The volume of the Antarctic continental ice sheet(s) varied
substantially during the Oligocene and Miocene (
Numerical paleoclimate models predict that with the current rate of ice
volume loss (up to
On both glacial–interglacial (Parrenin et al., 2013) and longer-term Cenozoic
timescales (Pagani et al., 2011; Zachos et al., 2008), Antarctic ice volume
changes have been mostly linked to changes in atmospheric
EAIS volume changes have been suggested for the Oligocene and Miocene based
on a number of deep-sea
Lithology, TEX
In 2010, the Integrated Ocean Drilling Program (IODP) cored a sedimentary
archive at the boundary of the continental rise and the abyssal plain
offshore Wilkes Land with a well-dated Oligocene and Miocene sequence: IODP
Site U1356 (Fig. 1), suitable for paleoclimatological analysis. In this study
we use the now well-established ratio between several isoprenoid glycerol
dialkyl glycerol tetraethers (GDGTs), the so-called TEX
Together with the companion papers by Salabarnada et al. (2018) and Bijl et al. (2018a) on the lithology and dinocyst assemblages of Site U1356, we contribute significantly to the limited knowledge that exists on Oligocene–Miocene paleoceanographic conditions close to the Antarctic margin.
Integrated Ocean Drilling Program (IODP) Expedition 318 Site U1356 was cored
about 300 km off the Wilkes Land coast (63
At present, IODP Site U1356 receives sediments transported from the shelf and the slope as well as the in situ pelagic component. Although we have no quantitative constraints on the water depth during the Oligocene and Miocene, the sediments as well as the biota suggest a deep-water setting at Site U1356 during these times (Houben et al., 2013; Escutia et al., 2014). Sedimentary units of Hole U1356A have been defined in the shipboard report (Escutia et al., 2011). Detailed logging of the sediments recovered in Hole U1356A has revealed that the Oligocene and Miocene sedimentary record (between 95.40 and 894.80 m below sea floor, m b.s.f.) consists mostly of alternations of (diatomaceous) laminated and bioturbated sediments, gravity flow deposits, and carbonate beds (Salabarnada et al., 2018; Sangiorgi et al., 2018) (Fig. 2). Gravity flow deposits include mass-transport deposits (MTDs) formed by the slump and debris flow sediments of the Miocene, Oligocene and Eocene–Oligocene transition (EOT), and the late Oligocene–Miocene turbidite-type facies as defined by Salabarnada et al. (2018). Samples from the MTDs contain the largest contribution of reworked older material transported from the continental shelf (Bijl et al., 2018a), while in the other lithologies, this component is reduced or absent.
Between 593.4 and 795.1 m b.s.f., there are clear alternations between
greenish carbonate-poor laminated and grey bioturbated deposits with some
carbonate-rich bioturbated intervals. These deposits are interpreted as
contourite deposits recording glacial–interglacial environmental variability
(Salabarnada et al., 2018). Above 600 m b.s.f., sediments mostly consist of
MTDs with low to abundant clasts (Fig. 2). However, between the MTDs greenish
or grey laminated deposits and greenish or grey bioturbated deposits are
preserved. Near the bottom of Unit III as defined in the shipboard report
(around 433 m b.s.f. and below), a different depositional setting is
represented with alternations between pelagic clays and (ripple)
cross-laminated sandstone beds (Escutia et al., 2011). These sandy (ripple)
cross-laminated beds are interpreted as turbidite deposits (Salabarnada et
al., 2018). Above these turbidite deposits, there are diatomaceous silty
clays that are characterized by an alternation of green laminated and grey
homogeneous (bioturbated) silty clays. Apart from their diatom content, these
deposits are very similar to the Oligocene alternations between
carbonate-poor laminated and carbonate-containing bioturbated deposits and
are therefore interpreted likewise (Salabarnada et al., 2018). Up-core within
the Miocene section, the alternations between laminated and homogeneous
diatomaceous silty clays become more frequent. In the upper Miocene sections
(95.4–110 m b.s.f.) laminations become less clear as the sediments become
less consolidated; however green and grey alternations can still be
distinguished. The more diatomaceous green deposits are interpreted as
interglacial stages. Samples analyzed for TEX
The Oligocene and Miocene Southern Ocean paleoceanographic configuration is
still obscure and controversial. Some studies suggest that most Southern
Ocean surface and deep-water masses were already in place by Eocene–Oligocene
boundary times (Katz et al., 2011). Neodymium isotopes on opposite sides of
Tasmania suggest that an eastward-flowing deep-water current has been present
since 30 Ma (Scher et al., 2015). A westward-flowing Antarctic Circumpolar
Counter Current (ACCC) was already established during the middle Eocene
(49 Ma; Bijl et al., 2013) (Fig. 1). The Tasmanian Gateway opening also allowed
the proto-Leeuwin current (PLC) flowing along southern Australia to continue
eastward (Carter et al., 2004; Stickley et al., 2004) (Fig. 1). However,
numerical modeling studies show that throughflow of the Antarctic Circumpolar
Current (ACC) was still limited during the Oligocene (Hill et al., 2013)
because Australia and South America were substantially closer to Antarctica
(Fig. 1) than today (Markwick, 2007). Moreover, tectonic reconstructions and
stratigraphy of formations on Tierra del Fuego suggest that following open
conditions in the middle and late Eocene, the seaways at Drake Passage
underwent uplift starting at 29 Ma and definitive closure around 22 Ma
(Lagabrielle et al., 2009). Evidence for active spreading and transgressional
deposits in the Tierra del Fuego area records the widening of Drake Passage
from 15 Ma onwards. The timing of the Drake Passage opening, which allowed
for significant ACC throughflow, is still heavily debated (Lawver and
Gahagan,
2003; Livermore et al., 2004; Scher and Martin, 2006, 2008; Barker et al.,
2007; Maldonado et al., 2014; Dalziel, 2014). Contourite deposits suggest that
strong Antarctic bottom-water currents first appeared in the early Miocene
(21.3 Ma) and that Weddell Sea Deep Water has been able to flow westwards into
the Scotia Basin since the middle Miocene (
Antarctica was positioned more eastward during the Oligocene and Miocene
relative to today (due to true polar wander; van Hinsbergen et al., 2015),
and Site U1356 was more to the north during the Oligocene and Miocene
compared to today (approximately 59
Oligocene sediments were recovered in the section from 894.68 m b.s.f.
(first occurrence (FO)
For the Miocene section of Hole U1356A we follow Sangiorgi et al. (2018), who applied the constrained optimization methodology (CONOP) of Crampton et al. (2016) to diatom and radiolarian biostratigraphic events to construct an age model. Based on the application of CONOP to the diatom and radiolarian biostratigraphic events, a second hiatus was identified spanning approximately the interval between 13.4 and 11 Ma.
In addition to the 29 samples from the Miocene section presented in Sangiorgi
et al. (2018), a total of 132 samples from the Oligocene and early Miocene
part of the sedimentary record (Table S1 in the Supplement) were processed
for the analysis of GDGTs used for
TEX
We have used the branched and isoprenoid tetraether (BIT) index (Hopmans et
al., 2004) to verify the relative contribution of terrestrial GDGTs in our
samples, compared to marine GDGTs. As isoprenoid GDGTs (isoGDGTs), used for
the TEX
Oxic degradation of GDGTs does not affect the relative amounts of individual
isoGDGTs (Huguet et al., 2009; Kim et al., 2009). However, oxic degradation
may lead to an increased relative influence of soil-derived isoGDGTs, which
could bias the TEX
The TEX
In addition to the TEX
Despite these recent efforts in improving the TEX
To obtain an estimate for the long-term average SST trends and confidence levels, a local polynomial regression model (LOESS) has been applied using R, which is based on the local regression model “cloess” of Cleveland et al. (1992). This method of estimating the long-term average trend is preferred over a running average because it accounts for the variable sample resolution. For the parameter “span”, which controls the degree of smoothing, a value was automatically selected through generalized cross-validation (R package fANCOVA; Wang, 2010).
A total of 116 samples spanning the Oligocene and earliest Miocene were
analyzed for TEX
Although sampling of disturbed strata was avoided, a total of 46 Oligocene and Miocene samples proved to be obtained from MTDs after detailed logging by Salabarnada et al. (2018). Hence, samples from these beds may not reflect in situ material exclusively. For the EOT slumps, this is supported by a high degree of reworked Eocene specimens within the dinoflagellate cyst assemblage below 880.08 m b.s.f. (Houben et al., 2013). In addition, the clast-bearing deposits of Units II and IV and decimeter-thick granule-rich interbeds of Unit VIII (Fig. 2) are interpreted as ice-rafted debris (IRD) deposits (Escutia et al., 2011; Sangiorgi et al., 2018) and thus indicate the presence of icebergs above the site during deposition of these intervals. To avoid potential bias due to allochthonous input and reworking of older sediments, all samples from MTDs are excluded from the SST reconstructions.
A contribution of terrestrial isoGDGTs can also bias the marine pelagic
TEX
Lithology and paleomagnetic polarities as in Fig. 2, but plotted
against age (Ma), updated to the GTS2012 timescale (Gradstein et al., 2012).
Sections with IRD and samples with sea ice and
After excluding samples with potentially biased TEX
Based on the linear temperature calibration of Kim et al. (2010) (black curve
in Fig. 3a), our TEX
The SST record for Site U1356 based on the BAYSPAR model shows the same trend
as the SST record generated with the linear calibration, but for SST values
below 20.5
Our TEX
In general, Oligocene SST estimates are higher than the SST estimates
reconstructed with other proxies at other high-latitude Southern Ocean Sites
511 and 689 (Fig. 3a). However, the reconstructed
Sangiorgi et al. (2018) compared TEX
Biota-based temperature reconstructions at such high latitudes are likely
skewed towards summer conditions, as has also been suggested for Site 689 and
Site 511 (Petersen and Schrag, 2015; Plancq et al., 2014). An important
reason for this could be the light limitation at high latitudes during winter
(e.g., Spilling et al., 2015), which is unfavorable for the growth and bloom
of phytoplankton and organisms feeding on phytoplankton. The potential
summer bias in high-latitude TEX
We aim to explore the implications of the long-term trends in our
TEX
A global benthic foraminiferal stacked
The LOESS curves through the glacial and interglacial data show similar
trends and show a good resemblance to the global benthic
The fact that the LOESS temperature trends mirror the benthic
Considering (1), in the modern-day Southern Ocean, bottom water forms
through mixing along the Antarctic Slope Front (ASF) of Circumpolar Deep
Water (CDW) and High-Salinity Shelf Water (HSSW), which forms as a consequence
of sea ice formation (Gill, 1973; Jacobs, 1991). Associated with the ASF is
the westward-flowing Antarctic Slope Current (ASC), which contributes to the
bottom-water formation and results from the geostrophic adjustment of Ekman
transport to the south, which is driven by the predominantly easterly winds
around Antarctica (Gill, 1973). It has been suggested that, after the
establishment of its shallower westward-flowing counterpart, the ACCC, around 49 Ma (Bijl et al., 2013), an ASC
was established near Site U1356 in the early Oligocene (Scher et al., 2015).
In areas where sea ice was formed during the Oligocene and Miocene the ASC
could have enhanced mixing between HSSW and CDW similar to today. For the
Wilkes Land margin this might have been the case for the earliest Oligocene
and MMCT for which we find sea ice indicators in the dinoflagellate cyst
assemblages (Bijl et al., 2018b; Houben et al., 2013). These sea ice
dinoflagellate cysts seem to indicate that winter temperatures at the Wilkes
Land margin were cold enough to allow sea ice formation and therefore maybe
formation of deep waters along the Wilkes Land coast during the earliest
Oligocene and MMCT. However, most of the record is devoid of sea ice
indicators, suggesting that modern-day processing of deep-water formation is
unlikely to have occurred at the Wilkes Land margin. Still, neodymium
isotopes of fossil fish teeth from Site U1356 have suggested that deep-water
formation took place at the Adélie and Wilkes Land margin during the
Eocene (Huck et al., 2017), when pollen indicates near-tropical warmth (Pross
et al., 2012) . Model studies have suggested that during such warm periods
seasonal density differences may still induce deep-water formation or
downwelling of waters around Antarctica (Goldner et al., 2014; Lunt et al.,
2010). Alternatively and more likely, sea ice may have formed in the cooler
Ross Sea and been transported along the Wilkes Land coast similar to today
during the Oligocene and Miocene, meaning that deep water formed in the Ross
Sea where glaciers extended onto the Antarctic shelf (see Sorlien et al.,
2007). In that case, the absence of sea ice dinoflagellate cysts during most
of the Oligocene and the MMCO at Site U1356 would mean that, in contrast to
the earliest Oligocene and MMCT, sea ice coming from the Ross Sea was
prevented from reaching Site U1356 by too warm winter SSTs. This is in
accordance with the relatively warmer (summer) SST values for Site U1356
during most of the Oligocene and the MMCO. If the reconstructed SST trends of
Site U1356 are representative for the climatic trends of a larger region
(i.e., including the Ross Sea as a potential region for deep-water formation),
this climatic signal may have been relayed to the deep ocean and recorded in
the stable oxygen isotope composition of benthic foraminifera at far-field
sites. In fact, Southern Ocean-sourced deep waters may have reached as far as
the north Pacific during the Oligocene (Borelli and Katz, 2015). If this is
the case, the consistent long-term trends between the SST of Site U1356 and
the benthic
Only one BWT record is available for the Oligocene, which is based on
Mg
Alternatively, long-term SST trends as well as Southern Ocean BWT trends
(Sites 747 and 1171) are governed by large-scale tectonic processes, such as
the opening and closure of the Drake Passage, as was suggested by Lagabrielle
et al. (2009). Opening of the Drake Passage could result in increased
isolation of the Antarctic continent through the establishment of a
(proto-)ACC. In turn, this would result in effective blocking of northerly
sourced warmer waters as well as ice sheet expansion, thereby resulting in a
simultaneous benthic
As an alternative hypothesis, reconstructed SSTs at Wilkes Land may depend
on the volume of the ice sheet in the hinterland. In that scenario most of
the long-term trends in the
For the Oligocene, the offset between the glacial and interglacial LOESS
curves is constant over time (Fig. 4). Irrespective of the chosen calibration
(i.e., TEX
This glacial–interglacial SST difference is also smaller than the observed
amplitude of the variability in our temperature record (2
The average consistent offset between glacial and interglacial values seems
to disappear for each of the Miocene data clusters at
If the recorded glacial–interglacial SST variability in the Oligocene is
representative for the SST variability at the region of deep-water formation,
it should be considered when interpreting benthic foraminiferal
A considerable influence of deep-sea temperature on benthic
We reconstruct (summer-biased) SSTs of around 17
GDGT concentrations are available in
the Supplement Table S1 as integrated peak values obtained from the
(U)HPLC-MS chromatograms. Based on these concentrations, TEX
The supplement related to this article is available online at:
FS, PKB, HB, and SS designed the research. JDH, PKB, and AJPH carried out Oligocene GDGT analyses. CE and AS incorporated the lithological data. PKB, FP, and SS assisted in GDGT analytical procedures and interpretation. JDH wrote the paper with input from all authors.
The authors declare that they have no conflict of interest.interest, please state what competing interests are relevant to your work.
Julian D. Hartman, Francesca Sangiorgi, Henk Brinkhuis, and Peter K. Bijl acknowledge the NWO Netherlands Polar Program project number 866.10.110. Stefan Schouten was supported by the Netherlands Earth System Science Centre (NESSC), funded by the Dutch Ministry of Education, Culture and Science (OCW). Peter K. Bijl and Francien Peterse received funding through NWO-ALW VENI grant nos. 863.13.002 and 863.13.016, respectively. Carlota Escutia and Ariadna Salabarnada thank the Spanish Ministerio de Econimía y Competitividad for grant CTM2014-60451-C2-1-P. We thank Alexander Ebbing and Anja Bruls for GDGT sample preparation during their MSc research. This research used samples from the Integrated Ocean Drilling Program (IODP). IODP was sponsored by the US National Science Foundation and participating countries under management of Joined Oceanographic Institutions Inc. This research is a contribution to the SCAR PAIS program. We thank Stephen Gallagher and the two anonymous reviewers for their thorough review and constructive comments, which helped improve this paper. Edited by: David Thornalley Reviewed by: Stephen Gallagher and two anonymous referees