Macrofauna Activity in Quaternary Bottom Water Environments off Western Australia: Fecal Pellets Evidence

Abstract

Fossil macrofauna are difficult to assess quantitatively compared with micro- and meiofaunas because of their large body size and rare occurrence, particularly in small samples (e.g., sediment core). To estimate the quantitative activity of the macrobenthos, we focused on fossil fecal pellet abundance in Quaternary sediments. For this investigation, we used a sediment core obtained off Western Australia in the eastern Indian Ocean (IODP Exp. 356). We focused on horizons which clearly exhibited sedimentary cycles. The upper part of the core mainly comprised of alternating beds of dark-colored packstone/wackestone and light-colored wackestone/mudstone. Well-preserved molluscan fossils and peloids occurred in the light-colored wackestone/mudstone in the upper part of the sedimentary sequence. The morphological character and size of the peloids within the studied sediments are similar to modern fecal pellets of shallow water polychaetes. The abundance of fossil fecal pellets shows fluctuating trends similar to those of macrofossils (e.g., bivalve, gastropod, scaphopod, and echinoderm) and it is likely that the fecal pellet abundance is an indicator of paleo-macrobenthos activity. This activity was compared to ostracode abundance (Fig.) and temporal changes in fossil fecal pellet abundance were found to be inversely correlated. At other intervals, the pellets changed to superficial ooids and occurred in conjunction with Larger Benthic Foraminifera, indicating deposition within the photic zone. Thus, these intervals may indicate a shallow water environment during deposition. In this research, we reveal that the abundance of fossil fecal pellets shows macrobenthic activity, reflecting the evolutions of bottom water environments during the Quaternary period.