The use of macrofaunal microfossils to unveil past Holocene changes: a case study from Edisto Inlet (Ross Sea, Antarctica).

Paleoenvironmental reconstructions are essential for the understanding of the evolution of past climatic conditions. These reconstructions rely on various proxies to gain a comprehensive view of environmental characteristics, ranging from geochemical to micropaleontological analyses. Traditionally, micropaleontological studies have focused on foraminifera, diatoms, and other microorganisms, relating changes in species compositions to changes in the physiochemical characteristics of the environment. However, the stratigraphical distribution of macrofaunal-related microfossils (150 μm – 1 mm) has been largely overlooked, particularly for the Holocene and within remote areas like Antarctica. In this study, we examine a Late Holocene (3.6 kyrs BP) high-resolution marine sediment core from a fjord in the northwestern part of the Ross Sea, Antarctica. With a core length of around 14 m, this record provides exceptional subcentennial resolution. Our findings demonstrate that macrofaunal remains, specifically echinoderms, can offer detailed insights into recent past environmental conditions. We identified two distinct echinoderm groups: ophiuroids, or brittle stars (Ophionotus victoriae), and irregular echinoids. Despite challenges in estimating population size due to the abundance of ossicles releases upon decay, we employed a statistical approach (Generative Additive Models) on presence/absence data to estimate the probability of occurrence of these organisms. By comparing results with other proxies derived from the same record and nearby cores, we successfully employed echinoderms as a novel proxy for polar environment reconstruction, tracing the evolution of the sea-ice cover. The presence of O. victoriae was associated with interannually stable sea-ice cycles with yearly organic matter deposition, owing to its peculiar reproductive physiology. Additionally, its presence indicated periods of a mature benthic community, as it is a widespread component of Antarctic benthos. Irregular echinoids were linked to organic matter presence on the seafloor. Through proxy comparisons, we distinguished a specific climatic phase (The Ophiuroid Optimum), providing additional insights into the environmental evolution of the area beyond standard proxies. In conclusion, this study offers compelling evidence that incorporating macrofaunal-related proxies in paleoenvironmental reconstructions is essential for capturing complex environmental dynamics.