Temporal variation in radiocarbon pathways caused by sea-level and tidal changes in the Bonaparte Gulf, northwestern Australia

Radiocarbon dating, applied to carbon-containing material such as shells, foraminifers, corals, peats, and bulk organic matter, is an essential method for constraining Late Quaternary age models. However, selection of appropriate material requires paleoenvironmental knowledge because past climate and sea-level variability may affect depositional processes. To illustrate this issue, we present radiocarbon dates performed on marine and terrestrial macrofossils, foraminifers, and acid-insoluble carbon (AIC), as well as paleo-tidal modeling results, from the Bonaparte Gulf in northwestern Australia. The varied topography and large tidal range of the Bonaparte Gulf strongly influence the past sedimentary environment in response to sea-level change. At sea level −70 m below present, carbonate fossil material is a more reliable indicator of the depositional age than AIC, with little lateral transport because of isolation from the open ocean and reduced tidal activity. In contrast, high tidal activity occurs when sea level is above −70 m, with significant redeposition of older fossil material. In this case the AIC dates are more reliable. This is the opposite relationship typically observed in less dynamic settings because the varied and uncertain AIC pathways. This result underscores the need to understand the past depositional environment, particularly on the continental shelves, which are highly affected by sea-level changes.