Revised Miocene splice, astronomical tuning and calcareous plankton biochronology of ODP Site 926 between 5 and 14.4 Ma
Highlights
► The stratigraphy of ODP Site 926 is improved. ► The orbitally tuned age model of ODP Site 926 is improved. ► The new age model is used to improve the calcareous plankton biochronology.
Introduction
The multiple hole drilling strategy is undoubtedly one of the greatest innovations in deep-sea drilling since the invention of the Kullenberg piston coring device (Kullenberg, 1947, Ruddiman et al., 1987), as it allows the recovery of complete if not continuous successions suitable for high-resolution paleoclimatic studies using detailed (astronomical) age models. ODP Site 926 was drilled at Ceara Rise (see Fig. 1) during Leg 154 with the main objective to reconstruct the history of the Atlantic Ocean and Amazon River output during the late Cenozoic (Shipboard Scientific Party, 1995a). Continuous successions were constructed by splicing of cores from the three (A–C) holes; some distortions were identified and excluded from tuned records (Shackleton and Crowhurst, 1997).
The spliced records from Leg 154 formed the backbone of an astronomically tuned time scale for the entire Neogene and Oligocene (Bickert et al., 1997, Shackleton and Crowhurst, 1997, Tiedemann and Franz, 1997, Weedon et al., 1997, Shackleton et al., 1999), and were used for the calibration of bioevents to obtain an age for the Oligocene/Miocene boundary at DSDP Site 522 (Shackleton et al., 2000). Unfortunately no magnetostratigraphy could be established for the Ceara Rise Sites. Therefore the astronomically tuned timescale provides only an accurate timeframe for bioevents (Backman and Raffi, 1997, Chaisson and Pearson, 1997, Pearson and Chaisson, 1997, Weedon et al., 1997, Shackleton et al., 1999, Turco et al., 2002). The biochronology from this time scale was incorporated into the Neogene chapter of the Standard Geological Time Scale (Gradstein et al., 2004, Lourens et al., 2004). In addition, the time scale provided a solid base for paleoclimatic investigations including among others the Oligocene/Miocene boundary interval (Paul et al., 2000, Zachos et al., 2001) and was used to constrain values for tidal dissipation (Td) and dynamical ellipticity (dE) in the La93 solution (Laskar et al., 1993, Pälike and Shackleton, 2000). In 2004, a minor adjustment of the time scale was proposed as part of the Astronomical Tuned Time Scale 2004 based on tuning to the La2004 (Laskar et al., 2004) solution with present day values for Td and dE (Lourens et al., 2004). More recently, updated biochronologies for Leg 154 Sites were presented by Raffi et al. (2006) and Wade et al. (2011).
Shackleton and Crowhurst (1997), when evaluating their orbital tuning of the Miocene by complex amplitude demodulation, state that “correlation errors of one or two precession cycles could be present” and “the evaluation of additional data […] may improve the accuracy of the correlations to the orbital template”. Around 10 Ma and between 11 and 12.5 Ma, the results of complex demodulation of the precession related physical property signal from ODP Site 926 differs from eccentricity, and a shift by one 100-kyr eccentricity cycle seems possible from ~ 12.5 to 13.5 Ma (see Shackleton and Crowhurst, 1997, their Fig. 3). These inconsistencies may be the result of either an erroneous splice or a wrong tuning (potentially as a result of an erroneous splice). Also for other sites it has become apparent that the shipboard splice may require revision (e.g. Evans et al., 2004, Westerhold and Röhl, 2006, Westerhold and Röhl, 2009). For this reason, a detailed re-evaluation of the ODP Site 926 stratigraphy is presented and the problems encountered in the original splice are discussed and solved. In some intervals with a complicated stratigraphy the construction of a continuous (cyclo)stratigraphy requires a detailed comparison with other Sites from ODP Leg 154 (Sites 927 and 928). A new splice for ODP Site 926 is constructed; the resolution of available physical property data sets is improved by carrying out high-resolution (1 cm) colour and magnetic susceptibility (MS) core scanning. Another potential complication for orbital tuning of this time interval is that astronomical target curves, with present-day values for the Td and dE, do not produce a good fit with the cycle patterns in the Mediterranean for intervals older than ~ 10 Ma (in detail shown by Hüsing et al. (2007) and applied by Mourik et al. (2010) and Hüsing et al. (2010)). Hence, different values for Td and/or dE are applied where they give a better fit with physical property data, without the intention to revise values for Td/dE from this record in detail. Finally, improved ages for calcareous plankton events are presented.
Section snippets
Setting and cores
ODP Leg 154 Sites were drilled at Ceara Rise located in the western equatorial Atlantic northeast of the Amazon outflow (see Fig. 1). A depth transect was drilled to reconstruct paleoceanography, especially the influence of North- and South Atlantic bottom waters. Generally the deeper Sites show relatively strong carbonate dissolution and are therefore condensed. Sediments from Ceara Rise date back to the early Maastrichtian (Perch-Nielsen et al., 1977, DSDP Site 354); Neogene sediments have
Methods
High-resolution MS- and image-scanning was carried out employing the GEOTEK linescan imaging device at the MARUM in Bremen. Control software supplied by GEOTEK was used. Cores were manually cleaned of fungi and other visible contamination before scanning the half-cores without foil for colour-, and with foil for MS-scans (to prevent the MS measuring apparatus from being contaminated upon contact with the core during measurements). The MS is given in 10− 5 SI units. From the scanned images colour
Revising the composite record
Since the astronomical control of the Miocene Ceara Rise record is already established (Shackleton and Crowhurst, 1997), the splice of Site 926 is evaluated by comparing (1) the core photographs, MS and lightness (L*) data of the individual holes, and (2) the cyclostratigraphic patterns with the normalised precession (p) minus half the tilt/obliquity cycle (t) of the La2004 solution. To improve the existing splice for Site 926, physical property data were compared with records from Sites 927
Proxy records
High values of MS and dark colours both indicate relatively low carbonate content and hence a relatively high fraction of terrestrial matter in the Ceara Rise sediments. In most intervals these proxy records covary, but in some intervals one of these parameters reveals higher amplitude variations than the other. Both MS and GS records are used for tuning; the colour data reflects the visible colours of the cores taken at Ceara Rise (see Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14). Maxima of
Quality assessment of the tuning and consequences for stratigraphy
Following Shackleton and Crowhurst (1997), the quality of the tuning is assessed by comparing the amplitude modulation of the precession related signal to the orbital eccentricity (Fig. 16). The fit between the amplitude of physical property data and the eccentricity is convincing throughout the record. In some intervals, especially where eccentricity has no strong expression of the ~ 100 kyr component (about 7.2–7.7, 9.3–9.7, and 14.2–14.4 Ma), the amplitude variations of the precession related
Conclusions
A reliable and comprehensible stratigraphy was generated for ODP Site 926, using high resolution MS, colour and XRF core scanning records and a comparison with other ODP Leg 154 Sites. The revised splice of ODP Site 926 is tuned to the La2004 and 2001b10 solutions applying the previously established phase relations (Bickert et al., 1997, Shackleton and Crowhurst, 1997, Tiedemann and Franz, 1997, Weedon et al., 1997, Shackleton et al., 1999). Values for tidal dissipation are varied, while
Acknowledgements
This is a contribution made within the GTSnext project (www.gtsnext.eu). The research leading to these results has received funding from the [European Community's] Seventh Framework Programme [FP7/2007–2013] under grant agreement no. [215458]. This research used data provided by IODP; funding was provided by NWO and the Deutsche Forschungsgemeinschaft. We thank Walter Hale and Alex Wülbers (BCR) for core handling and Vera Lukies for technical support during the generation of colour scans at the
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