J. J. Pospichal: Minylitha in Antarctic margin sediments ..., p. 195 - 197.

Journal of Nannoplankton Research, 24, 3, 2002.

SPECULATIONS ON THE OCCURRENCE OF THE
LATE MIOCENE SPECIES, MINYLITHA CONVALLIS,IN ANTARCTIC MARGIN SEDIMENTS
James J Pospichal
Dept. ofGeology, Florida State University, Tallahassee, FL 32306

Key words: Late Miocene, Minylitha conva/lis, Prydz Bay, Antarctic, climatic warming, calcareous
nannofossils
Abstract: Rare to few specimens of the Late Miocene nannolith, Minylitha convallis, were noted incontinental rise drift deposits recovered during Ocean Drilling Program Leg 188, Prydz Bay, Antarctica,
at 64°S latitude. This nannofossil species has not been previously reported at such an extremely highlatitude, and this occurrence extends its known geographic range to the margin of Antarctica. Although
its ecologic constraints are not well known, its presence in one sample, along with even more-rare
specimens of Discoaster spp. and Sphenolithus abies, possibly indicates a very brief warming of seasurface temperatures near the Antarctic margin during the Late Miocene (- 10.3-9.0Ma).

Introduction
Few calcareous nannofossil-bearing
sections of Neogene age have been
recovered from the extreme high-latitudesof the Southern Ocean, particularly along
the Antarctic margin. Haq (1976, 1980) was
one of the first to report on the occurrence
and biogeographic implications ofcoccoliths of this age from high austral
latitudes and near the Antarctic margin.
More recently, Miocene nannofossils havebeen retrieved during Legs 113, 119, 120 and178 (Wise, 1983; Wei & Wise, 1990; Wei &Thierstein, 1991 ; Wei & Wise, 1992a, b;
Barker, Camerlenghi, Acton et al., 1999). Thepaucity of nannofossils in deposits of thisFigure 1: Location map ofODP Leg 188 Sites 1165, 11 66 and 1167

age generally reflects cooling, associated with theincreased thermal isolation of the Southern Ocean aroundthe Antarctic continent, consequent to the establishmentof the Antarctic Circurnpolar Current (e.g. Kennett, 1977).
Ocean Drilling Program (ODP) Leg 188 recovered
piston and rotary cores at three southern Indian Ocean
sites on the continental margin in Prydz Bay, Antarctica(Figure 1). The sites were cored along a shelf-slope-risetransect in order to obtain a detailed sedimentary record
reflecting Cenozoic Antarctic glacial history andpalaeoenvironments. Calcareous nannofossils were mostcommon, although sporadic, at continental rise Site 1165
(water-depth= 3537m; 64°2'S). Here, a thick Neogene
section, comprising predominantly fine-grained terrigenous
and diatom-bearing hemipelagic sediments (sediment-driftdeposits), was collected (O'Brien, Cooper, Richter et al.,2001 ). In addition to nannofossils, diatoms, radiolaria andplanktonic foraminifera indicate a relatively continuous,
nearly 1OOOm, Lower Miocene to Upper Pliocene sectionthat underlies a thin Quaternary cover. The presence ofnannofossils at Site 1165, although intermittent, does
indicate that sea-surface conditions near the Antarctic
margin were favourable for nannoplankton productivity at
various times during the Neogene and Quaternary. This

brief report summarises the occurrence of nannofossils,
most notably the Late Miocene species, Minylitha
conval/is, in a single sample from Site 1165 (Plate 1).
Biostratigraphy
As expected, mid- and low-latitude, age-diagnostic
nannofossil marker -species are absent from, and a detailed
nannofossil biostratigraphy could not be achieved at, Site
1165. Diatoms and radiolaria provide the comprehensive
biostratigraphic framework for much of the section andindicate a nearly continuous Quaternary to Lower Miocene
section(O'Brien, Cooper, Richter et al., 2001) . Within thisframework, only generalised, combined nannofossil zones
could be applied. Nannofossil assemblages were key,
however, in determining the Early Miocene age for thelower 400m of the section down to the bottom of the holeat~ I OOOm below sea floor (mbsf).
The focus of this report is core-catcher sample
1165B-14H-CC (124.5mbsf). This sample contains few M
convallis, whose known range is confined to Nannofossil
Zones CN7b-CN9a (NN9-NN1la) (Gartner, 1992; Young,1998). The nannofossil zones approximately correlate withthe top of magnetic Chron 5 to near the top of Chron 4, andcan be assigned an age-range of 11.2-7.2Ma (Berggren et195

Journal of Nannoplankton Research, 24, 3, 2002 .

J. J. Pospichal: Minylitha in Antarctic margin sediments... , p. 195 - 197.

al., 1995). There are no other markers present in the sample
to further resolve this nannofossil zonal assignment. In
regard to other microfossils, the sample is placed in theAsteromorphalus kennetti Diatom Zone, which has an ageof 10.3-9. OMa, and in theAcrosphaera australis Radiolarian
Zone, withanageof10.36-9.2Ma(O'Brien, Cooper, Richteret al. , 2001 ; Whalen&Lazarus, 2001).
Nannofossil Obsen>ations
For Leg 188 drill and piston core samples, smear-slides
were prepared for calcareous nannofossil study using
standard techniques. Slides were examined using a lightmicroscope, under crossed-polarisers, transmitted light,
and phase-contrast light, at I OOOx and 1200x magnification.
At Site 1165, calcareous nannofossils are sporadic but occur
somewhat more regularly below ~500mbsf, within theLower Miocene. Overall, preservation is moderate andabundance high in only a few discrete samples. In theMiddle and Upper Miocene section, assemblages arecharacterised by low diversity, with one or two dominanttaxa, notably Reticulofenestra perplexa with fewer
Reticulofenestra producta, Reticulofenestra spp. (R.
haqii, R. minuta and R. minutula), and Coccolithus
pelagicus. Lower Miocene nannofossil assemblages lackR. perplexa, but contain rare sporadic Cyclicargolithus
jloridanus and even more rare Cyclicargolithus abisectus.
Reticulofenestra spp. is the dominant group throughout
the section.
Sample 1165B-14H-CC(l24.5mbsf) lies within an~200m interval barren of nannofossils, except for thissample which contains common to abundant, moderatelypreserved specimens. This single, relatively nannofossilrich sample (3-5 specimens/field of view (fov)) revealed
the unexpected presence of rare to few specimens (~ 1specimen/50 fov) of the Late Miocene nannolith, M.
convallis. Also present are very rare Discoaster spp. anda single observed specimen of Sphenolithus abies. Theassemblage is dominated by R. perplexa andR. producta,with few to common C. pelagicus, and small- and mediumsized Reticulofenestra spp. Very rare Reticulofenestra
pseudoumbilica is also present.
Discussion
M. convallis has not been previously observed in Upper
Miocene sediments from the high austral latitudes and, inparticular, the Antarctic margin, thus this discovery
considerably extends its known geographic range. Thisoccurrence could possibly indicate a brief warming of
Antarctic sea-surface temperatures during Late Miocene
times. Warming is supported by the presence of, albeit
very rare, Discoaster spp. and S. abies. The remaining
assemblage can be classified as a cold-water end-member,dominated by R. perplexa, as previously described fromthe southern Indian Ocean and other Southern Ocean drillsites (Haq, 1976; Wise, 1983; Wei & Wise, 1990; Wei &Thierstein, 1991; Wei & Wise, 1992a, b).
The nannofossil assemblage in sample 1165B14H-CC was likely to have been transported downslope,as suggested by the presence of a well-developed fauna
ofneritic benthic foraminifers (O'Brien, Cooper, Richter etal., 2001 ). Although most likely redeposited, the age of thenannofossil assemblage, based on the known range of M.
convallis (CN7b-CN9a), is in good agreement with diatom
and radiolarian stratigraphy. In fact, the original depositionin shallower water, and subsequent rapid transport,
deposition and burial downslope, may have aided
nannofossil preservation. Site 1165 is located in fairly deep
water (3537m), with the sea-floor presently below thecarbonate compensation depth (CCD) (Quilty, 1985). Thismay also have been the case at the time of deposition
during the Late Miocene. Rapid burial within a debris- or
mud-flow would facilitate preservation.
Leg 188 shipboard sedimentologists interpreted
several carbonate-rich intervals throughout the section at
Site 1165 as distal mud-flow deposits, although not
specifically the core-catcher sample in question (0 'Brien,Cooper, Richter et al., 2001) . Moreover, the lack ofnannofossils throughout much of the Middle to Upper
Miocene section may be due to dissolution below the CCD.
Nannofossils are more prevalent at age- and latitudeequivalent, but shallower, sites on Maud Rise, and ageequivalent, shallower sections of the Kerguelen Plateau(Wei & Wise, 1990; Wei & Thierstein, 1991). At deeperwater Site 1165 , the few intervals that do containnannofossils may have been deposited during times ofmuch higher nannofloral productivity, which may havesuppressed the CCD and permitted the deposition ofcoccoliths (see discussion in Wei & Wise, 1992b). It ispossible that this higher productivity coincided withwarmer sea-surface temperatures.
Wei & Wise (1992b) noted large fluctuations in
the abundance of R. perplexa relative to C. pelagicus in
sediments of 8-1 OMa from Kerguelen Plateau Sites 7 44,
747, 748, and 751. They attributed the sharp decrease inrelative abundance of R. perplexa to warmer sea-surface
temperatures, but did not note any M. conval/is in theirsamples, which encompass the age of the sample studied
here. Two brief, warmer intervals are implied in theKerguelen Plateap nannofossil data for this time period.
The specific eco1ogic preferences of M. convallis
are not well known. Gartner (1992) speculated that M.
convallis is an analog of the Pleistocene-Holocene taxon,
Florisphaera profunda, which is known to thrive innutrient-rich, lower photic zone waters, in well-stratified,oligotrophic oceans (Okada & Honjo, 1973). Its presencein Antarctic waters may reflect this as well, however,importantly, it perhaps denotes a broader temperature
tolerance.
Overall, the mere presence ofM. convallis, alongwith extremely rare Discoaster sp. and S. abies, most likely
indicates that, for at least a very brief period during theLate Miocene, sea-surface temperatures along the Antarctic
margin warmed. Although the ecologic limits of M.
convallis remain largely speculative, its presence at Site
1165 extends its known geographic range to the extreme
high-southern latitudes and indicates its tolerance for lower
sea-surface temperatures.
Acknowledgements
I thank P. Bown, S.W. Wise and W. Wei for helpful
suggestions, and J. Arney and K. McGonigal for assistancewith digital photography. I also thank T. Janecek for use of196

J. J. Pospichal: Minylitha in Antarctic margin sediments ..., p. 195 - 197.

the Antarctic Marine Geology Research Facility equipment
and M. Covington of BugWare, Inc. This study wassupported by a JOIIUSSF grant.
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Journal of Nannoplankton Research, 25, 1, 2003.

JJPospichal: Speculations on Minylitha . ., p. 5 - 6.

Plate 1plan-view

Minylitha convallis\Discoaster sp.

Reticulofenestra
perplexa

Cross-polarised (XPL) and plain-light (PL) micrographs of plan-views ofMinylitha convallis (Figs 1-2) and side-views (Figs 3-4). Fig. 5: Discoaster sp. (PL).
Fig. 6: R. perplexa (XPL). All specimens from ODP sample 1165B-14H-CC. Scale bar= 3!J.m6