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Armitage - Mineral Bioselectivity of Agglutinated

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					                   Abstract – The common mineral selected by agglutinated foraminifera in the pre-1991 eruption of Mount Pinatubo is identified as quartz.. The post-eruption chambers of the analysed tests show a distinctive change
                   in mineralogy, often being composed entirely of minerals that occur as a minor fraction in the sediment. The enhanced concentration of such minerals implies a preferential selection involving a selective
                   mechanism. This study shows that the minerals selected can be related to the mode of living of the agglutinated foraminifera.

                   Foraminifera studied from the Cretaceous-Tertiary Boundary (KTB) interval in Italy also show an affinity for dense mineral phases that occur as a minor fraction in the sediment. Several analysed tests contain
                   minerals such as Ni-spinel and shocked diamonds, indicative of the globally distributed Chicxulub ejecta. Additionally, there is evidence of redistribution of the KT ejecta by burrowing foraminifera.

Study Area - K/T Samples:                                                                                                                                                                               Discussion and Conclusions:
The Monte Conero section is located in the Umbria-Marche Basin in central Italy (right). The section was selected for study due to the minute amount of clastic                                         Mt. Pinatubo Samples:
detrital minerals. We hypothesized that where the availability of detrital minerals may be limiting, the agglutinating foraminifera living on the sea floor would then                                   Analysis of ash samples (see left) that were
make use of any detrital grains that are made available – in this case from the K/T impact ejecta. The hypotheses to be tested were (a) the agglutinated                                                 collected by [4] found no detectable pyroxene
foraminifera use ejecta-derived mineral grains, and (b) owing to their mobility within the sediment, can redistribute these grains.                                                                      minerals. However, clinopyroxene is present
                                                                                                                                                                                                         as the major component in the final chamber
                             Study Area - Mt. Pinatubo Samples:                                                                                                                                          of the Reophax specimen (photo under title)
                                                                                                                                                                                                         occurring as relatively large grains (up to 150
                             Mt. Pinatubo (Philippines) erupted violently on June 15, 1991 spreading a fan of ash several cm thick far into the South China Sea                                          microns, larger than normal choice).            Core of volcanic
                             (see map, left). An unexpected discovery during a cruise in 1994 was that many species of agglutinated foraminifera can
                                                                                                                                                                                                                                                         ash, 10cm in di
                             preferentially utilise dark-coloured grains of volcanic origin for the construction of the test wall [1]. Additional hypotheses to be tested
                             were (a) agglutinated foraminifera can preferentially select minerals and (b) the minerals selected can be related to the specimens life                                   The infaunal specimens, such as Reophax, which are burrowing
                             position in the sediment (morphogroup). E.g., a burrowing form would choose a denser mineral then an epifaunal form.                                                       forms appear to show an affinity for denser phase minerals. The
                                                                                                                                                                                                        epifaunal genera, e.g. Aschemonella (below), show a preference
                                                                                                                                                                                                        toward the least dense phases present, e.g. volcanic glass. It is
Methods:                                                                                                                                                                                                thought that the minerals are chosen to aid the position in the
The KTB interval at Monte Conero was sampled regularly both
above and below the clay layer. The samples of limestone were
                                                                                                Mineral Bioselectivity of Agglutinated                                                                  sediment.

dissolved in dilute HCl. The boundary clay was disaggregated in a
peroxide solution. The largest and most complete infaunal forms
                                                                                               Foraminifera from the 1991 Mt. Pinatubo
were picked from each sample residue and mounted on aluminium
stubs and examined in the microprobe. The infaunal forms (largely
                                                                                                Eruption and the Cretaceous/Tertiary
belonging to the genera Reophax and Subreophax) were selected                                                 Boundary                                                                                                           A.             B.              C.
because of their higher mobility and their known ability to bioturbate
the sediment [2].                                                                                                      DOMINIC ARMITAGE
                                                                                                                                                                                                        K/T Samples:
                                                                                                                    (University College London)
                                                                                                                                                                                                        Psammosphaera fusca taken from the KTB is a shallow infaunal
                                                                                                       Supervisors: Drs. MIKE KAMINSKI and ADRIAN JONES                                                 type. Minimal detrital material increases the probability of impact
         Photo right: KTB section at                                                                                                                                                                    ejecta being agglutinate (e.g. shocked diamonds and Ni-Co bearing
         Monte Conero.                                                                                                                                                                                  compounds).
                                                                           Results: Pinatubo Samples:
                                                                                                             Image left: SEM Image of infaunal
                                                                                                             Reophax dentaliniformis (photograph under                                                  Reophax cf. parvulus is a mobile infaunal form which must explain
From the Mt. Pinatubo ash layer surface, sediment samples were                                               title) taken from the surface of the ash.                                                  the redistribution of detrital grains, scavenged from the KTB and
taken on Leg SO-114 (1996) at depths of >4000 m using box coring                                             Final test composed predominantly of                                                       elevated to the surface. Modern species of Reophax have been
techniques. The sediment was preserved in methanol and stained                                               clinopyroxene (see Armitage, 2004 MSci                                                     observed to preferentially select heavy mineral grains to construct
with Rose Bengal to identify the living foraminifera which were to be                                        Thesis for individual mineral identification).                   A.                   B.
                                                                                                                                                                                                        their tests [5].
studied to identify any mineral changes in their tests from both                                             Spectra below shows composition of CPX.
before and after the eruption. The sediment was washed over a 63                                                                                                                                        Selective Mechanism:
µm sieve and dried at 50°C. The preserved specimens were then                                                Images Right: A. Epifaunal Aschemonella.
                                                                                                                                                                                                        How a single celled organism distinguishes between single grains
mounted on aluminium stubs to be introduced into the microprobe.                                             B and C are diatoms incorporated into the
                                                                                                                                                                                                        remains totally elusive. It is undoubtedly due to pseudopodia-
                                                                                                             test. D shows the primary mineral selected
                                                                                                                                                                              C.                   D.   mineral surface interaction detecting either composition or density.
A sample of the volcanic ash form the same core was also analysed                                            – glass.
                                                                                                                                                                                                        It was recently discovered [6] that bacteria show mineral
to determine the mineral components and relative proportions.                                                                                                                                           preferences. Perhaps foraminifera can be bacteria selective, which
                                                                                                                                                                                                        grow on specific mineral surfaces.
Results: K/T Samples:
                          Image A (left): Psammosphaera fusca collected from the KTB
                          composed predominantly of the heavy mineral barites. However,                                                                                                                 Acknowledgements:
                          microdiamonds are also present. B,C and D (left) show the
                          microdiamond images taken with electron backscatter,                                                                                                                          I wish to thank the following for their appreciated contribution:
                          secondary electron imaging and cathode luminescence                                                                                                                           Adrian Jones, Mike Kaminski and Andy Beard (UCL); Wolfgang
                          respectively. Spectra, far right, shows the mineral is pure carbon                                                                                                            Kuhnt (Kiel); Rodolpho Coccioni and the micropalaeontology
                          (diamond).                                                                                                                                                                    postgraduates at the University of Urbino.
 A.                                                                                            B.                        C.                        D.

                                                                                                                                                                                                        References:
                                               The same specimen also gave this electron microprobe
                                                                                                                                             A specimen of Reophax cf. parvulus (left) collected        [1] Hess et al, 2000
                                               spectrum (EDS - left) showing Ni, Co and Mn with minor
                                                                                                                                             6cm ABOVE the boundary also yielded shocked                [2] Kaminski, M. 1988 Abh Geol Bund 41, 229-243
                                               sulphur and silicate element contamination possibly from
                                                                                                                                             diamonds. Other specimens showed Ni-bearing                [3] Kearsley A, et al. (2004) Meteoritics and Planetary Sci 39, 247-
                                               adjacent clay minerals. Such fractionated siderophile element
                                                                                                                                             magnetite which are assumed to be from the K/T             265
                                               residues (especially Ni and Co) are important fingerprints of
                                                                                                                                             boundary.                                                  [4] Wiesner et al. (1995)
                                               meteoritic impact events [3].
                                                                                                                                                                                                        [5] Heron-Allen E. (1915) Proc. Roy. Micros
                                                                                                                                                                                                        [6] Lower et al. 2001.

				
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