A test of the gardening hypothesis as behavioral explanation for the Zoophycos trace

The trace fossil Zoophycos has long been considered an archetypical example of a deposit-feeding trace. The important discovery that at least some types of Zoophycos actively introduce surface material into the burrow sparked a new interest in alternative ethological explanations. Recently proposed ethological explanations for the trace fossil Zoophycos include gardening of microorganisms. In the gardening model, organic-rich material is collected on the sediment surface and introduced into the burrow as substrate for the cultivation of microorganisms. Because microorganisms are known to fractionate strongly against 13C, especially under low oxic to anoxic conditions, it is argued that any gardening activity in the trace would result in a noticeable shift in δ13Corg between spreiten and adjacent host sediment. In order to test this hypothesis, δ13Corg of spreiten material and directly adjacent host material was measured in 12 host-spreite couples from three cores from the eastern North Atlantic. The results show δ13Corg values ranging from -23.6% to -21.6% for host sediment and between -23.4% and -21.8% for Zoophycos material. The difference in the couples is usually only a few tenths of a permil. The minimal difference between Zoophycos and host material suggests that gardening plays an insignificant role. However, the trace material generally displays a significant enrichment in organic carbon compared to surrounding host sediment. Therefore, the gardening hypothesis is rejected in favor of a cache model, where food is squirreled away for poorer times.