THREE EXTANT GENERA OF FRESH-WATER THALASSIOSIROID

DIATOMS FROM THE MIDDLE EOCENE OF NORTHERN CANADA

2,4 3 ALEXANDER P. WOLFE AND PETER A. SIVER

2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB

T6G 2E3, Canada; 3Botany Department, Connecticut College, New London, CT 06320,

USA

1Manuscript received ___ November 2007; revision accepted ___. The authors thank BHP Billiton Diamonds Inc. (Kelowna), the Geological Survey of Canada (Calgary), John Westgate (University of Toronto), Anne Lizarralde (Connecticut College), George Braybrook (University of Alberta), and Bianca Perren for Greenland material containing Puncticulata bodanica. This work was supported by the Natural Sciences and Engineering Research Council (Canada), and the National Science Foundation (USA; NSF-DEB-0716606). 4Author for reprint requests (e-mail: [email protected]).

1 Numerous specimens of , Puncticulata, and Discostella (:

Thalassiosirales) have been recovered in Middle Eocene lacustrine sediments from the

Giraffe Pipe locality in the Northwest Territories, Canada. These extend the record of the family by at least 20 Ma (million years before present), and thus provide a new evolutionary milepost for the thalassiosiroid diatoms, an important clade of centric diatoms with both marine and fresh-water representation.

Furthermore, the Eocene forms do not differ morphologically from extant counterparts, suggesting complete morphological stasis for over 40 Ma. The quality of the fossil material enables detailed investigations of areolae, rimoportulae, and fultoportulae, revealing no compelling differences with the modern species C. michiganiana, P. bodanica, and D. stelligera. Finally, the modern autecologies of these diatoms match well with the Middle Eocene paleoenvironment of the site inferred from a host of complementary biostratigraphic and lithologic data. These observations extend the antiquity of several characters of phylogenetic importance within the thalassiosiroid diatoms, including the fultoportula, and imply that the entire lineage is considerably older than previous constraints from the fossil record, as suggested independently by several recent molecular phylogenies.

Key words: ; diatoms; Eocene; evolution; morphology; fultoportula; Cyclotella; Discostella; Puncticulata.

2 The order Thalassiosirales Glezer and Makarova contains 35-40 validly published genera of living and fossil diatoms that are distributed globally in fresh-water and marine environments (Table 1). Morphologically, these diatoms display an impressive array of apomorphic characters, including fultoportulae (strutted processes), rimoportulae (labiate processes), and loculate areolae with internal cribrae. Valve-face areolae form striae with either linear or more commonly radial arrangements. In several genera areolae are organized as distinct fascicles with or without internal radial costae on the interstriae.

Despite considerable morphological and ecological diversity, Thalassiosirales form a robust natural group in all recent molecular phylogenies, with the non-fultoportulate

Lithodesmiales Round and Crawford as sister clade (Fox and Sörhannus, 2003;

Sörhannus, 2004; Kaczmarska et al., 2005; Sims et al., 2006). Within the thalassiosiroid lineage, the families Glezer and Makarova and Stephanodiscaceae

Lebour are the most common and diversified (Table 1). Cleve is the most speciose in the order, with about 180 and 12 marine and fresh-water species, respectively (Round et al., 1990; Kaczmarska et al., 2005). This genus includes the only to date for which the entire genome has been sequenced, T. pseudonana Cleve, yielding a relatively small genome (34.5 Mbp) that nonetheless elucidates the complexity of diatom origins (Armbrust et al., 2004). Furthermore, the culturing of marine

Thalassiosira has been central to the development of algal toxicology, physiology, and biochemistry (e.g., Rueter and Morel, 1981; Harrison and Morel, 1986; Lane et al.,

2005). Marine representatives of Thalassiosira appear closely related to several fresh- water genera of the family Stephanodiscaceae, principally Cyclostephanos Round,

Cyclotella Kützing ex de Brébisson, Discostella Houk and Klee, Puncticulata

3 Håkansson, and Stephanodiscus Ehrenberg. Molecular phylogenies derived from silicon transporter genes suggest that initial colonization of fresh-water habitats by thalassiosiroid diatoms was likely a polyphyletic series of events (Alverson, 2007).

However, the timing of such inferred evolutionary radiations remains poorly constrained.

Interrogation of the fossil record is therefore necessary to furnish mileposts for various evolutionary events among the thalassiosiroid diatoms.

In this communication, we report unambiguous representatives of the genera

Cyclotella, Puncticulata, and Discostella preserved in lake sediments ≥40 Ma from northern Canada. These specimens are morphologically identical to the modern species

C. michiganiana Skvortzow, D. stelligera (Cleve and Grunow) Houk and Klee, and P. bodanica (Grunow in Schneider) Håkansson. These observations not only suggest protracted evolutionary stasis among these diatoms, but furthermore extend the fossil record of the fultoportula by at least 20 Ma. In this light, diatom phylogenetic schemes that rely on integrations of paleontological and molecular data are in need of substantial revision.

MATERIALS AND METHODS

Study site—The Giraffe Pipe locality (64º44’ N, 109º45’ W) is a kimberlite diatreme that has been infilled by a sequence of Middle Eocene lacustrine and paludal sediments, and subsequently back-filled by Neogene glacigenic sediments (Fig. 1). The

Giraffe Pipe is one of many kimberlites in the Lac de Gras field, most of which have

Cretaceous or Paleogene emplacement ages (Heaman et al., 2004). A 165 m drill core collared at 47˚ was obtained by BHP Billiton Diamonds Inc. in 1999 (core BHP 99-01),

4 in order to assess the diamond potential of kimberlite underlying the crater fill. Core BHP

99-01contains 113.1 m of stratified organic sediment of Middle Eocene age, including

44.8 m of peaty material underlain by 68.3 m of lacustrine facies, in many places finely laminated. Conversion of these core depths to their vertical equivalent implies stratigraphic thicknesses of 32.7 m for the peat and 51.1 m for lake sediments (Fig. 1).

Two air-fall tephra beds occur at the transition between lacustrine and paludal sedimentation, which marks the final infilling of the maar and incipient colonization by a forest dominantly composed of Metasequoia Miki ex Hu and Cheng (Cupressaceae).

The age of the lacustrine sediments is assessed in several ways. First, a 87Rb/87Sr model age of 47.8±1.4 Ma from kimberlitic phlogopite provides a maximum age for the overlying sediments (Creaser et al., 2003). Second, diameter-corrected (n=2) and isothermal-plateau (n=1) fission track dates from the tephra beds are highly-coherent and statistically indistinguishable from 40 Ma (J. Westgate, pers. comm., 2007). This provides a minimum age for the underlying lacustrine sequence. Third, pollen assemblages throughout the core contain a number of diagnostic Middle Eocene taxa

(Rouse, 1977; Hamblin et al., 2003). We therefore envisage that, following the pheatomagmatic kimberite emplacement event, a lake formed within the crater and persisted for ~7-8 Ma before becoming a terrestrialized maar crater. The present study is based on samples from every ~50 cm in the lower core (100-110 m equivalent vertical depth, Fig. 1C). Below this depth, diatoms and other siliceous microfossils are scarce, but colonies of Botryococcus braunii Kützing (Chlorophyceae) are profuse. Above the studied interval, diatoms belonging to the families Aulacoseiraceae Crawford and

Eunotiaceae Kützing become abundant, as do scaled chrysophytes of the class

5 Synurophyceae Andersen, all of which are exquisitely preserved (Siver and Wolfe, 2005,

2007; Wolfe et al., 2006). The interval considered here thus captures the lake’s initial diatom communities.

Sample preparation—Specimens were prepared for scanning electron microscopy (SEM) as follows. Small chips (~200 mg) of organic mudstone were first oxidized with 30% H2O2 overnight and then centrifuged and rinsed several times.

Aliquots of cleaned slurry were air-dried onto heavy-duty aluminum foil, trimmed, and mounted to aluminum stubs with Apiezon® wax. Additional examinations were made of unprepared material. In this case, fresh fractures perpendicular to the bedding plane were mounted onto stubs with silver paint. These stubs were sputter-coated with Au or Au-Pd mixtures, prior to examination with either a Leo 982 (University of Connecticut) or a

JEOL-6301F (University of Alberta) field-emission SEM. Voucher material is available from the authors upon request.

RESULTS

Diatoms are uncommon in the lower lacustrine facies of the Giraffe pipe core.

Chrysophycean stomatocysts, testate protozoan scales (rhizopods), sponge spicules, and sponge gemmoscleres are the most abundant siliceous microfossils. The most common diatoms are araphid fragilarioid taxa, including Fragilaria Lyngbye, Staurosira

(Ehrenberg) Williams and Round, and Staurosirella Williams and Round, which will be reported upon separately. However, following intensive SEM efforts (i.e., >80 hours), numerous observations of several morphotypes of thalassiosiroid diatoms were made.

Given striking resemblance between these diatoms and modern forms, initial concerns

6 were raised concerning potential laboratory contamination. Additional independent preparations were thereafter conducted in our respective laboratories, revealing the presence of these diatoms as rare but consistent occurrences across a range of samples from the lower core. Subsequently, fragments of these diatoms, including broken valves and copulae, were observed embedded within the matrix of undigested and freshly- fractured sediment surfaces. Furthermore, surfaces of these valves are lightly but visibly etched, confirming that they are part of the primary material. Fortunately, this degree of etching does not impede observations of fine structures.

Cyclotella michiganiana Skvortzow 1937 (Fig. 2-3)—Several specimens observed from Giraffe pipe sediments conform in every way to the original description of

Cylotella michiganiana. Valves of these diatoms have an undulating central area, and robust radial costae. Valve margin fultoportulae bearing two satellite pores occur adjacent to every fourth stria (occasionally fifth or sixth), whereas one or two simple sessile rimoportulae occur on the valve face (Kling and Håkansson, 1988) or occasionally on the mantle (Fig. 3). C. michiganiana is morphologically similar to C. meneghiniana Kützing, but has far fewer marginal fultoportulae and rimoportulae, and furthermore lacks rimoportulae on the costae, as seen in the latter taxon. Moreover, C. meneghiniana inhabits high conductance and even brackish environments, whereas C. michiganiana has oligotrophic to mesotrophic ecological affinities (Krammer and Lange-Bertalot, 1991).

Puncticulata bodanica (Grunow in Schneider) Håkansson 2002 (Figs. 4-9)—

Numerous specimens from the Giraffe pipe belong to a species cluster of morphologically-similar diatoms that formerly included Cyclotella bodanica (Grunow)

Lemmermann, C. comta (Ehrenberg) Kützing, and C. radiosa Grunow, all of which have

7 been transferred to Puncticulata by Håkansson (2002). Upon close examination, the

Giraffe specimens can be assigned confidently to P. bodanica. The valve face is heterovalvar (either convex or concave), and marginal fultoportulae with two satellite pores rise internally from every fourth (rarely fifth or sixth) interstria. One or two rimoportulae open internally to a labium on the valve face, marginally to the central area.

Valve face fultoportulae occur in clusters and generally have three satellite pores each, but occasionally two (compare Figs. 5 and 7), and more rarely four. The number of satellite pores on valve-face fultoportulae is not considered taxonomically diagnostic within this group (Håkansson, 2002). Because these processes are scattered and not arranged in a circular ring, the illustrated specimens can be differentiated from forms hitherto referred to as Cyclotella bodanica var. affinis Grunow. It is noteworthy that neither the latter taxon, nor C. bodanica var. lemanica (Müller ex Schröter) Bachmann, has been formally transferred to Puncticulata, and it remains uncertain whether indeed these should be considered distinct from the nominate variety. Furthermore, costae on specimens from the Giraffe pipe do not fork distally, which enables differentiation from

P. radiosa (Grunow) Håkansson. Ecologically, P. bodanica is similar to C. michiganiana, also being found mainly in oligotrophic lakes. Summarily, Middle Eocene specimens from the Giraffe pipe appear identical to modern P. bodanica from lakes in the

Sondre Strömfjord region of west Greenland (Figs. 8-9), New England (Siver et al.,

2005) and elsewhere (Kling and Håkansson, 1988; Krammer and Lange-Bertalot, 1991).

This is the most abundant thalassiosiroid diatom in the Giraffe pipe material, for which at least 40 specimens (or fragments thereof) have been observed.

8 Discostella stelligera (Cleve and Grunow) Houk and Klee 2004 (Figs. 10-12)—

We illustrate the internal view of the most complete specimen encountered in the Giraffe pipe material (Fig. 10). Additional fragments of this diatom were also observed with

SEM, and additional whole valves were confirmed by light microscopy. The stellate organization of the central area, the position of rimoportulae on the valve mantle, as well as the spacing and location of marginal fultoportulae are all consistent with the description of Discostella stelligera (Houk and Klee 2004), as well as with myriad previous accounts of Cyclotella stelligera (Cleve and Grunow) Van Heurck (e.g., Kling and Håkansson, 1988; Krammer and Lange-Bertalot, 1991; Siver et al., 2005).

DISCUSSION

The documentation of Middle Eocene fresh-water thalassiosiroid diatoms with modern morphological expressions has several fundamental implications for the evolution of the lineage. The emergence of these diatoms in lakes of western North

American had been hitherto assigned to the Middle and Late Miocene (Cylotella sensu lato followed by Mesodictyon Theriot and Bradbury), with further speciation during the

Pliocene as Stephanodiscus and Cyclostephanos evolved (Krebs et al., 1987). These same authors had stated (p. 506) that they “are confident, however, that the general outline of lacustrine diatom biochronology presented herein will remain intact”. While their biostratigraphic scheme has proven correct with respect to Mesodictyon, which appears restricted to the Late Miocene globally (Khursevich, 2006), it is now clear that the cyclostephanoid genera Cyclotella, Puncticulata, and Discostella existed by the Middle

Eocene, at least 20 Ma older than previously documented.

9 Central to arguments concerning thalassiosiroid evolution has been the emergence of the fultoportula, which, while representing the defining character of the order

Thalassiosirales, mandates both excellent fossil preservation and the use of SEM to document satisfactorily. Indeed, no prior published records of fultoportulae in diatoms older than Miocene are confirmed, as reviewed by Kaczmarska et al. (2005). With respect to the fultoportula, the latter authors have conceded cautiously (p. 132) that: “the sequence of events leading to its formation is not known either from the fossil record or ontogenetically. It appears quite suddenly in the Miocene with no known precursors”.

The present study demonstrates that both marginal and valve-face fultoportulae with morphologies and arrangements that are identical to modern counterparts were already present in non-marine diatoms of Middle Eocene age. Although tubular fultoportula-like processes lacking satellite pores are recognized in certain Paleogene thalassiosiroid diatoms (Hasle and Syvertsen, 1985; Kaczmarska et al., 2005), it now becomes uncertain whether these features are indeed plesiomorphic to, or even homologous with, the fultoportulae of modern Thalassiosirales. While the morphological identity between

Middle Eocene and modern congeners precludes any meaningful contribution to the hypothesized origins of the fultoportula (Kaczmarska et al., 2005), the observations from the Giraffe pipe convincingly indicate that the full complement of morphological characters that define modern thalassiosiroid diatoms had evolved much earlier than previously suspected. Two hypotheses can be invoked to explain this suite of observations.

First, the evolution of Thalassiosirales from marine centric diatoms of Cretaceous age (e.g., Harwood and Nikolaev, 1995, Tapia and Harwood, 2002) may have occurred at

10 an accelerated tempo, if indeed the latter are considered to represent the ancestral stock.

Three extinct genera of Aptian-Albian age (~115-110 Ma) erected by Gersonde and

Harwood (1990), namely Gladiopsis, Praethalassiosiropsis, and Rhynchopyxis, are conceivably ancestral to the Thalassiosirales, should their processes be proven synapomorphic to those of modern thalassiosiroid diatoms. We note that strong support exists for the potential of rapid speciation within the Thalassiosirales, as demonstrated convincingly by a wealth of molecular (Kooistra et al., 1996; Armbrust and Galindo,

2001; Alverson, 2007) and stratigraphic (Khursevich, 2006; Theriot et al., 2006) evidence. The scenario of accelerated evolution during the Cretaceous is not only consistent with these and other elements of the diatom fossil record (Sinninghe Damsté et al., 2004), but also mirrored by the spermatophyte and pteridophyte lineages on land

(Schneider et al., 2004). Possibly, accelerated diversification occurred broadly across autotrophic clades at this time, in response to unique but unspecified environmental cues.

A second possibility is that the entire thalassiosiroid lineage is considerably older than currently established, and inferences concerning its history have been complicated by the incomplete nature of the fossil record. This view is consistent with molecular clocks using small subunit rRNA and various constraints from the fossil record, which suggest that the order Thalassiosirales is older than implied by the available fossil record

(Kooistra et al., 1996; Sörhannus, 2007). Of course, the two hypotheses presented above are not mutually exclusive. Moreover, the observations from the present study allow only the second to be addressed, until a fuller range of Cretaceous diatoms, including putative fresh-water forms (Chacón-Baca et al., 2002), can be appraised in full morphological detail using SEM.

11 We argue here that it is primarily the limiting quality of the diatom fossil record that has led to discrepant age estimates for the origin of phylogenetically-important characters such as the fultoportula. In this regard, exceptional fossil localities with well- constrained ages, such as the Giraffe pipe locality for Middle Eocene lakes, gain considerable importance. For example, this site has already extended by tens of Ma the fresh-water fossil records of aulacoseiroid and eunotioid diatoms (Wolfe et al., 2006;

Siver and Wolfe, 2007), as well as that of scaled chrysophytes (Siver and Wolfe, 2005).

From the simple perspective of the outstanding preservation witnessed in this material, the discovery of thalassiosiroid diatoms is perhaps less surprising, all the more since the entire siliceous microfossil assemblage yielded from the site thus far differs little, if at all, from closest living relatives. It is not only the morphology of these organisms, but also their autecologies, that appear to have remained in evolutionary stasis since the Middle

Eocene. For example, the three genera of thalassiosiroid diatoms reported here commonly co-occur in modern oligotrophic to mesotrophic lakes (Kling and Håkansson, 1988; Siver et al., 2005), much as they apparently did in the Middle Eocene Giraffe lake ecosystem.

From the integration of these observations, we share the view that the diatom fossil record is likely considerably older than presently known, in keeping with the fact that molecular clocks place the origin of diatoms at least 100 Ma before the earliest unambiguous , but unlikely earlier than 266 Ma (Kooistra et al., 1996; Sims et al.,

2006). As for the thalassiosiroid lineage, an age of at least 100 Ma seems most probable, in keeping with current molecular estimates that predict an origin for the clade between

125 and 149 Ma (Sörhannus, 2007).

12 Finally, we are able to state with assurance that initial colonization of fresh-water habitats by cyclostephanoid diatoms having evolved from marine Thalassiosiraceae

(Alverson, 2007) had occurred by 40 Ma, considerably earlier than longstanding biostratigraphic estimates placing these events in the Miocene (e.g., Krebs et al. 1987).

Given the relative abundance of Cretaceous kimberlites in western and central North

America (Heaman et al. 2004), as well as in southern Africa (Smith et al. 1985), concerted efforts should be directed at exploring their post-eruptive sedimentary fills for well-preserved early diatom floras. Given the results obtained thus far from Giraffe pipe sediments, the kimberlite maar depositional environment will likely continue to inform the fossil record of non-marine diatoms.

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rapid morphological evolution of an endemic diatom in Yellowstone Lake, Wyoming.

Paleobiology 32: 38-54.

18 WOLFE, A. P., M. B. EDLUND, A. R. SWEET, AND S. CREIGHTON. 2006. A first account of

organelle preservation in Eocene nonmarine diatoms: observations and

paleobiological implications. Palaios 21: 298-304.

19 Fig. 1. Location of the Giraffe kimberlite pipe fossil locality (A), schematic stratigraphy of post-eruptive sediments (B) and detailed lithostratigraphy and chronology of core 99-01 (C). The interval of core from which the samples considered in the present study were collected is indicated in (C).

Figs. 2-9. Middle Eocene diatoms from the Giraffe pipe locality identified as

Cyclotella michiganiana (Figs. 2-3) and Puncticulata bodanica (Figs. 4-7). Holocene specimens of P. bodanica from west Greenland (Lake SS32, 66.97˚N, 49.80˚W) are shown for comparison (Figs. 8-9). Abbreviations are as follows: rp=rimoportula, mfp=marginal fultoportula, and vffp=valve-face fultoportula. Internal valve views

(Figs. 2-5) illustrate how marginal fultoportulae are typically positioned adjacent to every third stria in C. michiganiana, but every fourth interstria in P. bodanica. Valve- face fultoportulae in Fig. 5 have only two satellite pores, which contrasts the majority of specimens where three satellite pores are present per vffp (Fig. 7). The presence of both convex and concave valve surface morphologies (heterovalvy), as well as details of vffp, rp, and the internal cribrae of valve-face areolae, have expressions that are essentially identical in Middle Eocene and Holocene specimens of P. bodanica.

Figs. 10-12. Discostella stelligera from the Giraffe pipe material, internal view of a complete specimen. Abbreviations are as in previous micrographs. There are no compelling morphological differences between Middle Eocene forms and the type material from New Zealand (Lake Rotoaira, recent), with particular attention to the number, morphology and arrangement of mfp and rp, and the stellate central alveolate region (Houk and Klee, 2004).

20 Table 1. A classification scheme for the thalassiosiroid diatom genera, integrating

currently available molecular, morphological, and paleontological data, and including

both extant and fossil representatives. Genera only known from the fossil record and

presumed extinct are bolded. Five additional validly published genera are not

included due to either uncertain taxonomic positions at the family level, or possible

elements of synonymy. These are: Cymatodiscus Hendey 1958, Cymatotheca Hendey

1958, Ectodictyon Khursevich and Cherniaeva 1989, Pleurocyclos Casper and

Scheffler 1986, and Stephanocyclus Skabitschevsky 1975.

Taxon, authority, and year of publication

Domain Eukarya Whittaker and Margulis 1978 Kingdom Chromista Cavalier-Smith 1981 Subkingdom Chromobiota Cavalier-Smith 1991 Infrakingdom Heterokonta (Cavalier-Smith) emend. Cavalier-Smith, 1995 Phylum Ochrophyta Cavalier-Smith 1986 Subphylum Diatomeae (Dumortier) Cavalier-Smith 1991 Class Round, Crawford and Mann 1990 Subclass Thalassiosirophycidae Round, Crawford and Mann 1990 Order Thalassiosirales Glezer and Makarova 1986 Family Lauderiaceae (Schütt) emend. Medlin and Kaczmarska 2005 Genera Lauderia Cleve 1873 Porosira Jørgensen 1905 Family Lebour 1930 Genera Cyclotubicoalitus Stoermer, Kociolek and Cody 1990 Detonula Schütt ex DeToni 1894 Schroederella Pavillard 1913 Greville 1865 Family Stephanodiscaceae Glezer and Makarova 1986 Genera Concentrodiscus Khursevich, Moisseeva and Sukhova 1989 Crateriportula Flower and Håkansson 1994 Cyclostephanos Round 1987 Cyclostephanopsis Loginova 1993 Cyclotella Kützing ex de Brébisson 1838

21 Discostella Houk and Klee 2004 Mesodictyon Theriot and Bradbury 1987 Mesodictyopsis Khursevich, Iwashita, Kociolek and Fedenya 2004 Pelagodictyon Clarke 1994 Pliocaenicus (Round and Håkansson) emend. Flower, Ozornina, Kuzmina and Round 1998 Puncticulata Håkansson 2002 Stephanocostis Genkal and Kuzmin 1985 Stephanodiscus Ehrenberg 1845 Stephanopsis Khursevich and Fedenya 2000 Tertiariopsis Khursevich and Kociolek in Khursevich, Kociolek and Fedenya 2002 Tertiarius Håkansson and Khursevich 1997 Family Thalassiosiraceae Lebour 1930 Genera Bacteriosira Gran 1900 Coenobiodiscus Loeblich, Wight and Darley 1968 Coscinosira Gran 1900 Lomonycus Komura 1996 Microsiphona Weber 1970 Minidiscus Hasle 1973 Nephrodiscus Komura 1996 Parthaea (Schmidt) Gowthaman 1996 Planktoniella (Wallich) Schütt 1892 Roundia (Round) Makarova 1994 Takanoa Makarova 1994 Thalassiocyclus Håkansson and Mahood 1993 Thalassiosira Cleve 1873 Thalassiosiropsis Hasle and Syvertsen 1985

22 Giraffe pipe, Lac de Gras, A C 0 Lithology NT (64˚48’N, 101˚04’W) Quaternary 30 (till backfill) 50 40 Canada 60 Dates 70 50 Metasequoia woody peat 80 Diameter-corrected U.S.A. 60 fission-track ages 90 tephra 39.00 ±3.46 Ma 70 horizons BHP Billiton core 99-01 47˚ B 100 39.56 ±3.76 Ma 80 Lacustrine 41.63 ±3.73 Ma 110 shales and Glacial till Isothermal-plateau 120 90 mudstones fission-track age with opal-A Terrestrial peat 130 Country rock 100 nodules (Proterozoic Quiescent 140 lake sediments this study granodiorite) 150 110 } Post-eruptive 160 deltaic sediments 120 Granodiorite 87Rb/87Sr age: Kimberlite