlantic Oscillation) index suggest links between STT and westerlies intensity. Phytoplankton biomarker profiles (specific phytosterols and alkenones) are consistent with biological data acquired during annual surveys. They show significant increase in periods when Atlantic waters and subsequent mixing processes prevail during positive NAO phase, compared to low levels when Polar waters and stratified conditions occur. Over the last two millennia the SST curve depicts a broad cooling trend towards present, steepening over the last 500 years. Prolonged warm or cool centennial intervals are recognizable among which the Medieval Warm Period (MWP), the Little Ice Age (LIA), the Roman Warm Period (RWP) and cooler Dark Ages. The SST signal also shows intermittent 20-25 year oscillations that are likely reflecting the ocean response to wind forcing, presumably the NAO. Phytoplankton biomarkers show important high frequency oscillations. Diatom sterols infer a broad decrease of siliceous productivity since 2000 yr BP with minimum values during the LIA, and a sharp rise after 200 yr BP. Such a trend is not observed for coccolithophorid biomarkers (alkenones), which show low centennial scale variations since 1200 yrs BP, along with high abundance between 1600 and 1200 yrs BP. Similar trends are observed for dinoflagellate biomarkers (dinosterol). Overall, planktonic biomarkers show multidecadal to centennial scale fluctuations inferring modifications of phytoplankton community structure. These could be linked at least partly to hydrological changes indicated in the SST curve, namely modifications of the relative importance of IC and EIC, which likely reflect the ocean response to NAO forcing.
PP51B-1495
Implications of coccolithophores expanding to polar waters
There is much debate about the response of coccolithophores to decreased carbonate saturation state and decreased pH in the ocean as a result of increased CO2 partial pressure. It is still not known whether coccolithophores act as a sink or source for CO CO2. Nor is it known whether calcification will be reduced or increased in response to climate change. A proper understanding of the relationship between calcification and climate change is important not only because coccolithophores play an important role in determining the PIC:POC ratio of particle export into the deep ocean but also because climate change may affect the overall biodiversity of phytoplankton and the marine food chain. Because ocean acidification strongly affects polar regions, initially it seems unlikely that coccolithophores should prefer polar waters or even be a major component of phytoplankton in these regions. Yet there is much recent evidence that coccolithophores are increasingly expanding their range into polar oceans. This observation could be pivotal in improving our understanding of the mechanisms and rates of climatic adaptation by natural coccolithophore populations. We postulate that coccolithophores may be more sensitive to recent environmental changes, such as SSTs and salinity, than to factors more directly linked to changing ocean carbonate chemistry
PP51B-1496
The Scales of Coccolithophores: Adaptation to Climate Change
Rising ocean temperatures and lowering of ocean pH may disrupt marine productivity and calcification by coccolithophores, affecting natural biosphere-climate feedbacks. A better understanding of both the mechanisms and the rates of climatic adaptation by coccolithophores is critical for predicting future impacts of climate change. We will discuss how contrasts in the physiology and biogeography of modern coccolithophores could relate to different climatic adaptation strategies of their Cenozoic ancestors. On short time scales, experimental results highlight species-specific sensitivities to changing ocean carbonate chemistry, which is consistent with differences in cell size of the investigated taxa and likely related to intracellular pH control. On geological time-scales, coccolithophores appear to have adapted to a long- term decrease in atmospheric carbon dioxide (pCO2) and cooling ocean temperatures by decreasing their coccolith and cell size. We employed a novel, information-theoretic approach to quantify the relative influence of different environmental variables on coccolith size. This analysis suggests that the macroevolutionary size decrease primarily reflects a physiological adaptation to CO2 limitation, rather than decreased nutrient availability caused by large-scale changes in ocean stratification. The recent dominance of Emiliania huxleyi is likely due to its fast growing, small cells and light calcification. This allowed it to outcompete larger and heavily calcified coccolithophores under low pCO2 conditions of the Pleistocene. However, as the ocean carbonate system is rapidly reversing to more acidic pre-Pleistocene conditions, the fate of E. huxleyi and other modern prolific bloomers is uncertain. The potential expansion of the larger, pH-resistant species Coccolithus braarudii away from its restricted high- pCO2 niches remains untested.
PP51B-1497
Mesoscale Fossil Diversity and Ecosystem Modeling in the Cenozoic
Numerous experiments of extant ecosystems have tested aspects of modern niche theory as they relate to the development and maintenance of species richness in a geographic area. As such, species richness has often been observed to be a consequence of heterogeneous conditions within the ecosystem provided by environmental gradients, moderate levels of disturbance, and complex trophic interactions that give rise to niche partitioning. By contrast some studies of the fossil record have focused on identifying governing parameters for paleodiversity using instead simplified models of ecosystem interaction, which violate principles of niche theory. To examine ecosystem diversity within the most recent 60 Ma, we analyzed the depositional environment and lithologies of 17,984 globally distributed marine fossil assemblages, focusing on the relationship between diversity and ecosystem gradients. Our results indicate that although there is a myriad of factors that can provide for high fossil diversity within a geographic area, only a few ecosystem gradients are needed to explain the majority of that diversity. Our findings are consistent with modern niche theory and may extend the robustness of this concept significantly through time.
PP51B-1498
Global Cooling and its Impact on Phytoplankton Evolution: The Diatom Point of View
A major characteristic among skeletonized phytoplankton in the ocean is that they depend on the availability of bio-limiting nutrients in the photic zone. Competition (inter- and intra-group) for these bio-limiting nutrients together with present and past changes in oceanic conditions are potential dominant drivers of the adaptation and resulting evolution of these micro-organisms. Over the Cenozoic and particularly since the Neogene, diatoms have become the most efficient group for nutrient uptake, by far outcompetiting the coccolithophores in nutrient-rich areas and assuming dominance in the oceanic cycling of dissolved silica. Here we explore the utility of a quantitative measure, mean test sizes of centric diatoms and their variability, to characterize the macro-ecological patterns in today's oceans and the evolutionary response of this group in the past. Size and morphological characteristics of the centric diatom frustules (diameter >20 µm) were collected using recently developed automated light microscopy and image analysis techniques for a statistically representative number of specimens (i.e. 250-700) per sample. We present initial results of these size analyses by comparing our macro-ecological calibration in 46 Holocene core-top samples from tropical to polar environments with the macro-evolutionary patterns observed in DSDP/ODP sites from the Southern Ocean, the Equatorial and the North Pacific over the past 20 million years at a temporal sampling resolution of 0.3-2 m.y. Although our Holocene sample set shows large variability in environmental conditions, changes in the mean size of centric diatom assemblages are rather small. In contrast, during the Neogene mean sizes of centric diatoms show large variability through time with regional shifts occurring during well-known climatic and oceanic events (i.e. onset of northern hemisphere glaciation). These patterns imply an evolutionary trend possibly related to Neogene global cooling and related reorganizations of water masses and their dissolved silica pools. Since diatoms are the primary competitors of coccolithophores for nutrients among the skeletonized phytoplankton, their evolutionary emergence to dominance would imply coeval consequences for the evolution of the coccolithophores.
PP51B-1499
Significant Size Change of Coccoliths at 2.7 Ma: Paleoenvironmental and/or Evolutionary Controls?
Coccolithophores have been the major carbonate-producing marine microplankton group since the late Jurassic. Today coccolithophores are still essential in regulating marine carbon cycling and ocean- atmosphere CO2 exchange with the calcification of their calcareous skeleton and the subsequent settling of calcium carbonate to the ocean floor. Consequently size variations in the remains of marine planktic organisms through time may well have affected global biogeochemical cycles and might do so again in the near future. In this context we study the size variability of oval to circular coccolithophore platelets (placoliths) in 49 globally distributed Holocene sediments and compare these results with 100 assemblages from a Pliocene section from ODP Site 999A. The site, located in the southern part of the Caribbean Sea, was chosen because of its well-preserved nannofossil assemblages and the broad variety of paleoproxies available. Our newly discovered size increase at 2.7 Ma is correlated to changing abundances of taxa with small versus large coccoliths. Some paleoproxies at Site 999A (δ18Osw,δ11B) and abundances of dominant coccolith taxa show significant changes in the same time interval. However, our analyses of global size variability of Holocene coccolith assemblages show different environmental dependencies. We infer that we either have non-analog size controls in the Holocene and the Pliocene data sets or that other proxies not examined so far may control size distributions of coccoliths.
PP51B-1500
Simulating the Mid-Pliocene Warm Period: how similar are the models?
The mid-Pliocene Warm Period (MPWP, ca. 3.3 to 3 million years ago) provides an unparalleled opportunity to examine the long term response of the Earth System to elevated greenhouse gas concentrations and has been recognised by the Intergovernmental Panel on Climate Change as an accessible example of a world that is similar in many respects to what models estimate could be the Earth of the late 21st Century (Jansen et al., 2007). Understandably the MPWP has become an attractive target for palaeoclimate modelling, with a large number of studies published during the last decade. However, there has been no attempt to assess the degree of model dependency of the results obtained. Here we present a comparison of mid-Pliocene climatologies produced by the Goddard Institute for Space Studies and Hadley Centre for Climate Prediction and Research atmosphere-only General Circulation Models (GCMAM3 and HadAM3). Whilst both models are consistent in the simulation of broad-scale changes in mid- Pliocene surface air temperature and total precipitation rates, significant differences are noted on regional and local scales. Terrestrial data/model comparison using the BIOME 4 model, along with a new data set of Piacenzian Stage land cover (Salzmann et al., 2008), combined with the use of Kappa statistics, indicates that HadAM3 predicted biomes provide a closer fit to proxy data in the mid to high-latitudes. However, biomes predicted using GCMAM3 climatology provides the closest fit to proxy data in the tropics. Results from a terrestrial data/model comparison are also presented using new outputs from the National Center for Atmospheric Research (NCAR) climate model. This study is a contribution to the newly established Pliocene Climate Modelling Intercomparsion Project (Plio- MIP), which is part if the Palaeoclimate Modelling Intercomparison Project (PMIP).
PP51B-1501
Biotic Response of Ostracodes to the Middle Eocene Climatic Optimum (MECO) in the Southern Indian Ocean (ODP Site 748)
The long-term cooling trend through the Middle to Late Eocene was interrupted at ~40 Ma by a transient 4-5°C warming of the global ocean known as the Middle Eocene Climatic Optimum (MECO; Bohaty and Zachos, 2003). We examined ostracode assemblages through the MECO at ODP Site 748 (~800 m paleodepth) to assess its environmental impact on the benthic ecosystem of the southern Indian Ocean. Ostracode accumulation rates (OARs) markedly rise through the onset and maximum of warming, remain elevated for ~400 kyr, and then markedly decline to pre-warming values. These elevated OARs may be an artifact of the current age-model and broadly coincide with a transient decrease in the wt% coarse-fraction from ~10% to ~5%. Ostracode assemblages through the MECO show no faunal turnover, little change in various diversity measures, and are comprised predominately (~50- 70%) of varying proportions of Henryhowella, Cytheropteron, Bradleya, and Krithe. The relative abundance of Krithe, a cosmopolitan and often predominant genus in Neogene and Quaternary records, progressively decreases from pre-warming percentages of >20% to peak-warming percentages of less than 2% and subsequently recovers to nearly pre-warming percentages over the next 400 kyr - an environmental response consistent with the inferred cryophilic ecology of the genus. Bairdia, a less well- known genus, shows a similar stratigraphic pattern. Ongoing ostracode faunal analyses at higher temporal resolution and spanning a broader stratigraphic range will provide a more complete record of benthic metazoan response to this recently recognized episode of global warming.