Antanas Brazauskas

Integrated middle–upper Homerian (Silurian) stratigraphy of the Viduklė-61 well, Lithuania

Integrated lithostratigraphical, cyclostratigraphical, graptolite and conodont biostratigraphical and chemostratigraphical (δ13C) data are presented from the upper Jaagarahu, Gėluva and Dubysa regional stages, including the lundgreni Event (middle Homerian) in the Viduklė-61 well, Lithuania. The Viduklė-61 well is located in the west of Lithuania in the deep-water facies zone. Because of its completeness, this section is the “keystone” for middle–upper Homerian stratigraphy that allows us to combine different kinds of data into a coherent picture of large-scale biotic and environmental turnover. Three formations are distinguished: the upper part of the Riga Formation, with the Ančia Member in the uppermost part, the Siesartis Formation and the lower part of Dubysa Formation. The Grötlingbo bentonite is in the upper part of Ančia Member. In the Gėluva regional stage, two 4th-order and six 5th-order cycles have been distinguished. In the studied interval of the Viduklė-61 core, the following graptolite biozones were recognized: lundgreni (upper part), parvus, nassa, praedeubeli, deubeli, ludensis and nilssoni (lower part). Conodonts enabled recognition of the bohemica longa Biozone. A single find of Ctenognathodus murchisoni Pander in the nassa graptolite interzone indicates an earlier than previously thought origin of this species.

Application of Wavelets to the Cyclostratigraphy of the Upper Homerian (Silurian) G_ eluva Regional Stage in the Vidukl_ e-61 Deep Well (Western Lithuania)

The Gėluva age (late Homerian: Silurian) is an important interval of time during which one of the major episodes of mass extinction occurred, affecting many clades of marine organisms. In this study, we investigated a stratigraphic series of gamma logs in the Viduklė-61 well. To test the possible presence of sedimentary cycles, we applied wavelet and other time-series analysis techniques. As a result, we found two cycles with different periods: a 16.7-m-long cycle and a 6.7-m-long cycle. Based on our calculations, the cycles are best explained as being caused by two Milankovitch eccentricity cycles (400 and 100 kyr, respectively). If this interpretation is correct, then it gives us a good tool for understanding the tempo of the lundgreni extinction event and subsequent biotic recovery.

The Role of Temporal Abundance Structure and Habitat Preferences in the Survival of Conodonts during the Mid-Early Silurian Ireviken Mass Extinction Event

The Ireviken event was one of the most intense extinction episodes that occurred during the mid-Paleozoic era. It had a strong global effect on a range of clades, with conodonts, graptolites and chitinozoans affected most. Using geophysical proxies and conodont species parameters of their temporal abundance structure we investigate how they affected the selectivity of conodont species survival during this calamity. After performing bivariate logistic analyses on 34 species of conodonts, we find three variables that were statistically significantly associated with their odds of survival. These namely include spectral exponents that describe degrees of autocorrelation in a time series, the skewness of species abundance distribution, and average environmental preferences, which are mostly determined by ancient water depths at sampling sites. Model selection of multivariate logistic models found the best model includes species local abundance skewness and substrate preference. A similar pattern is revealed through the regression tree analysis. The apparent extinction selectivity points to a possible causes of environmental deterioration during the Ireviken event. The significant positive relationship between extinction risk and preferential existence in deeper environments suggests the open ocean causal mechanisms of biotic stress that occurred during the Ireviken event. Marine regressions, which were previously suggested as a causal factor in this extinction episode, on theoretical grounds should have had higher impact on species living in near-shore environments, through the processes of habitat loss which are associated with decreases of shelfal areas. In addition, the significant positive correlations found between skewness of abundance distributions and spectral exponent values and the probability of species survival suggest that community and ecosystem processes (which controlled species abundance fluctuation patterns) were significantly related to selectivity processes of this smaller mass extinction event.

Dynamics of abundance of the mid- to late Pridoli conodonts from the eastern part of the Silurian Baltic basin: Multifractals, state shifts, and oscillations

The Pridoli epoch is one of the most geobiologically unstable intervals in the Silurian period. However, the ecological and evolutionary dynamics of the biota from this time are still poorly understood. Here we present a comprehensive quantitative time-series analyses of conodont abundances during the middle and the upper parts of the Pridoli epoch. The study reveals that the stratigraphic series of conodont abundances has a multifractal structure. This feature implies the presence of multiple serially correlated processes hierarchically operating on different time scales and interacting in a multiplicative fashion. Estimation of a maximal Lyapunov exponent and a test for presence of determinism also revealed a chaotic and non-linear low-dimensional nature of long-term conodont abundance dynamics. The most probable (but not exclusive) mechanism for the origin of chaos in the time series of conodont abundance is that conodont abundances tracked changes in crucial environmental parameters, most probably related to the climate that exhibits long term chaos. The complex and non-linear nature of the conodont dynamics is also revealed by a moving-window correlation analyses of conodont abundances and by an environmental proxy time series. We found three alternating episodes of different correlation patterns between conodont abundances and the δ13C record. In the middle and upper most Pridoli interval there are positive but statistically non-significant correlations between these two variables. In contrast the lower part of the upper Pridoli (middle of the Jūra formation) exhibits negative and statistically significant correlation between the variables. In addition moving-window standard deviation analyses reveals that during the transitions between these episodes, there were abrupt changes in the variance in both the carbon isotopes and the conodont abundances, which implies that these episodes represent alternative states in the way the earth system functioned that were punctuated by the critical transitions between them. The interval with negative correlations between abundances and isotopes was also marked by a profound evolutionary turnover of acanthodians and to lesser extent conodonts. This interval most probably represented an anomalous time, with differing ecological rules and increased macroevolutionary rates, which can be interpreted as a turnover pulse. The spectral cyclostratigraphic analyses of the abundance data reveals that conodont abundances also exhibited periodic oscillations at several frequencies. The cycles determined for the analysis have the period lengths of 56, 99, 127 and 950 Ka, which probably were caused by the periodic changes in the obliquity and eccentricity of Earths orbit. The proposed proximate mechanism that controlled cyclic changes in the abundances of conodonts, based on the climatological deductions, was a variation in nutrients supplies that was due to changes in the degree of seasonality in the tropics, driven by Milankovitch cycles. It is probable that the described state shifts in the way that the ecosystem functioned were also forced by long-term Milankovitch cycles.