Abstract: The carbonate sediments of the Western Australian shelf in the Indian Ocean host diverse assemblages of benthic foraminifera. These shelf environments are dominated by the southward-flowing Leeuwin Current, which impacts near-surface circulation and influences biogeographic ranges of Indo-Pacific warm-water foraminifera. Analyses of outer-ramp to upper-slope sediments (127–264 m water depth) at four different sites (some with replicates) revealed 185 benthic species. A shift from benthic to planktonic foraminifera was accompanied by a decrease in “larger” benthic foraminifera below the lowermost euphotic zone. Fisher α and proportions of buliminid and textulariid taxa increased with water depth, as miliolids and rotaliids decreased in proportion. Cluster analyses on the 125 to 250 μm and 250 to 850 μm size fractions revealed distinct assemblages, with the former distinguishing between deeper and shallower sites, and the latter distinguishing between the Carnarvon Ramp site and the three sites on the northwestern shelf (NWS). The assemblage shift with depth was likely caused by rapidly changing physical conditions in the upper thermocline. The assemblage differences between the NWS and the Carnarvon Ramp site indicate limited horizontal transport and migration rates on the outer shelf below the influence of the Leeuwin Current. Similarity in bottom-water temperature at the studied sites indicates that water mass characteristics, biogeographic history, and/or possibly diversity in benthic shelf habitats, rather than temperature and depth, are responsible for differences between the two regions.

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 Z o = 2.592 T/D -2.293 , where Z o 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, Z o must be corrected for mesotrophic environments where light penetration is more limited. In mesotrophic conditions, Amphistegina test shape can be expressed mathematically as Z m = 1.037 T/D -2.293 whereas in oligotrophic–mesotrophic transitional situations the equation is Z om = 2.046 T/D -2.293 . Z o Z m and Z om can be used as quantitative bathymetric indicators. In the Latium–Abruzzi and Menorca carbonate platforms, paleodepths inferred from Z m and Z om , 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.