HIGH-RESOLUTION STRATIGRAPHIC CORRELATION AND BIODIVERSITY DYNAMICS OF MIDDLE AND LATE ORDOVICIAN MARINE FOSSILS FROM BALTOSCANDIA AND POLAND

The Middle and Upper Ordovician rocks of Baltoscandia have been divided into spatially distinct, composite litho- and biofacies units called confacies belts. A precise regional correlation of outcrops and boreholes in different confacies belts has always been problematic due to the pronounced biogeographical and lithofacies differentiation. Correlation between sections in the graptolite-rich black shales of the Scanian confacies and the carbonate-rich North Estonian confacies belts has been particularly difficult. To overcome these problems we used Constrained Optimization (CONOP9, Sadler et al., 2003) to construct a high resolution correlation model and composite range chart compiled from the stratigraphic range data of 554 chitinozoan, conodont, ostracod, and graptolite species from 38 drill cores and outcrops in Poland and Baltoscandia. We also used the CONOP composite as a timescale in which to calculate biodiversity, extinction, and origination rates through the Middle and Late Ordovician. Traditional biodiversity metrics and more recent probabilistic methods based on capture-mark-recapture analysis were used to estimate biodiversity and fossil recovery patterns. We divided the CONOP composite into 860 kyr intervals spanning the Lasnamägi through Porkuni regional stages. Our data show that overall biodiversity increases steadily from the base of the Keila to the middle Rakvere, mainly due to an increase in ostracod diversity. Chitinozoan diversity reaches a peak in the Late Keila, drops through the Oandu Stage, and then gradually declines across the rest of the Ordovician. Chitinozoans exhibit constant origination but variable extinction rates and undergo a dramatic diversity decline associated with the δ13C isotope excursion known as the GICE event. Conodonts have diversity peaks in the lower Uhaku and lower Kukruse regional stages, and then decline gradually through the Late Ordovician. Conodonts exhibit constant extinction and origination rates and their diversity decline is attributable to higher extinction than origination rates. Graptoloid diversity peaks in the Uhaku (late Darriwilian) and declines thereafter. Interestingly, the preservation and recovery rate among fossil groups was highly variable and appears to exert a strong influence on the observed biodiversity pattern.