Development of an Orbital Calcareous Nannofossil Biochronology for the Paleocene to lower Oligocene

Development of an Orbital Calcareous Nannofossil Biochronology for the Paleocene to lower Oligocene PI: Tim Bralower, Pennsylvania State University Funded by: American Chemical Society – Petroleum Research Fund Grant type: AC Abstract Geologists increasingly focus on changes in Earth History that took place over orbital to millennial time scales Neogene time scales are approaching the level of resolution required to study such changes, but the time scale for the Paleogene and earlier intervals is far too coarse for this new order of problems This project seeks to improve the precision and accuracy of the Paleocene and Eocene time scale by: (1) compiling nannofossil stratigraphic schemes with higher resolution than previously attainable and integrating them with other fossil groups; (2) developing orbital stratigraphy and correlating it with biostratigraphy; (3) refining the calibration between biostratigraphy and orbital stratigraphy with magnetostratigraphy and chemostratigraphy, (4) scaling the time scale using astrochronology and radiometric age estimates; and (5) publishing the time scale and all data used in its development in a user-friendly format on the CHRONOS website so that they are readily available to the community The time scale developed will be widely applicable in investigations of high-resolution Paleocene and Eocene stratigraphy In particular, the level of synchroneity and diachroneity of datums has great significance to interpretations of relative changes of sea level in shelf and slope sections Moreover, the time scale will allow more precise interpretations of changes in carbon cycling and abrupt events during this transitional climate interval Development of an Orbital Calcareous Nannofossil Biochronology for the Paleocene to lower Oligocene Project Overview Microfossil biostratigraphy is often crucial to solving important problems related to ancient global change For example, our understanding of global carbon cycling and relative sea level changes is commonly based on interpretation of sections dated using biostratigraphies of planktonic foraminifers and calcareous nannofossils (e.g., Haq et al., 1987; Aubry et al., 1988; Bralower et al., 1993; Zachos et al., 1993; Hardenbol et al., 1999; Kurtz et al., 2003) Other stratigraphic techniques such as magnetostratigraphy and macrofossil biostratigraphy are also used to obtain time control in studies of global change, but these techniques are generally of more limited applicability Despite their common application, microfossil biostratigraphy suffers from significant limitations in resolution, typically several million years per zone (Figure 1) Lack of resolution limits interpretation of transient events and rapid paleoenvironmental changes that occur over periods less than zonal durations (e.g., Kennett and Stott, 1991; Thomas, 1998; Bains et al., 2000) Moreover, biostratigraphers commonly interpret unconformities where the sequences of datums in a section differs from that in traditional biostratigraphic schemes; these gaps may simply result from differing relative positions of datums as a result of spatial (i.e latitudinal and shelfopen ocean) diachroneity (see discussion in Aubry et al., 1996) To reach their full potential in the investigation of a range of important geologic problems, we need to increase significantly the biostratigraphic resolution of microfossil schemes and constrain the synchroneity/diachroneity of individual datums Orbital chronology or astrochronology offers a unique opportunity to determine the age of biostratigraphic datums approaching the resolution of Milankovitch periodicities, i.e 20-100 kyr (Cramer, 2001; Röhl et al., 2001, 2003) Such orbital tuning of biostratigraphic datums has been carried out in the Neogene (e.g., Hilgen, 1991; Shackleton et al., 1995; Shackleton et al., 1999) but to a limited degree in the older part of the timescale (e.g., Röhl et al., 2001, 2003; Cramer et al., 2003) Remarkably cyclic sedimentary sections have been recovered during recent legs of the Ocean Drilling Program (ODP); these sequences offer a unique opportunity to establish orbital chronology for microfossil biostratigraphic datums and to determine the synchroneity of datums across a wide latitudinal range The proposed investigation is designed to establish an orbital nannofossil chronology for the Paleocene to lowermost Oligocene and to apply the results to a set of important paleoenvironmental problems associated with carbon cycling and sea level change Background Paleogene Nannofossil biostratigraphy: Potential for increased resolution The Paleogene represents a key interval of time in the evolution of nannoplankton This period includes the adaptive radiation of the few taxa that survived the Cretaceous/Tertiary boundary extinction, continued diversification during the late Paleocene and early Eocene climatic optimum, and a slight decrease in diversity during cooiling coincident with the late Eocene and early Oligocene (e.g., Bramlette and Sullivan 1964; Perch-Nielsen 1977; Percival and Fischer 1977; Jiang and Gartner 1986; Pospichal and Wise 1990; Wei and Pospichal 1991; Aubry, 1992; Aubry, 1998) A large amount of work has been carried out on the taxonomy of Paleogene calcareous nannofossils (see review in Perch-Nielsen, 1985; Aubry, 1984, 1988, 1989, 1990) Despite significant attention, there are still numerous uncertainties concerning Paleogene biostratigraphy Even though original calcareous nannofossil zonations (Martini, 1971; Bukry, 1973, 1975; Okada and Bukry, 1980) have proven to be widely applicable, there are individual datums that are difficult to apply (e.g., Bralower in review) Also, the resolution of Paleogene nannofossil biostratigraphy lags significantly behind that of the Neogene and even that of parts of the Cretaceous (Moore and Romine, 1981; Bralower et al., 1993) Yet, the Paleogene is an interval of high species diversity (Haq, 1973) and, therefore, the potential for increased biostratigraphic resolution exists (e.g., Bralower and Mutterlose, 1995) Recent Ocean Drilling Program (ODP) cruises in high southern latitude sites and the subtropical and tropical Pacific and South Atlantic, have recovered expanded and largely continuous Paleogene sequences that have been the targets of a host of biostratigraphic investigations (Pospichal and Wise, 1990; Wei and Wise, 1992; Bralower and Mutterlose, 1995; Bralower et al., 2002a; Lyle et al., 2003; Erbacher et al., 2004; Zachos et al., 2004) Thus there is great potential for improving the quality and resolution of Paleogene nannofossil biostratigraphy For example, the precise range of ~60 zonal and nonzonal events were determined in the Paleogene section on Shatsky Rise, N.W Pacific (Bralower, in review) (Figure 1) One of the most significant questions in Paleogene nannofossil biostratigraphy is whether events are spatially diachronous (e.g., Bralower and Mutterlose, 1995), or whether apparent diachroneity results from errors in published ranges or extremely rare species abundances near the geographic limits of a range With potentially large numbers of datums, we need to consider ways to determine the synchroneity or diachroneity of events over broad areas The most precise method is to utilize the sequence of magnetostratigraphic polarity zones within sedimentary sequences This technique is dependant on the ability to identify clearly and correlate the sequence of polarity zones regardless of their biostratigraphic correlations The synchroneity/diachroneity of a number of Paleogene nannofossil events has been addressed in this fashion by Wei and Wise (1989) and Wei and Wise (1992) Where magnetostratigraphy is unavailable other approaches have to be used to assess the synchroneity/diachroneity of fossil datums One such approach is an application of the technique of Shaw (1964), in which x-y plots of various types of events in two different sequences are used to analyze the sedimentation histories of these two sections This technique has been widely applied in biostratigraphy Differences in order of events among sections may result from inaccuracy when determining an event, differing taxonomic concepts among various workers, and diachroneity of an event in different parts of the ocean Shaw plots also have the potential to yield information about unconformities For example plots of different sections from ODP Leg 198 show numerous events that cluster at a narrow range of depths at Site 1211 suggesting the presence of unconformities or extremely slow sedimentation (Figure 1) Cyclostratigraphy: Elapsed time between datum horizons Because magnetostratigraphy does not generally provide resolution less than about 500 kyr and Shaw plots not provide absolute time control, the most precise way to determine true synchroneity/diachroneity of datums in complete sections is using orbital cyclicity Fluctuations in carbonate and clay content expressed in deep sea sections have been shown to reflect precessional, obliquity and eccentricity cycles in the Earth’s orbit with frequencies ranging from 20 kyr to 400 kyr Such cycles have provided the framework for tuning of Neogene timescales (e.g., Hilgen, 1991) and for identification of the level of synchroneity of datums (e.g., Chepstow-Lusty et al., 1989; Raffi et al., 1993; Flores et al., 1995; Backman and Raffi, 1997; Raffi, 1999; Gibbs et al., 2004) Orbital cycles imprinted in sediments provide accurate and precise chronometers to measure events to perhaps 5-10 kyr resolution Cyclic patterns of oxygen isotopes, carbonate content, and other measures are the workhorses of Plio-Pleistocene stratigraphy (Hays et al., 1976; Raymo et al., 1989; Hilgen, 1991; Imbrie et al., 1984; Shackleton et al., 1990; Shackleton and Crowhurst, 1997) Moving farther back in geologic time, astronomical cycles no longer can be used as a time template, because the details of the planetary orbits are not known precisely (Laskar, 1999) However, orbital cycles still provide a very useful measure of elapsed time between datum horizons given by biostratigraphic, paleomagnetic, or chemostratigraphic studies Herbert et al (1995) showed that it is possible to obtain very good estimates of elapsed time in sediments simply by knowing the mean orbital repeat times For example, individual precessional cycles (modern mean repeat time of 21.2 kyr) can be used to measure time to  16% (~3 kyr) if one assumes that each cycle has a constant repeat time, and to considerably higher precision ( 4-7%) if one averages over 10 or more cycles The relative errors using obliquity (period 41 kyr) and eccentricity (periods at 95 and 404 kyr) cycles are significantly less Recent high-resolution work across the Paleocene/Eocene boundary (Norris and Röhl, 1999; Röhl et al., 2000; Cramer et al., 2003) shows that the paleoceanographic community now accepts the use of orbital tuning to help solve pre-Neogene problems Spectacular, cyclic Paleogene sections have been recovered on a number of recent ODP legs For example, drilling during Leg 198 at four sites on Shatsky Rise recovered a series of nearly complete Paleogene sections dominated by eccentricity cycles (Figure 2) These cycles can be correlated precisely between sites and likely reflect regional fluctuations of a combination of productivity and dissolution of calcareous microplankton PROPOSED RESEARCH: DEVELOPMENT OF AN ORBITAL TIMESCALE FOR THE PALEOCENE TO EARLY OLIGOCENE Stages of time scale construction: The proposed orbital Paleogene time scale will be constructed in a logical fashion in five steps (Figure 3) The foundation of the new scheme will be a high-resolution nannofossil biostratigraphy (Step 1) Step involves the refinement and development of orbital stratigraphy Higher resolution biostratigraphy will improve the correlation of all of the different time scale components, including other fossil biostratigraphies, magnetostratigraphy, chemostratigraphy, orbital stratigraphy, epoch boundaries, and especially radiometric dates (Step 3) Precise ties between biostratigraphy, magnetostratigraphy, and radiometric dates will be the foundation of scaling of the time scale (Step 4) Orbital “tuning” of events will be done within the constraints of magnetostratigraphy and radiometric dating 10 High-latitude Site 690 is critical in this assessment and fortunately has a well-developed magnetostratigraphy that will augment cyclostratigraphy in evaluating the diachroneity of datums However, extreme caution must be taken in avoiding circular reasoning Thus, individual cycles at different sites must either be correlated using distinctive patterns Alternatively, magnetic polarity chron boundaries must be used to correlate cycles If the numbers of cycles in individual polarity chrons match between sites, then cycles might be correlated directly Precessional cycles (Sites 690, 1262, 1267) can be correlated to eccentricity cycles (Sites 1209, 1210) in similar ways especially since several of the latter cycles show evidence for finer-scale alternations (Fig 2) Step Improving Correlation of Different Chronostratigraphic Elements Considerable effort has been devoted recently to the correlation between different time scale components: biostratigraphy, magnetostratigraphy, chemostratigraphy, stage boundaries, and radiometric age estimates (see detailed discussions in Aubry et al., 1988; Berggren et al., 1995; Cande and Kent, 1995; Berggren et al., 2000) The orbital nannofossil chronology developed as a part of this investigation will provide significant improvements in the correlation of the different time scale components Detailed studies of planktic foraminiferal biostratigraphy, magnetostratigraphy, and chemostratigraphy at the sites to be investigated 17 are in progress by shipboard scientists on ODP Legs 198 and 208 Such detailed stratigraphic datasets are available for Site 690 (Stott et al., 1990; Stott and Kennett, 1990) Some of the correlations between nannofossil events and other stratigraphic elements may be achieved via the CHRONOS database (see Step 5) Step Improving Time Scale Construction and Scaling Time scale construction will be based only on the highest precision radiometric ages (e.g., Cande and Kent., 1995); mostly 40 39 Ar/ Ar dates We will use the Cande and Kent (1995) GPTS as the basis of our investigation This time scale uses recently revised block models scaled assuming constant spreading rates between high-quality 40 39 Ar/ Ar date tie-points We will compare this approach with fits that maximize the match of radiometric ("absolute" ages) and orbital ("elapsed" time) data High-quality magnetostratigraphy is available at Sites 690, 1262 and 1267 and in parts of the section at Sites 1209 and 1210 (Spieb, 1990; Zachos et al., 2004; Evans and Channell, in review) Step Making the Time Scale User-Friendly Publication of a more precise time scale does not ensure that it will be applied correctly by all researchers There are numerous cases where zones have been correlated incorrectly in sections, where old boundary definitions have been used, or where workers have relied on a fossil group that provides 18 conflicting age information from other groups Problems will be compounded once a new zonal scheme is defined as the ranges of zonal markers will need to be recognized in published range charts and because the ranges of many of the new zonal markers have not been identified in sections that were studied a long time ago or even rather recently We will make the time scale available to the geologic community via the CHRONOS web site (www.chonos.org) as downloadable Excel files CHRONOS and affiliated database programs (SEDDB and Paleostrat) will provide correlation of datums with geochemical data, chemostratigraphic data, and magnetostratigraphy These programs are funded by NSF and provide stable, long-term, user-friendly archives for storing data and making them available to the geologic community We stress that time scales are constantly in need of revision as new data are acquired Thus all of the basic data collected as a part of this investigation will be archived on the web site This will allow other workers to obtain access to our data, simplifying future modifications We will also publish our results in the open literature APPLICATIONS OF ORBITAL CHRONOLOGY The developed orbital timescale has numerous potential applications in interpreting Paleogene paleooceanography, geochemical cycling and depositional systems Here, I highlight two of these 19 Constraining the controls of the carbon cycle The Paleogene included major modifications in the nature of carbon cycling as a result of changes in climate and oceanic circulation as well as of tectonic forces (e.g., Corfield and Norris, 1996; Zachos et al., 2001; Kurtz et al., 2003) One of the most diagnostic proxies for carbon cycle changes is the benthic and planktic foraminiferal carbon isotope record Penn State MS student Anna Hilting has been compiling carbon isotope data from different DSDP and ODP sites and has produced smoothed curves that will serve as inputs for biogeochemical models designed to constrain the controls of carbon cycles including burial of organic matter, ocean circulation, and continental weathering However, such compilation is inhibited by difficulties in correlation between different sites (Anna Hilting, pers comm.) Moreover, estimating precise accumulation rates of carbon and carbonate is also difficult as a result of poor chronology Much attention has been paid to short-term changes such as the Paleocene-Eocene thermal maximum (PETM) at 55 Ma (e.g., Kennett and Stott, 1991) where input of large quantities of greenhouse gas is thought to have driven profound changes in climate, biotas and geochemical cycling (e.g., Dickens et al., 1995; Koch et al., 1995; Thomas and Shackleton, 1996; Crouch et al., 2001; Thomas et al., 2002) There is a possibility that other “mini-PETM” events also occur in the Paleogene (Thomas et al., 2000; Bralower et al., 2002b; Bohaty and Zachos, 2003) The PETM has detailed orbital chronology (Röhl et al., 2000) and He-3 chronology (Farley and 20 Eltgroth, 2003), but other events have yet to be constrained by highresolution chronology By improving the resolution and precision of chronology, this investigation will help detailed study of the Paleogene carbon cycle We will input these data into compilations of carbon isotope data (A Hilting, MS thesis, in prep) and update modeling studies The chronology of the PETM is stable, however, the orbital timescale will allow detailed investigation of other mini-PETM events such events in the early Paleocene, the early late Paleocene and the early Eocene (Thomas et al., 2000; Bralower et al., 2002b; Bohaty et el., 2003) High-resolution chronology will allow: (1) detailed chronology of these events at different sites, and (2) precise estimates of accumulation rates of C and CaCO3 which will help us understand changes in carbon cycling Unconformities and sea level Considerable debate has occurred over the topic of whether differences in relative order of microfossil datums reflects diachroneity or the presence of unconformities in shelf, slope, and deep-sea sections (e.g., Aubry et al., 1986; Aubry, 1995; Aubry et al., 1996; Miller et al., 1996) High-resolution orbital chronology will allow us to rigorously determine which nannofossil events are synchronous over great distances and which are diachronous This will shed light on whether differences between the sequence of events in different sections result from diachrony or from unconformities Unconformities in shelf and upper slope sections are likely to 21 be related to landward-shifting depocenters during late transgressive and highstand phases of sea level (e.g., Loutit et al., 1988) Deep-sea unconformities are more likely a result of erosion by strong currents (e.g., Miller et al., 1987) Biostratigraphy of most of the relevant sections exists and will not be acquired during the current investigation; rather by determining the true order and level of diachrony of the datums we will be able to evaluate the validity of previous unconformities and their interpretation in terms of sea level change SYNTHESIS OF PROJECT OUTCOMES The Paleogene orbital time scale will provide both higher stratigraphic resolution and precision In addition, the time scale will be made available to the community so that workers can accurately estimate the ages of their samples The benefits of the new time scale include:  Fewer errors in age estimates of samples will result in more accurate interpretations  Greater time scale precision will enable more realistic estimates of timing and rates of geochemical cycling, sea level change, and evolution  Higher chronostratigraphic resolution will allow more detailed investigations of rapid paleoenvironmental changes 22  Refined biostratigraphy and, in intervals, orbital stratigraphy will allow precise correlation between sites enabling more accurate global reconstructions in environmental change LOGISTICAL CONSIDERATIONS Research Team: We have assembled an experienced team of scientists to collaborate in the proposed effort to rebuild the Paleocene to early Oligocene time scale The investigation will be directed by P.I Timothy Bralower at Pennsylvania State University (PSU) Bralower will oversee integration of the various time scale elements Nannofossil biostratigraphy will be carried out by a graduate student research assistant at PSU An undergraduate technician at PSU will be responsible for preparation of smear slides We will also collaborate with Isabella Raffi who is working on the Paleogene nannofossil biostratigraphy of ODP sections from ODP Leg 208 (Walvis Ridge, South Atlantic) The orbital stratigraphy which will be an integral part of the development of the Paleocene to early Oligocene time scale will be carried out by Ursula Röhl at Bremen University The investigation will be closely liaised with continuing studies of the biostratigraphy of other fossil groups including planktic foraminifera by Isabella Premoli Silva and Maria Rose Petrizzo (University of Milan), and D Clay Kelly (U Wisconsin) METHODS 23 Biostratigraphic analysis: Nannofossil biostratigraphy will be carried out on high-resolution sample collections from the relevant ODP sections In general sample spacing will be approximately four to five samples per orbital cycle Standard smear slides will be observed in polarizing light microscopes at PSU The microscopes are hooked up with digital cameras so images can be stored We already have biostratigraphies on samples at a resolution of 1.5 m We estimate that an additional 1000 samples will be studied from Sites 690, 1209, and 1210 Astrochronology: Detailed astrochronology is available for Sites 1209, 1210, 1262, and 1267 (Röhl et al., in review) We will send cores from Site 690 to Bremen University for detailed XRF core scanning and magnetic susceptibility measurements These techniques are documented in Röhl and Abrams (2000) Project Schedule: We propose a three-year program Years and will be focused on acquiring data, developing biostratigraphies, acquiring orbital stratigraphic data and refining correlations Year will be devoted to correlation, time scale construction, publication of all information on the CHRONOS web site, and 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