McMurdo Dry Valleys

Soil Apatite Grain Weathering Metrics from Taylor Valley, Antarctica

Abstract: 

Mineral apatite is the ultimate source of the essential nutrient phosphorus to the soil ecosystem. In order to assess the biogeochemical weathering of apatite grains in the dry, basic soils of the McMurdo Dry Valleys, we collected nine surface soil samples from the Fryxell and Bonney Basins of Taylor Valley. After separating more than 50 individual soil apatite grains from each sample, we used scanning electron microscopy to quantify the morphology and surface etching of apatite grains to determine the degree of weathering. We developed three metrics to quantify the degree of weathering: aspect ratio, percent crystal faces, and a qualitative pitting index. This dataset contains the raw data from analyzing the morphology of more than 600 grains using the software ImageJ. Samples were collected during January 2013. The samples from the Bonney Basin (LB) were collected 21 Jan 2013, and the samples from the Fryxell Basin (LF) were collected 18 January 2013. Samples were processed and analyzed 2014-2016.

Core Areas: 

Additional Project roles: 

789
790
791
792
793

Data set ID: 

261

Short name: 

soils_apatite

Data sources: 

SOILS_APATITE

Methods: 

We collected samples the Fryxell and Bonney Basins of Taylor Valley. In the Fryxell Basin, we collected 2 samples at low elevation near the mouth of Von Guerard Stream and 3 samples at high elevation along the southern valley wall. In the Bonney Basin, we collected 2 samples at low elevation near Wormherder Creek and 2 samples at high elevation along the southern valley wall.To isolate individual apatite grains from soil samples, we used standard sieving, heavy-liquids, and magnetic mineral separation techniques. Grains presented here are all within the 125-250 µm size fraction.To capture images of the grains for the weathering metrics presented here, we used a Hitachi TM3000 scanning electron microscope. We used the software ImageJ to analyze the images.We developed three weathering metrics that characterize grain shape and micromorphology: aspect ratio, percent crystal faces, and chemical etching (pitting index). We defined the aspect ratio as the length of the long axis of the grain divided by the length of the short axis, giving a measure of how elongated grains are. We defined the percent crystal faces metric as the percent of the total visible grain area that was a recognizable crystal face. Our pitting index ranged from 1 (grains with few to no etch pits) to 4 (grains with etch pits covering the entire grain surface).

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