Physical degradation and early diagenesis in foraminiferal tests after subaerial exposure in terrigenous-depleted beaches of Yucatan, Mexico

摘要

Degradation, diagenesis, and mineralogical changes in foraminiferal tests are recognized as mechanisms that control their preservation and destruction under the subaquatic/aerial conditions, commonly influenced by processes such as dissolution, abrasion, breakage, and mineralogical replacement. In this paper, we discuss test degradation and early diagenesis on the large benthic foraminifera Archaias angulatus from the Yucatan Peninsula beach sands, a region absent in surficial rivers and terrigenous input seawards. Two main sites of Yucatan beach were selected: the high-energy environment of the Gulf of Mexico coast (GOMC) and the low-wave-energy environment of the Mexican Caribbean coast (MCC). Test degradation in the GOMC and MCC is affected by chemical dissolution, bioerosion, and physical abrasion. Among other factors, we found the effects of the bacterial microboring and dissolution by lowering pH in seawater (i.e., nearshore springs discharging seaward), as well as longshore currents and waves, depending on the prevailing conditions before transport and resuspension of the tests landwards. The observed degradation features in the tests are wall loss, chamberlet exposure, surface pits, and highly abraded edges. However, we explain the presence of pristine A. angulatus tests in the MCC beach through its association with Thalassia seagrass patches. Very low Mg/Ca ratios were found, which suggest the early diagenesis in the foraminiferal tests and the absence of recrystallized processes. Rare-earth element analyses suggest reduced authigenic precipitation of Mn-Fe coatings in the tests, and the absence of freshwater discharges or terrigenous inputs in the beach sands.