Biostratigraphic utility of coiling direction in Miocene planktonic foraminiferal genus Paragloborotalia

Abstract

Trochospiral planktonic foraminifera will coil either sinistral (left) or dextral (right). The prevalence of sinistral or dextral coiling can change through the stratigraphic range of morphospecies with a preference in coiling direction. A number of coiling shifts have been applied as secondary marker events through the Recent to late Miocene (~0-7 Ma) biochronology. However, no such events have been applied beyond this age despite a number of species being known to adopt preferential coiling directions. Here we investigate selected Miocene species within the genus Paragloborotalia. Previous work in the tropical to subtropical realm has shown that the mayeri-siakensis group undergoes a shift from random to sinistrally dominated coiling in the mid Miocene (~15 Ma). We extend the investigation to other Miocene para- globorotaliids in the low (IODP Sites U1337, U1338, ODP Sites 871 and 925), mid (JOIDES-3 hole) and high latitudes (ODP Site 747) in order to assess whether there is global synchronicity and if the change is unique to the mayeri-siakensis group. In addition, a number of outcrop samples from the Cipero and Lengua formations in southern Trinidad are quantitatively compared to previously published trends. Our results show that in the low-mid latitudes the coiling shift is at ~15.37 Ma within planktonic foraminiferal Zone M5 within both Paragloborotalia siakensis and Paragloborotalia continuosa. In the high latitudes the absence of para- globorotaliids through a portion of the mid Miocene interval prevents accurate dating of a shift from early forms showing random coiling to later paragloborotaliids adopting a sinistral preference. We also find two coiling changes in the genus Globorotalia at high latitude Site 747, from random to sinistral in the mid Miocene (15.14 Ma) and sinistral to dextral (10.02 Ma) in the late Miocene. We propose the recognition of a coiling change in Paragloborotalia as a secondary bioevent in the mid Miocene at 15.37 Ma, and a useful means for the recognition of the base of the Langhian. The coiling shift as a biostratigraphic marker is likely to be particularly useful in regions where the currently applied bioevents, namely the Praeorbulina-Orbulina lineage, is rare or poorly represented.