Podzolisation preserves ichnofossils constructed by ghost shrimp

Abstract

Ichnofossils of ghost shrimps (e.g., Ophiomorpha nodosa and Callichirus major) are interpreted worldwide as records of sea level fluctuations and provide reliable evidence of sedimentation rate, salinity, and substrate type. The preservation of these ichnofossils was previously ascribed to the polysaccharide mucus produced by ghost shrimps when burrows were built. We found evidence that these ichnofossils were preserved due to the impregnation of the burrow walls by dissolved organic matter (DOM) during podzolisation that took place after the galleries had been abandoned. Our study was conducted on a tropical barrier island in southeastern Brazil with a spatial gradient of podzolisation intensity. We made a detailed macromorphological description of the ichnofossils at the soil profile scale. Then, we used thin sections of undisturbed soils to identify sand grain arrangements and organic matter (OM) accumulation. We detected three macromorphological structures, (i) tubular gallery wall, (ii) sandy infilling of the gallery, and (iii) soil matrix outside the gallery. In the micromorphology analysis, we identified that the tubular wall is impregnated with monomorphic OM also found outside the burrow but in lower concentration. Sand grains in the wall of the gallery are tightly packed, causing a lower porosity than inside and outside the trace fossil. The precipitation of monomorphic OM is due to stagnation of DOM that percolated through the soil. Water with DOM is trapped in the wall of the tubule because of its smaller pores. The ghost shrimps constructed their burrows in marine sediments without podzolisation. Then, with the progradation of the shoreline, the sediments were colonized by restinga forest adding DOM to the soil and podzolisation started. The extent to which DOM is produced depends on vegetation development, therefore, the accumulation of DOM in the B horizon that enables the preservation of the ichnofossils is constrained by vegetation development and progradation of the shoreline.