Low amplitude obliquity changes during the early Pliocene reflected in diatom fragmentation patterns in the ANDRILL AND-1B core

In the ANDRILL (ANtarctic geologic DRILLing project) AND-1B core, an ~ 80 m-long interval of early Pliocene diatomite was obtained (382.98–440.12 m below sea floor). To better understand the role of fragmentation on diatom preservation within this unit, data were collected from 154 samples on the size and number of fragments of specimens of the diatom genus, Rouxia. The number of frustule fragments was classified into categories based on the amount of intact raphe and then evaluated using a formula created to characterize the nature and degree of fragmentation in each sample. Variations in fragmentation were compared with the age model and indicate a relationship between fragmentation and sediment accumulation rates for the lower ~ 37 m with higher fragmentation related to lower accumulation rates of hemipelagic mud during warmer intervals. This relationship is not as pronounced in the upper ~ 20 m, which can be related to the frequent occurrence of sandstone layers, lenses and laminated intervals. After evaluating a number of hypotheses, the most plausible explanation is that the presence of greater proportions of hemipelagic mud produces a cushioning effect, which prohibits the interaction of diatom frustules with sand grains or other diatoms, thereby reducing fragmentation.