titanium oxide is one of the most stable component of the soil against weathering. Therefore it is a good indicator of weathering intensity. In most heavily weathered soil zones titanium oxide is relatively enriched because compounds which are more susceptible for weathering are dissolved. Therefore in soils with in-situ soil development normally a decrease in Titanium oxide concentrations from the soil surface to the bedrock is found. If in the subsoil a buried fossile soil layer with high weathering intensity is situated, there the titanium oxide concentration is high again.

I am not sure though...might be there is some biogenic processing. I have detected Ti in lithified soil layers that are bioturbated (sediment processing by bottom dwelling worms/crustaceans). Did you detect any such sediment-animal interactions in your sample locality?

Just a minor comment: the change of titanium concentration in soils with depth will be strongly related to the degree of weathering, low developed soils could not have significant changes within the soil profile. In developed soils, the main process is related to the relative increment of this "heavy" element in relation to the light ones.

There are always two ways of interpretation for this kind of results. Weathering from the parent material in the surrounding landscape could add to its concentration in top soil. Even anthropurgic input could do so. Otherwise, the concentration of any metal with a geo-genic origin is ought to decrease along the soil profile, i.e. topsoil followed up by sub soil, followed up by lower stratum and so on. Kindly check with the physiography and geology of the area, basically the lithology. That would provide you enough ponits to think off, which would propel you towards drawing an inference.

This is a not easy answer. In general a High Heavy metal concentration in top soil is associated to a pollution process of anthropogenic origin. If you consider that Ti has a natural origin, you have to evaluate the soil profile heterogeneity, it is possible that the top horizonts have are diferent with the more depth horizonts as a consecuence of deposition of diferent materials, and therefore, the subsoil representa diferent parent material with diferent Ti concentrations.

Like Francisco Martin Peinado, I think that the high concentration of Ti at the soil surface is related to soil development. More weathered and developed horizons are enriched in Ti because titanium oxides, for example rutile and ilmenite, are very resistant to weathering. In fact, they are used as indicators of the degree of weathering. Usual numbers for this oxides in temperate environments are 0.1-0.6%, while in highly weathered old soils values can rich 12%.

Let me clarify one point. There was a typographical error. Hence the sentence may be read as "Even anthropurgic input could do so. Otherwise, the concentration of any metal with a geo-genic origin is ought to increase along the soil profile".

titanium oxide is one of the most stable component of the soil against weathering. Therefore it is a good indicator of weathering intensity. In most heavily weathered soil zones titanium oxide is relatively enriched because compounds which are more susceptible for weathering are dissolved. Therefore in soils with in-situ soil development normally a decrease in Titanium oxide concentrations from the soil surface to the bedrock is found. If in the subsoil a buried fossile soil layer with high weathering intensity is situated, there the titanium oxide concentration is high again.

The question you need to ask is what is the source of Ti, and how are you measuring it. If you are completely digesting the sample, the elevated Ti is likely from sand sized oxides. In typical weathering, smaller particles will move downward in the soil profile. This is why you see clay accumulations in sub surface horizons. This could by subtraction cause higher sand fractions near the surface. If you have a fixed Ti-Oxide percentage in the sands, this would cause elevated Ti concentrations near the surface. That said, depositional history of the soil material is critical. If deposited in a location such as a coastal plane where specific coastline structure and beach migration occur there could be significant subsurface accumulations of Ti through the profile. See - Ti mining operations in Virginia as an example.

Titanium is a lithogenic element with conservative behavior in the soil.

However, this general pattern is strongly affected by 1. soil type, 2. geochemical processes, and 3. physical fractionation of the original material.

1. In organic soils, Ti will increase into the soil horizons closest to the rock or sediment. In soils of fluvial origin, Ti distribution be conditioned by the material to be deposited at any time and not by soil depth. In cambisols, often Ti increased cambic B horizon.

2. In processes such as podsolization, titanium can be increased in the wash horizons (horizon E) whenever there is no also clays mobilization. In clay soils with argillic processes may be a Tibilization towards argic B horizon (Bt horizon).

3. Physical fractionation of soil inorganic particles generate the fractionation of lithogenic elements. Thus, the zirconium accumulates in grus fractions (coarse sand and gravel) while titanium collects in the fine fraction (silt and clay).

Thank you to everyone for their input. I think I have a good starting point on how to move forward.

Some additional information:

Soils have developed on glacial till which lies on top of shale bedrock.

The soils are chernozems

Climate is sub-humid with a lot of the precipitation falling as snow, warm summers and cold winters

Primarily agricultural land use

Located in the northern great plains

At least in Porder et al 2007 examining elemental mass loss across the Hawaiian chronosequence the concentration of Titanium oxide increased as one moved down the depth profile. This was examined at several different soil ages and several different rainfalls, so at least in Hawaiian basalts the effect seems pretty consistent across weathering intensities. There would be virtually no anthropogenic sources in this case, and they also corrected for carbon dilution so only looked at relative elemental concentrations.

Maria Julia Mazzarino, would you appreciate giving me the references of your comment?. "Usual numbers for this oxides in temperate environments are 0.1-0.6%, while in highly weathered old soils values can rich 12%".