Temporal and spatial sensitivity of foraminifera-based paleobathymetry proxies: an Early-Middle Pleistocene Transition case

The study of sedimentary basins´ evolution, ocean circulation patterns, inter-basin connectivity and past marine ecosystems´ spatial distribution often relies on paleobathymetric reconstructions. Foraminiferal abundance and assemblage data can be powerful proxies to unravel the paleodepth history of a given location. In this study, we compare different routinely-used foraminifera-based paleodepth proxies, including the ratio between planktic and benthic foraminifera (P/B ratio), as well as habitat depth ranges of benthic foraminiferal taxa. The respective transfer functions were subsequently applied on a ~100 m sedimentary sequence, at International Ocean Discovery Program (IODP) Site U1460. The targeted IODP Site is situated near the shelf break of the Southwestern Australian continental margin and the selected interval recorded approximately ~450 kyr of glacial-interglacial sea-level variability spanning the Early-Middle Pleistocene Transition (EMPT; 1.25-0.6 Ma). The EMPT, characterized by the transition from a 41-kyr to a 100-kyr glacial-interglacial cyclicity and amplified glacio-eustatic fluctuations, provided the ideal framework for determining: i) the paleodepth evolution of the continental shelf; ii) the temporal and spatial sensitivity of these proxies to the glacial-interglacial forcing. Collectively, the analyzed proxies revealed a deepening of the continental shelf before the onset of Marine Isotope Stage (MIS) 24, followed by a progressive, step-wise shallowing trend. Our research also highlighted the effectiveness and limitations of foraminifera-based paleodepth proxies in a shallow-water setting, where ecological variability exerts a dominant role on foraminiferal assemblages. We observed that the ratio of planktic to benthic foraminifera and its derived paleodepth curves consistently track the global glacial-interglacial sea-level variability. Nevertheless, the calculated absolute depth values are unrealistically high for a continental shelf setting, likely implying the overlapping of an ecological signal dominating the P/B ratio. Conversely, absolute paleodepth values derived solely from the benthic foraminifera depth ranges reflected more realistic bathymetric estimates for a carbonate ramp than those provided by the P/B ratio. However, this approach failed to resolve the glacial-interglacial cyclicity due to a too broad depth zonation of the total benthic assemblage. For a better depth resolution, a more specific selection of benthic taxa is necessary.