Fish bone diagenesis in southeastern Brazilian shell mounds and its importance for paleoenvironmental studies

Abstract

Brazilian shell mounds are relicts of prehistoric settlements characterized as distinct features on the sandy coastal plain, sandy bars of coastal lagoons and rocky island. They are composed of shells of edible marine mollusks, fish remains and wildlife bones intermixed with sand. In the coastal lagoon region of the Rio de Janeiro state these shell mounds range in age between 5100 and 1300 cal BP. In this study we analyzed the geochemistry of the bones of the whitemouth croaker that primarily offers insight on the nature of bone diagenesis and its use for radiochronology and as isotope paleo-thermometer. The x-ray diffractograms of bones from the Tarioba shell mound exhibit elevated calcite profiles in contrast to a reduced hydroxyapatite concentration. Two peaks of quartz and the presence of feldspar provide evidence of exogenous materials, e.g., sand or silt particles, infilling hollow spaces within the fish bones. The Beirada and the Ponte do Girau shell mounds are characterized by the highest peaks of apatite relative to calcite, which is clearly different from the result obtained for Tarioba. The Manitiba profile is similar to the Tarioba profile except for calcite peaks that are approximately three times lower than those for Tarioba. At a depth of 25 cm in the Tarioba shell mound, the redox sensitive index of diagenetic alteration reaches its maximum value, suggesting a zone of accumulation in the soil horizon redox condition. The δ18O derived temperatures profile based on mollusks showed a consistent range from 18.1 to 24.1 °C, indicating seasonal events of intense upwelling of the South Atlantic Central Water breaking the mean warm surface conditions of the Brazil Current. Diagenetic imprints of Tarioba shell mound suggest humid burial conditions with the loss of hydroxyapatite and the enrichment of exogenous calcite. Drier depositional environmental conditions are indicated in the Ponte do Girau and the Beirada shell mounds, while intermediary depositional environmental conditions characterize the Manitiba shell mound. In the period represented by the shell mound samples, the sea level exhibited significant oscillations and portions of the inner shelf were exposed, which may have resulted in an anomalous derived temperature records. These results are compatible with events of anomalous temperatures occurring in the sea level during the time span of occupation of these shell mounds.