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Medientyp:
E-Artikel
Titel:
Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka
Beteiligte:
von Blanckenburg, Friedhelm;
Hewawasam, Tilak;
Kubik, Peter W.
Erschienen:
American Geophysical Union (AGU), 2004
Erschienen in:Journal of Geophysical Research: Earth Surface
Sprache:
Englisch
DOI:
10.1029/2003jf000049
ISSN:
0148-0227
Entstehung:
Anmerkungen:
Beschreibung:
<jats:p>Some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep, humid, tropical highlands of Sri Lanka. The total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites, bedrock from mountain crests, and bedrock from inselbergs. Denudation rates are in the range of 5–30 t km<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup> (2–11 mm ky<jats:sup>−1</jats:sup>). These rates average denudation over the last 50–250 ky. Weathering exports in rivers draining the mountainous Central Highlands show that silicate weathering rates are also low, varying from 5 to 20 t km<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup> today (2–7 mm ky<jats:sup>−1</jats:sup>), but they represent a significant fraction of the total denudation. All these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief, temperatures, and precipitation. We speculate that the high relief in Sri Lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 Ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup. It is possible that throughout this history, hillslopes, where not exposing bare bedrock, were protected by thick weathered profiles. Such clay‐rich layers would inhibit silicate weathering by shielding bedrock from weathering agents. In the absence of landscape rejuvenation, physical erosion rates are low, and fresh mineral surfaces are not being supplied. The observation that wet, steep, tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long‐term controls of atmospheric CO<jats:sub>2</jats:sub> budgets: High temperature and precipitation, which are much invoked though controversial agents for silicate dissolution and CO<jats:sub>2</jats:sub> drawdown, become ineffective in promoting weathering in areas that are not tectonically active.</jats:p>