Seasonal paleoclimates of the MIS 5E, 9 and 11 interglacials, using geochemical proxies in Porites and Tridacna
Date
2006
Authors
Ayling, Bridget Francisca
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Abstract
Interglacial climate state are an important aspect of Quaternary climate, representing time of decreased global ice volume, and warm conditions similar to the present. Acquiring high- resolution paleoclimate data from these interglacial period critical for establishing the natural range of climate variability that exist in the absence of anthropogenic greenhouse gas forcing and/or under variable orbital forcing regimes. In addition paleoclimate data can shed light on how the climate system responded to various external or internal forcing mechanisms in the past, and also the thresholds and feedback systems that are in place. Stemming from this knowledge is a greater understanding of the modern climate system and thus predictive capabilities for what the future climate holds.
Seasonality is an intrinsic component of all climate systems, and has been linked to both large scale change in the growth and decay of the N. Hemisphere ice sheets, and abrupt climate change event such as Heinrich events and the Younger Dryas. Massive corals and giant clams are fast growing. High -resolution tropical climate archives that precipitate aragonite skeletons from sea water and preserve environmental information in their skeletal geochemistry. Fossil coral and clam records thus have the ability to provide ‘snapshots' of tropical climate and seasonality during earlier interglacial interval. Several seasonally resolved paleoclimate records exist for the In terglacial (–125 ka) however prior to this, few records exist, reasons being that limited samples are preserved beyond the Last Interglacial (fossil corals older than the Last Interglacial arc generally uncommon on tectonically stable sites) and diagenetic effect can alter the primary geochemistry of a sample. In addition. the applicability of modern day SST calibrations fossil specimens is questionable over interglacial/glacial timescales due to possible oceanic baseline change (∂180 and Sr/Ca) or biological effect.
In this thesis I attempt to overcome these problems using a multi-faceted approach that includes analysis of well preserved MIS 9 (339-303 ka) Porites coral from Henderson Island (25°S . 128°W) and MIS 11 5e (128-116 ka) and MIS 11 (423-360 ka) Tridacna clam from Huon Peninsula. Papua New Guinea (PNG ) (6°S, 148°E). Both sites have experienced uplift, thus preserving fossil reefs, and difference in location offer a unique opportunity to examine contrasting aspects of the paleo-climate system (sub-tropical climate vs. WP\VP ocean- atmosphere interaction ). The amplitude of the seasonal cycle of SST was reconstructed in a MIS 9 fossil Porites using 8180 and Sr/Ca average-slope calibration equations. The results suggest SST seasonality was similar or slightly larger (+18%) than present. Using the latitudinal relationship between insolation seaonality and SST seasonality, a new approach is investigated that uses the orbital forcing of seasonal SST cycle to place suborbital constraints on the timing of reef growth. The MIS 9 Porites specimen is con trained to one of two brief intervals: 327-320 ka, or 306-299 ka, which correlate to the MIS 9.3 and MIS
9.1 interstadials respectively.
Climate signals are also investigated in modern and fossil Tridacna gigas from Huon Peninsula, PNG, through analysis of stable isotope and a suite of trace clement ratios (Ba/Ca, Mg/Ca. Sr/Ca and B/Ca). 8180 is found to be a robust indicator of interannual variability associated with ENSO. and by applying modern threshold limits to a MIS 11 fossil
T. giga, a high-resolution paleo-ENSO record is generated. This MIS II record suggests El Nino events were reduced in frequency compared to present( 14 events/century vs. -26 events/century for the current regime). The ability of Tridacna to maintain geochemical integrity and allow high resolution climate records to be reconstructed over the last 400 k)•r, highlights the significant potential of this climate archive for future paleoclimate studies. Tridacna Ba/Ca and Mg/Ca also show potential as indicators of productivity and sea surface temperature respectively, though additional work is required to determine their fidelity as climate proxies. Although problems relating to baseline uncertainties prohibit absolute SST reconstructions in both fossil Porites and Tridacna, signal seasonality is a robust characteristic of these fossil records and provides some interesting insights into these Late Quaternary interglacial climates.
Paleoclimate studies are critically dependanton accurate dating method: in this the is the prospect of using a diffusion-adsorption model to obtain reliable U-series for fossil Tridacna is also investigated. by characterising the spatial distribution of secondary uranium in their skeletons using LA ICPMS techniques, and identifying possible uptake historic through simple open-system modelling. It is concluded that the D-A model can not be applied to fossil Tridacna to obtain age estimates, owing to their structural complexities and complex, multi-stage uranium-uptake histories.
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DOI
10.25911/5d78db388369e