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

Abstract

We use all available chronostratigraphic constraints – biostratigraphy, magnetostratigraphy,
\nradioisotopic dates, strontium-isotope stratigraphy, and correlation of compositional and physical properties
\nto well-dated global or regional records – to construct a preliminary age model for ANDRILL SMS Project’s
\nAND-2A drillcore (77°45.488’S, 165°16.605’E, 383.57 m water depth). These diverse chronostratigraphic
\nconstraints are consistent with each other and are distributed throughout the 1138.54 m-thick section,
\nresulting in a well-constrained age model. The sedimentary succession comprises a thick early and middle
\nMiocene section below 224.82 mbsf and a condensed middle/late Miocene to Recent section above
\nthis. The youngest sediments are Brunhes age (<0.781 Ma), as confirmed by a radioisotopic age of
\n0.691±0.049 Ma at 10.23 mbsf and the occurrence of sediments that have normal magnetic polarity down
\nto ~31.1 mbsf, which is interpreted to be the Brunhes/Matuyama reversal (0.781 Ma). The upper section
\nis punctuated by disconformities resulting from both discontinuous deposition and periods of extensive
\nerosion typical of sedimentary environments at the margin of a dynamic ice sheet. Additional breaks in
\nthe section may be due to the influence of tectonic processes. The age model incorporates several major
\nhiatuses but their precise depths are still somewhat uncertain, as there are a large number of erosional
\nsurfaces identified within the stratigraphic section. One or more hiatuses, which represent a total 7 to 8
\nmillion years of time missing from the sedimentary record, occur between about 50 mbsf and the base of
\nLithostratigraphic Unit (LSU) 3 at 122.86 mbsf. Similarly, between about 145 mbsf and the base of LSU
\n4 at 224.82 mbsf, one or more hiatuses occur on which another 2 to 3 million years of the sedimentary
\nrecord is missing. Support for the presence of these hiatuses comes from a diatom assemblage that
\nconstrains the age of the core from 44 to 50 mbsf to 2.06-2.84 Ma, two radioisotopic dates (11.4 Ma)
\nand a Sr‐isotope date (11.7 Ma) that indicate the interval from 127 to 145 mbsf was deposited between
\n11.4 and 11.7 Ma, and three diatom occurrence datums from between 225.38 and 278.55 mbsf that
\nconstrain the age of this upper part of Lithostratigraphic Unit (LSU) 5 to 14.29 - 15.89 Ma. Below the
\nboundary between LSU 5 and 6 sedimentation was relatively continuous and rapid and the age model is
\nwell-constrained by 9 diatom datums, seven 40Ar-39Ar dates, one Sr-isotope date, and 19 magnetozones.
\nEven so, short hiatuses (less than a few hundred thousand years) undoubtedly occur but are beyond
\nthe resolution of current chronostratigraphic age constraints. Diatom first and last occurrence datums
\nprovide particularly good age control from the top of LSU 6 down to 771.5 mbsf (in LSU 10), where
\nthe First Occurrence (FO) of Thalassiosira praefraga (18.85 Ma) is observed. The diatom datum ages
\nare supported by radioisotopic dates of 17.30±0.31 Ma at 640.14 mbsf (in LSU 9) and 18.15±0.35 and
\n17.93±0.40 Ma for samples from 709.15 and 709.18 mbsf (in LSU 10), respectively, and 18.71±0.33 Ma
\nfor a sample from 831.67 mbsf (in LSU 11). The sediments from 783.69 mbsf to the base of the hole
\ncomprise two thick normal polarity magnetozones that bound a thinner reversed polarity magnetozone
\n(958.59 - 985.64 mbsf). This polarity sequence most likely encompasses Chrons C5En, C5Er, and C6n
\n(18.056 - 19.772 Ma or slightly older given uncertainties in this section of the geomagnetic polarity
\ntimescale), but could be also be Chrons C6n, C6r, and C6An.1n (18.748 - 20.213 Ma). Either polarity
\nsequence is compatible with the 40Ar–39Ar age of 20.01±0.35 Ma obtained from single-grain analyses of
\nalkali feldspar from a tephra sample from a depth of 1093.02 mbsf, although the younger interpretation
\nallows a better fit with chronostratigraphic data up-core. Given this age model, the mean sedimentation
\nrate is about 18 cm/k.y. from the top of LSU 6 to the base of the hole.