PP32A-01 INVITED
Late Cretaceous Subtropical Paleotemperatures from 'Glassy' Foraminifera in East Africa and implications for sensitivity in Greenhouse Climates Models
Coring in southern coastal Tanzania has recovered extended Late Cretaceous intervals containing diverse planktonic foraminiferal assemblages exhibiting 'glassy' preservation (preservation where there is no apparent test recrystallization at the submicron scale). Previous δ18O data for specimens with 'glassy' preservation from Tanzania and elsewhere have demonstrated Late Cretaceous tropical temperatures were sometimes much higher than temperatures in the modern tropics. However, these data come largely from short stratigraphic intervals and/or from samples with unusual lithologic and faunal characteristics raising the possibility that the results reflect temperatures or seawater compositions that were not typical for the time. We are attempting to address this uncertainty by generating long records from multiple taxa preserved in unremarkable outer shelf claystones and silty claystones. For a section spanning most of the Turonian (Whiteinella archaeocretacea through Helvetoglobotruncana helvetica Zones), δ18O results are relatively stable with warmest temperatures estimated at ~34°C (assuming a seawater δ18O value of - 1‰SMOW). The planktic δ18O gradient is <1‰ among most samples. Species of Whiteinella consistently yield relatively low values whereas species of Dicarinella yield relatively high values. Our warmest calculated paleotemperatures are several degrees cooler than some Cretaceous tropical temperature estimates but are still ~6°C warmer than summer seawater temperatures in comparable modern settings. The data show no coordinated δ18O shifts that might indicate a Turonian glacial episode. In terms of the Cretaceous greenhouse climate, model experiments reproducing the temperatures calculated require atmospheric CO2 and/or CH2 higher than most estimates for the Cretaceous atmosphere supporting the conclusion that sensitivity to greenhouse gas forcing, at least on long time scales, is too low in the current climate models.
PP32A-02
Influence of Geography and Carbon Dioxide on Oceanic Circulation and Seawater δ18O
Oxygen isotope paleo-thermometry is one of the most powerful and ubiquitous tools for reconstructing past climate. It is well known that oxygen isotope paleotemperature estimates are subject to assumptions about past seawater δ18O. Previous modeling and empirical studies have shown that oceanic circulation was different during times when geography and CO2 differed from today; however, the influence of circulation on seawater δ18O and isotope paleo-thermometry has not been systematically investigated. To test the influence of CO2 and geography on oceanic circulation and seawater δ18O, we have conducted modern and Cretaceous simulations with a coupled ocean-atmosphere general circulation model, the GENESIS-MOM model. The model includes the ability to track water isotopes in and between the atmosphere, land surface, sea ice, and ocean. We report on two sets of simulations: 1) mid-Cretaceous experiments with varying levels of CO2 (355ppm, 1120ppm, 2240ppm and 3360ppm); and 2) modern experiments with CO2 levels of 355 and 2240ppm). Our model results indicate that changes in atmospheric CO2 and geography can significantly affect oceanic circulation and seawater δ18O distribution. In both experiments, the global meridional overturning circulation (MOC) consists of two overturning cells, located in the northern and southern hemisphere. However, the source of northern hemisphere sinking is geography dependent. In contrast to the modern, in the Cretaceous, northern MOC occurs predominantly in the North Pacific Ocean. These differences in the source of overturning lead to substantial differences in the shape of seawater δ18O stratification. Both Cretaceous and modern experiments respond similarly to increased CO2. With greater CO2, the southern branch of the global meridional overturning circulation (MOC) decreases in size and amplitude. However, the northern MOC changes very little. In addition, with greater CO2: 1) low-latitude zonal surface and intermediate seawater δ18O is depleted by ~0.1-0.2 permil with local depletions up to 1 permil; 2) high- latitude zonal surface and intermediate seawater δ18O is enriched by up to several per mil. These changes in seawater δ18O are partially attributed to switches in the location of deep-water formation from δ18O-depleted, high-latitude sites to δ18O-enriched subtropical areas. An additional factor is the reduction in equilibrium fractionation in a higher CO2 (warmer) world, which raises high-latitude precipitation δ18O by up to 10 permil. In sum, we conclude that geography and atmospheric CO2 influence ocean circulation and seawater δ18O distribution. Neglecting the effect of these factos on seawater δ18O can significantly bias oxygen isotope paleotemperatures.
PP32A-03
The Nd-Isotopic Composition of Late Cretaceous Bathyal–Abyssal Seawater From Fossil Fish Skeletal Debris
There is currently very little proxy data available for determining the inter-ocean mixing of deep-water masses during the Cretaceous, and thus uncertainty remains as to the importance of deep-water circulation in latitudinal heat transport and bottom-water oxygenation for that time. A solution lies in exploiting a geochemical water-mass tracer, such as the neodymium (Nd) isotopic composition of seawater. It has been shown that the distinct differences in the Nd-isotopic composition observed in modern deep and intermediate waters have persisted since the early Cenozoic, but currently our knowledge of the Cretaceous oceans is poor. Most of the existing Nd-isotope data for the Cretaceous are from shallow-water masses on the continental shelves of the Tethyan and Atlantic Oceans. It has previously been shown that biogenic apatites record the Nd-isotopic composition of bottom-waters during an early diagenetic reaction at the sediment- water interface. We present Nd-isotope data from fish teeth and skeletal debris picked from deep-ocean sediments recovered by DSDP and ODP drilling in the North and South Atlantic, Indian and Pacific Oceans. The sites chosen for this study were all deposited at bathyal-abyssal water depths. In conjunction with other recent studies, our data establish that the Pacific Ocean has likely maintained a constant range of Nd- isotopic values between ~-5 and -3 since at least 135 Ma. The data from the North Atlantic, South Atlantic and Indian Oceans show that bottom-waters in these basins had relatively radiogenic Nd-isotopic compositions for much of the mid-Cretaceous (~-8 to ~-5), before shifting to less radiogenic values (<-9) between 85 and 75 Ma. These data can be interpreted as reflecting either a decrease in the influence of Pacific waters via circum-equatorial surface currents, or as an increase in the contribution of a deep-water mass with a highly radiogenic value. We discuss the implications of these data for understanding the Late Cretaceous evolution of the Atlantic basins and for future higher resolution studies.
PP32A-04 INVITED
Quantifying Ocean Acidification During the PETM
The ocean will absorb increasing amounts of fossil fuel CO2 in the future, with the pH of surface waters decreasing by up to 0.5-0.6 pH [Caldeira and Wickett, 2003]. The Palaeocene-Eocene Thermal Maximum (PETM) has been suggested as a close palaeo-analogue for future climate change and ocean acidification [Zachos, et al., 2005] as the carbon release is thought to be comparable to that possible over the coming centuries. However, a prerequisite for the use of evaluated ecological response during the PETM as a constraint on future impacts on ecosystems of acidification due to fossil fuel burning is knowing how the paleo-pH changed at this time. The boron isotopic composition δ11B of foraminiferal calcite is a proxy for pH [Hemming and Hanson, 1992], but lack of sufficient amounts of un-recrystallized, singe species, foraminiferal calcite from this time interval has prevented the application of established pH proxies. We use in-situ, high-spatial resolution secondary ionization mass spectrometry (SIMS) to characterize the δ11B and B/Ca across the PETM in the benthic foraminifer Oridorsalis umbonatus at deep-sea Maud Rise (Site 690B) and shelf-depth Lenticulina sp. at Bass River. Mg/Ca indicates a two-step temperature increase from 12.7°C to 18.5°C, in agreement with previous work at Maud Rise. Since the boron isotope composition of Paleocene seawater is unknown, we applied the pH estimated by an Earth system model as a starting value. The reconstructed pH record across the PETM shows a large, two-step reduction coeval with temperature rise, with a recovery period to pre-event values significantly more drawn out than that of the isotopic composition of the ocean. Caldeira, K., and M. E. Wickett (2003), Nature, 425, 365. Hemming, N. G., and G. N. Hanson (1992), GCA, 56, 537-543. Zachos, J. C., et al. (2005), Science, 308, 1611-1615.
PP32A-05
Paleo-redox Reconstructions Across the Paleocene-Eocene Thermal Maximum
Manganese and Uranium enrichment factors (EF) relative to crustal averages from nine Deep Sea Drilling Project and Ocean Drilling Program sites are presented to examine paleo-redox changes across the Paleocene-Eocene Thermal Maximum (PETM). We use geochemical tracers to infer the bottom-water oxygen concentrations during this abrupt warming event. Determining redox changes will test the hypothesis that low oxygen concentrations caused the benthic foraminifera extinction event as well as contribute to our understanding of where the source of isotopically light carbon was released. The comparison of Mn EF before and after a reductive cleaning procedure allows us to determine the presence of Mn-oxides or Mn-carbonates. Mn EF representing Mn oxides and no U EF suggest oxygenated bottom-waters. Mn-carbonates and U EF suggest reducing conditions and an early diagenetic phase. We will present our results relative to the orbitally tuned age model of Site 1263. Mn EF range from 1-9 in Atlantic sites, 1-35 Southern Ocean sites, and 5-400 in Pacific sites. U EF range from 1-5 in Atlantic sites, 1-90 in Southern Ocean sites, and crustal averages EF =1 in Pacific sites. Our initial results indicate reducing conditions prior, during, and in the recovery of the PETM at intermediate depth sites in the Atlantic and Southern Ocean while the Pacific sites remain oxygenated. Reducing bottom- water conditions in the Atlantic and Southern Ocean may have contributed to the benthic foraminifera extinction event. Release of methane in the oceans oxidizes to carbon dioxide, depleting bottom-water oxygen concentrations. Because our sites in the Atlantic have the most reducing bottom-waters, we suggest the Atlantic Ocean could have been the source for isotopically light carbon release during the PETM.
PP32A-06
Statistical Analysis of Climate and Biotic Variability During the Paleocene Eocene Thermal Maximum
The Paleocene Eocene Thermal Maximum (PETM, 55Ma) was characterized by abrupt warming, wholesale turnover of plankton assemblages, mass extinction on the sea floor, and dramatic changes in the global carbon cycle. The interval has been studied at increasing levels of resolution requiring centimeter-scale sampling in low sedimentation rate marine sections. In addition, dissolution results in highly condensed section or possible unconformities at the base of deep-sea PETM intervals. As a result, many PETM records are characterized by sizeable variation in sample spacing in terms of depth and age. The large variations in sample spacing introduce nontrivial methodological challenges if one wants to characterize how the variability of climate and plankton communities changes over the PETM interval. Here we develop a Bayesian inversion technique that accounts for the effects of variable sample spacing, autocorrelated residuals, and the uncertainties about age-estimates and the onset and termination of the PETM interval. We apply this technique to PETM stable isotope and microfossil assemblage data (e.g., the intensively studied Ocean Drilling Program Site 690, Maud Rise, Southern Ocean). This technique allows us to determine, for example, the full nonparametric posterior probability density function of short term climate variability over the PETM interval. We use this technique to place probabilistic limits on the rate of warming and cooling at various stages of the PETM and to compare them between sites and with other intervals of abrupt climate change.
PP32A-07 INVITED
Organic and inorganic proxies for changes in the East African hydrological regime at the Palaeocene-Eocene Thermal Maximum
Recent onshore drilling expeditions in Tanzania have yielded sediments that span much of the Late Cretaceous and Paleogene and show exceptionally good preservation of both calcareous microfossils and organic matter. The interval of the Paleocene-Eocene Thermal Maximum (PETM), constrained by both carbon isotopic records and biostratigraphy, was recovered at Tanzania Drilling Project Site 14, in 2004. Planktonic foraminifera (Subbotina) show a negative carbon isotope excursion (CIE) of approximately 4.5 permil, and higher plant derived n-alkanes show a ca. 6.5 permil negative shift, both significantly larger than has been observed for most deep sea foraminifer records. Exploiting this expanded and well preserved section, lipid biomarker distributions and their hydrogen isotopic compositions were used to examine East African vegetation and hydrological responses to the global change occurring at the PETM. Although total organic carbon contents decrease, the abundances of both higher plant (n-alkanes, n-alkanoic acids) and soil bacterial (glycerol dialkyl glycerol tetraethers) biomarkers increase dramatically at the onset of the PETM, suggesting an increased discharge of fluvial sedimentary organic matter. Similarly, mineralogical indicators of terrestrial input – including Ti/Al and Si/Al ratios, quartz contents and, notably, the proportion of kaolinite – also increase at the onset of the CIE. However, the hydrogen isotopic composition of the higher plant biomarkers shift to more deuterium-enriched values, suggesting a more arid and/or hotter, rather than a more humid, environment. This evidence collectively suggests an East African PETM climate characterised by overall arid conditions punctuated by intense, perhaps seasonal, storm events. Such data match observations from other locations but are not consistent with a humidity effect amplifying the CIE recorded by terrestrial biomarkers. Thus, the Tanzanian record further suggests that the PETM CIE could have been larger than initially thought and provides evidence for dramatic hydrological change at this time.
PP32A-08
Reconstructions of Climatic and Topographic Gradients in the Sierra Nevada during the early Eocene using compound-specific stable isotopes and organic molecular temperature proxies
Terrestrial sediments from the early Cenozoic provide important records of continental temperature gradients during periods of high global temperatures and PCO2, yet these records are often interpreted in terms of either climatic or orographic effects. As a result, linking models of past global climate with terrestrial temperature information is hampered by a lack of detailed information on paleoelevation. Organic molecular proxies provide new tools to help distinguish between climatic and orographic information in terrestrial sediments. For this study, we used organic molecular proxies to determine paleoelevation and paleotemperature gradients in the Sierra Nevada during the early Eocene warm period. Specifically, we analyzed the hydrogen and carbon isotopes of n-alkanes in bulk sediments and fossil angiosperm leaf cuticle in overbank deposits of major drainages of the Eocene Sierra Nevada to quantify the changes in the isotopic composition of leaf water that potentially reflect changes in paleoelevation. We coupled this data with paleotemperature measurements across this landscape using the MBT/CBT organic molecular temperature proxy, as well as thermodynamic models of the isotopic evolution of rainwater during orographic ascent. Hydrogen isotopes of n-alkanes systematically decrease by more than 30 per mil with distance from the Eocene shoreline and temperature data show a decrease of more than 8 degrees across this ancient range, with temperatures near the ocean margin exceeding 22 degrees. Isotopic and temperature data provide evidence for steep topography and high temperature lapse rates at the California margin during the early Eocene. These results support model estimates of temperature and relative humidity for the early Eocene based on a four time doubling of atmospheric CO2.