MICROFOSSIL FAUNAL DIVERSITY DYNAMICS FROM THE MIDDLE AND UPPER ORDOVICIAN OF ROCKS BALTOSCANDIA

The early Late Ordovician was an interval of significant decline in marine biodiversity that has been variously attributed to sea level, facies, and climatic changes. In the East Baltic area, Kaljo et al., 1995, 1996, Ainsaar et al., 1999, 2004, and Meidla et al., 1999 have described a significant diversity decline and faunal turnover in marine microfossils at the Keila-Oandu Stage boundary, an event they called the Oandu Crisis. These studies have generally examined faunal changes in the shallow carbonate shelf sections of northern Estonia. In order to get a more complete understanding of microfossil diversity dynamics in the Middle and Upper Ordovician rocks of Baltoscandia we used the quantitative correlation method constrained optimization (CONOP9) to construct a composite range chart from the stratigraphic range data of 404 chitinozoan, conodont, ostracod, and graptolite species from 14 boreholes and one outcrop in Poland, Latvia, Estonia, and Sweden. These sections span three confacies belts, the Scanian (slope, black shale), Central Baltoscandian (outer shelf, argillaceous limestones), and North Estonian (carbonate platform) belts. We used the CONOP composite to calculate biodiversity, extinction, and origination rate changes through the Middle and Late Ordovician. In particular, we were interested in examining faunal dynamics across the late Keila and early Oandu stages, an interval associated with a prominent positive carbonate δ¹³C isotope excursion known as the GICE. Our data show that overall biodiversity (all groups) increases steadily from the base of the Keila Stage to the middle Rakvere, mainly due to an increase in ostracod diversity. Conodonts and chitinozoans exhibit biodiversity lows at the base of the Oandu and the base of the Rakvere Stages, respectively. Chitinozoans show climbing extinction rates through the Keila Stage, peaking at the Keila-Oandu boundary. Origination rates climb more slowly, peaking in the mid Oandu and dropping to a low at the top of the Oandu, resulting in a distinct faunal turnover. Conodonts exhibit a similar extinction pattern, but their origination rates crash at the top of the Keila and then climb rapidly to a peak in the middle Oandu. These results generally support the conclusions of previous studies, although in our study the faunal changes appear less dramatic.