Mid-cretaceous greenhouse environments and floral ecosystems of the south polar region (75-80°s): the tupuangi formation, Chatham Islands, Zealandia
thesisposted on 31.01.2017 by Mays, Chris
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For 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.