Zinc accumulation in phosphate granules of Ucides cordatus hepatopancreas

Amorphous phosphate granules are present in vertebrate and invertebrate organisms. The functions attributed to these structures depend on their mineral contents and organic matrix composition. In the present study we have determined zinc concentrations in the hepatopancreas of the crab Ucides cordatus from regions contaminated with zinc, and the elemental composition of hepatopancreal phosphate granules. Organisms were collected from the contaminated areas of Sepetiba Bay (SB) and Guanabara Bay (GB), and from a non-contaminated area, Ribeira Bay (RB). The first two sites are located near the metropolitan region of Rio de Janeiro city, Brazil. Atomic absorption spectroscopy (AAS) showed a significant difference (P<0.05) for zinc concentration in the hepatopancreas from organisms collected at the contaminated sites GB (210 ± 20 µg/g dry weight) and SB (181 ± 16 µg/g dry weight) compared to the non-contaminated site RB (76 ± 14 µg/g dry weight). Phosphate granules isolated from hepatopancreatic tissue were studied by electron diffraction (ED), energy dispersive X-ray analysis (EDX) and electron spectroscopic imaging (ESI). ED of granules presented no diffraction spots, indicating that these structures are in an amorphous state, while EDX of granules isolated from a contaminated area contained P, Ca and Zn. Mg, Cl and Fe were also found in some of the spectra. ESI showed that O, P and Ca were colocalized in the mineralized layers of most granules observed. The correlation between the results obtained by AAS and those obtained by microanalytical techniques suggests that the hepatopancreatic granules of U. cordatus may be related to the phenomenon of heavy metal retention

Tomographic analysis for bioerosion signatures in shallow-water rhodoliths from the Abrolhos Bank, Brazil

Bioerosion at all scales is now recognized as playing a major role in facies interpretation. Macroscopic and microscopic borings can provide an indication of water depth, if they can be attributed to the action of specific borers. Tomographic analysis is a new method for assessing bioerosion, for identifying boring taxa, as well as for calculating the volume and porosity in present-day rhodoliths. The tomographic system provides also a quantification of the calcium carbonate produced by bioerosion. Recent rhodoliths collected at 20-m water depth on the Abrolhos Bank, Brazil, have been multi-scanned analyzed. The study shows that rhodoliths from this site are characterized by a highly diversified assemblage of boring bivalves and sponges associated with the ichnogenera Gastrochaenolites and Entobia. The fauna from this boring assemblage can remove up to the 10% of the rhodolith volume. The method can be expected to yield similar results as applied to both modern and fossil rhodoliths from other localities and time frames

Rhodolith beds from the Abrolhos Bank, Brazilian platform: taphonomy, geochemistry, ecology

Recent rhodoliths from the Abrolhos Bank (Brazil) are described and discussed to assess the ecological setting where they are growing

Origin and sedimentary evolution of sinkholes (buracas) in the Abrolhos continental shelf, Brazil

Cup-shaped depressions (termed buracas by local fishermen) are common geomorphic features on the northeastern Abrolhos Continental Shelf (Brazil). Samples collected by technical diving from the walls of two depressions (Buraca Funda, B1, top at 59 m, and Buraca Rasa, B2, top at 26 m) and seismic profiles provide evidence of the processes leading to their formation. The top of the sedimentary succession consists of two units bounded by erosion unconformities. Unit 1 overlies an erosion surface (MR1) and is older than the radiocarbon dating limit. It is made up of packstone to rudstone accumulated on mid- to outer-shelf paleoenvironments. Voids in the limestone are filled by a meteoric cement 29,000 cal yrs BP in B2. It is assumed that Unit 1 formed in the late Pleistocene, mainly during MIS 5e. An erosion surface (MR2) carved sinkholes in Unit 1, with karstification taking place while the ACS was emergent during the last glacial period. The timing and span of subaerial exposure changes with depth within the shelf. Unit 2 accumulated on this karst surface in the Holocene, after postglacial sea level rise. At the B1 margin, Unit 2 consists of early-lithified packstone to rudstone with attached corals. In B2, the Holocene unit comprises a boundstone of encrusting invertebrates and calcareous algae similar to the living ones attached to the wall today. High productivity in the sinkholes probably promoted the growth of encrusting suspension feeders but also led to intense, multistory bioperforation of carbonates on the wall. The cup-shaped depressions are, therefore, the result of sinkhole formation during the last-glacial low sea levels and later carbonate accretion at sinkhole margins during the Holocene