Cyst size trends in the genus leiosphaeridia across the mulde (Lower silurian) biogeochemical event

Abstract

The upper Wenlock epoch (Homerian age) of the Silurian period was an interval of intense changes in biotic composition, oceanic chemistry and sea level, which also witnessed a double-peaked positive stable carbon isotopic excursion. These biotic and environmental perturbations are thought to have originated due to reorganizations of the ocean system and high-amplitude sea level fluctuations. However, the evolutionary responses of the size of the micro-phytoplankton, which would help comprehension of the oceanographic mechanisms of these global perturbations, are currently unknown. Therefore, in this contribution we present morphometric data on the size changes of cysts of the dominant acritarch genus Leiosphaeridia during the middle and upper parts of the Homerian, which includes the lundgreni and Mulde bioevents, from the deep shelf facies of the eastern Baltic Basin (western Lithuania, Viduklë-61 core). Three parameters were measured for size distributions. Those are namely: average size, range of sizes, and power law exponent, which measures degree of “heavy-tailedness” and thus complexity of the distribution of cyst sizes. The average of the cyst sizes increased in the post-lundgreni interval of the Homerian, which points to the fundamental shift in the acritarch communities. The uncovered trends in cyst size ranges and power law exponents of their cyst size distributions revealed their close correspondence to the 4th order sea level fluctuations. Probable paleoclimatic and paleoecological mechanisms for this connection are presented.