A MULTI-PROXY GEOCHEMICAL ANALYSIS OF THE MIDDLE FRASNIAN PUNCTATA ZONE EVENT: PALEOENVIRONMENTAL IMPLICATIONS

A suite of data including stable isotopes (C&N_org, C&O_carb), major and trace elements, and magnetic susceptibility (MS) were acquired from samples, collected in western Canada, that span a portion of the Middle Frasnian. This interval is characterized by several δ¹³C fluctuations culminating in an ~3-7‰ negative excursion (i.e. punctata zone event or PZE) at several localities worldwide. MS and proxies for paleoredox conditions, productivity, and detrital input display similar trends and we hypothesize that they are inherently linked. Evaluation of the data within a sequence stratigraphic framework implies that trends are also likely influenced by eustatic sea level and associated climate change. Most proxies display an initial increase during lowstand and early transgression of T-R cycle IIc1. Proxy values eventually return to background levels during sea level highstand. Clastic input proxies (Al, Ti, Zr) display similar trends to MS, corroborating hypotheses about MS and detrital input. Bioproductivity proxies (δ¹³C, δ¹⁵N, Ba, Cu, Ni, and TOC) also display similar trends implying that primary production was driven largely by detrital input. Similar overall trends in paleoredox proxies (Mo, U, V) and TOC indicate that low oxygen conditions were responsible for preservation of organic matter produced during detrital-driven bioproductivity. Recent paleoclimate data, based on δ¹⁸O from conodont apatite, indicates that the Givetian-Early Frasnian was relatively cool and that the PZE coincides with a warming trend that persisted until the first cooling pulse, or lower Kellwasser event (LKE), of the Frasnian-Famennian (F-F) mass extinction. Coeval evolutionary development of terrestrial vegetation and soils and the concomitant influence of those processes on weathering and detrital input are commonly invoked as drivers of the F-F event. The PZE appears to be a precursor to the F-F, but without a significant perturbation of the biota. The lack of a major bioevent within the punctata zone may relate to the paleoclimatic trend. Whereas the PZE represents an amelioration of long-term cooling that began in the Middle Devonian, the LKE records a rapid cooling/warming event following the long-term warming of the Frasnian. Rapid climate change may thus be a greater stressor than more gradual warming or cooling events.