J. Burdett | Geology Reports

USGS1997

Carbon isotopes in biological carbonates: Respiration and photosynthesis

Respired carbon dioxide is an important constituent in the carbonates of most air breathing animals but is much less important in the carbonates of most aquatic animals. This difference is illustrated using carbon isotope data from freshwater and terrestrial snails, ahermatypic corals, and chemoautotrophic and methanotrophic pelecypods. Literature data from fish otoliths and bird and mammal shell and bone carbonates are also considered. Environmental CO 2 /O 2 ratios appear to be the major controlling variable. Atmospheric CO 2 /O 2 ratios are about thirty times lower than in most natural waters, hence air breathing animals absorb less environmental CO 2 in the course of obtaining 0 2 . Tissue CO 2 therefore, does not isotopically equilibrate with environmental CO 2 as thoroughly in air breathers as in aquatic animals, and this is reflected in skeletal carbonates. Animals having efficient oxygen transport systems, such as vertebrates, also accumulate more respired CO 2 in their tissues. Photosynthetic corals calcify mainly during the daytime when photosynthetic CO 2 uptake is several times faster than respiratory CO 2 release. Photosynthesis, therefore, affects skeletal δ 13 C more strongly than does respiration. Corals also illustrate how “metabolic” effects on skeletal isotopic composition can be estimated, despite the presence of much larger “kinetic” isotope effects.

Nevada↗

USGS1997

Holocene eolian activity in the Minot dune field, North Dakota

Stabilized eolian sand is common over much of the Great Plains region of the United States and Canada, including a subhumid area of ~1500 km 2 near Minot, North Dakota. Eolian landforms consist of sand sheets and northwest-trending parabolic dunes. Dunes and sand sheets in the Minot field are presently stabilized by a cover of prairie grasses or oak woodland. Stratigraphic studies and accelerator mass spectrometry radiocarbon dating of paleosols indicate at least two periods of eolian sand movement in the late Holocene. Pedologic data suggest that all of the dune field has experienced late Holocene dune activity, though not all parts of the dune field may have been active simultaneously. Similar immobile element (Ti, Zr, La, Ce) concentrations support the interpretation that eolian sands are derived from local glaciofluvial and glaciolacustrine sediments. However, glaciolacustrine and glaciofluvial source sediments have high Ca concentrations from carbonate minerals, whereas dune sands are depleted in Ca. Because noneolian-derived soils in the area are calcareous, these data indicate that the Minot dune field may have had extended periods of activity in the Holocene, such that eolian abrasion removed soft carbonate minerals. The southwest-facing parts of some presently stabilized dunes were active during the 1930s drought, but were revegetated during the wetter years of the 1940s. These observations indicate that severe droughts accompanied by high temperatures are the most likely cause of Holocene eolian activity.

North Dakota↗