干旱区生态环境知识资源中心

From ongoing research examining weathering rates and mechanisms in the Paleozoic sandstones of Petra, Jordan, two possible thresholds have been identified. From a carved Roman Theater built under Vitruvian standards during the first century A.D., a two-meter sampling scheme was used to measure the presently weathered surfaces from a hypothetical false datum determined through originally documented Roman engineering and construction canon. Some 526 depth measurements were made on vertical and horizontal surfaces and correlated to the intrinsic variables of sandstone matrix-to-clast ratios, overall densities, matrix chemistry (Si, Ca, Fe, Al concentrations), and to the extrinsic accumulated annual insolation receipt (mega-joules/m(2)).

When matrix iron concentrations exceeded 2%, an abrupt decrease in overall weatherability is indicated until weathering is found to have decreased below measurable limits at 4%. It is speculated that matrix iron acts as a sandstone-clast binding agent, reducing clast disaggregation. In sandstone strata with matrix calcium concentrations exceeding 10%, weathering was accelerated with insolation >5200-5300 megajoules/ m(2). It is speculated that the increased heating from insolation is responsible for irregular calcite crystal expansion and contraction causing matrix/clast interface microfracturing, clast disaggregation, and subsequent weathering.

Mean recession rates ranged from 13 to 66 mm/millennium on horizontal surfaces and from 7 to 18 mm/millennium on vertical surfaces. Cross differences in recession rates were attributed to the extrinsic influences of moisture availability (slope) and insolation (aspect), while minor differences were attributed to intrinsic characteristics of matrix chemistry (Fe, Ca), sandstone density, and clast/matrix ratios.