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Mid-cretaceous greenhouse environments and floral ecosystems of the south polar region (75-80°s): the tupuangi formation, Chatham Islands, Zealandia
thesis
posted on 2017-01-31, 05:23 authored by Mays, ChrisFor analogues of global warming, the mid-Cretaceous is held up as both
an archetype and a warning. Using a combination of theoretical models and
geological evidence, a consensus is revealing that polar latitudes experience
the greatest degree of warming during global greenhouse conditions. The
Tupuangi Formation of the Chatham Islands, New Zealand, is the highest
latitude (~ 75-80°S) floral assemblage ever discovered from the mid-
Cretaceous; as such, the Chatham Islands provide an unprecedented
perspective into the most extreme effects of global warming.
This research has provided important climatic and evolutionary
implications for the flora and fauna in both a regional (e.g. Zealandia,
Australia and Antarctica) and global context. The Tupuangi Formation, and
its fossil flora record, reveals a benign terrestrial polar environment amid
the regional context of tectonic rifting, and a global context of extreme
greenhouse conditions.
Based on the timing of structural and sedimentological attributes
of the Tupuangi Formation, the tectonic and palaeoenvironmental context
of the region was established for the first time. It was concluded that
these sediments were deposited in a terrestrial, failed-rift basin of Eastern
Zealandia prior to the onset of seafloor spreading between Zealandia and
Marie Byrd Land, West Antarctica. Without ongoing orogenesis in the
region, a trend of fluvial aggradation and thermal subsidence followed this
failed-rift event; this resulted in an upsequence transition from high to low
fluvial flow-rate, with increasing prevalence of swamps, peats and paralic
settings.
This thesis includes a quantitative spore and pollen analysis to achieve
comprehensive biostratigraphic correlations of the Tupuangi Formation
and ecological interpretations of the mid-Cretaceous south polar region.
As typical for terrestrial basins, the Tupuangi Formation features several
age-old problems of spore and pollen biostratigraphy and ecological
reconstructions, including preservation biases and the inclusion of older,
reworked spores and pollen. This study demonstrates, that these variables
can be minimised by employing a range of novel statistical measures, and
the systematic removal of contaminated samples. This approach results in
more reliable biostratigraphic correlations and ecological interpretations.
This study documents seventy-seven distinct spore and pollen species,
including five new pollen species (Araucariacites saganii,
Balmeiopsis discus, Liliacidites exquisitus, Podocarpidites microradiatus
and Trichotomosulcites hemisphaerius) and two fern spores (Klukisporites
sphaerogoufus, Biretisporites labruplenus); a total of over one hundred
spore-pollen species was found.
Spore and pollen biostratigraphy was employed to derive the first
island-wide stratigraphic correlation of the Tupuangi Formation. Three
newly reported unconformities were identified; the uppermost of these
is concurrent with the Zealandian ‘breakup unconformity,’ a time-gap
of ~ 4-5 Ma demarcating the uplift phase of rifting between Zealandia
and Antarctica. Aligning the spore and pollen assemblage into the global
scheme, a Ngaterian to Mangaotanean (Cenomanian to Turonian; ~ 99-90
Ma) age was constrained for the sequence.
The age of this sequence is concurrent with the initiation of two
related global events: 1) the primary stage of flowering plant (angiosperm)
diversification; and 2) the mid-Cretaceous thermal maximum. The
palynological record of the Tupuangi Formation provides direct evidence for
the first event, and indirect evidence of the second. Abundance data of spore
and pollen species reveal an overall predominance of gymnosperm taxa in
the floral ecology; conifers as the primary overstorey components, and
an understorey comprised of smaller gymnosperms, ferns, angiosperms,
bryophytes, lycopods and ‘seed-ferns’. A tentative analogy is drawn to
the conifer-dominated boreal forests of the Northern Hemisphere. There
is increasing angiosperm diversity and abundance upsequence, and this
work corroborates that the angiosperms spread to the high latitudes instep
with the warming global climate of the time. As such, the increasing
diversity and abundance of angiosperm pollen of such a high palaeolatitude
locality corroborate an increasing global temperature. However, contrary
to previous findings, this study reveals that the angiosperm diversification
at south polar latitudes did not result in an ecological replacement of
understorey taxa, but instead increased the overall species richness. As
such, it is interpreted that the angiosperms formed new ecological niches,
rather than invade and outcompete the established flora in pre-existing
niches.
History
Campus location
AustraliaPrincipal supervisor
Jeffrey Darl StilwellAdditional supervisor 1
Mike HallYear of Award
2011Department, School or Centre
Earth, Atmosphere and EnvironmentCourse
Doctor of PhilosophyDegree Type
DOCTORATEFaculty
Faculty of ScienceUsage metrics
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