The mechanism of REE-Y impregnation on active carbonate normal fault scarps

Abstract

The fluctuations of the Rare Earth Elements and Yttrium (REE-Y) concentrations on exhumed carbonate normal fault scarps may reveal the number and size of paleoearthquakes that exposed the scarp subaerially. This is because, prior to each large-magnitude earthquake, narrow (<50 cm) sections of the fault plane which are in direct contact with the soil become enriched in REE-Y before they are exhumed co-seismically, together with deeper, non-enriched, scarp sections. Following exhumation, depletion in REE-Y commences on both the enriched (i.e. ‘soil rupture zone’) and non-enriched (i.e. ‘rock rupture zone’) scarp sections. Although these processes are commonly described to occur on carbonate scarps, the mechanisms through which they operate remains poorly understood. Here, we present a series of laboratory tests that mimic the natural process of REE-Y enrichment/depletion to elucidate the mechanism of REE-Y impregnation. Our results indicate a fast uptake of REE-Y by the carbonate plane, when in contact with soil, either as (REE, Y)₂(CO₃)₃ precipitate or by adsorption on calcite surfaces. The source of REE-Y in soil solution is released in a “pulses” due to alternations of dry and wet periods, characteristic of Mediterranean climatic conditions. Organic matter oxidation during the first rain events, triggers the Mn reductive dissolution and the release of REE-Y into the soil solution. The pH decrease due to organic matter dissolution is buffered by calcite, especially in the vicinity of the scarp, where calcite dissolution and re-precipitation occurs with a marked pH oscillation between 9.3 and 7.7. Further, comparison of these results with empirical data from three co-seismically exhumed fault scarps in Greece and Italy places quantitative constraints on the timing of these processes: the REE-Y enrichment within the ‘soil rupture zone’ may reach a maximum of ∼50% in about 500 years (+0.53 μg/kg/year), while the REE-Y depletion from the scarp is slow (−0.021 μg/kg/year), with a maximum recorded retention time of ∼16 ka. These enrichment and depletion characteristics work together to preserve paleoearthquake signal on carbonate scarps. Thus, this methodology is a valuable tool for quantifying the number of past earthquakes on carbonate fault scarps and allows more targeted use of expensive dating techniques (i.e. with cosmogenic nuclides) in order to derive the precise timing of these paleoearthquakes.