Exploring macrofaunal remains in Antarctic micropaleontological records

Paleoenvironmental reconstructions are an essential tool to understand the evolution of past climatic conditions. These studies often rely on microfossil analyses to discern different environmental characteristics. Conversely, the stratigraphical distribution of macrofaunal-related microfossils (150 µm – 1 mm) has been extensively overlooked, especially for the Holocene and within remote areas such as the polar regions. Here, we examine a Late Holocene (3.6 kyrs) high-resolution marine sediment core located in a fjord on the northwestern part of the Ross Sea. Our findings demonstrate that the use of macrofaunal remains, in the present case echinoderms, can yield detailed and complex information about recent past environmental conditions. Two distinct echinoderm groups were identified: ophiuroids (Ophionotus victoriae) and irregular echinoids. Despite the challenges to estimate population size due to the abundance of ossicles, we apply a statistical approach to presence/absence data to estimate the probability of occurrence of these organisms. By comparing the results with other proxies derived from the same record and from nearby cores, we were able to successfully employ echinoderms as a novel proxy for polar environment reconstruction. The presence of O. victoriae can be associated with interannually stable sea-ice cycles with yearly organic matter deposition, while simultaneously indicating periods of a mature benthic community. Irregular echinoid forms have been associated with the presence of organic matter on the seafloor. Through the comparison of different proxies, we were able to distinguish a specific climatic phase (The Ophiuroid Optimum), revealing additional insight into the environmental evolution of the area, unattainable with standard proxies. In conclusion, this study provides compelling evidence that the inclusion of macrofaunal-related proxies in paleoenvironmental reconstructions is crucial for capturing complex environmental dynamics.