There is growing interest in the scientific community in reconstructing the
paleoceanography of the Southern Ocean during the Oligocene–Miocene because
these time intervals experienced atmospheric CO
There
is a growing need to better understand the dynamics of the Antarctic
cryosphere and the paleoceanography of the Southern Ocean during the
Oligocene and Miocene, particularly in view of the apparent similarity
between Oligocene and Miocene atmospheric CO
For successions from the southern high latitudes, organic microfossils have repeatedly been shown to represent a useful biostratigraphic tool (e.g. Brinkhuis et al., 2003a, b; Sluijs et al., 2006; Tauxe et al., 2012; Pross et al., 2012; Houben et al., 2011, 2013; Stocchi et al., 2013). Recently, a calibration of Paleocene and Eocene dinocyst origination and extinction events to the geomagnetic polarity timescale (GPTS) has provided the potential for improved age control in the Southern Ocean for this part of the stratigraphic column (Bijl et al., 2013b, 2014). In contrast, a proper magnetostratigraphic calibration of organic microfossil events in the Southern Ocean for the Oligocene and Miocene epochs is still missing. This may be partially ascribed to the fact that Oligocene–Neogene dinocyst assemblages from the Southern Ocean (i) are of relatively low diversity, (ii) are dominated by stratigraphically long-ranging species (Escutia et al., 2011), (iii) contain a considerable number of formally un-described taxa (Escutia et al., 2011), and (iv) include endemic species that are highly specialised to the (paleo)oceanographic conditions prevailing around Antarctica (Houben et al., 2013). The latter means that the distribution of many dinocyst taxa found in Southern Ocean sediments is restricted to that region (e.g. Zonneveld et al., 2013; Houben et al., 2013). These typical Southern Ocean assemblages were essentially established during the Eocene–Oligocene transition, in conjunction with the onset of major Antarctic cryosphere growth (Houben et al., 2013). As a consequence of the abovementioned characteristics, these dinoflagellate cyst assemblages have yet yielded virtually no bio-events that could be calibrated to the internationally used geologic timescale (Gradstein et al., 2012).
Studies on Oligocene–Neogene dinocyst assemblages have been carried out on sediment cores from Prydz Bay (Hannah, 2006; Warnaar, 2006; Houben et al., 2013), Cape Roberts (Hannah et al., 1998, 2000, 2001a, b), within the Cenozoic Investigation in the Western Ross Sea (CIROS) program (Hannah, 1994, 1997), the Antarctic Geological Drilling (ANDRILL) program in the Ross Sea (Warny et al., 2009), and Ocean Drilling Program (ODP) Site 696 in the Weddell Sea (Warnaar, 2006; Houben et al., 2013). Further north in the Southern Ocean, dinocyst records have been generated from ODP Site 1168 (Brinkhuis et al., 2003a) and Deep Sea Drilling Project (DSDP) Site 511 (Goodman and Ford Jr., 1983; Houben et al., 2011). Efforts to accurately calibrate the dinocyst events encountered in these studies to existing timescales were not only hindered by taxonomical challenges, but also by other factors such as a lack of independent age control or relatively short and/or incomplete sedimentary archives. A recent revisit of sedimentary records from the Cape Roberts Project (CRP) has yielded formal descriptions of a number of dinocyst species, the ranges of which have been stratigraphically calibrated to some extent (Clowes et al., 2016).
Paleogeography of the southwestern Pacific Ocean and position of IODP
Site U1356 (red star) at
Clearly, a proper stratigraphic calibration of Oligocene–Miocene dinocyst events in the Southern Ocean could provide age constraints for many sedimentary records that have yet remained stratigraphically poorly dated and would also provide an opportunity to date sediments to be recovered during future drilling campaigns (e.g. McKay et al., 2016). In 2010, IODP Expedition 318 drilled a series of sedimentary archives off the Wilkes Land margin, East Antarctica (notably Site U1356; Fig. 1). In this paper, we present and describe dinocysts encountered in the Oligocene and Miocene part of the succession at Site U1356, with the aim of tying their distribution to the existing integrated bio-magnetostratigraphic age model (Tauxe et al., 2012). We critically evaluate the likelihood that certain species are reworked from older strata or deposited in situ using multivariate statistical analysis. We formally describe two new species, which we consider stratigraphically useful, while we informally report 23 other new taxa. Finally, we establish a first dinocyst zonation scheme for the Oligocene–Miocene of the Southern Ocean.
This research uses samples from IODP Hole U1356A, which was drilled on the
base of the continental rise of the Wilkes Land continental margin (Fig. 1a;
present latitude 63
Sediments consist of alternations of laminated and bioturbated siltstones, occasional claystones and coarser beds, the latter of which are occasionally deformed. Such deposits indicate contourite deposition (Escutia et al., 2011). Occasionally, carbonate is so abundantly present that limestone beds were formed. Diatoms are abundant down-core to about 400 m b.s.f. but are not preserved below this level. The presence of calcareous nanoplankton indicates that the carbonates were derived from a pelagic, biogenic source. Outsized clasts are confined to specific depth intervals. These intervals are 890–875, 735–675, 580–450 and 200–150 m b.s.f. (Escutia et al., 2011).
Integrated Ocean Drilling Program Hole U1356A. Core recovery, lithostratigraphic units, age–depth plot and position of samples taken for palynology. Palaeomagnetic data were obtained from Tauxe et al. (2012), in which black is normal polarity, white is reversed, lilac is uncertain polarity and grey is no data. Age model has been (re-)calibrated to GTS2012 of Gradstein et al. (2012); see text and Table 1.
Age constraints for the Oligocene–Miocene of Hole U1356A. Ages printed in bold are adjusted relative to the information given in Tauxe et al. (2012).
List of dinocyst species and their PCA scores.
Continued.
Continued.
Stratigraphic constraints for the Oligocene–Miocene succession from IODP Hole U1356A comprise calcareous nanoplankton, radiolarian, diatom, sparse dinocyst biostratigraphy and magnetostratigraphy (Tauxe et al., 2012; Fig. 2). The work of Tauxe et al. (2012) was calibrated to the Gradstein et al. (2004) timescale. Here we update the age model of the succession to the GPTS of Gradstein et al. (2012; Table 1). Furthermore, the age model for the Miocene part of the succession had been updated by Crampton et al. (2016) using constrained optimisation of available stratigraphic datums (CONOP; see also Sangiorgi et al., 2017); here we follow this updated age model for the Miocene part of the succession.
These revisions suggest that the sediments between 895.4 and 95.4 m b.s.f.
(i.e. Cores U1356A-95R-3w, 82 cm to U1356A-11R) were deposited between the
earliest Oligocene (33.7 Ma) and early late Miocene (10.7 Ma). A
14 Myr long hiatus at 895.4 m b.s.f. spans the mid-Eocene to earliest
Oligocene (46–33.7 Myr). Crampton et al. (2016) identified a hiatus between
Cores 14R and 15R that was previously unaccounted for. Tauxe et al. (2012)
interpreted a hiatus between Cores 46R and 47R, lasting from
In total, 272 samples were processed and studied for palynology. Sample
resolution varies between 20 cm in Core 84R-85R to over 9 m in intervals of
low core recovery (Fig. 2). Palynological processing of freeze-dried, crushed
and weighed samples (13 g on average, with a standard deviation of
3.4 g) involved decalcification overnight with 30 % hydrochloric acid
(HCl), followed by decanting, rinsing with water and centrifuging (2100
rotations per minute (rpm) for 5 min). The decanted residue was further
processed by adding 38 % hydrofluoric acid (HF) to remove silicates.
After completion of the chemical reaction with HF, samples were placed on a
shaker table for about 2 h, and subsequently were allowed to settle
overnight. An excess of 30 % HCl was added to remove fluoride gels, after
which samples were centrifuged (2100 rpm for 5 min) and decanted. The
entire HF step was repeated for full digestion of silicates. Organic residues
were sieved over a 250
Dinoflagellate cyst taxonomy follows that cited in Williams et al. (2017;
available through AASP – The Palynological Society at
Principal component analysis plot showing species scores of the first two axes. Species that are a priori assumed to be reworked in are in red; species that are a priori assumed to be in situ are in blue (see Table 2 for species list, which species are assumed reworked, eigenvalues, axes scores and cumulative % of variance explained). Dot sizes indicate the total number of encounters in our analyses and gives an indication of the importance of that species in our data.
A prominent feature in the palynological associations from the Oligocene and
Miocene of IODP Site U1356 is the abundance of dinocysts that were initially
assumed reworked from older strata. Reworked Eocene taxa dominate the
palynological assemblages in the lowermost 30 m of the Oligocene (Cores 95R
to 93R; Houben et al., 2013). In the lower Oligocene, the percentage of
reworked elements decreases to around 10–15 % of the total palynomorph
count (Houben et al., 2013). Dinocyst studies of Oligocene and younger
sediments in the circum-Antarctic area are scarce, which makes the assumption
that certain taxa recorded in the Oligocene strata in our record are reworked
uncertain. For instance, at IODP Site U1356
Informal diagnoses of new dinocyst species found and reference to corresponding images in the plates.
Of the 167 dinocyst species identified, 67 have positive scores on the first
two PCA axes (Table 2; Fig. 3). Of these 67 species, we have a priori
inferred 57 (85 %) of them to be reworked (Table 2 and Fig. 3). We do
note that some of these species (e.g.
Stratigraphic chart of Oligocene–Miocene dinocyst species encountered at Site U1356, plotted in the order of first appearances. Age scale in Gradstein et al. (2012). Based on key events of species highlighted in boldface, a dinocyst zonation scheme is proposed (see Table 6 for definitions of zone boundaries).
Schematic drawings of the six stratigraphically important species of
List of
Numerous previously unknown dinocyst species were encountered (Tables 2, 3;
Fig. 4). Here we formally describe the new species
Division Subdivision Class Subclass Order Suborder Family Genus Species
Genus Species
Dinocyst zone boundaries: depths and calibrated ages.
Summary of the dinocyst zones proposed in this paper for the
Oligocene–Miocene Southern Ocean. See Table 5 for a summary of the depth and
age data. M.esc:
Our high-resolution dinocyst biostratigraphy (Fig. 4) generally confirms the
published age model for the Oligocene and Miocene interval of Hole U1356A
(Tauxe et al., 2012); we have refined the age control around the
Oligocene–Miocene boundary. New dinoflagellate cyst events recognised in
this study enabled the identification of the position and constrained the
duration of the hiatus around the Oligocene–Miocene boundary. Specifically,
the presence of
We have identified 13 key (regional) dinocyst extinction or origination events (bold in Figs. 4, 6) from all the in situ dinocysts (Fig. 4). We prioritised those dinocyst events that had relatively high abundances, had sharp appearances or extinctions and can be calibrated well to magneto-subchrons. These index events form the basis for the Oligocene–Miocene Southern Ocean dinocyst zonation proposed here (see Table 5 for an overview of zone boundary definitions), which comprises 10 zones for the Oligocene and 3 zones for the Miocene. We name the zones as Southern Ocean Oligocene Dinocyst Zone (SODZ) or Southern Ocean Miocene Dinocyst Zone (SMDZ). We acknowledge that the zonation is incomplete due to a hiatus covering the early Miocene (22.5–17 Ma) and one encompassing the mid-Miocene (13–10.8 Ma) and the occasional low core recovery. We calibrate the ages of the zone boundaries to the GPTS by indicating foremost where in existing magnetostratigraphic chrons the zone boundaries occur. This is consistently indicated as a percentage into a certain chron, measured from the bottom, and by assuming linear sedimentation rates between reversals in our record.
Selected dinocyst species found in the present study, which have well-calibrated events elsewhere that are broadly consistent with their range at U1356.
Although the majority of dinocyst taxa we find in our record comprise species that are endemic to the Southern Ocean, or are long-ranging and cosmopolitan, some species have well-calibrated stratigraphic ranges elsewhere. We list a selection of these species in Table 6. The consistency between our dinocyst stratigraphy and those of others lends some support to our dinocyst zonation and ties our zonation at least to some extent to other dinocyst stratigraphies. However, we are unable to tie our dinocyst events very precisely to other stratigraphies because many magnetic reversals in the record at U1356 fall within core gaps, which make the interpretation of magnetic reversals uncertain, and therewith, the position of the dinocyst events with respect to the magnetostratigraphy. Moreover, comparison of dinocyst events across latitudes has proven complicated because dinocysts tend to be very sensitive to environmental conditions, which may not necessarily change in tandem over the globe through time. Therefore, dinocysts generally tend to have asynchronous originations and extinctions across different latitudinal bands over the globe (e.g. Williams et al., 2004). Nonetheless, with an error bar of about 200 kyr, some dinocyst events (Table 6) seem to be consistent between Site U1356 and a variety of locations over the globe, which adds to their significance as a biostratigraphic marker, and may help to stratigraphically date sediments from other parts of the Southern Ocean as well.
We formally describe two new dinoflagellate cyst species and present a first
zonation scheme for Oligocene–Miocene dinocysts of the Southern Ocean.
Temporal resolution of the proposed zonation is in some intervals
sufficiently high to allow accurate (i.e. ca. 500 kyr resolution)
stratigraphic calibration. However, in some intervals, poor core recovery, two
hiatuses covering the early Miocene (
Data are available at
PKB, FS and JP designed the research. AJPH, PKB, AB and FS carried out dinocyst analyses for the earliest Oligocene, the mid- to upper Oligocene, the Oligocene–Miocene boundary and the Miocene respectively. PKB integrated, cross-validated and compiled the data and wrote the paper with input from all co-authors.
The authors declare that they have no conflict of interest.
We thank the constructive reviews of Michael J. Hannah and Kasia S. Śliwinśka, which really improved our paper. This research used samples and data from the Integrated Ocean Drilling Program. IODP was sponsored by the US National Science Foundation and participating countries under management of Joined Oceanographic Institutions Inc. PKB and FS thank NWO-NNPP grant no. 866.10.110, NWO-ALW VENI grant no. 863.13.002 for funding and Natasja Welters for technical support. We thank Margot Cramwinckel for producing the illustrations in Fig. 5. Edited by: Luke Mander Reviewed by: Michael Hannah and Kasia K. Sliwinska