Preliminary Integrated Chronostratigraphy of the AND-2A Core, ANDRILL Southern McMurdo Sound Project, Antarctica

We use all available chronostratigraphic constraints – biostratigraphy, magnetostratigraphy,
radioisotopic dates, strontium-isotope stratigraphy, and correlation of compositional and physical properties
to well-dated global or regional records – to construct a preliminary age model for ANDRILL SMS Project’s
AND-2A drillcore (77°45.488’S, 165°16.605’E, 383.57 m water depth). These diverse chronostratigraphic
constraints are consistent with each other and are distributed throughout the 1138.54 m-thick section,
resulting in a well-constrained age model. The sedimentary succession comprises a thick early and middle
Miocene section below 224.82 mbsf and a condensed middle/late Miocene to Recent section above
this. The youngest sediments are Brunhes age (<0.781 Ma), as confirmed by a radioisotopic age of
0.691±0.049 Ma at 10.23 mbsf and the occurrence of sediments that have normal magnetic polarity down
to ~31.1 mbsf, which is interpreted to be the Brunhes/Matuyama reversal (0.781 Ma). The upper section
is punctuated by disconformities resulting from both discontinuous deposition and periods of extensive
erosion typical of sedimentary environments at the margin of a dynamic ice sheet. Additional breaks in
the section may be due to the influence of tectonic processes. The age model incorporates several major
hiatuses but their precise depths are still somewhat uncertain, as there are a large number of erosional
surfaces identified within the stratigraphic section. One or more hiatuses, which represent a total 7 to 8
million years of time missing from the sedimentary record, occur between about 50 mbsf and the base of
Lithostratigraphic Unit (LSU) 3 at 122.86 mbsf. Similarly, between about 145 mbsf and the base of LSU
4 at 224.82 mbsf, one or more hiatuses occur on which another 2 to 3 million years of the sedimentary
record is missing. Support for the presence of these hiatuses comes from a diatom assemblage that
constrains the age of the core from 44 to 50 mbsf to 2.06-2.84 Ma, two radioisotopic dates (11.4 Ma)
and a Sr‑isotope date (11.7 Ma) that indicate the interval from 127 to 145 mbsf was deposited between
11.4 and 11.7 Ma, and three diatom occurrence datums from between 225.38 and 278.55 mbsf that
constrain the age of this upper part of Lithostratigraphic Unit (LSU) 5 to 14.29 - 15.89 Ma. Below the
boundary between LSU 5 and 6 sedimentation was relatively continuous and rapid and the age model is
well-constrained by 9 diatom datums, seven 40Ar-39Ar dates, one Sr-isotope date, and 19 magnetozones.
Even so, short hiatuses (less than a few hundred thousand years) undoubtedly occur but are beyond
the resolution of current chronostratigraphic age constraints. Diatom first and last occurrence datums
provide particularly good age control from the top of LSU 6 down to 771.5 mbsf (in LSU 10), where
the First Occurrence (FO) of Thalassiosira praefraga (18.85 Ma) is observed. The diatom datum ages
are supported by radioisotopic dates of 17.30±0.31 Ma at 640.14 mbsf (in LSU 9) and 18.15±0.35 and
17.93±0.40 Ma for samples from 709.15 and 709.18 mbsf (in LSU 10), respectively, and 18.71±0.33 Ma
for a sample from 831.67 mbsf (in LSU 11). The sediments from 783.69 mbsf to the base of the hole
comprise two thick normal polarity magnetozones that bound a thinner reversed polarity magnetozone
(958.59 - 985.64 mbsf). This polarity sequence most likely encompasses Chrons C5En, C5Er, and C6n
(18.056 - 19.772 Ma or slightly older given uncertainties in this section of the geomagnetic polarity
timescale), but could be also be Chrons C6n, C6r, and C6An.1n (18.748 - 20.213 Ma). Either polarity
sequence is compatible with the 40Ar–39Ar age of 20.01±0.35 Ma obtained from single-grain analyses of
alkali feldspar from a tephra sample from a depth of 1093.02 mbsf, although the younger interpretation
allows a better fit with chronostratigraphic data up-core. Given this age model, the mean sedimentation
rate is about 18 cm/k.y. from the top of LSU 6 to the base of the hole.

Acton G., Florindo F., Jovane L., Lum B., Ohneiser C., Sagnotti
L., Strada E., Verosub K.L., Wilson G.S., & the ANDRILL-SMS
Science Team, 2008-2009. Palaeomagnetism of the AND-2A
Core, ANDRILL Southern McMurdo Sound Project, Antarctica.
Terra Antartica, 15, this volume, 193-210.
Cande S.C. & Kent D.V., 1995. Revised Calibration of the
Geomagnetic Polarity Timescale for the Late Cretaceous and
Cenozoic. J. Geophys. Res., 100, 6093-6095.
Cody R.D., Levy R.H., Harwood D.M. & Sadler P.M., 2008. Thinking
Outside the Zone: High-Resolution Quantitative Biochronology
for the Antarctic Neogene. Palaeogeography, Palaeoecology,
Palaeoclimatology, 260, 92-121.
Cooper R.A., Crampton J.S., Raine J.I., Gradstein F.M., Morgans
H.E.G., Sadler P.M., Strong C.P., Waghorn D. & Wilson G.J.,
2001. Quantitative Biostratigraphy of the Taranaki Basin, New
Zealand: a Deterministic and Probabilistic Approach. AAPG
Bull., 85, 1469-1498.
Fielding C.R., Henrys S.A. & Wilson T.J., 2006. Rift history of
the western Victoria Land Basin: a New Perspective Based
on Integration of Cores with Seismic Reflection Data. In:
D.K. Futterer, D. Damaske, G. Kleinschmidt, H. Miller & F.
Tessensohn, (eds.), Antarctica: Contributions to Global Earth
Sciences, Springer-Verlag, Berlin, 309-318.Acton G., Florindo F., Jovane L., Lum B., Ohneiser C., Sagnotti
L., Strada E., Verosub K.L., Wilson G.S., & the ANDRILL-SMS
Science Team, 2008-2009. Palaeomagnetism of the AND-2A
Core, ANDRILL Southern McMurdo Sound Project, Antarctica.
Terra Antartica, 15, this volume, 193-210.
Cande S.C. & Kent D.V., 1995. Revised Calibration of the
Geomagnetic Polarity Timescale for the Late Cretaceous and
Cenozoic. J. Geophys. Res., 100, 6093-6095.
Cody R.D., Levy R.H., Harwood D.M. & Sadler P.M., 2008. Thinking
Outside the Zone: High-Resolution Quantitative Biochronology
for the Antarctic Neogene. Palaeogeography, Palaeoecology,
Palaeoclimatology, 260, 92-121.
Cooper R.A., Crampton J.S., Raine J.I., Gradstein F.M., Morgans
H.E.G., Sadler P.M., Strong C.P., Waghorn D. & Wilson G.J.,
2001. Quantitative Biostratigraphy of the Taranaki Basin, New
Zealand: a Deterministic and Probabilistic Approach. AAPG
Bull., 85, 1469-1498.
Fielding C.R., Henrys S.A. & Wilson T.J., 2006. Rift history of
the western Victoria Land Basin: a New Perspective Based
on Integration of Cores with Seismic Reflection Data. In:
D.K. Futterer, D. Damaske, G. Kleinschmidt, H. Miller & F.
Tessensohn, (eds.), Antarctica: Contributions to Global Earth
Sciences, Springer-Verlag, Berlin, 309-318.Fielding C.R., Whittaker J., Henrys S.A., Wilson T.J. & Naish
T.R., 2008. Seismic Facies and Stratigraphy of the Cenozoic
Succession in McMurdo Sound, Antarctica: Implications for
Tectonic, Climatic and Glacial History. Palaeogeography,
Palaeoclimatology, Palaeoecology, 260, 8-29.
Fielding C.R., et al. & the ANDRILL-SMS Science Team,2008-2009.
Sedimentology and Stratigraphy of the AND-2A Core, ANDRILL
Southern McMurdo Sound Project, Antarctica. Terra Antartica,
15, this volume, 77-112.
Gradstein F.M., Ogg J. & Smith A.G, 2004. A Geologic Time
Scale 2004. Cambridge University Press, Cambridge, United
Kingdom, 610 pp.
Howarth R.J. & McArthur J.M., 1997. Statistics for Strontium Isotope
Stratigraphy. A Robust LOWESS Fit to the Marine Sr-isotope
Curve for 0 - 206 Ma, with Look-up Table for the Derivation
of Numerical Age. J. Geol., 105, 441-456.
McArthur J.M., Howarth R.J. & Bailey T.R., 2001. Strontium Isotope
Stratigraphy: LOWESS Version 3. Best-fit Line to the Marine
Sr-isotope Curve for 0 to 509 Ma and Accompanying Look-up
Table for Deriving Numerical Age. J. Geol., 109, 155-169.
Mukasa S.B., Shervais J.W., Wilshire H.G. & Nielson J.E., 1991.
Intrinsic Nd, Pb, and Sr Isotopic Heterogeneities Exhibited by
the Lherz Peridotite Massif, French Pyrenees. J. Petrol. Spec.
Lith. Issue, 117-134.
Rosenthal Y.M., Field M.P. & Sherrell R.M., 1999. Precise
Determination of Element/calcium Ratios in Calcareous
Samples Using Sector Field Inductively Coupled Plasma Mass
Spectrometry, Analytical Chemistry, 71, 3248–3253.
Scherer R.P., Bohaty S.M. & Harwood D.M., 2001. Oligocene and
lower Miocene siliceous microfossil biostratigraphy of Cape
Roberts Project Core CRP-2/2A, Victoria Land Basin, Antarctica.
In: P.J. Barrett & C.A. Ricci, (eds.). Studies from the Cape
Roberts Project, Ross Sea, Antarctica, Scientific Report of
CRP-2/2A, Terra Antartica, 7, 417-442.
Naish T. & the ANDRILL-MIS Science Team, 2007. Synthesis
of the Initial Scientific Results of the MIS Project (AND-1B
Core), Victoria Land Basin, Antarctica, Terra Antartica, 14(3),
317-327.
Taviani M. & Beu A.G., 2003. The Paleoclimatic Significance
of Cenozoic Marine Macrofossil Sssemblages from Cape
Roberts Project drillholes, McMurdo Sound, Victoria Land
Basin, East Antarctica. Palaeogeography, Palaeoclimatology,
Palaeoecology, 198, 131-143.
Taviani M., et al. & the ANDRILL-SMS Science Team, 2008-2009.
Palaeontological characterization and analysis fo the AND-2A
Core, ANDRILL Southern McMurdo Sound Project, Antarctica.
Terra Antartica, 15, this volume, 113-146.
Wessel P., and Smith, W.H.F., 1998, New, Improved Version of the
Generic Mapping Tools Released, EOS Trans. AGU, 79, 579.
Wilson G., Levy R., Browne G., Cody R., Dunbar N., Florindo F.,
Henry S., Graham I., McIntosh W., McKay R., Naish T., Ohneiser
C., Powell R., Ross J., Sagnotti L., Scherer R., Sjunneskog C.,
Strong C.P., Taviani M., Winter D. & the ANDRILL-MIS Science
Team, 2007. Preliminary Integrated Chronostratigraphy of the
AND-1B Core, ANDRILL McMurdo Ice Shelf Project, Antarctica.
Terra Antartica, 14, 297-316.