Relations Between Iron Oxides, Soil Color, and Soil Formation

Laboratory measurement of soil color: theory and practice.

Eolian dust derived from the desert regions of North Africa is blown far into the tropical Atlantic Ocean by persistent easterly and northeasterly winds. In this paper, we demonstrate that the iron oxides, hematite and goethite, are a worthwhile addition to proxy monitors of eolian sedimentation in the tropical Atlantic. Iron oxides are identified by diffuse reflectance spectrophotometry, a technique capable of identifying these minerals in concentrations as low as 0.01% by weight. We analyze samples from both the modern and last glacial maximum (LGM) synoptic levels from 178 sample locations yielding a total of 356 samples distributed throughout the Atlantic Ocean. To determine the relative contribution of the iron oxides, we factor analyzed the modern and LGM levels as a single data set. The iron oxide factor explains about 25% of the variance in the combined core top and LGM data set. Mapped factor scores for the LGM and modern ocean indicate high iron oxide values are present in just two regions, one off eastern North America and the other off northwest Africa. In the region off eastern North America, iron oxide occurs primarily during the LGM as the previously noted “brick red lutite,” a unique sediment type derived from the erosion of Permo-Carboniferous red beds in Atlantic Canada. A larger, lobe-shaped area of iron oxide rich sediment is present off northwest Africa in both the modern and LGM levels. The modern iron oxide lobe is coincident with the distribution of eolian dust as determined by observations from ships, satellites, and analysis of air samples. During the LGM, iron oxides exhibit a similar distribution except the southern margin of the region shifts equatorward and iron oxide concentration increases.

Modern and Last Glacial Maximum eolian sedimentation patterns in the Atlantic Ocean interpreted from sediment iron oxide content

Durations of soil formation and soil development indices in a Holocene Mediterranean floodplain

Clarifying stages of alluvial fan evolution along the Sfakian piedmont, southern Crete: New evidence from analysis of post-incisive soils and OSL dating

The iron oxide mineralogy of soils of two river terrace sequences of the rivers Jarama and Henares, Central Spain, which are in close proximity, was studied. The degree of weathering increases with age, as shown by several mineralogical and geochemical indicators, such as the increase in the ratio dithionite extractable (free) Fe / total Fe. Low values for oxalate extractable Fe indicate that the Fe oxides are essentially crystalline. In the Jarama sequence goethite is the dominant pedogenic Fe oxide. In the Henares sequence both goethite and hematite are present in significant amounts. The difference is attributed to pedoenvironmental differences, such as the lower pH and the moister pedoclimate of the Jarama sequence soils. The sensitivity of the goethite-hematite system to pedoenvironmental factors shows the limited value of iron oxides for the estimation of paleoclimatic conditions or relative age in chronosequences, even in relatively small geographical areas.

M.C. Diaz

Papers

Mineralogy of iron oxides in two soil chronosequences of Central Spain