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Abstract

Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones

SEPM Special Publication No. 93, Copyright © 2009

SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 67–82.

Solar energy (light) is essential for organisms that host algal symbionts. Hence, growth of these organisms is restricted to the photic zone. Among the larger benthic foraminifers, large rotaliids show changes in their test shapes when depth increases, becoming thinner and flatter in deeper environments. This morphological variability is shown clearly in the genus Amphistegina. In oligotrophic waters from the Indo-Pacific region, test shape can be mathematically expressed by the function Zo = 2.592 T/D-2.293, where Zo represents depth and T/D is the thickness-to-diameter ratio. Amphistegina test-shape distribution is strongly correlated with light extinction with depth. Light penetration depends on water transparency, which diminishes as biological productivity increases. Thus, Zo must be corrected for mesotrophic environments where light penetration is more limited. In mesotrophic conditions, Amphistegina test shape can be expressed mathematically as Zm = 1.037 T/D-2.293 whereas in oligotrophic–mesotrophic transitional situations the equation is Zom = 2.046 T/D-2.293. Zo Zm and Zom can be used as quantitative bathymetric indicators. In the Latium–Abruzzi and Menorca carbonate platforms, paleodepths inferred from Zm and Zom, respectively, are highly consistent with those obtained from the distribution of the red-algae associations. Thus, paleobathymetric models for both carbonate platforms have been constructed using Amphistegina T/D values as main indicators, supported by information from red algae and other biota. According to the inferred paleobathymetry, in the Latium–Abruzzi platform the inner ramp went from shoreline down to 10 m depth, the middle ramp from 10 m down to 35 m depth, and finally, the outer ramp corresponds to depths greater than 35 m. The Menorca platform paleobathymetric reconstruction indicates an inner ramp from 0 m down to 20 m depth, a middle ramp from 20 m down to 40 m, a ramp slope from 40 m down to 80 m, and, finally, an outer ramp no deeper than 100 m. Bathymetric ranges in the Menorca platform, deeper than those from the Latium–Abruzzi ramp, are consistent with greater light penetration.

The Amphistegina T/D index can also be used as a sediment-transport indicator. In the Menorca platform, sediment transport from the inner ramp down to the middle ramp, and even down to the lower ramp slope, are indicated by thick, ex situ Amphistegina specimens in relatively deep environments. The distribution of red algae and the co-occurrence of shallow organisms such as epiphytic foraminifers and fragments of hermatypic corals (Porites) confirm downslope transport and the role of the inner-ramp factory as an important sediment source.

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