Dramatic Holocene ecologic change on the Colorado Plateau inferred from a laminated pedogenic carbonate record
Abstract
Terrestrial paleoclimate records are critically important for testing hypotheses of climate dynamics and verifying climate simulations. However, unlike their oceanic counterparts, terrestrial records are short, more commonly discontinuous, and require specific geographic conditions not necessarily ideal for proposed questions (e.g., speleothem records must come from wherever a cave occurs). We instead utilize laminated soil carbonate rinds as a high resolution (100s yr) paleoclimate archive because rinds are apparently continuous over 10s ka, maintain stratigraphic order, and are common in arid and semi-arid regions.
We used this methodology at Torrey, Utah, to understand hydroclimatic effects on soils and ecologic conditions on the Colorado Plateau over 33-3 ka. Our field site sits at the northern edge of the North American Monsoon. Modern calibration data provided evidence of a strong summer seasonality bias in soil carbonate formation and "clumped" isotope data showed this bias was consistent through time, so we interpret data in the context of summer soil conditions. The record consists of radiocarbon-dated δ13C- and δ18O-soil carbonate transects obtained via Secondary Ion Mass Spectrometry. Our record provided evidence of the abundance of plants using the C3 vs. C4 pathways and of contemporary soil conditions. Soil conditions from 33-10 ka were stable and contemporaneous with a stable, mixed vegetation landscape (≈40-60 % C3). However, in the Holocene (<11 ka), C3 vegetation increased to 70-80% of the landscape. This was followed at 6 ka by a dramatic, generally decreasing trend in C3 vegetation until by 3 ka the landscape was void of C3 plants (i.e., 100% C4 plants). The overall decrease in C3 plants was broken by a brief pause at 6-4 ka. The timing of inferred early- to mid-Holocene ecologic changes are consistent with oxygen isotope data and regional proxy records and suggest that (a) variability in the inferred onset of Holocene aridity may in part be due to elevation sensitivity of locations and (b) decreasing summer rainfall and increasing soil aridity favored C4 plants. However, further increases in C4 vegetation occurred after 5 ka despite apparently ameliorating soil moisture conditions, indicating a potentially complex vegetation response that should be further investigated.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP21B..04C
- Keywords:
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- 3335 North American Monsoon;
- ATMOSPHERIC PROCESSESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4914 Continental climate records;
- PALEOCEANOGRAPHYDE: 4938 Interhemispheric phasing;
- PALEOCEANOGRAPHY