Ocean Drilling Program (ODP) Site 982 represents a key location for
understanding the evolution of climate in the North Atlantic over the past
12 Ma. However, concerns exist about the validity and robustness of the
underlying stratigraphy and astrochronology, which currently limits the
adequacy of this site for high-resolution climate studies. To resolve this
uncertainty, we verify and extend the early Pliocene to late Miocene
shipboard composite splice at Site 982 using high-resolution XRF core
scanning data and establish a robust high-resolution benthic foraminiferal
stable isotope stratigraphy and astrochronology between 8.0 and 4.5 Ma.
Splice revisions and verifications resulted in
The North Atlantic remains a crucial location for understanding past climate change. It has been a prime region of deep water formation since at least the middle Miocene (Woodruff and Savin, 1989) and a key location for correlating between the progressively enclosed Mediterranean Basin and the major oceanic basins (Andersson and Jansen, 2003; Hodell et al., 2001). North Atlantic Ocean Drilling Program (ODP) Site 982, situated on the Rockall Plateau, was recovered to document the evolution of intermediate water circulation in the North Atlantic Basin (Shipboard Scientific Party Leg 162, 1996). Palaeoclimate records generated at this location have been fundamental to improving our understanding of climate for the past 12 Ma (Bolton et al., 2011; Herbert et al., 2016; Hodell et al., 2001; Hodell and Venz-Curtis, 2006; Khélifi et al., 2014; Lawrence et al., 2009; Lisiecki and Raymo, 2005; Venz et al., 1999; Venz and Hodell, 2002). In particular, the late Miocene benthic stable isotope stratigraphy and astrochronology (Hodell et al., 2001) has been used as the stratigraphical and/or astrochronological foundation for a wide range of studies (Bickert et al., 2004; Drury et al., 2016; Herbert et al., 2016; Hodell and Venz-Curtis, 2006; Kontakiotis et al., 2016; De Vleeschouwer et al., 2017). However, there is an ongoing debate about the presence of potential stratigraphic issues in the Pliocene successions of Site 982 (Khelifi et al., 2012; Lawrence et al., 2013) and concerns exist that individual cycles may be missing from the late Miocene isotope stratigraphy (Bickert et al., 2004; van der Laan et al., 2005, 2012); these concerns remain unresolved.
Site 982 is characterised by high carbonate content, which is above 90 % for most of the Pliocene and Miocene (Hodell et al., 2001; Shipboard Scientific Party Leg 162, 1996). The standard shipboard physical properties measurements, such as spectral reflectance, gamma ray attenuation (GRA) bulk density, magnetic susceptibility and natural gamma radiation are difficult to use for stratigraphic correlation in such high carbonate settings. At Site 982, the early Pliocene to late Miocene composite splice was constructed using solely GRA bulk density and spectral reflectance in the 650–700 nm band; however, the shipboard composite record for this interval has never been independently verified. The stratigraphic discrepancies between Site 982 and other deep-sea and Mediterranean sites could originate from errors in the shipboard composite record. Verification of the shipboard composite splice is essential to resolve these issues. Non-destructive X-ray fluorescence (XRF) core scanning is an ideal technique for recovering the high-quality and high-resolution data that are required for composite splice verification.
Here, we aim to verify and establish a robust high-resolution stable isotope
stratigraphy and astrochronology at Site 982 that can continue as a reference
section for North Atlantic climate change. Firstly, we use XRF core scanning
to successfully test and extend the early Pliocene to late Miocene shipboard
splice at Site 982. Following splice revision, we generate new benthic stable
isotope data to fill gaps in the existing stable isotope stratigraphy at Site
982. Using the new complete benthic
Map showing the location of ODP Sites 982 and 926, as well as the Ain el Beida quarry section. New data were produced at the red site location; yellow site locations indicate the use of published data.
This study utilises early Pliocene to late Miocene sediments recovered from
ODP Site 982 (Fig. 1; 57
XRF core scanner data were collected every 2 cm down-core over a 1 cm
To assist with splice verification and data interpretation, core composite
images for Site 982 were compiled from core table photos using Code for Ocean
Drilling Data (CODD –
To fill the observed gaps in the original Hodell et al. (2001) stable isotope
stratigraphy and re-establish a robust stratigraphy for the North Atlantic,
an additional 263 samples were taken for isotope analysis between 200 and 280
revised m composite depth (rmcd). These samples were selected to fill the
11 m of missing sections in the original stable isotope stratigraphy and overlap with the original data to ensure the two datasets could
be integrated. Each measurement consisted of one to six translucent specimens of
Revised and shipboard offsets for ODP Site 982 (Holes A–D). Bold case indicates changes compared to shipboard splice. Italic case indicates where the shipboard splice was shifted to accommodate new offsets higher in the section. m b.s.f. stands for metres below sea floor.
Continued.
Continued.
Revised splice for ODP Site 982. Bold case indicates a new splice tie point. Italic case indicates an old splice tie point with revised composite depth.
Panels showing
The natural logarithm of the Si intensity data (ln(Si)) shows significant
variability on both
The XRF ln(Si) data and core photos reveal mismatches, overlaps and gaps
in the shipboard composite section (Figs. 2a, 3b, S2 and S3). Following
splice revision, the agreement of the composite core photos and ln(Si) is
remarkable between the three holes (Figs. 2b and 3a; S2 and S3). The shipboard
composite splice is altered and verified between 120.13 and 271.58 revised
metres composite depth (rmcd) and additionally extended to 280.23 rmcd
(Fig. S2; Tables 1 and 2). The splice revisions resulted in
Overview of the new XRF and updated stable isotope datasets on the
revised composite depth at Site 982. The complete benthic
On the revised composite splice, the ln(Si) data show strong
Using the new isotope data produced here (Fig. 3b), the resulting Site 982
benthic
Multitaper method (MTM) power spectra of the complete stable isotope
stratigraphies (8.0–4.5 Ma) generated using Astrochron (Meyers, 2014; for
the specific code, please see the Supplement) for
The original astrochronology presented in Hodell et al. (2001) was
constructed by respectively tuning the benthic
Constructing an astrochronology at Site 982 was an iterative process. To
establish whether the strong cyclicity observed in the benthic
Fine-tuned astrochronology for Site 982, with minimal tuning tie
points indicated in purple and fine-tuning tie points indicated in grey.
To construct the astrochronology for Site 982, we correlated the benthic
Initially, a minimal tuning strategy was applied (see also Holbourn et al.,
2007; Drury et al., 2017), where distinctive
Surprisingly, following splice revision, the new, complete, high-resolution
(
The Site 982
Between 6.4 and 5.4 Ma, the
Site 982 is a key reference section for palaeoceanographic and palaeoclimate
study, as it is the only deep-sea location in the North Atlantic with
high-resolution benthic oxygen isotope stratigraphies for the late Miocene to
early Pliocene. As such, it is frequently used as a North Atlantic end member
to investigate past deep-sea circulation (Bickert et al., 2004; Drury et al.,
2016; Hodell et al., 2001; Hodell and Venz-Curtis, 2006; Khélifi et al.,
2014). Additionally, it reflects a crucial link to tie regional records of the
enclosed Mediterranean Basin, such as those from Ain el Beida and Salé
Briqueterie, to the global signal preserved more strongly in deep-sea
sediments (Hodell et al., 1994, 2001; Kontakiotis et al., 2016; van der Laan
et al., 2005, 2012). However, although the long-term trends at Site 982
remained useful for investigating long-term, million-year changes in
deep-water circulation patterns, on inspection at higher-resolution,
misalignments between distinct excursions become apparent (Bickert et al.,
2004; van der Laan et al., 2005). Bickert et al. (2004) constructed their ODP
Site 999 age model by correlating their isotope data with the isotope
stratigraphy from Site 982; however, to account for obvious mismatches
between Site 999 and nearby ODP Site 926, they had to adjust the original
Site 982 astrochronology to resolve some inconsistencies with respect to
obliquity. The isotope stratigraphy at 926 has recently been extended back to
Comparisons between late Miocene Mediterranean records and Site 982 also
raised concerns about the validity of the original astrochronology and
splice. A high-resolution comparison between stable isotope data from Ain el
Beida (AEB; north-western Morocco) and Site 982 indicated that there were both
mismatches in the number of cycles present at both locations, as well as
inconsistencies between the age models with respect to the age of clearly
identical cycles (Fig. 8c; van der Laan et al., 2005). Again, following the
splice revisions and new orbital tuning, the agreement between these two
records is radically improved (Fig. 8d). The misalignment in age between
TG12/14 (
The new stratigraphy does not resolve all misalignments between Site 982 and AEB. Between TG12/14 and TG20/22, van der Laan et al. (2005) identified an additional cycle at Site 982 that was not visible in AEB (Fig. 8c). In the new stratigraphy, the agreement between Site 982 and AEB is still not perfect, with Site 982 displaying one to two cycles more than AEB (Fig. 8d). Low sedimentation rates at AEB in this interval (van der Laan et al., 2005), as well as lower sampling resolution, could indicate cycles may be missing at this location. Splice uncertainty at Site 982 is unlikely to be the cause of this discrepancy, as the XRF core scanning splice verifications show that the splice is robust in this interval. Also the interval where Site 982 shows additional cycles compared to AEB occurs within a single core. Although sedimentary disturbances cannot be excluded and are difficult to identify in low-contrast deep-sea pelagic sediments with high carbonate content, the stable isotope and XRF core scanning datasets support that the sedimentary succession at Site 982 in this interval is undisturbed.
Our study has resolved many of the issues surrounding the stratigraphy and astrochronology at Site 982. New cycles were identified, which correlate well with isotope records from the Mediterranean and equatorial Atlantic. Past attempts have been made to extend the Marine Isotope Stage (MIS) identification scheme first developed by Shackleton et al. (1995) into the late Miocene (Drury et al., 2016; van der Laan et al., 2005). The identification of the new cycles at Site 982 highlights the need for a redefinition of the current MIS scheme for this interval. Considering the number of high-resolution, robust benthic foraminiferal stratigraphies that now exist for the latest Miocene (Bickert et al., 2004; Drury et al., 2016, 2017; Vidal et al., 2002; Westerhold et al., 2005), a redefinition of late Miocene MIS should soon be attempted, following the global approach applied for the Pliocene by Lisiecki and Raymo (2005).
Here, we present a new high-resolution benthic stable isotope stratigraphy
and astrochronology for ODP Site 982 between 8.0 and 4.5 Ma. Splice revisions using XRF core
scanning data resulted in
Around 6.4–6.3 Ma, the splice revisions also reveal a significant
The revised splice and astrochronology resolve key stratigraphic issues that
have hampered correlation between Site 982 and other locations. Comparisons
of the revised Site 982
All data, including a Site 982 CODD experiment and all
composite core images, are available on the open-access PANGAEA database at
The project was designed by AJD, TW and DH. AJD, TW and UR contributed to the XRF dataset, AJD and TW generated the new stable isotope data and AJD cut the core images from the core box photos. All authors contributed to data collection, quality control and analysis. All authors were involved in scientific discussions. AJD and TW wrote the manuscript with additional contributions from all authors.
The authors declare that they have no conflict of interest.
This research used data acquired at the XRF Core Scanner Lab at the MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany. We thank Vera Lukies (MARUM) for assistance with XRF core scanning, Henning Kuhnert and his team (MARUM) for stable isotope analyses, Alex Wülbers and Walter Hale (IODP Bremen Core Repository) for core handling, and Barbara Donner (MARUM) for providing foraminiferal expertise. We additionally thank two anonymous reviewers and Pierre Francus for their constructive reviews, which helped to improve the manuscript. This research used samples and data provided by the Ocean Drilling Program (ODP), sponsored by the US National Science Foundation (NSF) and participating countries. The Deutsche Forschungsgemeinschaft (DFG) provided financial support for this research (We5479/1). The article processing charges for this open-access publication were covered by the University of Bremen. Edited by: Pierre Francus Reviewed by: two anonymous referees