Chad Wittkop

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

A lacustrine carbonate record of Holocene seasonality and climate

Annually laminated (varved) Holocene sediments from Derby Lake, Michigan, display variations in endogenic calcite abundance reflecting a long-term (millennial-scale) decrease in burial punctuated with frequent short-term (decadal-scale) oscillations due to carbonate dissolution. Since 6000 cal yr B.P., sediment carbonate abundance has followed a decreasing trend while organic-carbon abundance has increased. The correlation between organic-carbon abundance and the sum of March-April-October-November insolation has an r 2 value of 0.58. We interpret these trends to represent a precession-driven lengthening of the Holocene growing season that has reduced calcite burial by enhancing net annual organic-matter production and associated calcite dissolution. Correlations with regional paleoclimate records suggest that changes in temperature and moisture balance have impacted the distribution of short-term oscillations in carbonate and organic-matter abundance superimposed on the precession-driven trends.

Archean to Anthropocene

This volume of 25 field guides plus one paper on field instruction was prepared in conjunction with the 2011 GSA Annual Meeting in Minneapolis, Minnesota. The diverse slate of field trips spans a geologically broad range of topics, including the Precambrian geology of the southern Canadian Shield; the economic geology of the Lake Superior region; Phanerozoic strata in Minnesota, Wisconsin, Iowa, and North Dakota; glacial geology; hydrogeology and limnology; undergraduate and K12 geoscience field education; archaeological investigations in the upper Mississippi River valley; and geology by bicycle.