Improvements in our capability to reconstruct ancient
surface-ocean conditions based on organic-walled dinoflagellate cyst
(dinocyst) assemblages from the Southern Ocean provide an opportunity to
better establish past position, strength and oceanography of the subtropical
front (STF). Here, we aim to reconstruct the late Eocene to early Miocene
(37–20 Ma) depositional and palaeoceanographic history of the STF in the
context of the evolving Tasmanian Gateway as well as the potential influence of
Antarctic circumpolar flow and intense waxing and waning of ice. We approach
this by combining information from seismic lines (revisiting
existing data and generating new marine palynological data from Ocean Drilling Program (ODP) Hole 1168A)
in the western Tasmanian continental slope. We apply improved taxonomic
insights and palaeoecological models to reconstruct the sea surface
palaeoenvironmental evolution. Late Eocene–early Oligocene (37–30.5 Ma)
assemblages show a progressive transition from dominant terrestrial
palynomorphs and inner-neritic dinocyst taxa as well as cysts produced by
heterotrophic dinoflagellates to predominantly outer-neritic/oceanic
autotrophic taxa. This transition reflects the progressive deepening of the
western Tasmanian continental margin, an interpretation supported by our new
seismic investigations. The dominance of autotrophic species like
Late stages of continental break-up between Australia and Antarctica in the late Eocene–early Miocene led to gradual deepening and widening of the Tasmanian Gateway (Lawver et al., 1992; Cande and Stock, 2004; Whittaker et al., 2013). This process redirected Southern Ocean surface water circulation (Stickley et al., 2004a; Bijl et al., 2013; Sijp et al., 2014, 2016) and regionally redistributed ocean heat (Sijp et al., 2014, 2016) with notable consequences for Antarctic surface water temperatures (Houben et al., 2019; Sauermilch et al., 2021). While the general oceanographic consequences of the opening of the Tasmanian Gateway – the breakdown of gyres and the onset of a wind-driven, eastward-flowing current – are now broadly understood (Stickley et al., 2004a; Bijl et al., 2013; Hill et al., 2013; Sijp et al., 2014; Houben et al., 2019), the development of Southern Ocean frontal systems (Nelson and Cooke, 2001), such as the subtropical front (STF), and ultimately the evolution and strengthening of the Antarctic Circumpolar Current (ACC) as the gateway widens (Hill et al., 2013) are largely unexplained.
To a large extent, our knowledge on the development of Paleogene Southern Ocean surface
circulation is built on biogeographic patterns in organic-walled dinoflagellate cyst assemblages (Wrenn and Beckman, 1982; Bijl et
al., 2011) as well as supporting information from other microfossil groups
(Pascher et al., 2015), both backed up by numerical model simulations (e.g.
Huber et al., 2004; Sijp et al., 2014). Dinoflagellates are single-celled,
predominantly marine, eukaryotic protists, which represent an important
group of marine plankton. During their life cycle,
In 2000, Ocean Drilling Program (ODP) Leg 189 drilled five sites around
Tasmania – on the western Tasmanian margin (Site 1168), the South Tasmanian
Rise (sites 1169, 1170 and 1171) and the East Tasmanian Plateau (Site
1172) – with the goal of reconstructing the timing, nature and climatic
consequences of the opening of the Tasmanian Gateway (Fig. 1a; Exon et al., 2001a). Site 1168 is presently located north of the STF and contains a
near-continuous sediment record of late Eocene–Quaternary age. The site is
in the path of the Zeehan Current, a saline, warm surface water flowing off
western Tasmania that originates from the Leeuwin Current (Ridgway and
Condie, 2004). Brinkhuis et al. (2003) provided a preliminary overview of
the late Eocene–Quaternary dinocyst assemblage distribution and illustrated
the main trends in palynomorph distribution. The study by van Simaeys et al. (2005) detected
the typical boreal taxon
Empirical relationships between extinct species abundance and proxy data for oceanographic conditions (e.g. Wall et al., 1977; Pross and Brinkhuis, 2005; Sluijs et al., 2005; Frieling and Sluijs, 2018) have strengthened the use of dinocysts for past oceanographical reconstructions. These have been used, for example, to detect a circum-Antarctic biotic turnover across the Eocene–Oligocene transition (EOT) with the origination of a sea-ice ecosystem (Houben et al., 2013) and a shift towards upwelling-related primary productivity (Houben et al., 2019; Bijl et al., 2018) in the Southern Ocean. Meanwhile, knowledge of dinocyst ecological preferences in the modern ocean has greatly advanced in the last couple of decades (e.g. Zonneveld et al., 2013), particularly for the Southern Ocean (Prebble et al., 2013; Marret et al., 2020). These modern affinities of dinocysts have been successfully used to resolve Antarctic-proximal oceanic conditions in the Oligocene and Miocene (Sangiorgi et al., 2018; Bijl et al., 2018; building on pioneering work from studies such as Hannah et al., 2000). Moreover, recently, the tectonic history of Australian–Antarctic separation and the palaeoenvironmental consequences have been reconstructed in more detail (Whittaker et al., 2013; Williams et al., 2019; Sauermilch et al., 2019a, 2021). Together, this knowledge facilitates reinterpretation of the original shipboard reconstruction of the depositional environments at Site 1168 (Exon et al., 2001b).
Here, we revisit Site 1168 (Exon, 2001b; Brinkhuis et al., 2003; Hill and Exon, 2004) and the first results, and focus in more detail on the sediments deposited during the late Eocene–early Miocene. Integrating new interpretations of the seismo-stratigraphy, an improved integrated bio-magnetostratigraphic age model and new interpretations from high-resolution dinocyst assemblage data, we provide an updated view of the evolving depositional environment and palaeoceanographic conditions. We analyse our results in the context of those from other sites in the Australo-Antarctic Gulf (AAG) (IODP Site U1356 – Bijl et al., 2018; ODP Site 1128 and Browns Creek – Houben et al., 2019; east of Tasmania, ODP Site 1172 – Sluijs et al., 2003; in and offshore of the Ross Sea, Cape Roberts Project – Clowes et al., 2016; and Deep Sea Drilling Project, DSDP, Site 274 – Hoem et al., 2021a) to build a picture of the late Eocene–early Miocene (37–20 Ma) palaeoceanographic evolution in the region of the widening Tasmanian Gateway.
Ocean drilling at the western Tasmanian margin ODP Site 1168 (42
Age–depth model of ODP Site 1168, based on magneto- and biostratigraphy (calcareous nannofossils, planktic foraminifera, diatoms and dinocysts) after Stickley et al. (2004b), here recalibrated to GTS2012 of Gradstein et al. (2012) (Table S1 in the Supplement). A smoothed line is drawn through the age constraints using the loess smoothing method with a span of 0.1. Lithological units are indicated to the right of the palaeomagnetic chrons and the indication of epochs (Exon et al., 2001b). The arrows indicate the sedimentation rate.
Palaeolatitudes for the western Tasmanian margin between 37 and 20 Ma changed
from 63 to 55
We present seismo-stratigraphic interpretations of two multichannel seismic reflection profiles crossing (line SO36-47) and closely passing (line N417) Site 1168 (Fig. 1c, d). We follow the seismic interpretation of Sauermilch et al. (2019a) and Hill et al. (1997) for the sedimentary units and continental crust, and the exhumed subcontinental mantle part is our own (following the characteristics of McCarthy et al., 2020). Key reflections are linked to lithological units, largely following interpretations by Sauermilch et al. (2019a) and the Shipboard Scientific Party (Exon et al., 2001b). We used the time–depth relationship derived from downhole sonic p-wave velocities measured at Site 1168 (Exon et al., 2001b) for the core–seismic correlation. This method of velocity measurements is most accurate for seismic integration, particularly for calcareous-rich sediments (Sauermilch et al., 2019b).
ODP Site 1168 is located within an enclosed sedimentary basin, surrounded by now buried bathymetric highs (Fig. 1c). Characteristic seismic features, amplitude and internal reflection patterns can be used to follow key reflections and sedimentary units across this embedded basin. However, some uncertainties remain, as the basement highs disrupt the continuity of the reflection pattern. In order to calculate the regional thickness extensions of the sedimentary units in metres, we use interval velocities from downhole sonic measurements at Site 1168. We interpreted the exhumation of subcontinental mantle material offshore of western Tasmania, along its ocean–continent transition zone (OCT; Fig. 1). The interpretation of the mantle domain followed the analytical scheme of Gillard et al. (2015) and McCarthy et al. (2020), who investigated exhumed mantle domains of the offshore central southern Australian and conjugate East Antarctic margins (Seamount B, east of Adélie Rift Block) respectively.
We studied 123 samples for palynological content: 84 were samples that were revisited
and recounted from the previously studied palynological record by Brinkhuis
et al. (2003), and 39 were additional samples processed to increase the
resolution. The processing of sedimentary samples for palynological analysis
followed standard procedures at the Laboratory of
Palaeobotany and Palynology, Utrecht University (e.g. Brinkhuis et al., 2003; Bijl et al.,
2018). Dried sediment samples were crushed and weighed (on average 10 g,
standard deviation, SD, of
We provide detailed counts of dinocysts and other aquatic palynomorphs (acritarchs and prasinophyte algae) (Table S2 in the Supplement). Dinocyst taxonomy as cited in Williams et al. (2017) is followed. Acritarch and prasinophyte taxonomy follows that of Hannah (1998, 2006), Prebble et al. (2006) and Hartman et al. (2018a). We also included a broad count of terrestrial palynomorphs in order to calculate the relative abundances between terrestrial and marine palynomorphs. Detailed analyses of terrestrial palynomorphs and terrestrial palaeoenvironmental evolution at Site 1168 is the focus of another paper (Amoo et al., 2021).
The present-day distribution of dinocysts (Fig. 3) depends mostly on surface water temperature but also on nutrient availability, salinity, water depth, bottom-water oxygen, primary productivity and sea-ice cover (e.g. Dale, 1996; Prebble et al., 2013; Zonneveld et al., 2013; Marret et al., 2020). In order to use dinocyst assemblages as palaeoceanographic proxies, we assume that the habitat affinities and trophic strategy of modern dinoflagellate species have remained similar through time; therefore, we utilise the modern relationship between dinocysts and overlying water properties as a model for the “deep-time” ecological niches (Sluijs et al., 2005; Prebble et al., 2013; Sangiorgi et al., 2018). Palaeoecological preferences of extinct species are uncertain. To tackle this problem, the ecological affinity of extinct dinocyst taxa was reconstructed across well-defined climatic transitions (Brinkhuis, 1994; Houben et al., 2013; Egger et al., 2018), using the co-occurrence of extinct species with those for which the ecological information is still available, e.g. modern species (e.g. Schreck and Matthiessen, 2013), or in comparison to other palaeoceanographic proxies for temperature, runoff/freshwater input and nutrient conditions (Bijl et al., 2011; De Schepper et al., 2011; Frieling and Sluijs, 2018).
The present-day distribution of dinoflagellate cyst
assemblages in the southwest Pacific sector of the Southern Ocean, derived
from surface sample data generated by Prebble et al. (2013) and displayed in
Sangiorgi et al. (2018). The black lines illustrate the location of oceanic
fronts (Orsi et al., 1995), and yellow dots mark the present-day locations of
the drill sites studied (ODP Site 1168) and discussed in this paper (IODP Site
U1356, ODP Site 1172 and 1128, DSDP Site 269 and 274, Cape Roberts Project –
CRP, and Browns Creek). STF is the subtropical front, SAF is the
subantarctic front and AAPF is the Antarctic polar front. Land masses are
indicated in black, and the continental lithosphere is shown in light grey in the location
map generated by GPlates freeware (
In this paper, we apply this principle to divide dinocyst taxa into
eco-groups (Table 1), based on the context of the Southern Ocean. We utilise
the notion that protoperidinioid cysts and peridinioid dinocysts in general
(e.g.
Finally, we track Southern Ocean frontal systems' movements using the
environmental associations of modern dinocysts by Prebble et al. (2013) as a
model. Today, a pronounced latitudinal separation of dinocyst assemblages
across the Southern Ocean exists (Prebble et al., 2013) (Fig. 3). Surface
samples from the Antarctic margin, south of the subantarctic front (SAF),
are dominated by the peridinioid cysts
Dinocyst assemblage groups and their ecological and frontal system affinity.
Acritarchs represent “acid resistant organic walled microfossils of unknow
affinity” (Evitt, 1963) and likely represent organic remains of polyphyletic
origin, including prasinophyte algae. As of 2021, the latter group has
received more attention, and some life cycle stages are indeed composed of
preservable organic matter (Parke et al., 1978; Mudie et al., 2021). These
may be single-celled and/or multicellular, colonial stages. Examples include
the multicellular
The lithological boundaries from ODP Site 1168 are tied to the crossing seismic reflection profile (Fig. 1c) using the time–depth relationship from downhole sonic velocities (Exon et al., 2001a, b). The drilled lithological units V and III–I are clearly visible in the seismic lines, whereas the very thin (13.4 m) Unit IV is only detectable as a strong reflection pattern (Fig. 1c). An additional key unconformity underlies the drilled sections, dated to the Palaeocene, extrapolated from industry well site Cape Sorell 1 (Boreham et al., 2002) (Fig. 1c). Here, we focus on the sedimentary units V–II, deposited between the late Eocene and early Miocene.
Unit V reaches a maximum thickness of about 0.5 s TWT (two-way travel time)
or
The seismic characteristics of the upper section of Unit V are significantly different on each side of the Tasmanian Fracture Zone (TFZ) and the connected basement highs. The deposition east of the TFZ, around Site 1168, shows chaotic, hummocky features and reworking. Particularly the hummocky reflections have been previously interpreted as deltaic influence (Exon et al., 2001a, b). West of the TFZ, the deposition of the late Eocene material is more homogenous and undisturbed along the upper slope (Fig. 1d). Further offshore along the lower continental slope to abyssal plain, this sedimentary unit has been slightly reworked. This structure shows onlapping reflections onto the slope and is likely part of the contourite drift system observed along the Australian and Antarctic margins that formed due to strengthening clockwise currents in the AAG (Sauermilch et al., 2019a).
Stacked relative abundance of the three major palynomorph
groupings (terrestrial palynomorphs, acritarchs and dinocysts), plotted
against depth (m b.s.f.), next to the magnetostratigraphic framework
(Gradstein et al., 2012), lithological units, and shipboard total organic
carbon (TOC, wt %) and calcium carbonate (CaCO
The overlying Unit III, dated to the early Oligocene, is thinner (maximum
thickness
Unit II is
The most oceanward (SW) section of the seismic line (Fig. 1c) shows features
characteristic of subcontinental mantle exhumation. A high-amplitude,
continental dipping reflection is detected at
We record a fair coupling between changes in lithology and changes in the three major palynomorph groups: dinocysts, acritarchs and terrestrial palynomorphs (Fig. 4). The terrestrial palynomorphs are dominant in the upper Eocene–lower Oligocene part (units V–III) of the record, which also has the highest TOC levels. In the middle of lithological Unit III, below 700 m b.s.f. (30.5 Ma), the high relative abundance of terrestrial palynomorphs rapidly decreases to 5 %–15 %, concomitant with a decrease in TOC, and remains low in the overlying nannofossil chalks and silty claystones (Unit II). In the Oligocene, from 700 to 400 m b.s.f. (30.5–22.4 Ma), dinocysts are the most abundant palynomorph group and comprise up to 88 % of all palynomorphs. In the Miocene, acritarchs dominate the assemblages, reaching up to 90 % of the total palynomorph assemblage. Acritarchs reach the highest relative and absolute abundances at 540–520 m b.s.f. and above 400 m b.s.f. The second peak in the lower Miocene is synchronous with increased amounts of calcium carbonate (40 wt %).
From the 123 samples counted, 106 samples yield abundant and well-preserved dinocysts (Fig. 5). Identification of dinocysts on a species level was possible in most cases; however, some dinocysts were only categorised on a genus level when distinctive features were lacking. The quantitative requirement of counting to a minimum of 200 dinocyst was met for most of the samples, except for those from the upper Eocene, where terrestrial palynomorphs dominate the record, and from the lower Miocene, where acritarchs dominate the palynomorph assemblages (Table S2 in the Supplement).
Dinocyst assemblages are dominated by the Gonyaulacoid (autotrophic) taxa
Acritarch assemblages are dominated by skolochorate forms. We informally
name occasionally abundant unidentified small (10
Based on changes in the relative abundance of dinocyst assemblage groups, acritarchs and terrestrial palynomorphs (Fig. 5), we divide the record into three phases (P1–P3) of regional palaeoenvironmental change, independent of the lithological units, and compare these to the seismic and lithological investigations at Site 1168: (1) 37–30.5 Ma – transition from mid-shelf to outer-neritic conditions; (2) 30.5–25.5 Ma – outer-neritic conditions with transport of detrital material from the shelf; and (3) 25.5–20 Ma – transition from outer-neritic to more oligotrophic, oceanic conditions and frontal system development.
Dinocyst assemblages with abundant
During the rift to drift transition of Australia and Antarctica, the sedimentary environment along the western Tasmanian margin was likely strongly affected by the activity of the Sorell Fault Zone which later extended to the Tasman Fracture Zone with the onset of seafloor spreading (Fig. 1a, b) (e.g. Miller et al., 2002). This tectonic feature likely acted as a bathymetric barrier for ocean circulation to reach Site 1168 (e.g. Hill and Exon, 2004; Sauermilch et al., 2019a). In addition, recently published petrological and geophysical data directly along the conjugate Antarctic margin revealed that the late rifting stage was affected by the exhumation of subcontinental mantle (Seamount B, east of Adélie Rift Block; McCarthy et al., 2020). Petrological constraints tentatively indicate that melt infiltrated into the subcontinental mantle along the OCT during its rift to drift transition. This likely led to heating of the subcontinental mantle and changing petrological characteristics, increasing the buoyancy of the region. Consequently, this could have led to uplift and/or slower thermal subsidence compared with those of “normal” (mid-ocean ridge basalt) oceanic crust (e.g. Müntener et al., 2010). Seismic data along the western Tasmanian margin indicate a similar mantle exhumation pattern (Fig. 1c). As Seamount B and Site 1168 are directly conjugate, it can be assumed that the mantle uplift could also have affected the western Tasmanian margin, leading to an uplift and/or delayed thermal subsidence of the Tasmanian margin during the time of continental break-up. Organic-rich sediments indicate eutrophic, poorly ventilated bottom conditions, possibly sluggish circulation. This is in line with the palynological and seismic interpretation of a shallow and enclosed graben system.
In the glauconitic interval (Unit IV) that straddles the EOT, lithology and
dinocyst assemblages become more variable, probably as a result of the
progressive deepening of the continental slope. Unit IV is very thin and
only visible as a strong seismic reflection. Therefore, it is difficult to
distinguish between upper Unit V and Unit IV in the seismic profiles.
However, the boundary reflection is clearly wavy with hummocky features
(Fig. 1c, insets), which could be an indicator for winnowing. The early Oligocene section overlying the glauconite layer corresponds to seismic Unit
III with sediment waves along the landward basement high (Fig. 1c). These
indicate some bottom current activity and increased oxygen delivery,
although signs of winnowing have now ceased. As a result, TOC decreases
while CaCO
In this interval, the cosmopolitan dinocysts
The constant abundance of
In the late Oligocene, we note a sharp decrease in the neritic dinocyst
groups (mostly
Dinocyst biogeographic patterns in the Tasmanian sector in
three time slices in the late Eocene–early Miocene:
A shallow connection between the AAG and the southwest Pacific across the
southwestern South Tasmanian Rise has existed since
The strongly different seismic structure of the continental slope on either side of the TFZ indicates that the TFZ and the extended Sorell Fault Zone played a significant role in the depositional evolution of the region during the Eocene and early Oligocene, and likely also affected ocean circulation flow. The South Tasman Rise and adjacent margins started to subside between 35.5 and 30.2 Ma, in conjunction with the final break-up between Australia and Antarctica (Cande and Stock, 2004; Whittaker et al., 2013; Williams et al., 2019; Mccarthy et al., 2020). This coincided with the sediment filling of embedded basins, draping the local expressions of the basement highs and leading to a depositional environment of a deepening continental slope.
Results from high-resolution ocean model simulations suggests that, during
the time between 36 and 33.6 Ma, deepening of one of the gateways (Drake
Passage or Tasmanian Gateway) below 300 m whilst the second gateway was
already open (Sauermilch et al., 2021) could, in part, explain some of the
oceanographic changes showed in available proxy records from Southern Ocean
sediment drill cores (e.g. Houben et al., 2019; Westerhold et al., 2020). In
the simulation (Sauermilch et al., 2021), depth changes from 300 to 600 m
led to prominent surface water cooling offshore of Antarctica, whilst the
subpolar gyres (Weddell and Ross gyres) weakened and shrank significantly
and an eastward proto-Antarctic Circumpolar Current that was weaker than the present day current could
be established. Model–data comparison suggests further gateway deepening
(
To provide a wider, Southern Ocean perspective to the results of ODP Site 1168 during these times of Tasmanian Gateway widening, we compare palynological data from different drill sites in the region for the same time intervals (phases 1–3) as in Sect. 5.2 (Fig. 6).
Ice-proximal palynological records from the Ross Sea shelf, Cape Roberts
Project (CRP) (Prebble et al., 2006; Clowes et al., 2016), are dominated by
The intensified deepening and widening of the Tasmanian Gateway between
The peak of Acritarch sp. 2 around 26.8–25.8 Ma (550–520 m b.s.f.) and
Acritarch sp. 1 bloom from 22.3 Ma (400 m b.s.f.) (Fig. 5) occur around the
same time as a slight decrease in sedimentation rate (Fig. 2) and coincide
with the high carbonate (CaCO
We present new seismic interpretations and marine palynological association data to reconstruct the late Eocene to early Miocene palaeoceanographic conditions on the western Tasmanian continental margin. Upper Eocene sediments are embedded in a sedimentary basin enclosed by two basement highs, whereas material of EOT–earliest Oligocene age draped basement highs and show some indications of sediment reworking and winnowing. In the Oligocene to lower Miocene units, seismic features show evidence of bottom current activity. The Tasmanian Fracture Zone and the Sorell Fault Zone likely acted as a bathymetric barrier for ocean currents to reach Site 1168 during the late Eocene. Possible buoyancy-related uplift of the region through subcontinental mantle exhumation (Fig. 1c) during this time may have added to the bathymetric isolation effect. Palynological data confirm these seismic interpretations of the subsidence of the Tasmanian continental margin (Stickley et al., 2004a), with a transition from abundant terrestrial palynomorphs, Protoperidinioid cysts and extant neritic dinocysts in the total palynological counts in the late Eocene to the increasing abundance of modern oceanic dinocyst groups through the Oligocene–early Miocene. The different dinocyst assemblages north and south across the widening Southern Ocean reflect the onset of modern oceanographic conditions with a pronounced latitudinal temperature gradient starting in the late Eocene, manifesting itself in the Oligocene with more established frontal systems in the Southern Ocean. We suggest a northward broadening of the STF and subantarctic zone towards Site 1168 in the early Miocene. Aside from the gradual disappearance of inner-neritic–neritic species through the Oligocene record and the introduction of oceanic species common south of the STF (subantarctic zone), there are no significant changes to the surface-ocean properties or ocean currents at the western Tasmanian marginal Site 1168 in the Oligocene.
All microscope slides are housed at Utrecht University (LPP 069, 070, 129,
803 and 804). The data sets are stored on the Zenodo data archive:
The supplement related to this article is available online at:
PKB and FS designed the research. FSH, PKB and HB collected or requested the samples. PKB converted the age model to the GTS2012 timescale. IS requested, described and interpreted the seismic data. FSH processed samples for palynology and counted 105 slides. SH counted 18 of the slides from the Miocene. FSH, PKB and FS interpreted the palynomorph data. FSH wrote the paper with input from all co-authors.
Francesca Sangiorgi, co-author of this paper, is editor-in-chief of Journal of Micropalaeontology. Luke Mander operated as handling editor for this paper.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Sediment samples were provided by the Ocean Drilling Program (ODP). We thank Natasja Welters, Mariska Hoorweg and Giovanni Dammers for technical support at the Utrecht University GeoLab. We thank Spectrum Geo Ltd and the Federal Institute for Geosciences and Natural Resources Germany for providing the seismic reflection data sets. We acknowledge IHS Markit for the provision of the IHS Kingdom software used in this research.
This research has been supported by the Dutch Research councile, Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) polar programme (grant no. ALW.2016.001), and the European Research Council, H2020 programme (grant no. OceaNice 802835).
This paper was edited by Luke Mander and reviewed by Marcelo De Lira Mota and one anonymous referee.