HR: 0800h
AN: PP11B-0565    [Abstracts]
TI: How Were Southwest Pacific Pelagic Ecosystems Affected by Extreme Global Warming During the Initial Eocene Thermal Maximum?
AU: * Hollis, C J
EM: c.hollis@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, 6315 New Zealand
AU: Crouch, E M
EM: e.crouch@gns.cri.nz
AF: Institute of Geological and Nuclear Sciences, PO Box 30-368, Lower Hutt, 6315 New Zealand
AU: Dickens, G R
EM: jerry@rice.edu
AF: Rice University, Department of Earth Sciences, Houston, TX 77005 United States
AB: Four sections in eastern New Zealand provide the only South Pacific record of the initial Eocene thermal maximum (IETM): a siliciclastic outer shelf section (Tawanui, Hawkes Bay) and three pelagic-hemipelagic sections forming an outer shelf-upper slope transect across a carbonate ramp (Muzzle, Dee and Mead Streams, Clarence Valley). Although the rocks are too indurated to yield reliable oxygen isotope data, the IETM is identified by bulk carbonate carbon isotopes as a sharp negative excursion followed by gradual recovery over 0.6 to 4.0 m. In all sections, the excursion is mirrored by terrigenous sediment concentration, due to reduced biogenic (carbonate and silica) input and increased terrigenous input. Increased precipitation under warm humid conditions appears to have increased terrestrial discharge, recorded by deposition of smectitic marl in pelagic settings and illite/kaolinite-bearing smectitic mudstone in neritic settings. Eutrophic conditions are inferred for the IETM interval at Tawanui based on dysoxia, carbonate dissolution, an acme for the peridinioid dinocyst {\it Apectodinium} and abundant {\it Toweius} spp in nannofossil assemblages. Continued abundance of {\it Toweius} and replacement of {\it Apectodinium} by peridinioids of the {\it Deflandrea} complex suggests that eutrophic, albeit cooler, conditions persisted for at least 0.5 Ma after the IETM. In contrast, the IETM in Clarence Valley is marked by reduced biogenic silica content but little change in carbonate, and no evidence for carbonate dissolution. Sparse, poorly preserved palynomorphs assemblages suggest organic matter was oxidised under fully oxic conditions. Reduced numbers of upwelling indicators in the siliceous microfossil assemblage and common warm-water planktic foraminifera ({\it Morozovella} spp.), nannoplankton ({\it Discoaster} spp.) and radiolarians (e.g. {\it Podocyrtis} and {\it Theocorys} spp.) signal a switch from eutrophic to oligotrophic conditions and significant warming of near-surface waters. A progressive increase in neritic symbiotrophes within the radiolarian assemblage during the IETM recovery phase suggests warm, stratified, oligotrophic oceanic conditions. Radiolarians are scarce and upwelling indicators are very rare in sediments overlying the IETM. In the Southwest Pacific, global warming during the IETM increased terrestrial discharge, which enhanced productivity in shallow marine environments. Reduced productivity in deeper marine settings may have been caused by the poleward expansion of oligotrophic subtropical surface waters, impinging on a southern cyclonic system that had promoted upwelling along the eastern New Zealand margin through the Paleocene. Little evidence is found for local plankton productivity having a role in the gradual decrease in global temperatures that defines the upper IETM.
DE: 4855 Plankton
DE: 4267 Paleoceanography
DE: 3030 Micropaleontology
DE: 1615 Biogeochemical processes (4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting