Time averaging, stratigraphic disorder and geochemical proxy records: cautionary tale from the Holocene stratigraphic record of the Adriatic Sea

Variations in sediment supply are known to potentially alter the concentration of fossils or to mix up components of different environments or stratigraphic ages. The combination of shifts in sediment supply rates, sediment transport, bioturbation, erosion, and accommodation space can, controlled by the eustatic sea-level, lead to systematic changes in the extend of time averaging and consequently temporal resolution of the fossil record. The effect of this bias on fossil occurrence patterns is well studied, though less emphasis has been paid to its potential influence on the observed record of geochemical proxies, which are extracted from these fossils. To assess this potential bias in geochemical proxies we analysed δ¹³C and δ¹⁸O values from ¹⁴C- and amino-acid dated Holocene Corbula gibba shells from the Northern Adriatic Sea, covering the transgression and sea level highstand since the last deglaciation. As the fossil assemblages in transgressive deposits are typically more time-averaged compared to highstand strata, and thus capture larger absolute age span,  the range of δ¹³C and δ¹⁸O values should be higher in the transgressive core sections. However, our initial results suggest that while the age range indeed increases with sediment depth, the carbon and oxygen isotopic ranges decrease, with a few exceptions. This could be caused by higher variation or higher pace of environmental change during deposition of highstand strata, or by increased homogenisation due to the increased time of exposition to diagenetic fluids with sediment depth.