INFLUENCE OF PALAEOWEATHERING ON THE TRACE METAL CONCENTRATIONS AND STABLE ISOTOPE VALUES

The palaeoweathering of the Lower Carboniferous marine black shale has been documented in the Kowala quarry, the Holy Cross Mountains, Poland (Marynowski et al., 2011). It took place after the Upper Carboniferous folding and before the Upper Permian deposition. Within black shale three zones were appointed: 1) non-weathered zone, where total organic carbon (TOC) and total sulphur (TS) amount are similar and quite constant (TOC = ~5%, TS = ~0.8%) 2) partially weathered zone, where TOC values decreased by c.a. 50% and TS values are very low (below 0.1%) and 3) highly weathered zone, where both TOC and TS values dropped to near zero. All commonly using inorganic proxies change their values in partially- and highly weathered zones. For instance, U/Th ratio values varies from c.a. 0.9 for unweathered part of black shale to 0.3-0.4 for highly weathered zone. On the other hand values of Ni/Co ratio increased significantly from c.a. 1.0 to c.a. 4.0. Concentrations of many trace metals (e.g. Cu, Ni, Co, Sr, Mo, Pb, Zn, As, Cr) changed significantly their values while REE concentration fluctuations are less noticeable. Moreover, we documented the patterns of variation in organic δ¹³C and δ¹⁵N as well as δ⁹⁸Mo along the non-weathered to highly weathered zone. The δ¹³C_org values show the identical values across the non-weathered and partially weathered zone, but surprisingly just before the partially weathered - highly weathered zonal boundary the δ¹³C_org gradually increased by 2‰ with maximum at the beginning of the weathered zone. Organic N isotopes are from 0.5‰ to 4.6‰ with the highest values observed in the partially weathered zone. Nitrogen totally vanished in the highly weathered zone. The molybdenum isotope values are high (up to 1.6‰) only in the non-weathered zone while δ⁹⁸Mo values for the weathered and partially weathered zone significantly decreased. According to these results we conclude that in sedimentary rocks influenced by weathering, both inorganic proxies as well as isotope values should be interpreted with caution.

LM and AP acknowledge financial support from the Polish National Science Centre grants 2011/01/B/ST10/01106 and 2011/03/B/ST10/04602.

Marynowski et al., 2011. Chemical Geology 285, 144-156.