High-resolution paleoclimate of the MIS 11 interglacial (423–360 ka) using geochemical proxies in giant Tridacna clams

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High-resolution paleoclimate of the MIS 11 interglacial (423–360 ka) using geochemical proxies in giant Tridacna clams

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  What can zircon and monazite tell usabout metamorphic fluids? J.C. A YERS Department of Earth and Environmental Sciences, VanderbiltUniversity, USA (john.c.ayers@vanderbilt.edu) Zircon and monazite are potentially powerful tools for docu-menting episodes of fluid-rock interaction during contact andregional mid- to high-grade metamorphism. Both minerals havelow solubilities in all but highly acidic or alkaline aqueous fluids.Monazite has a higher solubility than zircon at all pH values, con-sistent with field evidence that monazite is more susceptible to flu-id alteration than zircon. For example, Th–Pb ages and oxygenisotope compositions of monazites in the contact metamorphicaureole of the Birch Creek pluton in the White Mountains of east-ern California record magmatic fluid infiltration, which causedmonazites near the contact to dissolve and reprecipitate. In con-trast, zircons from the same rocks experienced only minor Pb loss.The hydrothermal monazites have patchy internal zoning, whichmakes it difficult to document trends in fluid composition.If hydrothermal zircons grow in concentric zones like magmat-ic zircons, they could be used to not only date the episode of hydrothermal growth but also to reconstruct the elemental andisotopic evolution of hydrothermal fluids during growth. Forexample, ion microprobe studies of magmatic zircons show coreto rim changes in composition reflecting growth temperature(from Ti in zircon thermometry), oxidation state (Ce/Ce * ), andcompositional evolution of the melt (Hf concentration) (Loweryet al., in press, Mineral. Mag.; Wooden et al., this conference).Unfortunately, zircons that are unambiguously hydrothermalhave only been observed in settings where fluids were highly alka-line or F-rich. Furthermore, they usually have very irregularshapes or exist as irregular zones within otherwise unalteredgrains.In contrast, metamorphic zircons seem to grow in concentriclayers. A similarity in Th/U between hydrothermal and high-grade metamorphic zircons and the low rate of zircon growthunder anhydrous conditions suggest that metamorphic zirconsgrow from a fluid phase. Our experiments show that zirconsgrown from high-temperature, oxidizing aqueous fluids have neg-ative Eu anomalies and low Th/U; these characteristics can poten-tially be used to identify zircons grown in the presence of fluidduring high-grade metamorphism. Such zircons, if zoned contin-uously from core to rim, could be used to measure  T   t  pathsand trends in coexisting fluid composition (from measured  D  val-ues) and oxidation state. doi:10.1016/j.gca.2006.06.065 High-resolution paleoclimate of theMIS 11 interglacial (423–360 ka) usinggeochemical proxies in giant Tridacna  clams B.F. A YLING 1 , J. C HAPPELL 1 , M.T. M C C ULLOCH 1 ,M.K. G AGAN 1 , M. E LLIOT 2 1 Research School of Earth Sciences, The Australian NationalUniversity, Canberra, ACT 0200, Australia (Bridget.Ayling@anu.edu.au; John.Chappell@anu.edu.au; Malcolm.McCulloch@anu.edu.au; Michael.Gagan@anu.edu.au) 2 School of Geosciences, The Grant Institute, The University of Edinburgh, Edinburgh EH9 3JW, Scotland, UK (Mary.Elliot@ed.ac.uk) Interglacials are an important aspect of Late Quaternary cli-mate, representing times of decreased global ice volumes andwarm conditions similar to the present. The Marine Isotope Stage11 (423–360 ka) interglacial is of particular interest to paleoclima-tologists because isotopic records from ice cores and deep seacores suggest it was exceptionally long. Also, during this interval,Earth’s orbital eccentricity was similar to present-day, such thatMIS 11 is often considered to be a good analogue for the currentinterglacial. High-resolution paleoclimate records are essential forresolving climate seasonality, and can provide insight into the nat-ural range of climate variability that exists in the absence of anthropogenic forcing. However, such records are rare beyondthe Last Interglacial (  125 ka) owing to the effects of diagenesison coral archives and their limited preservation above sea-level.In this study, giant  Tridacna gigas  clams are investigated as ameans to reconstruct MIS 11 climate in the Western Pacific WarmPool (WPWP). Thickening of shell aragonite in  T. gigas  proceedsat rates up to 20 mm yr  1 during early growth years, and declinesto  2 mm yr  1 for later growth years. Given their dense skeleton, T. gigas  has the potential to be well preserved in the geologicrecord and provide high-resolution snapshots of past tropicalclimate.A modern specimen from Huon Peninsula, Papua New Guin-ea, was analysed to establish the fidelity of   d 18 O as a compositeproxy for sea surface temperature (SST) and sea surface salinity(SSS).  Tridacna  d 18 O displays good covariance with the SouthernOscillation Index, suggesting that this proxy can be used as anindicator of paleo-ENSO activity at this site. Using LA-ICPMS,trace element ratios Mg/Ca and Ba/Ca were investigated aspotential environmental proxies, and appear to be influenced bySST and productivity respectively. Skeletal  d 18 O in a MIS 11 fossil T.gigas  specimen collected at  1200 m elevation from the upliftedreef terraces of Huon Peninsula, suggests that during its 35-yearlifespan, El Nin˜o events were reduced in frequency compared topresent-day (  14 events/century vs.   26 events/century). doi:10.1016/j.gca.2006.06.066 A26  Goldschmidt Conference Abstracts 2006 
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