Jacalyn M. Wittmer

Differential responses of marine communities to natural and anthropogenic changes

Responses of ecosystems to environmental changes vary greatly across habitats, organisms and observational scales. The Quaternary fossil record of the Po Basin demonstrates that marine communities of the northern Adriatic re-emerged unchanged following the most recent glaciation, which lasted approximately 100 000 years. The Late Pleistocene and Holocene interglacial ecosystems were both dominated by the same species, species turnover rates approximated predictions of resampling models of a homogeneous system, and comparable bathymetric gradients in species composition, sample-level diversity, dominance and specimen abundance were observed in both time intervals. The interglacial Adriatic ecosystems appear to have been impervious to natural climate change either owing to their persistence during those long-term perturbations or their resilient recovery during interglacial phases of climate oscillations. By contrast, present-day communities of the northern Adriatic differ notably from their Holocene counterparts. The recent ecosystem shift stands in contrast to the long-term endurance of interglacial communities in face of climate-driven environmental changes.

Quantitative Bathymetric Models for Late Quaternary Transgressive-Regressive Cycles of the Po Plain, Italy

In marine settings, quantitative bathymetric models can be developed using various water depth proxies, including epibiont distribution, sedimentologic features, and the distribution of benthic taxa in time and space. Here, the late Quaternary bathymetric history of the Po coastal plain (Italy) has been reconstructed using mollusk samples from a network of 16 cores. Multiple analytical approaches have been applied in a comparative fashion. A direct ordination approach was used to estimate sample bathymetry using weighted averaging of genera with known preferred depth. Weighted averaging carries an advantage of analytical simplicity and produces direct ordination models expressed in environmentally meaningful units. Indirect ordination methods, based on depth estimates developed using posteriori-calibrated ordination strategies (correspondence and detrended correspondence analysis calibrated against present-day bathymetric data), yielded results consistent with weighted averaging. Regardless of the choice of analytical methods, mollusk assemblages yielded bathymetric proxies congruent with independent sequence stratigraphic interpretations derived previously for both Late Pleistocene and Holocene transgressive-regressive cycles. The mollusk-derived proxies quantify spatial bathymetric gradients across the basin and local trends in absolute water depth in response to relative changes in sea level. However, for cores located in the most proximal part of the basin, mollusk-based ordinations failed to provide viable estimates due to inclusion of mixed marine and nonmarine mollusk faunas and scarcity of fossiliferous horizons necessary for adequate quantitative sampling. The multiple analytical approaches cross evaluated in this study consistently suggest that high-resolution quantitative bathymetric estimates can be derived for mollusk samples independent of stratigraphy for fully marine settings. When applied simultaneously to both samples and taxa, these approaches provide a viable strategy for quantifying stratigraphic and paleontological patterns and enhancing interpretations of basin-scale depositional systems.

Stratigraphic signatures of mass extinctions: ecological and sedimentary determinants

Stratigraphic patterns of last occurrences (LOs) of fossil taxa potentially fingerprint mass extinctions and delineate rates and geometries of those events. Although empirical studies of mass extinctions recognize that random sampling causes LOs to occur earlier than the time of extinction (Signor–Lipps effect), sequence stratigraphic controls on the position of LOs are rarely considered. By tracing stratigraphic ranges of extant mollusc species preserved in the Holocene succession of the Po coastal plain (Italy), we demonstrated that, if mass extinction took place today, complex but entirely false extinction patterns would be recorded regionally due to shifts in local community composition and non-random variation in the abundance of skeletal remains, both controlled by relative sea-level changes. Consequently, rather than following an apparent gradual pattern expected from the Signor–Lipps effect, LOs concentrated within intervals of stratigraphic condensation and strong facies shifts mimicking sudden extinction pulses. Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges. Such effects of stratigraphic architecture, co-produced by ecological, sedimentary and taphonomic processes, can easily confound interpretations of the timing, duration and selectivity of mass extinction events. Our results highlight the necessity of accounting for palaeoenvironmental and sequence stratigraphic context when inferring extinction dynamics from the fossil record.

GEOCHEMISTRY, CLAY MINERALOGY, AND TAPHONOMY OF THE FOSSIL-RICH BUTTER SHALES

Abstract The Cincinnati Arch contains a suite of Upper Ordovician, mixed siliciclastic-carbonate, shallow marine strata, with alternating packages of packstones and shales that contain exceptionally well-preserved articulated fauna. The Waynesville Formation (C5 sequence) in Eastern Indiana includes distinctive clay-rich shales, which are locally referred to as butter shale. These bluish-green claystones are distinct in comparison to other shales because of their geochemistry, clay mineralogy, bulk composition, and well-preserved and biostratigraphically important fauna. This study focuses on an outcrop with exposed butter shale from the Waynesville Formation at Hanna Creek in Brookville, Indiana. Bulk-rock samples were collected and the geochemical and clay mineralogical composition of the claystones were analyzed using x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. On average, the butter shale clay mineralogy is 74% illite, 25% chlorite/kaolinite, and less than 1% smectite. The butter shale lacks expandable smectite, likely from post-depositional alteration of original smectite through illitization and chloritization. The presence of iron in the system as a result of diagenetic processes allowed for the immediate mineralization, and in some cases pyritization, of fossils. Bulk chemical analysis coupled with clay mineral analysis provides an explanation for the Fe substitution into chlorite for the butter shales and supports the interpretation of rapid sediment sealing that allowed for the pristine preservation of articulated fauna and mineralization observed in the butter shale.