The Middle Eocene Climatic Optimum (MECO) at ca. 40 Ma is one of the largest of
the transient Eocene global warming events. However, it is relatively poorly
known from tropical settings since few sites span the entirety of the MECO
event and/or host calcareous microfossils, which are the dominant proxy
carrier for palaeoceanographic reconstructions. Ocean Drilling Program (ODP)
Pacific Ocean Site 865 in the low-latitude North Pacific (Allison Guyot) has
the potential to provide a useful tropical MECO reference, but detailed
stratigraphic and chronological constraints needed to evaluate its
completeness were previously lacking. We have addressed this deficit by
generating new high-resolution biostratigraphic, stable isotope, and X-ray
fluorescence (XRF) records spanning the MECO interval (∼38.0–43.0 Ma) in two holes drilled at Site 865. XRF-derived
strontium / calcium (Sr/Ca) and barium / strontium (Ba/Sr) ratios and Fe count
records allow correlation between holes and reveal pronounced rhythmicity,
enabling us to develop the first composite section for Holes 865B and 865C
and a preliminary cyclostratigraphy for the MECO. Using this new framework,
the sedimentary record is interpreted to be continuous across the event, as
identified by a pronounced transient benthic foraminiferal δ18O
shift of ∼0.8 ‰. Calcareous microfossil
biostratigraphic events from widely used zonation schemes are recognized,
with generally good agreement between the two holes, highlighting the
robustness of the new composite section and allowing us to identify planktic
foraminiferal Zones E10–E15 and calcareous nannofossil Zones NP15–18.
However, discrepancies in the relative position and ordering of several
primary and secondary bioevents with respect to published schemes are noted.
Specifically, the stratigraphic highest occurrences of planktic foraminifera,
Acarinina bullbrooki, Guembelitrioides nuttalli, and Morozovella aragonensis, and calcareous nannofossils, Chiasmolithus solitus and Sphenolithus furcatolithoides, and the lowest occurrence of
Reticulofenestra reticulata all appear higher in the section than would be predicted relative to other
bioevents. We also note conspicuous reworking of older microfossils (from
planktic foraminiferal Zones E5–E9 and E13) into younger sediments
(planktic foraminiferal Zones E14–15) within our study interval consistent
with reworking above the MECO interval. Regardless of reworking, the
high-quality XRF records enable decimetre-scale correlation between holes
and highlight the potential of Site 865 for constraining tropical
environmental and biotic changes, not just across the MECO but also
throughout the Palaeocene and early-to-middle Eocene interval.
Introduction
The transition from peak Eocene “greenhouse” conditions at ∼49–53 Ma to the onset of the icehouse at ∼34 Ma is
punctuated by a number of transient climate events (e.g. Zachos et al.,
2008; Bohaty et al., 2009; Edgar et al., 2007; Sexton et al., 2011; Hollis et
al., 2019; Westerhold et al., 2018b). The most pronounced of these is the
MECO event at ∼40 Ma that represents a temporary reversal in
the long-term Eocene global cooling trend (Bohaty and Zachos, 2003; Bohaty
et al., 2009). The MECO event is characterized by ocean warming of
∼3–6 ∘C (Bijl et al., 2010; Bohaty et al.,
2009; Boscolo Galazzo et al., 2014; Edgar et al., 2010; Cramwinckel et al.,
2018; Rivero-Cuesta et al., 2019), ocean acidification evidenced by
>1 km shoaling of the Pacific calcite compensation depth (CCD),
and shifts in the global carbon cycle including a rise in atmospheric carbon
dioxide concentrations (Bohaty et al., 2009; Pälike et al.,
2012; Henehan et al., 2020; Bijl et al., 2010). Marine biotic responses vary
across the event, impacting floral and faunal composition, body size, and
ecology in the deep and surface ocean globally (e.g. Edgar et al.,
2013; Witkowski et al., 2014; Möbius et al., 2015; Boscolo Galazzo et al.,
2015; Luciani et al., 2010; Cramwinckel et al.,
2019; Arreguín-Rodríguez et al., 2016).
The MECO is well known from mid- and high-latitude sites (e.g. ODP Sites
689, 690, 702, 748, 738, 1051, 1172 and 1263; Alum Bay in the UK; the Baskil
section in Turkey; and the Alano, Contessa Highway, and Monte Cagnero
sections in Italy), many of which also are characterized by good
stratigraphic age control, comprehensive isotope stratigraphies, and
carbonate-rich sediments yielding microfossils useful for proxy
reconstructions (Edgar et al., 2010; Bohaty et al., 2009; Bohaty and
Zachos, 2003; Bijl et al., 2010; Boscolo Galazzo et al., 2014; Spofforth et
al., 2010; Jovane et al., 2007; Savian et al., 2013; Rivero-Cuesta et al.,
2019; Dawber et al., 2011; Giorgioni et al., 2019). However, we currently lack
a low-latitude site that unambiguously spans the pre-MECO, MECO, and
post-MECO intervals and has continuous carbonate sedimentation uninterrupted
by CCD or lysocline shoaling, thus recording environmental and biotic
responses in the tropics.
Of the sites that do exist, ODP Site 1260 in the equatorial Atlantic has a
high-resolution stable isotope stratigraphy that has been placed onto an
astronomically tuned age model (Edgar et al., 2010; Westerhold and
Röhl, 2013). However, interpretation of the MECO at this site is
complicated by the lack of a clear return to higher δ18O
values following the event and a hiatus which truncates the upper portion of
the record (Edgar et al., 2010; Shipboard Scientific Party,
2004; Westerhold and Röhl, 2013). ODP Site 1218 and IODP Site U1333 in
the equatorial Pacific Ocean both have good age control with comprehensive
magnetic stratigraphies and astronomically tuned age models across the
whole of the MECO interval (Westerhold et al., 2014; Expedition 320/321
Scientists, 2010), but their relatively deep palaeo-water depth (>3.3 km), coupled with a relatively shallow Pacific CCD in the middle Eocene,
resulted in little or no carbonate preservation across the peak of the
event, preventing detection of its true magnitude (Expedition 320/321
Scientists, 2010; Westerhold et al., 2014; Pälike et al., 2012; Toffanin et
al., 2013). Furthermore, no planktic foraminifera are present across the
entire MECO interval at Site U1333 to constrain surface-water conditions
(Expedition 320/321 Scientists, 2010). Similarly, at
shallower ODP Site 1209 (∼2 km palaeo-water depth) in the
subtropical North Pacific Ocean (Dutton et al., 2005) the interval
containing the MECO has poor carbonate microfossil preservation, very low
sedimentation rates, and thus, poor age control (Dawber
and Tripati, 2011; Shipboard Scientific Party, 2002). ODP Site 711, in the
tropical Indian Ocean has no carbonate preservation across the peak and
initial post-MECO intervals (∼3.5 km palaeo-water
depth) (Savian et al., 2016; Peterson and Backman, 1990; Bohaty et al.,
2009). Nearby ODP Site 709 has a much shallower palaeo-water depth of 2.2 km
and moderate carbonate preservation, but the MECO is absent (Bohaty et
al., 2009; Peterson and Backman, 1990). Finally, sediments from ODP Site 959
in the eastern equatorial Atlantic span the MECO but are organic-rich, poor
in carbonate microfossils (planktic foraminifera are absent), and lack a
robust magnetic stratigraphy (Cramwinckel et al., 2018).
Equatorial Pacific ODP Site 865 is an older site that has been rather
neglected in Eocene palaeoceanographic studies. Drilled on Allison Guyot in
1992 on ODP Leg 143, the recovered cores sample a pelagic sediment drape
(∼ 200 m thick at the centre of the guyot and ∼140 m at Site 865, which was drilled off-centre) that accumulated during the
Palaeogene and early Neogene. The recovered sediments at the site are
comprised of a succession of Palaeocene to Miocene foraminiferal-nannofossil
ooze and foraminiferal sands (Shipboard Scientific Party,
1993b). The relatively shallow palaeo-water depths (<2 km) on the
guyot top maximizes the preservation potential of carbonate microfossils,
thus avoiding problems of CCD and lysocline shoaling encountered elsewhere
at deeper sites in the Pacific (e.g. IODP Site U1333). Several
Palaeogene-focused studies have highlighted the potential of this site for
palaeoceanographic and evolutionary studies (Bralower et al., 1995; Tripati
et al., 2003; Pearson and Ezard, 2014; Coxall et al., 2000; Pearson and Palmer,
2000; Edgar et al., 2015; Arreguín-Rodríguez et al., 2016; Norris and
Nishi, 2001), but most of the high-temporal-resolution work has focused on
the Palaeocene–Eocene Thermal Maximum (PETM) (Kozdon et al., 2011, 2013; Kelly et
al., 1996; Tripati and Elderfield, 2004) since ODP Site
865 is one of the few open ocean sites with carbonate present throughout the
event.
Middle Eocene sediments at Site 865 have not received much attention for
palaeoceanographic studies, in large part because of (1) poor quality of
shipboard physical property records preventing the development of a
composite section necessary to develop continuous stratigraphic sections;
(2) the lack of magnetic reversal or cyclostratigraphy for age control; and
(3) evidence for reworking of older calcareous microfossils into younger
sediments, e.g. across the PETM onset and in the upper portions of the
sediment column (Shipboard Scientific Party, 1993b; Bralower and
Mutterlose, 1995; Kelly et al., 1998). Reworking and winnowing of sediment is
an acknowledged problem of guyot-top sites where ocean currents can be
locally intensified by the topography (Pearson, 1995).
However, evolutionary studies have demonstrated that the calcareous
microfossils (calcareous nannofossils and planktic and benthic foraminifera)
are diverse and abundant, albeit recrystallized (Pearson et al.,
2001; Sexton et al., 2006; Edgar et al., 2015; Arreguín-Rodríguez et
al., 2016; Bralower and Mutterlose, 1995) and in the case of planktic
foraminifera, provide an apparently complete sequence of tropical Palaeogene
biomarkers and their lineage evolution (Norris and Nishi, 2001; Pearson
and Ezard, 2014; Coxall et al., 2000).
Importantly, existing biostratigraphic assessments for Holes 865B and 865C
identify planktic foraminiferal Zone E12, defined by the total range of
Orbulinoides beckmanni and spanning the MECO interval (Berggren and Pearson, 2005), and
calcareous nannofossil Zone CN15/NP16 (Martini, 1971; Agnini et al.,
2014), although there are large (∼1 m) recovery gaps between
cores (Shipboard Scientific Party, 1993b; Bralower and Mutterlose,
1995; Pearson and Ezard, 2014). Core photos indicate the presence of
carbonate-rich sediments throughout the entire middle Eocene interval of
interest (i.e. there is no evidence of a clay horizon) providing a
promising target for geochemical and plankton assemblage work (Shipboard Scientific Party, 1993b; Edgar et al., 2015; Pearson
and Palmer, 2000).
Here we take the first step towards developing the ODP Site 865 MECO
sequence as a palaeoceanographic reference by adding new datasets that
allow development of a composite splice and a detailed chronostratigraphy
for the site. This involves integration of high-resolution X-ray
fluorescence (XRF) core scanning, benthic foraminiferal stable isotope
(δ13C and δ18O), and planktic foraminiferal and
calcareous nannofossil biostratigraphic data across the MECO interval at the
site. These data are combined with published calcareous nannofossil datums
(Bralower and Mutterlose, 1995) to (1) generate the first
composite sedimentary section across the middle Eocene interval of Site 865,
(2) identify and constrain the isotopic character of the MECO in the Pacific
Ocean, and (3) determine the position of key biostratigraphic datums
relative to the MECO event.
Materials and methodsRegional setting
ODP Site 865 was drilled during ODP Leg 143 and is located on Allison Guyot
in the western North Pacific Ocean at 18∘26.410′ N, 179∘33.339′ W and a modern water depth of 1516 m (Shipboard
Scientific Party, 1993b; Fig. 1). This guyot is just one of a number of
similar flat-topped volcanic seamounts in the region formed during the
Cretaceous (e.g. Resolution and Wōdejebato guyots), which stand up to
several kilometres above the surrounding abyssal (>4 km) seafloor
(Shipboard Scientific Party, 1993a; Matthews et al., 1974). Three holes
were drilled at Site 865 (A–C) with middle Eocene sediments only recovered
in Holes B and C, which were cored with the aim of creating a continuous
record of this interval. The middle Eocene sequence at Site 865 is
positioned at shallow burial depths (<100 m) with no significant
Quaternary cover. The foraminiferal nannofossil ooze and foraminiferal sands
were deposited at ∼ 1300–1500 m palaeo-water depth based on
benthic foraminiferal assemblages (Shipboard Scientific
Party, 1993b) in a fully pelagic ocean gyre setting characterized by year-round thermal stratification (Pearson et al., 2001).
Location map for ODP Site 865 (solid star) and other low-latitude sites discussed in the main text (open stars) on a 40 Ma
reconstruction. Palaeolatitudes for each site are calculated from
van Hinsbergen et al. (2015). Base map generated from http://www.odsn.de (last access: 8 June 2018).
X-Ray fluorescence (XRF) scanning
XRF data are routinely employed for stratigraphic correlation and the
construction of high-resolution composite depth scales because of their
higher signal-to-noise ratio and more consistent hole-to-hole character than
shipboard physical property measurements (Röhl and
Abrams, 2000). Prior to XRF analysis, the top ∼0.5 cm of the
split section surface of the archive halves was scraped off with a glass
slide. This revealed bioturbation structures, indicating that this interval
did not have significant coring disturbance (Fig. S1 in the Supplement), as
might be expected from the foraminifera-ooze lithologies and the massive
appearance of the unprepared cores. Using an Avaatech XRF scanner with a
Canberra X-PIPS SDD, Model SXD-150-500 X-ray detector, a suite of elemental
(including Fe, Ca, Sr, and Ba) intensity data were collected every 2 cm
down-core with a constant spot size (cross-core slot = 12 mm; down-core
slit = 10 mm) using a 6 or 10 s count time at 30 and 50 kV,
respectively, on the split surface of the archive half of each section using
the TAMU ODASES XRF scanner at the International Ocean Discovery Program
(IODP) Gulf Coast Core Repository. We measured the elemental composition of
sediments that encompass planktic foraminiferal Zone E12 from ODP Cores
865B-4H and -5H (27.52–44.58 metres below seafloor, m b.s.f.) and Sections 865C-4H-1 to 6H-2
(22.30–42.80 m b.s.f.) in order to capture overlapping sections and construct a
composite splice. We primarily used Sr/Ca and Ba/Sr ratios determined from
the 50 kV scans for splicing purposes. These are unconventional ratios for
hole-to-hole correlation, but the low detrital component of the Site 865
sediments necessitated the use of high-intensity elements associated with
the carbonate and biogenic barite fractions (Ca, Sr, and Ba) for detailed
correlation. All XRF elemental data are reported in Table S1 in the Supplement.
Cyclostratigraphy
To investigate if astronomical forcing drove patterns in the elemental data
records, the Fe intensity data, measured at 30 kV, and the natural logarithm
of the Sr/Ca ratio, ln(Sr/Ca), measured at 50 kV, were subjected to time
series analyses. The Fe intensity data were selected to allow comparison
with published datasets (Westerhold and Röhl, 2013), and the ln(Sr/Ca)
record was selected for its low signal-to-noise ratio (see Sect. 3.1). The
records were evenly sampled and periodicities larger than 7 m were removed
in the program AnalySeries (Paillard et al., 1996). Spectral analyses were
performed with Redfit 3.8 (Schulz and Mudelsee, 2002) using a Welch window.
The behaviour of periodicities over the length of the dataset was
investigated by evolutive harmonic analysis with the astrochron package in R
(Meyers, 2014), using window lengths of 5.5 m for ln(Sr/Ca) and 6 m
for Fe intensity. The Fe record displays a stronger imprint of short-term
(<1 m) variability than the ln(Sr/Ca) record. To test whether the
observed cycle hierarchy in the Fe record can be confidently linked to an
orbital origin, the interval with most persistent periodicities as
identified in the evolutive spectrum, between 32.5 and 44.5 “adjusted”
metres composite depth (amcd; see below), was investigated with the average
spectral misfit (ASM) method in astrochron (Meyers, 2014). Bandpass filters were
applied to the datasets centred at periodicities of 1.9 and 50 cm with
broad bandwidths of 1/3 of the centre frequency. The ln(Sr/Ca) record
displays a stronger imprint of longer (>1 m) periodicities than
the Fe record, with minima in the 1.9 m bandpass filter of ln(Sr/Ca)
generally coinciding with maxima in the 1.9 m bandpass filter of Fe. High Fe
intensities generally coincide with intervals of higher variability in the
Fe record, which are likely controlled by a larger amplitude of variation in
the orbital forcing parameters during eccentricity maxima. In concordance
with the approach of Westerhold and Röhl (2013), we
consider eccentricity maxima to coincide with maxima in the Fe record and,
therefore, minima in the ln(Sr/Ca) record. A tentative orbital tuning was
established by (i) anchoring the maximum in the 1.9 m bandpass filter of
ln(Sr/Ca) to the 405 kyr eccentricity minimum at 40.5 Ma in the La2011
eccentricity solution (Laskar et al., 2011) and (ii) connecting
consecutive maxima in the 1.9 m bandpass filter of ln(Sr/Ca) to 405 kyr
eccentricity minima. Astronomical tie points are presented in
Table S2, and resulting sedimentation rate estimates are in Fig. S2.
Planktic foraminiferal and calcareous nannofossil biostratigraphy
A planktic foraminiferal biostratigraphic scheme, with approximately one
sample per core section (every ∼1.5 m), was previously
presented for the middle Eocene of ODP Site 865 (presented in fig. 1 of Pearson and Ezard, 2014). Further analysis of the same sample
set, supplemented by new samples available at 10 cm spacing (as necessary)
between Sections 865B-4H-3 and -5H-2, (∼ 31.20–39.98 m b.s.f.) and between Sections 865C-4H-1 and -6H-2
(∼ 22.35–44.05 m b.s.f.), provided the opportunity to test and
refine existing datums. Higher-resolution biostratigraphic analysis also
afforded the opportunity to build on previous work to capture both primary
bioevents defining planktic foraminiferal zones and key secondary datums as
defined by Berggren and Pearson (2005) and Wade
et al. (2011). Planktic foraminiferal taxonomy follows Pearson et
al. (2006), and species range charts are presented in Tables S3
and S4. Representative specimens of selected biostratigraphically important
planktic foraminifera were imaged either under reflected light or using
scanning electron microscopy (SEM). Specimens were gold coated prior to SEM
analysis, which was conducted on a Philips XL-30 Environmental SEM at the
University of Birmingham.
We also define calcareous nannofossil datums (from Martini, 1971; Agnini
et al., 2014) based on semi-quantitative counts by
Bralower and Mutterlose (1995) at one sample per core
section, supplemented with new nannofossil data based on analysis of an
additional 20 samples from Sections 865B-4H-3 to -5H-1 and Sections
865C-4H-5 to -5H-5 (Tables S5 and S6). New calcareous
nannofossil data are derived from variable sampling intervals as they were
selected to test and refine the published datum events occurring within the
focal sampling interval of this study. Calcareous nannofossils were analysed
using simple smear slides and standard light microscope techniques
(Bown and Young, 1998). Data were collected semi-quantitatively using
a Zeiss Axiophot photomicroscope at ×1000 magnification. Species abundance
is estimated per field of view (FOV) after viewing at least five slide
transects (>500 FOV), with slides studied for at least 40 min. Species range charts are presented in Tables S5 and S6.
Taxonomy typically follows Nannotax (Young et al., 2017).
For defining both planktic foraminiferal and calcareous nannofossil datums,
we use Top (T) and Base (B) to describe the highest and lowest occurrences
of taxa and Top and Base common (Tc and Bc) for the highest and lowest
common occurrences, respectively (following Backman et al., 2012).
Stable carbon and oxygen isotope measurements
Benthic foraminiferal stable isotope (δ13C and δ18O) data were generated from the epifaunal taxon Nuttallides truempyi following the
taxonomic concept of Holbourn et al. (2013). Foraminifers were picked
from the 250–300 µm sieve size fraction and cleaned by
ultrasonication to remove any loose fine material prior to stable isotope
analysis. Benthic foraminifera used for stable isotope analysis were
partially recrystallized (with a “frosty” texture) but were not infilled or
heavily overgrown (also see Arreguín-Rodríguez et
al., 2016). All stable isotope measurements were determined using a Thermo
Scientific DELTA V Advantage mass spectrometer coupled to a Gas Bench II in
the School of Earth and Ocean Sciences at Cardiff University and are
reported relative to the Vienna Pee Dee Belemnite (VPDB) standard. External
analytical precision for δ13C and δ18O analyses is
0.06 ‰ and 0.07 ‰, respectively. No species–specific
corrections are applied. Stable isotope data for Holes 865B and 865C are
reported in Tables S7 and S8, respectively.
ResultsXRF data and composite splice
Despite initial concerns about coring disturbance and the lack of clear
signals in the shipboard physical property datasets
(Shipboard Scientific Party, 1993b), distinct features and
cyclicity are evident in the high-resolution XRF records throughout the
study interval aiding correlation between holes (Fig. 2; Table S1). Sr/Ca and Ba/Sr ratios are the most useful parameters for correlation
because of the higher signal-to-noise ratio compared to other elemental
ratios, e.g. Fe/Sr. For the first time at Site 865 these datasets enabled
the identification of several unambiguous tie points between holes and
construction of a composite section spanning a ∼ 25 m interval
(Tables 1 and 2). This is the first composite depth scale available for any
interval at Site 865. The biggest difference between the new composite depth
scale and the shipboard metres below seafloor (m b.s.f.) depth scale is that the core recovery gap
between Cores 865C-4H and -5H has increased from ∼ 1 to
>3 m, requiring significant offsets to be applied. In order to
maximize the sample material available for study, out-of-splice intervals
were also correlated to the in-splice intervals through development of an
“adjusted” metres composite depth (amcd) scale (Tables 3 and 4; Fig. 2).
This was achieved by aligning clearly identifiable common features in each
hole and defining mapping pairs (Tables 3 and 4) by stretching/squeezing of
the out-of-splice core segments to match the in-splice intervals (Fig. 2).
All interpretations below are considered relative to the final composite
amcd scale. However, the new amcd scale only applies to the intervals for
which XRF data were collected, and thus, some of the bioevents reported
within this study fall outside of this interval.
Sr/Ca and Ba/Sr records derived from XRF core scanning for ODP
Holes 865B and 865C on the new adjusted metres composite depth (amcd) scale.
Both ratios are calculated from 50 kV scans. Vertical blue lines indicate
positions of splice points, and the horizontal bars at bottom of the figure
provide a schematic representation of the composite section. Dark shading
indicates in-splice intervals of each core, and light shading indicates
out-of-splice intervals of each core.
Composite depth offsets within splice for ODP Site 865.
Mapping pairs for adjusting mcd to amcd for ODP Hole 865B.
Site core typeDepth (mcd)Offset depth (amcd)865B-4H28.4028.44865B-4H28.9029.02865B-4H30.0030.00865B-4H35.2035.20865B-4H36.2136.32865B-5H40.1239.82865B-5H40.5040.30865B-5H41.3241.20865B-5H41.7441.72865B-5H42.2442.24865B-5H45.7245.72865B-5H46.8046.81
Mapping pairs for adjusting mcd to amcd for ODP Hole 865C.
Site core typeDepth (mcd)Offset depth (amcd)865C-4H22.3022.30865C-4H30.0030.00865C-4H30.4230.64865C-5H35.2035.20865C-5H42.2442.24865C-5H43.8443.74865C-6H45.0645.12865C-6H45.7245.72865C-6H47.6747.67Cyclostratigraphy
Spectral analysis of the ln(Sr/Ca) record by Redfit displays the most
prominent periodicities at 1.9 and 73 cm and additional periodicities at
3.0 m, 2.4 m, 78 cm, and 40 cm above the 95 % confidence level, as well as
at 60 cm above the 90 % confidence level (Fig. 3). In the Fe record,
periodicities above the 95 % confidence level are detected at 6.3 m,
1.9–1.2 m, and 63 cm (Fig. 3), with an additional periodicity of 49 cm above
the 80 % confidence level. Evolutive analyses track the behaviour of the
∼1.9 m and 40–78 cm periodicities over the length of the
record, revealing reduced power at depths around 32.5 and 39.5 amcd. ASM
analysis of the Fe intensity record from 32.5 to 44.5 m reveals a hierarchy
of periodicities likely corresponding to an orbital imprint at a
sedimentation rate of 0.46 cm kyr-1 (Fig. S2). This is in close
agreement with the average sedimentation rate obtained from tuning to the
405 kyr eccentricity cycle at 0.48 cm kyr-1 on average over the entire record.
A sedimentation rate of 0.48 cm kyr-1 would translate the periodicity of 1.9 m, which is strongly present in the ln(Sr/Ca) record, to a duration of 406 kyr, which is close to the periodicity of long eccentricity at 405 kyr.
Time series analyses of the ln(Sr/Ca) and Fe intensity data and
preliminary orbital tuning. From left to right: the La2011 eccentricity
solution (dark blue) and its 405 kyr bandpass filter (light blue) and the
ln(Sr/Ca) record and the Fe intensity record, flanked by their 1.9 m (black)
and 50 cm (grey) bandpass filters and evolutive power spectra, with E1, E2,
and E3 indicating the potential imprint of the 405, 125, and 95 kyr
components of eccentricity, respectively. The evolutive spectra are topped
by Redfit power spectra with dashed lines showing the 99 %, 95 %, 90 %, and 80 %
confidence levels and with main periodicities indicated. Horizontal yellow
bands indicate the position of the MECO and the C19r event. Horizontal grey
dashed lines indicate tentative tuning tie points between the ln(Sr/Ca)
bandpass filter and the astronomical solution.
Planktic foraminiferal biostratigraphy
All of the samples analysed contain abundant recrystallized or “frosty”
planktic foraminifera (see Fig. 4 and Edgar et al., 2015,
for images of wall cross sections). The assemblages are diverse and typical
of tropical middle Eocene low-latitude environments (e.g. Pearson et
al., 2006; Berggren and Pearson, 2005). The dominant genera are Acarinina, Morozovelloides,
Turborotalia, Globigerinatheka, and Subbotina, with minor but conspicuous contributions from Hantkenina, Guembelitrioides, Globanomalina, and Pseudohastigerina. Thus, the
(sub)tropical planktic foraminifera zonation scheme of Berggren
and Pearson (2005) can easily be applied at this site, and, unusually, a
complete sequence of Eocene biomarkers and biozones can be identified. From
the pattern of evolutionary bioevents recognized we identify planktic
foraminiferal Zones E10–E15 within the study interval (Table 5; Figs. 5–6).
Images of planktic foraminiferal species of biostratigraphic significance
are shown in Fig. 4.
Scanning electron and light microscope images of
biostratigraphically important taxa at ODP Site 865. (a, b) Sample ODP
Hole 865C-4H-2, 25–27 cm, Morozovelloides lehneri – reworked specimens with distinctive
brown/orange discolouration. (c) Sample ODP Hole 865B-6H-1, 73–75 cm,
Hantkenina australis. (d) Sample ODP Hole 865C-4H-2, 85–87 cm, Globigerinatheka semiinvoluta, broken final chamber. (e) Sample
ODP Hole 865C-5H-1, 5–7 cm, Orbulinoides beckmanni. (f) Sample ODP Hole 865C-5H-4, 85–87 cm,
Acarinina bullbrooki. (g) Sample ODP Hole 865C-4H-3, 95–97 cm, a reworked Morozovella aragonensis. (h) Sample ODP Hole
865C-5H-2, 45–47 cm, Globigerinatheka index. (i) Sample ODP Hole 865C-5H-2, 45–47 cm,
Morozovelloides lehneri. All scale bars are 100 µm.
Benthic foraminiferal stable isotope records and age control
points in ODP Holes 865B and 865C. (a, b)δ18O and δ13C values are from multi-specimen analyses of Nuttallides truempyi (no vital effect
correction has been applied). Gaps in the records represent intervals where
no data were generated. (c) Planktic foraminiferal datums are from this study
(Table 5). (d) Calcareous nannofossil biostratigraphic markers are from Bralower and
Mutterlose (1995) with new data here (Table 6). Depth uncertainties in datums
are indicated by vertical lines (black lines: amcd scale; grey line: Top or Bottom depth falls outside of amcd scale and is shown in m b.s.f.) and
where not visible are smaller than the symbol. The
vertical yellow bars define the positions of the Middle Eocene Climatic
Optimum (MECO) and the possible C19r event. T: Top; B: Base; Bc: Base of common occurrence. Note that only datums that fall within the new
amcd scale are shown here.
Age–depth plot for ODP Site 865. Only datums falling within the
new adjusted metres composite depth (amcd: adjusted metres composite depth) scale are included here. To see
all calcareous microfossil datums from the study interval see
Fig. S3. Planktic foraminiferal datums are from this study and Pearson and
Coxall unpub. (presented in Pearson and Ezard, 2014) (Table 5).
Calcareous nannofossil datums are from this study and
Bralower and Mutterlose (1995) (Table 6). Depth
uncertainties in datums are indicated by vertical lines (black lines: amcd
scale; grey line: Top or Bottom depth falls outside of amcd scale and is
shown in m b.s.f.) and where not visible are smaller than the symbol. Black
diamonds are the tuning tie points to the orbital solution (Table S2). Ages are shown on the Gradstein et al. (2012) timescale. Abbreviations of datums are T, B, or Bc for
Top, Base, or Base of common occurrence, respectively, followed by the first letter of the genus and species name.
Planktic foraminiferal datums in ODP Holes 865B and C.
EventBase ofGTS2012TopTopTopTopBottomBottomBottomBottomMidpoint± (m)plankticage (Ma)sampledepthdepthdepthsampledepthdepthdepthdepthforaminiferal(m b.s.f.)(mcd)(amcd)(m b.s.f.)(mcd)(amcd)(amcd)zoneHole BT G. semiinvolutaE1536.183H-2, 75–7720.25––3H-2, 95–9720.45––––B G. semiinvoluta38.623H-CC27.52––4H-1, 60–6228.1228.9029.02––T O. beckmanniE1340.034H-3, 80–8231.3032.0832.084H-3,W, 90–9231.4232.2032.2032.140.06T A. bullbrooki40.494H-CC, 10–1235.3636.1436.244H-CC, 20–2235.4836.2636.3636.300.06B O. beckmanniE1240.495H-2, 20–2238.7040.9840.835H-2, 30–3238.8241.1040.9640.890.07B G. index42.647H-4, 110–11261.6––7H-5, 55–6062.60––––B Mo. lehneri43.157H-2, 87–8958.37––7H-3, 57–5959.59––––T M. aragonensisE1043.266H-1, 75–7747.25––6H-2, 75–7748.77––––Hole CT G. semiinvolutaE1536.183H-6, 46–4820.7620.76–4H-1, 5–722.3722.3722.37––B G. semiinvoluta38.624H-3, 95–9726.2526.2526.254H-3, 110–11226.4226.4226.4226.340.07T O. beckmanniE1340.034H-CC, 10–1230.730.730.915H-1, 5–731.8735.0935.1033.002.10B O. beckmanniE1240.495H-4, 145–14737.7540.9740.975H-5, 5–737.8741.0941.0941.030.05T A. bullbrooki40.495H-2, 55–5733.8537.0737.075H-2, 65–6733.9737.1937.1937.130.05T Gu. nuttalliE1142.076H-2, 25–2743.0546.5146.516H-2, 45–4743.2746.7346.7346.620.10B G. index42.647H-4, 110–11256.4––7H-5, 70–7257.52––––B Mo. lehneri43.157H-4, 110–11256.4––7H-5, 70–7257.52––––T M. aragonensisE1043.266H-2, 125–12744.0547.5147.517H-1, 110–11251.92––––
T: Top; B: Base; A: Acarinina; G: Globigerinatheka; Gu: Guembelitrioides; M: Morozovella; Mo: Morozovelloides; O: Orbulinoides. Full sample window in which datum could fall is reported, i.e. top depth of Top sample and lowermost depth in Bottom sample.
The planktic foraminiferal (and calcareous nannofossils, Sect. 3.4) biostratigraphic data serve two purposes: (i) comparison of bioevents in Holes 865B and 865C to provide a check on
XRF-based hole-to-hole correlations and (ii) critical age control for this
site. The relative sequence and depths of planktic foraminiferal datums in
Holes 865B and 865C are in relatively good agreement with one another on the
new amcd scale, indicating no major misalignments based on the XRF
correlations. The Top of Globigerinatheka semiinvoluta, defining the base of Zone E15, falls in samples
outside of the new splice (Table 5), occurring at 20.35±0.10 m b.s.f. in
Hole 865B and at 21.57±0.80 m b.s.f. in Hole 865C (and above 22.37 amcd).
Small offsets between the two holes support the minimal composite depth
offsets in the uppermost cores (<0.78 m; Tables 1, 2, and 5; Fig. S2).
Reworking of older into younger material is evident in the upper part of the
study section investigated here (above 31 amcd; Fig. 5). Specifically early
middle Eocene material is reworked into middle and late Eocene sediments
(Tables S3 and S4). The most noticeable example of this is in
the overlapping occurrences of G. semiinvoluta with the large Acarinina (e.g. A. praetopilensis and A. mcgowrani) and Morozovelloides (M. crassatus and M. lehneri), and even Morozovella aragonensis (from Zones
E5–9) in Sections 865C-4H-1 to -4H-4 and in Sections 865B-3H-5 and -3H-6.
Fortunately reworked specimens are relatively easy to discern as they
typically have a distinctive brown/orange stain, may contain small flecks of
pyrite, and/or are more poorly preserved, e.g. fragmented. We only find one
Plio-Pleistocene species (Globorotalia tumida) indicative of possible downhole contamination in
Section 865C-4H-1.
Conspicuous reworking has several consequences for the biostratigraphic
zonation in the study interval. First, the Top of M. crassatus used to define the base
of Zone E14 was not confidently identified here because significant numbers
of individuals are present in both Cores 865B-3H and 865C-3H. Thus, the Base
of G. semiinvoluta, which is calibrated to the same age as the Top of M. crassatus was instead used to
approximate the base of Zone E14 (27.82±0.10 m b.s.f. in Hole 865B and
26.34±0.07 amcd in Hole 865C; Table 5). The slight difference between
the two horizons may reflect the relative rarity of G. semiinvoluta and difficulty in
distinguishing this taxon from its immediate ancestor Globigerinatheka mexicana at the base of its
range (Premoli Silva et al., 2006) and/or that Core 865B-3H
falls outside of the new amcd scale. In contrast, the Top of O. beckmanni is an easily
identifiable datum that defines the base of Zone E13 and is well constrained
in Hole 865B (32.14±0.06 amcd; Table 5). This datum, however, falls
in the core gap between Cores 865C-4H and -5H, and hence there is a large
depth uncertainty for this datum in Hole 865C (33.00±2.10 amcd in
Hole 865C; Table 5; Fig. 5). The Base of O. beckmanni, used to define the base of Zone E12,
is more difficult to determine because there is a continuous chronocline
from its immediate ancestor Globigerinatheka euganea, and thus, there is no simple morphological
discrimination for “early” O. beckmanni forms (Premoli Silva et al., 2006; Edgar et
al., 2010). Here we distinguished O. beckmanni from G. euganea by the presence of a large,
hemispherical final chamber with more than four small and often irregular
supplementary apertures at its base, multiple supplementary apertures
between the chambers in the early whorl (sometimes poorly developed), and if
present, areal apertures in the final chamber (areal apertures are not known in G. euganea)
(Edgar et al., 2010; Premoli Silva et al., 2006). However, notably, here we
find highly developed spherical forms (i.e. with well-developed spiral
sutural apertures, areal apertures, and >4 supplementary
apertures at the base of the final chamber) in the lowermost part of the
species range at 40.89±0.07 amcd and 41.03±0.05 amcd in Holes
865B and 865C, respectively (Table 5). The Top of A. bullbrooki occurs ∼4 m
above the first occurrence of O. beckmanni at both sites (36.30±0.06 in Hole 865B
and 37.13±0.05 amcd in Hole 865C; Table 5) with highly quadrate A. bullbrooki
forms (Berggren et al., 2006) present that are not obviously reworked. This
provides an additional correlative horizon and a check on the correlation
between Holes 865B and 865C. Whilst not official biozone markers for this
interval of the Eocene, the Hantkenina assemblage at this site is very well developed,
and these taxa may be useful accessory markers (Coxall and Pearson, 2006).
For instance, we note that Hantkenina australis appears in samples at ∼43 amcd
coincident with a pronounced isotopic excursion, which may be correlative
with the C19r event (Fig. 5).
The lower portion of the Site 865 study interval is assigned to Zones E10
and E11. The Top of Guembelitrioides nuttalli, which defines the base of Zone E11, occurs at
46.62±0.10 amcd in Hole 865C but was not defined in Hole 865B (as it
fell outside of the available sample set; Table 5). Notably, a small number
of individuals that share the morphology of G. nuttalli with the distinctive high
spire, globular chambers, and a pronounced rim (Olsson et al.,
2006) but lack supplementary apertures on the spiral side are present
sporadically much higher in the site up to ∼22 amcd. The
extinction of the distinctive taxon Morozovella aragonensis defining the base of Zone E10 falls
outside of the new splice at 48.01±0.76 and 47.99±3.94 m b.s.f. in
Holes 865B and 865C, respectively (Table 5). In Hole 865C, this corresponds
to the extinction occurring below 47.51 amcd. The base of the secondary
marker species Globigerinatheka index occurs at 62.10±0.50 m b.s.f. in Hole 865B, and the bases
of G. index and M. lehneri occur at 56.96±0.56 m b.s.f. in Hole 865C (Table 5). These datums
typically occur in Zone E10 but occur well below the Top of M. aragonensis here and
therefore are within Zone E9.
Calcareous nannofossil biostratigraphy
Calcareous nannofossils at Site 865 are moderately well preserved, showing
signs of etching and/or recrystallization, but most specimens are
identifiable to species level throughout. Biostratigraphically important
calcareous nannofossil occurrences from Bralower and
Mutterlose (1995) that fall within our study interval are collated here and
where possible translated onto the new amcd scale (Tables 6, S5 and S6). Marker species in Holes 865B and 865C on the new amcd scale
are in broad agreement with one another; i.e. datums in both holes overlap
or are close to one another in depth space (Fig. 6). Datums that do not quite
overlap in terms of depth space include the following: Nannotetrina fulgens, Reticulofenestra (Dictyococcites) bisecta (>10µm),
Chiasmolithus solitus, and Sphenolithus furcatolithoides. These inconsistencies are likely a function of the relative rarity of
these taxa within the cores making it difficult to reliably determine the
highest or lowest true occurrence and/or in some cases this may be
exacerbated by etching and overgrowth of some specimens
(Bralower and Mutterlose, 1995). However, it is clear from
the datums that fall outside of the interval incorporated in the new amcd
scale in one or both Holes 865B and 865C (e.g. Bases of Chiasmolithus oamaruensis and Reticulofenestra umbilicus and Top of C. gigas and C. grandis; see
Fig. S3) that bioevents in Cores 865B-3H and 865C-3H occur at
similar levels, indicating limited offsets between Holes 865B and 865C. More
significant offsets of up to 5 m are present between Holes 865B and 865C
from Cores 865B-7H and 865C-7H downwards, indicating that significant
adjustments are needed to align the two holes (Table 6).
Calcareous nannofossil datums in ODP Holes 865B and C.
DatumBase ofGTS2012Top sampleDepthDepthDepthBottomDepthDepthDepthMidpoint± (m)calcareousage (Ma)sample(m b.s.f.)(mcd)(amcd)sample(m b.s.f.)(mcd)(amcd)(amcd)nannofossilzoneHole BT C. grandis38.013H-1, 70–72*18.70––3H-1, 132*19.32––––B C. oamaruensisNP1838.073H-3, 110*22.10––3H-4, 123*23.73––––T S. obtususCNE1638.694H-1, 16*27.6628.4428.494H-2, 40*29.4030.1830.1829.330.85B R.(D.) bisecta (>10µm)CNE1540.404H-4, 20–2232.2032.9832.984H-4, 107*33.0733.8533.8533.410.44T C. solitusNP1740.404H-1, 16*27.6628.4428.494H-2, 40*29.430.1830.1829.330.85B R.(D.) scrippsae (<10µm)40.234H-5, 90–9234.4035.1835.184H-5, 120*34.7035.4835.5135.350.16T S. furcatolithoides40.534H-1, 16*27.6628.4428.494H-2, 40*29.4030.1830.1829.330.85Bc R.(Cr.) reticulataCNE1442.204H-1, 16*27.6628.4428.494H-2, 40*29.4030.1830.1829.330.85Bc R. umbilicus (>14µm)CNE1342.845H-CC*44.5846.86–6H-1, 81–85*47.35––––T N. fulgensNP1642.875H-5, 70–72*43.7045.9845.985H-6, 54–566*44.5646.84–––B R. umbilicus43.326H-1, 81–85*47.31––6H-2, 73–75*48.75––––T C. gigasCNE12; NP15c43.686H-1, 81–85*47.31––6H-2, 73–75*48.75––––Hole CT C. grandis38.013H-5, 80*19.60––3H-6, 10–11*20.41––––B C. oamaruensisNP1838.074H-1, 10–11*22.4022.4022.404H-2, 10–11*23.9123.9123.9123.150.76T S. obtususCNE1638.694H-5, 75–7729.0529.0529.054H-6, 10–11*29.9129.9129.9129.480.43B R.(D.) bisecta (>10µm)CNE1540.404H-6, 65–6730.4530.4530.674H-CC*30.6030.6030.8130.740.07T C. solitusNP1740.404H-6, 65–6730.4530.4530.674H-CC30.6030.6030.8130.740.07B R.(D.) scrippsae (<10µm)40.235H-1, 45–4732.2535.4735.475H-1, 95–9732.7735.9935.9935.730.26T S. furcatolithoides40.534H-6, 65–6730.4530.4530.674H-CC30.6030.6030.8130.740.07Bc R.(Cr.) reticulataCNE1442.204H-5, 10–11*28.4028.4028.404H-5, 75–7729.0729.0729.0728.730.34Bc R. umbilicus (>14µm)CNE1342.846H-CC*44.2247.68–7H-1, 10–11*50.91––––T N. fulgensNP1642.876H-1, 10–11*41.4044.8644.896H-1, 75*42.0545.5145.5345.210.32B R. umbilicus43.327H-2, 10–11*52.40––7H-2, 100*53.30––––T C. gigasCNE12; NP15c43.686H-CC*44.2247.68–7H-1, 10–11*50.91––––
T: Top; Tc: Top common; B: Base; Bc: Base common; C: Chiasmolithus; Cr: Cribrocentrum; D: Dictyococcites; N: Nannotetrina; R: Reticulofenestra; S: Sphenolithus. *: from Bralower and Mutterlose (1995). All ages on the GTS2012 timescale from Gradstein et al. (2012) with those in bold recalculated based on revised calibrations from Agnini et al. (2014), in italics from Bohaty et al. (2009). Calcareous nannofossil zones: NP Zones are from Martini (1971), and CNE Zones are from Agnini et al. (2014). Full sample window in which datum could fall is reported, i.e. the top depth of the Top sample and the lowermost depth in the Bottom sample.
Consistent with Bralower and Mutterlose (1995), calcareous
nannofossil Zones NP15–18 (Martini, 1971) can be clearly identified at
Site 865 (Table 6). On the more recent “CNE” zonation scheme
(Agnini et al., 2014), the interval encompassing CNE Zones 12–17
is identified, but Zones CNE14–16 cannot be differentiated. This is because
the CNE primary marker species (1) are very rare at this site – Reticulofenestra (Cribrocentrum) erbae; (2) occur in the same narrow window at ∼29 amcd – T Sphenolithus obtusus and Bc Reticulofenestra (Cribrocentrum) reticulata, defining the bases of Zones CNE16 and 14, respectively (Fig. 6); or (3) appear out of sequence between ∼ 28 and 32 amcd – e.g. the Bc R. (C). reticulata is very shallow, as are the T S. furcatolithoides, T C. solitus, and B R. (D) bisecta; the last of which also has the
largest offsets between the two holes.
Benthic foraminiferal stable isotope results
Benthic foraminiferal stable isotope records in both holes show similar
patterns of change and absolute stable isotope values and are well aligned
on the new amcd scale (Fig. 5). Benthic foraminiferal δ18O
values vary between 0.3 ‰ and 1.2 ‰, with minimum δ18O values initially recorded at ∼43 amcd coincident
with an abrupt shift of 0.7 ‰ to more negative δ13C values. δ18O and δ13C values
subsequently increase and then plateau between 42 and 37 amcd. At
∼36.5 amcd, a second transient negative δ13C
excursion of ∼1 ‰ occurs, which is closely
followed by a ∼0.8 ‰ shift to lower
δ18O values at 35 amcd. δ13C values increase
through this same interval and reach maximum values at 34 amcd. δ18O values then gradually increase, and δ13C gradually
decrease towards the Top of the section at 24 amcd.
DiscussionReworking
Reworking of older planktic foraminifera (from Zone P14 now E13) into
younger material (Zones E13–E16 and Zone M1) in Cores 865B-1H to -3H was
reported during shipboard analysis (Shipboard Scientific
Party, 1993b) coinciding with the occurrence of relatively “soupy” sediments
with a high water content. A downcore transition to more cohesive sediments
coincided with reduced reworking. We demonstrate that reworking of planktic
foraminifera extends deeper than this transition but lessens considerably
with increasing depth and is not immediately evident below Sections 865B
4H-3 (∼32.8 amcd) or 865C 4H-4 (∼28.3 amcd)
(Fig. 5). Only discrete time intervals are obviously mixed into younger
sediments; i.e. reworked material is sourced from sediments deposited
during Zone E13 (as we find reworked Morozovelloides and Acarinina but not O. beckmanni from Zone E12) along
with much older sediments from Zones E8 to 9 (based on M. aragonensis and G. nuttalli). Reworking is also
present within nannofossil assemblages; for example, rare occurrences of C. gigas, Triquetrorhabdulus inversus, and
Chiasmolithus consuetus occur up to 15 m above their highest consistent occurrences in Cores
865B-3H and -4H and for C. gigas in Cores 865C-3H and -4H (Table 5; Bralower and Mutterlose, 1995). These occurrences are consistent with
remobilization of similar-aged sediments and depositional intervals to those
identified by planktic foraminifera (Bralower and
Mutterlose, 1995). Down-hole contamination was not obvious within planktic
foraminifera assemblages but was problematic within calcareous nannofossil
samples and was attributed to contamination from the saw used to split the
cores and the high water content of the cores (Bralower
and Mutterlose, 1995). Benthic foraminiferal assemblages appear relatively
unaffected by reworking or downhole contamination
(Arreguín-Rodríguez et al., 2016). However, many
deep-sea benthic foraminifera species are long-lived making reworking
difficult to recognize over the short intervals such as those investigated
here.
Reworking is very common in the pelagic cap sequences atop guyots drilled
during ODP Legs 143 and 144 in the equatorial Pacific, with the most intense
reworking reported in the lowermost ∼40 m of pelagic
sediments deposited above the drowned carbonate platforms related to changes
in local hydrography (Premoli Silva et al., 1993; Pearson, 1995). Less
intense reworking is also observed at higher (i.e. younger) levels in
pelagic cap sequences and is more limited to discrete horizons and time
intervals (Watkins et al., 1995; Pearson, 1995). Reworked material is
likely sourced from the edges of the guyot top and transported towards the
centre by intensification of bottom-water currents deflected up over the
guyot as well as other localized hydrographic features common to these
settings, ultimately helping to generate the characteristic low dome-shaped
sediment stack found on many Pacific guyots (Genin et al., 1989; Pearson,
1995).
ODP Site 865 sits in an oceanographically dynamic area, relatively close to
the edge of the guyot, where the sedimentary cap starts to thin; e.g. it
lacks the thick Miocene–Quaternary overburden of the guyot centre
(Shipboard Scientific Party, 1993b). The abraded appearance
of reworked foraminifera (e.g. dull lustre and fragmented) suggests that
they have been exposed to intense mechanical erosion, e.g. from currents
and/or a more intense transport history than the in situ assemblage (Pilkey et al.,
1969; Maiklem, 1967). However, the pervasive brown foraminiferal
discolouration on many specimens, an oxidized iron stain and pyrite, likely
reflect deposition in a low-sedimentation (and hence high current intensity)
area with iron sourced from the contemporaneous formation of
manganese–phosphate hardgrounds or remobilized from the lower limestone
platform (Shipboard Scientific Party, 1993b). Together these
lines of evidence suggest a similar reworking mechanism to that observed on
other Pacific guyots (Pearson, 1995; Watkins et al., 1995) with sediments
mobilized from the low sedimentation margins of the guyot top during
discrete intervals of increased local current intensity and then mixed into
sediments closer to the centre of the pelagic cap. Crucially, currents around
Site 865 were clearly sufficient to mix and remobilize sediments at times in
the late middle Eocene but not to scour the guyot top of sediments.
Consistent with this hypothesis is a reduction of sedimentation rates above
∼25 amcd or 25 m b.s.f. (Figs. 5 and S3) coincident with the zone
of most intense reworking. Lower sedimentation rates, in addition to being
consistent with elevated current activity either removing sediment or
preventing deposition, would also reduce dilution of mixed components in
sediments by in situ fauna making the reworking more evident. Regardless of
reworking, the ability to correlate the XRF records at the decimetre scale
and the coherent stable isotope stratigraphy through Cores 865B-4H to -6H
and 865C-4H to -6H (∼22 to 48 amcd) suggests that reworking
is relatively easy to avoid (benthic foraminifera were generally better
preserved than planktic counterparts, e.g. with no evidence of staining or
abrasion), not pervasive throughout the site, and most critically, has not
obscured primary environmental signals.
Climatic events at ODP Site 865
The long-term shift towards more positive benthic foraminiferal δ18O values observed through the Site 865 study interval (Fig. 5) is
consistent with a previously published low-resolution benthic foraminiferal
stable isotope record spanning the middle and upper Eocene at this site
(Coxall et al., 2000; Bralower et al., 1995), as well as the global Eocene
cooling trend (Zachos et al., 2008; Bohaty and Zachos, 2003).
Superimposed on this long-term trend is a transient negative δ18O excursion of ∼0.8 ‰ between
∼ 35–36 amcd, within calcareous nannofossil and planktic
foraminiferal Zones NP16 and E12, respectively, which is identified here as
the MECO event (Fig. 5). The onset of the event is defined at the point
where δ18O begins to show a transition to more negative values.
The end of the event is less clearly defined, as at some other sites
(Bohaty et al., 2009; Giorgioni et al., 2019; Boscolo Galazzo et al.,
2014). However, the distinctive δ13C maximum (of
∼0.6 ‰), which immediately follows the
MECO (e.g. Bohaty et al., 2009; Edgar et al., 2010; Spofforth et al.,
2010; Boscolo Galazzo et al., 2014; Giorgioni et al., 2019), suggests that the
abrupt increase to more positive δ18O values at 35 amcd likely
marks the end of the event (see vertical yellow bar in Fig. 5). Indeed, the
gradual increase to more positive δ18O values that follows
between ∼ 30–35 amcd is also seen at multiple ODP sites,
including Sites 738, 748, and 1051 where the end of the event is clearly
defined (Edgar et al., 2010; Bohaty et al., 2009).
The new composite record constructed using XRF data enables us to capture
the entire MECO event at Site 865. However, very low sedimentation rates
(<0.6 cm kyr-1; Table S2) at this site compared to many other
deep-sea sites (Bohaty and Zachos, 2003; Edgar et al., 2010; Boscolo
Galazzo et al., 2014) mean that the event is highly condensed and only spans
a ∼1 m interval (Fig. 3). This is consistent with the reduced
magnitude of the δ18O excursion (∼0.8 ‰ vs. 1.0 ‰–1.5 ‰) compared to
elsewhere, the lack of a short (<50 kyr) negative δ13C
excursion coincident with peak MECO conditions (defined by the δ18O minimum), and the rapid apparent onset of the event
(Boscolo Galazzo et al., 2014).
Constraining the magnitude, timing, and pattern of CCD change across Eocene
global warming events provides vital clues as to the source, mass, and rate
of carbon input driving these events. This is particularly true for the
MECO, dubbed a “carbon cycle conundrum” (Sluijs et al., 2013), because,
over the relatively “long” event duration (∼ 300–500 kyr),
increasing atmospheric CO2 levels, and associated warming should have
increased weathering rates leading to CCD deepening rather than the observed
shoaling (Sluijs et al., 2013; Bohaty et al., 2009). Because of its
relatively shallow water sedimentation, Site 865 is a critical end-member
for constraining the Pacific CCD response during the MECO and other Eocene
shoaling events. At >900 m, the CCD shoaling associated with the
MECO is the largest known in the middle-to-late Eocene interval (shoaling
from ∼4.2 km to above 3.3 km palaeo-water depth; Pälike et
al., 2012). Relatively continuous carbonate sedimentation across the event
at ODP Site 865 suggests that the CCD did not shoal above ∼ 1300–1500 m (the estimated palaeo-water depth of the site;
Shipboard Scientific Party, 1993b) in the Pacific Ocean at this time
providing a maximum limit of 3 km change.
The negative carbon and oxygen isotopic excursions represented here by only
a single sample at ∼43 amcd (Fig. 5), in planktic
foraminiferal Zone E11, immediately before a shift to more positive δ18O values likely corresponds to the “C19r event”, a transient
hyperthermal-style event (<100 kyr in duration) initially
recognized at ODP Site 1260 in the equatorial Atlantic (Edgar
et al., 2007). The C19r event is now also known from the South Atlantic at
ODP Sites 1263 and 702 and referred to as the Late Lutetian Thermal Maximum
(Westerhold et al., 2018a). Similar to the MECO, the
relatively small stable isotope excursions compared to elsewhere
(0.4 ‰ and 0.2 ‰ vs.
1.5 ‰ and ∼1.8 ‰, in
δ13C and δ18O at ODP Sites 865 and 1260,
respectively) are likely a function of the low sedimentation rates at this
site (∼ 0.4–0.6 cm kyr-1 vs. 2.0 cm kyr-1 at ODP Site 1260)
compounded by lower sampling frequency and time averaging. This is the first
record of the C19r event outside of the Atlantic Ocean and suggests that the
C19r event is in fact global in nature and, as such, may be a valuable
stratigraphic marker within the long >1 Myr planktic
foraminiferal Zone E11 in which it falls.
A tentative astronomical tuning for the MECO interval of Site 865 based on
correlation of the bandpass filter of ln(Sr/Ca) to the 405 kyr component of
eccentricity, anchoring the record near the onset of the MECO to the
eccentricity minimum at 40.5 Ma (Westerhold and Röhl, 2013), is in close
agreement with available biostratigraphic age control (Fig. 6). This
interpretation places the peak δ18O excursion during the MECO
within a 405 kyr eccentricity maximum at 40.3 Ma, and the carbon-isotope
maximum directly following the MECO coincides with the 405 kyr eccentricity
minimum at 40.1 Ma, in agreement with Westerhold and Röhl (2013). The duration of the interval between the potential C19r event and
the onset of the MECO is estimated at three and a half 405 kyr cycles or 1.4 Myr, which is longer than the duration estimated by Westerhold and Röhl (2013) at two and a half 405 kyr
cycles, ∼1 Myr. Unfortunately, a detailed comparison to
existing cyclostratigraphic studies is hampered by uncertainty in the
detection of the C19r event. The apparent differences in these duration
estimates are in line with existing discrepancies between astrochronologies,
notably in the reported length of magnetochron C19r, with duration estimates
varying between 0.9 Myr (Westerhold and Röhl, 2013; Westerhold et al.,
2014, 2015) and 1.5 Myr (Boulila et al.,
2018).
Integrated biostratigraphic schemes and relationship to the MECO
Whilst both calcareous nannofossil and planktic foraminiferal zonation
schemes can be applied to ODP Site 865 and are in generally good agreement,
there is significant disagreement in the post-MECO interval between
∼35 and 30 amcd (Fig. 6). Taken at face value, planktic
foraminifera indicate constant sedimentation rates of ∼0.5 cm kyr-1 throughout this interval, whereas calcareous nannofossil datums
suggest a 3.5 Myr hiatus between 38.6 and 42.1 Ma. However, XRF-derived
elemental and stable isotope datasets and cyclostratigraphic analysis do not
indicate any large shifts at this level that might indicate an abrupt shift
in environmental conditions (Figs. 2, 3, and 5). The identified presence of
the MECO event itself, which falls within this potential gap, also implies
that a hiatus is unlikely.
It is instead likely that low sedimentation rates and sampling frequency
make it difficult to discern closely spaced calcareous nannofossil bioevents
at Site 865, e.g. Tops of C. solitus and S. furcatolithoides, which are calibrated to a 130 kyr interval
(40.40–40.53 Ma). However, these events also occur much higher in the
section than expected relative to the MECO event at this site, and they also
overlap with the much younger Top of S. obtusus and Base of R. (C.) reticulata (Fig. 5). Explanations
could include the following: these bioevents are diachronous and/or are not well
calibrated or there is subtle reworking of calcareous nannofossils. As
discussed above, this interval does indeed show evidence of reworking (Fig. 3), which could have extended the Tops of species to higher in the section than
expected and introduced a degree of subjectivity in determining what is in situ
versus reworked. More intense bioturbation and/or higher core water content
through this interval may have further affected the datum levels. However, a
number of recent studies have questioned the accuracy of long-standing
calcareous nannofossil age calibrations (e.g. Agnini et al., 2014; Tori
and Monechi, 2013). For instance, many Chiasmolithus bioevents (including C. solitus) are no longer
included in the most recent calcareous nannofossil zonation scheme because
typically low and sporadic abundances of these species at many sites
introduces significant uncertainty to reported datum levels (Agnini et
al., 2014; Larrasoaña et al., 2008; Villa et al., 2008). We also note
differences in the relative positions of the first occurrences of R. (D.) bisecta (<10µm) and R. (D.) bisecta/scrippsae (>10µm) in multiple studies, but this may
also reflect taxonomic problems that are associated with the definition of
these taxa (e.g. Mita, 2001; Tori and Monechi, 2013; Larrasoaña et
al., 2008; Backman, 1987). The relatively high position of the Base of R. (C.) reticulata at Site 865 is also consistent with the highly variable position of this
event reported from other sites, ranging from magnetochron C20r to C18n.2n
(a ∼5 Myr interval; Fornaciari et al., 2010; Rivero-Cuesta
et al., 2019; Fioroni et al., 2012). However, given the sporadic and rare
presence of R. (C.) reticulata in our own observations here, the most likely explanation
is ecological bias at Site 865. Similarly, N. fulgens is typically rare with an
infrequent distribution at Site 865 and elsewhere, reducing its
biostratigraphic utility (Bown, 2005; Shamrock, 2010).
The Base of O. beckmanni is diachronous, with the species first appearing in the tropics
prior to the MECO but subsequently expanding to higher latitudes across the
onset of the MECO and inferred surface water warming (Edgar et al.,
2010; Luciani et al., 2010; Jovane et al., 2010). At Site 865, the Base of O. beckmanni
precedes the MECO event, which is consistent with this hypothesis (Fig. 3).
The Top of O. beckmanni occurs after the MECO event at Site 865, as at ODP Site 1051 and
in the Alano section in the Atlantic and Tethyan oceans, respectively
(Luciani et al., 2010; Edgar et al., 2010), suggesting that cooling of
surface waters following the MECO was not directly responsible for its
extinction. Regardless, planktic foraminiferal Zone E12 is a good marker for
the MECO interval in (sub)tropical sediments that lack a stable isotope
stratigraphy. The first occurrence of R. (D.) scrippsae(R. (D.) bisecta< 10 µm)
at low- and midlatitude sites is also used to approximate the beginning of
the MECO event based on close correlation at a number of sites in the
Atlantic Ocean (Bohaty et al., 2009). The Base of R. (D.) bisecta<10µm does occur within the MECO event at ODP Site 865
(Fig. 5).
The robust and cosmopolitan planktic foraminifera A. bullbrooki is a valuable secondary
marker that is approximately coincident with the base of Zone E12 at 40.04 Ma at many sites (Wade et al., 2011; Gradstein et al., 2012) and can be
applied when O. beckmanni is absent either due to its relatively high susceptibility to
mechanical damage and dissolution or limited (sub)tropical distribution
(Edgar et al., 2010). However, here we find that
the Top of A. bullbrooki and the Base of O. beckmanni are not synchronous as implied by the current
biozonation scheme (Wade et al., 2011). Whilst there can be
a degree of subjectivity in the definition of O. beckmanni between workers at the start
of its range (Premoli Silva et al., 2006) this is not the case
at ODP Site 865 where highly spherical and distinctive forms are present in
the lowermost samples (assuming that downhole contamination is not a major,
issue; Fig. 4). There are relatively few sites where both taxa are reported
but at ODP Sites 1051 and 1260, in the (sub)tropical Atlantic Ocean, the Top
of A. bullbrooki also occurs significantly higher than the Base of O. beckmanni before (Site 1051) or
even after (Site 1260) the MECO event (Shipboard Scientific Party, 2004,
1998; Edgar et al., 2010). In the Contessa section, Italy, the Top of A. bullbrooki occurs
significantly below the Base of O. beckmanni and the MECO (Jovane et al.,
2010). Either way, the events are not contemporaneous, suggesting that a
significant revision of this datum is required.
Whilst Hantkenina is present in low abundance throughout the middle-to-upper Eocene
section of Site 865 (Coxall et al., 2000), there is a
transient appearance of rare Hantkenina australis, a distinctive middle Eocene form with
recurved tubulospines, within the inferred C19r “hyperthermal” event. This
is surprising because this taxon is found globally, but, unlike most other
Hantkenina spp., is most abundant at higher latitudes and, thus, in cooler waters (Coxall and Pearson, 2006). The appearance of Hantkeninaaustralis may prove to be a
future useful biostratigraphic marker for C19r, within an otherwise “datum-poor” interval. Notably, previous transient incursions of more tropical
species of Hantkenina to high northern latitudes (>50∘ N) in the
middle Eocene at ODP Site 647 have previously been used to infer surface
ocean warming around the C19r interval (Shipboard Scientific
Party, 1987). However, stratigraphic revision of ODP Site 647 by
Firth et al. (2012) places the lower Hantkenina incursion at
the base of calcareous nannofossil Zone NP16 and close to the magnetochron
18r/19n boundary, which is significantly later than the C19r event but
before the MECO – and inconsistent with the hypothesis of expansion of the
Hantkenina range in response to warming surface waters. Thus, these incursions may
instead reflect changes in upwelling or increased primary production
(Coxall et al., 2007).
The Top of G. nuttalli was introduced as the marker for the base of planktic
foraminiferal Zone E11 in 2005, based on correlations at ODP Sites 1050 and
1051 (R. D. Norris, personal communication, cited in Berggren and Pearson, 2005) to
break up the otherwise very long multi-million-year Zone P12 from the
earlier zonation scheme (Berggren et al., 1995). However,
occurrences of G. nuttalli (albeit small individuals) have now been found as high as
Zone E14 (i.e. above the MECO) in the Alano section in northern Italy, with
the highest consistent occurrence in Zone E13 (Agnini et al.,
2011). The datum level reported in Table 5 (46.62±0.10 amcd) is the
highest consistent occurrence of this taxon at Site 865, but, similar to
Alano, we find at least sporadic occurrences of small individuals up through
Zones E13 and E14 that are not obviously reworked.
A number of bioevents within Zone E10 at Site 865 are out of sequence with
respect to the primary datum – the Top of M. aragonensis (43.26 Ma) – occurring above the
Bases of younger taxa G. index and M. lehneri (at 42.64 and 43.15 Ma, respectively). It is the
highest occurrences of taxa that are most likely to be impacted by
reworking. Yet M. aragonensis specimens do not show distinctive staining or worn
appearance at this level (as they do higher up in Hole 865C where they are
clearly reworked), and, whilst individuals could be remobilized locally, a
higher Top of M. aragonensis compared to G. index is also evident in the Contessa section, Italy
(Jovane et al., 2010).
Clearly more work is needed to confidently understand the relative patterns
of calcareous nannofossil and planktic foraminiferal datums in the middle
Eocene. This is particularly true for the Pacific Ocean since most (if not
all) calibrations are currently based on Atlantic or Tethyan sediment
sequences and indeed few sites where both groups are intercalibrated
(e.g. Agnini et al., 2014; Berggren and Pearson, 2005). Whilst many key
tropical middle Eocene bioevents are identified at Site 865, the lack of
magnetic or confident astrochronology hinders determination of the absolute
age of the events reported here. However, in the future, the high-resolution
stable isotope and XRF records should enable the development of a more
robust orbitally tuned age model here or allow correlation to sites
elsewhere that possess an independent reliable stratigraphy.
Conclusions
In this study, we have developed new biostratigraphic, stable isotope, and
XRF records that span the MECO event at ODP Site 865 in the tropical Pacific
Ocean. This site possesses a surprisingly coherent signal in XRF-derived
elemental counts and ratios permitting us to construct a reliable composite
section for Holes 865B and 865C spanning the late middle Eocene time
interval (∼ 38–43 Ma). Cyclicity observed in XRF datasets is
likely orbitally paced, and a tentative astronomical tuning of the study
section indicates consistently low but steady sedimentation rates of
∼ 0.4–0.6 cm kyr-1, in agreement with bio- and
chemostratigraphic age calibrations and correlations to other sites. Benthic
foraminiferal stable isotope data indicate that the MECO and the C19r events
are present at Site 865, albeit relatively condensed. Planktic foraminiferal
biostratigraphic events from the classic zonation schemes are recognized
here and are generally in good agreement with calibrated stratigraphies,
with the exception of the tops of G. nuttalli, A. bullbrooki, and M. aragonensis, which occur anomalously high in the
section. When considered alongside available calcareous nannofossil datums,
there is some disagreement in terms of both the relative ordering and
position of calcareous microfossil events, particular with respect to the
MECO event, which, in the absence of an independent age model, acts as a key
stratigraphic marker. Thus, further calibrations of these bioevents are
necessary, particularly from the Pacific Ocean, which is poorly represented
in current calibration schemes. Microfossil reworking is also evident in the
uppermost ∼30 m of Site 865 above the MECO interval,
indicating a dynamic local hydrography capable of remobilizing and mixing
sediments during the middle Eocene. ODP Site 865 is the first tropical
record where the apparent entirety of the MECO event is preserved in
carbonate-bearing sediments, and, despite the reworking at this site, the
primary environmental signals are preserved. Thus, ODP Site 865 represents a
valuable site for future investigations into environmental and biotic change
in the tropics during the MECO.
Data availability
All raw data files are available in the Supplement and also at 10.1594/PANGAEA.920115 (Edgar et al., 2020).
The supplement related to this article is available online at: https://doi.org/10.5194/jm-39-117-2020-supplement.
Author contributions
KME conceived and coordinated the study, processed and analysed the samples
for stable isotope analysis, developed a revised planktic foraminiferal
biostratigraphy, drafted figures, and wrote the article; SMB carried out XRF
analyses and constructed the composite depth scales and drafted associated
figures; SB conducted spectral analysis, constructed the orbital
stratigraphy, and drafted associated figures; PB conducted new calcareous
nannofossil analyses; PNP and HKC developed the initial planktic
foraminiferal biostratigraphy for this site and contributed to the revised
planktic foraminiferal biostratigraphy and taxonomy; CL supported the stable
isotope work. All authors contributed to the writing and editing of the
article.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This research used samples provided by the International Ocean Discovery
Program (IODP). The authors would like to thank Sandra Nederbragt for
technical assistance with δ18O and δ13C analyses
and Cardiff University undergraduate students Bo Fan Peng, Jonathon Gamble,
and Holly Welsby for help picking benthic foraminifera.
Financial support
This research has been supported by the Natural Environment Research Council (grant nos. NE/H016457/1 and NE/P013112/1) and the Leverhulme Trust (grant no. ECF-2013-608).
Review statement
This paper was edited by Laia Alegret and reviewed by two anonymous referees.
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