J. D. Gulley

Natural variations in calcite dissolution rates in streams: Controls, implications, and open questions

Models of bedrock channel evolution typically assume that chemical erosion is negligible in comparison to mechanical erosion. While this assumption is reasonable for channels in silicate rocks, it is questionable within highly soluble strata such as carbonates. The magnitude and variability of calcite dissolution rates in streams has remained as a critical unknown for models of bedrock incision and karst conduit formation. Here we use U.S. Geological Survey data to estimate calcite dissolution rates from 77 different streams located in a wide range of settings. The calculated rates are commonly on the order of ∼1 mmyr−1, which is 1 to 2 orders of magnitude larger than previous estimates. We also find that PCO2 is the strongest control on at-a-site variability, though some sites also display dilution-controlled variability. Typically, dissolution rates vary within a relatively narrow range, which has important implications for the relative importance of chemical and mechanical erosion.

A conceptual model of supraglacial lake formation on debris-covered glaciers based on GPR facies analysis.

Jordan R. Mertes acknowledges funding from Michigan Technological University and The Michigan Technological University 2016 Fall Finishing Fellowship. Sarah S. Thompson acknowledges funding from the University Centre in Svalbard (UNIS) and the European Commission FP7-MC-IEF.

Using Structure from Motion to create Glacier DEMs and Orthoimagery from Historical Terrestrial and Oblique Aerial Imagery

Increased resolution and availability of remote sensing products, and advancements in small-scale aerial drone systems, allows observations of glacial changes at unprecedented levels of detail. Software developments, such as Structure from Motion (SfM), now allow users an easy and efficient method to generate 3D models and orthoimages from aerial or terrestrial datasets. While these advancements show promise for current and future glacier monitoring, many regions still suffer a lack of observations from earlier time periods. We report on the use of SfM to extract spatial information from various historic imagery sources. We focus on three geographic regions, the European Alps, High-Arctic Norway and the Nepal Himalaya. We used terrestrial field photos from 1896, high oblique aerial photos from 1936 and aerial handheld photos from 1978 to generate DEMs and orthophotos of the Rhone glacier, Broggerhalvoya and the lower Khumbu glacier, respectively. Our analysis shows that applying SfM to historic imagery can generate high quality models using only ground control points. Limited camera/orientation information was largely reproduced using self-calibrated model data. Using these data, we calculated mean ground sampling distances across each site which demonstrates the high potential resolution of resulting models. Vertical errors for our models are ±5.4 m, ±5.2 m and ±3.3 m. Differencing shows similar patterns of thinning at lower Rhone (European Alps) and Broggerhalvoya (Norway) glaciers, which have mean thinning rates of 0.31 m a-1 (1896-2010) to 0.86 m a-1 (1936-2010) respectively. On these clean ice glaciers thinning is highest in the terminus region and decreasing upglacier. In contrast to these glaciers, uneven topography, exposed ice-cliffs and debris cover on the Khumbu glacier create a highly variable spatial distribution of thinning. The mean thinning rate for the Khumbu study area was found to be 0.54±0.9 m a-1 (1978-2015).

Hypogene Karst Influences in the Upper Floridan Aquifer

Dissolution of eogenetic carbonates in the Upper Floridan aquifer has produced the world’s densest assemblage of first-magnitude cave springs. Conceptual and numerical models of cave origin in the aquifer have emphasized epigenic and mixing-dissolution processes. We draw upon recent research concerning phreatic caves in the Suwannee River Basin, and dry caves in the west and central Florida, to suggest instead that many caves in the aquifer formed by hypogenic processes. Formerly, undersaturation generated in the subsurface has usually been ascribed to the mixing of, or temperature changes in, subsurface fluids. Here, we describe an alternate process. Cave formation at modern water tables in the aquifer has been linked to respiration of CO2 in the deep vadose zone and at water tables. Respired CO2 generates carbonate mineral undersaturation when it hydrates to carbonic acid at water tables. Because undersaturated waters are created at the water table, caves form as isolated macropores. Caves at modern water tables in the aquifer lack initial connections to the surface (e.g., entrances) and have morphologies that are unrelated to surface drainage, making them similar in many respects to flank margin caves (see Mylroie and Mylroie, Chap. 51). Since many caves that are below the modern water table have similar morphologies to caves that are at the modern water table, it is likely that they formed by similar processes operating at water tables associated with lower sea levels. These caves became sources of springs and flooded sinkholes when Holocene sea-level rise elevated water tables close to, and above, the land surface.

Sea level rise and inundation of island interiors: Assessing impacts of lake formation and evaporation on water resources in arid climates

Coasts of many low-lying islands will be inundated should sea level rise by 1 m by 2100 as projected, thereby decreasing water resources through aquifer salinization. A lesser known impact occurs if rising sea-level elevates water tables above interior topographic lows to form lakes. Impacts of lake formation on water resources, however, remain unquantified. Here we use hydrological models, based on islands in the Bahamian archipelago, to demonstrate that, on islands with negative water budgets, evaporation following lake inundation can cause more than twice the loss of fresh groundwater resources relative to an equivalent amount of coastal inundation. This result implies that in dry climates, low-lying islands with inland depressions could face substantially greater threats to their water resources from sea-level rise than previously considered.

The role of antecedent groundwater heads in controlling transient aquifer storage and flood peak attenuation in karst watersheds: Controls on transient aquifer storage in a karst watershed

Transient storage of floodwaters in aquifers is known to attenuate peak flows in rivers and drive subsurface dissolution. Transient aquifer storage could be enhanced in watersheds overlying karst aquifers where caves facilitate surface and groundwater exchange. Few studies, however, have examined controls on, or magnitudes of, transient aquifer storage or flood peak attenuation in karstic watersheds. Here we evaluate flood peak attenuation with multiple linear regression analyses of 10 years of river and groundwater data from the Suwannee River, which flows over the karstic upper Floridan aquifer in north-central Florida and experiences frequent flooding. Regressions show antecedent river stage exerts the dominant control on magnitudes of transient aquifer storage, with recharge and time to peak having secondary controls. Specifically, low antecedent stages result in larger magnitudes of transient aquifer storage and thus greater flood attenuation than conditions of elevated antecedent stage. These findings suggest subsurface weathering, including cave formation and enlargement, caused by transient aquifer storage could occur on a more frequent basis in aquifers where groundwater table elevation is lowered due to anthropogenic or climatic influences. Our work also shows that measures of groundwater table elevation prior to an event could be used to improve predictive flood models.